Vinyl azides, addition reactions

Elimination of HI, which in the presence of an excess of IN3 can form hydrazoic acid, followed by its addition to the vinyl azides can give an intermediate triazide 75. The same compound could arise directly by substitution of one iodine atom by an azido group. The intermediate 75 has been considered to undergo a transannular reaction with homolytic cleavage of the weak C—I bond to form the radical 76, which loses a nitrogen atom to a radical 77. Combination of the two radicals leads to the 2-tetrazene 74 (equation 77). 

To 0.2 mole of an olefin dissolved in 30 ml of glacial acetic acid is added 19.5 gm (0.3 mole) of sodium azide in 75 ml of water. The addition to acrolein and jf -nitrostyrene underwent rapid addition, requiring cooling with an ice-salt bath and slow addition of the sodium azide. Addition to methyl acrylate, acrylic acid, and acrylonitrile required 1-3 days at room temperature, a-vinyl pyridine and mesityl oxide required heating for 24 hr on a steam bath. Other olefins underwent no reaction even after 7 days of heating and were recovered unchanged. 

Azirines are also made by carbene addition to nitriles (89 — 90) and by thermal or photochemical (68JA2869) elimination of N2 from vinyl azides (e.g. 91 — 92). Vinyl azides are prepared by the Hassner reaction (68JOC2686, 71ACR9), where iodine azide is first added to an alkene and the resultant (3-iodoazide is dehydrohalogenated with base (Scheme 37) (86RTC456). 

Replacement in vinylic derivatives is, however, facilitated by electron-withdrawing groups to the site of substitution and addition-elimination reactions of this type have been reviewed by Rappoport and by Patai and Rappoport Such reactions have been applied in the direct synthesis of vinyl azides. For example the synthesis in 81 % yield of the )5-azidovinyl ketone (25), of unspecified stereochemistry, from the chloro derivative and azide ion in dimethylformamide at 40° has been described by Maiorana . 

Although there are virtually no experimental data relating to the influence of the azido group on addition reactions it is pertinent to consider the probable consequences particularly in the case of vinyl azides where both the inductive and conjugative effects of the azido group would be of significance. 

The most obvious route to vinyl azide synthesis lies in the addition of hydrazoic acid to an alkyne. Unfortunately, this reaction has been found to be successful with only one group of compounds, the esters of acetylene dicarboxylic acid (equation 1). Acetylene mono-... 

The major problem in this sequence of reactions is the preparation of the necessary precursors. Fortunately, the addition of iodine azide to olefins has provided a convenient route to many of these compounds. In the majority of the olefiris studied, this addition has been found to be regiospecific and, assuming iodine to be the cation, in accordance with Markownikov s rule. Thus, addition of iodine azide to a terminal olefin followed by dehydroiodination leads to the formation of the internal vinyl azide (equation 3). Moreover, the base-... 

It is apparent, then, that precursors for terminal vinyl azides are obtained following iodine azide addition to olefins only when rather unusual steric or electronic factors are operating. A number of other reaction sequences have been investigated in the search for a general approach to the terminal compounds. The opening of a terminal epoxide by azide ion leads to an azidohydrin which can be readily dehydrated (equation 4). Unfortunately, this reaction sequence... 

A useful, although somewhat limited, method of preparing vinyl azides involves the addition of azide ion to vinyl halides with subsequent elimination of halide ion. This reaction is only possible when the halogen is located to a group which can effectively stabilize the intermediate carbanion. A number of alkyl and aryl substituted )3-chlorovinyl ketones (23) have been found to react in this way ". In many cases, however, the resulting /3-azidovinyl ketones (26) cannot... 

In a reaction quite similar to the aforementioned addition-elimination reactions, conjugated vinyl azides have been obtained from certain terminally unsubstituted allenic esters (equation 9)... 

The isolation of the furoxan 107 following reaction of 1,2-dinitro-olefins 105 and sodium azide was explained by assuming addition of azide ion to the double bond, followed by elimination of nitrite ion to form l-azido-2-nitro6lefins 106. These vinyl azides could undergo loss of nitrogen concerted with cyclization to give 107. 

The photoreactions of azides can in most cases be rationalized in terms of initial loss of nitrogen with the formation of nitrenes which undergo rearrangement, insertion or addition reactions. Further examples of rearrangement of the nitrenes derived from vinyl azides to 2H-azirines have been reported ... 

Tandem procedures have recently been described for alkynyl azides in a simple reaction with an equimolar amount of the appropriate thiol in the presence of AIBN as initiator, to afford cyclic compounds in good yields. The authors previously used vinyl azides for simple addition procedure [49a] and have extended this reaction to tandem procedures. Scheme 10 describes an example with thiophenol and alkynyl... 

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... 

Later, Narasaka and coworkers reported that the scope of this transformation could be substantially improved with the use of Mn(III) catalysis [313]. These new reaction conditions allowed highly efficient employment of differently substituted simple non-activated alkyl-, aryl-, hetaryl- and even cydic vinyl azides 328 (Scheme 8.117). In addition, previously unreactive 1,3-diketones 33S could serve as feasible 1,3-dicarbonyl components in this formal [3 + 2] cycloaddition reaction, affording 2,3,5-tri-and 2,3,4,5-tetra-substituted pyrroles 336 in moderate to excellent yields... 

In 2009, Chiba s group reported a manganese-mediated synthesis of pyridines from cyclopropanols with vinyl azides [70]. In the presence of 1.7 equivalents of Mn(AcAc)3, a variety of pyridine derivatives have been prepared in good yields at room temperature. In their further studies, they realized this transformation in a catalytic manner [71]. In their mechanistic studies, they found the reactions were initiated by a radical addition of /3-carbonyl radicals, generated by the one-electron oxidation of cyclopropanols with Mn(III), to vinyl azides to give imi-nyl radicals, which cyclized with the intramolecular carbonyl groups. Additionally, the application of this newly developed methodology in the synthesis of quaternary indole alkaloid, melinonine-E, was accomplished as well (Scheme 3.33). 

The synthesis of azidocyclopropanes has been accomplished with varying success via the nitrogen transfer reaction from toluene-p-sulphonyl azide to cyclopropylamine anions or by the addition of carbenes or carbenoids to vinyl azides. 

Vinyl azides were seldom prepared by nucleophilic addition at alkynes lacking in strongly electron-withdrawing groups. Thus, the reaction of butadiyne (24) with lithium azide in methanol is such a rare case, which allows the isolation of enyne 25 as a yellow liquid. ... 

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