Allenyl azides

Heated in methanol for an extended period of time, propargyl azide 1147 experiences a [3,3] sigmatropic shift to allenyl azide 1148 that undergoes rapid cyclization to triazafulvene 1149. Addition of a molecule of methanol converts reactive intermediate 1149 to triazole 1150 that is isolated in 68% yield. In concentrated solutions, two molecules of intermediate 1149 may undergo cycloaddition to form dimer 1151 as a side product (Scheme 189) <2005EJ03704>. 

The pentacyclic alkaloid ( )-meloscine (134) was prepared by Feldman and Antohne using a clever allenyl azide cycloaddition/cyclization cascade to deliver the core azabicyclo[3.3.0]octadiene substructure (2012OL934). Strain-driven release of nitrogen from the dipolar cycloadduct 129 derived from 128 promotes formation of the azatrimethyl-enemethane diradical 130 en route to the bicyclic product 131 (Scheme 30). For the synthesis ofmelo-scine 134, the thermolysis of a dilute solution of allene 132 in toluene gave the desired bicycle 133 whose structure was estabfrshed by single crystal X-ray analysis. Subsequent manipulation of the peripheral functionality in 133 then delivered ( )-meloscine 134. 

Faza ON, Feldman KS, Lopez CS (2010) Cyclization cascade of allenyl azides synergy between theory and experiment. Curr Oig Chem 14(15) 1646-1657... 

Also nitrenes, generated from the allenyl azides 99, undergo the [2+1] cycloaddition reaction to give the annelated indole derivatives 100. ... 

Allenes react with azides across one of their C=C= bonds to give a mixture of the expected stereoisomeric 1,2,3-triazolines. For example, from 1,2-nonadiene and phenyl-and 4-bromo-phenyl azide mixtures of the two stereoisomeric cycloadducts are obtained °. Also, picryl azide adds to tetramethylallene to give 4-isopropylidene-5,5-dimethyl-1 -picryl-1,2,3-triazoline . In contrast, heating of the allenyl azides 77 at 100 °C leads to the formation of tricyclic imines, formed by loss of nitrogen from the initial cycloadducts. 

Scheme 5.33 Generation of allenyl azides by [3,31-slgmatroplc rearrangement of propargyl azides and transformation of these short-lived intermediates into 1,2,3-triazoles and Diels-Alder products (NuH = nudeophllef ...

Starting with the substrate 274, for example, treatment with sodium azide and then with nucleophiles NuH, such as methanol, water, or ammonia, in the presence of the base sodium hydroxide furnished the triazoles 277 via the cascade B (Scheme 5.36). On the other hand, the products 276 were formed from 274 through the sequence A without the strong base sodium hydroxide. Thus, the nucleophile anunonia was dominating over sodium hydroxide. But without the latter, base-induced generation of allenyl azides of... 

Scheme 5.38 Photolysis of allenyl azides and generation of 2-methylene-2H-...

Most probably, the rapid electrocyclic ring closure of allenyl azides was responsible for the fact that the chemistry of these azides was discovered so lately. This cyclization of azidoallenes allowed to create cascade reactions for the synthesis of functionalized NH-l,2,3-tiiazoles. 

---