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Ab initio and semiempirical modelling of stereoselectivities of Diels-Alder cycloadditions of furan and cyclopentadiene with norbornenes

Ab initio (RHF/3-21G and RHF/6-31G* basis sets) and semiempirical (AM1 and PM3) quantum chemical calculations have been applied to a study of Diels-Alder reactions of furan and cyclopentadiene as 1, 3-dienes, with norbornene and 7-oxanorbornene as dienophiles, in order to model  -facial selectivities and stereoselectivities of these reactions. To achieve this goal, transition states for these reactions are located and activation energies estimated. The exclusive exo- pi- facial selectivity exhibited in these cycloadditions are readily predicted using semiempirical or ab initio methods. To determine stereochemical outcomes following pi-facial attack, ab initio calculations are required. Exo, endo- stereoselectivities found experimentally, are successfully predicted by using RHF/6-31G*//3-21G or higher levels of calculations. Secondary orbital interactions are postulated to be responsible for the experimental stereoselectivities.

Diels-Alder; cycloaddtions; transition states; calculations; ab initio; semiempirical

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