Nitrones. Part III. Photolysis of cyclic nitrones

Abstract

Ultraviolet irradiation of 2-substituted 1 -pyrroline 1 -oxides yielded corresponding bicyclic oxaziridines. Prolonged irradiation of the 3,3,5-trimethyl-6-oxa-l -azabicyclo[3.1 ,O] hexane gave an N-acetylazetidine and a pyrroline. Acid hydrolyses of 2,2,5and 3,3,5-trimethyl-6-oxa-l -azabicyclo[3,1 .O] hexane yielded acetonylacetone and 3.3-dimethylpentan-5-aI-2-one. respectively. The synthesis of the nitrone 3.35-trimethyl-1 -pyrroline 1 -oxide and its irradiation to form an oxaziridine is described. Prolonged irradiation of the latter yielded a pyrrolidinone.IRRADIATION of acyclic nitrones gives isomeric oxaziridines,2 but Todd and his co-workers3 reported that whilst irradiation of the cyclic nitrone 5,5-dimethyll-pyrroline l-oxide (Ia) produced the oxaziridine (IIa), irradiation of 2-substituted l-pyrroline l-oxides, e.g., (Ic) failed to yield oxaziridines.The reason was obscure, but is not electronic since the 2-substituted l-pyrroline l-oxides absorb strongly at almost the same ultraviolet wavelength and with the same intensity as the unsubstituted nitrones, so excitation of the x-electrons in the nitrone system must still occur. Molecular models show that a 2-methyl substituent does not appear to offer sufficient steric hindrance to prevent oxaziridine formation.With this in mind, 2,5,5-trimethyl-l-pyrroline l-oxide (Ib) was irradiated. In the solvent used by Todd and his co-workers (1 : 1 ethanol-cyclohexane) , overnight

A Computational Study on the Photochemistry of 2,4,4‐Trimethyl‐1‐pyrroline 1‐Oxide and Investigation on the Reaction Paths of Its Photoproduct Oxaziridine

“…A portion of the electronic charge due to the breaking of the C 2 ‐C 3 bond is utilized in increasing the bond order of the N‐C 2 and C 2 ‐O bonds. This closely resembles the prediction done by Lamchen group in their description of path b for azetidine formation in the photochemical route. The N‐C 3 bond formation finally happens through the low‐lying conical intersection geometry CI 1 .…”

“…A high energy barrier was required to form lactam through breaking of the C–CH 3 bond adjacent to the nitrogen. These results supported the experimental observations of Todd group and Lamchen group, reported almost sixty years back. The latter group carried out photo‐irradiation and pyrolysis studies on several other cyclic nitrone systems, as well.…”

“…At a highly stretched C−O bond length, an optimized geometry has been obtained which is comparable to the pyrroline optimized structure (discussed in previous section). A similar species with separated oxygen atom from the 5‐membered ring was predicted by Lamchen etal in their description of the deoxygenated pyrroline formation through path c . This geometry lies far above the oxaziridine and N‐acetyl azetidine structures (without considering any solvent effect).…”

“…This geometry lies far above the oxaziridine and N‐acetyl azetidine structures (without considering any solvent effect). It is difficult to understand precisely whether this geometry arises from the parent nitrone or from the oxaziridine; however, the low yield of pyrroline reported experimentally, may have some relation with its stability. In case, this is a photoproduct of nitrone then it will be a competitor of oxaziridine as the photoproduct.…”

“…It must be emphasized here that the azetidine formed in all the cases were clearly reported as the photo‐product of the oxaziridine where the latter is obtained from the photo‐irradiation of its corresponding nitrone. This is a ring contraction process and the azetidine ring formation mechanism was predicted by Lamchen etal to happen through a biradical species which forms from the oxaziridine, though it was merely a prediction without much evidence at that time.…”