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Pushing Chemical Boundaries with N‑Heterocyclic Olefins (NHOs): From Catalysis to Main Group Element Chemistry Matthew M. D. Roy and Eric Rivard* Department of Chemistry, University of Alberta, 11227 Saskatchewan Dr., Edmonton, Alberta Canada, T6G 2G2 S Supporting Information *

CONSPECTUS: N-Heterocyclic olefins (NHOs) have gone from the topic of a few scattered (but important) reports in the early 1990s to very recently being a ligand/reagent of choice in the far-reaching research fields of organocatalysis, olefin and heterocycle polymerization, and low oxidation state main group element chemistry. NHOs are formally derived by appending an alkylidene (CR2) unit onto an N-heterocyclic carbene (NHC), and their pronounced ylidic character leads to high nucleophilicity and soft Lewis basic character at the ligating carbon atom. These olefinic donors can also be structurally derived from imidazole, triazole, and thiazole-based heterocyclic carbenes and, as a result, have highly tunable electronic and steric properties. In this Account, we will focus on various synthetic routes to imidazole-2ylidene derived NHOs (sometimes referred to as deoxy-Breslow intermediates) followed by a discussion of the electron-donor ability of this structurally tunable ligand group. It should be mentioned that NHOs have a close structural analogy with Breslow-type intermediates, N-heterocyclic ketene aminals, and β-azolium ylides; while these latter species play important roles in advancing synthetic organic chemistry, discussion in this Account will be confined mostly to imidazole-2-ylidene derived NHOs. In addition, we will cover selected examples from the literature where NHOs and their anionic counterparts, N-heterocyclic vinylenes, are used to access reactive main group species not attainable using traditional ligands. Added motivation for these studies comes from the emerging number of low coordinate main group element based compounds that display reactivity once reserved for precious metal complexes (such as H−H and C−H bond activation). Moreover, NHOs are versatile precursors to new mixed element (P/C and N/C), and potentially bidentate, ligand constructs of great potential in catalysis, where various metal oxidation states and coordination environments need to be stabilized during a catalytic cycle. The most active area of recent growth for NHOs is their use as nucleophiles to promote efficient organocatalytic transformations, including transesterification, carbonyl reduction, and the conversion of CO2 into value added products. Polyesters have also been generated through the NHO-promoted ring-opening polymerization of lactones, and the highly tunable nature of NHO organocatalysts allows for the rapid screening and enhancement of catalytic performance. Therefore, the growing utility of NHOs in the realm of organic and polymer chemistry can be viewed as evidence of the widespread impact of Nheterocyclic olefins on the chemical community. It is hoped that through this Account others will join this flourishing research domain and that the rapid recent growth of NHO chemistry is sustained for the foreseeable future.

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INTRODUCTION N-Heterocyclic olefins (NHOs) represent a compound class that formally contains an alkylidene unit terminally appended to a heterocyclic carbene framework. This molecular architecture leads to substantial polarization of the exocyclic CC array and a concomitant increase in nucleophilic character at the ylidic carbon atom (as shown by the canonical forms in Figure 1). While the term NHO was introduced to the chemical community in 2011,1 pioneering work by the Kuhn group2a,b in 1993 provided a general synthetic route to the carbon-based nucleophile ImMe4CH2 (ImMe4 = (MeCNMe)2C) and afforded 1:1 adducts with BH3 and M(CO)5 (M = Mo and W).2 Despite early reported examples of N-heterocyclic olefins (NHOs) as effective donors, research in this area remained mostly dormant until 2010 when Beller and coworkers generated sterically encumbered NHOs in situ and © 2017 American Chemical Society

Figure 1. Salient canonical forms of N-heterocyclic olefins (NHOs) demonstrating the polarized nature of the CC double bond and a related Breslow intermediate.

used these species to form cationic phosphine ligands, such as [IPr−CH2−PR2]I (IPr = (HCNDipp)2C; Dipp = 2,6-iPr2C6H3; R = Cy, tBu, and Ph); the resulting palladium complexes were active and recoverable catalysts for C−C, C−O, and C−N Received: May 25, 2017 Published: August 4, 2017 2017

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Accounts of Chemical Research bond forming reactions.3 These important studies caught the attention of one of us (E.R.) who was at the time a pretenure academic studying the stabilization of inorganic methylenes (EH2) and ethylenes (H2EEH2) with the aid of N-heterocyclic carbenes (NHCs).4 Our decision to subsequently explore NHOs for this application1 led to a continual interest in this ligand class, and some of the results are summarized in this Account. One must also mention the structural link between the above-mentioned NHOs and Breslow intermediates (Figure 1) formed during carbene-catalyzed conjugate addition reactions.5,6 It is also expected that NHOs will be soft Lewis bases by virtue of electron density being donated from a polarized CC π orbital, making NHOs promising ligands for coordinating soft Ni(0) and Pd(0) centers often present during catalysis. NHOs have also been recently investigated as organocatalysts for a variety of transformations, including lactone polymerization and ketone hydroborylation.7 Motivated by this surge in research activity, we will now describe recent advances in NHO chemistry with a focus on highlighting key properties of NHOs, with the hope that others will be inspired to study this readily accessible and synthetically useful class of ligand.

alkylimidazolium salts; however separation of the NHO from excess carbene can be problematic. If nBuLi is used as a base in THF, then the highly soluble byproduct Li(THF)xI can be difficult to separate from the desired NHO. Robinson and coworkers reported a novel alternate preparation of IPrCH2, where they initially formed the anionic N-heterocyclic carbene ligand Li[IPr] (Scheme 1, Procedure 2) by the deprotonation of a backbone-positioned alkene C−H unit in IPr (Scheme 1, Procedure 2).10 Subsequent addition of H3C−I to a slurry of Li[IPr] affords IPrCH2; however this protocol was only reported on a small scale. Given the aforementioned challenges with synthesizing NHOs in high yield and purity or on a large scale, our group recently developed an improved synthesis of NHOs containing bulky substituents at nitrogen.9b Accordingly, we found that ClCH2SiMe3 could be used directly as a methylene source (Scheme 1, Procedure 3), and addition of an excess of this chloromethylsilane to IPr yields IPrCH2 and unreactive ClSiMe3 as a volatile byproduct, thus greatly simplifying the workup procedure. This protocol is now routinely used in our laboratory to yield pure IPrCH2 in a ca. 80% yield on a >30 g scale. The donor properties of NHOs were evaluated in relation to their precursor NHCs and other widely explored ligands, such as phosphines. Accordingly, Tolman electronic parameters (TEPs)11 were obtained from various square planar complexes, [(NHO)Rh(CO)2Cl]. Specifically, a stronger electron donor should weaken the C−O π-bonding to a larger extent (relative to the case when a less donating ligand is present) via an increase in Rh(d)−CO(π*) backbonding, leading to lower average carbonyl stretching frequency in the IR spectrum and thus a lower TEP value. In line with prior observations in the field,9a we observed that the TEP of IPrCH2 was lower than that of its carbene counterpart IPr, implying that NHOs are stronger electron donors than NHCs. In order to directly probe the relative Lewis basicity12 of IPr and IPrCH2, a 1:1 mixture of both ligands was added to [Rh(CO)2(μ-Cl)]2 and it was found that [(IPr)Rh(CO) 2 Cl] was formed exclusively, illustrating that IPr is a stronger Lewis base than IPrCH2. These seemingly conflicting observations are best explained by considering the difference in π-accepting ability of NHCs and NHOs. While NHCs are known to be moderate π-acceptors, NHOs have little to no π-accepting ability at the ligating carbon atom.9b The added π-acidity of NHCs removes electron density from the rhodium center, making the TEP of NHCs higher than that if only σ-donation was considered. Moreover in the case of the Rh−NHC adducts, added Rh(d)−CNHC(p/π*) backbonding leads to an overall stronger Rh−C interaction with NHCs in relation to NHOs and thus explains the thermodynamic preference (and higher Lewis basicity) of NHC complex formation in this system.12 A similar trend in Lewis basicity (IPr > IPrCH2) is found throughout our work on main group element complexation by NHO and NHC donors.1,13 Therefore, one has to be cautious when using Tolman electronic parameters alone to estimate relative donor strengths between two ligand classes with very different πaccepting properties. In terms of the Brønsted basicity,14 the computed proton affinities (PA) of IPr and IPrCH2 in the gas phase are very similar: PA = 275.8 kcal/mol for IPr vs 277.0 kcal/mol for IPrCH2. Thus far, attempts to determine the nucleophilicity12 of NHOs (a kinetic property) by examining the rate of complexation with Lewis acids have not been documented in the literature, and efforts along these lines are underway.

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N-HETEROCYCLIC OLEFIN SYNTHESIS AND DONOR PROPERTIES There are presently several methods known in the literature for the synthesis of NHOs, and they can be grouped into three general protocols, summarized in Scheme 1. At the nexus of Scheme 1. Commonly Employed Syntheses of NHeterocyclic Olefins (NHOs)

each synthesis is the initial formation of an N-heterocyclic carbene (NHC) via established methods.8 For example, the b r o a d l y ex p l o r e d h i n d e r e d c a r b e n e I P r ( I P r = [(HCNDipp)2C:]) can be converted into its corresponding N-heterocyclic olefin counterpart IPrCH2 via deprotonation of the imidazolium salt [IPrCH3]I with a strong Brønsted base, such as nBuLi; [IPrCH3]I can be readily generated by the addition of H3C−I to IPr (Scheme 1, Procedure 1).1,2,9 Another variant of this procedure involves using excess carbene (e.g., IPr) as a base to deprotonate the precursor 2018

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Accounts of Chemical Research

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NEW VISTAS IN THE N-HETEROCYCLIC OLEFIN-STABILIZATION OF REACTIVE INORGANIC SPECIES The above-mentioned founding studies by Kuhn2 and Beller3 created two new branches of NHO-related research: the exploration of NHOs as alternative stabilizing coligands in main group element chemistry and the use of either free NHOs or their transition metal complexes in catalysis.7 In 2009, our group reported the stabilization of germanium(II) dihydride in the form of the donor/acceptor complex IPr· GeH2·BH3,4a followed later by the isolation of SiH2 and SnH2 complexes.4b,15 Motivation for studying these species comes from their key role as intermediates in the high temperature chemical vapor deposition of pure semiconducting metals starting from the respective tetrelanes EH416 and the recent preparation of luminescent germanium nanoparticles via the mild thermolysis of GeH2 complexes.17,18 Accordingly, we were interested to see whether NHOs would be able to stabilize these reactive fragments as attempts to form EH2 adducts with commonly used phosphines and 4-dimethylaminopyridine donors were unsuccessful.19 As shown in Scheme 2, a two-

again formed with the NHO as a donor (Scheme 3).22 Robinson and co-workers have also been active in exploring the coordination of NHOs to electron-deficient boron centers. Notably, the boron tribromide adduct IPrCH2·BBr310 can be formed in hexanes, while in THF a rapid ring-opening reaction transpired to yield the borenium cation [IPrCH 2 ·B(OC4H8Br)2]Br. It is likely that the labile nature of the CIPrCH2−B bond in IPrCH2·BBr3 leads to some dissociation of BBr3, which then ring-opens THF to give bromoalkoxides, in line with prior work in the field.23 The corresponding NHC adduct IPr·BBr3 remains unaltered in THF,24 suggesting stronger coordination of the IPr ligand to BBr3 in comparison to IPrCH2. The Ghadwal and Chiu groups have also explored borocation chemistry supported by NHOs and representative examples of borohydride cations (e.g., [IPrCH2·H2B(μ-H)BH2·IPrCH2]+)25 and Cp*-substituted boron dications (e.g., [IPrCH 2 · BCp*]2+)26 are listed in Scheme 3. Remarkably, reduction of IPrCH2·BI3 resulted in borylene insertion into a C−N bond of the imidazolium heterocycle, followed by Dipp group migration and hydrogen abstraction from the solvent (Scheme 4a). This transformation is of great interest as the IPr motif (and many NHCs in general) are often stable under reducing conditions.27 Parallel reduction of IPrCH2·BHCl2 led to the formation of the known diborane adduct IPr·H2B−BH2·IPr24 with the formal loss of the terminal CH units from the NHO ligands (Scheme 4b).

Scheme 2. Synthesis of IPrCH2-Capped EH2 Adducts (E = Ge and Sn) and Lewis Base Exchange with the NHeterocyclic Carbene IPr

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ANIONIC N-HETEROCYCLIC VINYLENE LIGANDS AND PHOSPHINE- AND AMINE-CAPPED N-HETEROCYCLIC OLEFINS In 2011, we reported the parallel reactivity of IPr and IPr CH2 toward the cyclic dichlorophosphazene trimer [Cl2PN]3.28 In the presence of IPr, reduction of one phosphorus(V) center in [Cl2PN]3 is possible by sodium metal to afford the stable monoadduct [IPr·PN(PCl2N)2] (Scheme 5). Somewhat to our surprise, IPrCH2 replaced one chlorine atom in [Cl2PN]3 to yield a formally anionic N-heterocyclic vinylene ligand [IPr CH]− (Scheme 5); this reaction proceeded with or without the presence of sodium metal.28 Later Ghadwal and co-workers noted a similar reaction between IPrCH2 and HSiCl3 to give the silyl-substituted NHO, (IPrCH)SiHCl2 (Scheme 5).29 These early studies led to the eventual discovery of new ligand types based on N-heterocyclic olefin frameworks (vide inf ra). Given the ability of NHOs to stabilize low valent main group element centers, we wanted to take advantage of the potentially strong electron-releasing nature of the [IPrCH]− unit by merging this structural motif with phosphine or amine donors. Following an analogous procedure as reported by Beller,3 the cationic phosphines [IPrCH2−PR2]+ (R = iPr and Ph) were prepared as their chloride salts and then deprotonated to yield the isolable N-heterocyclic olefin-phosphine (NHOP) donors (IPrCH)PR2 (R = iPr, Ph) as air-sensitive solids.30 As summarized in Scheme 6, these NHOPs can preferentially bind the Lewis acids BH3 and AuCl through phosphorus rather than the terminal NHO carbon atom; however the remaining carbon donor site can be coaxed to interact with excess AuCl to yield the bis(adduct) IPrCH(AuCl)−PPh 2 (AuCl), featuring (IPrCH)PPh2 as a four-electron donor ligand.31 In relation to possible future catalysis based on N-heterocyclic olefin scaffolds, the palladium complex {(IPrCH)PiPr2}PdCl(cinnamyl) was also prepared and displayed preferential P−

step synthetic route to the target NHO-stabilized GeH2 and SnH2 complexes, IPrCH2·EH2·W(CO)5 (E = Ge and Sn) was developed. In addition, we were able to show that IPrCH2/IPr exchange was possible while preserving the EH2·W(CO)5 unit, thus providing evidence for the dative nature of the CNHO−E linkages in these species (and the stronger Lewis basicity of IPr).1 Inorganic ethylenes can also be stabilized by Nheterocyclic olefins, as demonstrated by the synthesis of the parent digermene complex IPrCH2·H2Ge−GeH2·W(CO)5.20 One inherent property of N-heterocyclic olefins is their generally lower coordination volume relative to their carbene counterparts due to the presence of an added CR2 spacer between the imidazolium moiety and the site of complexation. For example, Gandon and co-workers evaluated the relative percent buried volume (%V bur) of ImMe 2 (ImMe 2 = (HCNMe)2C) and ImMe2CH2 and noted values of 26.1% and 18.7%, respectively.21 Accordingly, one might expect differences in the coordination behavior of NHCs versus NHOs toward main group element centers. As depicted in Scheme 3, the N-heterocyclic carbene ImMe2 combines with GaCl3 to afford the expected 1:1 adduct ImMe2·GaCl3, while ImMe2CH2 interacts to yield the ion pair [(ImMe2CH2)2GaCl2]GaCl4 containing two less hindered NHO units bound to gallium.21 Furthermore, when IPr and IPrCH2 are combined with excess equivalents (>4) of Cl2Ge· dioxane, vastly different products are obtained with ion pairs 2019

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Accounts of Chemical Research Scheme 3. Differing Reactivity Profiles of NHCs and NHOs with Main Group Halides

Scheme 4. Activation of NHOs upon Reduction of Boron Halide Adducts

Scheme 6. Phosphine- and Amine-Appended NHOs and Their Versatile Coordination Behavior

Scheme 5. Installation of the Vinylic NHO Ligand [IPr CH]− onto Main Group Centers

salt [H2CNMe2]I, and in contrast with the analogous NHOPs, the −NMe2 donor site in the 1:1 gold adduct IPrCH(AuCl)−NMe2 remains free, while the softer olefinic carbon coordinates an equivalent of AuCl (Scheme 6). The availability of flexible (mixed element) donor sites in NHOderived phosphines and amines, coupled with their ability to act as four-electron donors, makes these ligands promising in the context of ligand-supported metal catalysis, where different metal oxidation states and variable coordination environments have to be stabilized throughout a catalytic cycle. We recently reported two methods to install the electronreleasing [IPrCH]− ligand onto main group element (E) centers (Scheme 7).32 The first route involves the combination

Pd ligation with the NHOP.31 The amine-functionalized NHO (IPrCH)NMe2 was synthesized from IPr and Eschenmoser’s 2020

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Accounts of Chemical Research Scheme 7. Low Coordinate Germylium and Phosphenium Cation Environments Stabilized by Anionic NHO Donors

Figure 2. Molecular structure of [(IPrCH)2Ge:] and relevant molecular orbital demonstrating the π interaction between the ligand and germanium. Thermal ellipsoids were plotted at the 50% probability level and flanking −Dipp groups plotted as wireframes for clarity.

of an NHO, such as IPrCH2, with an element halide (e.g., GeCl4) in the presence of DABCO (1,4-diazabicyclo[2.2.2]octane) as a base to remove the HCl byproduct generated. The second route is direct halosilane elimination between a preformed terminally silylated N-heterocyclic olefin MeIPr CH(SiMe3) and an element halide to form (MeIPrCH)-ERx products.32 In the same study, the sterically encumbered germanium(IV) dihalide (IPrCH)2GeCl2 was prepared (Scheme 7) and then reduced with KC8 to give the first example of a stable base-free divinylgermylene, (IPr CH)2Ge:, as a deep red solid (Figure 2, Scheme 7). Methylation of (IPrCH)2Ge: with MeOTf afforded an isolable germylium ion [(IPrCH)2GeMe]+ wherein the cationic three-coordinate Ge center is stabilized by added C− Ge π-interactions involving the adjacent carbon atoms on the flanking [IPrCH]− ligands (Figure 3);32 thus one can regard [IPrCH]− as both an effective σ- and an effective π-donor. Recently the Kinjo group prepared the first isolable example of a bis(NHO) chelate (Scheme 7)33 and used this species as a precursor to a novel cyclic phosphenium cation via related deprotonation chemistry as described above.

Figure 3. Molecular structure of [(IPrCH)2GeMe]+ and relevant molecular orbital demonstrating the π interaction between the ligand and germanium. Thermal ellipsoids were plotted at the 50% probability level and flanking −Dipp groups plotted as wireframes for clarity.

NHO-based organocatalysis have already been reviewed,7b we will summarize more recent advances in this field. In this regard, Saptal and Bhanage expanded on previous CO2 sequestration work to show that NHOs can effectively catalyze the synthesis of oxazolidinones from CO2 and aziridines (Scheme 8).35 Of the catalysts studied, ImiPr2CH2 (ImiPr = (HCNiPr)2C) was found to be the most active while maintaining a high degree of regioselectivity. The authors also used NHOs to catalyze the formamidation of amines using

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N-HETEROCYCLIC OLEFINS IN ORGANOCATALYSIS One exciting new avenue of study for N-heterocyclic olefins is their increasing use in organocatalysis.34 While aspects of 2021

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Accounts of Chemical Research Scheme 8. Recent Examples of Organocatalysis Mediated by NHOs

borylation with HBpin (Bpin = B(OCMe2)2) in the presence of 5 mol % of IPrCH2 in THF to give quantitative conversion to the hydroborylation product Ph2HCO(Bpin) after 18 h at room temperature; when IPr was used as a catalyst under similar conditions, only trace amounts of Ph2HCO(Bpin) formed.39 A range of functionalized substrates, including the sterically hindered aldehyde MesC(O)H (Mes = 2,4,6Me3C6H2) and alkyl-functionalized ketones (such as cyclohexanone) could also be hydroborylated under mild conditions. Parallel NHO-catalyzed hydrosilylation chemistry between MesC(O)H and PhMeSiH2 was also reported, albeit with moderate conversions (28% after heating for 48 h at 60 °C). The general mechanism of these transformations is now being investigated as part of ongoing work. Following initial reports of NHO-promoted polymerization reactions,7,34b Naumann and Dove compared the reactivity of five structurally distinct NHO organocatalysts for the ringopening polymerization of lactones and trimethylene carbonate.41,42 In the case of NHOs with exocyclic −CH2 units, the polymerization activity increased as the expected nucleophilicity of NHO became higher; however these polymerizations were nonliving due to competing monomer deprotonation by the NHO. The corresponding methylated NHO ImMe4 CMe2 was found to be an active polymerization catalyst; however a lower degree of control was observed, leading to multimodal polymer distributions from δ-valerolactone. Naumann and co-workers subsequently investigated NHOs as Lewis basic cocatalysts in the cooperative acid/base mediated polymerization of ε-caprolactone and δ-valerolactone in the presence of metal-based Lewis acids.42 Following abovementioned results, the NHOs studied were all CMe2 capped, and the most controlled polymerizations transpired in the

CO2 as an alternate source of CO, and a mixture of 9borabicyclo[3.3.1]nonane (9-BBN) and poly(methylhydro)siloxane as the reducing agents (Scheme 8).35 Nguyen and co-workers explored NHO-catalyzed transesterifications,36 (Scheme 8) taking advantage of the significant Brønsted basicity of N-heterocyclic olefins, which renders the alcohol reactants more nucleophilic.36 Using Kuhn’s backbone methylated NHO2a ImMe4CH2 as a catalyst (5 mol %), the authors found that ethyl benzoate could be prepared from methyl benzoate and ethanol in an appreciable yield of ca. 50% when the reaction was conducted in neat alcohol; no conversion to ethyl benzoate was observed in the absence of ImMe4CH2. The same authors also evaluated the Brønsted basicity of ImMe4CH2 through NMR titration experiments and found the basicity of this NHO to be higher than the widely used weakly nucleophilic base DBU (1,8diazabicyclo(5.4.0)undec-7-ene) but lower than the iminophosphorane “super base” (C4H8N)3PNtBu;36 thus if one increases the steric bulk at the terminal CR2 unit in NHOs, further applications for these species as weakly nucleophilic bases should be possible. Later, Nguyen and co-workers explored NHOs as organocatalysts for the dehydrogenative silylation of alcohols and the hydrosilylation of ketonic substrates, with MeIPrCH2 as the most effective catalyst.37 The Brønsted basicity of ImMe4CH2 has also been used to promote alkylation reactions in the presence of stoichiometric KOtBu.38 Very recently, we noted that the organocatalytic hydroborylation of ketonic substrates can be accomplished with hindered NHOs such as IPrCH2 and its backbone methylated counterpart MeIPrCH2 (Scheme 8).39,40 As a benchmark reaction, benzophenone underwent catalytic 2022

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Accounts of Chemical Research presence of MgI2. For comparison, ImMe4CMe2 gave uncontrolled ring lactone polymerization in the absence of Lewis acid,41 and this observation was attributed to the formation of Lewis acid/base adduct (with MgI2), which limits the concentration of free NHO in solution.

of Alberta in 2008. He is currently an Associate Professor and an Alexander von Humboldt (AvH) Fellow. In addition to his AvH Fellowship and various awards for teaching and research, he was an RCMS Visiting Professor at Nagoya University in 2016 and a JSPS Long-Term Visitation Fellow at Chuo University (2015) in Japan.

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CONCLUSIONS After the initial syntheses of N-heterocyclic olefins by Kuhn two decades ago, this field was largely dormant until the recent resurgence of NHOs as ligands and organocatalysts. Aided by recent improvements in the synthesis of NHOs, research utilizing these ylidic donors has greatly expanded. As highlighted in this Account, the reactivity of the potentially softer and less hindered NHO donors differs greatly from their parent NHCs, and this feature has been used to open new vistas of chemical reactivity, including metal-mediated catalysis.43 Furthermore, NHOs can be used to prepare mixed element (C/N and C/P) four-electron donors when linked with amines or phosphines. Methods now exist to formally convert NHOs into their deprotonated vinylic analogues, for example, [IPrCH]−, and the highly electronreleasing nature of this new anionic ligand is useful in gaining access to reactive low coordinate centers for small molecule activation and catalysis. The lower Lewis basicity of NHOs in comparison to NHCs has been exploited in organocatalysis, where NHOs are now considered to be the carbon-based nucleophile of choice for promoting carbon-element bond formation in the absence of metal centers.

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ACKNOWLEDGMENTS E.R. thanks the highly talented students and postdoctoral fellows who have worked tirelessly to advance some of the research described in this Account. In addition, E.R. thanks NSERC of Canada (for Discovery, CRD, and CREATE grants), the Faculty of Science at the University of Alberta, Alberta Innovates-Technology Futures, the Canada Foundation for Innovation, the donors of The American Chemical Society Petroleum Research Fund, the Japan Society for the Promotion of Science, and the Alexander von Humboldt Foundation for support. M.M.D.R. also thanks NSERC for a CREATE stipend. We also thank Sarah Parke in the Rivard group for painting the duck image found in the TOC graphic, used with her permission.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.accounts.7b00264. Details of computational studies (PDF)

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REFERENCES

(1) Al-Rafia, S. M. I.; Malcolm, A. C.; Liew, S. K.; Ferguson, M. J.; McDonald, R.; Rivard, E. Intercepting low oxidation state main group hydrides with a nucleophilic N-heterocyclic olefin. Chem. Commun. 2011, 47, 6987−6989. (2) (a) Kuhn, N.; Bohnen, H.; Kreutzberg, J.; Bläser, D.; Boese, R. 1,3,4,5-Tetramethyl-2-methyleneimidazoline-an ylidic olefin. J. Chem. Soc., Chem. Commun. 1993, 1136−1137. (b) Kuhn, N.; Bohnen, H.; Bläser, D.; Boese, R. Derivate des Imidazols, XI. (C8H14N2)M(CO)5 (M = Mo, W) − Terminale Koordination eines Olefins in Pentacarbonylmetall-Komplexen. Chem. Ber. 1994, 127, 1405−1407. (c) For a related pioneering study on triazolylidene-based NHOs, see: Enders, D.; Breuer, K.; Raabe, G.; Runsink, J.; Teles, J. H.; Melder, J. − P.; Ebel, K.; Brode, S. Preparation, Structure, and Reactivity of 1,3,4Triphenyl-4,5-dihydro-1H-1,2,4-triazol-5-ylidene, a New Stable Carbene. Angew. Chem., Int. Ed. Engl. 1995, 34, 1021−1023. (3) Dumrath, A.; Wu, X.-F.; Neumann, H.; Spannenberg, A.; Jackstell, R.; Beller, M. Recyclable Catalysts for Palladium-Catalyzed C-O Coupling Reactions, Buchwald-Hartwig Aminations, and Sonogashira Reactions. Angew. Chem., Int. Ed. 2010, 49, 8988−8992. (4) (a) Thimer, K. C.; Al-Rafia, S. M. I.; Ferguson, M. J.; McDonald, R.; Rivard, E. Donor/acceptor stabilization of Ge(II) dihydride. Chem. Commun. 2009, 7119−7121. (b) Al-Rafia, S. M. I.; Malcolm, A. C.; Liew, S. K.; Ferguson, M. J.; Rivard, E. Stabilization of the Heavy Methylene Analogues, GeH2 and SnH2, within the Coordination Sphere of a Transition Metal. J. Am. Chem. Soc. 2011, 133, 777−779. (c) Al-Rafia, S. M. I.; Malcolm, A. C.; McDonald, R.; Ferguson, M. J.; Rivard, E. Trapping the Parent Inorganic Ethylenes H2SiGeH2 and H2SiSnH2 in the Form of Stable Adducts at Ambient Temperature. Angew. Chem., Int. Ed. 2011, 50, 8354−8357. (d) Rivard, E. Donoracceptor chemistry in the main group. Dalton Trans. 2014, 43, 8577− 8586. (5) (a) Schedler, M.; Wurz, N. E.; Daniliuc, C. G.; Glorius, F. NHeterocyclic Carbene Catalyzed Umpolung of Styrenes: Mechanistic Elucidation and Selective Tail-to-Tail Dimerization. Org. Lett. 2014, 16, 3134−3137. (b) Berkessel, A.; Elfert, S. On the Involvement of a Spiroepoxide Intermediate in N-Heterocyclic Carbene (NHC)Catalyzed Benzoin Condensations - An Approach by Oxygenation of Deoxy-Breslow Intermediates. Adv. Synth. Catal. 2014, 356, 571−578. (c) Zhang, Y.; Schmitt, M.; Falivene, L.; Caporaso, L.; Cavallo, L.; Chen, E. Y.-X. Organocatalytic Conjugate-Addition Polymerization of Linear and Cyclic Acrylic Monomers by N-Heterocyclic Carbenes: Mechanisms of Chain Initiation, Propagation, and Termination. J. Am. Chem. Soc. 2013, 135, 17925−17942. (d) Maji, B.; Mayr, H. Structures and Reactivities of O-Methylated Breslow Intermediates. Angew.

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* E-mail: [email protected]. ORCID

Eric Rivard: 0000-0002-0360-0090 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest. Biographies Matthew Roy was born in Fredericton, New Brunswick, Canada, and completed his B.Sc. (Honours) in Chemistry at the University of New Brunswick in 2014. He is presently a Ph.D. candidate under the supervision of Prof. Eric Rivard with a specific interest in the isolation of reactive inorganic fragments using strong donor ligands for catalysis and small molecule activation. Eric Rivard received his B.Sc. (Hon.) from the University of New Brunswick and his Ph.D. from the University of Toronto (with I. Manners). After NSERC postdoctoral work at Caltech (J. C. Peters) and UC Davis (P. P. Power) and a research stay with C. Jones at Monash University, he became an Assistant Professor at the University 2023

DOI: 10.1021/acs.accounts.7b00264 Acc. Chem. Res. 2017, 50, 2017−2025

Article

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the potential energy surface by frequency analysis. In the calculation of proton affinity, the thermal contribution of a proton was assumed to be 0 kcal/mol. The reported reaction enthalpies (PAs) were calculated using the sum of the thermal and electronic enthalpies of the ligand and the protonated ligand; see the Supporting Information for more details. For comparison, the computed PA for H3PCH2 using a SCF approximation was 272.3 kcal/mol, see: Lischka, H. Electron Structure and Proton Affinity of Methylenephosphorane by ab Initio Methods Including Electron Correlation. J. Am. Chem. Soc. 1977, 99, 353−360. (15) Al-Rafia, S. M. I.; Malcolm, A. C.; McDonald, R.; Ferguson, M. J.; Rivard, E. Efficient generation of stable adducts of Si(II) dihydride using a donor-acceptor approach. Chem. Commun. 2012, 48, 1308− 1310. (16) Rivard, E. Group 14 inorganic hydrocarbon analogues. Chem. Soc. Rev. 2016, 45, 989−1003 and references therein. (17) Purkait, T. K.; Swarnakar, A. K.; De Los Reyes, G. B.; Hegmann, F. A.; Rivard, E.; Veinot, J. G. C. One-pot synthesis of functionalized germanium nanocrystals from a single source precursor. Nanoscale 2015, 7, 2241−2244. (18) Swarnakar, A. K.; McDonald, S. M.; Deutsch, K. C.; Choi, P.; Ferguson, M. J.; McDonald, R.; Rivard, E. Application of the DonorAcceptor Concept to Intercept Low Oxidation State Group 14 Element Hydrides using a Wittig Reagent as a Lewis Base. Inorg. Chem. 2014, 53, 8662−8671. (19) Al-Rafia, S. M. I.; Shynkaruk, O.; McDonald, S. M.; Liew, S. K.; Ferguson, M. J.; McDonald, R.; Herber, R. H.; Rivard, E. Synthesis and Mössbauer Spectroscopy of Formal Tin (II) Dichloride and Dihydride Species Supported by Lewis Acids and Bases. Inorg. Chem. 2013, 52, 5581−5589. (20) Al-Rafia, S. M. I.; Momeni, M. R.; Ferguson, M. J.; McDonald, R.; Brown, A.; Rivard, E. Stable Complexes of Parent Digermene: An Inorganic Analogue of Ethylene. Organometallics 2013, 32, 6658− 6665. (21) El-Hellani, A.; Monot, J.; Guillot, R.; Bour, C.; Gandon, V. Molecular versus Ionic Structures in Adducts of GaX3 with Monodentate Carbon-based Ligands. Inorg. Chem. 2013, 52, 506−514. (22) Al-Rafia, S. M. I.; Momeni, M. R.; McDonald, R.; Ferguson, M. J.; Brown, A.; Rivard, E. Controlled Growth of Dichlorogermanium Oligomers from Lewis Basic Hosts. Angew. Chem., Int. Ed. 2013, 52, 6390−6395. (23) Gerrard, W.; Lappert, M. F. Reactions of Boron Trichloride with Organic Compounds. Chem. Rev. 1958, 58, 1081−1111. (24) Wang, Y.; Quillian, B.; Wei, P.; Wannere, C. S.; Xie, Y.; King, R. B.; Schaefer, H. F., III; Schleyer, P. v. R.; Robinson, G. H. A Stable Neutral Diborene Containing a B=B Double Bond. J. Am. Chem. Soc. 2007, 129, 12412−12413. (25) Ghadwal, R. S.; Schürmann, C. J.; Andrada, D. M.; Frenking, G. Mono- and di-cationic hydrido boron compounds. Dalton Trans. 2015, 44, 14359−14367. (26) Lee, W.-H.; Lin, Y.-F.; Lee, G.-H.; Peng, S.-M.; Chiu, C.-W. NHeterocyclic olefin stabilized boron dication. Dalton Trans. 2016, 45, 5937−5940. (27) (a) Ghadwal, R. S.; Schürmann, C. J.; Engelhardt, F.; Steinmetzger, C. Unprecedented Borylene Insertion into a C-N Bond. Eur. J. Inorg. Chem. 2014, 4921−4926. (b) For related work, see: Lui, M. W.; Merten, C.; Ferguson, M. J.; McDonald, R.; Xu, Y.; Rivard, E. Contrasting Reactivities of Silicon and Germanium Complexes Supported by an N-Heterocyclic Guanidine Ligand. Inorg. Chem. 2015, 54, 2040−2049. (28) Al-Rafia, S. M. I.; Ferguson, M. J.; Rivard, E. Interaction of Carbene and Olefin Donors with [Cl2PN]3: Exploration of a Reductive Pathway toward (PN)3. Inorg. Chem. 2011, 50, 10543−10545. (29) (a) Ghadwal, R. S.; Reichmann, S. O.; Engelhardt, F.; Andrada, D. M.; Frenking, G. Facile access to silyl-functionalized N-heterocyclic olefins with HSiCl3. Chem. Commun. 2013, 49, 9440−9442. For more recent studies on N-heterocyclic vinylene ligation, see: (b) Li, Z.; Chen, X.; Andrada, D. M.; Frenking, G.; Benkö, Z.; Li, Y.; Harmer, J. R.; Su, C.-Y.; Grützmacher, H. (L)2C2P2: Dicarbondiphosphide Stabilized by N-Heterocyclic Carbenes or Cyclic Diamido Carbenes. 2024

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Accounts of Chemical Research Angew. Chem., Int. Ed. 2017, 56, 5744−5749. (c) Rottschaefer, D.; Neumann, B.; Stammler, H.-G.; Ghadwal, R. S. N-Heterocyclic Vinylidene-Stabilized Phosphorus Biradicaloid. Chem. - Eur. J. 2017, 23, 9044−9047. (d) Causero, A.; Elsen, H.; Pahl, J.; Harder, S. Calcium Hydride Reactivity: Formation of an Anionic N-Heterocyclic Olefin Ligand. Angew. Chem., Int. Ed. 2017, 56, 6906−6910. (30) Paisley, N. R.; Lui, M. W.; McDonald, R.; Ferguson, M. J.; Rivard, E. Structurally versatile phosphine and amine donors constructed from N-heterocyclic olefin units. Dalton Trans. 2016, 45, 9860−9870. (31) Lui, M. W.; Shynkaruk, O.; Oakley, M. S.; Sinelnikov, R.; McDonald, R.; Ferguson, M. J.; Meldrum, A.; Klobukowski, M.; Rivard, E. Engaging dual donor sites within an N-heterocyclic olefin phosphine ligand. Dalton Trans. 2017, 46, 5946−5954. (32) Hering-Junghans, C.; Andreiuk, P.; Ferguson, M. J.; McDonald, R.; Rivard, E. Using N-Heterocyclic Vinyl Ligands to Access Stable Divinylgermylenes and a Germylium Cation. Angew. Chem., Int. Ed. 2017, 56, 6272−6275. (33) Chong, C. C.; Rao, B.; Ganguly, R.; Li, Y.; Kinjo, R. Bis(Nheterocyclic olefin) Derivative: An Efficient Precursor for Isophosphindolylium Species. Inorg. Chem. 2017, DOI: 10.1021/acs.inorgchem.6b02991. (34) (a) Wang, Y.-B.; Wang, Y.-M.; Zhang, W.-Z.; Lu, X.-B. Fast CO2 Sequestration, Activation, and Catalytic Transformation Using NHeterocyclic Olefins. J. Am. Chem. Soc. 2013, 135, 11996−12003. (b) Naumann, S.; Thomas, A. W.; Dove, A. P. N-Heterocyclic Olefins as Organocatalysts for Polymerization: Preparation of Well-Defined Poly(propylene oxide). Angew. Chem., Int. Ed. 2015, 54, 9550−9554. (35) Saptal, V. B.; Bhanage, B. M. N-Heterocyclic Olefins as Robust Organocatalyst for the Chemical Conversion of Carbon Dioxide to Value-Added Chemicals. ChemSusChem 2016, 9, 1980−1985. (36) Blümel, M.; Noy, J.-M.; Enders, D.; Stenzel, M. H.; Nguyen, T. V. Development and Applications of Transesterification Reactions Catalyzed by N-Heterocyclic Olefins. Org. Lett. 2016, 18, 2208−2211. (37) Kaya, U.; Tran, U. P. N.; Enders, D.; Ho, J.; Nguyen, T. V. NHeterocyclic Olefin Catalyzed Silylation and Hydrosilylation Reactions of Hydroxyl and Carbonyl Compounds. Org. Lett. 2017, 19, 1398− 1401. (38) Blümel, M.; Crocker, R. D.; Harper, J. B.; Enders, D.; Nguyen, T. V. N-Heterocyclic olefins as efficient phase-transfer catalysts for base-promoted alkylation reactions. Chem. Commun. 2016, 52, 7958− 7961. (39) Hering-Junghans, C.; Watson, I. C.; Ferguson, M. J.; McDonald, R.; Rivard, E. Organocatalytic hydroborylation promoted by Nheterocyclic olefins. Dalton Trans. 2017, 46, 7150−7153. (40) For related hydroborylation/hydrosilylation catalysis from our group, see: (a) Lummis, P. A.; Momeni, M. R.; Lui, M. W.; McDonald, R.; Ferguson, M. J.; Miskolzie, M.; Brown, A.; Rivard, E. Accessing Zinc Monohydride Cations through Coordinative Interactions. Angew. Chem., Int. Ed. 2014, 53, 9347−9351. (b) Roy, M. M. D.; Ferguson, M. J.; McDonald, R.; Rivard, E. Investigation of N-Heterocyclic CarbeneSupported Group 12 Triflates as Pre-catalysts for Hydrosilylation/ Borylation. Chem. - Eur. J. 2016, 22, 18236−18246. (41) Naumann, S.; Thomas, A. W.; Dove, A. P. Highly Polarized Alkenes as Organocatalysts for the Polymerization of Lactones and Trimethylene Carbonate. ACS Macro Lett. 2016, 5, 134−138. (42) Naumann, S.; Wang, D. Dual Catalysis Based on N-Heterocyclic Olefins for the Copolymerization of Lactones: High Performance and Tunable Selectivity. Macromolecules 2016, 49, 8869−8878. (43) (a) Iglesias, M.; Iturmendi, A.; Sanz Miguel, P. J.; Polo, V.; Pérez-Torrente, J. J.; Oro, L. A. Tuning PCP-Ir complexes: the impact of an N-heterocyclic olefin. Chem. Commun. 2015, 51, 12431−12434. (b) Iturmendi, A.; García, N.; Jaseer, E. A.; Munárriz, J.; Sanz Miguel, P. J.; Polo, V.; Iglesias, M.; Oro, L. A. N-Heterocyclic olefins as ancillary ligands in catalysis: a study of their behaviour in transfer hydrogenation reactions. Dalton Trans. 2016, 45, 12835−12845.

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DOI: 10.1021/acs.accounts.7b00264 Acc. Chem. Res. 2017, 50, 2017−2025