Nitrenes from azidoformates

Carboalkoxynitrenes generated from azidoformates undergo insertion reactions with saturated hydrocarbons (Scheme 6.29). Triplet nitrenes do not insert into alkyl C-H bonds. 

On the other hand, poly(ethoxycarbonylimino-4-vi-nylpyridinium ylide) (Scheme 13) was prepared essentially by the same method from 1-ethoxycarbonylimino-pyridinium ylide, as described by Hafner [15] from the reaction of poly (4-vinylpyridine) with nitrene, generated from the pyrolysis of ethyl azidoformate. 

The nitrene can be generated by a variety of methods, the most popular being the thermal or photolytic decomposition of azidoformates. Other methods, particularly the base-catalyzed a-elimination of arylsulfonate ion from 7V-[(arylsulfonyl)oxy]urethanes, are useful as they avoid the use of the potentially explosive azido esters. 

The nitrenes that most consistently give addition and insertion reactions are carboalkoxynitrenes generated from alkyl azidoformates. 

The reaction of ethyl azidoformate (93) with tetramethylallene yielded triazoline 94 and oxazoline 95 [88]. The triazoline 94 was formed by [3 + 2]-cycloaddition of azide 93 to the allene. The oxazoline 95 may result from [3 + 2]-cycloaddition of car-bethoxynitrene (96), which is formed from 93 by nitrogen evolution, to the allene or by the [2 +1] addition of the nitrene and subsequent rearrangement. 

An interesting development of this azepine synthesis is the intramolecular addition of acylnitrenes to arenes, as exemplified in Scheme 27 for the nitrene (231) generated from benzyl azidoformate by spray pyrolysis (80CC1161). 

Although the functionalization of unactivated C—H bonds by intramolecular nitrene insertion has been aj Iied to the synthesis of diterpene alkaloids and in the modification of steroids as described below, it has also been used to good effect in simpler systems. For example, 1-adamantyl azidoformate, readily prepared from 1-adamantanol, gives the oxazolidinone (17) on irradiation in cyclohexane by in-... 

Of particular mechanistic interest is the photochemistry of ethyl azidoformate which does not normally undergo the Curtius rearrangement (the migratory aptitude of the . oxy group is low), and where the nitrene can be efficiently trap. d, e.g. by acetylenes and nitriles . Lwowski was able to establish the spin state of carbethoxynitrene (ethoxycarbonyl nitrene ) by an elegant method adapted from the work of Skcll on carbenes " - It is based on the assumption, fully justified by the results, that a singlet species deficient in two electrons will add stereospecifically to a double bond. A triplet reactant can accept only one electron at a time and will close the bond with the second electron only after one of the spins has... 

The singlet lifetime has been experimentally established only for cyanonitrene which in the gas phase converts to the ground state triplet at the rate of about lO sec . An estimate of singlet lifetimes in solution can be obtained from the work of McConaghy and Lwowski . They find that in the thermolysis of ethyl azidoformate in the presence of cfj-4-methylpent-2-ene the rate of spin reversal of the nitrene is about 10 times slower than the rate of addition to the... 

All these products are compatible with the formation of a nitrene, yet on their own they do not constitute conclusive proof for the real existence of a discrete intermediate. It was only when Lwowski was able to show that the nitrene produced by a-elimination from N- -nitrobenzenesulphonoxyurethane (74) led to the same products as the photolysis of ethyl azidoformate that the evidence for a photolytic nitrene mechanism became entirely convincing, the more... 

A cyclopropane containing a C-N bond was also obtained when a mixture of ethyl azidoformate and 1-ethoxy-1-trimethylsiloxycyclopropane in acetonitrile was photolyzed at room temperature. The product, l-ethoxy-2-(ethoxycarbonylamino)-l-trimethylsiloxycyclopropane(19), formally resulted from a substitution reaction, but the product is in fact generated by insertion of (ethoxycarbonyl)nitrene into a cyelopropyl C-H bond, keeping the cyclopropane ring intact.Interestingly, if the reactants are dissolved in dimethylformamide or dimethyl sulfoxide and are heated instead, quite different reactions take place. 

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