Z‐Selective Alkene Formation from Reductive Aldehyde Homo‐Couplings

Abstract

AbstractCurrent methodologies for the direct reductive coupling of two aldehydes to alkenes afford almost exclusively the thermodynamically favouredE‐isomer. Recent efforts to find phosphorus‐based reagents as replacements for the low‐valent Ti species in McMurry couplings present opportunities to change this shortcoming, and to design new reagents that allow for the formation of high proportions ofZ‐alkenes under kinetic control. Here, we report the first example of such a reagent, a phosphanyl phosphonate MesFP(H)P(O)(OEt)2,6, with an electron‐deficient MesF=2,4,6‐(CF3)3Ph substituent that promotes the reductive homo‐coupling of (hetero)aromatic aldehydes to alkenes with highZ‐selectivity. Computational results indicate that the selectivity stems from the electron deficient MesF, which results in lowered activation barriers for the collapse of a cis‐oxaphosphetane intermediate. In the absence of MesF, theE‐isomer is exclusively observed experimentally. Directing the isomeric outcome of alkene formation by introducing electron withdrawing P‐substituents bears resemblance to the Still‐Gennari modification of the Horner‐Wadsworth‐Emmons reaction where perfluorinated ethoxy substituents in the former also lead to high proportions of theZ‐isomer.