Olefin are an important class of unsaturated ligands that bind to a metal by σ−donating its C=C π−electrons and also accepts electrons from the metal in its π* orbital of C=C bond. These symbiotic σ−donation and π−back donation in metal bound olefin complexes have a significant impact on their structure and reactivity properties. The olefin-metal bonding interaction is best explained by the Dewar−Chatt model, that takes into account two mutually opposing electron donation involving σ−donation of the olefinic C=C π−electrons to an empty dπ metal orbital followed by π−back donation from a filled metal dπ orbital into the unoccupied C=C π* orbital. The first metal olefin complex dates back a long time to the beginning of 19th century, in 1827, the Danish chemist Zeise synthesized the famous Zeise's salt K[PtCl3(C2H4)]∙H2O containing a Pt bound ethylene moiety, which is first metal−olefin complex. Nowadays, a large number of complexes using alkenes as ligands have been developed and have a wide range of applications.
Fig.1 The 3D ball structure of Zeise's salt
Although there are many classification way of olefin ligands, the most common and most clear way is based on their structure, based on this, olefin ligands can be divided into alkene ligands, alkyne ligands, polyene ligands and cycloalkene ligands. The details are as follows:
The main application of olefin ligands is catalysis, and the most popular and extensive is in asymmetric catalysis. During the last century, in addition to many coordination studies, olefin complexes involving late-transition metals such as Rh, Ir, Pt, Pd or Ni are mostly employed as catalyst precursors in catalysis. Today, chiral olefins have recently emerged as among the most promising ligands for asymmetric catalysis, which exhibit higher activity and selectivity than the traditional nitrogen, phosphine, oxide ligands in some reactions, and solved some problems that other ligands could not do.
The types of chiral olefins ligands commonly used in asymmetric catalysis are bicyclic dienes, chain diene ligands, phosphorus/terminal-olefin hybrid ligands, and sulfur/olefin hybrid ligands. They have been successfully utilized in a series of transition-metal-catalyzed asymmetric reactions, such as iridium-catalyzed asymmetric hydrogenation of imines, allylic substitution; rhodium-catalyzed conjugate addition of organoboron reagents to α, β-unsaturated compounds, 1,2-addition of organoboron reagents to imines/carbonyl compounds, intramolecular hydroacylation; and palladium-catalyzed asymmetric allylic alkylation/amination/etherification of allylic esters, as well as Suzuki-coupling reactions. In many cases, the reactions occur with high enantioselectivities, allows for access to a broad range of valuable chiral products.
Fig.3 The asymmetric catalysis by coordination compound based on chiral-chain diene ligand
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-  Yu Y N.; et al. Chiral phosphorus-olefin ligands for asymmetric catalysis[J]. Acta Chim. Sinica, 2017, 75(7), 655-670.