Photochemical generation of iminoquinone methides by 1,4-hydrogen migration in derivatives of o-tolylnitrene

The photochemistry of a series of derivatives of o-tolyl azide, bearing a v ariety of substituents in the benzylic positions, has been investigated usi ng matrix isolation spectroscopy and density functional calculations. It ha s been found that introduction of any substituent possessing a lone pair (i .e., R = Br, Cl, MeO, Me2N) allows a 1,4-hydrogen shift to take place, yiel ding iminoquinone methides. Additional methyl groups in the benzylic positi on, however, do not promote a photo-chemical conversion into iminoquinone m ethides. If the benzylic substituent itself is part of a ring system, the s ize of this ring plays an important role. Thus, 2-methyl-8-nitrenotetrahydr oisoquinoline rearranges very easily, whereas 4-nitrenophthalan does not gi ve the reaction. Density functional calculations [B3LYP/6-31G(d)] have been used to gain an understanding of the reaction. It has been found that the activation energies depend strongly on the nature of the substituent, being lowest if R = NMe2. Incorporation of the benzylic substituent into a ring reduces the flexibility of the system and results in significantly raised b arriers.