Nitrogen silver-catalyzed synthesis

Silver-Catalyzed Synthesis of Nitrogen-Containing Heterocycles... 

Several reviews on the synthesis of aziridines have been published in the previous year. These publications include a review on the silver catalyzed addition of nitrenes (among other intermediates such as carbene) across a double bond <06EJOC4313> a review on sulfur ylide addition to imines to form aziridines <06SL181> a review on nitrogen addition across double bonds <06ACR194> a general review on functionalization of a,p-unsaturated esters with some discussion of aziridination <06TA1465>... 

As a part of a program directed toward the synthesis of the potent topisomerase I inhibitors, the lamellarins (e.g., 153 and 154), Porco has reported the silver triflate-catalyzed tandem cycloisomerization-azomethine ylide cycloaddition of 155 (Scheme 2.42).75 The postulated mechanism of this intriguing and highly efficient process is shown in Scheme 2.43. Silver-catalyzed addition of the imine nitrogen to the alkyne results, on subsequent deprotonation, in the formation of an azomethine ylide 160. This ylide participates in [3+2] cycloaddition with the alkyne component leading to formation of a dehydropyrrole 161. Finally, oxidation by adventitious oxygen leads to formation of the product 162. 

In addition, silver-catalyzed asymmetric aza-Diels-Alder reactions provide a useful route to optically active nitrogen-heterocyclic compounds such as piperidines or pyrid-azines. Substituted dihydrobenzofurans can also be enantioselectively prepared through silver-promoted allylation of aldehydes. Other types of silver-mediated cyclizations can also be used in the synthesis of tetrahydrofnrans, tetrahydropyrans, 1,2-dioxetanes, 1,2-dioxolanes, medium-sized lactones, dihydroisoqninolines, and so on. Silver salts can also be used as cocatalysts with other transition metals. Unique activity was observed for these silver-based systems in several cases. Conseqnently, the use of silver can enrich several available heterocyclization methods, and fnrther developments in the application of chiral silver complexes will hopefnlly appear in the near future. 

Intramolecular nitrogen attack in propargylated enaminones allows silver-catalyzed access to functionalized pyrroles. This Ag-promoted hydroamination can also be used to obtain A bridgehead pyrroles. Silver nitrate-mediated cyclization of allenylamines, available from Uthiated alkoxy allenes and imines or through reaction of l-(lV-carbamoyl)-alkylcuprates with propargyl substrates, provides access to 2,5-dihydropyrrole derivatives. Iminoallenes can be used for the synthesis of substituted p)moles in moderate yields in the presence of potassium carbonate. ... 

The synthesis of nitrogen heterocycles has been achieved through variations of intramolecular imino-Nazarov cycUzations (Schemes 3.26 and 3.27) [26], A silver-catalyzed version of the reaction generated indolines in moderate to good yields from a range of functionalized aldehydes and secondary amines, while a gadolinium-promoted reaction afforded fused tetrahydroquinoline derivatives in moderate yields. 

Nitrogen heterocycles can also b e prepared by silver(I)-catalyzed cyclization reaction. Asao, Yamamoto, and their colleagues have shown synthesis of 1,2-dihydro-isoquinoline derivatives by addition of pronucleophiles to ortho-alkynylaryl ald-imines employing AgOTf as a catalyst [29]. For example, treatment of imine (19) with 2 equiv of nitromethane (20) in the presence of 3 mol% of the catalyst at 80 °C, 1,2-dihydroisoquinoline derivative (21) is produced in 85% yield (Scheme 18.7). Terminal alkynes and carbonyl compounds possessing activated methylene groups are also usable as the pronucleophiles [29]. 2-Alkoxyazetidines are efficiently synthesized by Ag(fod)-catalyzed [2-F2] cycloadditions of imines to (alkoxymethylene)... 

---