Chiral palladacycle

There had been doubts about the utility of palladacycles in asymmetric catalysis, raised by the failure to achieve enantioselectivity as a result of a slow release of low ligated Pd(0) (naked Pd) [54]. However, recent success of several planar chiral palladacycles in highly enantioselective aza-Claisen reactions and in a number of other applications proves that the coordination shell of the Pd(II) species is not necessarily destroyed during the catalytic action. 

An interesting application of the chiral cyclopalladated complexes is palladium-promoted asymmetric Diels-Alder reactions of l-phenyl-3,4-dimethylphosphole (Scheme 4). In the original report on the Diels-Alder reaction of the phosphole reported by Nelson and co-workers, the dichloropalladium species was employed as a promoter.f In 1994, Leung showed that a chiral cyclopalladated complex was capable of promoting the Diels-Alder reaction. In this reaction, the chiral palladacycle worked as a chiral auxihary and showed almost perfect diastereoselectivity. It has been revealed that simultaneous precoordination of a diene and a dienophile to the palladium center is essential for the reaction. Thus, both dienes and dienophiles should possess Lewis basic functionahty. The diene l-phenyl-3,4-dimethylphosphole has been the only substrate examined so far, while a variety of dienophiles, such as vinylphosphine, vinyl-sulfoxide, vinylsulfide, acrylamide, vinylarsine, vinylpyridine, vinyl-pyrrole, or methylenequinuchdinone, have successfully been applied to this asymmetric Diels-Alder reaction. 

Chen S, Ng JK-P, Pullarkat SA, Liu F, Li Y, Leung P-H (2010) Asymmetric synthesis of new diphosphines and pyridylphosphines via a kinetic resolution process promoted and controlled by a chiral palladacycle. Organometallics 29 3374—3386... 

Scheme 2.5 Resolution of monophosphines by means of chiral palladacycles.

A chiral TRIP anion combined with a palladacycle provides an active catalyst for enantioselective rearrangement of allylic imidates to the corresponding amide products (Scheme 98). Although the catalyst contains a chiral palladacycle, the stereoselectivity is induced by the chiral phosphate anion (Scheme 98). 

New applications of chiral palladacycle catalysts were carried out by Liu and Shi [67], They synthesized axially chiral cyclometalated bidentate NHC-Pd(II) complexes derived from binaphthyl-2,2 -diamine and used them for the asymmetric addition of indole to N-tosylaryliniines. 

Palladacycles are defined as compounds with a Pd-C CT-bond with the Pd being stabilized by one or two neutral donor atoms, typically forming 5- or 6-membered rings [51]. Ferrocenyl palladacycles constitute a particularly attractive catalyst class partly due to the element of planar chirality. The first diastereoselective cyclopallada-tion of a chiral ferrocene derivative was reported in 1979 by Sokolov [52, 53]. 

In 1997 the first asymmetric aza-Claisen rearrangement was reported by Overman et al. [55], which made use of diamines as bidentate ligands for Pd(II), allowing for moderate enantioselectivities. In the same year, Hollis and Overman described the application of the planar chiral ferrocenyl palladacycle 38 as a catalyst for the enantioselective aza-Claisen rearrangement of benzimidates 39 (Fig. 19) [56]. A related ferrocenyl imine palladacycle provided slightly inferior results, while a benzylamine palladacycle lacking the element of planar chirality was not able to provide any enantioselectivity [57]. 

In 2005, Moyano et al. [60] reported a new type of chiral dimeric ferrocene palladacycle 43 that lacked the element of planar chirality and involved three... 

A related planar chiral Co-based oxazoline palladacycle COP-X (46) was later found to be of higher synthetic utility as it permitted the use of benzimidates, [62] as well as allylic trifluoro- [63] and trichloroacetimidates [64, 65]. 46 was found to be superior to its ferrocene analogue 41 [61] in a number of aspects such as ease of... 

Anderson CE, Donde Y, Douglas CJ, Overman LE (2005) Catalytic asymmetric synthesis of chiral allylic amines. Evaluation of ferrocenyloxazoline palladacycle catalysts and imidate motifs. J Org Chem 70 648-657... 

Palladacycle (109) and its analogues induce chirality of A -allylalkanamides during rearrangement of allylic imidates. For a synthesis of a-branched chiral 4-alkene-thioamides, it is convenient to allylate thioamides of (-F)-frany-2,5-diphenylpyrroli-dine. This process involves a thio-Claisen rearrangement. 

Cyclopalladated complexes, which are most conunonly prepared from tetrachloropaUa-date(II) and appropriate amines or imines, have drawn the attention of inorganic, coordination, and organometallic chemists since the first report in 1965 (for imines) and in 1968 (for amines) by Cope and co-workers. ° These palladacycle species can easily be transformed into chiral form by employing corresponding chiral amines or imines. Two representative examples of the chiral cyclopalladated species are illustrated in Figure 1. This section will describe three examples that represent applications of these chiral orthometallated palladium complexes in organic synthesis. 

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