Photoacetylation of Diamondoids: Selectivities and Mechanism

The photoacetylation of diamondoids (nanodiamonds), in particular [1212]pentamantane, [1(2,3)4]pentamantane, and [123]tetramantane, with diacetyl almost exclusively gives apical acetyl derivatives. The mechanism of the photoacetylation was studied on the basis of a comparative analysis of the experimental deuterium kinetic isotope effects as well as the observed C–H bond substitution selectivities, which were also computed at the B3LYP, B3PW91, and MP2 levels of theory including polarized continuum modeling of solvent effects. The high apical C–H bond selectivities in the photoacetylation of diamondoids are determined by the higher polarizabilities of the cages along the apical direction in the transition structures for hydrogen ion with the triplet diacetyl diradical. Steric effects play only a minor role, and in combination with solvation, outweigh charge‐transfer effects that usually favor medial substitution with electrophiles and electrophilic radicals.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009) Despite many nonequivalent C–H bonds present in the structures of hydrogen‐terminated nanodiamonds (diamondoids), photoacetylation with diacetyl gives apical derivatives almost exclusively as a result of the higher polarizabilities of the structures in the apical molecular direction.