CATALYSIS IN APROTIC-SOLVENTS - INTERMOLECULAR AND INTRAMOLECULAR HYDROGEN-BONDING COMPLEXATION

A mechanistic investigation is reported of aminolysis reactions of 2-hydroxy-5-nitro-alpha-toluenesulfonic acid sultone (1) in aprotic solvents. The n-butylaminolysis of 1 in acetonitrile and in toluene requires two and three molecules of amine, respectively. In the latter solvent, general bases strongly catalyze the reaction, and their catalytic constants are well correlated by the hydrogen bonding parameter pKm. These results are interpreted by a multistep mechanism where each intermediate can be stabilized via hydrogen bonding by general bases. The mechanistic features depend on the stability of the intermediates and on the solvent characteristics. When diamines such as polyoxyethylenediamines H 2NCH2(CH2OCH2)nCH2NH2 (2, n=2; 3, n = 4; 4, n = 6) are used as nucleophiles for the reaction with sultone 1 in toluene, much higher reactivities are observed when compared to reactions of monoamines and alkylenediamines. This represents a novel type of intramolecular catalysis due to intramolecular hydrogen bonding complexation between oxygen atoms and the ammonium group of the reaction intermediates (Scheme 111). In toluene 2-4 also display a large basicity

Neutral Cobalt Carbonyl Catalysts for the Equilibration between CO and P(OPh)3 Ligands in Carbonylcobaltates

The substitution reaction of [Co(CO)(4)](-) by P(OPh)(3) was investigated in THF tinder a CO atmosphere, the reaction consists of an equilibrium, which, in order to be established, needs [Co-2(CO)(6)(P(OPh)(3))(2)]. The catalysis by this neutral carbonyl is formally related to a previously reported case, where either [Co-2(CO)(8)] or [Co-4(CO)(12)] was found to be necessary for the (CO)-C-13 scrambling in [Co(CO)(4)](-). The kinetic analysis reported in this paper indicates, however, that the two CO labilizations differ in their mechanism-the substitution by P(OPh)(3) is ascribed to a disproportionation-synpro-portionation of [Co-2(CO)(6)(P(OPh)(3))(2)]

Degenerate lithium-hydrogen exchange reactions: an alternative mechanism for metalation of CH4 in gas phase and in THF solution

We report here the results of an ab initio study of the lithium-hydrogen exchange reaction of CH4 + (CH3Li)(2) both in gas phase and in tetrahydrofuran (THF) solution. All the species involved in the reaction have been characterized at the Hartree-Fock, second-order Moller-Plesset (MP2)(full), and density functional theory (B3LYP) levels using the 6-31G(d,p) basis set. The effect of the solvent (THF) has been modeled using the Polarizable Continuum Model developed by the group in Pisa that includes both electrostatic and nonelectrostatic (cavitation and dispersion repulsion terms) contributions to the solvation energy. A main result of this study is the finding of a nonplanar transition state structure that leads to a barrier similar to 2 kcal/mol lower at the MP2 level than the one calculated based on a C-s six-membered ring transition state previously reported by Schleyer et al. for the same reaction (J. Comput. Chem. 10, 437 (1989)). We include here a detailed discussion of the differences between these two mechanistic alternatives and the effect of the solvent on both of them. The performance of the B3LYP hybrid functional is examined against our MP2 results to assess whether this methodology is reliable for the study of more complex metalation reactions in which the size of the reactant system prevents the use of MP2 methods as a way for including electron correlation