Boranes cyclic, coupling with

Under the phase-transfer catalysis conditions, 2-bromo-8-methylquino-line (67) was coupled with 2-pyridylboronic ester 68 to furnish 2-(2-pyridyl)-8-methylquinoline (69) in 56% yield (91JOC6787). At this point, it is opportune to mention that the simple 2-pyridylborane, in contrast to 3- and (4-pyridyl)boranes, is considered an unsuitable Suzuki coupling partner because it forms an unusually stable cyclic dimer resembling a dihydroanthracene. In this case, the obstacle was circumvented by using 2-pyridylboronic ester in place of 2-pyridylborane (Scheme 9). 

The synthesis of enantiomerically pure compounds is the challenging problem for organic chemists. The synthesis becomes obsolete if the intermediates produce racemic mixtures. The problem is particularly acute when the asymmetric centers do not reside in a rigid cyclic or polycyclic framework. To be able to carry out efficient syntheses of complex molecules, chemists have to control the sense of chirality at each chiral center as it is introduced in the course of synthesis. Monoalkyl- or dialkyl-boranes exhibit a remarkable chemo-, stereo-, and regioselectivity for the hydroboration of unsaturated compounds. This property, coupled with the capability for asymmetric creation of chiral centers with chiral hydroboration agents, makes the reaction most valuable for asymmetric organic synthesis. In some of the cases, however, this has been achieved by diborane itself as shown in the synthesis of monensin by Kishi et al. A stereospecific synthesis of its seven carbon. component has been accomplished by two hydroboration reactions (Eq. 129) 209. ... 

A series of 12- to 15-membered C-substituted cycles was prepared according to Scheme 12. Amide-benzylimines 70 were reductively coupled to the cycles 71 in a moderate yields and the cyclic amides were reduced with borane to give the free amines 72 <1995JOC3980>. Similarly, some other C-substituted derivatives were obtained <1995JOC3980>. 

As the overall cross-coupling reaction proceeds with inversion of stereochemistry and reductive ehmination is well known to undergo retention of stereochemistry, the result imphes that transmetaUation in this reaction proceeds predominantly with retention of stereochemistry. In addition to this study, in 1998, Woerpel and Soderquist [102] independently studied the stereochemistry of transmetaUation for the Suzuki-Miyaura cross-coupling reactions of alkyl boranes with aryl or alkenyl hahdes. Their deuterium labehng study revealed that the transmetaUation of alkyl boranes 163 or 166 proceeds with retention of stereochemistry to give products 165 or 167. Soderquist proposed a closed four-membered cyclic transition state 168 to account for the retention of stereochemistry observed during the reaction. 

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