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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. |
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