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 13 C-13 C spectra (for the transformation 1a f 1b E act ) 35.5 ( 2.5 kJ mol -1 , ∆G q 298 ) 53.8 ( 0.5 kJ mol -1 ; for the transformation 1b f 1a E act ) 33.8 ( 2.5 kJ mol -1 , ∆G q 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 2af2b ) 0.30 ( 0.03 s -1 , k 2bf2a ) 6.3 ( 0.3 s -1 ).

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