Aliphatic C-H Borylation

Simultaneous C-H activation and functionalization of hydrocarbons allows abundant, inert hydrocarbons to serve as direct feedstocks for functionahzed compounds [50]. The versatility of organoboron compounds in organic synthesis makes the borylation of hydrocarbons an attractive goal [20, 25-27]. 

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SchemeZ4 Catalytic cycle for aliphatic C-H borylation with Cp Re complexes.

Borylation of benzene by pin2B2 takes place under the same conditions as those used for aliphatic C-H borylation (Scheme 2.6) [51]. The reaction may proceed through a mechanism similar to that postulated for aliphatic C-H borylation. 

Under conditions similar to those used for aliphatic C-H borylation, rhodium complexes catalyze the borylation of arenes by pin2B2 [53], Cp Rh( n -QMeJ (19), which in situ generates a coordinatively unsaturated rhodium species (20) active for oxidative addition, was found to be the best catalyst, giving a 92% yield after 2.5 h with 5 mol% catalyst loading and an 82% yield (328 turnover number of the catalyst, TON) with 0.5 mol% loading at 150 °C. 

Notably, Rh catalysts have also been employed by Hartwig for efficient C—H borylations, which enable even C—H functionalizations of aliphatic C—H bonds in alkanes (42)." "" C—H borylations of alkanes are remarkably selective for functionalization at primary C—H bonds. 

A classic example of C-H functionalization is the familiar NBS bromination of a benzyUc site. Recent npdates of this approach allow for direct alkoxylation J. Am. Chem. Soc. 2008, 130, 7824) and net animation (Organic Lett. 2008,10, 1863). For the amination of simple aliphatic H s, Holger F. Bettinger of Ruhr-Universitat Bochum developed (Angew. Chem. Int. Ed. 2008, 47, 4744) the boryl azide 2. The insertion with 1 proceeded to give a statistical mixture of the nitrene insertion products 3 and 4. The tethered C-H functionalization devised (J. Am. Chem. Soc. 2008,130, 7247) by Phil S. Baran of Scripps-La Jolla is selective, as in the conversion to 5 to 6, but appears to be limited to tertiary and benzyUc C-H sites. 

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