On the behavior of alpha,beta-unsaturated thioaldehydes and thioketones inthe Diels-Alder reaction

alpha,beta-Unsaturated thioaldehydes and thioketones, R1CH=CH-C(=S)R-2, are prepared in situ by the reaction between the corresponding carbonyl compou nds and bis(dimethylaluminum) sulfide. These compounds undergo [4 + 2] self -dimerization reactions in which one molecule serves as the heterodiene com ponent and the other as the dienophile to afford different types of dimeric products depending on the R-1 and R-2: 1,2-dithiin and 1,3-dithiin (R-1 = R-2 = H), 1,2-dithiin (R-1 = Ph, R-2 = H, CH3), or dihydrothiopyran (R-1 = R-2 = Ph). These differences in selectivity are explained on the basis of t he relative energies evaluated by molecular orbital (MO) calculations at th e DFT (density functional theory) level. The calculations show that in the dimerization reaction of thioacrolein (I) the head-to-tail (S-C-S bonded) d imers are kinetically more stable by about 5 kcal/mol but slightly thermody namically unstable by about 2 kcal/mol than the head-to-head (S-S bonded) d imers. The calculations on thiocinnamaldehyde (IV) indicate that the dimeri zation reactions of phenyl-substituted a,e-unsaturated thioaldehydes and th ioketones are almost equally controlled by thermodynamic and kinetic factor s. These unsaturated thiocarbonyl compounds also function as heterodienes ( C=C-C=S) in the cycloaddition reaction with norbornadiene and as dienophile s (C=S) in the reaction with cyclopentadiene.