Photochemistry of 2,2,12-trimethylcyclododecanone and 2,2,12,12-tetramethylcyclododecanone: product distribution, photo-CIDNP and magnetic isotope effect

Abstract

The primary photochemical process of 2,2,12-trimethylcyclododecanone ( 1a) and 2,2,12,12-tetramethylcyclododecanone ( 1b) is exclusively C 1C 2 α cleavage to form biradicals 2a and 2b which then disproportionate to form enals. This contrasts with the strong preference for γ-hydrogen abstraction for the parent cyclododecanone. The preference of α cleavage over γ-hydrogen abstraction for the methyl-substituted large ring cycloalkanones is explained by the faster rate of α cleavage due to stabilization of the intermediate biradical by methyl substitution. No nuclear polarization can be observed in the photochemically induced dynamic nuclear polarization (photo-CIDNP) of 1a in the absence of radical scavengers. Measurement of the 13C isotopic content of the photoproducts shows that the major enal product is slightly enriched in 13C in the absence of radical scavengers through isotopomèr separation via the magnetic isotope effect. However, the recovered 1a is not enriched in 13C. The addition of radical scavengers, such as molecular oxygen, CBrCl 3 or CCl 4, causes dramatic nuclear polarization and an increase in the 13C enrichment of the enal product. The presence of radical scavengers opens up a competitive pathway which serves as the spin (or isotopomer) sorting process required for both the generation of T 0S biradical CIDNP and isotope enrichment. In the absence of radical scavengers, the observation of the slight 13C enrichment in the enal product and the absence of nuclear polarization result from the slow decarbonylation process which is fast enough to serve as an isotopomer sorting process, but not fast enough to serve as a spin sorting process.