C-C and C-N Bond Activation, Lewis-Base Coordination and One- and Two-Electron Oxidation at a Linear Aminoborylene

Formation and Metallomimetic Reactivity of a Transient Dicoordinate Alkylborylene

While existing literature has primarily focused on carbene-stabilized amino- and arylborylenes of the form [(carbene)BR] (R = substituent), herein we report the generation and metallomimetic reactivity of the first carbene-stabilized alkylborylene [(CAACMe)BCy] (CAACMe = 1-(2,6-diisopropylphenyl)-3,3,5,5-tetramethylpyrrolidin-2-ylidene, Cy = cyclohexyl). Furthermore, the transition metal-like decarbonylation reactions of a borylene complex, [(CAACMe)BCy(CO)], derived from borylene [(CAACMe)BCy] and CO, are described. Additional findings described include i) the identification of the coordination stages of the ligand to boron towards forming complexes [(CAACMe)BCyL] in the reduction route from starting material [(CAACMe)BCyBr2] and in the photolysis route from carbonyl complex [(CAACMe)BCy(CO], and ii) insights from quantum-chemical computations regarding the molecular and electronic structure of the borylene at various stages.

Attempts toward a Silyl-Stabilized Dicoordinate Borylene: Insertion of Carbon Dioxide into the B-Si Bond

One-electron reduction of the carbene-stabilized borane (Me2-cAAC)B(Cl)2Si(SiMe3)3, 1, with potassium naphthalenide gave the radical (Me2-cAAC)B(Cl)Si(SiMe3)3, 2. A subsequent one-electron reduction of 2 yielded the dicoordinate borylene (Me2-cAAC)BSi(SiMe3)3, 3, which rapidly underwent intramolecular C-H activation to give cyclo-(Me2-cAAC)B(H)Si(SiMe3)3, 4, irrespective of the employed reaction conditions. Compound 3 could be stabilized as the carbonyl complex (Me2-cAAC)B(CO)Si(SiMe3)3, 5, that gave 4 upon irradiation with a UV light under a CO2 atmosphere. In contrast, the two-electron reduction of 1 under an atmosphere of CO2 yielded a mixture of products of which (Me2-cAAC)B(Cl)(H)C(O)OSi(SiMe3)3, 6, could be separated and structurally characterized. Compound 6 is a rare example of CO2 insertion into a B-E (E = heavier main group element) bond in which boron functions as a nucleophile, thereby mimicking transition metal-mediated carboxylation. The mechanism for the formation of 6 from the purported boryl anion intermediate [(Me2-cAAC)B(Cl)Si(SiMe3)3]-, 2 -, was analyzed computationally.

Harnessing transient CAAC-stabilized mesitylborylenes for chalcogen activation

Newly synthesized adducts of CAAC-bound mesitylborylene with carbon monoxide (CO) and trimethylphosphine (PMe3) are established as efficient precursors for the in situ generation of the dicoordinate borylene [(CAAC)BMes] (CAAC = cyclic(alkyl)(amino)carbene), as demonstrated by their ability to activate elemental chalcogens. Upon thermal or photolytic activation, these precursors readily react with sulfur and selenium, yielding boron chalcogenides characterized by terminal boron-chalcogen double bonds. In contrast, the reaction with tellurium leads to the formation of an unusual diradical ditelluride species with a Te-Te bond. Quantum chemical calculations of its electronic structure indicate an open-shell singlet ground state characterized by significant diradical character. Further investigations into the redox behavior of these boron chalcogenides reveal intriguing transformations, including the redox-induced formation and cleavage of E-E bonds.

Binding of base-stabilized borylenes with transition metals and formation of metal only Lewis pairs

Computational investigations employing Density Functional Theory (SMD(benzene)-M06-2X-D3/6-311+G*) predict that stable metal-free mono (Lewis base)-stabilized borylenes could strongly bind with transition metal (Fe and Ni) complexes. The binding results in increase in the Lewis basicity of the metal centers thus facilitating the formation of metal only Lewis pairs (MOLPs) of the form [L(CO)4Fe → GaCl3] and [L(CO)3Ni → GaCl3] (L = electron donating ligands). Further, the binding of different ligands with the metals as well as bonding in the MOLPs have been further investigated with the help of QTAIM and EDA-NOCV analyses.

Intermolecular 1,2-Aminoboration of Alkynes and the Critical Role of Electron-Rich Alkynes

The intramolecular 1,2-aminoboration of alkynes by aminoboranes is rare and invariably requires a catalyst to proceed, while the intermolecular aminoboration of alkynes is yet entirely unknown. Through an exploration of the significance of electronics in alkynes for activating the B-N σ-bond of aminoboranes, we demonstrate in this work the first intermolecular 1,2-aminoboration of alkynes. These reactions employ a series of (amino)dihaloboranes and aminoboronic esters, mild reaction conditions, and no catalysts, yielding syn-addition alkene products with the incorporation of two crucial functionalities: amino and boryl. While highly electron-rich examples can afford the aminoborated products (Z)-2-borylethenamines, other alkynes, including unactivated and less electron-rich examples, do not lead to the corresponding aminoborated products due to the fundamental impediment that the reactions are significantly endergonic.

Transition-metal-like coordination chemistry of dicoordinate borylenes with organic azides

The photolytic or oxidative liberation of a cyclic (amino)(alkyl)carbene (CAAC)-stabilized arylborylene in the presence of organoazides yielded borylene-organoazide complexes (4a,b) has been achieved in a manner akin to the first step of the Staudinger reaction. Similarly, a CAAC-stabilized aminoborylene also afforded borylene-organoazide complexes (6a-c), which further undergo rearrangement to produce aminoborane triazene species (7a,b).

Small molecule activation by sila/germa boryne species

The inability of p-block elements to participate in π-backbonding restricts them from activating small molecules like CO, H2 , and so forth. However, the development of the main group metallomimetics became a new pathway, where the main-group elements like boron can bind and activate small molecules like CO and H2 . The concept of the frustrated Lewis pair, Boron-Boron multiple bonds, and borylene are previously illustrated. Some of these reported classes of boron species can mimic the jobs of the metal complexes. Hence, we have theoretically studied the binding of CO/N2 molecules at B-center of elusive species like sila/germa boryne stabilized by donor base ligands (cAAC)BE(Me)(L), where E  Si, L  cAACMe , NHCMe , PMe3 , E  Ge, L  cAACMe and (NHCMe )BE(Me)(cAACMe )). The substitutional analogues of (cAACR )BSiR1 (cAAC) and E  P, L  cAACMe ) have been studied by density functional theory (DFT), natural bond orbital, QTAIM calculations and energy decomposition analysis (EDA) coupled with natural orbital for chemical valence (NOCV) analyses. The computed bond dissociation energy and inner stability analyses by the EDA-NOCV method showed that the CO molecule can bind at the B-center of the above-mentioned species due to stronger σ-donor ability while binding of N2 has been theoretically predicted to be weak. The energy barrier for the CO binding is estimated to be 13-14 kcal/mol by transition state calculation. The change of partial triple bond character to single bond nature of the BSi bond and the bending of CBSi bond angle of sila-boryne species are the reason for the activation energy. Our study reveals the ability of such species to bind and activate the CO molecule to mimic the transition metal-containing complexes. We have additionally shown that binding of Fe(CO)4 and Ni(CO)3 is feasible at Si-center after binding of CO at the B-center.

Cyclic (Alkyl)(Amino)Carbene-Iminoboryl Compounds with Three Formal Oxidation States

BN/CC isosterism is an effective strategy to build hybrid functional molecules with unique properties. In contrast to the alkynyl iminium salts derived from cyclic (alkyl)(amino)carbenes (CAACs) that feature only one reversible reduction wave, the isoelectronic cationic CAAC-iminoboryl adducts could be singly and doubly reduced smoothly. Both the resultant neutral radical and anionic azaborataallenes bear NBC-mixed allenic structures. The former radical has a high spin-density of 0.55e at CCAAC carbon, yet exhibits formal boron-centered radical reactivity. The latter azaborataallenes feature the nucleophilic CCAAC center and polar N(δ-)═B(δ+)═C(δ-) unit, and readily undergo nucleophilic substitution, isocyanide insertion, dipolar addition and cycloaddition reactions etc. The N-substituents have been shown to have a significant influence on the solid-state structure, thermal stability, and reactivity of azaborataallenes. This work showcases the allenic BN-unsaturated species as versatile building blocks in organic synthesis.

Isolation of (Aryl)-(Imino) Phosphide and (Aryl)-(Phosphaalkene) Amide Complexes of Alkali Metals from Carbene-Phosphinidenes under Reductive-Thermal Rearrangements

Two-electron reduction of cyclic alkyl(amino) carbene (cAAC)-supported chloro-phosphinidene cAAC=P-Cl (1) followed by unprecedented thermal rearrangements afforded the alkali metal complexes of (aryl)-(cyclic alkyl(imino)) phosphides 3 a-3 c, 4 a-4 b through migration of the 2,6-diisopropylphenyl (dipp) group from N to the P centre, and the (aryl)-(cyclic alkyl(phosphaalkene)) amide 5 through cleavage of the CMe2 -N bond followed by energetically favoured 5-exo-tet ring-closure in the presence of the alkali metals Cs (3 a-3 c), K (4 a, 4 b), and Li (5). Compound 3 a was found to be photoluminescent (PL), emitting bright orange light under a laboratory UV lamp of wavelength 365 nm with PL quantum yield (ϕPL ) of 2.6 % (λem =600 nm), and an average lifetime (τ) of 4.8 μs. Reaction of 3 a with CuCl and AgOTf afforded (aryl)-(cyclic alkyl(imino)) phosphide-stabilized tetra-nuclear CuI (6), and octa-nuclear AgI (7) clusters, respectively. Moreover, complexes 3 a-3 c provided a direct route for the stabilization of cyclic alkyl(aminoboryl) phosphaalkenes 8 a-8 c when treated with 1-bromo-N,N,N',N'-tetraisopropylboranediamine.

Experimental and Computational Study of a Confirmed Borylene-to-Diborene Dimerization

While the dimerization of heavier group 13 carbene analogues to the corresponding alkene analogues is known and relatively well understood, the dimerization of dicoordinate borylenes (LRB:, L = neutral donor; R = anionic substituent) to the corresponding diborenes (LRB═BRL) has never been directly observed. In this study we present the first example of a formal borylene-to-diborene dimerization through abstraction of a labile phosphine ligand from the tricoordinate hydroborylene precursor (CAAC)(Me3P)BH (CAAC = cyclic alkyl(amino)carbene) by bulky Lewis-acidic dihaloboranes (BX2Y, X = Cl, Br, Y = aryl, boryl), generating the corresponding dihydrodiborene (CAAC)HB═BH(CAAC) and (Me3P)BX2Y as the byproduct. An in-depth experimental and computational mechanistic analysis shows that this seemingly simple process (2 LL'BH + 2 BX2Y → LHB═BHL + 2 L'BX2Y) is in fact based on a complex sequence of finely tuned processes, involving the one-electron oxidation of and PMe3 abstraction from the borylene precursor by BX2Y, multiple halide transfers between (di)boron intermediates and BX2Y/[BX3Y]-, and multiple one-electron redox processes between diboron intermediates and the borylene precursor, which make the reaction ultimately autocatalytic in [(CAAC)(Me3P)BH]•+. The findings suggest that [LBXR]• boryl radicals are more likely coupling partners than dicoordinate LRB: borylenes in the reductive coupling of base-stabilized LBX2R boranes to LRB═BRL diborenes.