Investigation of the dynamic equilibrium between 2-(dialkylboryl)-1-methylenecyclobutane and 1 - ((dialkylboryl) methyl)-1-cyclobutene

The reaction of potassium methylenecyclobutanide with dibutylboron chloride and 9-bromo-9-borabicyclo[3.3.1]nonane yields the corresponding allylic type triorganoboranes 1 and 2. Compounds 1 and 2 exhibit a facile [1,3]-B sigmatropic shift, which results in the reversible coexistence of isomeric 2-(dialkylboryl)-1-methylenecyclobutanes 1a and 2a with 1-((dialkylboryl)methyl)-1-cyclobutenes 1b and 2b. For both compounds 1 and 2, the 2-(dialkylboryl)-1-methylenecyclobutane form (1a, 2a) is predominant at low temperatures, while raising the temperature leads to a shift of the equilibrium in favor of the 1-((dialkylboryl)methyl)-1-cyclobutene form (1b, 2b). Activation parameters for the equilibrium in 1 were found from the 2D ¹³C-¹³C spectra (for the transformation 1a → 1b E_act = 35.5 ± 2.5 kJ mol^-1, ΔG ^± ₂₉₈ = 53.8 ± 0.5 kJ mol^-1; for the transformation 1b -1a E_act = 33.8 ± 2.5 kJ mol^-1, ΔG ^± ₂₉₈ = 52.8 ± 0.5 kJ mol^-1). For the equilibrium in 2, the rate constants of the direct and reverse reaction were determined at 180 K (k_2a→_2b = 0.30 ± 0.03 s^-1, k_2b→_2a = 6.3 ± 0.3 s^-1).