Nitrenes cycloaddition reactions

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... 

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. 

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... 

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). 

Compounds with a Si=Si or Ge=Ge bond (i.e., disilenes and digermenes) can be isolated when the double bond contains bulky substituents. Only a few cycloaddition reactions with diazo compounds are known, and two reaction modes have been observed. One of the paths leads to the formation of disiliranes 184 (242) and digermiranes 185 (243) (Scheme 8.42), probably by ring contraction of an initially formed [3 + 2] cycloaddition product. The other path involves a 1,1-cycloaddition of the diazoalkane to give disilaaziridine 186 (244) and digermaazir-idine 187 (245). This nitrene-like reactivity is rather uncommon although some intramolecular examples are known (see Section 8.6.1). 

Although it has been established that the HOMO (diazoalkane)-LUMO (alkene) controlled concerted cycloaddition occurs without intervention of any intermediate for the reactions of simple diazoalkanes with alkenes, Huisgen once proposed a mechanistic alternative 4 namely an initial hypothetical nitrene-type 1,1-cycloaddition reaction of phenyldiazomethane to styrene followed by a vinylcyclopropane-cy-clopentene-type 1,3-sigmatropic rearrangement Control experiments, however, excluded this hypothesis for the bimolecular 1,3-dipolar cycloaddition reaction of diazomethane (Scheme 60).204... 

Based on results presented in Scheme 37, Logothetis suggested that the thermal decomposition products from the olefinic azides in the scheme are derived from triazoline intermediates formed by an intramolecular cycloaddition reaction and not by fragmentation of the azido group to a nitrene.100 However, allyl azide and 4-azido-l-pentene do not undergo internal cycloaddition because of the strain in the corresponding triazoline they fail to give aziridines and imines upon thermolysis.100... 

Triplet nitrenes mostly react by hydrogen abstraction (inter- and intramolecularly). Cycloaddition reactions with Ti-bonds occur nonstereoselectively. 

The only known example of a useful preparative intramolecular cycloaddition reaction is the formation of ds,ds-trialkyltriaziridines by the reaction of the nitrene with the N=N-double bond (Sch. 2) [13]. In this example, the fixed position of the reactants enables the fast intramolecular reaction. 

The nitrene 28 is not produced from the azide precursor, but from heterocycles via photolysis and thermolysis as shown in Sch. 11 [20]. Iminoacyl nitrenes react intramolecularly giving benzimidazoles with good yields (Sch. 11), and, dependending on the precursor used and the reaction conditions, varying amounts of carbodiimides are obtained. The reactivity of the acyl nitrenes is influenced by the substituent connected to the acyl group (see Sch. 10), however all acyl nitrenes are quite reactive and therefore rather unselective. Apart from cycloaddition reactions with Tt-bonds, insertion reactions into a-bonds, additions to lone pair electrons of... 

Acyl azides which produce the nitrenes 3 and 4 upon photolysis are the most suitable for synthetic applications of cycloaddition reactions. Therefore, this subchapter will mainly deal with alkoxycarbonyl and aroyl nitrenes. 

The outcome of the nitrene addition reaction depends on the type of 7i-bond involved. In contrast to electron deficient olefins [26] and nonpolar olefins forming aziridines, electron rich olefins react with alkoxycarbonyl nitrenes to give oxazolines (Sch. 14) [22]. The same type of cycloaddition reaction leading to the production of five-membered rings has also been observed with nitriles [27] (such as compound 35 in Sch. 14) and isocyanates [28] as illustrated in Sch. 15. 

Alkylcarbonyl nitrenes have only rarely been used for intermolecular cycloaddition reactions. 2,2-Dimethylpropanoyl nitrene reacts in the usual manner with cyclohexene giving the aziridine (45%) as well as the... 

It is often the case that the oxygen and nitrogen atom of the acyl nitrene are involved in the cycloaddition reaction. In this respect, acyl nitrenes are able to react like a 1,3-dipole [37] however this is different to other species commonly classified as 1,3-dipoles as they are not octet stabilized. 

Intermediate three-membered rings have never been detected in the cycloaddition reaction and a direct route to the five-membered rings is probable in most cases. For example, benzoyl nitrene adds to triple bonds and in contrast to the addition to nonpolar double bonds, a five-membered oxazole is obtained (Sch. 22) [21,38]. An azirine intermediate is not detected, however it cannot be excluded that the three-membered ring containing a 7i-bond rearranges rapidly giving rise to the oxazole. 

In contrast to the above, the more reactive ethoxycarbonyl nitrene 4 is able to attack the carbonyl group of ketones forming three-membered rings such as compound 90 shown in Sch. 24. The ground state for this nitrene is the triplet state, meaning that the cycloaddition reaction occurs in at least two steps. Indeed is has been found that acetone can be attacked by the reactive intermediate 4 primarily at the carbonyl O-atom (see Sch. 24) the dipolar intermediate is able to add a second acetone molecule to yield the dioxazoline (compound 91) [21]. 

The triple bond of nitriles is attacked by aroyl nitrenes to give rise to oxadiazoles as illustrated in Sch. 25 [22,41]. However, additions of acyl nitrenes to olefmic double bonds can be carried out in acetonitrile solution because the cycloaddition reaction to the solvent is much slower. 

Accordingly, cycloaddition reactions can be carried out in solvents such as alkanes, cycloalkanes and acetonitrile. The insertion reaction into the O-H-bond of alcohols is, in every case, faster than the cycloaddition reaction. Furthermore, 100% production of isocyanate was observed upon irradiation of benzoyl azide in dichloromethane solution, however, nitrenes that can be trapped by compounds with double bonds are also formed in this solvent [21]. 

The combination of nitrene and substrate chirality improves the diastereoselectivity of the cycloaddition reaction. For example, due to the use of both chiral aroyl azides and chiral alkenes, the pure exo-product (compound 108) was obtained (Sch. 32) [43]. 

Reactions with Radicals, Carbenes, Nitrenes, and Silylenes. 2.8 Cycloaddition Reactions 3 Reactivity of Nonconjugated Rings... 

The intramolecular reaction of alkyl nitrenes with a 1,3-diene is unsuitable for the preparation of fused bicyclic dihydropyrroles142. However, the intramolecular 1,3-dipolar cycloaddition reactions of the same substrates to give vinylaziridines, followed by rearrangement, can be utilized to achieve the desired target. 

Methyltrioxorhenium has been found to be a universal catalyst for a number of [2-1-1] cycloaddition reactions, including nitrene, carbene, or oxo-atom addition to olefins <2001GC235>. Typically, to increase the chemical yield of the reaction, at least 5 equiv of an olefin is required. As with most nitrene transfer reactions, simple cyclic olefins such as cyclohexene produce a low chemical yield of aziridine. The authors assume that the intermediate of the reaction is a reactive rhenoxaziridine intermediate. 1,2-Dihydronaphthalene provides aziridine 28 in 43% chemical yield under these reaction conditions (Equation 11). 

Nitrene generation from nitroso and nitro compounds with triethyl phosphite can also be used. For example, (304 R = NO) and TEP give (305) in 98% yield (63JCS42). The pyrrole (309) and TEP, refluxed in xylene (7 d), give the mesomerically stabilized heteropen-talene derivative (310) which undergoes cycloaddition reactions (Scheme 91) (79JOC622). 

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. 

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