Ambident nucleophilic reactivity in a-complex formation. Part 5.' Reaction of 1-phenyl-2,4,6-trinitrobenzene with methoxide and phenoxide ions

ERWIN BUNCEL, SURESH KUMAR MURARKA, AND ALBERT RICHARD NORRIS Departmer~t of Chemistry, Queen's Universih. Kingston, Ont., Canada K7L 3N6

Received June 13, 1983

ERWIN BUNCEL, SURESH KUMAR MURARKA, and ALBERT RICHARD NORRIS. Can. J . Chem. 62, 534 (1984). The reactions of 1-phenyl-2,4,6-trinitrobenzene (4) with methoxide and phenoxide ions in DMSO and DMSO-methanol

solutions have been investigated. MeO- gives rise to a 1,3 adduct through kinetic control and a 1,l adduct through thermo- dynamic control. These processes persist also in the reaction of 4 with potassium phenoxide in DMSO-methanol. However, reaction of 4 with potassium phenoxide in DMSO gives rise to a 1,3 adduct as the only observed species in which phenoxide is bonded to the nitroaromatic moiety via the para phenoxy carbon, in accord with previous observations on the ambident reactivity of phenoxide ion. The results are considered in terms of various steric and electronic effects and it is concluded that F-strain, relief of steric compression, and delocalizability considerations play dominant roles in accounting for the observed reaction course.

ERWIN BUNCEL, SURESH KUMAR MURARKA et ALBERT RICHARD NORRIS. Can. J . Chem. 62, 534 (1984) On a CtudiC les reactions du phtnyl-l trinitro-2,4,6 benzkne (4) avec les ions methanolates et phenolates en solution dans

le DMSO et dans le melange DMSO-methanol. L'ion MeO- donne des adduits 1,3 qui sont des produits de contrBle cinktique et des adduits 1 ,I rtsultant du contrBle thermodynamique. Ce processus se manifeste Cgalement dans la reaction du compose 4 avec le phenolate de potassium dans le melange DMSO-mtthanol. Cependant, cette m&me rtaction effectuee dans le DMSO conduit a un adduit 1,3; c'est la seule espkce observee dans laquelle I'ion phtnolate est lie a la portion nitroaromatique par I'intermediaire du carbone en position para du groupe phtnoxy en accord avec les observations anterieures relatives a I'ion phtnolate qui posskde deux positions de reaction. On considkre ces rksultats en fonction des effets stCriques et tlectroniques et on conclut que I'encombrement F, la dCcompression sterique et la possibilite de dClocalisation jouent un rBle majeur dans I'evolution de la rtaction.

[Traduit par le journal]

As part of a continuing study on the interactions of nitro- aromatic compounds with bases (1-3), in this paper we present results pertaining to the formation of 1,l and 1,3 a-complexes and the ambident reactivity of phenoxide ion (4).

Among the various structure-reactivity relationships in a-complex formation processes (5-1 l ) , one which has re- ceived considerable attention is the generally observed kinetic preference for 1,3, and thermodynamic preference for 1 , l adduct formation, in the reactions of bases with 1-X-2,4,6- trinitrobenzenes and related substrates. The first instance of this type of result was the discovery by Servis in 1965 (12) through 'H nmr spectroscopy that 2,4,6-trinitroanisole (TNA, I) , on reaction with methoxide ion in dimethyl sulfoxide (DMSO) - methanol solution (90: 10 v/v), yielded initially the 1,3 adduct (2), which then decayed and was replaced by the more stable 1, l adduct (3), Scheme 1.

While in the majority of subsequent studies this trend in kinetic versus thermodynamic control was confirmed, several exceptions have been observed ( 13 - 15). Hence it appeared that a unique theory for this phenomenon could not apply and several proposals evolved to account for its origin (vide infra). It occurred to us that structural modification could produce useful information and, while use of alkyl substituted nitro- aromatics could be considered, study of 2,4,6-trinitrotoluene has shown that alkoxide ion can partake in proton abstraction to give a nitrobenzylic anion in addition to a-complex for- mation (16- 19). On the other hand, use of 1-phenyl-2,4,6- trinitrobenzene (4) would preclude proton abstraction from oc- curring, while providing a model to test the importance of steric and delocalization effects. In addition to choosing methoxide as the standard nucleophile, we selected phenoxide ion in the expectation that this could act either as an oxygen nucleophile

' Parts 3 and 4: see refs. 39 and 40.

to yield 5 and/or 6, or as a carbon nucleophile to give 7 and/or 8. The ability of phenoxide to act as an ambident nucleophile towards 1,3,5-trinitrobenzene (TNB) has been reported pre- viously (20-23). Similar observations have been made with naphthoxide (24) and indolide (25) anions.

Previous work on anionic a-complexes ( 5 -7) has shown that structural characterization of these species can generally be accomplished through 'H nmr spectroscopy. However, uv-visible absorption spectroscopy can provide valuable sup- porting evidence, especially in differentiating between bonding via oxygen or via carbon in the case of a potential ambident nucleophile such as phenoxide ion. As will be shown in the following, the results of these studies have in fact allowed complete structural characterization of the u-complexes formed in the reaction of 4 with methoxide and phenoxide ions as nucleophiles. The significance of the findings is considered.

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Results u-Complex formation between 4 and methoxide

An orange-red colored solution resulted on addition of equi- molar methanolic sodium methoxide to 4 in (CD3),S0 (final solvent composition, 90: 10 v/v (CD3)?SO/MeOH; reactant concentrations, 0.46 M). An nmr spectrum recorded 3 min after mixing showed the absence of a signal at 6 9.10 due to H-3(5) of 4, and the appearance of new peaks at 6 8.68 (d, J = 1.5 Hz) and 6 6.25 (d, J = 1.5 Hz), integral ratio 1.1 : I . An

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unresolved multiplet centered around 6 7.3 (5H) also present could be ascribed to the phenyl ring protons of the resulting complex. The region below 6 4.5 was blanked out by the solvent. On further monitoring of the spectra with time, the doublets at 6 6.25 and 6 8.68 were found to decrease while a singlet at 6 8.72 increased in intensity; after 8 h only the 6 8.72 singlet was present and had an integral value twice that of the doublets at 6 8.68 and 6 6.25 seen initially.

The above spectral changes can be interpreted as follows. The appearance of two doublets at 6 8.68 and 6.25 is ascribed to the Ha and Hp protons of 9, while the singlet at 6 8.72 is assigned to Ha, of 10. The chemical shift values observed correspond closely to literature data obtained for these types of protons (5-7). As well, the rapid formation of 9 followed by its slow transformation into 10 is in accord with previous obser- vations on the kinetic preference for formation of 1,3 adducts, and their transformation into the thermodynamically more sta- ble 1 , l complexes.

Complementary evidence was obtained by following the re- action course through uv-visible spectroscopy. In Fig. 1 are shown the visible absorption spectra, as a function of time, of solutions in DMSO-MeOH (90: 10 v/v) obtained by ca. 1000-fold dilution of a reaction mixture having the same com- position as in the nrnr experiment. It is seen that initially the spectra exhibit absorption maxima at 428 and 506 nm but in time these maxima shift to 436 and 520 nm with a concurrent

0.0 550 350 L 50

Xnm FIG. 1. Ultraviolet-visible absorption spectra as function of time

in reaction of 1-phenyl-2,4,6-trinitrobenzene with methoxide ion in DMSO-methanol indicating transformation of 1,3 and 1 , l a-complexes: (1) 1.5 min; (2) 3.5 h; (3) 24 h.

change in intensities. Addition of acid caused the absorptions to disappear. The acid lability and the general characteristics of these spectra, i.e. two absorptions with the one at lower wave- length the more intense, are typical of methoxide adducts of trinitrobenzene derivatives. Though these spectra do not differ- entiate between 1,3 and 1 ,I adducts, they are, however, in complete agreement with our assignment. It is also of interest that, in the majority of systems studied in the past, the 1,3 adduct was formed as a transient intermediate whose visible spectrum could only be reconstructed through repeated stopped-flow experiments performed as a function of wave- length. Thus the present system appears to be one of the few where the spectrum of the metastable species, as well as its transformation, were readily measurable by standard spec- trophotometric techniques.

Another estimate of the relative stabilities of the complexes 9 and 10 was obtained as follows. The diluted solutions whose spectra are given in Fig. I were kept in the dark for 24 h, following which the spectra were again recorded. The resulting spectra correspond exactly, in the positions of the absorption maxima and their relative intensities, to the final spectrum shown in Fig. 1. Thus the 1,3 adduct 9 decomposed on stand- ing, while the 1 , l adduct 10 was completely stable for the extended period of time.

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536 CAN. J . CHEM.

u-Complex formation by phenoxide ion Due to the ambident nature of phenoxide ion, its reaction

with 4 can potentially lead to theformation of both okygen- bound and carbon-bound a-complexes. The use of nmr and electronic absorption spectroscopy, when coupled with other properties such as stability (or lack of) to acidification, enables one to differentiate among the various possibilities.

The nmr spectrum of a solution of 4 and potassium phenox- ide in (CD,),SO (final concentrations 0.49 M and 0.57 M, respectively), when recorded immediately on mixing and then monitored with time, was found to exhibit no further changes during the next 24 h. A typical spectrum showed two some- what poorly resolved doublets at 6 8.60 and 6 5.65 with inte- gral ratios of 1.1 : 1 .O. A complex multiplet between 6 6.2 and 6 7.8 was also present, arising from the phenyl protons in the complex as well as any excess phenoxide ion that was present.

The appearance of the doublets at 6 8.60 and 5.65 is indica- tive of the formation of a 1,3 complex but not of a 1, l complex. The two possibilities consistent with these features are the 0-bound complex 6 and the C-bound complex 8. The following considerations serve to differentiate between these possi- bilities. The shielding of the H, protons in 6 and 8 will depend on the electronegativity of the atom bonded to C-3. o n the basis of literature data (5-7), one would expect 6 values close to 6.2 for 6, and 5.7 for 8. The observed value, 6 5.65 for the resonance assigned to Hp, is thus in accord with complex 8. Furthermore, literature data indicate that the difference in the Ha and Hp shieldings for 0-bound complexes lies between 2.2 and 2.6 ppm, whereas for C-bound complexes this value is -3.0 ppm. The observed value in the present case is 2.95 ppm, which again favors structure 8.

Further evidence concerning the nature of the complexes was obtained from their uv-visible spectral characteristics. When the reaction between 4 and PhOK in DMSO was monitored by

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uv-visible spectroscopy, a characteristic spectrum was ob- I I tained immediately on mixing and then underwent no further I

I changes for the duration of the experiment (24 h), as had pre- I viously been found in the nmr experiment. The resulting spec-

trum exhibited maxima at 465 and 556 nm, the former being the more intense. From literature data (5-7) one would predict that for an 0-bound complex the two absorption maxima should lie in the region of 430 and 495 nm, which is not in accord with the present result. In the case of the C-bound phenoxide adduct of TNB (20), the corresponding absorption maxima occurred at 468 and 570 nm (broad). Thus the uv-visible spectra are strongly indicative of the formation of a carbon-bound complex, 8, rather than an oxygen-bound com- plex, 6. Assuming that the molar extinction coefficient of 8 is of the same order of magnitude as that of the phenoxide adduct of TNB, one estimates that 8 was formed in -25% yield.

Acidification of the solutions from the reaction of 4 with phenoxide caused no change in the spectrum of the resulting solution. This result rules out the possibility of formation of an 0-bound complex since such species are known to be destroyed by acid. ~ h u s the evidence from nmr and uv-visible spec- I troscopy fully supports structure 8 for the phenoxide complex of 4.

A contrasting result from the TNB-phenoxide system was found when the reaction of 4 with potassium phenoxide was conducted in DMSO-methanol (80 : 20 v/v). In the TNB sys- tem it was found (20) that reaction gave rise immediately to the TNB-methoxide a-complex, through solvolysis, which was then slowly transformed to the TNB-phenoxide adduct. When

the reaction was performed with 4 and followed by both nmr and uv-vis spectroscopy, as above, it was found that the meth- oxide complex 9 was immediately obtained and was replaced over 1 h by 10, which then remained stable over an extended period (24 h). Thus formation of the C-bounded phenoxide adduct was not detected in this system. The significance of this result will be considered further in the following discussion.

Discussion Steric and electronic effects in u-complex formation

A number of theories have been proposed to explain the origin of the generally observed kinetic preference for 1,3 adduct formation and the thermodynamic preference for 1 , l adduct formation (26-29). The nitroaromatic substrate under investigation in this study differs structurally from compounds previously investigated in a-complex formation, which enables us to critically assess the various proposals, as discussed under the five headings below.

1. Resonance stabilization irlvolving alkoxy and nitro groups

Resonance structures involving conjugation between the me- thoxy and nitro functionalities could potentially contribute to- wards stabilization of the reactants as well as products in nitroaromatic-base interactions. Thus contribution by struc- tures la and 2a would serve to stabilize the ground state of TNA (1) and the 1,3 adduct (2), respectively, but this inter- action would no longer be possible in the 1, l adduct (3). The transition state leading up to 2 could similarly benefit from this stabilization, but not that for formation of 3. Therefore the transition state for 1,3 adduct formation would be effectively lowered in energy relative to that for 1, l adduct formation.

In the present system, for ground state stabilization to b e effective, one would have to invoke structure 4a as a con- tributing form. However, it could be expected that the phenyl group in 4 would be disposed substantially out-of-plane relative to the TNB ring, noting that one ortho nitro group in biphenyl derivatives results in an angle of rotation between the two rings of the order of 50-60" (30-32). On this basis, the extent of contribution by 4a would be minimal. One can hence effec- tively eliminate this possibility as a source of the kinetic pref- erence for 1,3 adduct formation in this system.

2. Geminal alkoxy substitution as a stabilizing effect Multiple substitution by electronegative groups at an sp3

carbon center is recognized as a stabilizing electronic influence through a negative type of hyperconjugation (33, 34), and this has been considered to contribute to the greater stability of 1 , l relative to 1,3 dimethoxy complexes. However, although the phenyl group can be considered as an electron-withdrawing substituent, it appears unlikely that this could assume a role similar to OMe, F, etc. since resonance structures involving a phenyl anion would be prohibitively high in energy. It is also noted that in the case of attack by CN- on TNA, the 1 , l adduct was found to be of lower stability than the 1,3 adduct (13),

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which could indicate that two unlike electron-withdrawing sub- stituents are not effective in stabilizing u-complexes by this type of mechanism.

3. Structural effects on delocalizing a b i l i ~ A variety of evidence points to the fact that, in u-complex

formation by 1-X-2,4,6-trinitrobenzene, negative charge is predominantly delocalized onto the nitro group which is para with respect to the formed sp3 carbon center. In 1, l adduct formation, this para nitro group bearing negative charge can achieve maximum coplanarity with respect to the overall TT electron system. However, in a 1,3 complex, the NOz situated para to the sp3 carbon is also ortho to the substituent X, which will result in deviation from coplanarity depending on the de- gree of steric interaction between X and NO,. In the present system, steric inhibition of resonance should be of considerable importance due to the large steric requirement of the C-1 phenyl, thus leading to stabilization of the 1 , l adduct 10 rela- tive to the 1,3 adduct 9.

4. Relief of steric strain A substituent X in 1-X,2,4,6-trinitrobenzene will tend to

cause the ortho nitro groups to deviate from coplanarity with the ring as a result of steric compression. For example, in 1-ethoxy-2,4,6-trinitrobenzene, the 2- and 6-nitro groups are rotated from the ring plane by 32" and 61" respectively (35). Formation of the u-complex results in relief of this steric inter- action as the C-1 centre becomes tetrahedral. In the present system, the phenyl ring in 4 could be expected to be substan- tially out-of-plane relative to the TNB ring and there can only be slight relief of this type of steric strain on formation of the 1 , l adduct. However, there will nevertheless be an appreciable increase in librational motion compared to the reactant where rotation about the C-1 bond should be severely restricted. This effect will serve to stabilize the 1 , l adduct relative to the 1,3 adduct since in the latter case this relief of steric strain would not arise.

5. F strain Formation of a 1 , l adduct will in general be subject to

greater steric hindrance ( F strain) in the approach of the nucleo-

phile compared to formation of a 1,3 adduct. Since the nucleo- phile will approach from a direction perpendicular to the TNB ring plane, this effect will be particularly important in the present system owing to interaction between the approaching nucleophilic reagent and the phenyl group at C- 1 . On the other hand, attack at C-3 will be relatively unhindered, thus favoring 1,3 adduct formation. We conclude that F-strain is probably a dominant factor leading to kinetic preference for 1,3 adduct formation in this system.

The ambident reactivi5 of phenoxide ion The results obtained in the reaction of 4 with phenoxide can

be discussed in relation to Scheme 2 which considers both oxygen-bound and carbon-bound u-complexes. Of the various possibilities shown in this scheme, the pathway leading to formation of 8 is the only one which was detected in the PhOK/DMSO system. The finding of the C-bonded adduct is in agreement with the results obtained by us previously for the TNB/PhOK/DMSO system (20), but contrasts with the report of Shein et al . (22, 23) that reaction of TNB gives rise initially to the 0-bonded phenoxide adduct observable by standard nmr techniques. On the other hand, 2,4,6-trimethylphenoxide ion was found by us to react with TNB to give the 0-bonded u-complex (2 1).

When the present reaction was performed in DMSO-MeOH (80: 20 v/v) we observed the formation, through solvolysis, of the methoxy complexes 9 and 10 as the intial and final products of reaction, respectively. A phenoxy complex was not obtaiqed in this system, contrasting with the course of reaction observed with TNB under corresponding conditions (20).

It has been shown by Bemasconi and Muller (15), using fast reaction techniques (stopped flow and T-jump), that the reac- tion of TNA with phenoxide ion in DMSO-water media gives rise in a rapid process to the 0-bonded 1 , l phenoxide adduct as a transient species which is then converted to the 1,3 hydrox- ide adduct of TNA. Comparison with the results for reaction of MeO- with TNA showed that PhO- attack ( H 2 0 ) is faster than MeO- attack (MeOH) by a factor of 2.9, but PhO- expulsion is faster by 4.5 X lo6, with the result that the equilibrium

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538 CAN. J . CHEM. VOL. 62 . 1984

constant for 1,l phenoxide adduct formation is smaller than for 1 , l methoxide adduct formation by 1.5 x lo6.

Our own observation, that in the PhOK/DMSO-MeOH sys- tem MeO- attack on 4 is preferred over PhO- attack, contrasts with Bernasconi's report (15) that kPhO- > kMeO- for reaction with TNA, although the different time scales of the two studies could affect the overall result. It could thus be possible that, at extremely short reaction times, attack by PhO- to form an 0-bonded complex might be detected in our system as well, and that the observed result is the outcome of a secondary rather than a primary process. However, our observation in the PhOK/DMSO-MeOH system of the initial 1,3 and subsequent 1,l MeO- adduct formation, with the latter not replaced by a C-bonded phenoxide adduct, is remarkable in view of the con- trasting result observed in the TNB/PhOK/DMSO-MeOH system (20), and may reflect widely different stabilities of the MeO- adducts in the two cases.

Perhaps the most significant difference between the present work and the results obtained by Bernasconi (15) is that attack by PhO- on TNA occurs preferentially at C-1 whereas PhO- attacks 4 at C-3. Presumably attack by phenoxy oxygen at C-1 of 4 to give 5 would be subject to steric interactions fairly comparable to those for attack by MeO-, although attack at C- 1 of 4 through carbon of phenoxide to give 7 would be sterically much less favorable. This result further demonstrates the im- portance of F-strain in the present system. It appears that this factor overrides the delocalizability criterion which normally results in 1 , l adducts being thermodynamically more stable than 1,3 adducts.

Conclusions

The study has shown that the title compound 4 reacts with phenoxide ion in DMSO to form a carbon-bonded a-complex 8 through an irreversible process, whereas oxygen attack by

I phenoxide would occur reversibly (Scheme 2). This is in ac- cord with our previous observations with TNB (20). In contrast to the observations with TNB, reaction of 4 with potassium phenoxide in DMSO-methanol gives rise, through solvolysis, only to the 1,3 and 1 , l methoxy adducts 9 and 10, formed sequentially. Under the same conditions, TNB gave rise first to the methoxy adduct and then, in a slower process, to the phen- oxy adduct. The ease of formation of the 1 , l methoxy adduct of 4 indicates that 4 reacts in a conformation in which the C-1 phenyl and the C-2,6 nitro groups are oriented in a propeller- like configuration, thus minimizing steric hindrance to ap- proach of the nucleophile. The a-complex formation reaction is expected to occur through an early transition state, as should be the case also for S,Ar processes (36, 37) in highly activated nitroaromatics such as 1-X-2,4,6-trinitrobenzene derivatives where X is a bulky substituent (C5H5Nf, S02Ph, etc.).

While reaction of 4 with methoxide ion gives rise to the 1,3 adduct 9 and the 1 , l adduct 10 through kinetic and thermo- dynamic control, as in the case of TNA, the two sets of obser-

I vations differ in origin. Dominant factors in the present system I I are believed to be the delocalizing ability of para nitro substit- I uents in absorbing of negative charge, F-strain with respect to

approach of nucleophile, and the relief of steric compressions in the ground state in passing to the transition state for a-complex formation.

Experimental Materials and methods

Dimethyl sulfoxide was distilled from calcium hydride under nitro-

gen at reduced pressure. Methanol was distilled from magnesium metal. Potassium phenoxide was prepared according to Komblum and Lurie (4). Sodium methoxide solutions were prepared from freshly cut sodium metal and methanol, or methanol-0-d, and standardized be- fore use. DMSO-d6 was dried before use over molecular sieves.

I-Phenyl-2,4,6-trinitrobenzene (4) was prepared by the Ullmann synthesis from picryl chloride, iodobenzene, and copper bronze using the method described by Gull and Turner (38). The product, re- crystallized from ethanol, was obtained as pale yellow needles, mp 129" (lit. (38) rnp 130°C); nmr (CD3)2SO, 6: 9.12 (s, 2H), 7.22-7.56 (m, 5H).

The nrnr spectra were recorded on a Varian EM360 or HAIOO spectrometer. Chemical shifts are given in ppm downfield from tetramethylsilane (TMS) used as an internal standard. Ultra- violet-visible spectra were recorded on a Unicam SP800B spectro- photometer.

Reaction of 4 with methoxide ion in DMSO-MeOH For the nmr experiments, reaction solutions were prepared by addi-

tion of NaOMe/MeOH to 4 in (CD3)2S0, giving a solution 0.46 M in 4 and in NaOMe, and a solvent composition 90:lO (v/v) DMSO-MeOH. Visible spectra were taken following 7 x lo3 dilu- tion with DMSO of reaction solutions with concentrations of 4 and NaOMe of same order as in the nmr experiments. The uv-vis spectra were recorded immediately on dilution and again after standing for 1 day.

Reaction of 4 with potassium phenoxide in DMSO The (CD3)2S0 solution in the nmr experiment was 0.49 M in 4 and

0.57 M in PhOK. Ultraviolet-vis spectra were obtained following 3 x lo3 dilution. To test the reversibility of complex formation, 3 rnL of the diluted solution was acidified with 50 y L 1 N HCI and the uv-visible spectrum recorded.

Reaction of 4 with potassium phenoxide in DMSO-MeOH The experiments were performed as above but the solution in the

nmr experiment was prepared using (CD3)2S0 and methanol-0-d (80: 20 v/v). The concentration of PhOK was varied between 0.5 1 and 0.98 M while keeping 4 constant at 0.46 M.

Acknowledgements

Continuing financial support of this research by the Natural Sciences and Engineering Research Council of Canada is gratefully acknowledged, as are discussions with Drs. S . Hoz and R. Y. Moir.

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