Ruthenium-Catalyzed Regiospecific Borylation of Methyl C−H Bonds

Abstract

We report the regiospecific, ruthenium-catalyzed borylation of saturated terminal C-H bonds. Alkylboronates were obtained in 78-98% yields. The borylations of alkanes, trialkylamines, protected alcohols, and fluoroalkanes occurred regiospecifically at the methyl group that is least sterically hindered. In contrast to most organometallic C-H activation, the reactions of alkanes occurred in higher yields than the reactions of arenes. Reactions were conducted that probed steric and electronic effects on the alkyl borylation. These reactions showed that the borylation occurred preferentially at the methyl group that is least sterically hindered and most electron-deficient. Ruthenium compounds containing boryl ligands were synthesized, and one was characterized by X-ray crystallography. One of these compounds contained a rare bridging boryl ligand and served as a catalyst precursor for the borylation of octane.

Catalyst-controlled selectivity in the C–H borylation of methane and ethane

“…We next examined Ir and Ru complexes 2/3 and 4 as potential catalysts for methane C-H borylation. These complexes were selected on the basis of their known catalytic activity for the C-H borylation of liquid alkanes (18,19,24,33,34). Under the optimal conditions for catalyst 1, the combination of Ir complex 2 and ligand 3 (18) afforded moderate yield (45%) of CH 3 Bpin, whereas Ru complex 4 provided 67% yield ( Table 2, entries 1 and 2).…”

“…However, the initial reaction rate with Rh catalyst 1 is approximately four times faster than that with 2/3. Furthermore, 4 displays a lengthy induction period (~2 hours), suggesting that it serves as a precatalyst for this transformation (24,35). In the Table 2 data, the choice of catalyst has a major impact on the selectivity of C-H borylation, both for methane versus cyclohexane and for methane versus CH 3 Bpin.…”

Improving Transformations Through Organotrifluoroborates

“…More recently developed methods, which avoid the use of organic halides, include Rh-and Ru-catalyzed C-H borylations of unactivated alkanes and arenes [12][13][14]. As one example, Hartwig et al have demonstrated the application of Ir-catalysis toward aryl C-H borylation [15].…”

Reactions of the titanium-carbide species CpTi(μ2-Me)(μ2-NPi-Pr3)(μ4-C)(AlMe2)3

“…Since the early 1980s [1,2], a variety of approaches to C-H activation have been examined [3][4][5][6][7][8]. In the 1990s and into the current decade the research groups of Smith [9][10][11][12][13][14] and Hartwig [15][16][17][18][19][20][21][22][23][24][25] among others have developed a range of catalysts that utilize C-H bond activation to effect functionalization of alkanes. Our interest in C-H bond activation lies in the potential to generate unusual transition metal derivatives.…”