Thermodynamic trends in carbon-hydrogen bond activation in nitriles and chloroalkanes at rhodium.

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.

Selective C-H activation of haloalkanes using a rhodiumtrispyrazolylborate complex.

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.

The activation of alkyl cyanides using a rhodiumtrispyrazolylborate complex.

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.

Palladium-catalyzed cross-coupling of pyrrole anions with aryl chlorides, bromides, and iodides.

[reaction: see text] A general method for the conversion of pyrrole anions to 2-arylpyrroles has been developed. Using a palladium precatalyst and sterically demanding 2-(dialkylphosphino)biphenyl ligands, (pyrrolyl)zinc chloride may be cross-coupled with a wide range of aryl halides, including aryl chlorides and aryl bromides, at low catalyst loadings and under mild conditions. A high degree of steric hindrance is tolerated. Certain ring-substituted pyrrole anions have also been arylated with aryl bromide substrates.

Novel process for generating useful electrophiles from common anions using Selectfluor fluorination agent.

In the present work, the electrophile equivalents Cl+, Br+, SCN+, and NO2+ are generated from their respective sodium, potassium, and in some cases ammonium salts (M+X-) by reaction with Selectfluor electrophilic fluorination agent in acetonitrile solution at room temperature. These generated electrophilic species subsequently react in situ with a variety of aromatic substrates containing one or more substituent groups including H, F, Cl, CH3, COOH, C(O)CH3, NO2, and OR' and NR'R' ' where R' and R' ' are H or CH3. The resulting substitution products are, in most cases, isolable as pure compounds in high yield. Variations in the process include the use of other anions, electrophilic fluorination agents, and solvents.