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Ruthenium-catalyzed transfer hydrogenation of 2-substituted dienes 1a-i in the presence of paraformaldehyde results in reductive coupling at the 2-position to furnish the hydroxymethylation products 3a-i, which embody all-carbon quaternary centers. Reductive coupling of diene 1g to paraformaldehyde under standard conditions, but employing deuterio-paraformaldehyde, 2-propanol-d(8), or both, corroborated a catalytic mechanism involving rapid, reversible diene hydrometalation with incomplete regioselectivity in advance of C-C coupling. The present method provides an alternative to the hydroformylation of conjugated dienes, for which efficient, regioselective catalytic systems remain undeveloped.
Šmejkal et al. (Mon,) studied this question.