Abstract
| - Diisopropyl azodicarboxylate (DIAD) undergoes a cycloaddition reaction with the cyclic phosphitesCH2(6-t-Bu-4-Me-C6H2O)2PX (1) [X = NCS (a), N3 (b), Cl (c), NHMe (d) and Ph (e)] to afford thenovel pentacoordinate phosphoranes 2a−e as crystalline solids. This result is different from thereaction of PPh3 with DIAD used in the well-known Mitsunobu reaction. X-ray crystallography of2a, 2b, and 2d reveals that the nitrogen, rather than the oxygen, occupies an apical position of thetrigonal bipyramidal phosphorus. This is in violation of the commonly accepted preferences forsubstituents in trigonal bipyramidal phosphorus. In 2e, although the oxygen of the five-memberedring 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 structureis assigned.
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