Reductive cleavage of the carbonphosphorus bond with alkali metals. III. Reactions of arylalkylphosphines

Abstract

Abstract The reductive cleavage of phenylalkylphosphines Ph 2 PR, PhPR 2 (R = Bu, iPr) with Na/NH 3 is unselective: both phenyl and alkyl groups can be cleaved and Birch reduction may occur. Reaction of Ph 2 tBuP gives a high yield of diphenylphosphide. Polar groups (CO 2 Na, SO 3 ,Na) at the ω position of primary alkyl groups may lead to an increase in selectivity; Birch reduction is suppressed and a functionalised secondary phosphide is obtained. From diarylbenzyl‐ and diarylallylphosphines, the benzyl and allyl groups are selectively removed; Ar 2 PH and ArRPH are formed in high yield after hydrolytic work‐up unless the aryl group bears F, CF 3 , or (CH 3 ) 2 N substituents. From the reaction mixture of Ph 2 PCH 2 Ph we have isolated 1,2‐ ‐diphenylethane. 2‐Methoxyphenyl and 2,6‐dimethoxyphenyl groups are selectively removed from Ar 2 BuP, ArPhBuP and Ar 2 P(CH 2 ) 3 PAr 2 , forming ArBuPH, PhBuPH and ArP(H)(CH 2 ) 3 (H)PAr, respectively. A double‐cleavage reaction of Ar 2 RP may occur in low yield. 2,6‐(Dimethoxyphenyl‐dibutylphosphine gives dibutylphosphine in moderate yield. When compounds with a 2,6‐dimethoxyphenyl moiety are allowed to react with Li/THF, removal of a methyl group leads to novel phosphinophenols. It is concluded that cleavage of alkyl groups R selectively occurs when R° is relatively stable (tBu, PhCH 2 > iPr > Bu).