Reductive cleavage of the carbon-phosphorus bond with alkali metals. III Reactions of arylalkylphosphines

Article abstract of DOI:10.1002/recl.19921110402

The reductive cleavage of phenylalkylphosphines Ph2PR, PhPR2 (R = Bu, iPr) with Na/NH3 is unselective; both phenyl and alkyl groups can be cleavaged and Birch reduction may occur.Reaction of Ph2tBuP gives a high yield of diphenylphosphide.Polar groups (CO2Na, SO3Na) 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 radical is relatively stable (tBu, PhCH2> iPr > Bu).