ABSTRACT
Several halogenated substrates are found to participate in C-H bond cleavage reactions with the photochemically generated fragment [Tp'Rh(CNR)] (Tp' = hydrotris(3,5-dimethylpyrazolyl)borate; R = CH(2)CMe(3)). Reaction with 1- or 3-chloropentane gives only terminal C-H activation products. Reaction with 2-chloropentane gives a mixture of 4-chloropentyl activation product and Tp'Rh(CNR)HCl, arising from beta-chloride elimination of the 2-chloropentyl activation product. Activation of chloromethane gives Tp'Rh(CNR)(CH(2)Cl)H, with no activation of the C-Cl bond. Dichloromethane, however, gives only C-Cl cleavage product Tp'Rh(CNR)(CH(2)Cl)Cl. By comparing the kinetic stabilities of a series of 1-chloroalkane activation products (C(1)-C(5)), it was found that the chlorine substituent dramatically decreases reductive elimination rates as the substitution is closer to the metal center. With 1-chloroalkanes, there is evidence for the formation of small quantities of C-H cleavage products alpha to the chloro substituent. Reactions of neopentyl chloride also showed evidence for small quantities of alpha-chloro C-H activation product. Reactions with the cyclic substrates 1-chlorocyclopentane and 1,1-dichlorocyclopentane yielded a mixture of diastereomeric activation products.
ABSTRACT
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(CH(3))H, Tp'RhL[(CH(2))(n)CN]H (n = 1-5), and Tp'RhL[(CH(2))(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(CH(2)X)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.
ABSTRACT
The photochemical reaction of Tp'Rh(L)(eta2-PhN=L) [Tp' = tris-(3,5-dimethyl pyrazolyl)borate, L = CNCH(2)CMe(3)] to form the coordinatively unsaturated reactive fragment, [Tp'Rh(L)], in the presence of alkylnitriles has been studied. The [Tp'Rh(L)] complex has been shown to selectively activate the primary C H terminus of acetonitrile, propionitrile, butyronitrile, and valeronitrile. The resulting hydrides showed uncharacteristic stability in the presence of C(6)D(6) and their rates of reductive elimination were monitored by (1)H NMR spectroscopy. Competition reactions permit the establishment of the relative stabilities of the activation products.
ABSTRACT
The competitive activation of C-H bonds of linear, cyclic, and branched hydrocarbons using the coordinatively unsaturated 16-electron [Tp'RhL] reactive fragment have been studied (Tp' = tris-(3,5-dimethylpyrazolyl)borate; L = CNCH2CMe3). Activation of the hydrocarbons leads to the formation of Tp'Rh(L)(R)(H) alkyl complexes, which were converted to the stable chlorides immediately following the activation of the bonds via photolysis of Tp'Rh(L)(PhN=C=NCH2CMe3) in the solvent mixture. The products were analyzed by 1H NMR spectroscopy. The experiments described provide relative rates for the coordination of primary and secondary C-H bonds to the Rh metal center, indicating a 1.5x preference for the latter.