Studies on cycloaddition reactions of ketenes: further investigations of pseudopericyclic reaction mechanisms
Cycloaddition reactions of ketenes are widely used in organic synthesis. The cycloaddhions of kelene with ethylene, formaldimine and formaldehyde were studied theoretically using B3LYP/6-31G*, MP2/6-31G* and MCSCF/6-31G* methods. A pseudopericyclic mechanism was proposed for the reactions between ketene with ethylene and formaldehyde. A pseudopericyclic transition state was also located for the reaction of kelene with formaldimine even though h has a higher energy barrier than a conrotalory eleclrocyclization transition state, which is a concerted in the gas phase and a stepwise in solvent confirmed by IRC calculations. The cycloadditions of formaldimine with conjugated ketenes were studied theoretically al the B3LYP/6-3IG* level. The concerted and stepwise [4 + 2], [2 + 2] cycloaddition reactions were examined systematically. For the reaction of formaldimine with vinylkelene, the stepwise [2 + 2] and the concerted [4 + 2] are all pericyclic cycloaddhion reactions and have similar energy barriers. For the cycloaddition reactions between formaldimine with imidoylketene and formylketene, the stepwise [4 + 2] pathways are the lowest energy barrier ones. The concerted [4 + 2] and the second step of the stepwise [4 + 2], eleclrocyclization steps are pseudopericyclic with dramatically low barriers. In addition, the choice of formaldimine led the 1, 5- and I, 3-hydrogen shift reaction possible in some zwitterions intermediate, therefore transition stales of them were located. These transition states have low energy barriers and planar geometries. Thus these reactions are best interpreted as pseudopericyclic as well. The least developed conjugated ketenes, imidoylkelenes were studied more experimentally and theoretically. For the first lime, a novel reaction condition was developed to generate a variety of substituted imidoylkelenes. The first biomolecular reactions observed are dimerizations of imidoylkelenes, which were studied experimentally and theoretically. A series of trapping reactions with other reagents were studied as well. In addition, a new method to generate oxokelenes from p-kelo carboxylic acids were investigated and some experimental evidence for the pseudopericyclic reaction mechanism of the thermal chelelropic reactions were found by analyzing the Xray structures of ground states of pyrrolediones, furandiones and their derivatives. Finally, sequential transition states were located in the formation of a semibullvalene and the valley-ridge inflection point was located on its potential energy surface as well.