Synthesis of Silaketenimine Anion and Its Coupling with Isocyanide

A Masked Boryl-Substituted Oxo-Bridged Bis-Silylene: Synthesis and Reductive-Elimination and Synergistic Oxidative-Addition Reactivity

Controlled oxidation of NHB-stabilized disilyne (NHB)Si ≡ Si(NHB) (1, NHB = [ArN(CMe)2NAr]B, Ar = 2,6-iPr2C6H3) with one equivalent of trimethylamine N-oxide (Me3N+─O-) in dry n-hexane gave oxo-bridged bis-silepin 2 in high yields. DFT calculations disclosed that silepin 2 is only more stable by 13.4 kcal/mol than the corresponding oxo-bridged bis-silylene intermediate 2' (NHB)Si(μ-O)Si(NHB), and 2 was very likely to be formed by the insertion of the two divalent Si atoms into the pendant aryl rings in bis-silylene intermediate 2'. The two silicon atoms in bis-silepin 2 could undergo formal reductive-elimination of the aryl rings and sequential oxidative-insertion reactions with small molecules and organic substrates. Treatment of 2 with H2O, S8, and P4 at 60 °C yielded compounds 3-5 via reductive-elimination of the aryl rings, followed by the sequential oxidative-addition of these molecules at the two Si(II) centers. Similarly, reactions of 2 with PhSiH3, a diphenylalkyne, pyridines, 1,3,4,5-tetramethylimidazolin-2-ylidene (IMe4), Ph2CO, and thiophene yielded the corresponding polycyclic bis-silanes 6-12 via reductive-elimination and oxidative-addition of C-H, Si-H, C≡C, and aromatic C═C, C-S, and C═N bonds at the two Si atoms. These novel reactions indicated the pronounced bis-silylene reactivity of bis-silepin 2, consistent with the low-energy barrier for the interconversion between 2 and 2', as disclosed by DFT calculations.

Isolable T-Shaped Planar Silyl Anion

Silyl anions have garnered significant attention due to their synthetic abilities. However, previously reported silyl anions have been limited to either trigonal-pyramidal or trigonal-planar geometries, which confine them primarily as nucleophiles in substitution reactions. Herein, we report the isolation of the unprecedented T-shaped planar silyl anion salt 2 by employment of a geometrically constrained triamido pincer ligand. Theoretical calculations disclosed that the silicon centre in 2 possesses both a lone pair of electrons and an empty 3pz orbital. In addition to nucleophilic substitution reactions with Ph3PAuCl and W(CO)6, 2 readily undergoes oxidative additions with CO2 and 2,6-dimethylphenylisonitrile at room temperature. Furthermore, under mild conditions, compound 2 cleaves Csp2-H, Csp2-H, and H-H bonds in 1,2,4,5-tetrafluorobenzene, an intramolecular iPr group, and dihydrogen, representing the first examples of C-H and H-H activations mediated by a silyl anion, respectively. This work unveils new reactivity of silyl anions owing to the non-classical geometry and electronic structure.

Yb/Si frustrated Lewis pairs with a labile naphthalenyl bridge

The first examples of RE/Si FLPs (RE: rare-earth metal, FLPs: frustrated Lewis pairs), namely Yb/Si FLPs were synthesized, where Yb⋯Si distances are in the range of 3.55 to 3.72 Å. These FLPs react with triphenylphosphine sulfide and aryl isocyanide to produce novel silylyne group transfer products through dissociation of naphthalene.

Carbene-Stabilized Phosphagermylenylidene: A Heavier Analog of Isonitrile

Phosphagermylenylidenes (R-P═Ge), as heavier analogs of isonitriles, whether in their free state or as complexes with a Lewis base, have not been previously identified as isolable entities. In this study, we report the synthesis of a stable monomeric phosphagermylenylidene within the coordination sphere of a Lewis base under ambient conditions. This species was synthesized by Lewis base-induced dedimerization of a cyclic phosphagermylenylidene dimer or via Me3SiCl elimination from a phosphinochlorogermylene framework. The deliberate integration of a bulky, electropositive N-heterocyclic boryl group at the phosphorus site, combined with coordination stabilization by a cyclic (alkyl)(amino)carbene at the low-valent germanium site, effectively mitigated its natural tendency toward oligomerization. Structural analyses and theoretical calculations have demonstrated that this unprecedented species features a P═Ge double bond, characterized by conventional electron-sharing π and σ bonds, complemented by lone pairs at both the phosphorus and germanium atoms. Preliminary reactivity studies show that this base-stabilized phosphagermylenylidene demonstrates facile release of ligands at the Ge atom, coordination to silver through the lone pair on P, and versatile reactivity including both (cyclo)addition and cleavage of the P═Ge double bond.

Recent advances in the chemistry of isolable carbene analogues with group 13-15 elements

Carbenes (R2C:), compounds with a divalent carbon atom containing only six valence shell electrons, have evolved into a broader class with the replacement of the carbene carbon or the RC moiety with main group elements, leading to the creation of main group carbene analogues. These analogues, mirroring the electronic structure of carbenes (a lone pair of electrons and an empty orbital), demonstrate unique reactivity. Over the last three decades, this area has seen substantial advancements, paralleling the innovations in carbene chemistry. Recent studies have revealed a spectrum of unique carbene analogues, such as monocoordinate aluminylenes, nitrenes, and bismuthinidenes, notable for their extraordinary properties and diverse reactivity, offering promising applications in small molecule activation. This review delves into the isolable main group carbene analogues that are in the forefront from 2010 and beyond, spanning elements from group 13 (B, Al, Ga, In, and Tl), group 14 (Si, Ge, Sn, and Pb) and group 15 (N, P, As, Sb, and Bi). Specifically, this review focuses on the potential amphiphilic species that possess both lone pairs of electrons and vacant orbitals. We detail their comprehensive synthesis and stabilization strategies, outlining the reactivity arising from their distinct structural characteristics.

Palladium-catalyzed ligand-regulated divergent synthesis of pyrrole[2,3-b]indoles and ureas from 2-ethynylanilines and isocyanides

Herein, a palladium-catalyzed and ligand-controlled protocol for the divergent synthesis of pyrrole[2,3-b]indole and urea derivatives has been described. Pyrrole[2,3-b]indoles ("cyclization on" products) via tandem cyclization of o-alkynylanilines with isocyanides in the absence of a ligand and ureas ("cyclization off" products) via oxidative amination of anilines with isocyanides in the presence of a ligand were obtained both in moderate to good yields with high selectivity. In this chemistry, cyclic and acyclic products were easily accessed with the same starting materials under the regulation of the ligand.

Boryl-substituted low-valent heavy group 14 compounds

Low valent group 14 compounds exhibit diverse structures and reactivities. The employment of diazaborolyl anions (NHB anions), isoelectronic analogues to N-heterocyclic carbenes (NHCs), in group 14 chemistry leads to the exceptional structures and reactivity. The unique combination of σ-electron donation and pronounced steric hindrance impart distinct structural characteristics to the NHB-substituted low valent group 14 compounds. Notably, the modulation of the HOMO-LUMO gap in these compounds with the diazaborolyl substituents results in novel reaction patterns in the activation of small molecules and inert chemical bonds. This review mainly summarizes the recent advances in NHB-substituted low-valent heavy Group 14 compounds, emphasizing their synthesis, structural characteristics and application to small molecule activation.

Homocoupling of Isocyanide at the Si(II) Center of Borylaminoamidinatosilylene

Two borylaminoamidinatosilylenes (L)[(1,5-C₈H₁₄)B(Ar)N]Si (L = PhC(NtBu)₂, Ar = 2,6-iPr₂C₆H₃ (1)) and (L)[(1,5-C₈H₁₄)B(Ar')N]Si (Ar' = 2,4,6-Me₃C₆H₂ (2)) have been prepared and utilized to investigate the reaction toward isocyanide. Reactions of 1 with the respective CN-2,6-Me₂C₆H₃ and CNCy (Cy = cyclo-C₆H₁₁) produced compounds (L)Si(NAr)C(N-2,6-Me₂C₆H₃)B(1,5-C₈H₁₄)(CN-2,6-Me₂C₆H₃) (3) and (L)Si(NAr)C(NCy)C(NCy)B(1,5-C₈H₁₄)(CNCy) (4). Reactions of 2 with the respective CNCy and CN-2,6-Me₂C₆H₃ yielded compounds cyclo-(L)SiN(Ar')C(NCy)B(1,5-C₈H₁₄)C(NCy) (5) and cyclo-(L)[(1,5-C₈H₁₄)B(Ar')N]SiC(CN-2,6-Me₂C₆H₃)N(2,6-Me₂C₆H₃)C(N-2,6-Me₂C₆H₃) (6). Compounds 3-6 have different compositions and structures from each other. Density functional theory (DFT) calculations suggest initial formation of (L)[(1,5-C₈H₁₄)B(←:CN-2,6-Me₂C₆H₃)(Ar)N]Si (A), (L)[(1,5-C₈H₁₄)B(←:CNCy)(Ar)N]Si (A'), (L)[(1,5-C₈H₁₄)B(←:CNCy)-(Ar')N]Si (A″), and (L)[(1,5-C₈H₁₄)B(←:CN-2,6-Me₂C₆H₃)(Ar')N]Si (A‴) as the respective intermediates. The as-followed transition states TS, TS1', TS1″, and TS‴ all feature probable Si:→C(═N):→B bonding with different Gibbs energies of 7.24, 2.46, 3.86, and 6.59 kcal/mol, respectively, due to variation among the Ar, Ar', 2,6-Me₂C₆H₃, and Cy groups in these species, and reacted in different ways.

Isolation of a planar 1,2-dilithio-disilene and its conversion to a Si-B hybrid 2π-electron system and a planar tetraboryldisilene

Lithium reagents have long played important roles in synthetic chemistry. However, unsaturated organosilicon lithium reagents are few in number. Herein, we describe the first isolation of a 1,2-dilithiodisilene: [(boryl)SiLi]₂ (2) was prepared in 73% yield by the reduction of (boryl)tribromosilane (1, boryl = (HCArN)₂B, Ar = 2,6-iPr₂C₆H₃) with lithium in Et₂O. The salt elimination reaction of 2 with dihaloboranes RBX₂ afforded disilaborirenes [(boryl)Si]₂BR (3a–c), whereas the reaction with two equivalents of B-bromocatecholborane ((cat)BBr) yielded the first tetraboryldisilene [(boryl)(cat)BSi]₂ (4). X-ray diffraction analysis and density functional theory calculations indicated that the disilene 2 and tetraboryldisilene 4 feature an almost planar geometry and disilaborirenes 3a–c are aromatic with a silicon–boron hybrid 2π-electron delocalized structure. The results indicate that 1,2-dilithiodisilene 2 is a powerful synthetic reagent for the construction of novel silicon multiply bonded species with unique electronic structures and that the boryl substituents have significant electronic effects on the structure of silicon multiple bonding.

Dianionic disilyne: reduction of boryltribromosilane yielded the 1,2-dilithio-disilene 2, which is a powerful transfer reagent for the synthesis of a novel 2π aromatic system and the first tetraboryldisilene.

(NHC)Si═C═N-R: A Two-Coordinated Si0-Isocyanide Compound as Si(NHC) Transfer Reagent

Experimental and theoretical studies are reported of the first two-coordinated Si⁰-isocyanide compound (SIDipp)Si═C═N-Ar^Mes (1: SIDipp (NHC) = C[N(Dipp)CH₂]₂, Ar^Mes = 2,6-dimesitylphenyl), supported by an N-heterocyclic carbene (NHC). A Si atom economic two-step synthesis of 1 involves a 2e reduction of the isocyanide-stabilized silyliumylidene salt [SiBr(CNAr^Mes)(SIDipp)][B(Ar^F)₄] (2[B(Ar^F)₄], Ar^F = B(C₆H₃-3,5-(CF₃)₂)₄) with KC₈. 2[B(Ar^F)₄] was obtained from SiBr₂(SIDipp) after bromide abstraction with an equimolar mixture of Na[B(Ar^F)₄] and Ar^MesNC. Exact adherence to the stoichiometry is crucial in the latter reaction, since 2[B(Ar^F)₄] reacts with SiBr₂(SIDipp) via isocyanide exchange to afford the disilicon(II) salt [Si₂Br₃(SIDipp)₂)][B(Ar^F)₄] (3[B(Ar^F)₄]), the reaction leading to an equilibrium that favors 3[B(Ar^F)₄] (K_eq(298 K) = 10.6, ΔH° = -10.6 kJ mol^-1; ΔS^° = -16.0 J mol^-1 K^-1). 3[B(Ar^F)₄] was obtained selectively from the 2:1 reaction of SiBr₂(SIDipp) with Na[B(Ar^F)₄] and fully characterized. Detailed studies of 1 reveal an intriguing structure featuring a planar C_NHC-Si-C-N skeleton with a V-shaped geometry at the dicoordinated Si⁰ center, a slightly bent Si═C═N core, a C_NHC-Si-C_CNR 3c-2e out of plane π-bond (HOMO), and an anticlinal conformation of the SIDipp and Ar^Mes substituents leading to axial chirality and the presence of two enantiomers, (R_a)-1 and (S_a)-1. Compound 1 displays structural dynamics in solution, rapidly interconverting the enantiomers. The silacumulene 1 is a potent Si(SIDipp) transfer agent as demonstrated by the synthesis and full characterization of the NHC-supported germasilyne (Z)-(SIDipp)(Cl)Si═GeAr^Mes (4) from 1 and Ge(Ar^Mes)Cl.

Isolation of a 1-Magnesium-2,3-disilacyclopropene and a Related Bis(disilenide)

Herein we report the first synthesis of a 1-magnesium-2,3-disilacyclopropene and a related bis(disilenide). Reduction of (boryl)tribromosilane (1, boryl = (HCArN)₂B, Ar = 2,6-iPr₂C₆H₃) with magnesium in THF afforded boryl-substituted magnesium complex [(boryl)Si]₂Mg(THF)₃ (2) in good yield, whereas reduction of (boryl)trichlorosilane (3) with KC₈ in THF led to the isolation of bridged alkoxy alkyl bis(disilenide) (THF)K(boryl)Si═Si(boryl)O(CH₂)₄(boryl)Si═Si(boryl)K(THF) (4) via ring opening of a THF molecule. X-ray diffraction analysis of 2 confirmed the presence of the novel Si₂Mg three-membered ring as well as the Si═Si double bond, which existed in a noticeably twisted B-Si-Si-B array. Complex 2 also represents the first reported example of a stable disilyne dianion.

[3+1+1] type cyclization of ClCF2COONa for the assembly of imidazoles and tetrazoles via in situ generated isocyanides

A facile synthesis of imidazoles and tetrazoles via [3+1+1] type cyclization of ClCF₂COONa is developed.