Spontaneous trans‐Selective Transfer Hydrogenation of Apolar Boron–Boron Double Bonds

Abstract

The transfer hydrogenation of N-heterocyclic carbene (NHC)-supported diborenes with dimethylamine borane proceeds with high selectivity for the trans-1,2-dihydrodiboranes.D FT calculations,s upported by kinetic studies and deuteration experiments,s uggest as tepwise proton-first-hydride-second reaction mechanism via an intermediate m-hydrodiboronium dimethylaminoboratei on pair.Since the 1925 landmark discovery by Meerwein and Verley of the aluminum alkoxide-promoted hydrogenation of ketones using alcohols as sacrificial hydrogen donors, [1] transfer hydrogenation has become an attractively mild and selective alternative to direct hydrogenation. [2] Being easy-tohandle hydrogen-storage materials, [3] ammonia borane (AB = H 3 N·BH 3 )a nd amine boranes (R 2 NH·BH 3 )h ave also demonstrated their usefulness in catalytic transfer hydrogenation reactions. [4] While the majority of these reactions are applied to polar substrates,s uch as imines and ketones, [5] there are several examples of transition-metal-and main-group-catalyzed transfer hydrogenations of apolar N=N, [6] C=C, [7] and CC bonds [8] using AB or dimethylamine borane (DMAB) as the hydrogen source.S pontaneous,u ncatalyzed transfer hydrogenation reactions with AB or DMAB have only been reported for imines, [9] highly polarized olefins, [10] aminoboranes [11] and, most recently,i minoboranes. [12] Detailed experimental and theoretical studies have shown that, in the case of imines and iminoboranes,t he reaction proceeds via pre-coordination of the protic amine hydrogen to the more electronegative nitrogen atom, followed by concerted delivery of both the protic and hydridic hydrogen atoms to the unsaturated bond, via the six-membered transition state depicted in Figure 1a. [9,12] In contrast, the Angewandte Chemie Communications 9783

Dialumenes – arylvs.silyl stabilisation for small molecule activation and catalysis

“…74 Furthermore, Braunschweig and co-workers recently showed that Me 2 NHBH 3 can be used to isolate hydrogenated diborenes; thus, analogous reactivity is anticipated. 75 However, in our hands, stoichiometric reactions of 3 and Me 2 NHBH 3 resulted in a mixture of species, whilst reaction with a higher number of equivalents of Me 2 NHBH 3 resulted in the dehydrocoupling products. We therefore conclude that the dialumene acts as a pre-catalyst in this reaction and the active catalyst is generated in situ.…”

trans‐Selective Insertional Dihydroboration of a cis‐Diborene: Synthesis of Linear sp3‐sp2‐sp3‐Triboranes and Subsequent Cationization

“…The reductive coupling of N‐heterocyclic carbene (NHC)‐stabilized (NHC)BX 2 R (R=Br, Ph) precursors, for example, provided access to neutral di‐ and tetrahydrodiboranes of the form [(NHC)R′HB‐BHR′(NHC)] (R′=H, Ph), whereas that of [ArBH 2 ] 2 diborane(6) precursors yielded [ArH 2 B‐BH 2 Ar] 2− dianions which were in turn converted via double hydride abstraction to neutral dihydrodiboranes(4) . Milder routes to diboranes with terminal B−H bonds include the dehydrocoupling of boranes, selective dimethylamino‐hydride exchange at B 2 N 2 C 2 heterocycles, the spontaneous transfer hydrogenation of diborenes with Me 2 NH⋅BH 3 , or the insertion of a borylene into a B−H bond at a boron cluster …”

“…While studying the stoichiometric transfer hydrogenation of the ferrocene-bridged diborene 1 [15] with Me 2 NH•BH 3 , we observed, beside the expected 1,2-dihydrodiborane (d 11B = À18.0 ppm), a second product (d 11B = 88.6, À29.6 ppm, 1:2 ratio), which we deemed to result from the reaction of 1 with the dehydrocoupling byproduct Me 2 N = BH 2 . [10] Similarly, the reaction of 1 with 1 equiv pyrrolidinoborane (PyrBH 2 ) in C 6 D 6 at 60 8C overnight resulted in quantitative formation of the triborane 2-Pyr (Scheme 2), which shows two broad 11 B NMR resonances at d 11B = 87.8 (sp 2 -B) and À28.8 (sp 3 -B) ppm in a 1:2 ratio and a 1 H{ 11 B} NMR BH resonance (2 H) at…”

Trans‐selektive Dihydroborierung eines cis‐Diborens durch Insertion: Synthese eines linearen sp3‐sp2‐sp3‐Triborans und anschließende Kationisierung

“…So ermöglichte beispielsweise die reduktive Kupplung von N‐heterocyclischen Carben (NHC)‐stabilisierten (NHC)BX 2 R (R=Br, Ph)‐Vorstufen den Zugang zu neutralen Di‐ und Tetrahydrodiboranen der Form [(NHC)R′HB‐BHR′(NHC)] (R′=H, Ph), während [ArBH 2 ] 2 Diboran(6)‐Spezies zu Dianionen [ArH 2 B‐BH 2 Ar] 2− reduziert wurden, welche wiederum durch doppelte Hydridabstraktion in neutrale Dihydrodiborane(4) überführt wurden . Mildere Reaktionsbedingungen für die Bildung von Diboranen mit terminalen B‐H‐Bindungen beinhalten die Dehydrokupplung von Boranen, den selektiven Dimethylamino‐Hydrid‐Austausch an B 2 N 2 C 2 Heterocyclen, die spontane Transferhydrierung von Diborenen mit Me 2 NH⋅BH 3 oder die Insertion eines Borylens in eine B‐H‐Bindung eines Borclusters …”