Organocatalytic boronate exchange methodology for the enantioselective synthesis of alchohols
Barnett, David S.
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Allyl-, allenyl-, and propargylboronate nucleophiles react with carbonyl-containing compounds in a predictable and regioselective manner. Nucleophilic borations using these reagents are known to occur through six-membered transition states, where the carbonyl carbon is activated from the complexation to Lewis acidic boronate species. The ability of boronates to participate in facile and dynamic ligand exchange reactions with chiral alcohols provides opportunity for asymmetric catalysis through the formation of highly reactive chiral boronate nucleophiles in situ. Improved ketone allylborations were achieved using stable cyclic allyldioxaborinanes and BINOLs as chiral catalysts. Enhancement of boronate ligand exchange was revealed through the addition of the sterically hindered Lewis base tert-butanol to promote highly enantioselective allyl- and crotylborations to produce optically enriched tertiary homoallylic alcohols. The asymmetric boronate exchange method was later extended to ketone allenylborations using cyclic allenylboronates with chiral BINOLs to provide tertiary homopropargyl alcohols in excellent enantioselectivities and yields. Cyclic propargylboronates with varying ysubstitutions were then applied to aldehyde allenylborations using vaulted biphenol VANOL as a chiral catalyst in the catalytic boronate exchange method. Propargylborations with dioxaborinanes were determined to be highly enantioselective for a broad range of aldehydes providing various substituted α-allenic alcohols as reaction products.
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