Azides, propargylic

Monoaddition of 1,3-butadiene followed by instantaneous halide transfer from the counter anion and selective formation of the trans-l,4-adduct (PIB-AUyl-X) was observed in hexanes/MeCl 60/40 (v/v) solvent mixtures at —80 °C at [1,3-butadiene] <0.05 mol 1 ([1,3-butadiene]/[chain end] <12). Simple nucleophilic substitution reactions on these chloro or bromoallyl functional PIBs allowed the syntheses of end functional PIBs including hydroxy, amino, carboxy, azide, propargyl, methoxy, and thymine end groups [131]. 

HG. 51. Azides and propargylated dendrons for the rapid assembly of glycodendrimers. 

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>. 

Azides 1253 obtained from propargyl halides or sulfonates 1252 undergo sigmatropic rearrangement to azidoal-lenes 1254, which subsequently undergo cyclization to triazafulvenes 1255. Under the reaction conditions, species 1255 react with another molecule of sodium azide to furnish triazoles 1256. Products 1256 are isolated in 65-97% yield (Scheme 209) <2005S1514>. 

Propargylated tosylamides also isomerize efficiently [221, 281]. On the other hand, rare examples are propargylated hydrazines [251], N-propargylated imines [282], isonitriles (must be N-propargylated) [283], ammonium salts [284] and azides [285],... 

Several other propargylic substrates have successfully undergone [3,3]-sigmatropic rearrangements. Examples are thiocyanates to isothiocyanates [580], the opposite conversion [581] and the equilibrium between such species as exemplified by 173 (Scheme 1.77) [582], cyanates to isocyanates [582] and the formation of isoselenocya-nates [583] and azides [584]. 

Potassium cyanide Propargyl alcohol Sodium azide Sodium cyanide... 

In general, acyl azides are too unstable to survive at the temperatures required for addition to acetylenes, although benzoyl azide adds readily to ynamines in toluene. Ethoxycarbonyl azide also gives triazoles in good yield with ynamines. The azide adds to propargylic alcohols in boiling ethanol, and to acetylene at 100° under pressure. Addition to phenylacetylene and to electron-deficient acetylenes has been carried out at 130°. Oxazoles are also formed at this temperature by competing thermal decomposition of the azide, and addition of ethoxycarbonylnitrene to the acetylenes. The triazole obtained from phenylacetylene is 2-ethoxycarbonyl-4-phenyltriazole the two 1-ethoxycarbonyltriazoles can be isolated if the addition is carried out at 50° over several weeks. Since the IH- to -triazole isomerization takes place readily in these systems, a IH-structure cannot be assumed for a triazole formed by addition of these azides. 

Organic azides belong to the propargyl-allenyl category of dipoles, and are popular for synthetic transformations because of their ready availability (Scheme 9.1) (1). 

The preparation of the allene bis-epoxide 1 started with isovaleraldehyde 9. Addition of the protected propargyl alcohol 10 under the Carreira conditions led to 11 in > 95% . Mesylation followed by displacement with methyl cuprate provided the allene without loss of enantiomeric excess. Oxidation of the allene 12 with dimethyldioxirane could have led to any of the four diastereomers of the spiro bis epoxide. In the event, only two diastereomers were observed, as a 3 1 mixture. That 1 was the major diastereomer followed from its conversion to 3. The configuration of the minor diastereromer was not noted. Exposure of 1 to nucleophilic azide then gave the easily-purified 2. 

Sixteen-Electron. Those for which the dipolar canonical form has a double bond on the sextet atom and the other resonance structure has a triple bond. Examples are azides (R—N3), diazoalkanes (R2C=N=N), and nitriloxides (R—C= N—O). These have also been labeled as propargyl/allenyl anion type [270]. 

Organic azides are octet-stabilized 1,3 dipoles of the propargyl-allenyl type, la<- lb.15 The dipolar character of the azido group9,15 enables it to... 

Diazoalkanes, like azides, are 1,3-dipoles of the propargyl-allenyl type (Scheme 87)15 and their reaction with imines provides a route for building the triazoline framework from the C—N—N and C—N fragments. Although diazomethane addition to the carbon-carbon double bond was achieved by von Pechmann in 1898,325 its reaction toward the carbon-nitrogen double bond was investigated only 50 years later. 

---