Aziridine nitrogen

With aziridine, nitrogen pentoxide forms a compound that is very unstable and is thought to have the following structure ... 

Millar and co-workers studied the reaction of dinitrogen pentoxide in chlorinated solvents with a number of different A-substituted aziridines and found that reactions are highly dependent on the nature of the A-substituent on the aziridine nitrogen (Table 5.9). A-Alkylaziridines give good yields of 1,2-nitramine-nitrate product (Table 5.9, Entry 1). These... 

In a parallel study, Wipf and Fritch11041 have shown that also urethane-protected (Boc), and even amino acid segments, are tolerated as acyl compounds on the aziridine nitrogen. The best results were obtained with alkylcopper reagents derived from CuCN and an alkyl-lithium in the presence of boron trifluoride-diethyl ether complex. Some 6-alkylated compounds (11-15%) were isolated as well. This work was extended to a solid-phase procedure that resulted in resin-bound alkene isosteres that could immediately be used in further peptide synthesis.11051 For this purpose, the 2-nitrophenylsulfonyl (oNbs) group was used for nitrogen protection and aziridine activation. It could be readily cleaved with benzenethio-late, which was compatible with the acid-sensitive Wang linker used. 

Electrophilic Reactions on the Aziridine Nitrogen. The generalised reaction of aziridines with an electrophile (R+) is as follows. 

When the aziridine nitrogen is substituted, Michael-type addition to the acetylene is not possible. Reactions are then usually preceded by ring cleavage with the formation of an azomethine ylid. The product obtained depends on the nitrogen substituent. Heine and co-workers 45,46 used m-l,2,3-triphenylaziridine (27) with DMAD, DEAD, EP, and EPP. The DMAD gave 98% of compound 28 after refluxing in toluene... 

In the absence of organic solvents, Sc(OTf)3 catalysed the cycloaddition of aziridines to nitriles to produce substituted imidazolines in good to excellent yields at room temperature and in an air atmosphere. The reaction is believed to progress through a highly reactive cationic intermediate in which the aziridine nitrogen is coordinated to Sc(OTf)3.65 The phosphine-catalysed enantioselective 3 + 2-cycloaddition of buta-2,3-dienoates with arylimines yielded 2-aryl-3-pyrrolidines with 64% ee.66... 

Orlov et al. [112] concluded that the protonation took place on the azomethine group nitrogen atom on the basis of IR spectral data. In concentrated sulfuric acid, the formation of the only acidolysis product, 1,3-diarylpropan-l,2-one 107, was observed (Scheme 1.29). Orlov et al. [112] assumed that first protonation took place on the ketimine nitrogen atom (intermediate 106), but in strong acidic media the aziridine nitrogen atom is protonated as well, making the system unstable and promoting additional transformations. 

The H and 13C spectra of this material revealed the characteristic presence of two invertomers at the aziridine nitrogen. For a more detailed discussion on a similiar system, see Wang, Z. Jimenez, L. J. Org. Chem. 1996, 61, 816. 

The s character of the aziridine nitrogen s lone pair has other effects too. The lone pair interacts very poorly with an adjacent carbonyl group, so N-acyl aziridines such as the one you saw on p. 1125 behave not at all like amides. The nitrogen atom is pyramidal and not planar, and the stretching frequency of the C=0 bond (1706 cm-1) is much closer to that of a ketone (1710 cm-1) than that of an amide (1650 cm-1). 

Despite this tendency, examples of alkylation, acylation, sulfonylation, halogenation, silylation, and phosphorylation of aziridines at nitrogen abound. Aziridinium salts can be prepared by further alkylation of the aziridine nitrogen. Aziridines can also be alkylated on nitrogen with epoxides producing -hydroxyamines. 

Several thiazolidines were synthesized via titanium tetrachloride catalytic cyclization <2005JOC227>. The reaction proceeds via an intramolecular attack on the nitrile by the aziridine nitrogen to provide bicyclic aziridinium intermediate 91. Subsequent ring opening by chloride yields thiazolidine 92 (Scheme 25). 

FurtW eridenoe for the relatively weakly bsaic character of the aziridine nitrogen atom was provided by a proton magnetic resonance study of a series of cyclic amines,and measuiements of the basicity of cyclic and branch soninca toward the Lewis acid trimethylborou. >... 

Conversely, the presumed heterolytic C-N bond separation should involve a prior protonation step since this reaction takes place only under strong acid catalysis when it is run at room temperature. In the absence of acid, however, the same process may be thermally induced. There are two potential sites for proton attachment The aziridine nitrogen and the carbonyl oxygen. Both—as protonated species—are suitable to initiate fragmentation of the three-membered ring by way of intermediates VII and VIII, respectively (see Scheme 14.2). While evidence supporting the existence of VIII is available from proton nmr spectral analysis of a V-acrylaziridine in superacid media, other researchers ... 

The s character of the aziridine nitrogen s lone pair has other effects too. The lone pair interacts very poorly with an adjacent carbonyl group, so N-acyl aziridines such as the one you saw on p. 

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