Allylboronates from Alkenylmetal Intermediates

In the last example, cis-carbocupration of alkynoic ester 14 provides a weakly nucleophilic and configurationally imstable 1-alkoxycarbonyl alkenylcopper intermediate (15) the presence of HMPA as additive was crucial to avoid erosion of the E/Z stereoselectivity in the alkylation step with electrophile 16 [39, 40]. Under these conditions, 3,3-disubstituted allylboronates 17 were prepared in over 20 1 selectivity, and these reagents were subsequently employed in the stereoselective preparation of quaternary carbon centers (Section 6.4.2.2). 

The use of optically pure a-chloroalkyl boronic esters as electrophiles, e.g. the dicyclohexyl boronate 18 obtained from Matteson s elegant methodology [41], lends access to a-alkyl substituted allylboronates (e.g., 19) with very high diastereomeric purity by way of a net inversion of configuration (Equation 9) [42]. Likewise, alkenylmetal fragments react with chiral dichloromethylboronate 20 to afford optically pure a-chloroallylboronates such as 21 (Equation 10) [43]. Subsequent addition of these a-substituted reagents to aldehydes is highly stereoselective. Furthermore, the chloride 

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). 

As exemplified with 23, the highly regioselective platinum-catalyzed hydroboration of alkoxy-substituted allenes with pinacolborane provided the corresponding 2)-y-alkoxyallylboronates by total anti-Markovnikov addition at the terminal double bond (Equation 12) [45]. Alkyl- and aryl-substituted allenes devoid of an alkoxy group, how- 

4 All]rlboronates from the Transition-metal Catalyzed Diboration and Silabora-tion of Dienes and Allenes 

---