Boron metalation/quenching method

Because of the aggressive nature of the metalated intermediates involved in the direct metalation/quenching method shown above, it has proven advantageous in many instances to generate, instead, silicon-, tin-, or mercury-based heterocyclic intermediates that can be employed in subsequent metathesis reactions to exchange the heteroatom for a boron center (Scheme 10.3). Such reactions... 

This method has not yet found widespread use for the preparation of allylboronates. In fact, uncatalyzed hydroborations of dienes tend to provide the undesired regioiso-mer with the boron atom on a terminal carbon, i.e., homoallylic boranes. By making use of certain transition metal catalysts, however, Suzuki and co-workers found that (Z)-allylic catecholboronates such as 22 can be obtained in high yield from various substituted butadienes (e.g., isoprene. Equation 11) [44]. Whereas a palladium catalyst is the preferred choice for acyclic dienes, a rhodium catalyst (Rh4(CO)i2) was best for the hydroboration of cyclohexadiene. A suitable mechanism was proposed to explain the high regioselectivity of this process. In all cases, a reaction quench with benzaldehyde afforded the expected homoallylic alcohol product from a tandem hy-droboration/allylation (Section 6.4.1.4). 

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