The first use of 1,4-pentadiene and 1,5-hexadiene as
allylmetal
pronucleophiles in regio-, anti-diastereo-, and enantioselective
carbonyl addition from alcohol proelectrophiles is described. As corroborated
by deuterium labeling experiments, primary alcohol dehydrogenation
delivers a ruthenium hydride that affects alkene isomerization to
furnish a conjugated diene, followed by transfer hydrogenative carbonyl
addition. Hydrometalation appears to be assisted by the formation
of a fluxional olefin-chelated homoallylic alkylruthenium complex II, which exists in equilibrium with its pentacoordinate η1 form to enable β-hydride elimination. This effect confers
remarkable chemoselectivity: while 1,4-pentadiene and 1,5-hexadiene
are competent pronucleophiles, higher 1,n-dienes
are not, and the olefinic functional groups of the products remain
intact under conditions in which the 1,4- and 1,5-dienes isomerize.
A survey of halide counterions reveals iodide-bound ruthenium-JOSIPHOS
catalysts are uniquely effective in these processes. This method was
used to prepare a previously reported C1–C7 substructure of
(−)-pironetin in 4 vs 12 steps.