Homogeneous Hydrogenation Catalyzed by Boranes

DeWitt, E. J.; Ramp, Floyd L.; Trapasso, L. E.

doi:10.1021/ja01483a049

Cited by 42 publications

(15 citation statements)

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“…In some respects, this sequence is reminiscent of mechanisms proposed for transition metal C–H insertions involving aromatic substrates,31 but the 3c2e bonding interaction between the mostly vacant borenium p -orbital of 14a and the arene σ C–H bond leads to a cationic borenium-hydrogen complex 38 . Related hypervalent species may be involved in high temperature hydrogen transfer reactions catalyzed by trialkyl boranes,32 the recent hydrogen activation experiments of Stephan et al,33 and gas phase equilibria involving BH 2 (+), H 2 , and BH 4 (+) 34. The relationship between 38 and 16a has a close parallel in the cationic ammonia borane derivatives [H 3 N•BH 4 ] + (borenium ion hydrogen adduct) and [H 3 N•BH 2 ] + (borenium ion), structures that have been evaluated computationally 35…”

Section: Discussion; Evaluation Of Potential Reactive Intermediatesmentioning

confidence: 99%

Superelectrophilic Intermediates in Nitrogen-Directed Aromatic Borylation

et al. 2009

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The first examples of borylation under conditions of borenium ion generation from hydrogen-bridged boron cations are described. The observable H-bridged cations are generated by hydride abstraction from N,N-dimethylamine boranes Ar(CH2)nNMe2BH3 using Ph3C+ (C6F5)4B− (TrTPFPB) as the hydride acceptor. In the presence of excess TrTPFPB, the hydrogen-bridged cations undergo internal borylation to afford cyclic amine borane derivatives with n = 1-3. The products are formed as the corresponding cyclic borenium ions according to reductive quenching experiments and 11B and 1H NMR spectroscopy in the case with Ar = C6H5 and n = 1. The same cyclic borenium cation is also formed from the substrate with Ar = o-C6H4SiMe3 via desilylation, but the analogous system with Ar = o-C6H4CMe3 affords a unique cyclization product that retains the tert-butyl substituent. An ortho-deuterated substrate undergoes cyclization with a product-determining isotope effect of kH/kD 2.8. Potential cationic intermediates have been evaluated using B3LYP/6-31G* methods. The computations indicate that internal borylation from 14a occurs via a C–H insertion transition state that is accessible from either the borenium π complex or from a Wheland intermediate having nearly identical energy. The Ar = o-C6H4SiMe3 example strongly favors formation of the Wheland intermediate, and desilylation occurs via internal SiMe3 migration from carbon to one of the hydrides attached to boron.

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Section: Discussion; Evaluation Of Potential Reactive Intermediatesmentioning

confidence: 99%

Superelectrophilic Intermediates in Nitrogen-Directed Aromatic Borylation

et al. 2009

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“…Combining the FLP approach and previous knowledge about borane-catalyzed hydrogenation of alkenes 11,12,13,14 and polyarenes 15,16,17,18 , we propose herein a new general catalytic pathway to the hydrogenation of unsaturated hydrocarbons ( Fig. 1b) and demonstrate its validity by the highly selective hydrogenation of alkynes into cis-alkenes.…”

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confidence: 85%

A frustrated-Lewis-pair approach to catalytic reduction of alkynes to cis-alkenes

et al. 2013

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Recently, a new approach to dihydrogen activation known as frustrated Lewis pairs (FLPs) concept has been introduced 1, 2, 3 . A combination of highly Lewis acidic boranes and sterically hindered bases can split hydrogen heterolytically generating onium (phosphonium, ammonium, etc.) borohydrides. These compounds show reduction activity resembling that of inorganic borohydrides like NaBH 4 , i. e. they are suitable mostly for reduction of polarized multiple bonds. Imines, enamines, silyl ethers 4, 5,6 , α,β-enones 7 , ynones 8 , Nalkylanilines 9 were hydrogenated using stoichiometric or catalytic amounts of FLPs. Due to heterolytic nature of FLP-H 2 adducts, hydrogenation of unactivated multiple C-C bonds using FLPs has some natural limitations, since during the respective step of the catalytic cycle a proton transfer from catalyst to substrate should take place (Fig. 1a). Although Greb et al. have implemented this approach to hydrogenation of alkenes under ambient conditions, this method is predictably restricted to the alkenes with high proton affinity 10 .

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“…The observed selectivity in these olefinic bonds reductions and the lack of polymerization product suggest also that there was no radical or any Ar 2 BH mediated side reactions. [33] Following the demonstration that Lewis acid 1 c can catalyze the hydrogenation of a broad range of functionalized quinolines, the synthetic streamlining ability of this metalfree reduction was then probed in the three-step synthesis of racemic cuspareine [34] (15) (Scheme 7). The synthesis commenced with a ZnCl 2 -catalyzed condensation of quinaldine (10 b) with the aldehyde 12.…”

Section: Resultsmentioning

confidence: 99%

Catalytic Hydrogenation with Frustrated Lewis Pairs: Selectivity Achieved by Size‐Exclusion Design of Lewis Acids

Erős

Nagy

Hasan

et al. 2011

Chemistry A European J

156

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Catalytic hydrogenation that utilizes frustrated Lewis pair (FLP) catalysts is a subject of growing interest because such catalysts offer a unique opportunity for the development of transition-metal-free hydrogenations. The aim of our recent efforts is to further increase the functional-group tolerance and chemoselectivity of FLP catalysts by means of size-exclusion catalyst design. Given that hydrogen molecule is the smallest molecule, our modified Lewis acids feature a highly shielded boron center that still allows the cleavage of the hydrogen but avoids undesirable FLP reactivity by simple physical constraint. As a result, greater latitude in substrate scope can be achieved, as exemplified by the chemoselective reduction of α,β-unsaturated imines, ketones, and quinolines. In addition to synthetic aspects, detailed NMR spectroscopic, DFT, and (2)H isotopic labeling studies were performed to gain further mechanistic insight into FLP hydrogenation.

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Homogeneous Hydrogenation Catalyzed by Boranes

Cited by 42 publications

References 0 publications

Superelectrophilic Intermediates in Nitrogen-Directed Aromatic Borylation

Superelectrophilic Intermediates in Nitrogen-Directed Aromatic Borylation

A frustrated-Lewis-pair approach to catalytic reduction of alkynes to cis-alkenes

Catalytic Hydrogenation with Frustrated Lewis Pairs: Selectivity Achieved by Size‐Exclusion Design of Lewis Acids

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