Reactions Compounds with Unlike Substituents

One model for bonding in a diazo compound would be die ylide (89 equation 37)." Unlike many other ylides, diazoalkanes are stable to air and water. With acid, however, protonation can lead to the highly reactive salt (90), the functional equivalent of the corresponding carbocation. As the substituents on the diazo group are made increasingly electron withdrawing, the ylide becomes less basic, and thus more stable to acid. Reaction of a diazo compound with a transition metal can also often be understood as proceeding via initial donation of electron density by (89) to a coordinatively unsaturated metal center. 

One consequence of tetrahedral geometry is that an amine with three different substituents on nitrogen is chiral, as we saw in Section 9.12. Unlike chiral carbon compounds, however, chiral amines can t usually be resolved because the two enantiomeric forms rapidly interconvert by a pyramidal inversion, much as an alkyl halide inverts in an Sfg2 reaction. Pyramidal inversion occurs by a momentary rehybridization of the nitrogen atom to planar, sp2 geometry, followed by rehybridization of the planar intermediate to tetrahedral, 5p3 geometry... 

The reaction of Rudy and Cramer referred to earlier involved the preparation of the alloxan anil (183), analogous to the known blue-black para isomer (184).88 However, unlike the latter compound, the major product was pale yellow and did not revert to the starting materials (181) and (182) with acid. Rudy and Cramer decided that the compound was the anil (183) and that its unusual properties were due to the ortho substituent.82,89-91 They observed91 a second product... 

There are several reasons why it is unlikely that loss of nitrogen from the diazo compound, followed by direct addition of the carbene to the double bond, is the mechanism for the reaction. (1) Even though the carbene has two alkyl substituents, it is still quite electrophilic and would not be expected to react with one of the double bonds to the exclusion of the other. (2) There is no catalyst or ultraviolet light to aid in the generation of the carbene. 

The products of reaction are nitrogen and compounds B and C. Solvent effects cause significant differences in rate which are consistent with a competing ionization process. In contrast to the thermal isomerization mentioned above, the rate of thermal decomposition is insensitive to changes in substituent. Unlike the thermal isomerization reaction there is no satisfactory correlation between substituent constants and activation enthalpies or entropies. 

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