Synthesis and Reactivity of a Base-Free N-Heterocyclic Silanimine

Reaction of the N-heterocyclic silylene (HCNDipp)2Si (1, Dipp = 2,6-iPr2C6H3) with the terphenyl azide ArN3 (Ar = 2,6-Mes2C6H3, Mes =2,4,6-Me3C6H2) in THF yielded the base-free silanimine (HCNDipp)2SiNAr (2) with the almost linear SiNC geometry in high yield. Reaction of 2 with sulfur and H2O resulted in the 1,3-addition of S2 to the C2N2Si ring and 1,2-addition of H2O to the SiN bond, respectively

Synthesis and Reactivity of a Base-Free N-Heterocyclic Silanimine

Reaction of the N-heterocyclic silylene (HCNDipp)2Si (1, Dipp = 2,6-iPr2C6H3) with the terphenyl azide ArN3 (Ar = 2,6-Mes2C6H3, Mes =2,4,6-Me3C6H2) in THF yielded the base-free silanimine (HCNDipp)2SiNAr (2) with the almost linear SiNC geometry in high yield. Reaction of 2 with sulfur and H2O resulted in the 1,3-addition of S2 to the C2N2Si ring and 1,2-addition of H2O to the SiN bond, respectively

Synthesis and Reactivity of a Base-Free N-Heterocyclic Silanimine

Reaction of the N-heterocyclic silylene (HCNDipp)2Si (1, Dipp = 2,6-iPr2C6H3) with the terphenyl azide ArN3 (Ar = 2,6-Mes2C6H3, Mes =2,4,6-Me3C6H2) in THF yielded the base-free silanimine (HCNDipp)2SiNAr (2) with the almost linear SiNC geometry in high yield. Reaction of 2 with sulfur and H2O resulted in the 1,3-addition of S2 to the C2N2Si ring and 1,2-addition of H2O to the SiN bond, respectively

New Approaches to N‑Heterocyclic-Carbene-Coordinated Iminoborane and Borenium Species

The synthesis, characterization, and reactivity of an iminoborane–N-heterocyclic carbene (NHC) adduct were described. The reaction of DmpNHB­(OEt)­Br [1; Dmp = 2,6-bis­(2,4,6-trimethylphenyl)­phenyl] with 2 equiv of 1,3-diimethyl-4,5-dimethylimidazol-2-ylidene (IMe4) resulted in the formation of an iminoborane–NHC complex 2. Both X-ray analysis and density functional theory calculations revealed the double-bond character of the BN bond in 2. Interestingly, compared with the corresponding Lewis-base-free iminoborane, 2 features a nitrogen atom with increased electron density, which could be attributed to coordination of the NHC. Similar to the isoelectronic species imine, this nitrogen center in 2 can be easily attacked by electrophiles. Indeed, the reaction of 2 with trimethylsilyl triflate (Me3SiOTf) afforded an NHC-stabilized borenium cation 3, representing a facile strategy to prepare cationic tricoordinate boron species

Dehydrochlorination to Silylenes by N-Heterocyclic Carbenes

Reaction of cyclic diaminochlorosilanes with 1,3-bis(tert-butyl)imidazol-2-ylidene resulted in the facile formation of the corresponding stable and transient diaminosilylenes. This novel dehydrochlorination route could be applied for the generation of four- and five-membered N-heterocyclic silylenes with a range of different substituents under very mild conditions. Activation of an olefinic C−H bond and reduction of a cyclic diaminochlorosilane have been observed for these new transient silylenes

Formation of Boron–Main-Group Element Bonds by Reactions with a Tricoordinate Organoboron L₂PhB: (L = Oxazol-2-ylidene)

The reactivity of L₂PhB: (<b>1</b>; L = oxazol-2-ylidene) as well as its transition-metal (chromium and iron) complexes toward main-group substrates have been systematically examined, which led to the construction of B–E (E = C, Ga, Cl, H, F, N) bonds. The combination of <b>1</b> and triethylborane smoothly captured carbon dioxide concomitant with the formation of B–C and B–O bonds. The soft basic boron center in <b>1</b> readily reacted with soft acidic gallium trichloride (GaCl₃) to afford the extremely stable adduct <b>4</b> involving a B–Ga dative bond. Electrophilic alkylation of a neutral tricoordinate organoboron was first achieved by the treatment of <b>1</b> with dichloromethane and methyl trifluoromethanesulfonate (MeOTf), both of which afforded ionic species featuring an additional B–C bond. Comparatively, redox reactions took place when halides of heavier elements such as germanium dichloride, dichlorophenylphosphine, and chlorodiphenylbismuth were employed as substrates, from which cationic species <b>7</b> bearing a B–Cl bond was obtained. In addition, reactions of metal complexes [<b>2</b>, Cr­(<b>1</b>)­(CO)₅; <b>8</b>, Fe­(<b>1</b>)­(CO)₄] with cationic electrophiles were investigated. With HOTf and FN­(SO₂Ph)₂, the corresponding ionic species featuring a B–H bond (<b>9</b>) and a B–F bond (<b>10</b>) were formed via a formal electrophilic substitution reaction, whereas the reaction of <b>1</b> with F·Py-BF₄ resulted in the formation of a dicationic boron species <b>11</b> with a newly formed B–N bond

Formation of Boron–Main-Group Element Bonds by Reactions with a Tricoordinate Organoboron L₂PhB: (L = Oxazol-2-ylidene)

The reactivity of L₂PhB: (<b>1</b>; L = oxazol-2-ylidene) as well as its transition-metal (chromium and iron) complexes toward main-group substrates have been systematically examined, which led to the construction of B–E (E = C, Ga, Cl, H, F, N) bonds. The combination of <b>1</b> and triethylborane smoothly captured carbon dioxide concomitant with the formation of B–C and B–O bonds. The soft basic boron center in <b>1</b> readily reacted with soft acidic gallium trichloride (GaCl₃) to afford the extremely stable adduct <b>4</b> involving a B–Ga dative bond. Electrophilic alkylation of a neutral tricoordinate organoboron was first achieved by the treatment of <b>1</b> with dichloromethane and methyl trifluoromethanesulfonate (MeOTf), both of which afforded ionic species featuring an additional B–C bond. Comparatively, redox reactions took place when halides of heavier elements such as germanium dichloride, dichlorophenylphosphine, and chlorodiphenylbismuth were employed as substrates, from which cationic species <b>7</b> bearing a B–Cl bond was obtained. In addition, reactions of metal complexes [<b>2</b>, Cr­(<b>1</b>)­(CO)₅; <b>8</b>, Fe­(<b>1</b>)­(CO)₄] with cationic electrophiles were investigated. With HOTf and FN­(SO₂Ph)₂, the corresponding ionic species featuring a B–H bond (<b>9</b>) and a B–F bond (<b>10</b>) were formed via a formal electrophilic substitution reaction, whereas the reaction of <b>1</b> with F·Py-BF₄ resulted in the formation of a dicationic boron species <b>11</b> with a newly formed B–N bond

Isolation of a Diborane(6) Dication: Formation and Cleavage of an Electron-Precise B(sp³)–B(sp³) Bond

One-electron oxidation of organoboron L₂PhB: <b>1</b> (L = oxazol-2-ylidene) afforded a dicationic diborane(6) species [L₂PhB–BPhL₂]·2X (X = OTf, BF₄, AlCl₄) <b>3</b>, representing a new strategy to construct a B­(sp³)–B­(sp³) covalent bond. Each boron atom in <b>3</b> is in the formal oxidation state +II, and tetracoordinate with a Ph group and two oxazol-2-ylidenes. The cyclic voltammetry of <b>3</b> shows irreversible reduction and oxidation. Indeed, two-electron reduction of <b>3</b> with potassium graphite (KC₈) afforded <b>1</b>, making a fully reversible <b>1</b> ↔ <b>3</b> redox system, whereas two-electron oxidation with AuCl produced a boronium [L₂PhBCl]­OTf <b>4</b>. Moreover, the reactions of <b>3</b> with isonitrile derivatives RNC: under heating conditions gave a cyano-substituted boronium [L₂PhBCN]­BF₄ <b>5</b> and a 2-boranyl-indole derivative <b>6</b>, depending on the substituent R. The proposed reaction mechanism involves a borinylium radical <b>1</b>^•+ which is generated via a homolytic cleavage of the B–B bond of <b>3</b>

Bifurcated Hydrogen-Bond-Stabilized Boron Analogues of Carboxylic Acids

The reactivity of a bulky m-terphenylboronic acid, DmpB­(OH)2 [1; Dmp = 2,6-bis­(2,4,6-trimethylphenyl)­phenyl], toward three different N-heterocyclic carbenes has been examined. The reaction of 1 with 1 equiv of bis­(2,6-diisopropylphenyl)­imidazol-2-ylidene (IPr) leads to the formation of a hydrogen-bonded carbene boronic acid adduct, 2, featuring strong O–H···C contacts. In contrast, more basic 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene (IPr2Me2) and 1,3-di-tert-butylimidazol-2-ylidene (ItBu) deprotonate 1 smoothly to afford the rare anionic boranuidacarboxylic acids 3 and 4, respectively. Structural determination reveals that 3 and 4 bear unprecedented bifurcated hydrogen bonds with a BO– unit as a double hydrogen-bond acceptor, which contribute significantly to stabilization of the highly reactive BO double bond. Quantum-mechanical calculations were conducted to disclose the unique electronic properties of the multiple bonds, as well as the important hydrogen bonds in these compounds

Formation of Boron–Main-Group Element Bonds by Reactions with a Tricoordinate Organoboron L₂PhB: (L = Oxazol-2-ylidene)

The reactivity of L₂PhB: (<b>1</b>; L = oxazol-2-ylidene) as well as its transition-metal (chromium and iron) complexes toward main-group substrates have been systematically examined, which led to the construction of B–E (E = C, Ga, Cl, H, F, N) bonds. The combination of <b>1</b> and triethylborane smoothly captured carbon dioxide concomitant with the formation of B–C and B–O bonds. The soft basic boron center in <b>1</b> readily reacted with soft acidic gallium trichloride (GaCl₃) to afford the extremely stable adduct <b>4</b> involving a B–Ga dative bond. Electrophilic alkylation of a neutral tricoordinate organoboron was first achieved by the treatment of <b>1</b> with dichloromethane and methyl trifluoromethanesulfonate (MeOTf), both of which afforded ionic species featuring an additional B–C bond. Comparatively, redox reactions took place when halides of heavier elements such as germanium dichloride, dichlorophenylphosphine, and chlorodiphenylbismuth were employed as substrates, from which cationic species <b>7</b> bearing a B–Cl bond was obtained. In addition, reactions of metal complexes [<b>2</b>, Cr­(<b>1</b>)­(CO)₅; <b>8</b>, Fe­(<b>1</b>)­(CO)₄] with cationic electrophiles were investigated. With HOTf and FN­(SO₂Ph)₂, the corresponding ionic species featuring a B–H bond (<b>9</b>) and a B–F bond (<b>10</b>) were formed via a formal electrophilic substitution reaction, whereas the reaction of <b>1</b> with F·Py-BF₄ resulted in the formation of a dicationic boron species <b>11</b> with a newly formed B–N bond