9-Borabicyclo[3.3.2]decanes and the Asymmetric Hydroboration of 1,1-Disubstituted Alkenes

摘要

In their seminal study, Brown and Zweifel ushered in the modern era of reagent-controlled asymmetric synthesis with the hydroboration of cis-alkenes employing diisopinocampheylborane (Ipc_2BH, Figure 1, A). The instability of this reagent with respect to dehydroboration, led to the development of monoisopinocampheyl-borane (IpcBH_2) (Figure 1, B) which was found to be effective for trans- and trisubstituted alkenes. In some cases, the intermediate mixed dialkylborane dimers can be crystallized to provide enan-tiomerically pure organoboranes for many synthetic applications. Masamune's C_2-symmetric borolane (DMB, Figure 1, C) is highly selective for all of these substrates, namely cis, trans, and trisubstituted alkenes. Unfortunately, none of theses reagents, including a Rh-BINAP catalyzed process, is effective for the hydroboration of 2-substituted-l-alkenes (e.g., α-methylstyrene, Ipc_2BH, 5% ee; 2-methyl-1-butene, IpcBH_2, 1% ee; 2,3-dimethyl-1-butene, DMB, 1.4% ee; 2-phenyl-1-butene, catecholborane, Rh-BINAP, 46% ee;).