Hydrogenation of acrylic acids catalysed by complexes of rhodium(I) containing mixed anhydrides of acrylic and diphenylphosphinous acids

Abstract

Complexes of the form [RhCl(PPh₃)_n(Ph₂PO₂CCRCR′R″)](n= 1 or 2) or [Rh(PPh₃)₂(Ph₂PO₂CCRCR′R″)]PF₆(R,R′,R″= H, alkyl or aryl) have been shown to be more effective for the catalytic hydrogenation of acrylic acids in the presence of added base (KOH or NEt₃) than is [RhCl(PPh₃)₃]. Mechanistic studies show that the active species is [Rh(O₂CCRCR′R″)(PPh₃)(Ph₂PO₂CCRCR′R″)], in which the mixed anhydride is bound through both the phosphorus atom and the double bond. After hydrogenation of the double bond of the mixed anhydride, new substrate is introduced by a base-catalysed transesterification reaction at the phosphorus atom of the mixed anhydride. This is modelled in the reactions of [RhCl(PPh₃)₂(Ph₂PO₂CCH₂CH₂Me)], which exists in two isomeric forms, one with binding of the mixed anhydride only through P and the other with binding of the mixed anhydride through P and O, or of [Rh(PPh₃)₂(Ph₂PO₂CCH₂CH₂Me)]PF₆ with MeCHCHCO₂^–, which give [Rh(O₂CCHCHMe)(PPh₃)(Ph₂PO₂CCHCHMe)] in high yield. Hydrogenation of hexa-2,4-dienoic acid gives hex-4-enoic acid and hexanoic acid and evidence is presented that these two products are obtained from hydrogenation of the mixed anhydride (hex-4-enoic acid as product) or of the directly bonded anion (hexanoic acid as product). Attempts to promote high enantioselectivity in the hydrogenation of PhCHCMeCO₂H by using [Rh(P–P*)(Ph₂PO₂CCMeCHPh)]PF₆(P–P*= a chiral diphosphine) were frustrated by the displacement of the mixed anhydride and the formation of [Rh(P–P*)(Ph₂POPPh₂)]PF₆. Enantiomeric excesses of only ca. 10% were achieved.