The reaction of the 16e half-sandwich complex {CpCo[S 2 C 2 B 10 H 10 ]} (1S) with methyl acetylene monocarboxylate at ambient temperature led to {CpCo[S 2 C 2 B 10 H 10 ] [CHC(CO 2 Me)CHC(CO 2 Me)]} (2S) and {CpCo[S 2 C 2 B 10 H 8 ][CHCH(CO 2 Me)] 2 } (3S). In2S the alkyne is 2-fold inserted into one of the Co-S bonds. 3S is a 16e B-disubstituted complex with the olefinic units in a Z/Z configuration. In comparison, {CpCo[Se 2 C 2 B 10 H 10 ]} (1Se) reacted with the alkyne to give rise to {CpCo[Se 2 C 2 B 10 H 10 ]-[CHC(CO 2 Me)CHC(CO 2 Me)]}(2Se),{CpCo[Se 2 C 2 B 10 H 9 ][CH 2 C(CO 2 Me)]}(4Se),and{CpCo[Se 2 C 2 B 10 H 8 ]-[CH 2 C(CO 2 Me)][CHCH(CO 2 Me)]} ( 5Se). 2Se is the analogue of 2S. Upon heating, 2S and 2Se catalyze cyclotrimerization of the alkyne to generate 1,3,5-and 1,2,4-tricarboxylatebenzenes. 4Se is an 18e B-substituted species with a B-CH 2 unit. 5Se is analogous to 4Se, but contains an olefinic substituent at the B(3)/B(6) site of the carborane in a Z configuration. Mechanistic implications on metal-induced B-H bond activation and catalytic cyclotrimerization of alkyne were elucidated. All new complexes were characterized by NMR spectroscopy ( 1 H, 11 B, 13 C), and X-ray structural analyses were reported for 2S, 2Se, 3S, 4Se, and 5Se.

The molecular mechanism of H(2) activation by two transition metal thiolate complexes [Cp*M(PMe(3))(SDmp)](BAr(F)(4)) (M = Ir, Rh) (Ohki, Y; Sakamoto, M; Tatsumi, K. J. Am. Chem. Soc., 2008, 130, 11610-11611) has been investigated using density functional theory calculations. According to our calculations, the reaction of the iridium thiolate complex with H(2) is likely to proceed through the following steps: (1) the oxidative addition of H(2) to the iridium center to generate a dihydride intermediate; (2) the reductive elimination of one Ir-bound hydrogen to produce the hydride thiol product. For the rhodium thiolate complex, its reaction with H(2) is to form the dihydrogen intermediate first, and then the H-H bond is heterolytically cleaved at the Rh-S bond via a four-center transition state to yield the hydride thiol product. The rate-determining step is the oxidative addition step (with a barrier of 18.0 kcal/mol in the solvent) for the iridium complex, and the formation of the dihydrogen complex (with a barrier of 13.9 kcal/mol in the solvent) for the rhodium complex. The calculated free energy profiles for both metal thiolate complexes can reasonably account for the observed reversible H(2) activation by two metal thiolate complexes under mild conditions.

The mechanism of a typical Petasis-type boronic mannich reaction (the styrylboronic acid, dibenzylamine, and α-hydroxylpropionaldehyde) has been investigated using density functional theory calculations. According to our calculations, the reaction is most likely to proceed through the following steps: 1) the nucleophilic addition of the amine to the aldehyde to form the carbinolamine; 2) the dehydration of the carbinolamine; 3) the formation of the tetra-coordinated borate intermediate; 4) the C-C bond formation by the intramolecular transfer of the styryl group; 5) the hydrolysis of the resulting intermediate to give the final products. The highest point on the energy profile is the transition state for the C-C bond formation (118.8 kJ•mol -1 above the reactants in ethanol). Our results cangive reasonable explanations on some experimental facts observed for many Petasis-type boronic Mannich reactions.

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