jo0703707_si_001.pdf (113.43 kB)
Mechanistic Aspects of the Formation of Aldehydes and Nitriles in Photosensitized Reactions of Aldoxime Ethers
journal contribution
posted on 2007-05-25, 00:00 authored by H. J. Peter de Lijser, Natalie Ann Rangel, Michelle A. Tetalman, Chao-Kuan TsaiThe photooxidation of a series of aldoxime ethers was studied by laser flash photolysis and steady-state
(product studies) methods. Nanosecond laser flash photolysis studies have shown that chloranil (CA)-sensitized reactions of the O-methyl (1), O-ethyl (2), O-benzyl (3), and O-tert-butyl (4) benzaldehyde
oximes result in the formation of the corresponding radical cations. In polar non-nucleophilic solvents
such as acetonitrile, there are several follow-up pathways available depending on the structure of the
aldoxime ether and the energetics of the reaction pathway. When the free energy of electron transfer
(ΔGET) becomes endothermic, syn−anti isomerization is the dominant pathway. This isomerization pathway
is a result of triplet energy transfer from CA to the aldoxime ether. For substrates with α-protons (aldoxime
ethers 1−3), the follow-up reactions involve deprotonation at the α-position followed by β-scission to
form the benziminyl radical (and an aldehyde). The benziminyl radical reacts to give benzaldehyde, the
major product under these conditions. A small amount of benzonitrile is also observed. In the absence of
α-hydrogens (aldoxime ether 4), the major product is benzonitrile, which is thought to occur via reaction
of the excited (triplet) sensitizer with the aldoxime ether. Abstraction of the iminyl hydrogen yields an
imidoyl radical, which undergoes a β-scission to yield benzonitrile. An alternative pathway involving
electron transfer followed by removal of the iminyl proton was not deemed viable based on charge densities
obtained from DFT (B3LYP/6-31G*) calculations. Similarly, a rearrangement pathway involving an
intramolecular hydrogen atom transfer process was ruled out through experiments with a deuterium-labeled benzaldehyde oxime ether. Studies involving nucleophilic solvents have shown that all aldoxime
ethers reacted with MeOH by clean second-order kinetics with rate constants of 0.7 to 1.2 × 107 M-1
s-1, which suggests that there is only a small steric effect in these reactions. The steady-state experiments
demonstrated that under these conditions no nitrile is formed. This is explained by a mechanistic scheme
involving nucleophilic attack on the nitrogen of the aldoxime ether radical cation, followed by solvent-assisted [1,3]-proton transfer and elimination of an alcohol, similar to the results obtained for a series of
acetophenone oxime ethers.