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

Abstract

AIBN initiated reaction of N-acylaziridines 1 with Bu 3SnH 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 Bu 3Sn . 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 Bu 3SnH. 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.