Photoreduction of Carbonyl Compounds

Carbonyl compounds can be converted into 1,2-diols (pinacol) and alcohols by irradiation in presence of a hydrogen donating compound such as alcohols, amines or hydrocarbons. Reaction occurs through (n, π*) triplet excited state of the carbonyl compound which abstracts a hydrogen atom from the second reactant. Hydrogen atom abstraction is much more efficient for n, π* than π, π* for ketones. The lowest singlet excited state of ketones is S₁(nπ*) state.

Excited ketones can be reduced by tertiary amines. The key reaction in this case is electron transfer from the amine to the ketone producing ketyl radical and the cation radical from the amine.

The photolysis of propanone (acetone) in propan-2-ol results in hydrogen atom transfer to the ketone and two identical ketyl radicals are formed. The formed ketyl radical undergo disproportion and dimerisation reaction to get stabilised. The photolysis yields 2, 3-dimethylbutan-2, 3-diol (pinacol) as the main product of the reaction.

Reductive coupling occurs for aromatic ketones also. Benzophenone gives benzpinacol as main product when the reaction is carried out in benzhydrol (Diphenylmethanol) as solvent.

Carbonyl compounds like ketones, aldehydes and quinones can also add photochemically to activated methylene groups by hydrogen atom abstraction and subsequent radical recombination.

Photo reduction generally proceeds from n → π* triplet state. However, with aromatic ketones both n → π* and π → π* triplet states can be populated and π → π* triplet state is more reactive and efficient in Photo reduction.

The reactivity of these two triplet states is different, the ³(n, π*) state behaves like alkoxy radical with one unpaired electron on the oxygen atom. While on ³(π, π*) state, the unpaired electrons are delocalised over both the carbon and the oxygen atom. For example, Photo reduction of acetophenone in isopropanol proceeds from (n, π*) triplet state while of 2-acetylnaphthalene not.

Source: Pericyclic Reactions and Organic photochemistry by V P Sharma and R Kumar
Photochemistry and Pericyclic Reactions by Jagdamba Singh and Jaya Singh