Conformations of favorable transition states in the thermal pyrolysis of alkyl acetates and xanthates

Abstract

Conformations of favorable transition states in thermal β-eliminations, including the pyrolysis of alkyl acetates and xanthates, and the decarboxylative reaction of 3-oxoalkanoic acids, were investigated by density functional theory (DFT) calculations. To systematically understand the syn-elimination in these thermal pyrolysis, different regio- and stereoselective favorable transition states in thermal β-eliminations of distinct representative acetates, xanthates, and 3-oxoalkanoic acids were calculated. The results indicate that the thermal β-eliminations investigated prefer syn-elimination through a favorable six-membered, approximately flat half-boat/half-chair cyclic transition state rather than a six-membered chair cyclic transition state. It is attributed to the participating orbitals in the favorable transition states. The favorable transition states involve a hydrogen atom transfer from a carbon or an oxygen atom to the lone electron pair-occupied orbital in the oxygen atom of the CO bond or on the sulfur atom of the CS bond, and the lone electron pair-occupied orbitals are perpendicular to their π-bonds. Thus, the transition states had to adopt the six-membered, approximately flat half-boat/half-chair cyclic conformation rather than a six-membered chair cyclic conformation, resulting in the thermal β-eliminations being syn-eliminations. The observed anti-eliminated products of cis-substituted cyclohexyl xanthates under thermal conditions are generated through a tandem C[1,3] sigmatropic shift and subsequent syn-β-elimination process. The current investigation provides a theoretical rationale for the thermal syn-β-eliminations and useful information for teaching organic chemistry.