Azole transfer

A phosphitylation instead of an azole transfer occurred in the reaction of a guanosine derivative with phosphorous diester tetrazolide (diethoxytetrazolylphosphine) [2003,[2013... 

Triarylmethylazoles can also be synthesized by azole-transfer reactions of triaryl-methyl halides or acetates with azolides. In a similar manner 2,4,6-trinitro-phenylazoles are obtained from 2,4,6-trinitrophenyl chloride ... 

Azole-Transfer Reactions with Benzylalcohols and Secondary Alcohols... 

This azole transfer has been used for expedient gas chromatographic determination of 2-ethylhexanoic acid.[34]... 

Any doubt about the existence of individual tautomers is now long past some tautomers can be crystallized separately (desmotropy), and others can be observed simultaneously in the same crystal (Section V,D,2) in summary, tautomers are not intrinsically different from isomers. Maybe it is worth mentioning that even two identical tautomers can differ. This is the case for the two intramolecular hydrogen-bonded (IMHB) enol tautomers of acetylacetone and for many NH-azoles they correspond to a doublewell profile for the proton transfer with both wells having the same energy (autotrope). 

The imbalance between and NMR studies in the solid state (Section VI,F) partly reflects the fact that it is easier to introduce N than into heterocyclic compounds, particularly azoles (DNMR in the solid state usually requires isotopic enrichment). Compared to solution studies, solid-state intermolecular proton transfer between tautomers has the enormous advantage that the structure of the species involved is precisely defined. 

The oxygen of azole A-ox ides produces a nucleophilic attack to C=S C-atom of a cycloalkanethione forming a zwitterion, which cyclizes and spontaneously decomposes, yielding the product of this sulfur transfer reaction201 (Scheme 16). 

Quatemized azines and azoles have also been investigated as potential phase-transfer catalysts. 1-Alkyl-2- and 4-dialkylaminopyridinium salts [38, 39] have... 

Several examples of the A-arylation of azoles by activated haloarenes under phase-transfer catalysed conditions have been reported (see Chapter 5). The reaction is often aided by the use of ultrasound in the absence of a solvent [e.g. 68]. 

TV-Aminoethylazoles, which are frequently encountered as intermediates in the preparation of pharmacologically active compounds, are readily obtained by solid liquid phase-transfer catalysed reaction of the azole, diazole or triazole and their benzo derivatives with 2-chloroethylammonium chloride at reflux temperature using a procedure analogous to 5.3.3 [25]. 

Scheme 4 Proposed stacking structure. The plane-to-plane distance d for the polymer shown in No. 19 is about 3.8 A [93-95], which is somewhat shorter than the d value (about 3.85 A) reported for head-to-tail-type poly(3-alkylthiophene) HT-P3RTh [128,129] and somewhat longer than the d values (3.6-3.65 A) observed with head-to-head-type poly(4-alkylthiazole-2,5-diyl) [124,129,130] and the charge-transfer-type copolymer of thiophene and 4-alkylthi-azole [131]. Poly(ethylenedioxythiophene) gives a similar d value (3.8 A) [132]. The existence of S in the aromatic ring seems to make the distance d longer...

Trimethylsilyldiazomethane, as a stable and safe substitute for hazardous diazomethane, is useful both as a reagent for introducing a Cj-unit and as a C-N-N synthon for the preparation of azoles. Many methods are described in the literature for the preparation of trimethylsilyldiazomethane, including the trimethylsilylation of diazomethane (7-74S), the alkaline decomposition of N-nitroso-N-(trimethylsilylmethyl)amides (25-61%) and the diazo group transfer reaction of trimethylsilylmethyllithium with p-toluenesulfonyl azide (38%). The present modified diazo group transfer method appears to be the most practical, high-yield, and large scale procedure for the preparation of... 

Another very instructive case concerns the alleged initiation of a cationic polymerisation by a charge-transfer complex formed by the compound chloranil (2,3,5,6-tetrachloroquinone) with the monomer N-vinyl-carb-azole. It was shown (Natsuume et al., 1969 1970) that this compound is not an initiator, but that the polymerisations were caused by a hydrolysis product, 2-hydroxy-3,5,6-trichloroquinone, which is a strong acid. One has learnt from this finding to be extremely suspicious of any claims for charge-transfer catalysis and to test one s suspicions by appropriate experiments involving progressive purification of the putative catalyst. 

Bensaude et al. (78T2259) have used T-jump relaxation spectrophotometry to determine the rates of protonation and deprotonation of 3(5)-methyl-5(3)-phenylpyrazole anion (416") and cation (416H ), respectively. This study is a fundamental cornerstone in understanding annular tautomerism in azoles. The nondissociative intramolecular proton transfer in azoles is not observed (78T2259 86BSF429). 

In addition to these few examples of kinetic studies related to acid-base equilibria in azoles, there are reports in the literature on activation energies calculated theoretically for proton transfer involving azoles (80CCC3482 80MI3 84JPC5882 86BSF429). Generally, the inclusion of solvent molecules is necessary to find a double minimum potential function. 

Utility systems for equipment and space heating and cooling frequently use heavy metal corrosion inhibitors in their heat transfer fluids. Chromate compounds are among the best corrosion inhibitors available. Nonchromate inhibitors that have proved to be feasible substitutes include polyphosphates, organophosphates, zinc, molybdates, and aromatic azoles. Some of these compounds have their own environmental impacts, however. Azoles, for instance, can be quite dangerous to human health. 

Apart from structural factors annular tautomerism is strongly influenced by the aggregate state of the compound. Gas-phase and solid state studies of tautomerism have received much attention recently because of the development of methods such as X-ray crystallography, CPMAS NMR, microwave spectroscopy, etc. Low temperature H NMR is the simplest and most straightforward method to determine Kx it only requires that proton transfer be slow enough to observe separate signals for both tautomers, and that the equilibrium is not shifted too much towards one of the tautomers (about 5% of the minor tautomers seems the limit). More detailed information on tautomerism of key azole systems is given below. 

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