Intermolecular Hydroacylation: High Activity Rhodium Catalysts Containing Small-Bite-Angle Diphosphine Ligands

Readily prepared and bench-stable rhodium complexes containing
methylene bridged diphosphine ligands, viz. [Rh­(C6H5F)­(R2PCH2PR′2)]­[BArF4] (R,
R′ = tBu or Cy; ArF = C6H3-3,5-(CF3)2), are shown to be practical
and very efficient precatalysts for the intermolecular hydroacylation
of a wide variety of unactivated alkenes and alkynes with β-S-substituted
aldehydes. Intermediate acyl hydride complexes [Rh­(tBu2PCH2PtBu2)­H­{κ2(S,C)-SMe­(C6H4CO)}­(L)]+ (L = acetone,
MeCN, [NCCH2BF3]−) and the
decarbonylation product [Rh­(tBu2PCH2PtBu2)­(CO)­(SMePh)]+ have been characterized
in solution and by X-ray crystallography
from stoichiometric reactions employing 2-(methylthio)­benzaldehdye.
Analogous complexes with
the phosphine 2-(diphenylphosphino)­benzaldehyde are also reported.
Studies indicate that through judicious choice of solvent and catalyst/substrate
concentration, both decarbonylation and productive hydroacylation
can be tuned to such an extent that very low catalyst loadings (0.1
mol
%) and turnover frequencies of greater than 300 h–1 can be achieved. The mechanism of catalysis has been further
probed by KIE and deuterium labeling experiments. Combined with the
stoichiometric studies, a mechanism
is proposed in which both oxidative addition of the aldehyde to give
an acyl hydride and insertion of the hydride into the alkene are reversible,
with the latter occurring to give both linear and branched alkyl intermediates,
although reductive elimination for the linear isomer is suggested
to have a considerably lower barrier.