Mechanism of photochemistry of alkanones with .gamma.- hydrogens

Structural Causes of Singlet/triplet Preferences of Norrish Type II Reactions in Carbonyls

“…8 Additionally, triplet quenching studies show that NYC photoproducts arise purely from triplet NTII reactions, since the T 1 biradical can undergo conformational rearrangement before cleavage occurs. 12,33 This T 1 biradical cyclisation mechanism is consistent with the lifetime of the biradical measured in femtosecond mass spectrometry experiments on a series of ketones: the 1,5-H-shift occurs on a 70-90 fs timescale, while subsequent cleavage from this biradical is an order of magnitude slower at 400-700 fs. 34 The photolysis data by Kharazmi on 1methylcyclobutanol formation from pentan-2one are also consistent with a triplet cyclisation mechanism.…”

“…However, rationalising experimental data on the prevalence of cyclisation, cleavage, or H-backtransfer from the 1,4-biradical requires dynamic modelling. 8,12,35,94,102 More theoretical studies of the NTII biradical 42 are required, and ab initio dynamics has only been performed for the NTII reaction in pentanal. 43,44 Furthermore, in order to assess the competition between NTII and other photolysis reactions on S 1 , T 1 , and S 0 kinetic modelling is required.…”

“…The different lifetimes of the singlet and triplet biradicals are also reflected in different NTII products observed for reaction on S 1 and T 1 , such as cyclisation or retention of stereochemistry. 10 The 1,4-biradical can undergo different subsequent reactions: either cleavage of the α, βbond to form an enol and an alkene (Scheme 1), or Norrish Yang cyclisation (NYC) to form a substituted cyclobutanol 11,12 (Scheme 2). Additionally, reaction from the biradical can be frustrated, as the abstracted H-atom can backtransfer to re-form the original carbonyl.…”

“…Norrish Yang cyclisation occurs in many carbonyl species, 10,12,16 and requires a conformation with in-phase overlap of the biradical SOMOs to form a new C C bond (Scheme 2). The NYC reaction is common for solvated carbonyls, 15,16 and a synthetic strategy to form 4-membered rings is to use carbonyl substituents that promote conformations with overlapping SOMOs for cyclisation.…”

The gas-phase thermal decomposition of cyclobutanol

“…In the phoIn a few systems, alternative reactions of the bi-tolysis of the appropriate aldehydes and ketones radical (other than isomerization) can compete (5,6) isomeric cyclobutanols are produced, and it with reaction [I]. In cyclobutanone, for example, is clear that these photolyses and the thermal deminor products are cyclopropane and CO, although composition of the cyclobutanols are intimately it is not clear whether they are formed from the related, and warrant further investigation.…”