The Preparation of Phosphorus Esters and Thioesters from White

47 The Preparation of Phosphorus Esters and Thioesters from White Phosphorus

Downloaded by UNIV QUEENSLAND on October 14, 2014 | http://pubs.acs.org Publication Date: November 11, 1981 | doi: 10.1021/bk-1981-0171.ch047

CHARLES BROWN, ROBERT F. HUDSON, and GARY A. WARTEW Chemical Laboratory, University of Kent at Canterbury, Kent, CT2 7NH, England HAROLD COATES Albright & Wilson Limited, P.O. Box 3, Oldbury, Worcestershire, England

Many attempts have been made to synthesise organophosphorus compounds directly from the element with varying degrees of success. In general mixtures of products are obtained, and the yields are frequently low and variable. The main difficulty is the insolubility of phosphorus in organic liquids and consequently the reactions are heterogeneous with the attendant problems of diffusion and local concentration differences. Our approach was based on the following considerations. 1) White phosphorus, a strained tricyclic system has a low nucleophilic reactivity but is highly electrophilic. 2) Attack by a nucleophile produces a reactive phosphide ion which is rapidly protonated in hydroxylic media. For high yields of a required product it is essential to trap this ion with a suitable electrophile. 3) Owing to the mutual reaction of nucleophile and electrophile the number of useful combinations is limited. In principle a combination of hard nucleophile and soft electrophile is preferred. Phosphines and phosphides react rapidly with positive halogen and hence polyhalogen compounds, e.g. tetrachloromethane, are particularly suitable and have the advantage that the chlorophosphine formed is rapidly attacked by the nucleophile. In principle therefore the phosphorus atoms can be fully substituted by reactions of the following type, 1,2

3

0097-6156/81/0171-0231$05.00/ 0 © 1981 American Chemical Society In Phosphorus Chemistry; Quin, L., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

PHOSPHORUS

232

CHEMISTRY

The reactions were carried out in an excess of alcohol and tetrachloromethane, and the concentration of alkoxide varied, the white phosphorus being added, under nitrogen, in the form of a fine sand. The progress of the reaction was followed by GLC after the products of reaction had been identified by P NMR. In initial experiments, stoicheiometric quantities of phosphorus and alkoxide were used, according to the equation, 3

31


Pi»

+ 6NaOR + 6R0H + 6CCH —» 4P (OR) 3 + 6CHCI3 + 6NaCl

Although quantitative yields of chloroform were found, low yields of trialkyl phosphite were produced and these decreased with time as the yields of trialkyl phosphate and dialkyl phosphate increased, e.g. in the reaction of sodium n-butoxide (0.06 mol) with phosphorus (0.009 mol) in 30 ml of methanol and 50 ml of n-butanol 47% of phosphite was produced in 3 h. together with 19% of phosphate and 7% of phosphonate. The product composition changed to 34%, 22% abd 21% respectively after 7 h. The former is no doubt produced by the well known reaction! +

+

^(R0) P=0 + RC1 3

DAU

(R0) P + CCU—»(R0) P-C1 3

3

> (R0KPC1 (R0) P=0 + R 0 3

2

De-alkylation by alkoxide ion increases the acidity of the medium and this accounts for the formation of dialkyl phosphonate in the later stages of the reaction. For these reasons, higher concentrations of alkoxide were used, firstly to increase the initial rate and secondly to preserve alkalinity throughout the reaction. With two equivalents of alkoxide, high yields of trialkyl phosphite were obtained within 1-2 h. at 25°. Again, the yield decreased with time owing to the subsequent oxidation, e.g. 82% tr ime thylpho sph i te and 76% t r i ethylphosphite after 1 h. When the reaction was carried out in the probe of a P NMR spectrometer, no evidence for the accumulation of intermediates was obtained (vide infra). These results establish the conditions for the formation of trialkyl phosphites in high yields. However attempts to distil the ester from the reaction mixture were unsuccessful as codistillation and some decomposition always occurred. Other workers who carried out similar experiments independently,! report a 50% yield of isolated trimethyl phosphite. Attention was turned to the analogous reaction of thiols. Here the subsequent oxidation can be neglected, and the products were readily separated by distillation. Again, low yields of triester were obtained when equivalent quantities of thioalkoxide and phosphorus were used.I. Evidence of incomplete conversion was obtained from the P NMR spectra of the reaction mixtures using 31

31

In Phosphorus Chemistry; Quin, L., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

47.

BROWN

ET AL.

Phosphorus

Esters

and

Thioesters

233

ethane t h i o l . Three absorptions i n the r a t i o 2:1:1 were observed corresponding to t r i e t h y l p h o s p h o r o t r i t h i o i t e (δρ 114.1), d i e t h y l p h o s p h o r o c h l o r i d o d i t h i o i t e (δρ 184.5) and d i e t h y l t r i c h l o r o m e t h y l phosphonodithioite (δρ 121.6). The two intermediates probably a r i s e i n the l a t e r stages o f the r e a c t i o n when the t h i o a l k o x i d e c o n c e n t r a t i o n i s low. The presence o f these intermediates i s evidence o f the intermediate formation of the t r i c h l o r o m e t h y l anion, i n r e a c t i o n s of the following kind.


DO"

(RS) P-P(SR) 2

2

— — » CCI.

(RS) P + (RS) PC1 — » ( R S ) P + (RS) P.CC1 3

2

3

+ CCI

2

3

+ Cl

3

These r e a c t i o n s occur because the t h i o a l k o x i d e i s depleted by r e a c t i o n with tetrachloromethane i n the f o l l o w i n g side reactions ,Ζ 4NaSR

+

CCI +

RSH

* CH(SR)

3

+

RSSR

+

4NaCl

The c h l o r i d o d i t h i o i t e does not r e a c t with n e u t r a l t h i o l , whereas the corresponding d i a l k y l p h o s p h o r o c h l o r i d i t e r e a c t s r a p i d l y with ethanol. Consequently the l a t t e r r e a c t i o n proceeds to completion even when the alkoxide has been n e u t r a l i s e d . With two equivalents o f t h i o a l k o x i d e , the t r i e s t e r only i s produced and t h i s can be d i s t i l l e d from the r e a c t i o n mixture i n high y i e l d (e.g. 97% from e t h a n e t h i o l and 82% from b u t a n e t h i o l ) . The f o l l o w i n g scheme i s suggested f o r the breakdown o f the Pi# molecule and the formation o f t r i e s t e r (X = 0, S ) .

1X1 P'

Ρ

— —

fx/F

IXI

—

Ρ

î

Ρ

t

^P

Ρ—XR II

RX

I

XR 4(RX) P * 3

2(RX) P—P(XR) 2

2

+

(RX) P — Ρ — Ρ — Ρ (XR) 2

2

U

XR III

The absence of detectable r e a c t i o n intermediates suggests that the i n i t i a l heterogeneous r e a c t i o n i s rate determining. The b i c y c l i c intermediate, I , gives the c y c l o t e t r a p h o s p h i t e I I with r e l e a s e of r i n g s t r a i n . C y c l i c molecules of t h i s k i n d have been i s o l a t e d from r e a c t i o n s o f elemental phosphorus, e.g. t e t r a a l k y l cyclotetraphosphines from the combined a c t i o n o f Grignard reagents and n - a l k y l bromides.JL


234

PHOSPHORUS

CHEMISTRY

The subsequent stages i n v o l v i n g tetraphosphine, triphosphine and diphosphine d e r i v a t i v e s proceed r a p i d l y owing t o the high r e a c t i v i t y o f the P-P bond. These r e a c t i o n s appear to be r e s t r i c t e d to s t r o n g l y b a s i c n u c l e o p h i l e s , as we found no r e a c t i o n with phenoxides and t h i o phenoxides. This l a c k o f r e a c t i v i t y i s a t t r i b u t e d to the r e v e r s i b i l i t y o f the n u c l e o p h i l i c s u b s t i t u t i o n , promoted by the increased l e a v i n g group a b i l i t y of the n u c l e o p h i l e , e.g.


:-)

LITERATURE CITED 1. 2. 3. 4. 5. 6. 7. 8.

Rahut, M.M. Topics in Phosphorus Chemistry Vol. 1, p.1, Interscience, Ν.Y., 1964 Maier, L. Fortsch. Chem. Forsch. 1971, 19, 1 Brown, C.; Hudson R.F.; Wartew, G.A.; Coates, H. Phosphorus and Sulphur 1979, 6, 481 Burn, A.J.; Cadogan, J.I.G. J . Chem. Soc. 1963, 5788 Lehmann, H.A.; Schadow, H.; Richter, H.; Kurze, R.; Oertel, M. Ger. Patent, 127,188, 1977 Brown, C.; Hudson, R.F.; Wartew, G.A. J.Chem. Soc. Perk. I., 1979, 1979 Backer, J.H.; Stedehonder, P.L. Rec. Trav. Chim. 1933, 52, 437 Cowley, A.H.; Punnell, R.P. Inorg. Chem. 1966, 5, 1463

RECEIVED

June 30, 1981.


The Preparation of Phosphorus Esters and Thioesters from White

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