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

The reductive cleavage of phenylalkylphosphines Ph2PR, PhPR2 (R = Bu, iPr) with Na/NH3 is unselective: both phenyl and alkyl groups can be cleaved and Birch reduction may occur. Reaction of Ph2tBuP gives a high yield of diphenylphosphide. Polar groups (CO2Na, SO3,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; Ar2PH and ArRPH are formed in high yield after hydrolytic work‐up unless the aryl group bears F, CF3, or (CH3)2N substituents. From the reaction mixture of Ph2PCH2Ph we have isolated 1,2‐ ‐diphenylethane. 2‐Methoxyphenyl and 2,6‐dimethoxyphenyl groups are selectively removed from Ar2BuP, ArPhBuP and Ar2P(CH2)3PAr2, forming ArBuPH, PhBuPH and ArP(H)(CH2)3(H)PAr, respectively. A double‐cleavage reaction of Ar2RP 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, PhCH2 > iPr > Bu).