Nitrenes cycloaddition

Nitrene cycloaddition was also used by Yinghuai and co-workers to append C2B10 carborane cages to SWCNTs [31]. One of the derivatives showed very interesting potential in medicinal applications as it was found that boron atoms had higher concentrations in tumor cells than in blood or other organs when administered to mice. This finding could inspire further research on the use of these systems as nanovehicles for... 

Accordingly, many reactions can be performed on the sidewalls of the CNTs, such as halogenation, hydrogenation, radical, electrophilic and nucleophilic additions, and so on [25, 37, 39, 42-44]. Exhaustively explored examples are the nitrene cycloaddition, the 1,3-dipolar cycloaddition reaction (with azomethinylides), radical additions using diazonium salts or radical addition of aromatic/phenyl primary amines. The aryl diazonium reduction can be performed by electrochemical means by forming a phenyl radical (by the extrusion of N2) that couples to a double bond [44]. Similarly, electrochemical oxidation of aromatic or aliphatic primary amines yields an amine radical that can be added to the double bond on the carbon surface. The direct covalent attachment of functional moieties to the sidewalls strongly enhances the solubility of the nanotubes in solvents and can also be tailored for different... 

This review does not cover multiple eliminations at the same (cf.156)) or at different centres proceeding without rearrangement and which, [for e.g., the Ramberg-Backlund- 2) or the benzotriazinone-arylazide-reaction 3) (Scheme 2)], can be classified in simple stages ([l,3]-e. and [l,2]-e., or [l,4]-e., valence tautomerism, [l,l]-e., intramolecular nitrene cycloaddition, respectively). 

K. In addition, it was found that the irradiation of 22 in benzene in the presence of both cis- and rans-pentenes (0.05 to 3 M) produces a mixtiu-e of aziridines, which is typical of the presence of both singlet and triplet nitrene cycloaddition reactions. The triplet-triplet absorption spectrum of azide 22 was detected, and it was found that naphthalene inhibits the photoreaction of 22 by quenching of its triplet state. It was concluded, therefore, that the ground state of nitrene 23 is a triplet and that it is formed exclusively upon sensitized photolysis of azide 22 through the triplet state of 22. 21 Nq isocyanate was detected in the photolysis products of 22. 

In view of the presence of unsaturated -electrons in CNTs, Smalley and coworkers and Holzinger et al. have functionalized the sidewall of SWCNTs directly by fluorination and azide-thermolysis, respectively. Direct addition of functionalities to the CNTs was enabled by nitrene cycloaddition, nucleophilic carbine addition, and radical addition. 

FIGURE 6.8 Schematic presentations of (a) nitrene cycloaddition, (b) nucleophilic carbine addition, and (c) radical addition. (Adapted from M. Holzinger et al., Angew. Chem. Int. Ed. 2001,40,4002. Copyright Wiley-VCH Verlag GmbH Co. KGaA. With permission.)... 

In contrast to the nitrene cycloadditions shown in Schemes 6.11 and 6.12, C ) reacts with aryl azides at room temperature to give isolable triazolines. These extrude nitrogen on photolysis to give the corresponding fulleroaziridines whereas thermal decomposition leads predominantly to ring opened products. ... 

The two-bond disconnection (re/ro-cycloaddition) approach also often works very well if the target molecule contains three-, four-, or five-membered rings (see section 1.13 and 2.5). The following tricyclic aziridine can be transformed by one step into a monocyclic amine (W. Nagata, 1968). In synthesis one would have to convert the amine into a nitrene, which-would add spontcaneously to a C—C double bond in the vicinity. 

Thiophene, 2-amino-3-cyano-5-phenyl-synthesis, 4, 888-889 Thiophene, 3-amino-4,5-dihydro-cycloaddition reactions, 4, 848 Thiophene, 2-amino-3-ethoxycarbonyl-ring opening, 4, 73 Thiophene, 2-amino-5-methyl-synthesis, 4, 73 Thiophene, 2-anilino-synthesis, 4, 923-924 Thiophene, aryl-synthesis, 4, 836, 914-916 Thiophene, 2-(arylamino)-3-nitro-synthesis, 4, 892 Thiophene, azido-nitrenes, 4, 818-820 reactions, 4, 818-820 thermal fragmentation, 4, 819-820 Thiophene, 3-azido-4-formyl-reactions... 

Another conceptually unique approach in alkene aziridination has come from Johnston s labs. These workers shrewdly identified organic azides as nitrene equivalents when these compounds are in the amide anion/diazonium resonance form. Thus, when a range of azides were treated with triflic acid and methyl vinyl ketone at 0 °C, the corresponding aziridines were obtained, in synthetically useful yields. In the absence of the Bronsted acid catalyst, cycloaddition is observed, producing triazolines. The method may also be adapted, through the use of unsaturated imi-des as substrates, to give anti-aminooxazolidinones (Scheme 4.25) [32]. 

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. 

Cycloadditions are one class of reactions that provide a wide variety of differently functionalized nanotubes (Fig. 3.8). As forhalogenations, cycloadditions are generally carried out directly on the nonoxidized pristine CNTs, which requires the use of highly reactive species or/and harsh conditions. The most common cycloaddition reactions include the addition of carbenes, nitrenes, 1,3-dipolar cycloadditions and Diels Alder reactions. 

However, in the ground state, the main contributions to the resonance hybrid are due to forms 3a,b. Their importance increases going from diazopyrroles to the diazotetrazole, so that the diazo structure with cumulated double bonds, which has been extensively employed as a shortened form for the diazoazoles, does not seem to depict them correctly any more. Therefore, in continuation of this review, the diazoazoles will be represented by structure 3a unless other limiting forms better account for the observed reactivity. In fact, even the nitrene-like form, a heteroanalogue of 17, which is the one with highest energy, has been invoked to explain the reactivity in some cycloaddition reactions (86CC1127). 

Cycloadditions to [6,6]-double bonds of Cjq are among the most important reactions in fullerene chemistry. For a second attack to a [6,6]-bond of a C q monoadduct nine different sites are available (Figure 10.1). For bisadducts with different but symmetrical addends nine regioisomeric bisadducts are, in principle, possible. If only one type of symmetrical addends is allowed, eight different regioisomers can be considered, since attack to both e - and e"-positions leads to the same product. Two successive cycloadditions mostly represent the fundamental case and form the basis for the regioselectivity of multiple additions. In a comprehensive study of bisadduct formations with two identical as well as with two different addends, nucleophilic cyclopropanations, Bamford-Stevens reactions with dimethoxybenzo-phenone-tosylhydrazone and nitrene additions have been analyzed in detail (Scheme 10.1) [3, 9, 10]. 

Azolines of type (13) undergo thermal decomposition in an analogous way to that already discussed for azolones (see Section 4.14.5.2) (Scheme 19). Path (i) is followed by those azolines having Z = S and path (ii) by those with Z = O. Path (i) is a typical retro-1,3-dipolar cycloaddition process, via an intermediate nitrile sulfide, while path (ii) might involve an acyl (Y = Z = O) or thioacyl (Y = S, Z = O) nitrene intermediate (136), which in turn rearranges to iso(thio)cyanate. However, no systematic attempts to trap this possible nitrene intermediate seem to have been made, and so a concerted pathway for the fragmantation cannot be ruled out. 

The reaction of l,3-bis(trimethylsilyl)-l,3-diaza-2-germa(II)-indane (365) with trimethylsilyl azide to give (366) may seem to be reminiscent of several preceding transformations but, as was shown with open-chain germylenes, the process is not a cycloaddition and probably involves nitrene intermediates <89CB245>. 

Trimethyl-2-azidobiphenyl thermolyzed in n-hexadecane gave only 4% 2,4,9-trimethylcarbazole, that is, the product of nitrene insertion into the ortho C-methyl bond the main products were 2 -amino-2,4,6-trimethyl-biphenyl and 250 (R = Me). An intramolecular dipolar cycloaddition com-... 

Das bei der Thermolyse von 7-Azido-3-phenylthio-, 7-Azido-3-silyloxy- oder 7-Azido-3-(l-ethoxy-ethoxy)-l,3-heptadien entstehende Nitren geht eine intramolekulare (doppel-te) Cycloaddition ein unter Bildung von 1-Phenylthio- (50-62%), l-(tert.-Butyl-dimeihyl-silyloxy)- (39%) bzw. I-(2-Ethoxy-ethoxy)-5,6,7,7a-tetrahydro-3H-pyrrolizin (55%) ne-ben anderen Produkten2 ... 

In contrast to considerations of 50 years ago, today carbene and nitrene chemistries are integral to synthetic design and applications. Always a unique methodology for the synthesis of cyclopropane and cyclopropene compounds, applications of carbene chemistry have been extended with notable success to insertion reactions, aromatic cycloaddition and substitution, and ylide generation and reactions. And metathesis is in the lexicon of everyone planning the synthesis of an organic compound. Intramolecular reactions now extend to ring sizes well beyond 20, and insertion reactions can be effectively and selectively implemented even for intermolecular processes. 

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