Regioselectivity in the ring-opening of 2-methylcyclopropylcarbinyl and 2-methylcyclobutylcarbinyl radicals

Abstract

The methylcyclopropylcarbinyl radical trans-(A) was generated by treating the corresponding chloride or bromide with tributyltin hydride at 25 and 45 °C, and the regioselectivity of ring-opening was determined by g.l.c. analysis of the alkenes which were formed. The intermediate radical (B) was also prepared from the corresponding acyclic bromide. [graphic omitted]

It is shown that the ring-opening is regioselective in favour of the primary alkyl radical (B) but that, when the concentration of tin hydride is low, the acyclic radicals (B) and (C) can equilibrate through the cyclic radical (A).

The radicals, cis- and trans-(A), and (B) and (C), were also generated photolytically at low temperature, and were monitored by e.s.r. spectroscopy. Ring-opening of (A) is now irreversible, and whereas cis-(A) gives principally the secondary alkyl radical (C), the trans-(A) compounds again give principally the primary alkyl radicals (B).

The cis- and trans-2-methylcyclobutylcarbinyl radicals (D) were generated by treating the corresponding chloride and bromide with tributyltin hydride, and again the course of the reaction was determined by g.l.c. analysis of the products. [graphic omitted]

Ring opening is now relatively slow and irreversible, and both cis- and trans-(D) give principally the secondary alkyl radical (E).