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

Abstract

Four paramagnetic 14-electron tetracoordinated Fe(II) complexes of 5-substituted-2-iminopyrrolyl ligands of the type [Fe{κ²N,N′-5-R-NC₄H₂-2-C(H)N(2,6-ⁱPr₂-C₆H₃)}(Py)Cl], with R = 2,6-Me₂-C₆H₃ (1a), 2,4,6-ⁱPr₃-C₆H₂ (1b), 2,4,6-Ph₃-C₆H₃ (1c) and CPh₃ (1d), were synthesized in moderate yields by reacting the respective 5-substituted-2-iminopyrrolyl potassium salts KLa-d with FeCl₂(Py)₄ in toluene. Complexes 1a–d were characterized by ¹H 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(HBEt₃), 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(HBEt₃) 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(HBEt₃) over 30 minutes yielded the mixture of hydride species 2 and 2₂ (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 + 2₂ (mixture II). These mixtures were characterized in solution by the Evans method and in the solid state by elemental analysis, ⁵⁷Fe Mössbauer and FTIR spectroscopies, compounds 2₂ 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.