engleski

A theoretical study of the Pi-facial and stereoselectivities in the Diels-Alder cycloadditions of cyclopentadiene and 1,3-cyclohexadiene with 7-azabenzonorbornadienes and 5-aza-benzobicyclo[2.2.2]oct-7-en-6-one

Ab initio (RHF/3-21G basis set) and semi-empirical (AM1) quantum chemical calculations have been applied to a study of the Diels-Alder reactions of 7-azabenzonorbornadienes 1 and 5,6-benzo-2-azabicyclo[2.2.2]oct-7-en-6-one 12 as dienophiles with cyclopentadiene and cyclohexadiene as cyclic 1,3-dienes. Heats of formation were determined and used to show that these reactions were not under thermodynamic control. Transition states for all modes of cycloaddition in all four reactions were located and activation energies determined, including the effect of NH-invertomer orientation in the cycloadditions with 1. The exclusive exo p-facial selectivity and exclusive formation of the exo, endo- stereomers in the cycloadditions with 7-azabenzonorbornadiene 1 are readily predicted using semi-empirical, RHF/3-21G or higher ab initio levels of theory. The lack of p-facial selectivity and the exclusive stereoselectivity exhibited in these cycloadditions with 5,6-benzo-2-azabicyclo[2.2.2]oct-7-en-3-one 12 is successfully predicted only by ab initio methods (RHF/3-21G calculations or higher theoretical levels). Secondary orbital interactions are postulated to play a significant role in determining the observed selectivities.

AM1; abinitio; calculations; norbornenes; cycloadditions

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