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.
The synthesis of the first phophanylphosphonate, Mes*PH-PO(OEt)2 (2-H), in which the P(III) centre is not coordinated by a M(CO)5 (M = W, Mo, Cr) fragment is reported. The title compound reacts with LDA under the formation of 2-Li which is best described as the enolate form with a high double bond character between the two phosphorus centres. 2-Li is shown to engage in the phospha-Wittig-Horner reaction and converts aldehydes into phosphaalkenes that are metal-free and thus available for future manipulations at the phophorus lone pair. Using a selection of aldehydes with aliphatic, aromatic or vinylic substituents as substrates, phosphaalkene formation proceeds in high yields and high E-selectivity. The selectivity is however compromised during purification on standard silica which was found to promote E/Z isomerization.
The first direct alkynylation of C,C-dibromophosphaalkenes by a reaction with sulfonylacetylenes is reported. Alkynylation proceeds selectively in the trans position relative to the P substituent to afford bromoethynylphosphaalkenes. Owing to the absence of transition metals in the procedure, the previously observed conversion of dibromophosphaalkenes into phosphaalkynes through the phosphorus analog of the Fritsch-Buttenberg-Wiechell rearrangement is thus suppressed. The bromoethynylphosphaalkenes can subsequently be converted to C,C-diacetylenic, cross-conjugated phosphaalkenes by following a Sonogashira coupling protocol in good overall yields. By using the newly described method, full control over the stereochemistry at the P=C double bond is achieved. The substrate scope of this reaction is demonstrated for different dibromophosphaalkenes as well as different sulfonylacetylenes.
A facile one-pot transformation of Dmp-I to Dmp-P=CBr 2 (Dmp = 2,6-dimesitylphenyl), a valuable precursor for other unsaturated phosphorus compounds, is described. VT-1 H-NMR experiments reveal a hindered rotation of the m-terphenyl structure with a rotational barrier of approx. 14 kcal/mol. Bromination of Dmp-I gives selective substitution of all aromatic protons of the mesityl substituents.
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