Efficient dehydration of primary amides to nitriles catalyzed by phosphorus-chalcogen chelated iron hydrides

A series of phosphorus-chalcogen chelated hydrido iron (II) complexes 1-7, (o-(R'P-2)-p-R-C6H4Y)FeH (PMe3)(3) (1: R = H, R' = Ph, Y = O; 2: R = Me, R' = Ph, Y = O; 3: R = H, R' = Pr-i, Y = O; 4: R = Me, R' = Pr-i, Y = O; 5: R = H, R' = Ph, Y = S; 6: R = Me, R' = Ph, Y = S; 7: R = H, R' = Ph, Y = Se), were synthesized. The catalytic performances of 1-7 for dehydration of amides to nitriles were explored by comparing three factors: (1) different chalcogen coordination atoms Y; (2) R' group of the phosphine moiety; (3) R substituent group at the phenyl ring. It is confirmed that 5 with S as coordination atom has the best catalytic activity and 7 with Se as coordination atom has the poorest catalytic activity among complexes 1, 5 and 7. Electron-rich complex 4 is the best catalyst among the seven complexes and the dehydration reaction was completed by using 2 mol% catalyst loading at 60 degrees C with 24 hr in the presence of (EtO)(3)SiH in THF. Catalyst 4 has good tolerance to many functional groups. Among the seven iron complexes, new complexes 3 and 4 were obtained via the O-H bond activation of the preligands o-(Pr2P)-Pr-i(C6H4)OH and o-(Pr2P)-Pr-i-p-Me-(C6H4)OH by Fe(PMe3)(4). Both 3 and 4 were characterized by spectroscopic methods and X-ray diffraction analysis. The catalytic mechanism was experimentally studied and also proposed.