Reactions of Vinyl Azides

Vinyl azides bearing a carbonyl group of a ketone or an ester in the a-position like 

171 (R =aryl) in aprotic solvents, for example, xylene at 140 °C, is more important and has been applied very frequently to prepare indoles of type 176 via intermediate 172. This method is connected with the names of Hemetsberger and Knittel and can be transferred to the synthesis of azaindoles, fused indoles, and fused pyrrol derivatives. Quite recently, reactions like 171 (R =Ph) — 176 have been conducted in a rhodium(II)-catalyzed way, which allows to decrease the necessary temperature into a range of 25-60 C. The transformation of 171 into isoquinolines is also possible if one or both ortho positions of the aryl group R are substituted appropriately.  

185a and 185b were successful under mild conditions. Of the four imaginable diaste-reomeric [4 + 2] cycloadducts exo-exo endo-exo exo-endo endo-endo), the exo-endo stereoisomers 189 were isolated exclusively. Thus, the cyclopentadiene was added to the exo side of the bridgehead aziiines to form 5-azanorbomenes with an attached three-membered ring in the endo position.  

The priority of vinyl azides over alkyl azides in light absorption and photochemical transformation was demonstrated by the irradiation of diazide 192, which led to the strained heterocycle 193 without degradation of the second azido group. The 

3-bridged 2//-azirines are stabilized by bulky substituents in the 2-position. This was shown, for example, by liberating compound 194 from its solution to get a slight yellow solid that was stable for a short time. On the other hand, the tricyclic azirine derivative 196 could be handled only in solution at low temperature because it is transformed quantitatively into the ring enlargement product 197 at room temperature. 

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Reactions of Vinyl Azides G. L abbe, Angew. Chem., Int. Ed. Engl., 1975, 14, 775-782. Cyclic Hydroxylamines Review of Preparative Methods and Properties R. T. Coutts and A. M. K. El-hawari, Heterocycles, 1974, 2, 669-743. 

Reaction of vinyl azides with nitrosyl tetrafluoroborate gives furazans in good yield, particularly when the alkene is 1,2-dialkyl-substituted. For example, 1-azidocyclooctene and 2-azidobut-2-enes yield (19 n = 6) and (1, R1 = R2 = Me), respectively 1,2,4-oxadiazoles may also be formed as by-products. 

Hassner and Belinka118 generated primary enamines in situ by reaction of vinyl azides 67 with alkyl lithium derivatives and converted them to ketones (equation 45). 

Reactions of vinyl azides stable at room temperature... 

In this chapter we will present a review of the literature through 1969 on the preparation and reactions of vinyl azides, then discuss briefly the conclusions which may be drawn concerning the mechanism of vinyl azide decomposition. A short section will also be included on the chemistry of azidoquinones. 

A limited number of reactions of vinyl azides are known in which molecular nitrogen is not extruded. These reactions, which will be discussed in Part C of this section, include nucleophilic displacement of the azido function and isomerization of the vinyl azide to a triazole. 

A. Thermally and Photochernically Induced Expulsion I. Reactions of vinyl azides stable at room temperature... 

The simplest member of the class of vinyl azides, H2C=CH-N3, has been known for about 100 years. However, it was not until the late 1960s that vinyl azides became an important and synthetically useful class of organic compound. The most interesting and important reaction of vinyl azides is the formation of azirines produced upon photolysis and thermolysis of vinyl azides. Smolinsky and Pryde first observed azirine formation, together with a small amount of keteneimine, by gas-phase pyrolysis of a-aryl-substituted vinyl azides. 

Scheme 23 A Cu-catalyzed reaction of vinyl azides and pyridines to form imidazo[1,2-o] pyridines.

A solid-supported carbonate reagent was used to initiate the formation of 2H-l,4-benzoxazin-2-ones 303 from P-nitroacrylate 301 and 2-aminophenol 302 (Scheme 7.72) [154]. This hetero-Michael addition/intramolecular transesteri-fication/elimination of HN03/[l,3]-proton shift domino reaction proceeded in moderate to excellent yields. In another study, a series of various functionalized pyrrolo[l,2-r]pyrazines 305 were made synthetically available through the reaction of vinyl azides 304 and lH-2-pyrrolecarbaldehyde (Scheme 7.73). The deprotonated pyrrole initially reacts in a Michael manner with the vinyl azides, with nitrogen as... 

An investigation of the reactions of vinyl azides with enamines has been published, in which the results are given extensive discussion with consideration... 

Not only azirines but also nitriles were obtained in thermal reactions of vinyl azides (see, for example. Schemes 5.15 and 5.17). Hassner and coworkers summarized some rules, ° which allow to expect the former or latter products after loss of dinitrogen from enazides. The 1-azidocyclopentadiene 202 tnmed ont to be highly unstable because fragmentation to furnish the nitrile 203 was observed already at room temperature (Scheme 5.24). On the other hand, the isomer 204 was isolated as a relative stable lemon-yellow solid, which was converted to the indole 205 by reflnxing in toluene or irradiation in chloroform. In some cases, the formation of cyano compounds from vinyl azides was... 

In addition, the Chiba group reported that the reaction of vinyl azides 72 and internal alkynes 73 in the presence of a catalytic amount of [Cp RhCl2]2 and Cu(OAc)2 with AcOH in dimethylformmide (DMF) could provide isoquinolines 74 in moderate to good yields (Scheme 16.33) [54]. It was speculated that the reaction could be initiated by the formation of N—H imines B from vinyl azides 72 via the Cu-mediated reductive ring opening of highly strained three-membered cyclic imines, 2//-azirines... 

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