Vinyl azides nucleophilic substitution

In most cases it is easy to rationalize the products obtained from vinyl azide reactions which do not involve the loss of molecular niaogen. For example, the substitution of the azido group by a nucleophilic reagent in the 2-azidovinyl ketone 73 (R = H) and the )3-dicyanovinyl... 

Many attempts at other modifications of poly(vinyl chloride) were reported in the literature. Often, the reactions are based on expectations that the polymers will react like typical alkyl halides. Unfortunately, in place of nucleophilic substitutions, the polymers often undeigo rapid and sequential eliminations of HCl along the chains. Nevertheless, many substitution reactions are still possible and can be successfully carried out. One example is a replacement of 43% of the chlorine atoms with azide groups... 

Alkyl-, vinyl-, and aryl-substituted acyl azides undergo thermal 1,2-carbon-to-nitrogen migration with extrusion of dinitrogen — the Curtius rearrangement — producing isocyantes. Reaction of the isocyanate products with nucleophiles, often in situ, provides carbamates, ureas, and other A-acyl derivatives. Alternatively, hydrolysis of the isocyanates leads to primary amines. 

The only alternative method for the synthesis of vinyl azides already known in 1923 was the nucleophilic substitution of vinyl halides bearing an electron-withdrawing group in the j8-position. However, this method was used at first mainly for the generation of azidoquinones as depicted in Scheme 5.2. ... 

Scheme 5.3 Synthesis of vinyl azides by nucleophilic substitution of vinyl halides bearing an electron-withdrawing group " ...

The aziridine aldehyde 56 undergoes a facile Baylis-Hillman reaction with methyl or ethyl acrylate, acrylonitrile, methyl vinyl ketone, and vinyl sulfone [60]. The adducts 57 were obtained as mixtures of syn- and anfz-diastereomers. The synthetic utility of the Baylis-Hillman adducts was also investigated. With acetic anhydride in pyridine an SN2 -type substitution of the initially formed allylic acetate by an acetoxy group takes place to give product 58. Nucleophilic reactions of this product with, e. g., morpholine, thiol/Et3N, or sodium azide in DMSO resulted in an apparent displacement of the acetoxy group. Tentatively, this result may be explained by invoking the initial formation of an ionic intermediate 59, which is then followed by the reaction with the nucleophile as shown in Scheme 43. 

The nucleophilic addition of alcohols [130, 204-207], phenols [130], carboxylates [208], ammonia [130, 209], primary and secondary amines [41, 130, 205, 210, 211] and thiols [211-213] was used very early to convert several acceptor-substituted allenes 155 to products of type 158 and 159 (Scheme 7.25, Nu = OR, OAr, 02CR, NH2, NHR, NRR and SR). While the addition of alcohols, phenols and thiols is generally carried out in the presence of an auxiliary base, the reaction of allenyl ketones to give vinyl ethers of type 159 (Nu = OMe) is successful also by irradiation in pure methanol [214], Using widely varying reaction conditions, the addition of hydrogen halides (Nu= Cl, Br, I) to the allenes 155 leads to reaction products of type 158 [130, 215-220], Therefore, this transformation was also classified as a nucleophilic addition. Finally, the nucleophiles hydride (such as lithium aluminum hydride-aluminum trichloride) [211] and azide [221] could also be added to allenic esters to yield products of type 159. 

Unfortunately, attempts to perform this substitution reaction on cyclohexenol and geraniol led to the exclusive formation of the corresponding silyl ethers. It thus would seem that one requirement for effective carbon-carbon bond formation is that allylic alcohols be secondary and have possess y,y-disubstitution. Pearson, however, discovered a method with less restriction on the natiue of the substrate he used allylic acetates with y-mono-substitution or primary alcohols [96]. Not only ketene silyl acetals but also a diverse set of nucleophiles including aUyl silane, indoles, MOM vinyl ether, trimethylsilyl azide, trimethylsilyl cyanide, and propargyl silane participate in the substitution of y-aryl allylic alcohol 90 to give allylated 91 (Sch. 45). Further experimental evidence suggests that these reactions proceed via ionization to allylic carboca-tions—alcohols 90 and 92 both afforded the identical product 93. 

Changing the identity of the ortho substituent of aryl azides dramatically influences its reactivity toward transition metals. Shen and Driver discovered that substitution of a vinyl group with an iminyl moiety enables Fe(II)-catalyzed benzimidazole (65) formation (Scheme 16.29) [48]. It was proposed that coordination of iron(II) bromide to the imino nitrogen of 64 facilitates the C—N bond formation through nucleophilic attack of an azido nitrogen to the activated imino carbon, leading to B. Expulsion of N2 followed by tautomerization affords the benzimidazoles 65. 

Other reactions of synthetic utility can be observed when suitable nucleophiles are added to the reaction system. Typical examples of these nucleophiles are azide, cyanate, thiocyanate, and cyanide anions and thioanisole. Reactions using these nucleophiles provide a reaction path to functionalized vinyl derivatives. The transformations are successfully achieved by using triaryl-substituted alkenyl halides. The reactions with such nucleophiles provide an efficient path to heterocyclic compounds, among others. Some examples, such as the pyrroHnes, isoxazoHnes, isoquinolones, and thio derivatives, are prepared by this method and are shown in Equation 11.11. 

---