Structure, Conformation and Self-Association of the Alumolanes and Borolanes Spiro Fused with Terpenes

The ¹H, ¹³C, ¹¹B and ²⁷Al NMR signals of several polycyclic spiro-fused terpene-alumolanes and spiro-fused terpene-borolane were assigned. Aluminolanes form the dimers predominantly at the cyclic Al(1)–C(5) bond in toluene according to DFT calculations of Gibbs free energy of dimerization reaction and ²⁷Al NMR spectroscopy (δ_Al ∼ 150–159 ppm). In tetrahydrofuran, the complexation of a solvent molecule on aluminolane metal atom proceeds, as evidenced by the shift of signals in the ²⁷Al NMR spectra to δ_Al ∼ 174–178 ppm. The signals of methylene groups located in α-position relative to aluminum and boron atoms are broadened or not observed in ¹H and ¹³C NMR spectra because of quadrupole relaxation of these heteroatoms. Theoretical conformational analysis showed that the energy barrier for inversion of terpene derivatives of spiro-fused 1-fluoroborolane is lower compared to those of spiro-fused 1-ethylalumolanes.