Addition of Nitrenes

The addition of nitrenes leads predominantly to the closed [6,6] bridged isomers. The corresponding [5,6] bridged isomer is - if at all - formed only in small amounts, probably via a direct addition to the [5,6] bond [394]. Nitrenes have been generated by thermolysis of azido-formic esters [172,395 00], photolysis of aroyl azides [401] or aryl azide [402], elimination of O-4-nitrophenylsulfonylalkylhydroxamic acid [403] or reaction of amines with Pb(OAc)4 [404]. 

The tert-butyl derivative 343 quantitatively eliminates iso-butene and carbon dioxide on a neutral alumina column. The obtained aziridine can be esterified in good yields with acid chlorides in ODCB in the presence of pyridine [398, 399]. 

With this technique, a broad range of aziridino-SWCNTs was obtained by our group by cycloadding addends such as alkyl chains, aromatic groups, crown ethers and oligoethylene glycol units (Fig. 1.8) [148, 154]. 

Nitrene additions led to a considerable increase in solubility in organic solvents the highest solubility of 1.2 mg mL4 in DMSO and TCE was found for SWCNT ethylene glycol-crown ether adducts. AFM and TEM revealed that the formation 

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Other potential synthetic routes to these unsaturated aziridine derivatives which involve the addition of nitrenes to allenes <75JOC224), carbenes to imines with subsequent hydrolysis <67JA362), and of carbenoid species to ketenimines <76TL1317,79TL559) have been investigated but are collectively of little or no preparative value. 

N-Aminobenzoxazolin-2-one (4), which was readily prepared by animation of benzoxazolin-2-one with hydroxylamine-O-sulfonic acid, is also a useful nitrene precursor (Scheme 2.2). Oxidation of 4 with lead(iv) acetate in the presence of a conjugated diene resulted in exclusive 1,2-addition of nitrene 5, to yield vinylazir-idine (6) in 71 % yield [6]. The formation of vinylaziridines through 1,2-additions of methoxycarbonylnitrene (2) or amino nitrene 5 contrasts with the claimed 1,4-ad-dition of nitrene itself to butadiene [7]. Since the reaction proceeded stereospecif-ically even at high dilution, the nitrene 5 appears to be generated in a resonance-stabilized singlet state, which is probably the ground state [8]. 

Those reactions that have found general use for the preparation of aziridines can be grouped into two broad classes addition and cyclization processes, and each of these categories can be further divided. Addition processes can be classified as being C2+N1 reactions (addition of nitrenes, or nitrene equivalents [ nitrenoids ], to alkenes Scheme 4.1) or (J N1+C1 reactions (addition of carbenes or carbenoids to imines Scheme 4.2). 

C2+N1 reactions addition of nitrenes, or nitrenoids, to alkenes Scheme 4.1... 

Notwithstanding the drawbacks to the method, the addition of nitrenes to alkenes is a well studied classical method for direct aziridination. The original reactions (often involving alkoxycarbonylnitrenes) employed harsh conditions, resulting in nonstereoselective transformations. In these pioneering reports, the requi-... 

Trifluoroethanol has been shown to promote the addition of nitrenes, generated by the reaction of nitroso-compounds with phosphites, to aromatic hydrocarbons, e.g. (57), (58), and (59) are formed from the reaction... 

Several reviews on the synthesis of aziridines have been published in the previous year. These publications include a review on the silver catalyzed addition of nitrenes (among other intermediates such as carbene) across a double bond <06EJOC4313> a review on sulfur ylide addition to imines to form aziridines <06SL181> a review on nitrogen addition across double bonds <06ACR194> a general review on functionalization of a,p-unsaturated esters with some discussion of aziridination <06TA1465>... 

The photochemistry of aryl azides is quite complex, suggesting that the nitrene 14 may not be the only reactive intermediate and that insertion reactions may not be the only route to form photoconjugates.Although aryl nitrenes are much less susceptible to rearrangements than acyl nitrenes, they may still occur and lead to the formation of reactive intermediates such as azepines, which may go on to react with nucleophiles.[911 141 Addition of nitrenes to double bonds will generate azirines, while dimerization will produce azobenzenesJ11 Aryl azides are stable to most of the procedures used in the course of peptide synthesis except for reduction reactions. Non-photochemical reduction of aryl azides to the primary amines by thiols has been reported by Staros et al.[15]... 

A review has appeared on the synthesis of enantiomerically enriched aziridines by the addition of nitrenes to alkenes and of carbenes to imines.45 A study of the metal-catalysed aziridination of imines by ethyl diazoacetate found that mam group complexes, early and late transition metal complexes, and rare-earth metal complexes can catalyse the reaction.46 The proposed mechanism did not involve carbene intermediates, the role of the metal being as a Lewis acid to complex the imine lone pah. Ruthenium porphyrins were found to be efficient catalysts for the cyclopropana-tion of styrenes 47 High diastereoselectivities in favour of the //-product were seen but the use of chiral porphyrins gave only low ees. 

E. Kozlowska-Gramsz and G. Descotes, Addition of nitrenes to cyclic vinyl ethers. Application to the synthesis of aminosugars, Tetrahedron Lett., 22 (1981) 563-566. 

In order to use photogenerated nitrenes for selective cycloaddition towards 7i-bonds, a number of possible side reactions must to be suppressed. In particular, the choice of the solvent is very important as it can serve as reaction partner. Singlet nitrenes are expected to be more suitable than triplet nitrenes for the above cycloaddition, to date, the addition of nitrenes 1, 2, and 6 to double bonds has not been demonstrated. 

Nitrenes add to carbon-carbon bonds to give aziridine (Scheme 2.60). Like carbenes, the addition of nitrenes to a carbon-carbon bond is stereospecific with singlet and nonstereospecific with triplet nitrenes. 

The classical methods utilized for the addition of nitrene to olefins generally suffer from several drawbacks including harsh conditions required to generate the nitrene and the lack of stereoselectivity in bond formation. Issues associated with triplet versus singlet nitrene generation have plagued this process and rendered it less useful than most epoxidation methods. 

A related reaction is the addition of nitrenes to olefins. Irradiation of ethyl azidoformate in cyclohexene yielded 7-carbethoxy-7-azabi-cyclo[2,l,0] heptane (129) (50%) °. 

An internal addition of nitrenes is also possible, when the double bond is located at a suitable distance. The highly strained fused aziridines formed undergo immediate hydrolysis to hydroxymethyl pyrrolidines. With acyl azides yields were low, as the respective isocyanates were the major products. Examples are the conversions... 

One method widely used for the synthesis of aziridines is the addition of nitrenes to olefins. Along these lines, sulfonamides serve as convenient nitrene precursors, which can be converted... 

Aziridines can also be prepared by addition of nitrenes to alkenes, or by the use of nitrogen-transfer agents analogons to epoxidising agents. ... 

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