Chiral Mo−Binol Complexes: Activity, Synthesis, and Structure. Efficient Enantioselective Six-Membered Ring Synthesis through Catalytic Metathesis
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Abstract
A new class of chiral Mo-based complexes 2a and 2b, bearing functionalized chiral binol ligands, is disclosed. Mo complex 2a promotes the asymmetric ring-closing metathesis (ARCM) of various dienes and trienes to afford six-membered carbo- and heterocycles efficiently and in high optical purity. The binol-based chiral Mo catalysts complement the previously reported biphen-based complexes, which are particularly effective in the enantioselective synthesis of five-membered rings by ARCM. Studies regarding catalytic kinetic resolutions and asymmetric desymmetrizations are described. It is possible to obtain optically pure products in high yield from catalytic reactions without the use of solvent (cf. eq 1). The structural attributes of these complexes are detailed on the basis of the data available from an X-ray structure and variable-temperature 1H NMR studies. The results of this investigation indicate the following: (i) The anti-Mo·THF complex exists as a mixture of diastereomers, whereas the syn isomer is formed stereoselectively. (ii) The anti-Mo isomers are likely more Lewis acidic.
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