Catalytic Decarbonylation of Aldehydes Using Cationic Complexes of Rhodium(I) with Chelating Diphosphine Ligands

Abstract

The decarbonylation of aldehydes, acyl halides, aroyl halides and alcohols is a useful and important reaction in organic synthesis.1,2 Although several methods not utilizing transition metals are known (including various deformylation reactions, thermal and photochemical decarbonylations ),3,4 they are not general and not usually applicable under mild conditions where undesirable side reactions are minimized. Stoichiometric homogeneous decarbonylation of aldehydes and acid chlorides using transition metal complexes of monodentate tertiary phosphine ligands such as RhCl(PPh3)3 5−7, [Rh−(PPh3)2(solvent)n]+ 8, and [Ru2Cl3(PEt2Ph)+ 9 is now well established. Of these, RhCl(PPt3)3 I has received the most study and has been proven useful as a reagent in organic synthesis.1,10 I decarbonylates aldehydes and acid chlorides under mild thermal conditions (<100°C) homogeneously in solution. The reactions are stoichiometric and are summarized by eq 1 and 2 where X = H or C1.6,11−13 Reaction Open image in new window 2 where X = CI occurs when a β-H is present while reaction 2 where X = H is of minor importance (e. g., decarbonylation of heptanal using I yields 86% hexane and 14% 1-hexene) .6 Hence, complex I is useful for the stoichiometric conversion of aldehydes into alkanes and of acid halides into alkyl or aryl halides and olefins (if β-H is present).