Silver complexes cycloadditions

Scheme 6.7 shows some other examples of enantioselective catalysts. Entry 1 illustrates the use of a Co(III) complex, with the chirality derived from the diamine ligand. Entry 2 is a silver-catalyzed cycloaddition involving generation of an azome-thine ylide. The ferrocenylphosphine groups provide a chiral environment by coordination of the catalytic Ag+ ion. Entries 3 and 4 show typical Lewis acid catalysts in reactions in which nitrones are the electrophilic component. 

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

As mentioned above, Grigg et al. had performed a brief reconnaissance into chiral silver complexes as inducers of chirality in the [3+2] cycloaddition.16 This work... 

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. 

Table 2.7).54 The tricyclic products 113 were formed in good yield and excellent enantioselectivity. Interestingly, the PHOX-silver complex performed less well in intermolecular cycloadditions.

Silver salts and phosphine complexes have proved themselves as catalysts and mediators of a range of cycloadditions. No doubt only the tip of the iceberg has been uncovered regarding the possible applications of silver-mediated cycloadditions. 

Silver salts or reagents have received much attention in preparative organic chemistry because they are useful catalysts for various transformations involving C-G and C-heteroatom bond formation.309 Especially, the silver(i)/ BINAP (2,2 -bis(diphenylphosphino)-l,T-binaphthalene) system is a very effective catalyst for a variety of enantio-selective reactions, including aldol, nitroso aldol, allylation, Mannich, and ene reactions. Moreover, silver salts are known to efficiently catalyze cycloisomerization and cycloaddition reactions of various unsaturated substrates. Recently, new directions in silver catalysis were opened by the development of unique silver complexes that catalyze aza-Diels-Alder reactions, as well as carbene insertions into C-H bonds. 

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. 

The reaction of VCP 79 illustrates the performance of the rhodium(I) dimer (Tab. 13.4). For reference, attempts to effect [5+2] cycloadditions with this substrate (79) and [RhCl(PPh3)3]/silver triflate resulted only in the formation of complex product mixtures. In remarkable contrast, when this same substrate was treated with 5 mol% [RhCl(CO)2]2 for 20 min in toluene at 110°C, the [5+2] cycloadduct 80 was obtained in 80% yield. Despite these significant advantages, tethered alkene-VCPs are not successfully converted with this catalyst. 

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,... 

In addition to participating in [2 + l]-cycloaddition reactions, divalent reactive intermediates can form ylides in the presence of carbonyl or other Lewis basic functionalities.108 These ylides participate in cycloaddition or other pericyclic reactions to furnish products with dramatically increased complexity. While carbenes (or metal carbenoids) are well known to participate in these pericyclic reactions, silylenes, in contrast, were reported to react with aldehydes or ketones to form cyclic siloxanes109,110 or enoxysilanes.111,112 Reaction of silylene with an a,p-unsaturated ester was known to produce an oxasilacyclopentene.109,113,114 By forming a silver silylenoid reactive intermediate, Woerpel and coworkers enabled involvement of divalent silylenes in pericyclic reactions involving silacarbonyl ylides115 to afford synthetically useful products.82,116,117... 

On the other hand, silver salts can act as a mild Lewis acid, promoting various reactions such as allylations, aldolizations, cycloadditions, and cyclizations. Silver salts can also be used as halide scavengers, acting as cocatalysts in cross-coupling reactions catalyzed by other metals, especially palladium. In the latter context, the exact role of silver salts is far from clear and may be more complex than just halide... 

It is more efficient to obtain cycloaddition products via a stable silver(I) triflate complex of trans-cycloheptenes that is preferably not isolated, e.g. formation of 21. ... 

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). ... 

In 2014 several reports described the use of inverse-electron-demand Diels-Alder reactions to furnish naphthalene-based compounds from phthalazine-containing ring systems. Employing nonprecious metal complexes, copper(l) and nickel(O) complexes, instead of silver salts to catalyze formal [4+ 2] cycloadditions of 1,2-diazines and siloxyalkynes, Rawal and his group developed an environmentally friendlier and more economical method for preparing siloxy derivatives of naphthalene, anthracene, and phenanthrene (Scheme 16) (140L3236). In addition, the copper catalyst could also be employed for the synthesis of the corresponding quinolines and isoquinolines. This method provided the respective products in... 

Another convenient synthesis of cyclobutanones employs cycloadditions of keten-immonium ions. These have previously been generated by the rather expensive method of dechlorination of an a-chloro-enamine with silver fluoroborate, but a new procedure which uses zinc chloride instead is reported to give high yields (Scheme 26). Even ethylene was found to react with the complex (319) at room temperature and atmospheric pressure. ... 

The highly substituted, chelating, vinylcarbene ruthenium complex 22 has been prepared by Fiirstner [40] via the reaction of 6 with ortho-isopropoxyethynylbenzene in dichloromethane at room temperature. Silver chloride (AgCl) was added to this reaction as a phosphine scavenger. Complex 22, which results from a formal insertion of the triple bond into the Ru=C via a [2-1-2] cycloaddition, was not evaluated as a metathesis catalyst in this report. 

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