Despite repeated efforts over many years and several tantalizing empirical results that suggest oxygen-directed hydroboration (ODHB), definitive examples of this process have been rare. Evans' metal-catalyzed reaction of catecholborane with several unsaturated alcohols, phosphinites, and carboxamides is the only method known to date with established synthetic potential for a range of substrates. Another case of ODHB involving an α-methoxy-β,γ-unsaturated ester was encountered by Panek et al. using Me_2S -BH_3. This example approaches the regioselectivity of the Evans result with a homoallylic alcohol (8:1 vs 11:1), but appears to be a special case reflecting unusual reactivity due to the presence of ester as well as methoxy groups in the starting material. The other historical examples reveal interesting perturbations of hydroboration stereoselectivity or regioselectivity by oxygen sub-stituents, but these reactions generally do not give useful product ratios.
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