| The emergence of phosphaalkenes as viable synthetic building blocks in phosphorus heterocyclic chemistry has prompted a number of experimental and theoretical studies on these systems. The results of ab initio theoretical studies on the pericyclic reactions of phosphaalkenes are presented and comparison made to the analogous hydrocarbon systems where appropriate.;The first project, calculation of the ring strain energy of tetraphosphacubane, shows that substitution of four phosphorus atoms for carbon atoms drastically reduces the ring strain energy of tetraphosphacubane relative to cubane by more than 100 kcal mol$sp{-1}$. This is attributed mainly to the ability of phosphorus to accommodate small bond angles.;The results of the second project, the determination of the relative energies of methyl- and vinyl-phospholes, clarified what seemed to be a contradiction between experiment and theory on the relative order of the stability of phospholes. Experimentally, $1H$-phosphole is observed to be the more stable isomer, whereas theoretically $2H$-phosphole is the most stable. Substitution at the phosphorus centers is shown to be favored. The relative stability order thus shows a substituent effect.;Project three, the Diels-Alder reaction of $2H$-phosphole with 1,3-butadiene, expanded upon the phosphorus-carbon analogy. The reaction mechanism, selectivity and regiochemistry is analogous to the hydrocarbon case. Substitution of phosphorus, however, is shown to lower the activation barriers relative to the hydrocarbon systems.;The fourth project was a study of the Thio-Wittig and Thio-Corey reactions. Unlike the analogous Oxa reactions, the Thio-Wittig reaction competes thermodynamically and kinetically with the Thio-Corey reaction, albeit only slightly. Also, unlike the Oxa reactions these reactions proceed via a betaine intermediate. Activation barriers are also lowered upon substitution of sulfur for oxygen, as observed for substitution of phosphorus for carbon in the Diels-Alder reactions. |
