Criss-cross mechanism

Unfortunately, the computational studies differ in quantitative detail regarding the importance of the mechanisms that involve either Glu 165 or His 95 as the acid-base catalysts to catalyze interconversion of the tautomeric enediolate intermediates. Friesner and coworkers concluded that the transition state for proton abstraction from DHAP is the highest point on the energy diagram after formation of the enediolate anion intermediate, the calculations predict that the barrier for the criss-cross mechanism catalyzed by Glu 165 is - 3 kcal moU lower than that for classical mechanism involving catalysis of tautomerization of the enediolate intermediates by His 95, so the criss-cross mechanism is predicted to be the favored mechanism. In contrast, Cui and Karplus concluded that transition state energies for tautomerization of the enediolate anion intermediates via an enediol intermediate are isoenergetic for both the classical and criss-cross mechanisms. 

Thermally induced intra-intermolecular criss-cross cycloaddition of nonsymmetrical azines 363 in the presence of phenyl isocyanate provides the corresponding products of the mixed criss-cross cycloaddition 364 (Scheme 55) <2002TL6431>. Two different reaction mechanisms, intra-intermolecular and inter-intramolecular, of the mixed criss-cross cycloaddition with opposite sequence of reaction steps are possible. Quantum chemistry calculations suggest the intra-intermolecular mechanism as the most probable mechanism of this reaction <2004CCC231>. 

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