Photochemical Generation of Iminoquinone Methides by 1,4-Hydrogen Migration in Derivatives of o-Tolylnitrene

Abstract

The photochemistry of a series of derivatives of o-tolyl azide, bearing a variety of substituents in the benzylic positions, has been investigated using matrix isolation spectroscopy and density functional calculations. It has 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, yielding iminoquinone methides. Additional methyl groups in the benzylic position, however, do not promote a photochemical conversion into iminoquinone methides. If the benzylic substituent itself is part of a ring system, the size of this ring plays an important role. Thus, 2-methyl-8-nitrenotetrahydroisoquinoline rearranges very easily, whereas 4-nitrenophthalan does not give 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 barriers.