Pentacoordinate Phosphoranes with Reversed Apicophilicity as Stable Intermediates in a Mitsunobu-Type Reaction

Diisopropyl azodicarboxylate (DIAD) undergoes a cycloaddition reaction with the cyclic phosphites
CH2(6-t-Bu-4-Me-C6H2O)2PX (1) [X = NCS (a), N3 (b), Cl (c), NHMe (d) and Ph (e)] to afford the
novel pentacoordinate phosphoranes 2a−e as crystalline solids. This result is different from the
reaction of PPh3 with DIAD used in the well-known Mitsunobu reaction. X-ray crystallography of
2a, 2b, and 2d reveals that the nitrogen, rather than the oxygen, occupies an apical position of the
trigonal bipyramidal phosphorus. This is in violation of the commonly accepted preferences for
substituents in trigonal bipyramidal phosphorus. In 2e, although the oxygen of the five-membered
ring occupies the expected apical position, the phenyl group also occupies (the other) apical position,
forcing the more electronegative oxygen atoms of the eight-membered ring to span equatorial−equatorial positions. In contrast to the above, the isocyanato compound CH2(6-t-Bu-4-Me-C6H2O)2PNCO (1f), upon treatment with DIAD, affords compound 3 to which a tetracoordinate structure
is assigned.