Triazenes, cyclic—

Tire only known example of 1,2,3-triazepine (32) behaves as a reactive cyclic triazene with a high degree of masked diazonium character [74JCS(P1)1248]. 277-1,2,4-Triazepine (33a) has been obtained from the 577-tautomer (33b) by a 1,5-hydrogen shift (74CC45,74TL2303), this being one of these rare cases that an antiaromatic tautomer 33a is more stable than the nonaromatic one 33b. But it must be noted that these 277-1,2,4-tri-... 

Because of the special structural requirements of the resin-bound substrate, this type of cleavage reaction lacks general applicability. Some of the few examples that have been reported are listed in Table 3.19. Lactones have also been obtained by acid-catalyzed lactonization of resin-bound 4-hydroxy or 3-oxiranyl carboxylic acids [399]. Treatment of polystyrene-bound cyclic acetals with Jones reagent also leads to the release of lactones into solution (Entry 5, Table 3.19). Resin-bound benzylic aryl or alkyl carbonates have been converted into esters by treatment with acyl halides and Lewis acids (Entry 6, Table 3.19). Similarly, alcohols bound to insoluble supports as benzyl ethers can be cleaved from the support and simultaneously converted into esters by treatment with acyl halides [400]. Esters have also been prepared by treatment of carboxylic acids with an excess of polystyrene-bound triazenes here, diazo-nium salts are released into solution, which serve to O-alkylate the acid (Entry 7, Table 3.19). This strategy can also be used to prepare sulfonates [401]. 

This review, which complements an earlier one (Part I) dealing with transition metal complexes of triazenes, tetrazenes, tetraazadienes, and pentaazadienes, examines the coordination chemistry of related cyclic catenated nitrogen ligands. Six-membered rings containing three, four, or five adjacent nitrogen atoms — 1,2,3-triazines, 1,2,3,4-tetrazines, and pentazines, respectively — are either unknown or are relatively unstable species whose coordination chemistry has yet to be explored. 

Triazene dyes in which the terminal nitrogen atom is cyclically bound are obtained by coupling heterocyclic diazonium salts with partially hydrogenated N heterocycles. In 17 alkylation was subsequently performed with ethylene oxide [40],... 

A ring contraction involving this reaction has been found in cyclic hydrazones, hydrazides, and triazenes. 

Substituted 1,4-diazooxides rect with secondary aliphatic amine to give the corresfxjnding triazene (diazoamino compounds) in high yield, while the products with primary amine are surprisingly unstable and deompose by a radical mechanism. Reactions with diazoalkanes yield asymmetrical azine 29 (15) in the case of 1,4-diazooxide, and cyclic benzoxadiazines 30 with 1,2-diazooxides (16) . 

The synthesis of various azido compounds with arene sulfonyl azides has, at least in a formal sense, to be considered as an electrophilic substitution of hydrogen by an azide function.By this route a number of amines, cyclic derivatives and bridgehead compounds included, after deprotonation with NaH or Grignard reagents, could be transformed into azides in high yield on treatment with arene sulfonyl azides." Details cannot be given here. Enolates can also be converted into azido derivatives in a very similar way, via triazene intermediates. Evans et alP did recently investigate the influence of the countercation, the structure of the transfer azide and work-up conditions on this process. 

Taylor and Raw recently designed a tethered imine-enamine cascade sequence that converts 1,2,4-triazenes into substituted pyridines. In the presence of molecular sieves, A-methylethylenediamine (147) underwent condensation with excess cyclic ketone 148 (n — 1-4) to give imine-enamine 150 (04CC508). The enamine portion of the molecule then participated in an inverse-demand Diels-Alder cycloaddition reaction with 149 to provide intermediate 151. Cycloreversion of 151 with loss of N2 then gave 152 in which the tertiary amino group underwent addition to the adjacent imine functionality to afford zwiterionic 153. Finally, an intramolecular Cope elimination produced 154 in 74-100% yield. Several other triazines were also shown to participate in this novel cascade (Scheme 27). 

Replacing the CH unit of diarylamidines with imino nitrogens leads to diaryltri-azenes. As illustrated in Fig. 6.46, the aryl groups are found to be coplanar with the triazene unit. A consequence is that an intermolecular steric interaction between aromatic CH arises, which prevents the formation of cyclic dimers. Thus, diaryltriazenes are not able to exchange protons without the help of a catalyst, as has been shown recently [33]. In order to obtain more information about catalytic proton exchange, the base catalyzed transfer of l,3-bis(4-fluorophenyl)[l,3-i N2]triazene was studied in more detail using H and F NMR. As catalysts dimethylamine, trimethylamine and water were studied, using tetrahydrofuran-dg and methylethylether-dg as solvents. The latter is liquid down to 130 K. 

For a number of condensed 1,2,3-triazines, it has been shown that an equilibrium can exist between the cyclic structure and an open-chain form.39-41 Depending on the substituents R1 and R2 and the solvent, 1,2,3-benzotriazines are in equilibrium with the open-chain triazene form.39-41 Since 1,2,3-benzotriazines, and the other fused 1,2,3-triazines which show reversible ring opening, are not aromatic systems, this problem will not be discussed in detail here. 

Miscellaneous Systems Many systems have been mentioned [li,m,2i] acyloxy and acylsilyl phosphine oxides, phosphine sulfides, cyclic compounds, benzoyloxa-ziridine derivatives, dibenzoylmethane derivatives, triazene and pentaazadiene moiety containing compounds. New developments include benzyl benzoin benzyl ethers [112], dithiocarbamates [113], ketoamides [114], phosphonates [115], bromo-acetylpyrene [116], alkylimides [117], aryloxy naphthalene [118], oligosilanes [119], bisphosphine sulfides [120], sulfamic esters of benzoin ethers [121], sulfur [122], or carbohydrate [123] containing compounds. 

The imidazotetrazinone temozolomide 28 is medicinally applied as cancer therapeuticum, the cyclic triazene moiety is considered to produce a cascade of ionic and radical species with antitumoral effects [179]. 

Partially hydrogenated 1,2,3-triazines (prototypes 6-11) and 3,4-dihydro-l,2,3-benzotriazines based on structure 12 are a second group of compounds of interest here. They did receive some consideration also in the previous editions however, neither the prototypes 6 and 9 nor substituted derivatives there of are known, and except for 11 (being a cyclic triazene), prototypes 7, 8, 10, and 12 are represented only in the form of numerous substituted derivatives. 

In a study on the electrophilic azide transfer to chiral enolates, Evans found that the use of potassium bis(trimethylsilyl)amide was crucial for this process. The KN(TMS)2 played a dual role in the reaction as a base, it was used for the stereoselective generation of the (Z)-enolate (1). Reaction of this enolate with trisyl azide gave an intermediate triazene species (2) (eq 4). The potassium counterion from the KN(TMS)2 used for enolate formation was important for the decomposition of the triazene to the desired azide. Use of other hindered bases such as Lithium Hexamethyldisilazide allowed preparation of the intermediate triazene however, the lithium ion did not catalyze the decomposition of the triazene to the azide.This methodology has been utilized in the synthesis of cyclic tripeptides. 

Fig. 13 Cyclic voltammogram (a) and temporal development of UV/Vis spectra during potential scan (b) of 3-(p-ethoxy-phenyl)-l,l-dimethyl-triazene (with kind permission of Wiley-VCH from Ref [91]) further details see text and Ref [91].

Cyclic diimide, triazene Diazo (hydrazone) Azomethine Oximate Imido (nitrene)... 

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