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Synthesis of Azides will be available on
Omura, Rapid 'SAR' via Click Chemistry: An Alkyne-Bearing Spiramycin is Fused with Diverse Azides to Yield New Triazole-Antibacterial Candidates, , 69, 55 (2006).
Azides are nucleophilic at their terminal nitrogen atoms, and may add to suitably activated electrophiles in the presence of a Brønsted or Lewis acid. Upon addition, the newly bound nitrogen atom becomes electron-deficient and is subject to 1,2-migration of a carbon or hydrogen substituent with loss of a molecule of dinitrogen. Historically, carbonyl compounds were the first electrophiles successfully employed in this context. Since the initial discovery of the Schmidt reaction, many variants employing alternative electrophiles and hydrazoic acid have been developed (Eq. 1). Related reactions of alkyl azides may yield substituted amides, lactams, or amines (after reduction of iminium ions). However, the scope of alkyl azides in the Schmidt reaction is limited compared to hydrazoic acid.
Catalytic Azide-Alkyne Cycloaddition: Reactivity and Applications.
The mechanism of the reaction of carboxylic acids with hydrazoic acid is well understood. Initial formation of an acylium ion is followed by the addition of hydrazoic acid to form an acyl azide intermediate. The only possible migration event produces an isocyanate, which undergoes hydrolysis to yield an amine and carbon dioxide (Eq. 3).
Described is the often neglected, but very efficient, procedure for the preparation of primary and secondary alkyl and cycloalkyl azides in excellent purity and high yields. At ambient temperature conditions, the alkyl azides were readily obtained from alkyl bromides by the nucleophilic substitution of bromide with 1.2 equivalents of sodium azide in dimethyl sulfoxide (DMSO). This facile and practical procedure provides highly pure products and eliminates the hazards associated with distillation of alkyl azides.
Synthesis and Versatile Reactions of β-Azidotetraarylporphyrins.
A variety of alternative electrophiles react with hydrazoic acid to afford products of Schmidt-type reactions. For instance, dithioketals activated by iodoazide form α-azido sulfides, which may be converted to amides via acidic hydrolysis or to thioimino ethers with stannic tetrachloride (Eq. 8).
The Schmidt reaction has been applied for the desymmetrization of symmetric ketones containing enantiotopic α-carbon atoms. Employing a chiral hydroxyalkyl azide resulted in highly diastereoselective migration, and subsequent removal of the nitrogen substituent produced the lactam as a single enantiomer.
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Jia, Ruthenium-Catalyzed Cycloaddition of Alkynes and Organic Azides,
Azides may be reduced to by or with a phosphine, e.g. , in the . This reaction allows azides to serve as protected -NH2 synthons:
Aziridines in Parallel- and Solid-Phase Synthesis.
The scope of reactions of alkyl azides is generally more limited than the scope of reactions of hydrazoic acid. Intermolecular Schmidt reactions of alkyl azides are plagued by poor site selectivity and strong sensitivity to steric effects. However, intramolecular reactions of alkyl azides are among the most synthetically useful applications of the Schmidt reaction. The range of ketones that engage in the intramolecular Schmidt reactions is exceptionally broad. For these reactions, the azide and carbonyl carbon must be separated by four or five atoms to facilitate cyclization, with four preferred (Eq. 9). Substrates containing five-carbon tethers react only in the presence of relatively strong Lewis acids.
J., Mechanism of Thio Acid/Azide Amidation.
One useful intermolecular method involves the use of hydroxyalkyl azides, whose pendant hydroxy groups may attack the intermediate oxocarbenium ion present after migration to form iminium ethers. Subsequent basic hydrolysis leads to the hydroxyalkyl-substituted amide, and nucleophiles other than hydroxide have been employed with some success.
V., Synthesis of substituted 1,2,3-triazoles via -sulfonylaziridines.
Organic azides engage in useful . The terminal nitrogen is mildly nucleophilic. Azides easily extrude diatomic , a tendency that is exploited in many reactions such as the or the or for example in the synthesis of γ-imino-β-enamino esters.
V., Synthesis of (azidomethyl)phenylboronic acids.
Many Schmidt reactions of cationic electrophiles are subject to carbocationic rearrangements. However, a judicious choice of reaction conditions (and utilizing an intramolecularly bound azide) can steer the reaction toward a single product (Eq. 11).. Intermolecular reactions of benzylic and tertiary cations with alkyl azides are also known.
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