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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 and 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 and 57Fe Mössbauer spectroscopy, compound 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.

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Publication details

The article was received on 10 Nov 2018, accepted on 15 May 2019 and first published on 16 May 2019


Article type: Paper
DOI: 10.1039/C8CY02319K
Catal. Sci. Technol., 2019, Accepted Manuscript

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

    P. T. Gomes, T. C. Cruz, L. C. J. Pereira, J. C. Waerenborgh and L. F. Veiros, Catal. Sci. Technol., 2019, Accepted Manuscript , DOI: 10.1039/C8CY02319K

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