page_title: Lewis acid catalysis
section_title: BINAP
hierarchical_section_title: Lewis acid catalysis / Asymmetric Lewis acid catalysis / Common Chiral Ligands / BINAP
caption_attribution_description: English: (R)-BINAP Ligand and Complex
context_page_description: In Lewis acid catalysis of organic reactions, a metal-based Lewis acid acts as an electron pair acceptor to increase the reactivity of a substrate. Common Lewis acid catalysts are based on main group metals such as aluminum, boron, silicon, and tin, as well as many early and late d-block metals. The metal atom forms an adduct with a lone-pair bearing electronegative atom in the substrate, such as oxygen, nitrogen, sulfur, and halogens. The complexation has partial charge-transfer character and makes the lone-pair donor effectively more electronegative, activating the substrate toward nucleophilic attack, heterolytic bond cleavage, or cycloaddition with 1,3-dienes and 1,3-dipoles.
  Many classical reactions involving carbon–carbon or carbon–heteroatom bond formation can be catalyzed by Lewis acids. Examples include the Friedel-Crafts reaction, the aldol reaction, and various pericyclic processes that proceed slowly at room temperature, such as the Diels-Alder reaction and the ene reaction. In addition to accelerating the reactions, Lewis acid catalysts are able to impose regioselectivity and stereoselectivity in many cases.
context_section_description: Developed by Noyori, BINAP (2,2'-diphenylphosphino-1,1'-binaphthyl) is a family of chiral diphosphine ligands featuring two triarylphosphine moieties installed on a binaphthalene backbone.  BINAP chelates onto a metal (usually a late transition metal) to form a C₂-symmetric complex. As shown below in the structure of an (R)-BINAP ruthenium complex, among the four remaining coordination sites on an octahedral metal center, the two equatorial sites (purple) are strongly influenced by the equatorial phenyl groups, while the two axial sites (green) are influenced by the axial phenyl groups.
  Based on the structure, models for the observed enantioselectivity in many BINAP-based Lewis acid-catalyzed reactions have been proposed. For example, in the palladium-catalyzed enantioselective Diels-Alder reaction shown below, the dienophile is thought to coordinate the metal center at the equatorial sites. Thus the equatorial phenyl group on phosphorus obstructs the Si-face, resulting in excellent enantioselectivity. A very similar model was used to rationalize the outcome of a nickel-catalyzed asymmetric enolate alkylation reaction, where the substrate also bears an auxiliary that allows it to chelate onto the metal. On the other hand, a copper(I)-catalyzed hetero-ene reaction is thought to proceed through a tetrahedral intermediate, offering an alternative mode of stereoinduction by changing the metal center.
caption: Left: Structure of (R)-BINAP. Right: Structure of an (R)-BINAP ruthenium complex, highlighting the equatorial (purple) and axial (green) coordination sites, and the equatorial and axial phenyl groups that enforce the asymmetric environment for incoming ligands.
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