Olefins hydroazidation reactions

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... 

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). 

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... 

Similar reaction conditions allowed the preparation of a-branched nitriles (Fig. 83). When mono-, di-, and trisubstituted olefins 355 bearing functionalities, such as esters, aldehydes, ketones, amides, or alcohols, were reacted with tosyl cyanide 356 and phenylsilane in the presence of 1 mol% of the (salen)Co catalysts 357a or 357b the corresponding nitriles 358 were isolated in 55-99% yield (entry 21) [402]. Although no mechanistic evidence was provided, the reaction may be assumed to proceed similarly as the hydroazidation. 

The first successful approach was reported by Heathcock in 1969 using mercury(ll) salts. The reaction proceeds via an olefin azidomercuration followed by a reductive demercuration in the work-up. The azides derived from terminal olefins were obtained in 50-88% yield with good Markovnikov selectivity, while non-terminal olefins gave lower yields (Figure 4.2). An obvious drawback of this procedure is the use of a stoichiometric amount of mercury salts. This procedure also leads to the formation of potentially explosive Hg(N3)2. Nevertheless, this method is unique and reliable for the hydroazidation of monosubstituted non-activated olefins. 

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