The unprecedented formation of unsymmetrical alkenes from the intermolecular reductive coupling of two different aldehydes is described. In contrast to the McMurry reaction which affords statistical product mixtures, selectivity in the reported procedure is achieved by a sequential ionic mechanism in which a first aldehyde is reacted with a phosphanylphosphonate to afford a phosphaalkene intermediate which, upon activation by hydroxide, reacts with a second aldehyde to the unsymmetrical E-alkenes. The described reaction is free of transition metals and proceeds under ambient temperature within minutes in good to excellent overall yields. It is a new methodology to use feedstock aldehydes for the direct production of C═C double bond-containing products and may impact how chemists think of multistep synthetic sequences in the future.
Triphenylphosphaalkenes 1a-c were prepared in good to excellent yields in a modified phospha-Peterson reaction between PhP(Li)TMS and benzophenones with different para-substituents at the C-phenyl groups (a: R = H, b: R = Ooctyl, c: R = F). Owing to the low kinetic stabilization that is provided by the P-phenyl group, compounds 1a-c engage in [a]
Stilbenes with push-pull electronics are directly accessible from an electron-rich and an electron-deficient benzaldehyde in a novel reductive aldehyde cross-coupling reaction. The one-pot procedure is enabled by the oxidation of a transient phosphinite to the corresponding phosphinate which exhibits sufficient reactivity towards deactivated aldehydes.
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 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 high Z‐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, 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.
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