Synthesis of Spiro[2.2]pentanes and Spiro[2.3]hexanes Employing the Me3Al/CH2I2 Reagent

Substituted alkylidenecyclopropanes reacted with 5 equivalents each of Me3Al and CH2I2 at room temperature in hexane to give 1‐mono‐ and 1,1‐disubstituted spiro[2.2]pentanes in high yields. Surprisingly, the same reaction with substituted alkylidenecyclopropanes in CH2Cl2 afforded exclusively 1,1‐disubstituted spiro[2.3]hexanes. The transformation of 1,1‐diphenylspiro[2.2]pentane into 1,1‐diphenylspiro[2.3]hexane was studied with the use of CD2I2 and a plausible mechanism was suggested. The reaction of substituted alkylidenecyclobutanes with the Me3Al/CH2I2 reagent in CH2Cl2 gave only 1,1‐disubstituted spiro[2.3]hexanes. Alkylidene‐cyclopropanes and ‐cyclobutanes with Me3Al/CH2I2 reagent in hexane gave substituted spiro[2.2]‐pentanes and ‐hexanes in high yields. The reaction with alkylidenecyclopropanes in CH2Cl2 afforded exclusively 1,1‐disubstituted spiro[2.3]hexanes. The transformation of 1,1‐diphenylspiro[2.2]pentane into 1,1‐diphenylspiro[2.3]hexane was studied with CD2I2, and a mechanism was suggested.