Norrish Type-II Reaction
Norrish Type II reaction is also known as the Yang reaction. This is a photochemical process of carbonyl compound having γ-hydrogen involves intramolecular abstraction of a γ-hydrogen from an excited ketone or aldehyde, resulting in the formation of a 1,4-biradical. This biradical can then undergo various transformations, leading to different products such as alkenes, enols, and cyclobutanols. Fragmentation of 1,4-diradical to give a methyl ketone via enol and an alkene is known as Norrish type II reaction.
When a molecule has two γ-carbons both having hydrogens, transfer of hydrogen in the Norrish Type II process is marked by a preference of cleavage of the weaker carbon-hydrogen bond as in case of below ketone. Intramolecular hydrogen abstraction is not possible if γ-carbon has no hydrogen.
For alkylaryl ketones the electron donating groups such as p-methyl and p-methoxy substituents decreases the rate and quantum yield for Norrish Type II cleavage. Following this trend p-hydroxy, p-amino and p-phenyl substituents inhibit the reaction completely. This is because in such cases energy for π → π* excitation is less than the n → π* excitation. The rate of radical recombination to give cyclobutanols compared with α, β-bond cleavage is often dependent on α-substitution.
| Substituents | Cyclization |
|---|---|
| R1 = R2 = H | 10% |
| R1 = H, R2 = CH3 | 29% |
| R1 = R2 = CH3 | 89% |
Thus substitution at α-position favours cyclisation reaction.
On the other hand substitutions at β-position favours Norrish Type II reaction. Thus, the below ketone mainly gives Norrish Type II reaction i.e., elimination reaction. In other words if 1,4-diradical is stable than this favours cyclisation reaction. If 1,4-diradical is unstable then this favours Norrish Type II reaction.
The singlet state photoelimination (Norrish Type II process) reaction occurs with a high degree of stereospecificity in threo and erythro form of ketone in the below reaction. Threo form of ketone in below reaction gives trans product whereas erythro form gives cis product.
Although the reaction occurs from both the singlet and the triplet states of n, π* transition, the quantum yields from the singlet state are generally lower than the triplet state. In case of aryl-alkyl ketones, the reaction occurs only with the triplet state because aromatic ketones can undergo rapid intersystem crossing. Solvents also affect the efficiency of the reaction. The singlet state reactions are unaltered in the presence of polar solvents. Polar solvents such as alcohol, on the other hand, enhance the reaction from the triplet state.
The quantum yield of the reaction is poor since radiation less deactivation from the S1 and T1 states and reversal of the hydrogen transfer can compete with reactions proceeding to products. The reversal process is confirmed by using the optical active ketone having a chiral γ-carbon. Such ketone undergoes racemisation. Racemisation reaction confirmed that the reaction intermediate is 1,4-diradical. This also confirmed the back transfer of hydrogen atom.
Back transfer of hydrogen atom i.e., photoracemisation can be quenched by the addition of 1,3-cyclopentadiene. This quenching experiment confirmed the formation of triplet state. Participation of a 1,4-diradical intermediate in the Norrish Type II reaction has been confirmed by trapping experiments and spectroscopic techniques. Formation of 1,4-diradical has also been proven chemically. Photoracemisation of a ketone with a γ-chiral carbon atom and loss of the chirality in the product was observed.
Source: Pericyclic Reactions and Organic photochemistry by V P Sharma and R Kumar
Photochemistry and Pericyclic Reactions by Jagdamba Singh and Jaya Singh
