| Abstract
| - The reaction of a chlorophosphorane (9-Cl) with primary amines produced anti-apicophilic spirophosphoranes(5, O-equatorial phosphoranes), which violate the apicophilicity concept, having an apical carbon-equatorial oxygenconfiguration, along with the ordinarily expected O-apical stereoisomers (6) with the apical oxygen-equatorial carbonconfiguration. Although the amino group is electronegative in nature, the O-equatorial phosphoranes were found tobe stable at room temperature and could still be converted to their more stable O-apical pseudorotamers (6) whenthey were heated in solution. X-ray analysis implied that this remarkable stability of the O-equatorial isomers couldbe attributed to the orbital interaction between the lone-pair electrons of the nitrogen atom (nN) and the antibondingσ*P-O orbital in the equatorial plane. A kinetic study of the isomerization of 5 to 6 and that between diastereomericO-apical phosphoranes 13b-exo and 13b-endo revealed that 5b bearing an n-propylamino substituent at the centralphosphorus atom was found to be less stable than the corresponding isomeric 6b by ca. 7.5 kcal mol-1. This valuewas smaller than the difference in energy (11.9 kcal mol-1) between the O-equatorial (1b) and the O-apicaln-butylphosphorane (2b) by 4.4 kcal mol-1. This value of 4.4 kcal mol-1 can be regarded as the stabilizationenergy induced by the nN → σ*P-O interaction. The experimentally determined value was in excellent agreementwith that derived from density functional theory (DFT) calculations at the B3PW91 level (4.0 kcal mol-1) betweenthe nonsubstituted aminophosphoranes (5g is less stable than 6g by 10.1 kcal mol-1) and their P-methyl-substitutedcounterparts (1a is less stable than 2a by 14.1 kcal mol-1).
- The O-equatorial C-apical aminophosphoranes (5b−5e), which violate the apicophilicity concept, were synthesized. X-ray crystallography and kinetic measurements for the stereomutation of 5 to its stable stereoisomer 6 (O-apical) revealed that the nN → σ*P-O interaction in the equatorial plane primarily contributed to the remarkable stability of 5. On the basis of the detailed kinetic study and density functional theory (DFT) calculations, the energy of the nN → σ*P-O interaction was quantitatively estimated to be ca. 4 kcal mol-1.
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