SINGLET-STATE AND TRIPLET-STATE PHOTODISSOCIATION OF C-O AND C-N BONDS IN AROMATIC ACETONES STUDIED BY H-1-CIDNP SPECTROSCOPY

The photolysis of (4-methoxyphenoxy)acetone (1a) and (N-methylanilino)acetone (1b) in fluid solution was investigated by photo H-1 chemically induced dynamic nuclear polarization (CIDNP) spectroscopy. The primary step is a dissociation of the C-O or C-N bond to give acetonyl and 4-methoxyphenoxy or N-methylanilino radicals respectively. For 1a the dissociative state is the singlet state on direct excitation while by triplet sensitization the triplet state of 1a shows similar reactivity. Additionally, via the triplet state in a separate reaction channel, acetone is formed. On direct excitation, 1b decomposes via both the singlet and the triplet states to the same primary radical pair. At temperatures below 290 K the triplet reaction dominates. With increasing temperature the singlet reaction becomes favoured and thus an unusual inversion of the CIDNP polarization sign is observed. These quite unexpected observations suggest that the photochemistry of these acetone derivatives should not be classified as those of a typical carbonyl compound. Rather, they show strong resemblance to the photo-Claisen reaction of unsatured ethers or the photo-Fries rearrangement of phenyl esters and anilides.