Silver azomethine complex

Computational studies yielded the models shown in Figure 2.2. These models illustrate how the existence or absence of hydrogen bonding between the silver/ligand complex and the dipolarphile may lead to selection of opposite faces in the reaction with the silver-bound azomethine ylide derived from 111. 

The mechanism proposed involves desilyation of the silver complexed imidate and cycloaddition by the azomethine ylide 35 to give 37 followed by elimination (22). 

Another approach employing chiral acyclic azomethine ylides was published in two recent papers by Alcaide et al. (85,86). The azomethine ylide-silver complex (51) was formed in situ by reaction of the formyl-substituted chiral azetidinone (50) with glycine (or alanine) in the presence of AgOTf and a base (Scheme 12.18). Azomethine ylides formed in this manner were subjected to reaction with various electron-deficient alkenes. One example of this is the reaction with nitrostyrene, as illustrated in Scheme 12.18 (86). The reaction is proposed to proceed via a two step tandem Michael-Henry process in which the products 52a and 52b are isolated in a... 

During the reaction silver(0) deposits on the sides of the reaction vessel. The sequence of events leading to the azomethine ylide is unclear. However, evidently single-electron transfer (SET) from the amine to silver takes place either prior or subsequent to fluoride-enabled silyl cleavage. This process is repeated with a second equivalent of silver fluoride resulting in the formation of 38 either in free form or more likely as its silver complex. The scope of the method was expanded to the synthesis of bicylic systems exemplified here by tropinone 42 (Scheme 2.11).19 Pandey has also extended the protocol to the synthesis of tricycloalkanes20 and applied it to a total synthesis of the poisonous frog alkaloid epibatidine.21... 

Zhou also reported a series of related P,S-ferrocenyl ligands and their use in the [3+2] cycloaddition of aryl-substituted azomethine ylides with A-phenylmalei-mide.52 While these silver complexes were able to efficiently catalyze the reaction, the enantioselectivity was lower than in the protocol described above. 

While virtually all of the research described above has focused on the inter-molecular cycloaddition of azomethine ylides, the intramolecular process holds considerable promise for the synthesis of polycyclic natural products. The Pfaltz group reported an intramolecular catalytic asymmetric cyclization of aryl iminoesters 112 using a complex of silver acetate with PHOX type ligand 100 (Scheme 2.29,... 

The cycloaddition of in sitM-generated azomethine yhdes with electron-deficient alkenes is a useful method for the generation of stereodefined, substituted pyrrolidines, and there has been some recent interest in the development of a catalytic asymmetric variant. While a variety of methods for the generation of azomethine ylides have been developed, treatment of an a-iminoester (8.200) with an amine base in the presence of metal salts is the process most commonly employed in the asymmetric variant, which generally uses an enantiomerically pure metal complex of copper, silver or zinc to give an N-metallated ylide (8.201) (Figure 8.6). ... 

Zhang and coworkers screened a variety of phosphine ligands in the silver-catalysed cyclo addition of the azomethine yhde generated from aryl imines such as (8.202) with dimethyl maleate (8.203), and discovered that the highest ees are achieved with the bis-ferrocinyl amide phosphine (8.204). ° During Zhang s studies it was shown that silver complexes with BINAP are poor catalysts in the addition with dimethyl maleate, however Sansano and coworkers have discovered... 

The silver complex prepared from 122 promotes dipolar cycloaddition reactions of azomethine ylides to give products such as 120 with excellent en-antioselectivity (95% ee). This class of ligands has been shown to be more conveniently prepared and, importantly, it is possible to gain access to a wide range of structural variants of the parent scaffold, permitting facile ligand optimization. 

---