- Rhodium boryl complexes in the catalytic, terminal functionalization of alkanes
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A series of studies have been conducted by experimental and theoretical methods on the synthesis, structures, and reactions of Cp*Rh boryl complexes that are likely intermediates in the rhodium-catalyzed regioselective, terminal functionalization of alkanes. The photochemical reaction of Cp*Rh(η6-C6Me6) with pinacolborane (HBpin) generates the bisboryl complex Cp*Rh(H)2(Bpin) 2 (2), which reacts with neat HBpin to generate Cp*Rh(H)(Bpin) 3 (3). X-ray diffraction, density functional theory (DFT) calculations, and NMR spectroscopy suggest a weak, but measurable, B-H bonding interaction. Both 2 and 3 dissociate HBpin and coordinate PEt3 or P(p-Tol)3 to generate the conventional rhodium(III) species Cp*Rh(PEt3)(H)(Bpin) (4) and Cp*Rh[P(p-tol) 3](Bpin)2 (5). Compounds 2 and 3 also react with alkanes and arenes to form alkyl- and arylboronate esters at temperatures similar to or below those of the catalytic borylation of alkanes and arenes. Further, these compounds were observed directly in catalytic reactions. The enthalpies and free energies for generation of the 16-electron intermediate and for the C-H bond cleavage and B-C bond formation have been calculated with DFT. These results strongly suggest that the C-H bond cleavage process occurs by a metal-assisted σ-bond metathesis mechanism to generate a borane complex that isomerizes if necessary to place the alkyl group cis to the boryl group. This complex with cis boryl and alkyl groups then undergoes B-C bond formation by a second σ-bond metathesis to generate the final functionalized product.
- Hartwig, John F.,Cook, Kevin S.,Hapke, Marko,Incarvito, Christopher D.,Fan, Yubo,Webster, Charles Edwin,Hall, Michael B.
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- Origins of the selectivity for borylation of primary over secondary C-H bonds catalyzed by Cp-rhodium complexes
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Detailed experimental and computational studies of the high selectivity for functionalization of primary over secondary sp3 C-H bonds in alkanes by borane reagents catalyzed by Cp*Rh complexes are reported. Prior studies have shown that Cp*Rh(X)(Bpin) (X = H or Bpin), generated from Cp*Rh(H)2(Bpin)2 and Cp*Rh(H) 2(Bpin)3, are likely intermediates in these catalytic reactions. To allow analysis of the system by H/D exchange, the current studies focused on reactions of Cp*Rh(D)2(Bpin)2 through the 16-electron species Cp*Rh(D)(Bpin). Density functional theory (DFT) calculations of the reaction between Cp*Rh(H)(BO2C 2H4) and the primary and secondary C-H bonds of propane indicate that the lowest energy pathway for C-H bond cleavage occurs to form an isomer in which the alkyl and boryl groups are trans to each other, while the lowest energy pathway for functionalization of the primary C-H bond occurs by formation of the isomer in which these two groups are cis to each other. The barrier for formation of the rhodium complex by cleavage of secondary C-H bonds is higher than that by cleavage of primary C-H bond. The alkyl intermediates are formed reversibly, and steric effects cause the barrier for B-C bond formation from the secondary alkyl intermediate to be higher than that from the primary alkyl intermediate. Experimental studies are consistent with this computational analysis. H/D exchange occurs between (Cp*d15)Rh(D) 2(Bpin)2 and n-octane, indicating that C-H bond cleavage occurs reversibly and occurs faster at primary over secondary C-H bonds. The observation of small amounts of H/D exchange into the secondary C-H bonds of linear alkanes and the clear observation of H/D exchange into the secondary positions of cyclic alkanes without formation of products from functionalization are consistent with the high barrier calculated for B-C bond formation from the secondary alkyl intermediate. A series of kinetic experiments are consistent with a mechanism for H/D exchange between (Cp*d*)Rh(D) 2(Bpin)2 and n-octane occurring by dissociation of borane-d1 to form (Cp*d15)Rh(D)(Bpin). Thus, the origin of the selectivity for borylation of primary over secondary C-H bonds is due to the cumulative effects of selective C-H bond cleavage and selective C-B bond formation.
- Wei, Carolyn S.,Jimenez-Hoyos, Carlos A.,Videa, Marcelo F.,Hartwig, John F.,Hall, Michael B.
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