Second-order rate coefficients and activation parameters are reported for the reaction in solution of triphenylphosphine with a series of diazoalkanes (Ar,CN,) having two aromatic groups attached to the diazo-carbon atom, 9-diazofluorene (DAF), dJcycloheptene (DBSE), its 10,ll -dihydro analogue (DBSA), and diazodiphenylmethane (DDM). Although the product phosphazine Ar,CNN PPh, arises formally by nucleophilic attack of phosphorus on the terminal nitrogen atom of the diazoalkane, the pattern of reactivity approximates to that for proton transfer from acetic acid to the diazoalkane under similar conditions, highlighting the biphilic nature of the reaction. The results can be rationalised in terms of qualitative FMO theory taking into account both HOMO( Ph3P)-LUMO (Ar, CN,) and HOMO(Ar,CN,)-LUMO (Ph3P) interactions. This is lent qualified support by MNDO calculations on Ar,CN, based on molecular parameters from the X-ray crystal structure of DAF itself and of the ketones corresponding to DBSE, DBSA, and DDM.The reactivity of DDM and the effect of 4,4'-disubstitution by CH30-, CH3-, and CI-, which gives rise to a U-shaped Hammett po correlation, is not easily interpreted within the FMO framework. Conformational changes which affect the interacting orbitals as the reactants are transformed into transition states offer an alternative interpretation.
Crystal structures are reported for three new sulfonyloxiranes 5, 6b and 7. Taken together with the structures previously reported for the two sulfonyloxiranes 1 and 2, these structures provide evidence of the existence of a generalised anomeric effect in 2-phenylsulfonyloxiranes, accounting for the substantial bond asymmetry in the oxirane ring, and thus for the reactivity of these oxiranes towards nucleophiles.
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