Diborabutatriene: An Electron‐Deficient Cumulene

Böhnke, Julian; Braunschweig, Holger; Ewing, William C.; Hörl, Christian; Krämer, Thomas; Krummenacher, Ivo; Mies, Jan; Vargas, Alfredo

doi:10.1002/anie.201403888

Cited by 141 publications

(157 citation statements)

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“…[12] Though neither 77 Se nor 125 Te nuclei could be detected by NMR spectroscopy with 2 and 3, likely a result of extensive quadrupolar broadening induced by proximity to multiple boron nuclei, the 1 H NMR spectra of the The B-B bonds in 2 (1.707(3) Å) and 3 (1.713(5) Å) are elongated with respect to the normal range of B=B lengths in diborene compounds (~1.58 -1.61 Å), [10,13] just slightly short of the normal range for B-B single bonds in base-stabilized neutral diboranes (1.72 -1.84). [13,14] This mirrors the geometries of oxiranes, which tend to have C-C bond distances (1.438(4) Å, ethylene oxide) [15] between those of alkanes (1.532 Å, ethane) [16] and alkenes (1.3142(3) Å, ethylene). [17] The typical C-C bonds in aziridines (~1.48 Å) and thiirane (~1.49 Å) are similarly situated between ethane and ethylene.…”

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confidence: 79%

Highly Strained Heterocycles Constructed from Boron–Boron Multiple Bonds and Heavy Chalcogens

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Abstract:The reactions of a diborene with elemental selenium or tellurium are shown to afford a diboraselenirane or diboratellurirane, respectively. These reactions are reminiscent of the sequestration of sub-valent oxygen and nitrogen in the formation of oxiranes and aziridines; however, such reactivity is not known between alkenes and the heavy chalcogens. While carbon is too electronegative to affect the reduction of elements of lower relative electronegativity, the highly reducing nature of the B=B double bond enables reactions with Se 0 and Te 0 . The capacity of multiple bonds between boron to donate electron density is highlighted in reactions where diborynes behave as nucleophiles, attacking one of the two Te atoms of diaryltellurides, forming salts consisting of diboratellurenium cations and aryltelluride anions.The energy stored in small, highly strained cyclic molecules has made them an integral part of modern synthetic chemistry. Since this "strain energy" increases with decreasing ring size, it is greatest for three-membered rings, and when these rings are heterocyclic the charge-asymmetry induced in the molecule provides sites ready for reaction. Accordingly, an enormous amount of research has gone into both the synthetic paths to, and reactions of, members of this class of compounds, most prominently oxiranes (C2O rings) and aziridines (C2N rings). The most common route to these materials is the oxidation of olefins using, in the case of oxirane formation, subvalent oxygen species such as O2, peroxides, peroxyacids, and ozone, or with reagents that impart a degree of electron deficiency to an oxygen atom, such as chlorite or iodosylbenzene. [1] Aziridination of olefins is most frequently accomplished through the in situ generation of nitrenes from azides or other electron deficient nitrogen sources such as iodinanes, hydroxylamines, and hydrazines. [1a,2] These alkene-oxidations are made possible by the relatively high electronegativity of oxygen and nitrogen, (χPauling = 3.44 and 3.04, respectively), relative to carbon (χPauling = 2.55).Thiiranes (C2S rings) are comparatively less common, and though examples of the direct addition of elemental sulfur to alkenyl double bonds are not unknown, [3] their syntheses are more likely than their first row neighbors to involve non-redox routes. [4] The similar electronegativities of carbon and sulfur (χPauling = 2.58) decreases the thermodynamic driving force for alkene oxidation, further exemplified by the noted willingness of thiiranes to thermally extrude atomic sulfur [5] and by their utility as sulfur atom transfer reagents. [6] Three-membered heterocycles featuring heavier chalcogens (Se and Te) are even less prevalent. Though seleniranes have been proposed as reactive intermediates in a handful of transformations, [7] the isolated examples of these compounds are few and none have been crystallographically verified. [8] To date, there are no known examples of telluriranes. The heavy chalcogens have roughly equal (χSe = 2.55) or smaller (χTe = 2.10...

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confidence: 79%

Highly Strained Heterocycles Constructed from Boron–Boron Multiple Bonds and Heavy Chalcogens

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“…Like its NHC analogue, 1, [21] compound 7 presents a planar (C1B1H1)2 core with the hydrides in trans-conformation but, unlike 1, the structure is C2-symmetric. 0.9 Å longer than that of the parent diboracumulene, 5, [24] indicating a substantial reduction in π-backbonding. Quantitative deuteration of 5 to the corresponding 1,2-deuterodiborene, (cAAC)2B2D2, was achieved using ultrapure D2, thus precluding C-H activation as the origin of the boron-bound hydrides.…”

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“…Examination of 11 B NMR data showed complete consumption of 5 and the appearance of a single new resonance at 40.7 ppm, ca 40 ppm upfield from that of 5, [24] but still significantly downfield from that of its NHC analogue, 1 (25.3 ppm).…”

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“…Previous studies in our group have shown that the cAACsupported diboracumulene 5, (cAAC)2B2 (in this case cAAC = 1-(2,6-diisopropylphenyl)-3,3,5,5-tetramethyl-pyrrolidin-2-ylidene), while formally analogous to its NHC counterpart 3a, may best be described as a diboracumulene presenting a C ... B=B ... C bonding motif, rather than a diboryne. [24] This is due to the considerably increased σ-donor and additional π-acceptor properties of cAACs resulting from replacement of one of the π-donor amino groups adjacent to the carbene carbon center by a purely σ-donor alkyl group. [25] As a result 5 also displays distinctly divergent reactivity towards CO than 3a.…”

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Uncatalyzed Hydrogenation of First‐Row Main Group Multiple Bonds

Arrowsmith

Böhnke

Braunschweig

et al. 2016

Chemistry A European J

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Room temperature hydrogenation of an and a CAAC-supported diboracumulene 3,5, Since the pioneering work of Sabatier over a century ago [1] catalytic hydrogenation has become the most used reaction type in industrial chemical synthesis [2] and still constitutes one of the most active research areas in chemistry. While the vast majority of industrial hydrogenation processes are based on well-established heterogeneous late transition metal catalysts (Pt, Pt, Rh, Ru, Ni) [2] the last decade has seen a revival of the field with the advent of metal-free frustrated Lewis pair catalysts capable of activating H2 [4] and transfer hydrogenation [5] for the reduction of polar multiple bonds.

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“…[5c,d,e] a doubly basestabilized diborene [5,7] with tricoordinate boron atoms and a B=B double bond, and a linear diboryne species bearing strongly π-acidic cyclic (alkyl)(amino)carbene (CAACs) donors, [8] effectively a diboracumulene [9] species with a B-B bond order between two and three. Interestingly, in the reaction of CO2, we were also able to isolate the thermally unstable 2+2 (C=O + B=B) cycloaddition product, which slowly undergoes cleavage of one C=O bond.…”

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CO₂ Binding and Splitting by Boron–Boron Multiple Bonds

et al. 2018

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[3] whose combination of nucleophilic and electrophilic sites are well-suited to combine with the carbon and oxygen atoms of CO2, respectively ( Figure 1A). Another common mechanism of main-group-based CO2 activation is the (initial) 2+2 cycloaddition of one C=O bond of CO2 with another E-E multiple bond, e.g. P=N (i.e. the Aza-Wittig reaction), Si=O, Si=N, Ge=O, Sn=O, and B=N bonds ( Figure 1B). [4] To our knowledge, no CO2 fixation or activation has been observed by solely utilizing a nonpolar multiple bond, despite the fact that a range of highly reactive compounds with E-E multiple bonds are known.[5] However, in 2011 Kato and Baceiredo reported [6] the reaction of CO2 with a disilyne bisphosphine adduct, a compound thought to possess some multiple bonding character between its two silicon atoms despite the clearly non-planar geometry around the silicon atoms. Herein we present fixation and splitting reactions of CO2 through its interaction with distinctly non-polar multiple bonds of two significantly different diboron species:[5c,d,e] a doubly basestabilized diborene [5,7] with tricoordinate boron atoms and a B=B double bond, and a linear diboryne species bearing strongly π-acidic cyclic (alkyl)(amino)carbene (CAACs) donors, [8] effectively a diboracumulene [9] species with a B-B bond order between two and three. Interestingly, in the reaction of CO2, we were also able to isolate the thermally unstable 2+2 (C=O + B=B) cycloaddition product, which slowly undergoes cleavage of one C=O bond. The apparently facile reaction of CO2 with B-B multiply bound species is attributed to the high reactivity of the latter, which is able to overcome the lack of polarity in the bond and effect the initial cycloaddition step. Dibromodiborenes (L(Br)B=B(Br)L), very few of which exist in the literature, [10,11] were chosen as candidates for CO2 binding due to their sterically unhindered B=B bonds and thus presumed high reactivity. Upon treatment with one atmosphere of CO2 at room temperature, after 7 min the 11 B NMR spectroscopic signal of diborene 1 [10] (dB 20) was found to have completely disappeared, replaced by two broad signals (dB ca. 0, -10). Removal of the solvent from this mixture and extraction of the residue into hexane provided a solution from which orange crystals (2) were grown. The solid-state structure of 2 (Figure 2, middle) confirms the combination of the diborene 1 with CO2 to form a dibora-b-lactone structure in which the two boron atoms form a slightly puckered four-membered B-B-C-O ring with one carbon and one oxygen atom of the CO2 unit. The remaining oxygen atom is part of a carbonyl group with a short C-O distance of 1.20(1) Å but a relatively wide O-C-B angle (136.2(8)º). Interestingly, the endocyclic B-B-C angle is strongly acute (73.7(8)º). The NHC and Br groups are each oriented in a trans fashion with respect to the ring.

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Diborabutatriene: An Electron‐Deficient Cumulene

Cited by 141 publications

References 47 publications

Highly Strained Heterocycles Constructed from Boron–Boron Multiple Bonds and Heavy Chalcogens

Highly Strained Heterocycles Constructed from Boron–Boron Multiple Bonds and Heavy Chalcogens

Uncatalyzed Hydrogenation of First‐Row Main Group Multiple Bonds

CO₂ Binding and Splitting by Boron–Boron Multiple Bonds

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Diborabutatriene: An Electron‐Deficient Cumulene

Cited by 141 publications

References 47 publications

Highly Strained Heterocycles Constructed from Boron–Boron Multiple Bonds and Heavy Chalcogens

Highly Strained Heterocycles Constructed from Boron–Boron Multiple Bonds and Heavy Chalcogens

Uncatalyzed Hydrogenation of First‐Row Main Group Multiple Bonds

CO2 Binding and Splitting by Boron–Boron Multiple Bonds

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CO₂ Binding and Splitting by Boron–Boron Multiple Bonds