Planar Chiral Disilanols and Diols as H-Bonding Asymmetric Organocatalysts
(2012) Planar Chiral Disilanols and Diols as H-Bonding Asymmetric Organocatalysts Eur. J. Org. Chem. , 3373-3376.
The successful development of planar-chiral bis-silanols 3a–d and their application as asymmetric organocatalysts in hetero-Diels–Alder (HDA) reactions is described. All precursors were easily prepared by addition of commerically available silyl electrophiles to a dilithiated [2.2]paracyclophane derivative followed by silane oxidations. The analogous bis-carbinols 7a–c have been prepared by addition of suitable Grignard reagents to bis-methoxycarbonyl derivative 6. Both racemic as well as enantiopure planar-chiral bis-silanols and bis-carbinols were obtained in good yields. The catalytic activities of the bis-silanols were analyzed by monitoring HDA reactions between Rawal's diene and aldehydes by in situ IR spectroscopy and comparing the resulting data with those obtained in catalyses with the corresponding bis-carbinol derivatives. The results show for the first time that planar-chiral bis-silanols with hydrogen-bonding capabilities can be applied as asymmetric organocatalysts leading to enantiomerically enriched products