Indian Journal of ChemistryVol. 37B, October 1998, pp. 965 - 978

Review Article

Phosphorous oxychloride (POCh): A key molecule in organic synthesis

S D Sharma* & Seema Kanwar

Department of Chemistry, Panjab University, Chandigarh 160014

Received 11 September 1996; accepted (revised) 3 June 1998

Of the many important compounds of phosphorous, phosphorous oxychloride (POCI3) finds tremendous usein organic synthesis. Among the overwhelming majority of reactions of this reagent are Vilsmeier - Haackreaction, Bischeler-Napieralski reaction, Regiospecific dehydration reaction, formation of heterocycles such asbenzoxazoles, quinazolines, ~-Iactarns, imidazoles and many others. It also finds use in the synthesis ofaminoacid amides, esters, lactones, phosphorylation of nucleosides, decarbonylation of u-aminoacids andpeptides.

Over the years POCh has been used as an importantmolecule for various organic synthesis, either alone or incombination with tertiary bases like dimethyl formamide(DMF), dimethyl acetamide (DMA), etc. In view oftremendous applications in organic synthesis for use byacademic and industrial houses, we are summerisingvarious reactions of this easily available andcomparatively inexpensive compound.

The structure of POCI3 consists of a pyramidal PCI3

group with the oxygen atom occupying the fourthposition to complete a distorted tetrahedron. Phos-phorous atom in POCh is Sp3 hybridised and theshortening of P-CI, P-O bonds are due to formation of 1t-

bond between P and 0 atoms (Chart 1).

oIIP

c( \~CI

Chart 1

(I) 3PCIS + P20S--~> SPOCI3

(11) 2PC1S+COOH ~ 2POCI3+(COCI)2+2HCIICOOH

(JIl) 3PCIS+2H3803 ~3POCI3+B203+ 6HCI

Chart 2

POCh is a colourless, strongly fuming liquid withpungent odour. It reacts exothermally with water andalcohols. It can act as solvent! as well as a reagent.Acceptor character of phosphorous, which is due to thepresence of empty d-orbitals, results in the possibility ofphosphoryl bridging P=O---P comparable to hydrogenbonding. Generally, POCI3 is prepared from PCIs Chart2 illustrates three important reactions of PCIs with P20S,

oxalic acid and boric acid yielding POCh.

Use of POCh in Organic Synthesis

Formylating agent. The reaction of aromaticcompounds with disubstituted formamides of POCh iscalled Vilsmeier - Haack reaction'. It is used fdorformylation of active aromatic rings, acetals,vinylcyclopropanes, alkoxy-benzenes and also forthioformylation. Whereas POCh is used as such fdorthe formylation of acetals, formylatiorr' ofalkoxybenzenes, vinylcyclopropanes as well asthioformylation'' occurs through a combination of POCI3

and DMF via the formation of Vilsmeier reagent -Me2W=CHCI(Scheme J).

It has also been reported' that various 2-cyclohexen-l-ones undergo formylation on treatment withfonnamides and POCI3 at 20°C. Two distinct reactivitypatterns were found designated as type A and type B.For example, 3-oxo-4-ene steroids give mono-, di-,triformylated oletins or benzenes or simplychlorooletines (Scheme II).

Dehydrating reagent. POCl3 can act as dehydratingagent? as depicted in Scheme 01. The observation ofLederle group? is worth mentioning, where it was

966 INDIAN J CHEM, SEC. B, OcrOBER 1998

observed that dehydration using SOCh - Py, brought;about dehydration of all the -OH groups, whereas usingPOCh-Py resulted in selective dehydration under similarconditions (Scheme IV).

Early stages of dehydration of tertiary alcohols wereshown to involve antielirnination. The high regio-specificity and stereospecificity of POCI3 dehydration ofsecondary alcohols is a synthetically useful reaction.The stereoselective sequence, syn-hydroboration-antidehydration offers a convenient method forisomerization of trisubstituted 0lefines8

• This methodalso provides a way for introducing hydrogen isotopes tovinyl position of a double bond. An interesting reactionof POCl3 as cyclodehydrating agent" is Bischeler-Napieralski reaction, where cyclization of amide occurs.However, aryloxyamides on treatment with POChfurnishes arninonapthalene (Scheme V).

Synthesis of isocyanides, cyanides and carbo-diimides: (i) Isocyanaide synthesis 10. Formation ofisocyanides involves dehydration of the correspondingforrnarnides (Scheme VI). The above reaction can alsobe carried out using variety of reagents like phosgene,diphosgene and POCI3 in combination with tertiaryarnines. Due to high cost of toxic nature of phosgeneand diphosgene, the POCh method for this purpose, is

comparatively more convenient and less expensive. Theuse of diisopropylamide instead of triethylamine, resultsin an increase in the reaction rate due to steric bulk ofisopropyl group and also increases the reaction yield. Itis because of the fact that isopropylamine acts as baseand not as nuc1eophile, whereas triethylamine due to lessbulkier groups can act as nucleophile also.

(ii)Cyanide synthesis!'. If POCh is used as reagentfor the dehydration of unsubstituted amides, cyanidesare obtained as depicted in Scheme. VII.

(iii)Carbomiimide synthesis I I Dehydration ofdisubstituted ureas using POCh furnishes thecarbodiimides (Scheme VIII).

Peptide synthesis'f. Acylated amino acids orpeptides on treatment with POCI3 in equimolar amountsin dry THF with two molar equivalents of Net, result inthe formation of peptide derivatives. Equimolaramounts of POCI3 and amino acids are taken so thatmixed anhydrides should not be formed by replacingtwo or three chlorine atoms.

J

j

•

POCI3 on reaction with carboxylic acids formsmixed anhydrides (carboxylicdichlorophosphoric acidanhydrides) the action of which on amines leads to thedesired products. A yield of 50% using SOClz was

S.D. Sharma was born in Hoshiarpur district of Punjab, India in 1940. Hereceived his Master's (1963) and Ph.D (1967) degrees in chemistry fromPanjab University, Chandigarh. He was a postdoctoral research associate withProf. A K Bose in US during 1968-1971, and joined the faculty of Chemistrydepartment, PU, Chandigarh, as a lecturer in 1971 where he is a professor oforganic chemistry since 1985. His research interests included total synthesis,synthetic reagents and reaction mechanism. Dr Sharma has publishedextensively in the area of terpenoids and beta-Iactams.

Seema Kanwar was born in Una district of Himachal Pradesh in 1976. For herMaster's degree (1998) in chemistry she worked on the synthesis of beta-Iactamsunder the guidance of Prof. S D Sharma. Ms. Kanwar is keen to pursue research for adoctorate degree in organic chemistry.

,r

SHARMA et at.: PHOSPHOROUS OXYCHLORIDE IN ORGANIC SYNTHESIS

MeI

Ph-N-C-HIIo

[Z = -OH, -NHll

967

Z

~CHOLQJ + PhNHMe

OR"POCl) I,

----.,; ...•~~ R-CH-C-RI I"CHO OR

IHydrolysis+ (H+)

os"I ,

R-C~-C-RI trOR

oII

+ Ph-N-C-HI

Me

oII ,

R-CH-C-RICHO

CI~H) I CH

CCX=CH-NM~ r: CHO,I Hydrolysis ~ ~

'..-- CH) ~CH)Me Me Me Me

S +

C1'fJ CS'I ~CH=N~MehN + I CIN

+CH=N(Meh II

CI- (aSH

VinylCyclopropane

DMF

Lt:)CHS+LyC:HS

(XoMe i) 82_83°C OHClQtoMe,.... 35 min. 0. I + N-methylformamide + POCI) ----~~ OMe ii) Hydrolysis OMe

35-40°C

increased to as high as 80% with the use of POCI,(Scheme IX).

POCl3 mediated condensation proceeds via theactivation of carboxylic acid reactant as mixedanhydride rather than via the activation of aminereactant because amine intermediates of POCh requiredrastic conditions for its initial formation and for its

Scheme I

subsequent participation in coupling reaction.Advantage of this methods lies in its ability to beemployed with amino acids which possess unprotectedhydroxyl function. For example, in threonine andothers.

Synthesis of benzoxazoles13• Benzoxazoles are

importasnt intermediates for the synthesis of polyethers,

968 INDIAN J CHEM, SEe. B, OCTOBER 1998

CHO

(Type A)

(Type B)

Scheme n

I POCl)• Quinoline

ii)KOHAcO

HHO

H

Scheme m

SHARMA et al.: PHOSPHOROUS OXYCHLORIDE IN ORGANIC SYNTHESIS

Me 0I II

CH-C-NE'00"(Aryloxyarnide)

R-NH-CHO ---(Keto form)

969

POCI)-Py4

.s-c

lSOC'2-Py

-S·C

Scheme IV

oR II CII ....O-p~

©(C, CIo r;m--.....CHlCH2

RI

©OoII

+ HO.PClz

Scheme V

OHI POCl)

R-N=CH ----+NEt

(Enol form) )

oII

HOPCIl + RN=C

Scheme VI

970 INDIAN J CHEM. SEC. B. ocrOBER 1998

R-C-NHII 1

o-- R-C=NH POCl]~

1OH

oR-C=N + II....ClHO-P,

Cl

Scheme VB

RNH-C-NHRIIo

-- RN=C-NHR1OH

HI

R-N=C-NRI ....ClO-P

II'CIo

o11

HOPCl1 + R-N=C=N-R

Scheme VIII

RCONHCHR'COzH+ POCl) + EI)N -+- EI)N. HCl+ RCONHCHR1C01POCl1(Mixed anhydride)

flourescent whitening agents and as dye releaser ininstant colour photography. POChlDMA is aconvenient method for the preparation of benzoxazolesand hdroxy derivatives. The reaction proceeds smoothlyat room temperature. Acetonitrile is used as solvent andDMA to increase yield because in the absence of DMAthere is formation of acetylamino resorcinol as a by-product and hence there is a decrease in yield. Thedifference in results, in the presence and absence ofDMA is due to attack of Vilsmeier type complex insteadof POCI3 and the efficient solvation of carbocationintermediate (Scheme X).

Synthesis of cx,~unsaturated esters and lactones'".Synthesis of a.[3-unsaturated esters and lactones fdromn-seleno carbanions via selenoxide is not stereospecificreaction when two alkyl groups are a to carbonyl group.With SOCl2 yields are very low but with POCI3 reactionproceeds smoothly.

Scheme IX

The reaction starts from c-seleno-Bvhydroxy esterswhich are obtained from n-seleno carbanions. Productsof E- and Z-isomers are obtained with R = -CH3• P:.CIC6~ because of lower nucleophilicity of p-chlorophenylselenyl moiety and its high acidifying effecton a-H (Scheme xi).

POCI3 is also used in regiospecific and stereo-specific synthesis of alkylidene lactones (Scheme XII).

Quinazoline synthesis'i', The synthesis of 2-phenylquinazoline-4(3H)-one is carried out by heating3-nitroso-2-phenylindole with POCI3 followed bydilution with water. The reaction involves ringexpansion which proceeds via the formation ofintermediate 2-benzoylaminobenzonitrile where the firststage of this reaction mechanism is reminescent of theBedford Partridge reaction in which isatin-3-oxime ispyrolysed into j-arninobenzonitrile. If this reaction iscarried out in the presence of amine. excess of POCh

SHARMA et al.: PHOSPHOROUS OXYCHLORIDE IN ORGANIC SYNTHESIS

(Oxime of2',4' dihydroxy acetophenone)

Mechanism

Me OHI I

cQ(C=N

OH

POCI)IDMA•

971

rCYN}-Me

HO~O

(6 Hydroxy benzoxazole)

MeMe 1

I ~-C-NM~I 1

©(0C'

OH

+ -+ (CH)lN=qCH)CI CI --+-

Vilsmeir type reagent

(Benzoxazole)

HO f!\ ", ..,.COOCH)

_-·C-CWJ I --+-HIlC: SeR POCl)

MeI

cQ(~C~o H

Scheme X

Beckmann

rearrangement

Me- I

+ O-C-NM~1

CI

70% 18%

O~ J--o\HI)Ci'~

Se-Ph

}bH0 0

H --

.HI] 6 Se-Ph

19%

Scheme XI

POCI)/Et)N •

POCI)/Et)N •

Scheme XU

81%

74%

77%

972 INDIAN J CHEM, SEC. B, OCTOBER 1998

and sulfolane, we get 4-aminoquanizoline in one step(Scheme XIII).

Elimination of iodohydrines'", Reaction ofiodohydrin with freshly distilled POCl3 in pyridinesolution first at 0 °C and then at room temperature givesbicyclic compounds. When threo-iodohydrine wastreated with POCh in pyridine, cis-olefin was obtainedand the yield increased at bp. The erythero-isomer at 0°C gives the trans-olefin (Scheme XIV).

Ester formation'", Substituted phosphoryl chloridesare used for ester formation from alcoholate anion andfrom ketone. Here, first BuLi gives alcoholate anion in

DME, which on reaction with N,N,N,N-tetramethyl-diamiado phosphorochloridate gives correspondingesters after they are reduced with lah. Ester of fluoro-phosphoric acid can be synthesized'" by the reaction ofPOCIF with alcohols. This reaction occurs due todifference in reactivity of F and CI attached to phospho-rous atom (Scheme XV).

Phosphoorylation of nucleosides'", Phosphorylationof unprotected nucleoside is ineffective for thepreparation of 5-nucleotide from 5'-nucleoside.However, with POCh in trimethyl phosphate,phosphorylation occurs easily and selectively on Cs-OH.

oNOH . II@c( POCI)~ ~3·PCll

N ~ ~h

~NO

~~)~ ~N PhH

+ Hel

(Enol form)

~OONH~

lSJl~T)-PhN

(II)2-Phenylquinazoline

-4(3H)-one

oII

©CC .....o ~PhN...•\

H OH

Scheme .xm

SHARMA et al.: PHOSPHOROUS OXYCHLORIDE IN ORGANIC SYNTHESIS

Along with 5f-phosphate, 2',(or 3'), 5'-diphosphate isalso formed which can be reduced. Trialkyl phosphateis used because it acts not only as solvent but alsoaccelerates the reaction (Scheme XVI).

Decarbonylation of a.-amino acids. Bates andRapoport'? have applied this method for thedecarbonylation of o-amino acids to generate imimiumion which cyc1izes to the bicyc1ic product (SchemeXVII).

Synthesis of ~-Iactams. Ziegler and coworkers'"used POCh in DMF as a condensing agent for thepreparation of ~-Iactam from dichloroacetic acid. Itmay be possible that acid chloride is formed as anintermediate, though it has been suggested thatphosphorous halide participates as an electrophiliccatalyst.

Nelson22 reported that both cis- and trans-azetidinones are formed depending upon the particularSchiff base used.' Haloacetic acid under these conditions

ODCto RT

(Biyclic lodolactone)

Scheme XIV

973

react with imine only in refluxing DMF using POCI3.

Sharma et al. 23, made use of POCh for the synthesisof ~-lactam ring 1. Here, POCl3 is used in the presenceof Net, at room temperature. POCl3 is also used in theformation of polycyclic ~-lactam rings such as II and III(Chart 3).

24 .Dane salts from glycine can be allowed to reactwith POCl3 and then with Schiff bases and Net, to form3-(~-carbonylvinylamino)-2-azetidinones. All ~-lactamswere obtained in cis-configuration (Scheme XVIII).

POCh is also used25 in the conversion of cephamanalogs to monocyc1ic cis-~-lactams and in the synthesisof cis-l-aza analogs of cepham'".

Substituted phosphoryl chlorides like diethylchloridate, phosphorochloridate, diphenyl phospho-chloridate have been found to convert suitablesubstituted acetic acids and imines into ~-lactams. Forexample: N, N-bis(2-oxo-3-oxazolidinyl)phosphorodi-

POCI/Py ~

o°0-CH'OCH'Ph(Biyclic olefins)

(w(Alcohol)

~);;;~;:N~':~(-rPoPO~(CH')'J'(Ester)

oii) n-BuLi, DME

Et)N,CIPO[N(Mehll

(Ketone)

2 ROH + POClzF ------I.. OPF(OR)2 + 2 HCI

Scheme XV

974 INDIAN J CHEM. SEC. B. ocrOBER 1998

N~N> N6--N) NLN10J-N POCl) ~ lYJ-N Hydrolysis LOJ-"HOH y:OJ -(C-H-)O-»)'-p·OCI10POH

1y~OJ i ~N N

'~ ~ (OH),POH,: 0"

OH OHOH

5'-Phosphorylatednucleoside

[9-(13- D-arabinofuranosyl)adenines]

Scheme XVI

o CA. 0POCl) Cl,ll II~ ,P-OH+CO+ +. ~

Cl . ~ CI (CH1).

CH)

)

(lminiumion)

CyclisationwithMeOH, H+

MeI

~COCH'

Scheme XVII

_~~XJtO 0

PhOJ1~)

o N~OCH)

(I)

amidochlorides have been used in Net/7 to form 13-lactams. Similarly. bis(2.2. 2-trichloroethyl)phosphoro-chloridates have been used to obtain l3-lactams28

(Scheme XIX).

Armatic carbamic acid aryl esters", These estersare us.eful as intermediates for isocyanates and areprepared by the reaction of aromatic nitro compounds.aromatic hydroxy compounds and carbon monoxide athigh temperature and pressure over a catalyst Pd.POCI3• vanadium compound at Net-,

Aromatic isocyanates'", These are prepared byautociaving-2,4(N02hC6H3Me, PhOH. dichlorobis(Py)-

RO

o

(n) (ill)

Chart III

Pd. POC]3. dichlorobis(Py)Fe. quinoline and o-ChC6H4

under CO at 220°C for 210 min. Here 60% of 2,4-tolyenediisocyanates and 25% of nitrotolylisocyanatesare obtained.

a-Aromatic group-substituted alkanoic acids andesters". Compounds of the type R R!R2CCOOH, whereR = substituted aryl, R!_R2 = H, alkyl, are prepareda bythe rearrangement of RC(OR3)(OR4)-CR!R20H whereR4_Rs=alkyl. R4_RS may form cyclic ketal in thepresence of lewis acids [PXs (Xehalogen) orphosphorous oxyhalide with base or amides] (SchemeXX).

SHARMA et al.: PHOSPHOROUS OXYCHLORIDE IN ORGANIC SYNTHESIS

Diphenyl malonates'i, These are prepared byesterification of sodium malonate with PHOH in thepresence of POCh at 65°-70°C (Scheme XX).

o-Diamino phosphinyl derivatives of oximes'".Compounds of the type R1R2CNOP(X)(NR3R4)(NR5R6),where X = O,S,Rl_R6, alkyl, aryl, are prepared as ureaseinhibitors, making use of POCh. For instance,Me2C=NOH was reacted with POCh in pyridine at 0 °Cto give Me2C=NOPOCh, which on treatment with liquidNH3 at - 30° gives M~C=NOP(O)(N02h (Scheme XXI).

N-Substituted imidazoles34. Compounds Ph2C(C~R)-Rl (R = H, halo, nitro, Rl = l-imidazolyl) are prepared lbytreating Ph2C(C6H4R)OH with R2R3R4pO ([R2 =(substituted) l-imidazolyl; R3, R4= (substituted 1-imidazolyl, NR5R6; R5, R6 = lower alakyl, cycIoalkyl orNR5R6] may form a ring containing 0 or s. Thus,imidazole in MeCN was treated at 0 °C with POCI3,mixed at room temperature for 1 hr, and refluxed with 0-

975

C;IC6~CPhOH for 7.5 hr to give 70% of 1-[(0-chlorophenyl) diphenyl methyl] imidazole.

Amino acid amides'" and anilides36j Amino acidamides of the type RC:ONR1R2 (RCO = amino acidresidue; R1, R2 = H, C1.7 alkyl allyal, arylalkyl), usefulas starting materials for bioactive peptides and ~-lactams, can be prepared by reacting an amine with N-protected amino acids using POCI3 followed by N-deprotection. Scheme XXII depicts such a reactionwith N-protected-L-valine.

POCI3 is also used in the preparation of anilides.Amino acid p-nitroanilides can be prepared using thisreagent and the condensation was free of racemisationwith amino acid derivatives of urethane type.

Use in pyrimidines'", An interesting reaction usingPOCI3 occurs when a triazolopyrimidine derivative I isreacted with PHNHR3 to afford compound II (SchemeXXIII).

H HE: : R'

OnR

==== E-,1 -+COOK

i)POClllR'

ii),,J(R I NEtl

E R'....CH--CH'+ -I +1KOC=N,

• R

Scheme xvm

n~n0yN-?--N 0 +

~I Yo 0

r> ~ n0yN-rNyO010

O=C-C~R'

H HRI~R'

~N-R'o<p Lactam)

R'CH=N-R'

oII/OC~CCll

CI-P'OC~CCI3

Scheme XIX

976

R R'I I

R30-C-C-OHI I

OR4 R2

INDIAN J CHEM, SEe. B, OCTOBER 1998

R' 0+p I IIPOCI3 Xs...R-C-C-OH

BASE ~2

Scheme XX

(oxime derivative)

N-(t butoxy carbonylr-L-valine

Scheme XXI

!POCI,INHR,R,

N-(t butoxy carbonyl}--L-valine amide

Scheme XXII

Preparation of polyhydroxy benzophenones", Thepolyhydroxy benzophenones are useful as lightstabilizers and UV absorbers. These are prepared bytreating hydroxybenzoic acid with phenols in sulfolanein the presence of ZnCI2 and POCI3 (Scheme XXIV).

Ring closure of 3-amino-5-methyl thiazolo-3, 2-pyridinium salts39

• Ring closure of 3-amino-5-methylthiazolo-3,2-pyridinium salts occurs with POCI3

DMF followed by treatment with water (Scheme XXV).

Nitrile additions to c,N-diacylamines: formationof 4-amidooxazoles4o• When trimethoxylamide washeated as a solvent under reflux for 1 hr in the presenceof catalytic amounts of borontriflouride etheratefollowed lby aqueous workup and flashchromatography, compounds 1 and 2 were isolated inquantitative yields.

POCl3 gave same results as BFretherate. Thereaction was found to be general for various nitriles(aliphatic, aromatic and unsaturated) producingamidooxazoles and some times 3. The reaction of otherN-acylamines with nitriles in the presence of POCl3were examined and found to lead to diamides (SchemeXXVI).

Summary

It is not possible to describe Ihere the detailedmechanism of all the reactions utilizing POCI3.

Potential for its use in organic synthesis has beendemonstrated and more useful synthetic applications ofthis easily available Ireagent have been highlighted.

x

N-N::C'RI)lN~tf R'JL

R', R' = H, Me, Et, Me, CF), Ph

X=C6~NK, R'=Me, Et

PhNHR',POCl)••

Scheme xxm

¥OHlnCl,l'OCI,o sulfolanc ~

all

OH HO

HO-©-n-©-OH

o

Scheme XXIV

SHARMA et al.: PHOSPHOROUS OXYCHLORIDE IN ORGANIC SYNTHESIS

i) poeLl' DMF••ii)H,O

Scheme XXVo

)lRxOMePh N XPh

HMeO OMe

977

~

RN

OHC . JlN S~J=l

N

Lewis Acid

PhyOJL °\\ IIN N-C-R'

H

(2)

+ 0

° R N)lR')l XH...PhPh N 1f

H °(3)

R'Jl H NH-<:Ph NXR 0

H

POCl,-N-Acylamine

R = COOH, X = OH

R= H. X = OMeR = H. R' = Ph

Scheme XXVI

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978 INDIAN J CHEM, SEe. B, OCTOBER 1998

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