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A Convenient Synthesis ofTertiary Isothiocyanates andAcyl Isothiocyanates UsingPhosphoryl IsothiocyanateLadislav Kniežo a & Juraj Bernát aa Department of Organic Chemistry, P. J. ŠafárikUniversity, 041 67, Košice, CzechoslovakiaPublished online: 24 Oct 2006.

To cite this article: Ladislav Kniežo & Juraj Bernát (1990) A ConvenientSynthesis of Tertiary Isothiocyanates and Acyl Isothiocyanates Using PhosphorylIsothiocyanate, Synthetic Communications: An International Journal forRapid Communication of Synthetic Organic Chemistry, 20:4, 509-513, DOI:10.1080/00397919008244898

To link to this article: http://dx.doi.org/10.1080/00397919008244898

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SYNTHETIC COMMUNICATIONS, 2 0 ( 4 ) , 509-513 (1990)

A CONVENIENT SYNTHESIS O F TERTIARY ISOTHIOCYANATES AND

ACYL ISOTHIOCYANATES USING PHOSPHORYL ISOTHIOCYANATE.

Ladislav Knieio- and Juraj Eernat

Oepartment of Organic Chemistry,P.J.Safirik University

041 67 KoSice,Czechoslovakia

Phosphoryl isothiocyanate,PO(NCS) reacts readily with tertiary alcohols and carboxylic ZAids ,giving the re- spective tertiary isothiocyanates and acyl isothiocyana- tes in good yields and purity.

It is generally known that POC13 belongs to reagents

(such as SOCl2,PCl3,PCl5,Ph3PC1 ) capable of converting

alcohols into the corresponding alkyl chlorides o r

carboxylic acids into acyl chlorides.In spite of the

fact that the -NCS group can be regarded as a pseudo-

halide groupl,phosphoryl isothiocyanate,PO(NCS)3,was

hitherto not used for the preparation of isothiocyana-

tes directly from hydroxy compounds.The reason may be

the possibility of undesired reactions (e.g. the known

2

~ ~~

+ To whom correspondence should be addressed.

509

Copyright 0 1990 by Marcel Dekker, Inc.

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5 10 KNIEZO AND BERNAT

addition of primary alcohols to PO(NCSl3 under forma-

tion of thiouretanes ) . 2

On the other hand,hydroxy derivatives react with

Ph P(NCS)2,primary alcohols giving the corresponding

thiocyanates,secondary alcohols a mixture of thiocyana-

tes and isothiocyanates and tertiary alcohols isothio-

~ y a n a t e s ~ ’ ~ . R e a c t i o n o f carboxylic acids with this

reagent leads to acyl isothi~cyanates~ 5 . It seems,

however,that the yield of tertiary isothiocyanates

3

depends on the substituents .Thus, it is reported’

that 2-phenylpropan-2-01 affords the corresponding

isothiocyanate in 80% yield whereas the yield of tert-

butyl isothiocyanate from tert-butyl alcohol is only 8%.

We have found that tertiary alcohols containig alkyl

o r aryl substituents react readily with phosphoryl iso-

thiocyanate,affording tertiary isothiocyanates in good

yields.The starting PO(NCS)) is easily accessible by

reaction of P0Cl3 with alkali metal isothiocyanates,

the yields and reaction time depending strongly on the

polarity of the solvent used 196-8.The best yields 1 (up to 90 % ) were achieved with NH,,SCN in liquid S O 2

However,we obtained comparable yields also by reacting

KSCN with POC13 in benzene in the presence of 18-crown-6

ether9.The tertiary isothiocyanates,prepared by us from

tertiary alcohols by reaction with PO(NCS)3,are listed

in Table 1.

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TERTIARY AND ACYL ISOTHIOCYANATES 511

Table 1 : Isothiocyanates R-NC5,prepared from tertiary alcohols R-OH

R Yield(%) B.p.('C/mmHg) Ncs(cm-l)

tert-8utyl 72 140/760 2095 1,l-Oimethylpropyl 91 164-166/760 2095 l,l-Dimethyl-3-oxobutyl 78 95-97/10 2075 1-Phenyl-1-methylpropyl 70 80-82/1.2 2100 Triphenylmethyl 75 m.p.137-138 2060

Table 2 : Acyl isothiocyanates prepared from carboxylic acids

NCS C=O Acyl Yield 8.p.

( % ) ('C/mmHg) (cm-') (cm-')

Benzoyl 70 135-137/18 1980 1685 2-Methylbenzoyl 90 90-92/0.6 1970 1685 3-Methylbenzoyl 8 5 86-87/0.6 1955 1685 4-Methoxybenzoyl 8 3 111-112/0.7 1980 1680

65 m.p.90-92 1920 1950 1690 4-Nitrobenzoyl

Cinnamoyl 62 m.p.42-44 1965 1680 2-Thenoyl 77 78-80/0.7 1950 1675 St earoyl 6 5 180-183/0.7 1965 1720

a

Phenylacetyl 85 138-140/10 1965 1710 4-tert-Butyl-1-cyclo- hexencarbonyl 90 100-101/0.7 1980 1680 Z-(Z-Propinyl)- -4-pentinoyl

aEquimolar mixture of the acid and PO(NCS13 was heated without solvent to 7OoC fo r 6 h and worked up by flash chromatography.

45 73-74/0.5 1950 1715 a

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5 12 KNIEZO AND BERNAT

Because of simplicity and effectivity we tried to

apply the described method to the preparation of acyl

isothiocyanates from carboxylic acids.Reaction of phenyl-

acetic acid with PO(NCS)3 in benzene was followed by

NMR spectroscopy directly in the NMR tube.After heating

to 7OoC for 7 h,the benzene solution did not contain

any starting acid and subsequent chromatography o n silica

gel afforded pure phenylacetyl isothiocyanate.The same

procedure was used in the preparation of other acyl

isothiocyanates,described in Table 2.

The easy availability of PO(NCS)3,as well as the

simplicity of the method,allow a general and efficient

synthesis of tertiary o r acyl isothiocyanates which are

obtained in high purity and good yields.

EXPERIMENTAL

General procedure for preparation o f tertiary isothio- cyanates.

Freshly distilled PO(NCS)3 (0.02 mol) was added dropwise under stirring to the alcohol (0.04 mol) so as the temperature did not exceed 8O0C.After heating to 80°C for 2.5 h,the reaction mixture was cooled and the product extracted with light petroleum.The iso- thiocyanates (with the exception of triphenylmethyl isothiocyanate) were isolated by evaporation of the solvent and distillation.

Triphenylmethyl isothiocyanate.

Freshly distilled PO(NCS)3 (0.02 mol) was added dropwise at room temperature to a stirred solution of triphenylmethanol (0 .02 mol) in ether (50 ml).After 2 h the ether was evaporated and the residue crystallized from ethanol to give 75% of the title compound.

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TERTIARY AND ACYL ISOTHIOCYANATES 513

General procedure for preparation of acyl isothio-

cyanates.

Freshly distilled PO(NCS13 (0.02 mol) was added dropwise to a stirred solution of the carboxylic acid (0.02 mol) in benzene (10 ml) and the reaction mixture was heated to 7OoC for 7 h.After standing at room temperature overnight,the mixture was worked up by flash chromatography on silica gel (ch1oroform:hexane = 3:l) and the product was distilled or crystallized.

REFERENCES.

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9.

Fluck,E.,Binder,H. and Goldmann,F., Z.anorg.Chern., 1963,=,58 Elmore,D.T. and Ogle,J.R., J.Chern.Soc.,1959,2286 Tamura,Y.,Kawasaki,T.,Adachi,M.,Tanio,M. and Kita,Y., Tetrahedron Letters,1977,4417 Burski,J.,Kieszkowski,J.,Michalski,J.,Pakulski,M. and Skowronska,A., J.Chern.Soc.Chern.Cornmun.,1978,940

Tamura,Y.,Kawasaki,T.,Tanio,M. and Kita,Y., Chem. and Ind.,1978,806 Oixon,A.E. and Taylor,J., J . C h e r n . S 0 ~ . , 1 9 0 8 , ~ , 2 1 4 8

Anderson,H.H., J.Arner.Chem.S0~.,1942,64,1757

Green,B.S.,Sowerby,D.B. and Wihksne,K.J., Chern. and Ind.,1960,1306 Refluxing a mixture of P0Cl3 (0 .05 mol),KSCN (0.2 mol),l8-crown-6 ( 0 . 0 0 5 mol) and benzene ( 5 0 ml) for 6 h afforded PO(NCS)3 (0.04 mo1,80 %),b.p. 120-122°C/1. 2 mmHg. The yield was not optimized.

(Received in UK 4 October, 1989)

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