Abstract
| - Computations on Diels−Alder (DA) reactions of phosphaethene with 1,3-butadiene and with isoprenereveal asynchronous transition structures. The DFT (B3LYP/6-311+G**) activation energies of thesereactions, 12−14 kcal/mol, are much lower than that of the parent ethene−butadiene reaction, 28kcal/mol, even though the exothermicities of all lie in the same range, from −29 to −33 kcal/mol.The transition states (TSs) for the phosphethene−butadiene or isoprene DA reactions are earlierthan the TSs of the butadiene−ethene cycloaddition. Due to the weakness of the CP π bondcompared to the CC π bonds, the energies required to reach the phosphaethene TSs are muchless than the carbocyclic cases. The computed 1H NMR chemical shifts and nucleus independentchemical shifts (NICS) quantify the aromatic character of the transition states. Regioselectivitiesof the neutral phosphaethene−isoprene DA reactions are modest, at best. However, computationson radical cation DA reactions of phosphaethene with isoprene, which proceed stepwise with openchain intermediates, can account for the high regioselectivities that have been observed in somecases.
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