Pushing Chemical Boundaries with N-Heterocyclic Olefins (NHOs): From Catalysis to Main Group Element Chemistry

Abstract

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 (CR) 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-2-ylidene 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 CO 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 N-heterocyclic 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.

N‐Heterocyclic Olefin‐Ligated Palladium(II) Complexes as Pre‐Catalysts for Buchwald–Hartwig Aminations

“…The exocyclic=CH−CH=CH 2 unit in 1 is in the same plane as the proximal five‐membered imidazole ring. The structure of the deep‐blue IPr(BIAN)CH 2 ( 2 ) was also determined by X‐ray crystallography (Figure ) and the exocyclic C1−C4 linkage (1.336(3) Å) is of a typical length for an N‐heterocyclic olefin; likewise standard metrical parameters for the backbone saturated SIPrCH 2 ( 3 ) were noted (Figure S6, Supporting Information)…”

“…N‐Heterocyclic olefins (NHOs) represent an emerging class of carbon‐based donors that each contain a polarized, ylidic, alkylidene unit (=CH 2 or =CR 2 ) terminally linked to an N‐heterocyclic carbene fragment (see Scheme for contributing resonance forms) . The first isolable example of an NHO, (MeCNMe) 2 C=CH 2 , was described by Kuhn and co‐workers in 1993, with nucleophilic/donor ability at the terminal carbon atom demonstrated . Moreover, N‐heterocyclic olefins are considered to be softer σ‐donors than NHCs and might yield stable coordination complexes with the soft Pd 0 centers found during cross‐coupling catalysis.…”

“…Although the seminal work by Kuhn and co‐workers introduced various NHO⋅M(CO) 5 complexes to the community (M=Cr, Mo, and W), the number of metal complexes comprising NHOs as ligands is still limited, with examples of Au, Ir, and Rh complexes now known . NHOs have also been used to stabilize reactive main‐group environments, and an exciting new direction is their use in organocatalysis …”

Thorium(IV) alkyl synthesis from a thorium(III) cyclopentadienyl complex and an N-heterocyclic olefin

“…The methylene groups of NHOs are known to be basic and nucleophilic, and so might be expected to coordinate to electron deficient metals such as thorium. In that scenario, a number of electronic structure formulations can be used to depict the methylene-metal and methylene-imidazole (Im) interactions, and thus such a derivative could be formally regarded to possess some alkylidene character [50] where the alkylidene is Fourier transform infrared spectra were recorded as Nujol mulls in KBr discs using a Shimadzu IRAffinity-1S spectrometer. Elemental analyses were performed by Mrs Anne Davies and Mr…”

S–H Bond Activation in Hydrogen Sulfide by NHC-Stabilized Silyliumylidene Ions

“…Low-valent main group chemistry is a rapidly developing field and the wealth of new structural motifs, which have been isolated in the past two decades, have increasingly gained interest in using these species for the activation of small molecules and, potentially, for catalysis (for representative reviews see [1][2][3][4][5][6][7]). Key to these developments have been the usage of suitable synthetic methodologies in combination with thermodynamic and kinetic stabilization by appropriately chosen ligands.…”