Quantum Chemical Studies of Radical Cation Rearrangement, Radical Carbonylation, and Homolytic Substitution Reactions

Sammanfattning: Quantum chemical calculations have been performed to investigate radical cation rearrangement, radical carbonylation, and homolytic substitution reactions of organic molecules.The rearrangement of the bicyclopropylidiene radical cation to the tetramethyleneethane radical cation is predicted to proceed with stepwise disrotatory opening of the two rings. Each ring opening is found to be combined with a striking pyramidalization of a carbon atom in the central bond.The isomerization of the norbornadiene radical cation to the cycloheptatriene radical cation (CHT.+), initialized by opening of a bridgehead–methylene bond, is investigated. The most favorable path involves concerted rearrangement to the norcaradiene radical cation followed by ring opening to CHT.+. The barrier of this channel is found to be significantly reduced upon substitution of the methylene group with C(CH3)2.Stepwise mechanisms are predicted to be favored over concerted isomerization for the McLafferty rearrangement of the radical cations of butanal and 3-fluorobutanal. The barrier for the concerted rearrangement is found to be lowered by 17.2 kcal/mol upon substitution, a result which is rationalized by the calculated dipole moments and atomic charges.Recent experiments showed that photoinitiated carbonylation of alkyl iodides with [11C]carbon monoxide may be significantly enhanced by using small amounts of ketones that have n?' character of their excited triplet state. DFT calculations show the feasibility of an atom transfer type mechanism, proposed to explain these observations. Moreover, the computational results rationalize the observed differences in yield when using various alcohol solvents.Finally, following photolysis of methyliodide, recent electron spin resonance spectroscopy experiments demonstrated that the SH2 reaction •CD3 + SiD3CH3 ? CD3SiD3 + •CH3 proceeds with high selectivity over the energetically more favorable D abstraction. The role of geometrical effects, especially the formation of prereactive complexes between methylsilane and methyliodide is studied, and a plausible explanation for the experimentally observed paradox is presented.