Sulfoxides are capable of forming stable complexes with transition metals and there have been many comprehensive studies into their binding properties. However, the use of sulfoxides, particularly chiral sulfoxides, as ligands in transition metal catalysis is rather less well developed. This review aims to describe these catalytic studies and covers new developments that are showing very promising results and that have led to a renewed interest in this field.

A family of chiral C(2)-symmetric disulfoxide ligands possessing biaryl atropisomeric backbones has been synthesized by using the Andersen methodology. Complete characterization includes X-ray crystallographic studies of all ligands and some of their rhodium complexes. Their synthesis, optical purity, electronic properties, and catalytic behavior in the prototypical rhodium-catalyzed 1,4-addition of phenylboronic acid to 2-cyclohexen-1-one are presented through an in depth study of this ligand class. Density functional theory calculations on the step of the catalytic cycle that determines the enantioselectivity are presented and reinforce the first hypothetical explanations for the high levels of asymmetric induction observed.

Into the groove: The introduction of a C2‐symmetric N‐heterocyclic carbene ligand with appropriately substituted naphthyl side chains enables the efficient Suzuki–Miyaura coupling to form bulky tetra‐ortho‐substituted biaryls from aryl bromides and chlorides at room temperature. DFT calculations uncover the subtle steric phenomena at play that lead to the superior catalytic performance. Cyoct=cyclooctyl.

Monodentate N-heterocyclic carbene (NHC) ligands have become ubiquitous in organometallic chemistry and catalysis. [1] Conversely, development of chiral monodentate NHC ligands that induce high selectivity in asymmetric metal catalysis is still at an early stage with relatively few reports detailing enantioselectivities of 90 % ee and higher. [2][3][4][5][6] The main difficulties in designing efficient ligands of this type reside in placing stereocontrol elements at positions near the metal center without affecting the overall reactivity of the catalysts. Scheme 1 shows some of the most promising ligand designs to date and highlights the fact that the inherent flexibility of the N substituents has to be restricted to afford ligands that efficiently transfer their chiral information. This restriction can be done by fusing these wingtips onto the N heterocycle, a design motif pioneered by Glorius et al., [2] and more recently developed further by Murakami et al. [3] Decreasing the rotation of the N substituents is also key in the successful C 2 -symmetric ligands reported by Kündig et al., [4] who have been able to show that such ligands can be used very effectively in palladium chemistry. [5] Probably the most versatile ligand system developed so far was first reported by Grubbs et al., [6,7] and they rely on transferring chirality from a chiral N-heterocyclic backbone onto unsymmetrically substituted aryl side chains and ultimately onto the metal coordination sphere. While the design permits easy access to the precursor imidazolinium salts, such side chains will in principle create three diastereomers which would have to be separated for optimal use in catalysis. Our own efforts, [8] have indeed highlighted the pivotal role the respective orientation of naphthyl wingtips can have on enantioselectivity and, contrary to what other groups have proposed or found, the best ligands with 2-alkyl-substituted naphthyl side chains position their alkyl substituents directly below the corresponding phenyl group of the backbone [(Ra,Ra)-isomer].Encouraged by our first results, we became interested in ways of exclusively accessing this particular diastereomer, as it would undoubtedly allow a more straightforward synthesis and use of these ligands. After testing several substitution patterns, we were pleased to find that placing a relatively rigid cyclooctyl group at the 2-position of the naphthyl moieties and ring-closing the corresponding chiral diamine A at 120 8C for 2 hours (Scheme 2) generated the virtually pure NHC salt (Ra,Ra)-B. The salt showed one set of signals and a diagnostic single peak for the C2 proton of the imidazolinium ring in the 1 H NMR spectrum.This salt was then used to synthesize the palladium cinnamyl complex (Ra,Ra)-C in high yield, the structure of which was unambiguously confirmed by single-crystal X-ray crystallography. [9,10] Qualitative assessment of the structure shows both the relative bulk and the C 2 symmetry of the ligand. The overall steric demand of the ligand was then quantified by it...

Two saturated N-heterocyclic carbene ligands with substituted naphthyl side chains were used for the preparation of Blechert-type ruthenium metathesis precatalysts. The resulting conformers of the complexes were separated and unambiguously assigned by X-ray diffraction studies. All new complexes were compared in terms of activity to the original, SIMes-derived Blechert catalyst and were shown to be superior. A study on the impact of solvent concentration in RCM reactions using the most active of these new catalysts ultimately led to the ring closing of a variety of substrates at very low catalyst loadings.

From zero to hero? Sulfoxides are generally not considered useful ligand entities in asymmetric metal catalysis. However, a chiral disulfoxide as a chelating ligand in the rhodium-catalyzed 1,4-addition of aryl boronic acids to cyclic, alpha,beta-unsaturated ketones and esters gives impressive catalytic results, thus opening the door to future applications of this new chiral ligand class.

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A new NHC x Pd-catalyzed asymmetric alpha-arylation of amides is reported that gives direct access to synthetically valuable, allylated oxindoles with quaternary carbon centers. The reaction is made possible by the introduction of a new chiral NHC ligand. The palladium complexes derived therefrom combine excellent reactivity with high chemo- and enantioselectivity for the title transformation.