The selective activation of a C–F bond with an auxiliary strong Lewis acid: a method to change the activation preference of C–F and C–H bonds
The selective activation of the C–F bonds in substituted (2,6-difluorophenyl)phenylimines (2,6-F2H3C6-(CNH)-n′-R-C6H4 (n′ = 2, R = H (1); n′ = 2, R = Me (2); n′ = 4, R = tBu (3))) by Fe(PMe3)4 with an auxiliary strong Lewis acid (LiBr, LiI, or ZnCl2) was explored. As a result, iron(II) halides ((H5C6-(CNH)-2-FH3C6)FeX(PMe3)3 (X = Br (8); Cl (9)) and (n-RH4C6-(CNH)-2′-FH3C6)FeX(PMe3)3 (n = 2, R = Me, X = Br (11); n = 4, R = tBu, X = I (12))) were obtained. Under similar reaction conditions, using LiBF4 instead of LiBr or ZnCl2, the reaction of (2,6-difluorophenyl)phenylimine with Fe(PMe3)4 afforded an ionic complex [(2,6-F2H3C6-(CNH)-H4C6)Fe(PMe3)4](BF4) (10) via the activation of a C–H bond. The method of C–F bond activation with an auxiliary strong Lewis acid is appropriate for monofluoroarylmethanimines. Without the Lewis acid, iron(II) hydrides ((2-RH4C6-(CNH)-2′-FH3C6)FeH(PMe3)3 (R = H (13); Me (14))) were generated from the reactions of Fe(PMe3)4 with the monofluoroarylmethanimines (2-FH4C6-(CNH)-2′-RC6H4 (R = H (4); Me (5))); however, in the presence of ZnCl2 or LiBr, iron(II) halides ((2-RH4C6-(CNH)-H4C6)FeX(PMe3)3 (R = H, X = Cl (15); R = Me, X = Br (16))) could be obtained through the activation of a C–F bond. Furthermore, a C–F bond activation with good regioselectivity in (pentafluorophenyl)arylmethanimines (F5C6-(CNH)-2,6-Y2C6H3 (Y = F (6); H (7))) could be realized in the presence of ZnCl2 to produce iron(II) chlorides ((2,6-Y2H3C6-(CNH)-F4C6)FeCl(PMe3)3 (Y = F (17); H (18))). This series of iron(II) halides could be used to catalyze the hydrosilylation reaction of aldehydes. Due to the stability of iron(II) halides to high temperature, the reaction mixture was allowed to be heated to 100 °C and the reaction could finish within 0.5 h.