Theoretical studies Bergman cyclization

A theoretical study on the reaction mechanism for the Bergman cyclization from the perspective of the Electron Localization Function and Catastrophe Theory has been reported.175 The authors argue that topological analysis of electron localization function can be used to complement the molecular orbital- or valence bond-based methods. 

A theoretical study of the biological activity of dynamicin A (52) dealt in part with the question of the cd distance. Tuttle et al. examined the Bergman cycliza-tion of 52 to 53 and compared it to cyclization of activated dynamicin A (54) to its diradical product (55). Given the size of these molecules, they optimized the structures at the relatively small B3LYP/3-21G level, but it is important to note that this level predicts the barrier for the cyclization of the 10-member ring enediyne (44) with an error of less than 1 kcal mol —clearly a situation of fortuitous cancellation of errors. Energies were then computed at B3LYP/6-31G(d). The activation enthalpy for the cyclization of 52 is 52.3 kcal mol while the barrier for cyclization of 54 is dramatically reduced to 17.9 kcal mol It is even much lower than the barrier for the cyclization of 44 (28 kcal mol ). 

Most of the unsaturated CpCo carbonyls are active catalysts in alkyne oligomerization (Section 5.1.4), as CO can be displaced for aUcynes. In addition to cyclization reactions, CpCo(CO) complexes have been studied as C-H bond activators. Bergman determined that, unlike the corresponding rhodium and iridium complexes, CpCo(CO) is unreactive toward C-H activation. Theoretical studies were used to probe the inability of CpCo(CO) to activate alkanes, and it... 

Extensive mechanistic studies of this cyclization reaction were carried out by Myers et al. and extended with theoretical work by Squire s et al. It is known that, in contrast to the Bergman cyclization of the ene-diyne (Chapter 4.2), this transformation proceeds as an exothermic process determined by the increased stability of a benzyl radical versus a phenyl radical. The barrier for cyclization from substrate to a diradical product is low and can further be reduced by an appropriate substitution at the allenic terminus of the substrate. The dichotomous (polar and free radical) reactivity is observed on pyrolysis in the presence of polar reactants. Both radical and polar products arise from a common intermediate, which is described as a polar diradical, a linear combination of limiting structure 7 and zwitterion 11. According to Squires, polar diradical singlet species are involved. Based on computational studies supported by experimental product distribution studies, it has been proposed that both the diradical 7 and... 

Theoretical studies of free-radical initiated cyclization of enediynes predicted that electron-deficient acetylenic substituents increases the Bergman cyclization barrier. The radical cyclization of enediynes containing electron-withdrawing substituents (such as acetyl and ethoxy-carbonyl groups) has been investigated. Heating a solution of l,2-bis(3-ethoxycarbonyl-l-butynyl)ethene 3.652 in the presence of stable radicals... 

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