Homogeneous Catalytic Dehydrocoupling/Dehydrogenation of Amine−Borane Adducts by Early Transition Metal, Group 4 Metallocene Complexes

Abstract

The efficient catalytic dehydrocoupling of a range of amine-borane adducts, R'RNH x BH(3) (R' = R = Me 1a; R' = R = (i)Pr 1b; R' = Me, R = CH(2)Ph 1c) by a series of group 4 metallocene type precatalysts has been demonstrated. A reduction in catalytic activity was detected upon descending the group and also on substitution of the cyclopentadienyl (Cp) ligands with sterically bulky or electron-donating substituents. Precatalysts Cp(2)TiCl(2)/2(n)BuLi and Cp(2)Ti(PMe(3))(2), which are believed to act as precursors to [Cp(2)Ti], were found to promote the transformation of 1a to [Me(2)N-BH(2)](2) (3a) in a homogeneous catalytic process. Mechanistic studies identified the linear dimer Me(2)NH-BH(2)-NMe(2)-BH(3) (2a) as a reaction intermediate, which subsequently undergoes further catalytic dehydrogenation to form cyclic dimer 3a. Synthesis of the (2)H-isotopologues of 1a allowed the extraction of phenomenological kinetic isotope effects for 1a --> 2a and 2a --> 3a from initial rate data, which permitted the proposal of a catalytic cycle with plausible intermediates. Support for the presence of an active Ti(II) catalyst was provided by the lack of reactivity of Ti(III) complexes TiCl(3) and Cp(2)TiCl or Ti(0) in the form of THF soluble colloids or bulk Ti powder toward 1a or 1b. Modeling of the rates of consumption of 1a and formation of 3a during catalysis by Cp(2)Ti(PMe(3))(2) supported this conclusion and allowed the proposal of a two cycle, four step reaction mechanism. The proposed first cycle generates 2a in a two step process. In the second cycle, interaction of 2a with the same catalyst then results in a catalytic dehydrogenative ring closing reaction to form 3a, also in a two step process.

[Ir(PCy3)2(H)2(H2BNMe2)]+ as a Latent Source of Aminoborane: Probing the Role of Metal in the Dehydrocoupling of H3B⋅NMe2H and Retrodimerisation of [H2BNMe2]2

“…[6] However, the role of metal in the formation of intermediate C has only recently been investigated using kinetic experiments, [20,21] and is in competition with the dimerisation of H 2 B À NMe 2 to give final product D, an equilibrium that lies well in favour of the dimer at room temperature and above [K eq(180 8C) > 100]. [28,29] Direct experimental evidence for the role played by B in these reactions is thus difficult to obtain.…”

“…[20] Interestingly Manners and co-workers have suggested that in Cp 2 Ti systems this dimerisation is only likely to be a minor contributor, with formation of intermediate C dominating. [21] Baker and co-workers have also suggested that the final products of dehydrocoupling of the parent amine-borane H 3 B·NH 3 (polyborazylene or pentamer products) depend on whether transient aminoborane H 2 B-NH 2 is free or coordinated to the metal respectively. [10] Clearly the identity of the catalyst has a large part to play in all these processes.…”

“…[4] There are now a good number of transition-metal and main group [5] systems that effect this transformation, some releasing hydrogen rapidly at low catalyst loadings [6][7][8] or giving polymeric materials of useful molecular weight and polydispersities. [4] Progress has also been made in understanding the mechanism of dehydrocoupling, [9] with intermediate and model complexes studied empirically, [10][11][12][13][14][15][16][17][18][19] kinetically [8,20,21] and through computational studies. [15,[22][23][24] Much of this work has centred upon the dehydrocoupling of H 3 B·NMe 2 H (A), which although it neither gives polymer (but rather dimeric [H 2 BNMe 2 ] 2 D instead) nor has high % H capacity, is attractive as it does afford a relatively straightforward set of products and intermediates that are soluble in common solvents.…”

Synthetic and mechanistic studies of metal-free transfer hydrogenations applying polarized olefins as hydrogen acceptors and amine borane adducts as hydrogen donors

“…• C for a prolonged reaction time (entries [13][14]. The dehydrogenated Hantzsch ester could not at all be directly hydrogenated by AB (1o, entry 15).…”

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

“…Under these conditions the adduct was shown by 11 B and 19 F NMR analysis to be unstable, partially dehydrogenating to produce the aminoborane 2, in low yield (23 %), along with other unidentified boron-containing products which proved to be inseparable from the desired product (Scheme 6). Attempts to prepare the aminoborane under milder conditions via the use of transition metal catalysts were unsuccessful, with no dehydrogenative reaction prevailing at 20°C with [Rh(μ-Cl)(cod)] 2 (cod = 1,5-cyclooctadiene), colloidal Rh, [10,74] "Cp 2 Ti", [23] IrH 2 POCOP (POCOP = κ 3 -1,3-(PtBu 2 ) 2 C 6 H 3 ) [19,75] or the Fagnou catalyst system (KOtBu and RuCl 2 L 2 [L = iPr 2 PCH 2 CH 2 NH 2 ]). [12] We postulate that this is due to two main factors which prevent the catalysis.…”