Hydroazidation reactions

Preparation of Alkyl Azides. A hydroazidation reaction similar to the reaction of Eq. 49 permits preparation of alkyl azides (Eq. 50), 215... 

Direct introduction of HN3, or its equivalents, onto olefins constitutes an efficient and straightforward approach. Unfortunately, the most efficient reactions are multi-step sequences, such as epoxidation followed by opening with azide ions or hydroboration followed by iodination and snbstitution. Several one-step methods, such as halo-azidation, diazidation, seleno-azidation, nitrato-azidation, ° formation of a-azido ketones," and carboazidation, have also been reported, bnt the conceptually simplest reaction, the hydroazidation reaction, has been much less developed (Figure 4.1). 

In 1915, Oliveri-Mandala first reported on the conjugate addition of hydrazoic acid onto a quinone derivative, but the product obtained was an aromatic azide. The first real hydroazidation reaction of electron-deficient double bond was then reported... 

The progress towards a mild general metal-based hydroazidation reaction has proven slow. Minor improvements have been reported by Kropp, who used trimethylsilyl azide together with triflic acid on silica gel and Sreekumar, who used zeolite bound sodium... 

The structural features of 11b are noteworthy even in the presence of excess amino acid, only the 1 1 complex of 12 and Co was detected by NMR spectroscopy in sharp contrast to complex 4 (Fignre 4.4). This is probably due to the steric bulk of ligand 12 and explains its increased reactivity and lower stability. Unfortunately, we were unable to obtain reproducible results using complex 11b, as yields (40-70%) and reaction time (8 8 h) were batch-dependent. In many cases, an initiation time was observed before the reaction started. Mukaiyama and co-workers used ferf-butyl hydroperoxide as a cobalt-catalyst for the hydration of certain olefins when initiation of the reaction was difficult. A similar effect was observed in the hydroazidation reaction when using catalyst 11 with ethanesulfonyl azide (7) for the hydroazidation of 4-phenylbut-l-ene (3), complete conversion was observed after 2-8 h using 30% of ferf-butyl hydroperoxide. In situ formation of complex 11b in the reaction mixture leads to reproducible reaction times (2h) and yields (70%). Co(BF4)2-6H20 was the best Co salt for this procedure, as complex formation was faster than with other salts and quick oxidation to the Co(III) complex occurred in the presence of tert-butyl hydroperoxide. 

The scope of the hydroazidation reaction was examined next, both with phenylsilane and TMDSO (General procedure A and B (Table 4.2)). All tested terminal olefins showed... 

Although reaction conditions are similar to the earlier developed hydrohydrazination reaction, the scope of the hydroazidation reaction appeared more limited. We hypothesized that this resulted from the lower reactivity of the tosyl azide towards a possible Co-alkyl (or radical) intermediate. Consequently, other potential azide sources were examined for the hydroazidation of 4-phenylbut-l-ene (3) (Figure 4.5). ... 

Azides by Olefin Hydroazidation Reactions 103 Table 4.2 The hydroazidation of monosubstituted olefins... 

Azides by Olefin Hydroazidation Reactions 105 Table 4.4 Comparison of TsN (17), azide 49 and azide 50 in the hydroazidation reaction... 

Table 4.5 Hydroazidation reaction with deprotonated suifonide azide 46... 

Scheme 4.5 Working model for the mechanism of the hydroazidation reaction...

---