Metal and Phosphine Mediated Transformations

By: Josephine Nakhla, Aldrich ChemFiles 2009, 9.2, 7.

Aldrich ChemFiles 2009, 9.2, 7.

Functionalized allyl boronates are useful building blocks in natural product synthesis. Olsson et al. reported the use of a palladium pincer complex in combination with diboronic acid for the boronation of readily available allylic alcohols. Under mild conditions, a variety of allylic alcohols were reacted with 5 mol% catalyst to yield the corresponding boronic acids, which were converted to the more stable trifluoroborate salt derivatives in good yields. Considering the widely available nature of allylic alcohols, the mildness of the reaction conditions, and the potential utility in natural product as well as fine chemical synthesis, this conversion of the typically difficult-to-substitute alcohol moiety into a functionalized alkylboronate is a remarkably efficient method for the preparation of these reagents (Scheme 1).

Scheme 1.

The authors suggest the diboronic acid is behaving as a Lewis acid (Scheme 2); however, since it is not as strong a Lewis acid as alkyl- or haloboranes, they suggest participation by a MeOH molecule. The sixmembered transition state facilitates esterification of the boronic acid and consequently converts the hydroxyl moiety into a better leaving group.1

Scheme 2.

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  1. Olsson, V. J. et al. J. Am. Chem. Soc. 2006, 128, 4588.

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