| Popis: |
Current methodologies for the direct reductive coupling of two aldehydes to alkenes afford almost exclusively the thermodynamically favoured E-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 of Z-alkenes under kinetic control. Here, we report the first example of such a reagent, a phosphanyl phosphonate Mes(F)P(H)P(O)(OEt)(2), 6, with an electron-deficient Mes(F)=2,4,6-(CF3)(3)Ph substituent that promotes the reductive homo-coupling of (hetero)aromatic aldehydes to alkenes with high Z-selectivity. Computational results indicate that the selectivity stems from the electron deficient Mes(F), which results in lowered activation barriers for the collapse of a cis-oxaphosphetane intermediate. In the absence of Mes(F), the E-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 the Z-isomer. |
