Reductive cleavage of the carbonphosphorus bond with alkali metals. II. Cleavage of mixed functionalized triarylphosphines; Birch reduction of arylphosphines

Abstract

Abstract The reductive cleavage of mixed ortho ‐ and para ‐functionalized triarylphosphines with Na/NH 3 and Li/THF depends strongly on the nature and positions of the substituents. Reduction occurs readily with phosphines PhAr 2 P ( 4, 6 and 9 ) and Ph 2 ArP 19 when the corresponding phosphine Ar 3 P is not reduced. Cleavage of para ‐substituted compounds 7 and 9 leads to mixtures of secondary phosphines. By contrast, cleavage of mixed ortho ‐substituted triphenylphosphines is very selective. The functionalized phenyl group is split off in high yield when it carries CH 3 , (CH 3 ) 2 N and CH 3 O substituents ( 2, 3, 5, 6, 8, 10, 11, 13, 14, 17, 19, 24, 25 ). Reaction of 4 is not selective due to loss of methoxy groups. By contrast, the phenyl group is split off when the mixed phosphine contains phenoxide groups ( 1, 15, 16 ). The reactions of bis‐ and tris(diphenylphosphino)benzenes with Li/THF lead predominantly to cleavage of the diphenylphosphino group from the respective substrates. In a number of cases, the product of a Birch reduction with an isolated diene system is formed in NH 3 ( 1, 9, 12, 21, 23 ) via a phosphino‐stabilized cyclohexadienyl anion. This reduction does not occur in the aprotic solvent THF. Base‐catalyzed isomerization leads to a conjugated double‐bond system with a vinylphosphine moiety. We also report interesting large a J (PP) couplings in 1,3‐diphosphinobenzenes and complicated 13 C resonances of para ‐substituted phosphines.