Selectivity in reactions of tricyclic phosphatranes

Five-coordinated phosphorus compounds are known to adopt the trigonal bipyramidal (TBP) configuration, in which the apical positions are preferred by electron-withdrawing groups, whereas electron-donating groups are situated in equatorial positions (1). In addition, the apical bonds are longer and weaker than the equatorial bonds originating apical entry and departure of groups (phosphorylation) (2). Little attention has been paid to reactions in which external nucleophiles discriminate between pseudo-equatorial and pseudo-apical carbon atoms in a TBP configuration. The observation of such selective reactions is hampered by the occurrence of pseudo-rotation which brings about ligand exchange in the TBP (3). In order to obtain a definite answer with respect to equatorial vs. apical reactivity for nucleophiles we have synthesized the rigid tricyclic phosphatranes 1-4 in which a transannular N ¿ P bond brings phosphorus in a TBP configuration. Previously the characterization of the related compound 5 was published by Verkade et al.