Hydroboration of terminal olefins with pinacolborane catalyzed by new 2-iminopyrrolyl iron() complexes

Four paramagnetic 14-electron tetracoordinated Fe( ii ) complexes of 5-substituted-2-iminopyrrolyl ligands of the type [Fe{κ 2 N , N′ -5-R-NC 4 H 2 -2-C(H)&z.dbd;N(2,6- i Pr 2 -C 6 H 3 )}(Py)Cl], with R = 2,6-Me 2 -C 6 H 3 ( 1a ), 2,4,6- i Pr 3 -C 6 H 2 ( 1b ), 2,4,6-Ph 3 -C 6 H 3 ( 1c ) and CPh 3 ( 1d ), were synthesized in moderate yields by reacting the respective 5-substituted-2-iminopyrrolyl potassium salts KLa-d with FeCl 2 (Py) 4 in toluene. Complexes 1a-d were characterized by 1 H NMR, FTIR spectroscopies, elemental analysis and by the Evans method, the corresponding effective magnetic moments showing a high-spin electronic nature. X-ray diffraction studies on complexes 1a and 1c showed distorted tetrahedral coordination geometries. Complexes 1a-c , activated with K(HBEt 3 ), were efficient catalyst systems for the hydroboration of several terminal alkenes with pinacolborane in good to high yields (50-90%). This system mainly yielded the respective anti-Markovnikov addition products, except when styrenes were used. A screening of the hydroboration of styrene catalyzed by complexes 1a-c activated with K(HBEt 3 ) showed that the selectivity in the Markovnikov product increased with increasing steric bulkiness of the R group, exhibiting selectivities up to 91%. Additionally, the stoichiometric reaction of complex 1b with K(HBEt 3 ) over 30 minutes yielded the mixture of hydride species 2 and 2 2 (mixture I). On the other hand, when reacting the same components over 16 h, the Fe( i ) complex 3 was also identified in the mixture, in addition to 2 + 2 2 (mixture II). These mixtures were characterized in solution by the Evans method and in the solid state by elemental analysis, 57 Fe Mössbauer and FTIR spectroscopies, compounds 2 2 and 3 being also analyzed by X-ray diffraction. These results suggest that the corresponding catalytic cycle follows the borane oxidative addition route to a Fe( i ) species. New Fe( ii ) mono(2-iminopyrrolyl) complexes catalyze the hydroboration of terminal olefins with pinacolborane via a borane oxidative addition pathway.