Triazole complexes numbering system

Synthesis of Triazoles A number of MCRs that use an epoxide-azide-alkyne Cu(I)-catalyzed cycloaddition sequence to provide substituted p-hydroxy-l,2,3-triazoles 70 (Scheme 3.36) have been recently developed. The azida-tion of an epoxide with sodium azide to give 2-azidoalcohols and subsequent reaction with a terminal alkyne have been carried out under mild conditions in environmentally friendly solvents, such as water, using the catalytic system CuSO sodium ascorbate [65] (see the example depicted in Scheme 3.36) [65a] or a porphyrinatocopper(II) complex... 

Chloromethyl-l,2,4-triazoles can be valuable intermediates in the synthesis of more complex compounds containing a 1,2,4-triazole moiety, and they can be accessed using a number of established methods for the synthesis of the triazole ring system. However, these processes often give variable yields and require much work to construct the starting material. A more convenient procedure has been developed, by which a hydroxymethyl-1,2,4-triazole is converted to the chloromethyl derivative by reaction with thionyl chloride (Equation 20 and Table 6) <2006S156>. 

For five-membered heterocycles other than thiazole, (such as pyrazole [27], imidazole [28], and triazole [29]) the effect of replacement of just one pyridine moiety in 1 is greater and the [Fe N6]2+ derivatives in these instances show crossover behaviour. The [Fe N6]2+ derivative of 2-(pyridin-2-yl)imidazole 19 (Dq(Ni2+) 1150 cm-1 [22]) was shown relatively early on to be a crossover system [28]. In solid salts and in solution the transition is continuous and centred above room temperature. The dynamics for the 5T2— Ai relaxation for this system have been investigated by a number of techniques [30-32] and Beattie and McMahon have shown that in solution there is not only a spin equilibrium but also a ligand dissociation process, very reasonably ascribed to the high spin form of the tris complex [32]. 

The latter number incorporates just the chemical step(s) of formation of triazole within cucurbituril. Since the product release step apparently is at least 100-fold slower than the actual cycloaddition, the net catalytic acceleration should be adjusted downward by that amount. An instructive alternative estimation of kinetic enhancement is to compare the extrapolated limiting rate for cycloaddition within the complex (i.e. cucurbituril saturated with both reactants, k — 1.9xl0 s ) with the uncatalyzed unimolecular transformation of an appropriate bifunctional reference substrate as in Eq. (3) (k, = 2.0x 10 s ). Such a comparison of first-order rate constants shows that the latter reaction is approximately a thousandfold slower than the cucurbituril-engendered transformation. This is attributable to necessity for freezing of internal rotational degrees of freedom that exist in the model system, which are taken care of when cucurbituril aligns the reactants, and concomitantly to an additional consideration which follows. 

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