Identical heterocycles

Abstract In this chapter, selected results obtained so far on Fe(II) spin crossover compounds of 1,2,4-triazole, isoxazole and tetrazole derivatives are summarized and analysed. These materials include the only compounds known to have Fe(II)N6 spin crossover chromophores consisting of six chemically identical heterocyclic ligands. Particular attention is paid to the coordination modes for substituted 1,2,4-triazole derivatives towards Fe(II) resulting in polynuclear and mononuclear compounds exhibiting Fe(II) spin transitions. Furthermore, the physical properties of mononuclear Fe(II) isoxazole and 1-alkyl-tetrazole compounds are discussed in relation to their structures. It will also be shown that the use of a,p- and a,C0-bis(tetrazol-l-yl)alkane type ligands allowed a novel strategy towards obtaining polynuclear Fe(II) spin crossover materials. 

Only for 4-R-substituted 1,2,4-triazoles, isoxazoles and 1-alkyl-tetrazoles (Fig. 1), has the Fe(II)N6 spin crossover chromophore been found to consist of six chemically identical heterocyclic ligands. These spin transition materials are of particular interest. Since only a single N-donor ligand is involved in the synthetic procedure, the formation of mixed ligand species is avoided, and hence rather high yields are usually obtained. In addition, the choice of such relatively small heterocyclic ligands favours almost regular Oh symmetry about the Fe(II) ion. This is especially so for low-spin Fe(II). 

Assemblies of two or more identical heterocyclic systems are named by placing the prefix bi- , ter- , quater- , etc., before the name of the heterocyclic system or radical. The numbering of the assembly is that of the corresponding heterocyclic systems, one component being assigned unprimed and the others primed, doubly primed, etc.,... 

Substituted 2-ethyl-l,2,3-triazinium salts 87c and 87e give poor yields of 2-ethyl-2,5-dihydro-l,2,3-triazines 120 (R = Ph or Me) and, additionally, in two cases, a pyrazole 121 bearing two identical heterocyclic substituents (Equation 40). The ring contraction generating 121 is rationalized in Equation (41). 

Identical) heterocycles linked by single bonds are treated like related carbocycles ... 

Many monomeric heterocyclic anhydrobases can be isolated now using specific methods (44), but application of these methods to thiazole ring did not succeed however, appropriate conditions lead to the separation of a dimer, the structure of which has been established by its NMR Spectra and chemical reactivity (26). The most probable mechanism of its formation appears identical with the one previously described in the benzothiazolium series (24). A second molecule of quaternary salt A3... 

The quatemization reaction of the thiazole nitrogen has been used to evaluate the steric effect of substituents in heterocyclic compounds since thiazole and its alkyl derivatives are good models for such study. In fact, substituents in the 2- and 4-positions of the ring only interact through their steric effects (inductive and resonance effects were constant in the studied series). The thiazole ring is planar, and the geometries of the ground and transition states are identical. Finally, the 2- and 4-positions have been shown to be different (259. 260). 

The strain energies of these five-membered heterocycles are relatively small with values of 23.5, 24.8 and S.SkJmoF estimated for tetrahydrofuran, pyrrolidine and tetrahy-drothiophene respectively (74PMH(6)199). The closeness of the values for the two former compounds reflects the almost identical covalent radii of oxygen (0.66 A) and nitrogen (0.70 A) atoms. The sulfur atom with a much larger covalent radius of 1.04 A causes a... 

The three possibilities of synthesizing a C5 chain, namely, C2 +Ci + C2. C2 -t C2 -I- Cl, or Cl + C3 + Cl, lead to pyrylium salts having identical substituents in positions 2 and 4 in the second case, or 2 and 6 in the first and third cases. Despite this limitation, such syntheses are very convenient because they make the pyrylium salts easily accessible (more so than other six-membered heterocyclic aromatics) from aliphatic starting materials. 

The authors have compared in every case the p-values for the heterocyclic series (ph) with the corresponding p-value for the benzene series (Pb) under identical conditions. The ratios ph/pb f e thiophene series vary between 0.63 and 1.34, with the two extreme values applying to the 5-R-3-Y systems. For the other systems the variation is between 0.83 and 1.20. with an average value of 0.99. The three furan-system values, all for 5-R-2-Y, are more divergent, between 0.89 and 1.39, with an average of 1.15, apparently slightly higher, than in the thiophene system. 

In comparing the reactivity at different positions in a heterocycle, a poly-substituted derivative is sometimes used with the idea that selective reaction of the same leaving group at different positions in a single molecule gives the most clear-cut answer. However, in a polychloroazine, the mutual activation of the chlorines by one another is not identical (unless the molecule is symmetrical, in which case the... 

In 2008, Grisi et al. reported three ruthenium complexes 65-67 bearing chiral, symmetrical monodentate NHC ligands with two iV-(S)-phenylethyl side chains [74] (Fig. 3.26). Three different types of backbones were incorporated into the AT-heterocyclic moiety of the ligands. When achiral triene 57 was treated with catalysts 65-67 under identical reaction conditions, a dramatic difference was observed. As expected, the absence of backbone chirality in complex 65 makes it completely inefficient for inducing enantioselectivity in the formation of 58. Similarly, the mismatched chiral backbone framework of complex 66 was not able to promote asymmetric RCM of 57. In contrast, appreciable albeit low selectivity (33% ee) was observed when the backbone possessed anti stereochemistry. 

Abstract This chapter focuses on carbon monoxide as a reagent in M-NHC catalysed reactions. The most important and popular of these reactions is hydro-formylation. Unfortunately, uncertainty exists as to the identity of the active catalyst and whether the NHC is bound to the catalyst in a number of the reported reactions. Mixed bidentate NHC complexes and cobalt-based complexes provide for better stability of the catalyst. Catalysts used for hydroaminomethylation and carbonyla-tion reactions show promise to rival traditional phosphine-based catalysts. Reports of decarbonylation are scarce, but the potential strength of the M-NHC bond is conducive to the harsh conditions required. This report will highlight, where appropriate, the potential benefits of exchanging traditional phosphorous ligands with iV-heterocyclic carbenes as well as cases where the role of the NHC might need re-evaluation. A review by the author on this topic has recently appeared [1]. 

HMBC experiments are not limited to proton-carbon interaction. With suitable hardware, it is possible to acquire 1H-I5N spectra which can be extremely useful for confirming the identity of nitrogen-containing heterocyclic compounds. The sensitivity of this technique is very low, probably only about... 

The research was oriented towards the development of biocatalysts for removal of recalcitrant sulfur heterocyclic compounds including benzothiophenes, naphthothio-phenes, and alkylbenzothiophenes. To begin with, they focused on asymmetric sulfur compounds in this class and developed a method for desulfurization of these compounds present in petroleum products [108], The identity of the microorganisms was not disclosed in the abstract but they do claim use of the enzymes as well in the application. 

To calibrate our thinking, the resonance or extra stabilization found in benzene as opposed to a species with three isolated double bonds is ca 151 kJ mol-1, which we note is fortuitously nearly the identical value as found in the comparison of these nitrogenous heterocycles. 

The objective of this paper is to present an up-to-date account of what we know about the chemistry of soil N. The first part of this paper will deal with the distribution in different soils of proteinaceous materials, amino sugars, and ammonia, while the second part will focus on recent data on the identities and roles of heterocyclic N compounds, which also appear to play a significant role in supplying N to plant roots and microbes. 

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