INERT CARBON FREE RADICALS

INERT CARBON FREE RADICALS

MANUEL BALLESTER

Patronato "Juan de Ia Cierva", C.S.I.C., University of Barcelona, Barcelona, Spain

INTRODUCTIONReactivity of the carbon free radicals

Since Professor Gomberg reported' the detection of the first free radical,triphenylmethyl (I), it has been known that the trivalent carbon compounds,even the so-called "stable" long-lived free radicals, are unstable, chemicallyaggressive species.

By means of modern techniques it is now possible to detect, and evento study, extremely unstable free radicals. However, it is our purpose hereto limit the present study to those radicals which can exist in a substantialconcentration and therefore can be detected by classical methods such asdirect chemical reactions, magnetic susceptibility measurements and evenisolation, i.e. the already-mentioned "stable" free radicals, and to examinethe importance and influence of the steric factors in relation to their reactivity.

It is well known that carbon free radicals may disappear in many differentways. They dimerize reversibly (giving ethanes) as well as irreversibly, andthey disproportionate. They also react with species from the environmentsuch as the solvents, the oxygen in the air, inorganic and organic compounds,halogens, metals, other free radicals, etc.4—9.

Gomberg's radical, triphenylmethyl, is not only the first free radical everdetected, but also a very representative one among its class. It reacts re-versibly with itself to give hexaphenylethane (II). In fact, it has never been

pdo

(I) (11)

0(III)

123

MANUEL BALLESTER

isolated as such but only in its dimer form. At room temperature a 01 Msolution of it contains only about 3 per cent of free radical10.

In this connection it is mentioned that triphenylmethyl also undergoesirreversible dimerization to 4-diphenylmethyltetraphenylmethane (111)11_la.

The rate of reversible dimerization is very large. In fact, it can easily becalculated from the equilibrium constant10' 14—17 and the first order ratefor the dissociation of hexaphenylethane (see below), and it turns out to bearound lO l.mole-1min'. A feeling for such a value is obtained by realizingthat if it were possible to make a one per cent solution of triphenylmethylradical it would be half dimerized in about one tenth of a second.

The aggressiveness of triphenylmethyl is also evident from its reactionwith oxygen. The rate of this reaction is even larger than that of its re-versible dimerization. In fact, it is so high that it has been used to measurethe dissociation rate of hexaphenylethane. It has been found that in thepresence of a chain-inhibitor of the phenolic type, the reaction of this ethanewith oxygen is a first order process, its rate being about independent ofthe oxygen partial pressure'8' 19 (see also ref. 20). This clearly shows (i)that the rate-determining step of this reaction is the dissociation of thehexaphenylethane; (ii) that the actual reacting species is the triphenylmethylradical, and (iii) that, as indicated, the reaction rate of the latter withoxygen far exceeds that of its dimerization.

r.d.s.(C6H5)3CC(C6H5)3 == 2(C6H)3C.

very rapid(C6H5)3C. + 02 —-—--—- Oxidation products

The reaction with oxygen is therefore an established method not only forthe measurement of the dissociation rate of ethanes2123 but also for that ofradical dimerizations23.

The rate of the reaction between triphenylmethyl and the halogens is alsovery high. Even iodine or nitric oxide compete so successfully with thedimerization reaction that under certain circumstances good rates ofdissociation can also be obtained from the relevant reactions24' 25

Most "stable" carbon free radicals behave similarly. Some, such asSchienk's trisbiphenylylmethyl (IV)26 27, Gomberg's trip-nitrophenyl-methyl (V)28' 29, Ziegler's pentaphenylcyclopentadienyl (VI)3°' 31, dioxadi-hydrocoranthryl (Vu)32 and tri-3-pyrenylmethyl (VIII)33, show, however,remarkably little tendency to associate. Nevertheless, they react quicklywith oxygen, halogens and other substances.

One interesting group of carbon free radicals that has attracted someattention recently is Koelsch's 1 ,3-bis-biphenylen-2-phenylallyl (TX)34 andrelated radicals3537. These are completely disassociated free radicals which,although they react with chlorine and bromine, are nevertheless inerttowards iodine. Also, their reactions with oxygen are relatively slow. Half-lives of 10 mm and 10 h in solution, in the air, have been reported forKoelsch's radical37 and Kuhn's 2(pbiphenylyl)bisbiphenylenylal1yl (X)36,respectively.

124


N 02

00

N 02

(V)

COllIO

(VI)

125

0(VII)

(Iv)

(VIII)

(IX)

CX)

MANUEL BALLESTER

Factors governing stability in cabon free radicalsIt is generally accepted that the stability of arylmethyl radicals is partly

due to delocalization of the "odd" electron in the aromatic nuclei, andpartly to "frontal" steric strain in the corresponding ethanes, i.e., stericrepulsion between their two composing arylmethyl groups.

A measure for the contribution of these two intervening factors is theresonance energy of the radical and the steric strain energy of the ethane.

Figure 1. Schematic representation of the "frontal" strain in the ethane Ar3C—CAr3.

In Figure 1 a schematic representation of the "frontal" strain in the ethaneAr3C—CAr3 is shown. The arrows represent the actual acting forces.

It has been estimated that the effective resonance in the triphenylmethylradical (two moles) and the steric strain in hexaphenylethane (1 mole) areequally important. In terms of energy they both amount to about 35kcal38'

If triphenylmethyl were a flat molecule, with all its benzene nuclei lyingin a plane, its resonance stabilization would be about 80 kcal (2 moles)40.The 45 kcal loss is caused by the partial rotation of the planes of the benzenenuclei around their bonds with the central carbon atom thus conferring tothe molecule a propeller-like array (Figure 2). Such tilting, which is assumedto be the same for the three nuclei, is measured by the angle between theirplanes and that of their three bonds with the central carbon, and it is due tosteric interaction between the ortho hydrogens.

Figure 2. Propeller-like array of triphenylmethyl molecule.

126


The degree of their tilting is still a matter of speculation. While Theil-acker41, by taking account of the distances between the atoms involved andtheir spheres of action, adopts an angle of about 200, McLachlan42 andChesnut and Sloan43 find that its e.p.r. data is best interpreted when avalue of around 60° is assumed. The theoretical calculations carried out byAdrian38 lead to the value of 32°.

It is remarkable that the idea of frontal strain in this class of radicals isalready in Gomberg's celebrated first paper:

"The existence of such a body means that when three valencies of carbon are taken up bythree phenyl groups it is difficult, or perhaps even impossible, to introduce as a fourth groupsuch a complicated radical as Ph3C—. . . . Whether this be due to the negative character ofthe three phenyl groups, or whether it is caused by the fact that these groups take up so muchspace around the carbon atom as to hinder the introduction of another complicated group,

need not be discussed here"

Frontal steric strain may also be present in a substance resulting fromcombination of a triphenylmethyl radical with a bulky group. Consequently,this can slow down the relevant reaction rate or even prevent the occurrenceof the reaction. We shall use the term, steric shielding, for this type ofhindrance. Conversely, it can be expected that the highest rates will beobserved in reactions with small particles, particularly with other (smaller)free radicals.

If the ortho hydrogens of triphenylmethyl are replaced by other, necessarilybigger, substituents, either partly or completely, it is clear that the frontalstrain in the ethane will increase. The steric energy will therefore be greaterthan 35 kcal. However, the twist angle in the radical will also increase and,consequently, the resonance energy will decrease. If such energy changescancel out, it should be expected that the equilibrium between the ethaneand the radical would not be significantly shifted. However, as we shall seenext, the evidence available shows that the frontal strain predominates,the net effect being an increase in the dissociation of the ethane.

Reactivity of arylmethyl radicals and steric factorsThere is no syin-diarylethane that dissociates appreciably at room tern-

perature.t Apparently, neither the resonance energy in the benzyl radicalsnor the frontal strain in the corresponding bibenzyls are important enough.

Most syin-tetraarylethanes are also not dissociated under such conditions.However, in a number of examples where frontal strain is also clearlyinvolved, the corresponding diarylmethyl radicals are detected. Such isthe case of radicals a-tert-butyldiphenylmethyl (XII)45, a-tert-butylbis-biphenylmethyl (XIII)46, pentaphenylethyl (XIV)47, and 9-triphenyl-methyifluorenyl (XXII)48. In these examples, the third substituent on the

f In this connection it is mentioned that 2,4,6-triisopropylphenylcthynyl (XI) has beendetected44.

,CH(CI-I,),

(CH3)Ct-1--(-—cc.CI-I(C}-l,),

(XI)

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MANUEL BALLESTER

central methyl contributes little to resonance, its role being mainly steric.Another type of stable diphenylmethyl is that which has sterically inter-

acting substituents in the ortho positions of the phenyl groups. Examples:di-2-xylylmethyl (XV)49, dimesitylmethyl (XVI)43 50, 51, di-3-duryl-methyl (XVII)49, bis(2,6-diethylphenylmethyl (XVIII) 23, bis(2,6-diisopro-pylphenyl)methyl (XIX)23, 2,2'-tert-butyldiphenylmethyl (XX)52, bis(2,5-di-tert-butylphenyl) methyl (XXI)53, and di-! ,8-di-tert-butylfluorenyl(XXIII)52.

(XII) RCH3; R'H (XV) RCH3; R'RH(XIII) RCH3 R'C6H5 (XVI) RRCH3; R'H(XIV) R=C6H5; R'H (XVII) RR'CH3; RH

,R

(XVIII) R C2H5; R'= R' H

(XIX) R/-C3H7; R'R"H

0H-

(XX) P-f C4H9, P—Fl (C6H)3C_!<(XXI) R=R'.t—C4H9 (XXII)

In this connection it should also be mentioned that radicals XIV,XIX, XXI and XXII are completely disassociated. The Stuart—Briegleb

128

(CH3)3C

H'0

(CH3)3C

(XXIII)


scale models show clearly that these radicals are also the most shielded aroundthe methyl carbon, i.e. that frontal strain of the corresponding ethanes ismaximal.

These radicals are, nevertheless, extremely sensitive to oxygen. In fact, therate of dissociation of some of the corresponding sym-tetraphenylethanes hasbeen ascertained from their reaction rates with oxygen23' 54, It has beenfound, for example, that the rate of association of radicals XV and XVIII iseven somewhat lower than that for triphenylmethyl23. It should, however,be mentioned that in equilibrium they are more associated than the latterradical10' 23

The steric effects of the substituents are, as expected, also important inthe case of the triarylmethyl radicals. Resonance is here greater. In fact thelarge or complete disassociation of some radicals of this type is predomi-nantly due to resonance, the best example being Schienk's tribiphenylyl-methyl radical IV already mentioned.

Of course, in the triphenylmethyls the frontal strain of the correspondingethanes (or shielding of the radicals) can only be varied by changing theort/zo substituents. It might be thought that replacement of all ortho hydrogensof triphenylmethyl by alkyl groups or halogens, for example, would lead toa large group of completely disassociated free radicals. Unfortunately,internal molecular overcrowding has prevented so far not only the synthesisof the relevant chemical precursors, but also even that of some less orthosubstituted ones. Actually, not only is the number of the latter radicals verylimited but also very few are completely disassociated. All of them reactwith oxygen and other chemically aggressive substances.

The following correlations show clearly enough, however, the importanceof the steric factor in controlling the association of such radicals. They havebeen taken from the data published by Preckel and Selwood'7, Theilackerand Wessel—Ewald41, and Marvel and coworkers55' 56,

Consideration of sequences 1, 2 and 3, as well as the results mentionedbefore for the diphenylmethly radicals, leads to the following general conclu-sions concerning the influence of the steric factors on the disassociation of freeradicals: (i) The effect of a substituent in the ortho position is much greaterthan that of one in the meta orpara position. (ii) The effect increases with thesize of the ortho substituents. (iii) The effect increases with the number ofortho substituents.

From the purely steric point of view, the effect of an a-naphthyl groupshould be close to that of an o-tolyl. Actually, the extent of association of theradicals a-naphthyldiphenylmethyl and o-tolyldiphenylmethyl is similar17' 57in spite of the greater resonance of the former40. This seems to indicate thateven here the steric factor predominates. Accordingly, the association of-naphthyldipheny1methyl is much greater.

Consideration of rather old data on phenanthryldiphenylmethyls alsosupports this conclusion. 1 -Phenanthryldiphenylmethyl, the steric equivalentof a-naphthyl, is less associated than its 2- or 3-phenanthryl isomers58.However, in the case of aromatic polynuclear groups, such as 3-pyrenyl, bothsteric and resonance effects seem to be of comparable importance33.

It is well known that the radical 9-phenyifluorenyl (XXIV) is completelyassociated at room temperature. The reason for this remarkable fact is that

129P.A.C.—K

MANUEL BALLESTER

uence 1

(C6H5)3C.

..,3% (20°;O03M)

(C6H5)2—200/o (20°;O03M)25°I (25°;O1M)

CH3GROUP .4

57%(25°;O1M) C6H5( 0 )2

91°/o(20°;003M)82°Io(25°;OIM)

(—)3C.!_.IQO%

(cH3———)3c. (__)3C.l6°fo(25°O1M) CH3

95% (19°;OO3M)87% (190; 0 1M)

()3C'/

CH3

1,00/ (25°;01M)

CH3 143

Group A; (C6H5' (C6H5)2CH3 C6H C6R5C(CH3)2.

the effect of the reduction of the frontal strain in the ethane outweighsthe relevant increase of the resonance in the radical59. The cause of thisreduction is that two of the three phenyl groups are being held in an almostcoplanar arrangement. This shows again how predominant the steric effectusually is. Accordingly, the introduction of a bulkier substituent in the

130

INERT CARBON FREE RADICALS.

Sequence 2

25°; O1M

(—Kh-_—l'-,c4Hg)2 <_—C E-_-—t-C4H9)2OH3

75°f. 65°!.

<

94 0/

Sequence 325°, O1P.l

(3c. <( )3C. <( )3c. <()3c.3 0f 7.5 0/ 12 °/. 17 °/.

(XXIV) OH3 (XXVI)

Associated (XXV)

Disassociated

9-position of fluorene not only restores stability of the radical at room tem-perature but in certain radicals, such as 9-triphenylmethyifluorenyl (XXII),9-mesityifluorenyl (XXV) and 9(2-terl-butylphenyl)fluorenyl (XXVI) doesso to such an extent that no appreciable association occurs52.

Radicals of Koelsh's type (IX and X) are sterically comparable toXXII, XXV and XXVI. However, they possess two equivalent centresof high electron spin density that make resonance a most important factorin conferring stability. As has already been indicated, not only are theycompletely disassociated but they even react at a moderate rate withoxygen3437. Half-lives in solution and in the air, of up to 10 h have beenreported36. Also, under normal conditions, they do not react with hydro-quinone, sodium suiphite, iodine, or vitamin C, and they can withstand

131

MANUEL EALLESTER

in solution temperatures close to 1000. However, they react immediatelywith chlorine, sulphuric acid, etc.

PERCHLOROARYLALKYL RADICALSIn 1954 we reported the synthesis of perchiorotoluene (XXVII)60. 61

This represented a major breakthrough leading to a new field full of potentialcompounds and new phenomena: the field of the alkaromatic chlorocar-bons62.

The first attempt to synthesize an alkaromatic chiorocarbon was probablythat of Beilstein about one hundred years ago63. Programmes with this endin view were undertaken by various research teams during World War H,but unfortunately they did not succeed. The difficulties encountered couldundoubtedly be traced to steric shielding in the process of introduction of thelast chlorines, steric compression in the molecule of the alkaromatic chioro-carbons, and electronic chemical deactivation. These effects are due to therelatively large size of the chlorines and to their accummulation. Actually,it is impossible to assemble a Stuart—Briegleb scale model for the simplestalkaromatic chlorocarbon, perchiorotoluene, because of the interferencebetween the alp/ia and the ortho chlorines (Figure 3). The steric strain

cL

for homomorphs of perchlorotoluene has been estimated64' 65 to bearound 24 kcal mole—1, a value consistent with the bathochromic shiftsobserved in its electronic absorption spectrum66. Such steric interactionsoccur also in many polynuclear aromatic chlorocarbons, such as perchioro-phenanthrene, which explain the fact that only a few of them were knownwhen we undertook our research programme (Figure 3).

182

Figure 3. Diagrammatic sketch of the Stuart—Briegleb scale models for perchiorotoluene andperchiorophenanthrene.


The synthesis of perchiorotoluene in good yield indicated that we had aselective, yet powerful, chlorinating agent capable of overcoming all thosehindrances appearing in the process of perchlorination. In fact, it has enabledus to synthesize other new aromatic and alkaromatic chiorocarbons. Fromthe study of their nature and behaviour we have learnt significant facts aboutthis new class of organic substances which, in turn, have helped us to devisenovel ways to synthesize them62.

We have mentioned that the greatest problem in the synthesis of stablepolyarylmethyl radicals is sometimes the unsurmountable steric difficultiesin the preparation of their chemical precursors. Consequently, certainalkaromatic chiorocarbons might prove to be valuable starting compounds forthe synthesis of stable free radicals, since the size of chlorine is practicallyidentical to that of the methyl group which, as it has been indicated, isvery effective in taming the aggressiveness of the di- and triaryl-methylradicals.

Perchlorobenzyl radicalOur first encounter with a perchiorinated arylmethyl radical took place

in 1 95967• By that time we were concentrating on the synthesis of new chloro-carbons and, consequently, our interest in free radicals was merely incidental.

ClCL2 ctCCt2CCt2C(XXX) (XXXI)/

Cl + Cl

Cl Cl C—Cl CE CL CLCl

Cl CL

CLC1(XXIX)

(XXVIII)

It was found that the reaction of perchlorotoluene (XXVII) with someinorganic reducing species gives a good yield of a mixture of cis- (XXVIII)and trans-perchlorostilbene (XXIX). This reaction was interpreted on thebasis of the formation of perchlorobenzyl radical (XXX), which, by dimeri-zation to perchlorobibenzyl (XXXI) and subsequent vicinal dechlorination,

133

(XXVII)

MANUEL BALLESTER

would finally yield the perchiorostilbenes.Evidence for such a mechanism was the fact that when the reaction was

performed in the presence of toluene, ocH-heptachiorotoluene (XXXII) wasisolated in a substantial yield, while that of the perchiorostilbenes wasconsequently diminished. In the reaction using iodide ion as the dechiorina-ting agent benzyl iodide was also isolated.

These results are consistent with a release of an oc-hydrogen from tolueneto the perchlorobenzyl radical, which competes with the formation of theperchiorostilbenes.

C6C15OC12 + C6H5CH3 -> C6C15CHC12 + C6FI5CH2

2 C6H5CH2 + '2 - C6H5CH2I

Substantial evidence for such a mechanism is obtained from the reactionof oH-heptachlorotoluene (XXXII) with iodide ion giving a mixture of

CL COt3 CL CCI2CCI2 CL

(XXXI!) (XXXII!)

— Cl CCL CCI Cl

(XXXIV)

2H,2'H-dodecachlorobibenzyl XXXIII and cis- and trans-2H,2'H-decachlorostilbene (XXXIV)67. It was also found that, under the reactionconditions, the bibenzyl XXXIII is readily converted into a mixture of thelatter products67.

It should be mentioned that various attempts to isolate the perchioro-bibenzyl in the reaction with perchiorotoluene have been unsuccessful.This is attributed to its high steric strain, higher than that of perchioro-toluene, which greatly assists the elimination of the vicinal chiorines. In thecase of the 2 H,2'H-dodecachlorobibenzyl, the presence of a hydrogen in anortho position on each phenyl ring provides room for the bulky alpha sub-stituents, and consequently reduces both its frontal steric strain and theassistance to chlorine elimination.

Figure 4 is a scale drawing of the perchlorobenzyl radical in the planarconformation which shows the "spheres" of action for the chiorines. Noticethat, on account of the steric interaction between the alpha and the orthochiorines, this conformation is impossible, the former chiorines beingnecessarily located above and below the plane of the benzene ring. Thescale models indicate that the angle between the latter and the plane of thethree bonds with the alpha carbon should be close to 45°.

134

Figure 4. Scale drawing of the perchlorobenzyl radical in the planar conformation showingthe "spheres" of action of chlorine.

Other reactions, which probably take place through the perchlorobenzylradical, are the thermolysis68 and the photolysis69 of perchiorotoluene.

The thermal decomposition of perchlorotoluene begins to take place at arather low temperature (185°). This is due to steric assistance to homolysisof an alpha-chlorine—carbon bond. The following sequence accounts forthe reaction products:

C6C15CC13 -÷ C6C15áC12 + C1 (1)

Cl + C6C15CCI3 - C6C16 + 'CC!3 (2)

C6C15CCI2 + 'CC13 -> C6C15CC1===CC12 + Cl2 (3)

Cl2 + C6C15CC13 -÷ C6C16 + CC14 (4)

Accordingly, the stoichiometry of the reaction is

3 C6C15CCI3 - 2 C6C16 + C6CI5CC1=CC12 + CC14

(3.000) (1.975) (0.967) (0.975)

The figures in parenthesis are the actual molar proportions of the products.Step 3, although sterically hindered, is assisted by the formation of mole-

cular chlorine in a concerted manner. This assumption is also supported bythe fact that 07 per cent (0.021) of trans-perchlorostilbene is isolated fromthe resulting reaction mixture.

As far as the photolysis of perchlorotoluene in carbon tetrachioride withultraviolet light is concerned69, the mixture obtained is consistent with thefollowing sequences

hvC6C15CC13 -÷ C6C15CC12 + Cl.

An alternative, rather unorthodox mechanism for this reaction has been reportedelsewhere68.

135


MANUAL BALLESTER

C6C15CC13 + Cl. — C6C16 + .CC13(isolated)

Cl. + Cd4 - Cl2 + .CC13

.cC13 + C6c15ãc12 -÷ C6C15c2c15(isolated)

2 C6C15Cl2 - C6C15CC1==CC1C6CJ5 + Cl2cis and trans(isolated)

hvC6C15C2C15 -÷ C6C15CC1=CC12 + Cl2

(isolated)

Other radicalsEvidence for sequence involving the formation of other perchiorocarbon

radicals, such as perchloro-3-phenylallyl, has also been obtained70.One remarkable reaction occurs through the formation of radicals of a

type describable by the general formula:

C13C(CC1CCI)p(Ar(CC1CC1)2p+lImAr(CC1=CC1)pCC12CL Cl Cl dcl CL

where r p— i

CL Cl CL dcl CL

The corresponding dimers have never been isolated; only the products oftheir vicinal dechlorination. When m is large they are polymeric chioro-carbons of extremely high thermal stability and chemical inertness71.

[—Ar(CC1=CC1)2p+i—]m

INERT CARBON RADICALSPerchlorodiphenylmethyl[PDM] radical

Two years ago, during the development of our programme on aromaticand alkaromatic chlorocarbons, it was decided to attempt the synthesis ofperchiorodiphenylmethane (XXXV) by means of our powerful chlorinatingagent. The odds were clearly against us, since such a chiorocarbon wouldpossess a huge steric strain. In fact, it can be assumed to be the result ofthe virtual substitution of an alpha chlorine in perchiorotoluene by a bulkypentachlorophenyl group. Nevertheless, perchiorodiphenylmethane wasobtained, although in a poor yield.

One most challenging question arose immediately: how would per-chiorodiphenylmethane behave under the same reaction conditions whichcause perchlorotoluene to yield the perchiorostilbenes?

If the reaction was formally analogous to that of perchlorotoluene,perchiorotetraphenylethylene (XXXVI) should be obtained via the per-

136


ctcIc Cl Cl

clc I CICl.CL*Cl(XXXV) (XXXVI)

Cl CCL—CC[ C

cl*cl Ct*C1(XXXVIU) (XXXVII)

chiorodiphenylmethyl radical. Perchiorotetraphenylethylene would be,however, a highly overcrowded substance, since the two central ethylenecarbons would be surrounded by forty-four atoms.

Another possibility was that the perchiorodiphenylmethyl radical(XXXVIII) might be obtained. Of course, from the data presented in theintroduction of this talk, it was unlikely that its dimer, perchiorotetraphenyl-ethane (XXXVII), would be obtained, since its frontal strain should neces-sarily be greater than that of the dimers of XV and XVI, which areknown to dissociate to a remarkable extent. It may be noted that here thealpha chlorines would contribute also to that strain. The fact that it has beenimpossible to isolate perchlorobibenzyl in the case of toluene was alsoagainst such a possibility.

The reaction was performed in dioxane using stannous chloride as adechiorinating agent (other dechiorinating agents, such as ferrous chlorideor mercury, could be used as well) and yielded a red crystalline materiaP2.This colour suggested strongly that the reaction product was a free radical.Elemental analyses were consistent with formula (C13Cli1) and the i.r.spectrum showed that its structure was entirely benzenoid. Also, its molecularweight in solution was, within the experimental error, that of the perchioro-diphenylmethyl radical. Furthermore, preliminary tests carried out withrudimentary equipment proved that the solid product was strongly para-magnetic.

Later, magnetic susceptibility and e.p.r. measurements confirmed that the

137

:1

Cl.

MANUEL BALLESTER

perchiorodiphenylmethyl radical, or PDM, was a completely disassociatedfree radical, even in the solid form.

Although such a result was not quite unexpected, a most surprising factwhich aroused our excitement from the moment we isolated PDM was,however, its remarkable chemical inertness. Its stability towards oxygen wasclearly greater than any known carbon free radical. In solid form, it can bekept for two years in the air without appreciable change. Also, it provedremarkably resistant towards chlorine, bromine, iodine, nitric acid,sulphuric acid, sodium hydroxide, nitric oxide, hydroquinone and p-quinone. However, although inert towards the usual organic solvents, it isslowly destroyed in boiling toluene giving ocH-undecachlorodiphenylmethane(XXXIX) and 1,1 -bis-pentachlorophenyl-2-phenylethylene (XL). Thisreaction is interpreted as an ochydrogen abstraction from toluene giving abenzyl radical, which is then captured by PDM to give XLI, a highlystrained compound that undergoes immediate elimination of hydrogenchloride.

(C6Cl5)2C1 + C6H5CH3 -÷ (C6C15)2CHC1 + C6H5H2(XXXIX)

(C6C15)2OC1 + C6H5CH2 -÷ (C6C15)2CC1CH2C6H5(XLI)

(C6C15)2C=CHC6H5 + HC1(XL)

It is possible that the condensation between these two radicals can be a onestep process.

The preceding reaction is essentially analogous to that of tri-p-biphenyl-methyl (or tri-p-tert-butylphenylmethyl) with toluene73 or xylene74. Theproduct was in these cases, the triphenylmethane and those which corres-pond to XLI.

In the air, at room temperature, and dilute solution, PDM oxidizesslowly. Under such conditions its half-life is around three days. However,if a stream of oxygen is passed through its boiling solution in hexane for anumber of hours, the radical is recovered almost quantitatively. Thisunusual result is probably due to the low concentration of oxygen in thesolution and, of course, to the low activity of PDM.

The e.p.r. spectrum of PDM consists, as expected, of four lines, whichcorrespond to spin—spin interaction between the odd electron and the alp hachiorinef. With higher gain a number of minor peaks appear that are dueto interaction with the 'C nuclei.

In Figure 5 the electronic absorption spectrum of PDM is shown. Theband around 390 mj and its tail extending beyond 500 m/L account for itscolour.

Figure 6 is the i.r. spectrum of PDM.

t The spin of chlorine and '3C are, respectively, 3/2 and 1/2.

138


Figure 5. Electronic absorption spectrum of PDM.

Figure 6. Infrared spectrum of PDM.

Perchiorotriphenylmethyl [PTMJ radicalBefore we had completed the preliminary study of PDM we were already

wondering whether perchiorotriphenylmethyl radical, or PTM (XLI), couldever be synthesized. The odds were much against us, since the obvious

139

CL CL

CL

C1CL

CL

my500

4000 3000 2 000Wave number, cm

1500 1000 900 800 700

UC

EU)C(0

- IIy-

bc , .

I: .i;ii1iiiiT- T

5 9 10Wavelength,

I 12 13 14 15

MANUEL BALLESTER

precursor for its synthesis, perchlorotriphenylrnethane, would possess suchan extremely high steric strain that the chances of overcoming it with ourperchlorinating agent would be practically non existent. In this connectionit should be recalled that the yield for the chlorination to perchiorodiphenyl-methane, the precursor of PDM, was very low and that this was attributedto steric strain. For this reason, the idea of preparing that chiorocarbonwas given up. However, it was decided to try the preparation of another,not so highly strained, possible precursor of PTM: xH-pentadecachlorotri..phenylmethane (XLII). This substance, by elimination of its hydrogen atom,should also yield PTM. Compound XLII was prepared without muchdifficulty, and when treated with sodium hydroxide in a mixture of ethylether and dimethyl suiphoxide gave quantitatively the perchiorotriphenyl-carbanion (XLIII), which in turn, by oxidation with iodine, was convertedin good yield into PTM.

(C6C15)3CHH0

(C6 Ct5)3C

(XLII) (XLIII)

(XLI) PTM

PTM is a garnet, crystalline chiorocarbon. Its molecular weight in solu-tion shows that it is completely disassociated. Magnetic susceptibilitymeasurements indicate that in solid state it is also completely disassociated.

Its electronic absorption spectrum (Figure 7) shows two absorption bandsaround 390 and 500 m to which its colour is traced. Its infrared spectrum isthat of a typical benzenoid chlorocarbon, which on account of the highmolecular symmetry consists of only a few peaks (Figure 8).

PTM is more inert than PDM. It does not react with oxygen at all. Evenin very dilute solution, at room temperature, it is perfectly stable in the air,as ascertained by precision spectral measurements. Under these conditions,its half-life is probably, at least, a few decades. Actually, all the operationsleading to its synthesis are performed in the air. In fact, a very effective wayof getting rid of some of the accompanying impurities consists of passing astream of oxygen through the reaction. mixture during certain stages of itspreparation.

It is also perfectly stable towards typical radical reagents such as chlorine(in the dark), bromine, and of course iodine, as well as nitric oxide, p-quinone, hydroquinone, etc. It is stable towards the usual solvents, even

140

C


boiling toluene. It does not decompose appreciably at temperatures as highas 3000.

Such chemical and thermal behaviour shows that PTM not only possessesa tremendous inertness, but also that its stability in the broadest sense isclearly greater than that for the majority of "normal" tetravalent carboncompounds.

UCCu

EU)CCu

600mp

Figure 7. Electronic absorption spectrum of PTM.

141

C:

300 400 500

Wave numbe cm-11000

Wavelength, iFigure 8. Infrared spectrum of PTM.

MANUEL BALLESTER

Scale models indicate that the phenyl rings should be in a propeller-like array around the central carbon, forming an angle with the plane ofthe three central bonds no smaller than 600. Such twisting is due to inter-action among the ortho chiorines and, accordingly, spin delocalizationshould be smaller not only than that of triphenylmethyl, its parent hydrocar-bon radical, but also than that of tri-o-tolylmethyl, since in the latter thereare only three ortho substituents and they are not sterically buttressed bymeta substituents as in the present case.

The e.p.r. spectrum of PTM consists of a main line with a number ofsatellite peaks due to spin—spin interaction with the 'C nuclei75. In thisconnection, it should be mentioned that quantum calculations on the 130splittings of PDM are consistent75 with an angle of about 60°.

In Figure 9 a top-view of a Stuart—Briegleb model of PTM is shown. Itcan be seen that the central carbon atom, where most of the spin densityresides, because of the tilting of the benzene rings, is almost perfectlyshielded by the three benzene nuclei and the six ortho chiorines—three ofwhich are on top and three (not seen) under—from attack by outsidechemical species. These are thus prevented from getting close enough to thatcentral carbon for effective bond formation.

These arguments apply also to PDM, although in this case the shielding isnot so perfect as in PTM and, consequently, its inertness is less.

One interesting fact concerning the behaviour of PTM, in contrast tothat of PDM, is the inertness towards boiling toluene. Presumably, thetransition state for the hydrogen abstraction possesses, in spite of the smallsize of the hydrogen, an important frontal steric strain.

PTM has, nevertheless, some reactivity. It reacts easily with potassium inethyl ether to give the perch1orotriphenylcrbanion.

K(C6C15)3C. - (C6C15)3C

/0(C6C15)3CH

Although as has already been indicated, PTM is stable towards sodiumhydroxide under usual conditions, in media containing dimethyl suiphoxidethis base converts it readily into the carbanion.

(C6C15)3C + HO- -* (C6C15)3C.. OHe

(C6C15)3C + H0

The role of this solvent is not clear, although it is probably related, atleast partly, to its efficiency as an ion stabilizer.

Notice that the two preceding reactions of PTM are electron-transferprocesses. Consequently, they do not necessarily require the formation of astable bond. Thus, if they occur through attack on the central carbon atom,the distance between the reacting species can be minimal in the transition

142

Xis . Lob-/9cM OL 2IflflE—BL!G&ICP UJOCJGJ °L BJJVI

I


state, and therefore, the steric shielding aiound PTM's central carbon isineffectual.

Such electron-transfer reactions seem to indicate that PTM and relatedinert free radicals are, essentially, highly reactive substances the aggressivityof which is well concealed behind their steric shielding. This is also consistentwith their low resonance stabilization caused by the high-angle propellor-like array.

Perchiorocarbanions -

The preparation of PDM is now being carried out advantageously fromocH-undecachiorodiphenylmethane through the corresponding perchioro-diphenylcarbanion (XLIV). The overall yield of this process is very good.

H0 e 02or12(C6C15)2CHC1 —- (C6C15)2CC1 ——-k (C6C15)2CC1

(XLIV)

In this connection, we wish to point out that the preparation of someoc-chioroalkyllithium compounds, which are prefectly stable at _1000 hasbeen reported76-81. That the halogen and the metal can coexist in thesecompounds has been considered a remarkable, unique fact. It has beenstressed that these preparations promise a wealth of opportunities forsynthetic applications and structural study that were not previouslypossible.

We wish to point out now that solutions of XLIV can be kept at roomtemperature for long periods without appreciable decomposition. Conse-quently, as far as we know, XLIV is by far the most stable x-chloroalkylalkali metal ever reported82. Under certain conditions, however, it can beslowly converted, through a carbene mechanism82, into the perchioro-tetraphenylethylene (XXXVI) and/or ocHxH-decach1orodiphenylmethane(XLV).

e(C6C15)2CC1 — (C6C15)2C=C(C6C15)2 + (C6C15) 2CH2

(XXXVI) (XLV)

This method of synthesis of radicals through the carbanion has allowedus to prepare a number of free radicals related to PDM or PTM. All ofthem show the characteristic chemical inertness of the parent chlorocarbonradicals.

PDM seriesThe radicals of the PDM series which have been synthesized through this

sequence are XLVI, XLVII, XLVIII and XLIX (formulae on p. 144).Perchloro-4-phenyldiphenylmethyl (PPDM, XLVIII)—This has been synthe-sized from cH-tetradecachloro-4-phenyldiphenylmethane (XLVI). It isan air-stable, orange solid. It is completely disassociated in both solid form(magnetic susceptibility) and in solution (osmometry). Its e.p.r. spectrumconsists of four lines due to the spin—spin splitting with the alpha chlorine.In solution, in the air and at room temperature its half-life is 125 days.In general, it is chemically more resistant than PDM.

143

MANUEL BALLESTER

(XLVIII) RC6C15;R'C1(XLIX)RR'=C6Ct5

.c1

Perchloro-4,4'-diphenyldiphenylmethyl (PDDM, XLIX)—This has been obtainedfrom ocH-octadecachloro-4,4'-diphenyldiphenylmethane. It is an air-stable,orange solid. It is completely disassociated in both solid form and in solution.Its e.p.r. spectrum also consists of four lines. In solution, in the air and atroom temperature its half-life is 19 days. It is also chemically more inertthan PDM.

PTM seriesThe radicals of the PTM series which have been synthesized in this

way are L—LV (formulae on p. 145).Perchloro-4-phenyltriphenylmethyl (PPTM, LIII)—This is obtained fromocH-nonadecachloro-4-phenyltriphenylmethane (L). It is an air-stable,crimson solid. It is completely disassociated in both solid form (magneticsusceptibility) and in solution (osmometry). Its e.p.r. spectrum consists of asingle line. Precision spectral measurements indicate that in dilute solution,in the air and at room temperature there is no appreciable change after onemonth. It displays the same inertness as PTM. It can be heated 3000with-out appreciable change.Perchloro-4,4'-dipheny1triphenylmet/y1 (PDTM, LIV)—This is obtained fromcH-tricosach1oro-4,4'-dipheny1triphenylmethane (LI). It is an air-stable,crimson solid. It is completely disassociated in both solid form and insolution. Its e.p.r. spectrum consists of a single line. In dilute solution, inthe air and at room temperature it is also perfectly stable (one month). Itdisplays also the inertness of PTM. It can be heated as high as 300° withoutdecomposition.Perchloro-4,4',4"-triphenyltriphenylmethyl (PTTM, LV)—This has been syn-thesized from xH-heptadecachloro-4,4',4"-triphenyltriphenylmethane (LII).

144

H0

(XLVI) R C6Ct5R' CL

(XLVII) R = R'= C6C15

:1


(L ) RC6CL5; R'R"CI(LI) RR'C6CL5;R" CL

(LII) R=R'R"zC5C[5

It is an air-stable, purple solid. It is completely disassociated (magneticsusceptibility). Its e.p.r. spectrum consists of one line. In dilute solution, inthe air and at room temperature it is also perfectly stable (one month). Inaddition, it displays the chemical inertness of PTM. It can be heated ashigh as 3000 without decomposition.

BiradicalsIn view of the preceding results it was decided to attempt the synthesis of

perchiorobiradicals.In 1907 Chichibabin prepared a hydrocarbon83 to which the structure

LVI was given. However, it appeared later that, within the experimentalerror of the measurements, this substance84 was diamagnetic. The quinoidstructure LVII was consequently proposed.

(C6H)2 (C6H5)2 (c6H5)2c C(C5H5)2

P.A.C.—L

(LVI) (LVII)

145

CL

CL

CL

RHO

.C1. -CL

CI CL

-CtCL CL

CL.

(LIII)(LIV)(LV)

RC6C5; R'R"CLRR'C6CL5; R"zCLR R' R" C6C15

MANUEL BALLESTER

Although this hydrocarbon behaved in many ways like a diradical, it wasnot until ten years ago that its paramagnetism was demonstrated85' 86However, the measurements indicated that only a small fraction of thesubstance is in the biradical form.

Introduction of bulky substituents in the central ortho positions of thebiphenyl system enhance the radical character so much that the para-magnetism of the relevant compounds, such as biradicals LV11152 andL1X87, is easily detected.

CH Cl Cl

(C6H5)2(CsH5)2(C6H5)2 (C6H5)

(LVII!) (LIX)

Steric interactions among such ortho substituents twist the central ethylenebond of form LVII to a great extent and, consequently, lower its stability(steric inhibition of resonance). Therefore, both biradical and polymeric(LX) forms become more important.

(LX)

It was found that Theilacker's biradical (LXI) is completely disassociatedat room temperature52. This shows that here not only the quinoid form isunimportant but also that polymerization of the biradical is stericallyhindered in essentially the same manner as the dimerization of tri-o-tolyl-methyl already mentioned (frontal steric strains in the polymer).

It seemed, therefore that if the corresponding Chichibabin chiorocarbon(LXIV) could be prepared, there would be a good chance of its being aninert, non-quinoid, completely disassociated biradical. Its synthesis waseffected as usual by conversion of the corresponding bis-ocH-polychlorinatedderivative (LXII) into the bicarbanion (LXIII), followed by oxidationwith iodine.

146

(LX!)


C dcl Cl

(C6C15)2HCCH(C6 015)2

(LXII)

Cl CICI Cl

(C5CL (C6 Cl5)2

CL CICI Cl

(LXIII)

CE CECI Cl

Cl 0101 Cl(LXIV)

Perchioro-Chichibabin, the nickname we use for perchloro-4,4'-bi-phenylenebisdiphenylmethyl biradical (LXIV), is a garnet chiorocarbon.Its visible and u.v. spectra are very similar to those of PTM (Figure 7).Its i.r. spectrum does not show any ethylene carbon—carbon bond stretchingpeak, in agreement with a non-quinoid, fully benzenoid structure. Magneticsusceptibility measurements prove also that, within the error of the measure-ments, it is a completely disassociated substance in solid state, and that it is adouble doublet.

Perchioro-Chichibabin biradical is perfectly stable towards oxygen,either in solid form or in solution. It also possesses the chemical inertnesscharacteristic of radicals of the PTM type. Its e.p.r. spectrum shows onesingle line with a g-value close to two.

Concerning biradicals, it should be mentioned that in the same mannerperchiorotoluene gives the perchlorobenzyl radical, already mentioned, itappears that perchloro-p-xylene gives perchloro-oc-xylyls which, by successivedimerizations and chlorine eliminations, yield finally a macromolecularchiorocarbon of exceptionally high thermal stability and chemical inertness71.Under certain conditions the product is, however, perchloro-p-xylyleneexclusively72' 88 This chiorocarbon is, as far as we know, the most stablep-xylylene ever reportedt. Its i.r. spectrum shows intense ethylene carbon—carbon bond stretching peaks and a complete lack of benzenoid "stretching"bands. Consequently, its structure is entirely of the quinoid type (LXV),

t The parent hydrocarbon, the p-xylylene, is an extremely reactive substance of quinoidstructure89.

147

MANUEL BALLESTER

(LXV)

even though its two "exo" ethylene bonds are highly twisted due to thesteric interactions among the chiorines.

It might be expected that the virtual substitution of all four chiorines ofthe exo ethylene bonds by bulky pentachiorophenyl groups would twistthem to such an extent that the triplet state or even the double doubletstate, as in the case of the perchioro-Chichibabin biradical, would beparticularly favoured. Therefore, it was considered worthwhile to attemptthe synthesis of such a chiorocarbon, the perchloro-o,oc,cL','-tetrapheny1-p-xylylene or, as we call it, the perchioro-Thiele chiorocarbon (LXVII) f.

For this purpose, the corresponding bis-xH-po1ych1orinated precursorLXVI has been synthesized and then processed as in the case. of the per-chioro-Chichibabin biradical. The resulting chiorocarbon is an almostinsoluble substance, which analyses well, the deep-red colour of whichstrongly suggests the expected biradical (LXVII). However, it is diamagnetic

(C6C15)2C H*CH (C6C 15)2(C6C15)2 :t65)2

(LXVI) (LXVII)

CL CL

(C6C1)2C C6C15)

(C5CL5)2C—K.fl--C(C6CL5)2CL CL

(LXVIII) (LXIX)

and its electronic absorption spectrum is different from that expected for abiradical. These two observations are consistent with the quinoid structureLXVIII. Nevertheless, its i.r. spectrum has been taken under a varietyof conditions but we have always failed to detect any ethylene carbon—carbon bond stretching peak, a fact hardly compatible with structureLXVIII.

We are currently performing additional work on this puzzling problem.At this moment it is tempting to conclude that the perchioro-Thiele chioro-carbon possesses a "non-classical" structure. The data indicate therefore

The parent hydrocarbon has been shown to be the x,;',oc'-tetrapheny1-p-quinodi-

methan86' (C6H) SC=<>C(COHS)2.

148


that there must exist a spin coupling that does not disturb significantly their-electron cloud on the central benzene nucleus. Consequently, most of therelevant electron density is located outside, mainly on the two alpha carbons.

There is no doubt whatsoever that these unique features of the perchioro-Thiele chlorocarbon are due to the very high ('—60°) twisting of the exoethylene bonds of form LXVIII, caused by the repulsions among theort/zo chlorines.

EPILOGUEIn conclusion, a number of carbon free radicals have been synthesized,

which not only are completely disassociated, but also possess a uniquethermal stability and chemical inertness.

Therefore, for the first time we have within our reach the possibility ofwide experimentation, and even use, of free radicals in certain fields ofscience and technology, such as electronics and medicine, for example.

However, the familiar statement that "perfection cannot be achievedin this world" can also be applied here. Although, as has been indicated,PTM and related radicals are more inert than the PDMs, they, have a draw-back the latter do not possess: light sensitivity in solution. Nevertheless, nocircumstance can be one hundred per cent to man's disadvantage. Such asensitivity has very recently allowed us to isolate in a very good yield a mostimprobable chiorocarbon provided with a colossal steric strain: the per-chiorotriphenylmethane (LXX). LXX gives back PTM in boilinghexane or carbon tetrachioride, or by illumination with white light, at roomtemperature.

(C6C15)3C'+C12 hV ci

(LXX)

In this connection it should also be pointed out that although PDM reactssluggishly with chlorine in the dark, in the presence of iodine this reactiontakes place much faster, giving an almost quantitative yield of perchioro-diphenylmethane. This is most probably due to the fact that iodine chlorideis the actual chlorinating species and, consequently, it shows how importantthe energy of the bond to be broken also is.

Cl2 + '2 —- IC!

(C6C15)2bCl + ICl —-÷ (C6C15)2CC12 + I149

CL

Cl

'CL

CL

MANUEL BALLESTER

While the bond energy of molecular chlorine is 58 kcal/mole that ofiodine chloride is 51 kcal/mole.

We are presently doing research work aimed at the synthesis of carbonradicals having both chemical stability of the PTMs and the light insensi-tivity .of the PDMs. We are also engaged in the synthesis of polyradicals,inert radicals with functional substituents, perchloroalkaromatic carbenes,and oxygen and nitrogen perchlorinated radicals.

I wish to acknowledge my appreciation to my former students at the University ofBarcelona, Dr. C. )Molinet and Dr. J. Rosa; my present collaborators, Dr. J.Castañer and Dr. J. Riera; postgraduate students, Mr. J. M. Monsó, Mr. C. Badiaand Air. D. Clivillé, for their valuable contribution in the development of the presentresearch work.

I also wish to thank our sponsors, the Aerospace Research Laboratories, Office ofAerospace Research, U.S. Air Force, and their scientists, particularly Dr. L. Spialter,for their kind interest in our research programme and their confidence in our ability todevelop it; the du Pont de Nemours and Co. and its scientists at the Central ResearchDepartment in Wilmington, Delaware, Dr. W. H. Sharkey, Dr. M. T. Jones andDr. J. F. Weiher, for the e.p.r. and magnetic susceptibility data.

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INERT CARBON FREE RADICALS

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