Catalytic B–N Dehydrogenation Using Frustrated Lewis Pairs: Evidence for a Chain-Growth Coupling Mechanism

Mo, Zhenbo; Rit, Arnab; Campos, Jesús; Kolychev, Eugene L.; Aldridge, Simon

doi:10.1021/jacs.6b01170

Cited by 88 publications

(86 citation statements)

References 48 publications

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“…The most interesting thing about these molecules is that they are reactive, activating H 2 and other bonds heterolytically. Linked amine-borane units have so far been relatively rare in FLP chemistry, but they are known (39)(40)(41)(42). We were thus encouraged to study the interaction of our substituted COTs with H 2 .…”

Section: Sidewaysmentioning

confidence: 99%

Stabilizing a different cyclooctatetraene stereoisomer

Lei

Xie

et al. 2017

Proc. Natl. Acad. Sci. U.S.A.

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An unconventional cis-cis-cis-trans or (Z,Z,Z,E) structure B of cyclooctatetraene (COT) is calculated to lie only 23 kcal/mol above the well-known tub-shaped (Z,Z,Z,Z) isomer A; one example of this type of structure is known. The barrier for B returning to A is small, 3 kcal/mol. However, by suitable choice of substituents, the (Z,Z,Z,E) isomer can be made to lie in energy below the tub-shaped structure. Steric, clamping, and electronic strategies are proposed for achieving this. In the steric strategy, the C 8 H 4 (CH 3 ) 2 (C( t Bu) 3 ) 2 structure B is predicted to lie 21 kcal/mol below structure A, which is separated from form B only by a small barrier. A simple clamping strategy, effective for COT planarization, does not influence the A/B isomerization much. But, if the clamping group is aromatic (a fused benzene, pyrrole, thiophene, furan), the subtle interplay of potential aromaticity with clamping can be used to confer persistence if not stability on the (Z,Z,Z,E) isomer. An electronic strategy of a different kind, push-pull substitution on the COT ring, was not very effective in stabilizing the B form. However, it led us to vicinal amine-borane-substituted normal COTs that proved to be quite good at activating H 2 in a frustrated Lewis pair scenario. The molecule is most certainly not aromatic, in accord with Hückel's rule (2). It features four cis-or Z double bonds in a characteristic nonplanar D 2d tub shape (3). From a combined femtosecond rotational coherence spectroscopy and theoretical study we have the molecule's precise gas-phase structure, with bond lengths r e (C-C) = 1.470, r e (C=C) = 1.337, and r e (C-H) = 1.079 Å (4). Potential ring inversion and bond-bond switching in COT have fascinated the organic community for decades (5-7). The planar D 4h structure of COT is a transition state (TS) for the ring inversion (calculated barrier height of 10-14 kcal/mol) (8-12). The D 8h bondswitching TS lies a few kilocalories per mole higher. The accepted automerization surface is summarized schematically in Fig. 1.Motivated by the recent determination of the structures of some asymmetrically substituted COTs (13), we began a systematic study of COTs. Some peculiar geometries, far away from the tub shape, emerged; in time we identified them as cis-cis-cistrans (Z,Z,Z,E) isomers of COT. Stabilizing these structures is the aim of this article.(Z,Z,Z,E)-COT structures already appear in the theoretical and experimental literature. Conesa and Rzepa in 1998 predicted this isomer to lie 21-29 kcal/mol above the conventional all-Z structure, with a small barrier to return to all-Z COT (14). In their studies of Möbius aromaticity, Havenith et al. (15) optimized a C 8 H 8 (Z,Z,Z,E) structure, finding it again 21-29 kcal/mol above (Z,Z,Z,Z). And, in a comprehensive paper on the photochemistry and thermal rearrangements of COT, Garavelli et al. (16) found the (Z,Z,Z,E) isomer as a reaction intermediate, 24 to 25 kcal/mol above normal COT. The primary interest in their wideranging paper was in the photochemistry o...

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Section: Sidewaysmentioning

confidence: 99%

Stabilizing a different cyclooctatetraene stereoisomer

Lei

Xie

et al. 2017

Proc. Natl. Acad. Sci. U.S.A.

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“…[31,32] As far as we can ascertain, this is the first example of as tructurally characterised Ga-keteniminate complex. [32] Conclusions The bulky trisalkylgallium, Ga(CH 2 SiMe 3 ) 3 ,h as been found to be av iable, effective Lewis acid for promoting small molecule activation when combined with as trong base such as 1,3-bis(tert-butyl)imidazol-2-ylidene (ItBu), thus furthere xpanding the versatility of frustrated Lewis pair chemistry.T he activation of carbonyl compounds can be achieved either by reduction of the C=Of unctionality affording zwitterionic compounds such as [ItBuCH 2 OGaR 3 ]( 1)b yf orming an ew CÀCb ond, or [{ItBuH} + {(Me 3 C 6 H 2 )C(CH 2 )OGaR 3 } À ]( 9)b yC ÀHb ond activation. [32] Conclusions The bulky trisalkylgallium, Ga(CH 2 SiMe 3 ) 3 ,h as been found to be av iable, effective Lewis acid for promoting small molecule activation when combined with as trong base such as 1,3-bis(tert-butyl)imidazol-2-ylidene (ItBu), thus furthere xpanding the versatility of frustrated Lewis pair chemistry.T he activation of carbonyl compounds can be achieved either by reduction of the C=Of unctionality affording zwitterionic compounds such as [ItBuCH 2 OGaR 3 ]( 1)b yf orming an ew CÀCb ond, or [{ItBuH} + {(Me 3 C 6 H 2 )C(CH 2 )OGaR 3 } À ]( 9)b yC ÀHb ond activation.…”

Section: Reactionswith Ketones: C4 Insertion Versus Càha Ctivationmentioning

confidence: 99%

Understanding the Subtleties of Frustrated Lewis Pair Activation of Carbonyl Compounds by N‐Heterocyclic Carbene/Alkyl Gallium Pairings

Uzelac

Armstrong

Kennedy

et al. 2016

Chemistry A European J

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This study reports the use of the trisalkylgallium GaR (R=CH SiMe ), containing sterically demanding monosilyl groups, as an effective Lewis-acid component for frustrated Lewis pair activation of carbonyl compounds, when combined with the bulky N-heterocyclic carbene 1,3-bis(tert-butyl)imidazol-2-ylidene (ItBu) or 1,3-bis(tert-butyl)imidazolin-2-ylidene (SItBu). The reduction of aldehydes can be achieved by insertion into the C=O functionality at the C2 (so-called normal) position of the carbene affording zwitterionic products [ItBuCH OGaR ] (1) or [ItBuCH(p-Br-C H )OGaR ] (2), or alternatively, at its abnormal (C4) site yielding [aItBuCH(p-Br-C H )OGaR ] (3). As evidence of the cooperative behaviour of both components, ItBu and GaR , neither of them alone are able to activate any of the carbonyl-containing substrates included in this study NMR spectroscopic studies of the new compounds point to complex equilibria involving the formation of kinetic and thermodynamic species as implicated through DFT calculations. Extension to ketones proved successful for electrophilic α,α,α-trifluoroacetophenone, yielding [aItBuC(Ph)(CF )OGaR ] (7). However, in the case of ketones and nitriles bearing acidic hydrogen atoms, C-H bond activation takes place preferentially, affording novel imidazolium gallate salts such as [{ItBuH} {(p-I-C H )C(CH )OGaR } ] (8) or [{ItBuH} {Ph C=C=NGaR } ] (12).

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“…An early study dealing with the reaction of an isolable NHSi with B(C 6 F 5 ) 3 revealed the formation of the respective silyleneborane Si II !B(III) adduct and its transformation into as ilylborane. [11] There is no phosphine-borane adduct formation even with less sterically hindered PPh 2 and P i Pr 2 group.T he unwanted adduct formation in the intermolecular systems and the ability of the 9,9-dimethylxanthene backbone to avoid adduct formation encouraged us to explore the chemistry of as ingle-component B III -a nd Si II -centered FLP.H erein we wish to report the synthesis and characterization of the first single-component NHSi borane LSi-R-BMes 2 1 with the 9,9dimethylxanthene spacer and its striking reactivity for small molecules activation. [9] Our group described the formation of azwitterionic NHSi-borane adduct from the reaction of aNHSi bearing a b-diketiminate-like ligand with B(C 6 F 5 ) 3 .…”

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confidence: 98%

“…Va riable-temperature 1 HNMR measurements show that the SiH 2 proton resonance signal at d = 4.92 ppm at 298 Kg ives as plitting into two doublets (d = 5.51 and 4.48 ppm) at 213 Kw ith the observation of ab road doublet 11 Br esonance at d = À15.4 ppm, indicating the presence of weak SiH···B interactions.Attempts to grow single-crystals of 5-H 2 were unsuccessful owing to its high solubility.I ts composition has been confirmed by high-resolution (HR) electrospray ionization (ESI) mass spectrometry,which shows the molecular ion peak at m/z 719.45593 (calcd 719.45625). According to 1 H, 29 Si DEPT,and 1 H-29 Si HMQC NMR spectroscopy,o nly the new silane-borane product L(H 2 )Si-R-BMes 2 (5-H 2 )c ould be obtained.…”

mentioning

confidence: 99%

An Intramolecular Silylene Borane Capable of Facile Activation of Small Molecules, Including Metal‐Free Dehydrogenation of Water

Szilvási

Zhou

et al. 2017

Angew Chem Int Ed

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The first single-component N-heterocyclic silylene borane 1 (LSi-R-BMes ; L=PhC(N Bu) ; R=1,12-xanthendiyl spacer; Mes=2,4,6-Me C H ), acting as a frustrated Lewis pair (FLP) in small-molecule activation, can be synthesized in 65 % yields. Its HOMO is largely localized at the silicon(II) atom and the LUMO has mainly boron 2p character. In small-molecule activation 1 allows access to the intramolecular silanone-borane 3 featuring a Si=O→B interaction through reaction with O , N O, or CO , and formation of silanethione borane 4 from reaction with S . The Si center in 1 undergoes immediate hydrogenation if exposed to H at 1 atm pressure in benzene, affording the silane borane 5-H , L(H )Si-R-BMes . Remarkably, no H activation occurs if the single silylene LSiPh and Mes B intermolecularly separated are exposed to dihydrogen. Unexpectedly, the pre-organized Si-B separation in 1 enables a metal-free dehydrogenation of H O to give the silanone-borane 3 as reactive intermediate.

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Catalytic B–N Dehydrogenation Using Frustrated Lewis Pairs: Evidence for a Chain-Growth Coupling Mechanism

Cited by 88 publications

References 48 publications

Stabilizing a different cyclooctatetraene stereoisomer

Stabilizing a different cyclooctatetraene stereoisomer

Understanding the Subtleties of Frustrated Lewis Pair Activation of Carbonyl Compounds by N‐Heterocyclic Carbene/Alkyl Gallium Pairings

An Intramolecular Silylene Borane Capable of Facile Activation of Small Molecules, Including Metal‐Free Dehydrogenation of Water

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