Ring–chain rearrangements of phosphirane

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Abstract
Ab initio molecular orbital calculations have been used to explore the C2H5P potential-energy surface. Geometries were optimized at the MP2/6-31G(d,p) level while relative energies were estimated using QCISD(T)/6-311 G(d,p) calculations and corrected for zero-point energies. Among C2H5P isomers, phosphirane 1, vinylphosphine, 2, 1-phosphapropene, 3, and 2-phosphapropene, 9, are low-energy isomers and have similar energy content. 1 lies only 5 kJ mol–1 above the most stable isomer, 9. This is in line with the experimentally observed equilibrium between substituted phosphiranes and vinylphosphines. The 1–2 rearrangement is a single-step process with an energy barrier of 235 kJ mol–1, which is not inconsistent with experimental thermal reactions at 500 °C. The 1–3 interconversion seems possible via ethylphosphinidene, 5, but 5 is not an equilibrium structure. The 2–3 isomerization is possible via either an antarafacial or a suprafacial 1,3-hydrogen shift with a barrier height of ca. 263 kJ mol–1. This represents the first example of an accessible suprafacial 1,3-hydrogen shift. The 1,3-hydrogen shift in 2-phosphapropene remains an antarafacial mode with an energy barrier of 288 kJ mol–1(in comparison with 345 kJ mol–1 in propene). Phosphinoethylidene, 4, is a high-energy isomer lying 201 kJ mol–1 above 1 and separated from 2 and 3 by moderate energy barriers (61 and 58 kJ mol–1). 4 is not involved in interconversions of 1, 2 and 3. While 1, 2-H2 loss from 3 giving CH3—CP or CH2 CPH is unlikely, cycloreversion of 1 giving an alkene plus a phosphinidene is a realistic thermal process and thereby a possible mechanism for the formation of CH3—CP upon thermolysis of vinylphosphirane at 700 °C. Conversion of phosphirane to 9 is possible by two distinct two-step pathways: the first involves (methylphosphino) methylene, 8, as an intermediate whereas the second pathway involves bis(methylene)phosphorane, 10. The latter is favoured over the former. Overall, these ring–chain isomerizations of phosphirane constitute a novel type of reaction of phosphorus compounds which do not exist in either carbon or nitrogen analogues.