Homolytic aziridine opening (aza variant of cyclopropylcarbinyl-homoallyl rearrangement) by addition of tributyltin radical to N-acylaziridines. Factors contributing to the regioselectivity

AIBN initiated reaction of N-acylaziridines 1 with Bu₃SnH in refluxing benzene provided products 5 and 8 of reductive ring opening. Yields (practically quantitative in most cases) fell drastically with steric hindrance of the addition of Bu₃Sn^. to the acyl oxygen of 1. They depended to some extent on the experimental conditions for hydrogen capturing when aziridine homolysis provided a primary radical 3 or 6. The regioselectivity of (probably reversible) ring homolysis can be understood in terms of the stability of the arising radical (3, 6), of stereoelectronic control (e.g. 1i as compared to 1h) and of frontier orbital interactions (1j). A possible difference in bond lengths as explanation for the formation of the primary radical from 1j did not find support from an X-ray structure analysis of N-tosyl-2-methyl-aziridine 11. Isomeric products were obtained only twice (1i, 1j) with a dependence of the ratio 5j:8j on concentration and hydrogen isotope of Bu₃SnH. No such dependence was found for the ratio 5:14 (reduction without and with an intervening cyclization of 3 leading to a pyrrolidone) obtained from the N-cinnamoylaziridine 11. This ratio (1:9 for 11 and 1:3 for 1n) must reflect the E-Z isomers in 3. The observed preference for the formation of E-3 from 2 can be explained by stereoelectronic and steric effects. A cinnamoyl double bond in 5 was saturated depending on experimental conditions.