- Mechanism of reduction of 1,1-diphenyl-2,2-dinitroethylene by 1-benzyl-1,4-dihydronicotinamide: Transition state with partial diradical and partial covalent bonding character
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The mechanism of reduction of 1,1-diphenyl-2,2-dinitroethylene (DPDN) by 1-benzyl-1,4-dihydronicotinamide (BNAH) in acetonitrile has been investigated. Based on product analysis, isotopic tracing and electrochemical analysis, the reaction takes place by a hydride transfer mechanism giving 1,1-diphenyl-2,2-dinitroethane (DPDNH). Single crystal X-ray analysis shows that DPDN conforms to idealized C2 symmetry, but steric repulsions between the bulky substituents result in an appreciable twist about the central bond, such that the phenyl rings make a dihedral angle of 77.7° and the planar C-NO2 fragments make a dihedral angle of 68.5°. A small kinetic H/D isotope effect was obtained, that we propose is due to steric hindrance. Reaction in oxygen-saturated acetonitrile produced DPDNH and benzophenone in the ratio of 59.2:22.0 as the final products with a total yield of 68.6% by GC. Control experiments were performed, by stirring a solution of DPDN or DPDNH alone in oxygen-saturated acetonitrile, or by stirring a solution of DPDN or DPDNH alone in aqueous acetonitrile containing a small amount of hydrochloric acid or in acetonitrile containing triethylamine. These produced no benzophenone. The results clearly indicate the trapping of a radical species by oxygen in the reaction. A curve-crossing model for the reaction projects that the transition state has partial diradical and partial covalent bonding character. As DPDN has a low-lying π* orbital (LUMO), the radical anion DPDN- is a stabilized radical. It is known that the reaction of alkyl and benzyl radicals with oxygen is exothermic with a rate close to the diffusion-controlled limit. Thus, with use of More O'Ferrall's two-dimensional potential energy diagram, the results are rationalized by a mechanistic change induced by steric hindrance so that the transition state collapses in two directions leading to the formation of DPDNH (polar pathway) and benzophenone (ET pathway), respectively.
- Liu, You-Cheng,Wang, Hong-Yi,Yang, Qing-Chuan,Mak, Thomas C.W.
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p. 649 - 653
(2007/10/03)
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- Nucleophilic Addition to Olefins. 5. Reaction of 1,1-Dinitro-2,2-diphenylethylene with Water and Hydroxide Ion in 50percent Me2SO-50percent Water. Complete Kinetic Analysis of Hydrolytic Cleavage of the C=C Double Bond in Acidic and Basic Solution
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Hydrolysis of 1,1-dinitro-2,2-diphenylethylene (2) to form benzophenone and dinitromethane (or its anion) was studied in 50percent Me2SO-50percent H2O and also in 50percent Me2SO-50percent D2O.Experiments were conducted over a pH range from ca.1 to ca.16.The data can be interpreted by Scheme I.Solvent isotope effects, observation of general acid and general base catalysis, and structure-reactivity relationships were used to assign rate-limiting steps under various conditions and to probe into the mechanistic details of the varios steps.The major conclusions are the following. (1) The equilibrium constants for OH- and water addition to 2 to form TOH- (Scheme I) are of comparable magnitude to those for the corresponding reactions of benzylidene Meldrum's acid (1), but the rate constants are much lower for 2 than for 1.This indicates a higher intrinsic kinetic barrier for the nitro-activated olefin and fits into a previously reported pattern, according to which activating substituents that are most effective in delocalizing negative charge lead to the highest kinetic barrier. (2) Carbon protonation of TOH- follows an Eigen curve similar to that for 1,1-dinitroethane anion (Figure 7) but which is displaced upward by nearly 1 log unit.This indicates a higher intrinsic protonation rate because of smaller charge delocalization in TOH- owing to an enhanced steric hindrance to coplanarity of the nitro groups in TOH-. (3) Intramolecular proton transfer from the OH group to the carbanionic site in TOH- (ki in Scheme I) is insignificant, which is in contrast to the behavior of the addition complex between 2 and morpholine. (4) The base-catalyzed breakdown of TOH0 into benzophenone and dinitromethane anion occures by rate-limiting oxygen deprotonation (k3B, k3OH in Scheme I), which implies that k4 for CH(NO2)2- departure from TOHO- is >>2*109 s-1, a remarkably high rate for a carbanionic leaving group.The water-catalyzed breakdown of TOH0 proceeds by a different mechanism, which is most likely concerted, with a transition state as 8 or possibly 9. (5) The acid-catalyzed breakdown of TOH2- occurs by rate-limiting carbon protonation (k6BH in Scheme I),, but the water-catalyzed breakdown follows a different mechanism.Various possibilities are discussed, and a slight preference is given to a preassociation mechanism (Scheme III).
- Bernasconi, Claude F.,Carre, David J.,Kanavarioti, Anastassia
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p. 4850 - 4860
(2007/10/02)
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