Two complementary Kumada coupling methods for the conversion of monotriflated 1,1′-binaphthalene-2,2′-diol (BINOL) into 2′-substituted binaphthyl monoalcohols under mild conditions are reported. A protocol using NiCl2(dppe), in combination with an improved preparation of the monotriflate, is effective for 1,1′-binaphthalene-2-ols containing unsubstituted or electron-poor aryl or benzyl 2′-substituents. An alternative procedure, using a potentially hemilabile-bidentate phosphinan-4-ol ligand, is superior for products containing neopentyl or electron-rich aryl 2′-substituents. The obtained binaphthyl alcohols represent potentially useful synthons for chiral ligands and auxiliaries.

Acyclic aminocarbenes have received much less attention as ancillary ligands in homogeneous catalysis compared with their cyclic relatives (i.e., N-heterocyclic carbenes, NHCs), despite having a longer history and greater structural variety. Although these species are generally more fragile than NHCs, recent advances in the synthesis and catalytic application of metal complexes of acyclic carbenes have brought increased attention to these underutilized ligands. It is increasingly clear that acyclic carbenes possess unique properties that distinguish them from other ligand classes and make them potentially valuable for catalysis. These include exceptional donor ability, conformational flexibility, and wide N–C(carbene)–N angles that can place bulky or chiral substituents near catalytic substrate binding sites. This purpose of this Perspective is to review and critically assess recent progress in this forefront area of catalyst design. Syntheses and ligand properties of acyclic diaminocarbenes, aminooxycarbenes, and other aminocarbenes are reviewed with a view toward catalytic relevance. A special focus is to highlight catalytic reactions in which acyclic carbene ligands confer unusual selectivity or activity on metal catalysts compared with conventional ligand types. Particularly promising catalytic results have been obtained with acyclic carbene complexes of gold, including some highly enantioselective catalytic transformations.Keywords: asymmetric catalysis; carbene ligands; gold catalysts; homogeneous catalysis; ligand design

Packing interactions in the crystal structures of a series of cis-M(CNAr)2Cl2 complexes (M = Pt, Pd; Ar = substituted phenyl) were examined and correlated with the luminescence properties of the Pt complexes. The structures of the PhNC and p-tolyl isocyanide complexes exhibit extended chains of metallophilic interactions with M···M distances of 3.24–3.25 and 3.34 Å, respectively, with nearly isostructural Pt and Pd compounds. Both structure types contain void channels running parallel to the M···M chains. The channels are 3–4 Å wide and vacant for the phenyl structures, while those in the p-tolyl structures are up to 7.6 Å wide and contain water. These channeled structures are stabilized by a combination of metallophilic bonding and aryl π–π stacking interactions. The Pt structure with 4-F substituents also features extended Pt···Pt chains, but with longer 3.79 Å distances alternating with shorter 3.37 Å contacts. Structures with 4-CF3 and 4-OMe substituents exhibit mostly isolated dimers of M···M contacts. In complexes with 2,6-dimethylphenyl isocyanide, steric hindrance precludes any short M···M contacts. The primary effect of aryl substitution is to provide alternative packing motifs, such as CF3···π and CH3···π interactions, that either augment or disrupt the combination of metallophilic contacts and π–π stacking needed to stabilize extended M···M chains. Differences in the Pt and Pd structures containing 4-F and 4-OMe substituents are consistent with a higher driving force for metallophilic interactions for Pt versus Pd. The M–C and M–Cl bond distances indicate a slightly higher trans influence for aryl isocyanides bound to Pt versus Pd. The three extended Pt···Pt chain structures display luminescence assignable to (dσ*→pσ) excited states, demonstrating the existence of substantial orbital communication along the metal–metal chains. Face-indexing shows that the preferred crystal growth axis is along the metal–metal chains for the luminescent structures. Variable temperature structural studies showed that both M···M and π–π interactions contract upon cooling. Overall, this study suggests that synergy with π–π and other interactions is necessary to stabilize extended M···M chain structures. Thus, efforts to design functional materials based on metallophilic bonding must consider the full array of available packing motifs.

Simple silver salts and dicationic palladium bis(N-heterocyclic carbene) complexes were found to be effective and complementary catalysts for Nazarov cyclizations of divinyl β-keto esters. A sterically hindered palladium bis(NHC) catalyst provided remarkable endo:exo selectivity in cyclizations of cyclohexenyl β-keto esters, whereas AgSbF6 gave improved yields with electron-deficient substrates. The high activities observed with silver salts are significant, given their widespread use as additives in catalytic Nazarov reactions.Keywords: catalytic electrocyclic reactions; N-heterocyclic carbene ligands; Nazarov cyclization; palladium catalysts; silver catalysts;

Two distinct polymorphs of cis-M(CNPh)2Cl2 (M = Pt,Pd) were obtained under different crystallization conditions. One polymorph of each compound contains 3−4 Å-wide, vacant channels stabilized by a synergistic combination of extended metallophilic interactions and ligand π−π stacking, whereas the other polymorph contains no discernible void space and only isolated, longer metal−metal contacts. The channeled polymorphs are favored under faster crystal growth conditions, and face-indexing revealed that the growth axes are along the M···M chains. Although the channels extend the length of a crystal, they do not absorb solvent because of their narrow widths.

Use of a bulky bidentate bis(NHC) ligand allows isolation of a stable cationic methylpalladium carbonyl complex that undergoes clean carbonylation to an acyl complex under excess CO, modeling a key step in CO/alkene copolymerization.

Palladium complexes of chiral bis(acyclic diaminocarbene) ligands with seven-membered chelate rings catalyze the aza-Claisen rearrangement of an allylic benzimidate to an allylic amide with moderate yields and enantioselectivities of 30–59% ee. The promotion of electrophilic catalysis by complexes of these ligands highlights their distinct stereoelectronic properties relative to those of common bis(N-heterocyclic carbenes).

Reaction of cis-dichlorobis(p-trifluoromethylphenylisocyanide)palladium(II) with hydrazobenzene afforded a complex with a monodentate aminohydrazino carbene ligand and an unreacted arylisocyanide. Upon heating, this complex partially converted into a chelating bis(acyclic diaminocarbene) complex. Both the monocarbene and bis(carbene) complexes were isolated and characterized by X-ray crystallography. In solution, an equilibrium between the monocarbene and bis(carbene) complexes was observed, with equilibrium constants of 2.56(15) in CD3CN and 10.0(5) in THF-d8 at 25 °C. A van’t Hoff plot provided thermodynamic parameters of ΔH° = −1.7(3) kcal mol−1 and ΔS° = −3.7(9) eu for the monocarbene to bis(carbene) conversion in CD3CN. The instability of the bis(carbene) complex toward chelate ring opening was rationalized on the basis of steric strain apparent in the X-ray structure. The crowded syn,syn arrangement of aryl groups on the bis(carbene) backbone results in widening of the carbene NCN angles to reduce aryl−aryl steric interactions. Thermodynamic calculations on density functional theory models of the carbene complexes were in reasonable agreement with experimental data, and they provided an estimate of 16 kcal mol−1 for the maximum amount of ligand strain that can be accommodated in forming a stable bis(carbene) ligand via diamine addition to cis isocyanides. The chelating bis(carbene) is thermodynamically favored despite an unfavorable reaction entropy and a substantial amount of ligand strain.

Addition of trans-N,N′-dimethyl-1,2-diaminocyclohexane to a palladium bis(arylisocyanide) complex leads to the one-step formation of the first chiral bis(acyclic diaminocarbene) complex, which is thermally stable under N2 but undergoes slow oxidation to a bis(amidine) complex under air.

Palladium-templated addition of phenylhydrazine to methylisocyanide led to a Chugaev-type chelating dicarbene–palladium complex that is an effective catalyst for Suzuki-Miyaura cross-coupling reactions of aryl bromides and activated aryl chlorides. In contrast, the use of isopropylisocyanide resulted in an unprecedented complex containing aminohydrazinocarbene ligands, which was characterized by X-ray crystallography. The latter complex represents a possible intermediate in the formation of chelating carbene ligands.The addition of phenylhydrazine to [Pd(CNR)4]2+ results in a chelating dicarbene ligand in the case of methylisocyanide, whereas the use of isopropylisocyanide affords an unprecedented complex with monodentate aminohydrazinocarbene ligands that represent intermediates in chelating dicarbene ligand formation.

Use of a bulky bidentate bis(NHC) ligand allows isolation of a stable cationic methylpalladium carbonyl complex that undergoes clean carbonylation to an acyl complex under excess CO, modeling a key step in CO/alkene copolymerization.

Addition of trans-N,N′-dimethyl-1,2-diaminocyclohexane to a palladium bis(arylisocyanide) complex leads to the one-step formation of the first chiral bis(acyclic diaminocarbene) complex, which is thermally stable under N2 but undergoes slow oxidation to a bis(amidine) complex under air.