| Abstract
| - Ab initio molecular orbital calculations at the RHF/3-21G(*),MP2/6-31G**//RHF/3-21G(*), and MP2/6-31G* levels of theory were used to examine the conformationalpreferences, transition structures,and products of the [2 + 2] electrocyclization reactions of1,4-pentadienyl systems bearingsubstituents on carbon 3. The susbstituents examined includeresonance-donating and -withdrawinggroups: NH2, OH, SH, H, PH2, BH2,and AlH2. The activation energy for the cyclization(thedifference in energy between the most stable acyclic conformation ofthe various 3-substitutedpentadienyl systems and the transition state) decreases in the ordershown, as does the energy ofcyclization (with the exception of the PH2 system).These results, as well as the conformationalpreferences, are explained by a combination of steric and electronicinteractions.
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