Journal
JOURNAL OF ORGANIC CHEMISTRY
Volume 74, Issue 18, Pages 6907-6914Publisher
AMER CHEMICAL SOC
DOI: 10.1021/jo9012223
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- U.S. Department of Energy Office of Basic Energy Sciences [FG02-86ER13569]
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Several transition-metal systems have been used to establish correlations between metal-carbon and carbon-hydrogen bonds. Here, the [Tp'RhL] fragment, where Tp' = tris(3,5-dimethylpyrazolyl)-borate and L = neopentyl isocyanide, is used to investigate C-H bond activation in a series of linear alkylnitriles and chloroalkanes. Using a combination of kinetic techniques, relative free energies can be found for the compounds TpRhL(CH3)H, Tp'RhL[(CH2)(n)CN]H (n = 1-5), and Tp'RhL[(CH2)(m)Cl]H (m = 1, 3,4, 5). It is found that the CN and Cl substituents dramatically strengthen the M-C bond more than anticipated if in the alpha-position, with the effect on bond strength diminishing substantially as the X group moves further from the metal (i.e, beta, gamma, delta). Examination of M-C vs C-H bond strengths shows that the Tp'RhL(CH2X)H Compounds (X = phenyl, vinyl, CN, Cl) all show a good correlation, as do the alkyl, aryl, and vinyl derivatives. The compounds in the former group, however, have stronger M-C bonds than expected based on the C-H bond strengths and consequently, their correlation is separate from the other unsubstituted compounds.
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