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

Abstract

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.