Beryllium-Induced CN Bond Activation and Ring Opening of an N-Heterocyclic Carbene

How to Decarbonize N-Heterocyclic Carbenes (NHCs): The simple Alane Adducts (NHC) ⋅ AlR3 (R=H, Me, Et)

The reaction of the amine-stabilized alane (NMe3) ⋅ AlH3 1 with the backbone-saturated N-heterocyclic carbene (NHC) SIDipp (SIDipp=1,3-bis-{2,6-di-iso-propyl-phenyl}-imidazolidin-2-ylidene) at 0 °C yielded the NHC alane adduct (SIDipp) ⋅ AlH3 2. Reaction at elevated temperatures or prolonged reaction at room temperature gave the product of a ring expansion reaction (RER) of the NHC, (NMe3) ⋅ AlH(RER-SIDippH2) 3 ⋅ (NMe3). Subsequent reaction of the latter with sterically less hindered NHCs (IMeMe {=1,3,4,5-tetramethyl-imidazolin-2-ylidene}, IiPrMe {=1,3-di-iso-propyl-4,5-dimethyl-imidazolin-2-ylidene}, and IiPr {=1,3-di-iso-propyl-imidazolin-2-ylidene}) afforded the NHC-stabilized RER-products (NHC) ⋅ AlH(RER-SIDippH2) 3 ⋅ (NHC) (NHC=IMeMe, IiPrMe, IiPr), while no reaction was observed with the sterically more demanding NHCs IDipp (=1,3-bis-{2,6-di-iso-propyl-phenyl}-imidazolin-2-ylidene), SIDipp and ItBu (=1,3-di-tert-butyl-imidazolin-2-ylidene). The compounds 3 ⋅ (NHC) were also obtained starting from (SIDipp) ⋅ AlH3 2 and NHC at room temperature. Heating solutions of (SIDipp) ⋅ AlH3 2 without additional base to 95 °C resulted in decarbonization of the NHC and substitution of the carbene carbon atom with aluminum hydride under loss of ethene. Subsequent dimerization afforded cis-[AlH{μ-N(Dipp)CH2CH2N(Dipp)}]2 4_dimer. Heating solutions of the NHC-ligated aluminum alkyls (SIDipp) ⋅ AlR3 2R (R=Me, Et) to 145 °C instead led to complete scission of the NHC backbone with evolution of ethene and isolation of the dialkylaluminium(III) amidinates {DippNC(R)NDipp}AlR2 5R (R=Me, Et).

Activation of Ge-H and Sn-H Bonds with N-Heterocyclic Carbenes and a Cyclic (Alkyl)(amino)carbene

A study of the reactivity of several N-heterocyclic carbenes (NHCs) and the cyclic (alkyl)(amino)carbene 1-(2,6-di-iso-propylphenyl)-3,3,5,5-tetramethyl-pyrrolidin-2-ylidene (cAAC^Me ) with the group 14 hydrides GeH₂ Mes₂ and SnH₂ Me₂ (Me=CH₃ , Mes=1,3,5-(CH₃ )₃ C₆ H₂ ) is presented. The reaction of GeH₂ Mes₂ with cAAC^Me led to the insertion of cAAC^Me into one Ge-H bond to give cAAC^Me H-GeHMes₂ (1). If 1,3,4,5-tetramethyl-imidazolin-2-ylidene (Me₂ Im^Me ) was used as the carbene, NHC-mediated dehydrogenative coupling occurred, which led to the NHC-stabilized germylene Me₂ Im^Me ⋅GeMes₂ (2). The reaction of SnH₂ Me₂ with cAAC^Me also afforded the insertion product cAAC^Me H-SnHMe₂ (3), and reaction of two equivalents Me₂ Im^Me with SnH₂ Me₂ gave the NHC-stabilized stannylene Me₂ Im^Me ⋅SnMe₂ (4). If the sterically more demanding NHCs Me₂ Im^Me , 1,3-di-isopropyl-4,5-dimethyl-imidazolin-2-ylidene (iPr₂ Im^Me ) and 1,3-bis-(2,6-di-isopropylphenyl)-imidazolin-2-ylidene (Dipp₂ Im) were employed, selective formation of cyclic oligomers (SnMe₂ )_n (5; n=5-8) in high yield was observed. These cyclic oligomers were also obtained from the controlled decomposition of cAAC^Me H-SnHMe₂ (3).

Reactions of a Four‐Membered Borete with Carbon, Silicon, and Gallium Donor Ligands: Fused and Spiro‐Type Boracycles

Donor‐acceptor cyclopropanes or cyclobutanes are dipolar reagents, which are widely used in the synthesis of complex organic (hetero)cycles in ring expansion reactions. Applying this concept to boron containing heterocycles, the four‐membered borete cyclo‐iPr₂N‐BC₁₀H₆ reacted with the carbon donor ligands 2,6‐xylylisonitrile and the carbene IMes :C(NMesCH)₂ with ring expansion and ring fusion, respectively. In particular, the tetracyclic structure formed with IMes displays zwitterionic character and absorption in the visible region. In contrast to the carbene IMes, the heavier carbenoids :Si(NDippCH)₂ and :Ga(AmIm) with a two‐coordinate donor atom afford spiro‐type bicyclic compounds, which display four‐coordinate geometry at silicon or gallium. (TD‐)DFT calculations provide deeper insight into the mechanism of formation and the absorption properties of these new compounds.

s-Block Multiple Bonds: Isolation of a Beryllium Imido Complex

A common feature of d- and p-block elements is that they participate in multiple bonding. In contrast, the synthesis of compounds containing homo- or hetero-nuclear multiple bonds involving s-block elements is extremely rare. Herein, we report the synthesis, molecular structure, and computational analysis of a beryllium imido (Be=N) complex (2), which was prepared via oxidation of a molecular Be⁰ precursor (1) with trimethylsilyl azide Me₃ SiN₃ (TMS-N₃ ). Notably, compound 2 features the shortest known Be=N bond (1.464 Å) to date. This represents the first compound with an s-block metal-nitrogen multiple bond. All compounds were characterized experimentally with multi-nuclear NMR spectroscopy (¹ H, ¹³ C, ⁹ Be) and single-crystal X-ray diffraction studies. The bonding situation was analyzed with density functional theory (DFT) calculations, which supports the existence of π-bonding between beryllium and nitrogen.

B-B Cleavage and Ring-Expansion of a 1,4,2,3-Diazadiborinine with N-Heterocyclic Carbenes

A 1,4,2,3-diazadiborinine derivative was found to form Lewis adducts with strong two-electron donors such as N-heterocyclic and cyclic (alkyl)(amino)carbenes. Depending on the donor, some of these Lewis pairs are thermally unstable, converting to sole B,N-embedded products upon gentle heating. The products of these reactions, which have been fully characterized by NMR spectroscopy, elemental analysis, and single-crystal X-ray diffraction, were identified as B,N-heterocycles with fused 1,5,2,4-diazadiborepine and 1,4,2-diazaborinine rings. Computational modelling of the reaction mechanism provides insight into the formation of these unique structures, suggesting that a series of B-H, C-N, and B-B bond activation steps are responsible for these "intercalation" reactions between the 1,4,2,3-diazadiborinine and NHCs.

Off the Beaten Track-A Hitchhiker's Guide to Beryllium Chemistry

This Minireview aims to give an introduction to beryllium chemistry for all less-experienced scientists in this field of research. Up to date information on the toxicity of beryllium and its compounds are reviewed and several basic and necessary guidelines for a safe and proper handling in modern chemical research laboratories are presented. Interesting phenomenological observations are described that are related directly to the uniqueness of this element, which are also put into historical context. Herein we combine the contributions and experiences of many scientist that work passionately in this field. We want to encourage fellow scientists to reconcile the long-standing reservations about beryllium and its compounds and motivate intense research on this spurned element. Who on earth should be able to deal with beryllium and its compounds if not chemists?

Adduct Formation, B-H Activation and Ring Expansion at Room Temperature from Reactions of HBcat with NHCs

We report the reactions of catecholborane (HBcat; 1) with unsaturated and saturated NHCs as well as CAAC(Me) . Mono-NHC adducts of the type HBcat⋅NHC (NHC=nPr2 Im, iPr2 Im, iPr2 Im(Me) , and Dipp2 Im) were obtained by stoichiometric reactions of HBcat with the unsaturated NHCs. The reaction of CAAC(Me) with HBcat yielded the B-H activated product CAAC(Me) (H)Bcat via insertion of the carbine-carbon atom into the B-H bond. The saturated NHC Dipp2 SIm reacted in a 2:2 ratio yielding an NHC ring-expanded product at room temperature forming a six-membered -B-C=N-C=C-N- ring via C-N bond cleavage and further migration of the hydrides from two HBcat molecules to the former carbene-carbon atom.

Unusual Fragmentation and Transformation of an N-Heterocyclic Carbene by a Stable Phosphonium-Borane peri-Functionalized Naphthalene

A 1-phosphonium-8-borane-decorated naphthalene molecule 2 has been found to react with N,N'-dimethylimidazol-2-ylidene (IMe), a popular member of the N-heterocyclic carbene (NHC) family, which converts it into two vinyl-amine fragments one of which is trapped between the phosphonium and borane unit by the formation of a C-C and a B-N bond. The same reactivity was not observed for larger NHC molecules. Control experiments and mechanistic studies have established the involvement of an ylide-borane molecule and an imidazolium salt in addition to IMe carbene in this new transformation of an NHC.

Dinuclear zinc hydride supported by an anionic bis(N-heterocyclic carbene) ligand

Methylene-linked bis(N,N′-di-tert-butylimidazol-2-ylidene) 1 reacted with diethylzinc to give dinuclear zinc ethyl compound 2, which contains a formally anionic bis(carbene) ligand as a result of deprotonation of the methylene bridge. The reaction of 2 with PhSiH3 gave the phenylsilyl compound 3. The zinc hydride 4 was obtained by the reaction of 2 with LiAlH4 or Ph3SiOH followed by treatment with PhSiH3. X-ray diffraction studies show that compounds 2, 3, and 4 all have a similar dimeric structure with D2h symmetry. The reaction of hydride 4 with carbon dioxide and N,N′-diisopropylcarbodiimide gave formato (5) and formamidinato (7) derivatives as a result of the insertion of the heterocumulene into both Zn-H bonds. Reaction with Ph2CO gave the diphenylmethoxy compound 6. Hydride 4 shows catalytic activity in the hydrosilylation of 1,1-diphenylethylene and methanolysis of silanes.

Room temperature ring expansion of N-heterocyclic carbenes and B-B bond cleavage of diboron(4) compounds

We report the isolation and detailed structural characterization, by solid-state and solution NMR spectroscopy, of the neutral mono- and bis-NHC adducts of bis(catecholato)diboron (B2 cat2 ). The bis-NHC adduct undergoes thermally induced rearrangement, forming a six-membered -B-C=N-C=C-N-heterocyclic ring via C-N bond cleavage and ring expansion of the NHC, whereas the mono-NHC adduct is stable. Bis(neopentylglycolato)diboron (B2 neop2 ) is much more reactive than B2 cat2 giving a ring expanded product at room temperature, demonstrating that ring expansion of NHCs can be a very facile process with significant implications for their use in catalysis.

Carbene-9-BBN ring expansions as a route to intramolecular frustrated Lewis pairs for CO2 reduction

Reactions of phosphine-derived carbenes with 9-borabicyclo[3.3.1]nonane (9-BBN) result in ring-expansion reactions to generate novel intramolecular frustrated Lewis pairs (FLPs). These FLPs effect the catalytic reduction of CO2 in the presence of boranes to give BOB and methoxy-borate species.

Zinc-catalyzed borylation of primary, secondary and tertiary alkyl halides with alkoxy diboron reagents at room temperature

A new catalytic system based on a Zn(II) NHC precursor has been developed for the cross-coupling reaction of alkyl halides with diboron reagents, which represents a novel use of a Group XII catalyst for CX borylation. This approach gives borylations of unactivated primary, secondary, and tertiary alkyl halides at room temperature to furnish alkyl boronates, with good functional-group compatibility, under mild conditions. Preliminary mechanistic investigations demonstrated that this borylation reaction seems to involve one-electron processes.