Z‑selective alkene formation from reductive aldehyde homo-couplings

Sammanfattning: Current methodologies for the direct coupling of two aldehydes to alkenes afford almost exclusively the thermodynamically favoured E-isomer. Our recent efforts to find phosphorus-based reagent as replacements for the low-valent Ti species used in McMurry couplings offers a possibility to change this shortcoming, and to design new reagents that allow for the formation of high proportions of Z-alkenes under kinetic control. Here, we report on the first such reagent, a phosphanyl phosphonate MesFP(H)P(O)(OEt)2 with an electron-deficient MesF = 2,4,6-(CF3)3Ph substituent. The reagent promotes the reductive homo-coupling of (hetero)aromatic aldehydes to alkenes with high Z-selectivity. The one-pot procedure passes through phosphaalkene intermediates, the 31P NMR chical shifts of which correlate with the proportion of formed Z-isomer, with the most deshielded phosphaalkenes giving more than 90% steroselectivity of the Z-alkene.