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

Abstract

Readily prepared and bench-stable rhodium complexes containing methylene bridged diphosphine ligands, viz. [Rh(C(6)H(5)F)(R(2)PCH(2)PR'(2))][BAr(F)(4)] (R, R' = (t)Bu or Cy; Ar(F) = C(6)H(3)-3,5-(CF(3))(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((t)Bu(2)PCH(2)P(t)Bu(2))H{κ(2)(S,C)-SMe(C(6)H(4)CO)}(L)](+) (L = acetone, MeCN, [NCCH(2)BF(3)](-)) and the decarbonylation product [Rh((t)Bu(2)PCH(2)P(t)Bu(2))(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.

Traceless Chelation‐Controlled Rhodium‐Catalyzed Intermolecular Alkene and Alkyne Hydroacylation

“…The amount of catalyst needed can be dramatically reduced if two separate catalysts are employed, each of which have been optimised for the individual steps of the process. We have previously shown that complex D (featuring the small bite‐angle di( t Bu)phosphine) can catalyse the hydroacylation of unactivated alkenes at as low as 0.1 % catalyst loading 15. The most efficient catalyst for the aryl sulfide reduction step is complex E ,17 which can efficiently deliver the reduced compound by using only 0.5 mol % loading (Scheme ).…”

“…Both compounds 7 and 8 are essentially inactive in catalysis, showing that once formed they represent a deactivation pathway for this process in CH 2 Cl 2 (or C 2 H 4 Cl 2 ). An active species is formed upon addition of 3 a to [Rh(Cy 2 PCH 2 PCy 2 )(MeCN) 2 ][BAr F 4 ] ( 9 )15 to form [Rh(Cy 2 PCH 2 PCy 2 )(SMe){σ,κ‐C 6 H 4 (OCMe)}(NCMe)][BAr F 4 ] ( 10 ), rather than the dimer. This reaction is rapid suggesting that dissociation of fluorobenzene is the limiting factor in the reaction of C with 3 a in CH 2 Cl 2 .…”

Well‐Defined and Robust Rhodium Catalysts for the Hydroacylation of Terminal and Internal Alkenes

“…That no branched final product is observed suggests a higher barrier to reductive elimination from the corresponding intermediate, as noted previously. [8, 13b] H/D exchange was observed into free 4 a during catalysis, suggesting reversible aldehyde and alkene binding. The measured KIE is similar to other systems in which reductive elimination is proposed to be turnover-limiting,[8b, 13] rather than irreversible C–H activation (KIE ≈2.5[7c, 18]), and is likely an equilibrium isotope effect.…”

“…We first demonstrated that employing this new catalyst allowed reduced catalyst loadings for substrates previously reported. [13b] For example, using complex 1 a catalyst loading of 5 mol % and 80 °C were required for the hydroacylation of methyl methacrylate with a yield of 68 %; however, when catalyst 3 a was employed, we were able to not only decrease the loading and the temperature but also significantly increase the yield (entry 1). A similar improvement was observed in the case of N -methyl maleimide (entry 2).…”

Synthesis of Highly Fluorinated Arene Complexes of [Rh(Chelating Phosphine)]+ Cations, and their use in Synthesis and Catalysis

“…We and others have shown cationic rhodium(I) complexes ligated by small bite‐angle diphosphine and η‐FC 6 H 5 to be effective pre‐catalysts for intermolecular hydroacylation . However, the substrate scope is limited to aldehydes or alkenes containing a β‐coordinating group . When on the aldehyde, this motif drives the pre‐equilibrium of aldehyde binding by chelation effect and also blocks a vacant site on the corresponding acyl hydride complex that comes from C−H activation.…”