Synthesis and reactivity of N-heterocyclic carbene (NHC)-supported heavier nitrile ylides.

The synthesis and isolation of stable heavier analogues of nitrile ylide as N-heterocyclic carbene (NHC) adducts of phosphasilenyl-tetrylene [(NHC)(TerAr)Si(H)PE14(TerAr)] (E14 = Ge 1, Sn 2; TerAr = 2,6-Mes2C6H3, NHC = IMe4) are reported. The delocalized Si-P-E14 π-conjugation was examined experimentally and computationally. Interestingly, the germanium derivative 1 exhibits a 1,3-dipolar nature, leading to an unprecedented [3 + 2] cycloaddition with benzaldehyde, resulting in unique heterocycles containing four heteroatoms from group 14, 15, and 16. Further exploiting the nucleophilicity of germanium, activation of the P-P bond of P4 was achieved, leading to a [(NHC)(phosphasilenyl germapolyphide)] complex. Moreover, the [3 + 2] cycloaddition and the σ-bond activation by 1 resemble the characteristics of the classic nitrile ylide.

Mechanistic Avenues in the Chan-Lam-Based Etherification Reaction: A Computational Exploration.

Invited for the cover of this issue is the group of Koley and co-workers at the Indian Institute of Science Education and Research (IISER) Kolkata. The image depicts the industrial generation of the product from the available starting materials in a catalytic cycle. Read the full text of the article at 10.1002/chem.202302983.

Mechanistic Avenues in the Chan-Lam-Based Etherification Reaction: A Computational Exploration.

Ongoing advances in CuII -catalyzed aerobic oxidative coupling reactions between arylboronic esters and diverse heteroatom nucleophiles have strengthened the development of the general Chan-Lam (CL)-based reaction protocol, including C-O bond formation methodologies. In-depth mechanistic understanding of CL etherification with specific emphasis on different reaction routes and their energetics are still lacking, even though the reaction has been experimentally explored. Here, we present a DFT-guided computational study to unravel the mechanistic pathways of CL-based etherification. The computational findings provide some interesting insights into the fundamental steps of the catalytic cycle, particularly the rate-determining transmetalation event. An aryl boronic ester-coordinated, methoxide-bridged CuII intermediate that acts as resting state undergoes transmetalation with an activation barrier of 20.4 kcal mol-1 . The energy spans of the remaining fundamental steps leading to the methoxylated product are relatively low. The minor p-cresol product requires an additional 14.2 kcal mol-1 energy span to surmount in comparison to the favored route. Hammett studies for the substituted aryl boronic esters reveal higher reaction turnovers for electron-rich aryl systems. The results agree with previously reported spectroscopic and kinetic observations. For a series of alcohol substrates, it was observed that, except for cyclohexanol, moderate to high etherification turnovers are predicted.

Palladium-Catalyzed para-C-H Arylation of Anilines with Aromatic Halides.

Controlling regioselectivity in C-H functionalizations is a key challenge in chemical method development. In arenes, functionalizations are most difficult to direct towards the C-H group furthest away from a substituent, in its para position. We herein demonstrate how the para-C-H arylation of anilines with non-activated aryl halides, elusive to date, is achieved by a base-assisted "metalla-tautomerism" approach. A proton is abstracted from the aniline substrate and replaced by an arylpalladium species, generated from the aryl halide coupling partner. In this step, the palladium is directed away from the N- to the tautomeric para-C-H position by a large phosphine ligand combined with a triphenylmethyl shielding group. The triphenylmethyl group is easily installed and removed, and can be recycled.

Monomeric Magnesium Catalyzed Alkene and Alkyne Hydroboration.

In this work, two monomeric magnesium alkyl complexes (1 and 2) were prepared using bis(phosphino)carbazole framework and among them 1 has been used as a catalyst for hydroboration of alkenes and alkynes with pinacolborane (HBpin). A broad variety of aromatic and aliphatic alkenes and alkynes were efficiently reduced. Anti-Markovnikov regioselective hydroboration of alkenes and alkynes was achieved, which was confirmed by deuterium-labelling experiments. The work represents the first example of the use of magnesium in homogeneous catalytic hydroboration of alkene with broad substrate scope. Experimental mechanistic investigations and DFT calculations provided insights into the reaction mechanism. Finally, the hydroboration protocol was extended to terpenes.

Organoaluminum hydrides catalyzed hydroboration of carbonates, esters, carboxylic acids, and carbon dioxide.

The reductive functionalization of the CO unit of carbonates, carboxylic acids, esters, and CO2, respectively has received great attention since its introduction. This method is often used industrially for the synthesis of high value-added energy products in chemistry. This opens up a new way forward to reduce greenhouse gases and the consumption of traditional energy sources. Herein, we report an earth-abundant, cheap, and readily available aluminum dihydride, which can catalyze the reduction of a range of carbonates, esters, carboxylic acids, and CO2, respectively in the presence of pinacolborane as a reducing agent. Moreover, we demonstrate that the reaction can proceed to obtain good yield products under mild conditions, with low catalyst loading and solvent-free reactions. The mechanism of the catalytic reduction of carbonates has been investigated.

Computational Exploration of Mechanistic Avenues in Metal-Free CO2 Reduction to CO by Disilyne Bisphosphine Adduct and Phosphonium Silaylide.

Recent years have seen a growing interest in metal-free CO2 activation by silylenes, silylones, and silanones. However, compared to mononuclear silicon species, CO2 reduction mediated by dinuclear silicon compounds, especially disilynes, has been less explored. We have carried out extensive computational investigations to explore the mechanistic avenues in CO2 reduction to CO by donor-stabilized disilyne bisphosphine adduct (R1M ) and phosphonium silaylide (R2) using density functional theory calculations. Theoretical calculations suggest that R1M exhibits donor-stabilized bis(silylene) bonding features with unusual Si-Si multiple bonding. Various modes of CO2 coordination to R1M have been investigated and the coordination of CO2 by the carbon center to R1M is found to be kinetically more facile than that by oxygen involving only one or both the silicon centers. Both the theoretically predicted reaction mechanisms of R1M and R2-mediated CO2 reduction reveal the crucial role of silicon-centered lone pairs in CO2 activations and generation of key intermediates possessing enormous strain in the Si-C-O ring, which plays the pivotal role in CO extrusion.

Isomerization of Functionalized Olefins by Using the Dinuclear Catalyst [PdI (µ-Br)(Pt Bu3 )]2 : A Mechanistic Study.

In a combined experimental and computational study, the isomerization activity of the dinuclear palladium(I) complex [PdI (µ-Br)(Pt Bu3 )]2 towards allyl arenes, esters, amides, ethers, and alcohols has been investigated. The calculated energy profiles for catalyst activation for two alternative dinuclear and mononuclear catalytic cycles, and for catalyst deactivation are in good agreement with the experimental results. Comparison of experimentally observed E/Z ratios at incomplete conversion with calculated kinetic selectivities revealed that a substantial amount of product must form via the dinuclear pathway, in which the isomerization is promoted cooperatively by two palladium centers. The dissociation barrier towards mononuclear Pd species is relatively high, and once the catalyst enters the energetically more favorable mononuclear pathway, only a low barrier has to be overcome towards irreversible deactivation.

Germyliumylidene: A Versatile Low Valent Group 14 Catalyst.

Bis-NHC stabilized germyliumylidenes [RGe(NHC)2 ]+ are typically Lewis basic (LB) in nature, owing to their lone pair and coordination of two NHCs to the vacant p-orbitals of the germanium center. However, they can also show Lewis acidity (LA) via Ge-CNHC σ* orbital. Utilizing this unique electronic feature, we report the first example of bis-NHC-stabilized germyliumylidene [Mes TerGe(NHC)2 ]Cl (1), (Mes Ter=2,6-(2,4,6-Me3 C6 H2 )2 C6 H3 ; NHC= IMe4 =1,3,4,5-tetramethylimidazol-2-ylidene) catalyzed reduction of CO2 with amines and arylsilane, which proceeds via its Lewis basic nature. In contrast, the Lewis acid nature of 1 is utilized in the catalyzed hydroboration and cyanosilylation of carbonyls, thus highlighting the versatile ambiphilic nature of bis-NHC stabilized germyliumylidenes.

Si(II) Cation-Promoted Formation of an Abnormal NHC-Bound Silylene and a cAAC-Silanyl Radical Ion.

The reaction of amidinatosilylene LSi(:)Cl [L = PhC(NtBu)2] with N-heterocyclic carbene IAr [:C{N(Ar)CH}2, where Ar = 2,6-iPr2C6H3] and NaOTf in tetrahydrofuran (THF) facilely afforded a silicon(II) cation [LSi(:)-aIAr]+OTf- (1+OTf-), where IAr isomerizes to abnormal N-heterocyclic carbene aIAr, coordinating to the silicon(II) center. Its Ge homologue, [LGe(:)-aIAr]+OTf- (2+OTf-), was also accessed via the same protocol. For the formation of 1+, we propose that an in situ-generated Si(II) cation [LSi(:)]+ under the treatment of LSi(:)Cl with NaOTf may isomerize IAr in THF. In contrast, the replacement of IAr with cyclic alkyl(amino) carbene (cAAC) furnished a cAAC-silanyl radical ion [LSi(H)-cAAC]•+(LiOTf2)- [3•+(LiOTf2)-], which may undergo an abstraction of the H radical from THF. All of the products were characterized by nuclear magnetic resonance spectroscopy, electron paramagnetic resonance, and X-ray crystallography, and their bonding scenarios were investigated by density functional theory calculations. These studies provide new perspective on carbene-silicon chemistry.

Yttrium germole dianion complexes with Y-Ge bonds.

The reactions of dipotassium 3,4-dimethyl-2,5-bis(trimethylsilyl)-germole dianion K2[1] with YCl3 and Cp*YCl2 (Cp* = cyclopentadienyl) in THF at room temperature afforded the dianion salt [(K-cryptand-222)2][1-YCl3] (K2[2]) and the dimeric complex [1-Y-Cp*]2 (3), respectively. While the polymeric complex {[(1)2-Y-K(toluene)]2}n (4) was obtained from the reaction of K2[1] and half molar equivalent of YCl3(THF)3.5 in toluene at 80 °C. The germole dianions in complexes 3 and 4 feature η5/η1 coordination interactions with the yttrium atoms. They represent the first examples of rare earth (RE) complexes containing RE-Ge bonds other than the RE-GeR3 structural type.

Computational Investigation of Multifaceted Cationic Rearrangement and Stereo- and Regioselectivity in the Formation of Dysideanone's Analogues.

Mechanistic studies of regiodivergent arylations of cycloalkanols to furnish enantioenriched dysideanone's analogues are performed by employing density functional theory (DFT) calculations (B3LYP-D3(SMD)/6-311++G**//B3LYP-D3/6-31+G** level of theory). On the basis of our calculations, remote γ'-C-H arylation is preferred for unsubstituted carbinol 1, an outcome from combined factors like carbocationic stability, less steric hindrance during C-C coupling, and facile dearomatization. Meanwhile, in the presence of dimethyl substituent 1Me, regioselective γ-arylation is favored by 3.4 kcal/mol, and both findings are in agreement with the reported experimental observations. Most importantly, we concur that the barrier associated with the formation of carbocation 6 and its substituted analogues correlates with the C-H arylation outcomes. Furthermore, the ß-arylation route remains unlikely for all the reaction pathways explored in this study.

Amidinate based indium(III) monohalides and ß-diketiminate stabilized In(II)-In(II) bond: synthesis, crystal structure, and computational study.

Synthesis and bonding aspects of mononuclear bis-amidinate indium(iii) monohalides L2InX (1-3), where L = PhC(NtBu)2; X is F (1), Br (2) or I (3) and ß-diketiminate (NacNac) stabilized In(ii) dimer (MesNacNac)2In2Br2 (4) with In-In bond are reported along with the single-crystal X-ray structures of 2-4.

Cyclic (Alkyl)(Amino)Carbene-Stabilized Aluminum and Gallium Radicals Based on Amidinate Scaffolds.

Neutral, mononuclear aluminum and gallium radicals, stabilized by cyclic (alkyl)(amino)carbene (cAAC), were synthesized. LMCl2 upon reduction with KC8 in the presence of cAAC afforded the radicals LMCl(cAAC), where L = PhC(NtBu)2 and M = Al (1), Ga (2). The radicals were characterized by X-ray crystallography, electron paramagnetic resonance (EPR) spectroscopy, and mass spectrometry. EPR, SQUID measurement, and computational calculations confirmed paramagnetism of the radicals with unpaired spin mainly on cAAC.

N-Heterocyclic Carbene-Stabilized Germa-acylium Ion: Reactivity and Utility in Catalytic CO2 Functionalizations.

The first acceptor-free heavier germanium analogue of an acylium ion, [RGe(O)(NHC)2]X (R = MesTer = 2,6-(2,4,6-Me3C6H2)2C6H3; NHC = IMe4 = 1,3,4,5-tetramethylimidazol-2-ylidene; X = (Cl or BArF = {(3,5-(CF3)2C6H5)4B}), was isolated by reacting [RGe(NHC)2]X with N2O. Conversion of the germa-acylium ion to the first solely donor-stabilized germanium ester [(NHC)RGe(O)(OSiPh3)] and corresponding heavier analogues ([RGe(S)(NHC)2]X and [RGe(Se)(NHC)2]X) demonstrated its classical acylium-like behavior. The polarized terminal GeO bond in the germa-acylium ion was utilized to activate CO2 and silane, with the former found to be an example of reversible activation of CO2, thus mimicking the behavior of transition metal oxides. Furthermore, its transition-metal-like nature is demonstrated as it was found to be an active catalyst in both CO2 hydrosilylation and reductive N-functionalization of amines using CO2 as the C1 source. Mechanistic studies were undertaken both experimentally and computationally, which revealed that the reaction proceeds via an N-heterocyclic carbene (NHC) siloxygermylene [(NHC)RGe(OSiHPh2)].

B(C6F5)3-catalyzed dehydrogenative cyclization of N-tosylhydrazones and anilines via a Lewis adduct: a combined experimental and computational investigation.

Tris(pentafluorophenyl)borane-catalyzed dehydrogenative-cyclization of N-tosylhydrazones with aromatic amines has been disclosed. This metal-free catalytic protocol is compatible with a range of functional groups to provide both symmetrical and unsymmetrical 3,4,5-triaryl-1,2,4-triazoles. Mechanistic experiments and density functional theory (DFT) studies suggest an initial Lewis adduct formation of N-tosylhydrazone with B(C6F5)3 followed by sequential intermolecular amination of the borane adduct with aniline, intramolecular cyclization and frustrated Lewis pair (FLP)-catalyzed dehydrogenation for the generation of substituted 1,2,4-triazoles.

Saturated N-Heterocyclic Carbene Based Thiele's Hydrocarbon with a Tetrafluorophenylene Linker.

The synthesis of a SIPr [1,3-bis(2,6-diisopropylphenyl)-imidazolin-2-ylidene] derived Kekulé diradicaloid with a tetrafluorophenylene spacer (3) has been described. Two synthetic routes have been reported to access 3. The cleavage of C-F bond of C6 F6 by SIPr in the presence of BF3 led to double C-F activated compound with two tetrafluoro borate counter anions (2), which upon reduction by lithium metal afforded 3. Alternatively, 3 can be directly accessed in one step by reacting SIPr with C6 F6 in presence of Mg metal. Compounds 2 and 3 were well characterized spectroscopically and by single-crystal X-ray diffraction studies. Experimental and computational studies support the cumulenic closed-shell singlet state of 3 with a singlet-triplet energy gap (ΔES-T ) of 23.7 kcal mol-1 .

(PhC(NtBu)2Al)2(SiH2)4 six-membered heterocycle: comparable in structure to cyclohexane.

A silicon-aluminum heterocycle LAl(SiH2SiH2)2AlL (L = PhC(NtBu)2) (1) was prepared. Compound 1 exhibits a unique (N2Al)2(SiH2)4 centrosymmetric six-membered ring structure with a chair conformation, which is comparable with that of cyclohexane. Furthermore, two intermediate analogues, silylene-alane adduct LSi(AlMe3)-Si(AlMe3)L (2) and silylene-alane oxidative product [LAlHSiH2Mes]2 (3) were obtained. Compound 3 has an interesting arrangement of an Al-H and an SiH2 unit, which are in close vicinity to each other. 3 might be important to function as a catalyst, due to the already activated bridging Al-H bonds.

Computational Exploration of Mechanistic Avenues in C-H Activation Assisted Pd-Catalyzed Carbonylative Coupling.

The detailed mechanism of the intermolecular Pd-catalyzed carbonylative coupling reaction between aryl bromides and polyfluoroarenes relying on C(sp2)-H activation was investigated using state-of-the-art computational methods (SMD-B3LYP-D3(BJ)/BS2//B3LYP-D3/BS1). The mechanism unveils the necessary and important roles of a slight excess of carbon monoxide: acting as a ligand in the active catalyst state, participating as a reactant in the carbonylation process, and accelerating the final reductive elimination event. Importantly, the desired carbonylative coupling route follows the rate-limiting C-H activation process via the concerted metalation-deprotonation pathway, which is slightly more feasible than the decarboxylative route leading to byproduct formation by 1.2 kcal/mol. The analyses of the free energies indicate that the choice of base has a significant effect on the reaction mechanism and its energetics. The Cs2CO3 base guides the reaction toward the coupling route, whereas carbonate bases such as K2CO3 and Na2CO3 switch toward an undesired decarboxylative path. However, K3PO4 significantly reduces the C-H activation barrier over the decarboxylation reaction barrier and can act as a potential alternative base. The positional influence of a methoxy substituent in bromoanisole and different substituent effects in polyfluoroarenes were also considered. Our results show that different substituents impose significant impact on the desired carbonylative product formation energetics. Considering the influence of several ligands leads to the conclusion that other phosphine and N-heterocyclic carbene, such as P nBuAd2 and IMes, can be used as an efficient alternative than the experimentally reported P tBu3 ligand exhibiting a clear preference for C-H activation (ΔΔ⧧ GLS) by 7.1 and 10.9 kcal/mol, respectively. We have also utilized the energetic span model to interpret the experimental results. Moreover, to elucidate the origin of activation barriers, energy decomposition analysis calculations were accomplished for the critical transition states populating the energy profiles.