Structure and conformations of alumolanes

What is it about?

The systematic structural analysis of cyclic organoaluminum compounds, 1-ethyl-3(R)-substituted alumolanes (R = n-Bu, n-Hex, n-Oct, i-Bu, Ph, Bn, SiEt3, cyclohexenyl) and 1-ethyl-2-phenylalumolane in polar solvents (Et2O, THF, pyridine), has been carried out by the means of NMR 1H, 13C, 27Al spectroscopy and quantum chemical methods (DFT, MP2). The NMR spectroscopic criteria for identification of alumolanes were established. Predominant conformation of the metallacycle as twist form with pseudo-equatorial location of a substituent was defined using experimental and theoretical approaches. The direct heteronuclear coupling constants J (1H-13C) in alumolanes were found. Relative stability of the alumolane solvates has been determined on the basis of calculated thermodynamic parameters of complexation reaction and from the dependence of the 1H, 13C, and 27Al NMR chemical shifts of metallacycle atoms vs the solvent nature.

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Why is it important?

Five-membered saturated metallacarbocycles represent a large family of organometallic compounds, which are frequently postulated as reactive intermediates in catalysis. We found that there is a lack of reliable 1H, 13C, or 27Al NMR structural information on aluminacycles in the literature. For the first time, conformations of 2-substituted and 3-substituted alumolanes, as well as the dynamics of their behavior in polar media were defined.

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