INVESTIGATION OF THE DYNAMIC EQUILIBRIUM BETWEEN 2-(DIALKYLBORYL)-1-METHYLENECYCLOBUTANE AND 1-((DIALKYLBORYL)METHYL)-1-CYCLOBUTENE

The reaction of potassium methylenecyclobutanide with dibutylboron chl oride and 9-bromo-9-borabicyclo[3.3.1]nonane yields the corresponding allylic type triorganoboranes 1 and 2. Compounds 1 and 2 exhibit a fac ile [1,3]-B sigmatropic shift, which results in the reversible coexist ence of isomeric 2-(dialkylboryl)-1-methylenecyclobutanes 1a and 2a wi th 1-((dialkylboryl)methyl)-1-cyclobutenes Ib and 2b. For both compoun ds 1 and 2, the 8-(dialkylboryl)1-methylenecyclobutane form (la, 2a) i s 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 equilibri um in 1 were found from the 2D C-13-C-13 spectra (for the transformati on 1a --> 1b E(act) = 35.5 +/- 2.5 kJ mol(-1), Delta G(298)(double dag ger) = 53.8 +/- 0.5 kJ mol(-1); for the transformation 1b --> 1a E(act ) = 33.8 +/- 2.5 kJ mol(-1), Delta G(298)(double dagger) = 52.8 +/- 0. 5 kJ mol(-1)). For the equilibrium in 2, the rate constants of the dir ect 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)).