Selectivity in the ring-expansion and ring-switching reactions of bicyclooxonium ions

Abstract

Treatment of ω-(2-oxacycloalkyl)alkyl bromides 5a–e with Ag₂O in the presence of a nucleophile (acetic or trifluoroacetic acid) yielded ring-expanded product 8, ring-switched product 7 or/and non-rearranged product 6. Regioselectivity in the nucleophilic attack on the intermediate bicyclooxonium ions 10 governs the product formation. Bicyclo [n.3.0] oxonium ions (n= 3, 4, 5) 10a–c, which can be formed from tetrahydrofuranylalkyl bromides 5a–c, underwent nucleophilic attack at the five-membered ring to produce a ring-switched product 7. Only bicyclo [3.3.0] ion 10a can undergo the preferential attack at the bridgehead position releasing ring strain to produce a ring-expanded product 8 in addition to the formation of 7. Similarly, in the reaction of tetrahydropyranylpropyl and oxepanylpropyl bromides 5d,e, initially formed bicyclo[m+ 2.3.0]oxonium ions (m= 2,3) underwent strain release by cleavage of a five-membered ring resulting in the exclusive formation of non-rearranged products 6d and 6e, respectively. The reason for the observed selectivity is discussed.