Carbamoyl azides rearrangement

RCON -+ RN = CO. The carbamoyl azides, as a group, were classified by Bertho (Ref 3) as resistant to the Curtius rearrangement. However, other work by Stolle (Ref 2) showed that while some members of this group (RL or Ra H and Ra or Ra alkyl/aryl) failed to become rearranged, others (Rt or Ra CjHs) did so. Scott et al offered a different interpretation for the resistance of carbamoyl azide to such change... 

Other types of carbonyl azides, such as azidoformates and carbamoyl azides, previously believed not to undergo the rearrangement, can also be induced to rearrange by photolysis in alcohols . Diaryl-carbamoyl azides have been found to rearrange even upon heating in /-butanol and this reaction was used for the synthesis of 1,1-diaryl-hydrazines, thus JV-aminocarbazole (41) was prepared in 80% yield from 40 . 

Carbamoyl azide, HgN—CO—has been known since 1894- °, made by nitrosation of semicarbazide. It is also formed in the partial decomposition of carbonyl azide, CO(N3)2 > and by partial Curtius rearrangement of oxalyl diazide . Curtius warns that carbamoyl azide, while relatively harmless can explode with unparalleled violence under certain conditions, such as contact with copper powder. Carbamoyl azide forms a highly explosive silver salt... 

Aldehydes can also be precursors for the Curtius acyl azide rearrangement tScheme 4.24) Aliphatic and aromatic aldehydes can be converted to the corresponding acyl azides by treatment with iodine azide. Subsequent heating gave various carbamoyl azides in high yields. A radical pathway, proceeding by homolysis of the iodine-azide bond, was proposed. The resulting carbamoyl azides 65 are relevant precursors for the synthesis of amines or ureas, for example. 

The same reagent allows to convert aldehydes into acyl azides. If the reaction is run in the presence of NaNs, the intermediate acyl azide is converted into a carbamoyl azide via Curtius rearrangement (Scheme 8.20). 

Moreover, Bols et al. developed another methodology for the synthesis of carbamoyl azides from aldehydes by treatment with iodine azide at reflux in acetonitrile [41]. The carbamoyl azides are obtained in 70-97 % yield from the aliphatic and aromatic aldehydes (Scheme 5.4). When the reaction of phenyl-propanal with IN3 at 25 °C was performed in the presence of the radical trap, no acyl azide was observed, which was taken as support for a radical reaction mechanism. The mechanism shown in Scheme 5.6 is proposed for the reaction. Iodine radicals are formed by homolysis of the weak iodine-azide bond, abstracting the aldehyde hydrogen atom. The resulting carbon-centered radical reacts with iodine azide to produce an acyl azide. The following Cuitius rearrangement provides carbamoyl azides. 

In 2010, Studer et al. reported A -heterocyclic carbene catalyzed oxidative amidations of various aldehydes to the corresponding acyl azides by using the readily available organic oxidant [43]. Acyl azides can readily be converted via thermal Curtius rearrangement to carbamoyl azides as shown for the transformation of benzaldehyde to phenyl carbamoylazide (Scheme 5.7), when r-BuOH was further added to the crude reaction mixture, which was then heated for 1 h to reflux. Therefore, a mild NHC-catalyzed oxidative azidation of aromatic aldehydes to form the corresponding acyl azides which can be rearranged to carbomoylazides in the same pot. 

In 2003, Bols and co-workers described that the reagent IN3 can easily transform the aldehydes into the acyl azides under mild conditions (Scheme 6.22a) [76]. Furthermore, they demonstrated that the synthesis of carbamoyl azides could be realized at reflux by combining the aldehyde C-H bond azidation and flie Cuilius rearrangement in a one-pot protocol (Scheme 6.22b). A possible radical mechanism were proposed for this transformation (Scheme 6.22c). The weak I-N3 bond homolysis can initiate the chain reaction. The generated iodine radical abstracts an aldehyde hydrogen atom from the substrates to produce the acyl radical A. The acyl radical A reacts with IN3 to afford the acyl azides and iodine radical, thereby sustaining the radical chain. 

Employing A-heterocyclic carbine (NHC) catalysis, in 2010, Sarkar and Studer presented a novel and efficient oxidative azidation of aromatic aldehydes by using 3,3 5,5 -tetra-tert-butyldiphenoquinone as the oxidant (Scheme 6.23) [77]. Reaction works with a low catalyst loading under mild conditions and the acyl azides are obtained in good yields. They further showed that the acyl azides can be rearranged to carbamoyl azides in the same pot. 

Brandt and Wirth performed the synthesis of carbamoyl azides in the presence of in situ prepared E 3 3 (Scheme 6.2), whereby subsequent thermal rearrangement of the isocyanate 4 in the presence of excess E 3 3 afforded the carbamoyl azide 5. To quench any residual E 3 3 or organic azides formed safely, the reaction products were collected in an aqueous solution of sodium thiosulfate and the products were extracted into dichloromethane. Using... 

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