Norbornene aziridination

It has been reported by R.Scheiner that phenylazide forms triazoline compounds by 1,3-cyclic addition to unsaturated olefines such as n-butylethylene and norbornen(9 ). These triazolines are decomposed photochemically or thermally to give imine compounds and aziridine as is shown in scheme 1. These facts suggest that phenylazide may react with 3-methyl-1-butene to give triazoline in a similar reaction to that with norbomen. 

Analogous methyl azidoformate forms with norbornene a thermal unstable triazoline.251 The decomposition products are 40% aziridine and 55% imide. Furthermore it has been observed that the rate of nitrogen evolution of the triazoline from methyl azidoformate increases threefold when triglyme and 20-fold when dimethyl sulfoxide are substituted for 1,1-diphenylethane as solvents. This fact supports a betaine intermediate in the thermal decomposition reaction. The triazoline from 2,4-dinitrophenyl azide and norbornene could just be isolated, but from picryl azide only the aziridine was obtained.252-254 Nevertheless, the high negative value of the activation entropy (—33.4 eu) indicates a similar cyclic transition state for both reactions. 

By treating norbornene with benzenesulfonyl azide in benzene at room temperature, a crystalline product is obtained in quantitative yield.264-265 On the basis of chemical and nmr evidence, an aziridine structure (85) similar to exo-2,3-epoxy-norbornene (86) is attributed to the adduct. The addition thus occurs at the least hindered exo side of the bicyclic nucleus. 

In some thermolysis reactions, amines (90 and 91) have been characterized.108,111,112,457 When R1 = CH2OH and R2 = H in Scheme 164, the aziridine undergoes spontaneous rearrangement (Scheme 166).125 An endo-triazoline adduct from norbornene yields a polymer as the principal... 

The influence of both anti and syn substitution in the 10-position on the thermolysis of triazoline adducts from a number of 7-substituted norbornenes (Scheme 14) has been investigated.128 Both the anti- and syn-substituted triazolines give only the exo-aziridine upon photolysis but pyrolysis yields endo-aziridines from the syn- and exo-aziridines from the anti compounds. The syn substituents have a strong endo directing influence and no exo-aziridine is detected in any case. Ketone formation in some cases apparently results from hydrolysis of the imines during workup.128... 

The thermolysis of triazoline adducts from other polycyclic bridgehead olefins is analogous to that of the norbornene-azide adducts (Scheme 164) and affords a route for the synthesis of various aziridine ring sys-... 

The cycloaddition of aryl, benzyl, and 1-phenylethenyl azides to strained bicyclic alkenes, e.g., norbornene, and their substituted derivatives afforded generally the exro-dihydrotriazoles 17, which on photolytic decomposition afforded cleanly the exro-aziridines 233,42-48,70,93. Thermal decomposition of the dihydrotriazoles resulted in a complex mixture of products, including both exo- and entfo-aziridine and the imine95,96. 

The use of Chloramine-T or Bromamine-T as the nitrogen source has been useful as the transfer reagent in the aziridination reaction. Bedekar and co-workers have found that simple cyclic olefins such as norbornene, cyclohexene, or cyclooctene produce low yields of aziridinated products. However, the same reaction run in a simple microwave oven increases the chemical yield dramatically <2001JOC30>. The reaction shown in Equation (13) was run without the assistance of microwave energy and provided approximately half of the chemical yield at 38%. 

Recently, Schmitz has reported aminations of weak nucleophiles such as alkenes to produce aziridines directly. Although simple aliphatic olefins do not react, styrene, indene, acrylonitrile, and norbornene were converted to aziridines. The reaction of 3-ethyl-3-methyloxaziridine with diphenylcyclopropenone may be a reaction of this type. ... 

Note, however, that the reaction of methyl azidoformate with the strained olefin norbornene (11 proceeds by a 1,3-cycloaddition to give the unstable triuzoline adduct (2), which decomposes in refluxing toluene to give the aziridine (3,40%) theimide (4, S3%),gndjtyn-2>norbomene-7-methylcurbamBte (5). ... 

It was recently demonstrated that 34 transfers its NH group to olefins. Aziridine 49 is obtained in 46% yield from a-methylstyrene 48 by 3 hours of heating with a solution of 34 in toluene. The same reaction was observed with styrene and its ring substitution products, indene, 1,1-diphenylethylene, and also norbornene and acrylonitrile. ... 

The addition of photochemically generated ethoxycarbonylnitrene to 16-dehydropregnenolone acetate to give three aziridines has been described.81 The corresponding addition of ethyl azidoformate to norbornene to give the aziridine (140) as the major product has been reported.82 The possible intervention of the triazoline (141) as an intermediate in the photoreaction has been excluded photoelimination of nitrogen from the triazoline (141) gave not only the aziridine (140) but also the imine (142) in a yield of 25%. 

No general pattern can be recognized in the reaction of azides with cycloalkenes. Angle strain can greatly enhance the rate of azide addition to cycloalkenes, as shown by work on norbornene and its derivatives (Huisgen et al., 1965), on hexa-methylbicyclo[2.2.0]hexa-2,5-diene (Dewar benzene) (Paquette et al., 1972) and related compounds (review Lwowski, 1984, p. 579ff). Normally, dihydrotriazoles are primary products, followed by a dediazoniation to aziridines. Diazo compounds are formed only in rare cases, e.g., with trifluoromethylated Dewar thiophenes (Kobayashi et al., 1977, 1980). 

The reaction is an important synthetic route to triazolines and their derivatives [5-7] such as cycUc imines or aziridines and is hence a valuable technique in the synthesis of heterocycles [8]. The reaction rate is dependent on the dipolarophile. Whereas strained olefins, such as norbornene, react readily, terminal alkenes react extremely slowly [9]. 

Aziridination of alkenes in the absence of conventional metal catalysts is particularly attractive from a green chemistry perspective. In this regard, the aziridination of norbornenes using electron-deficient sulfonyl azides was reported as a new bioconjugation reaction enabling the attachment of tags... 

The proposed mechanism involves the initial formation of the palladacycle 118 via the Catellani reaction sequence between the aryl iodide and norbornene (Scheme 3.32). Coordination of the aziridine 125 followed by oxidative addition of the N-C single bond by Pd leads to the intermediate 129. The fission of N-C single bond is commonly observed in transition-metal-catalyzed reactions with aziridines. Chemoselective C-C bond formation gives 130, which on decarbopalladation affords palladacycle 131 and extrudes norbornene. Reductive elimination furnishes... 

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