Influence of N-Substituents on the Adsorption Geometry of OH-Functionalized Chiral N-Heterocyclic Carbenes

Abstract

Adsorption of chiral molecules on heterogeneous catalysts is a simple approach for inducing an asymmetric environment to enable enantioselective reactivity. Although the concept of chiral induction is straightforward, its practical utilization is far from simple, and only a few examples toward the successful chiral induction by surface anchoring of asymmetric modifiers have been demonstrated so far. Elucidating the factors that lead to successful chiral induction is therefore a crucial step for understanding the mechanism by which chirality is transferred. Herein, we identify the adsorption geometry of OH-functionalized N-heterocyclic carbenes (NHCs), which are chemical analogues to chiral modifiers that successfully promoted α-arylation reactions once anchored on Pd nanoparticles. Polarized near-edge X-ray absorption fine structure (NEXAFS) measurements on Pd(111) revealed that NHCs that were associated with low enantioselectivity were characterized with a well-ordered structure, in which the imidazole ring was vertically positioned and the OH-functionalized side arms were flat-lying. OH-functionalized NHCs that were associated with high enantioselectivity revealed a disordered/flexible adsorption geometry, which potentially enabled better interaction between the OH group and the prochiral reactant.

Reversible Self‐Assembly of an N‐Heterocyclic Carbene on Metal Surfaces

“…The even stronger propensity towards σ‐donation of cyclic (alkyl)(amino) carbenes (CAAC) [25, 26] has been highlighted both in catalytic applications [27] and in pioneering on‐surface reactivity studies by Johnson and co‐workers, [28] but little is known about the fundamental behavior of such entities on metal surfaces [29] . A profound understanding of the binding modes, the conformation, and the supramolecular assembly of CAACs on surfaces will offer a springboard for the development of tailor‐made systems for catalysis [23] and advanced materials [30] . Kwon et al.…”

N‐Heterocyclic Carbene Based Nanolayer for Copper Film Oxidation Mitigation

“…NHC nanolayer was characterized by N 1s X‐ray photoelectron spectroscopy (XPS) measurement (Figure 1a, spectrum i). The N 1s XPS spectrum showed a broad peak that was fit by two Gaussians, centered at 398.9 eV and 400.7 eV and assigned to C−N=C and N−C bonds, respectively [17f] . For comparison, dimethyl‐benzimidazolium iodide (DMBI) was used as a precursor for monolayer formation on copper film due to its ability to form densely‐packed NHC monolayers [22] .…”

Reversible Selbstorganisation eines N‐heterocyclischen Carbens auf Metalloberflächen

“…Die noch stärkere Neigung zur σ‐Donation cyclischer (Alkyl)(Amino)‐carbene (CAAC) [25, 26] wurde sowohl in katalytischen Anwendungen [27] als auch in innovativen Studien zur Reaktivität auf Oberflächen von Johnson und Mitarbeitern [28] hervorgehoben, doch ist über das grundlegende Verhalten solcher Einheiten auf Metalloberflächen wenig bekannt [29] . Ein tiefgreifendes Verständnis der Bindungsmodi, der Konformation und des supramolekularen Aufbaus von CAACs auf Oberflächen könnte daher die Entwicklung maßgeschneiderter Systeme für die Katalyse [23] und fortschrittlicher Nanomaterialien beflügeln [30] . Kwon et al.…”