Trifluoromethylation of carbonyl compounds with trialkyl (trifluoromethyl) silanes in the presence of Lewis bases as catalysts

A variety of Lewis bases such as amines, phosphines, arsines, and stibines were effectively catalyzed trifluoromethylation of benzaldehyde using trimethyl (trifluoromethyl) silane (1). Among them amines exhibited highest activities (59-82% yield), and activities of phosphines, arsines, stibines and pyridine were rather low. When aldehydes such as aromatic aldehydes, aliphatic aldehydes and ol,P-unsaturated aldehyde were used as substrates, the trifluoromethylated products (2) were obtained in good yields (mostly>60% yield). Ketones were less reactive for the present reaction, however, ones bearing trifluoromethyl group attached directly to their carbonyl groups, exhibited higher reactivities to give the desired products in good yields. The Lewis base catalyzed reaction can be also applied to perfluoroalkylation of carbonyl compounds by using (perfluoroalkyl) silanes (3). Asymmetric trifluoromethylation of benzaldehyde was investigated by using chiral Lewis bases as catalysts. Many chiral Lewis bases such as alkaloids, amino acid derivatives, or chiral phosphines were effective for proceeding the trifluoromethylation, but enantiomer excesses of the products were low (<10%ee). The higher enantiomer excesses were observed by using trialkyl (trifluoromethyl) silanes bearing bulky alkyl groups on the silicon atom. When triethyl (trifluoromethyl) silane (5c) was used, the best result (21%ee) was obtained.