Development of Rhenium Catalysts for Amine Borane Dehydrocoupling and Transfer Hydrogenation of Olefins

Abstract

Five-coordinated rhenium(I) hydride complexes of the type [Re(Br)(H)(NO)(PR3)2] (R = Cy 2a, iPr 2b) were prepared from [Re(Br)2(NO)(PR3)2(η2-H2)] (R = Cy 1a, iPr 1b) via deprotonation of the η2-H2 ligands with various bases. Filling the vacant site of 2a or 2b by various less bulky two-electron donors produced the 18-electron complexes [Re(Br)(H)(NO)(PR3)2(L)] (L = O2 3, CH2CH2 4, acetylene 5, H2 6, CO 7, CH3CN 8). The influence of the trans-coordinated ligand on the Re−H bond was examined. The 1H NMR chemical shift of the hydride depends on L in the order O2 > acetylene > CH2CH2 > H2 > CO > CH3CN. The reactions of 2a or 2b with the IMes or SIMes ligands afforded the five-coordinated complex [Re(Br)(H)(NO)(PR3)(NHC)] (NHC = IMes 9 (IMes = 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene), SIMes 10 (SIMes = 1,3-bis(2,4,6-trimethylphenyl)-4,5-dihydroimidazol-2-ylidene)) via replacement of one phosphine. The reaction of 2a or 2b with n-BuLi leads to the formation of the n-butene-coordinated dihydride complexes [Re(H)2(NO)(PR3)2(η2-n-CH2CHC2H5)] (R = Cy 12a, iPr 12b). Species 1a and 1b reacted also with NaNMe2·BH3, affording the tetrahydride complexes [Re(H)4(NO)(PR3)2] (R = Cy 14a, iPr 14b) via the intermediacy of 2a and 2b. The molecular structures of complexes 8b, 10a, and 10b were established by single-crystal X-ray diffraction studies. The five-coordinated rhenium(I) hydride complexes 2a, 2b, 9a, and 9b catalyzed the dehydrocoupling of Me2NH·BH3 and the transfer hydrogenation of olefins using Me2NH·BH3 as a hydrogen donor, which showed high activities. Mechanistic studies were carried out indicating that these rhenium(I) hydride catalyses allowed formation of dihydrogen hydride complexes. A plausible catalytic cycle for both dehydrocoupling and transfer hydrogenation was proposed, which implies the ability of rhenium(I) complexes to activate B−H and N−H bonds by the facile redox interplay of Re(I) and Re(III) species.

Transfer Hydrogenation of Imines with Ammonia–Borane: A Concerted Double‐Hydrogen‐Transfer Reaction

“…Considering the manifold dehydrogenation products of AB, all the reactions were carried out with in situ NMR measurements under approximate pseudo-first-order conditions for AB with the imines in excess (0.5 mmol of 1 a reacting with 0.1 mmol of AB). According to the kinetic conversion chart based on 11 B NMR (Figure 1), the rate constants k were simulated and the DKIE values obtained [Eq. (1)].…”

“…In recent studies, it was demonstrated that rhenium catalysts can promote multistep transfer hydrogenations of olefins using AB and dimethylamine borane as a polar hydrogen donor. [11] Herein, we investigate the direct reaction between imines and AB, and reach the conclusion that concerted polar double H transfers are involved.…”

Experimental and Theoretical Studies of the Potential Interconversion of the Amine–Borane iPr2NH·BH(C6F5)2 and the Aminoborane iPr2N=B(C6F5)2 Involving Hydrogen Loss and Uptake

“…[4][5][6][7][8][9][10] More recently, the range of metal complexes reported to be capable of catalytically dehydrogenating amine-boranes has been significantly expanded. For example, homogeneous systems based on Rh, [11] Ru, [12,13] Re, [14] Ni [15,16] and Ir [17][18][19] are now known, along with those based on Group 2 and 4 metals, [20][21][22][23][24][25] and systems based on p-block metals. [26] In addition, photoactivated catalysts of the first-row transition metals have also recently been reported.…”

Reactions of Amine– and Phosphane–Borane Adducts with Frustrated Lewis Pair Combinations of Group 14 Triflates and Sterically Hindered Nitrogen Bases

“…The mixing of 2, 3 or 4 with a slight excess of either 5 or 6 in 1,2-dichlorobenzene at room temperature afforded no reaction as evidenced by 29 Si and 119 Sn NMR spectroscopy. [56,57] These combinations of Lewis acids and bases were therefore deemed appropriate for use in the FLP activation of amine and phosphane-borane adducts.…”

“…Support for the former deduction was provided by the absence from the 13 C{ 1 H} NMR spectrum of a signal that would be considered characteristic of a two-coordinate carbon atom (δ = 213.2 ppm). [58] Further evidence for the latter can be taken from the resonance at δ = -6.2 ppm in the 29 Si{ 1 H} NMR spectrum. The 19 F NMR spectrum indicated the inclusion of the triflate ion in the product by the presence of a singlet at δ = 78.2 ppm.…”