Silver catalysis cycloaddition

The chemical reactions possible with silver catalysis are multiple and cover cycloadditions, cycloisomerizations, allylations, aldol reactions, and even C-H bond activation. Also, asymmetric versions are known, even though they still need to be improved.3-10... 

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 intramolecular cycloaddition of the Simmons-Smith cyclopropanation product 30 of nor-born-5-en-2-one trimethylsilyl enol ether 29 is promoted by donor substitution and/or silver catalysis. ... 

AIkynyl(diisopropylsilyl)oxy-diazoacetates (295) undergo intramolecular 1,3-dipolar cycloaddition in good yield when R = H (isolation of silver pyrazolide 296 was possible) and R, R = Me,Me or (CH2)s, but no reaction occurred when R = H, R = H or R =Me (340). The silicon substimtion is apparently crucial. Replacement of the Si(i-Pr)2 in 295 (R = R = H) by Si(f-Bu)2 allowed an uncatalyzed intramolecular [3 + 2] cycloaddition to take place [xylene, 140-160 °C, 11% yield (340)], while Ag(I) catalysis led to decomposition. A diazoacetic acid (2-propyn-l-yl)oxysilyl ester also produced a bicyclic pyrazole, but in low yield. On the other hand, the same diazo compound 295, which reacted intm-molecularly under silver ion catalysis, underwent dimerization by an /nfermolecular... 

Cooperative catalysis using cinchona alkaloid derivatives in combination with metals such as silver have also been widely developed. On the basis of this concept, Escolano et al. have disclosed an enantioselective domino Michael-cyclisation reaction. This formal [3 + 2] cycloaddition occurred between isocyanoacetates and enones in the presence of a combination of a chiral hifunctional cinchona alkaloid, such as cupreine, and AgNOs to provide the corresponding chiral 2,3-dihydropyrroles in low to high yields and... 

More recently, a study by Lassaletta, Fernandez and co-workers showed that enantioselective NHC-Ag catalysis was not an impossible goal. Hence, the use of polydentate thiolato-NHCs 11-14 afforded good enantioselectivities in the 1,3-dipolar cycloaddition of azomethine ylides with acrylates (Scheme 11.4). ° Ligand 11 systematically produced the highest ee s (up to 80% ee) with a number of imino esters. The authors proposed that a tridentate SCS coordination of these C2-symmetric ligands onto the silver center was key for obtaining the good chiral induction observed in the cycloaddition. 

The cationic zirconium-mediated aldehyde addition methodology has recently been used by Maier and coworkers in their study toward the taxol skeleton [57] (Scheme 10). The diene precursor to a planned cycloaddition was prepared by first adding zirconocene 40 to aldehyde 41 under silver(I)-catalysis, which afforded alcohol 42 as a 77 23 mixture of diastereomers. Protection of the... 

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