Face selectivity in the Paterno–Büchi reactions of methacrylonitrile to 5-substituted adamantan-2-ones

The photocycloaddition of methacrylonitrile to 5-substituted adamantan-2-ones (1-X) produces two geometrically isomeric oxetanes in which the oxygen atom and the 5-substituent are in anti or syn positions. The substituent was varied from fluoro, chloro, bromo, phenyl to SiMe3 and the product ratios, except for SiMe3, were similar (ca. 56:44) in all instances. Structure determination of the anti and syn oxetanes were carried out by 1H and 13C NMR and mass spectroscopy. The structure of syn-5-bromospiro[adamantane-2,2′-oxetane](syn-5-Br) was further supported by single-crystal X-ray crystallography. The product formation bias resulting from the attack on the zu-face is discussed in terms of transition-state hyperconjugation, but new results for photocycloaddition reactions of 5-trimethylsilyl-substituted adamantan-2-one (1-SiMe3), with both fumaronitrile and methacrylonitrile, show an unprecedented σ-electron-withdrawing characteristic with this substituent.