Isomerism systems

An equihbrium mixture of the isomers usually contains a much higher proportion of the tme nitro compound. The equiUbrium for each isomeric system is influenced by the dielectric strength and the hydrogen-acceptor characteristics of the solvent medium. The aci form is dissolved and neutralized rapidly by strong bases, and gives characteristic color reactions with ferric chloride. 

An isomeric system is related to the benzenoid hydrocarbon phenanthrene. Both the [Pg.521]

A preparation of the parent system 4 has been claimed starting from the precursor 133. Thus, when treated with triethyl orthoformate under acidic conditions (H2SO4), 133 underwent transformatirai into 4 in 80% yield. This experiment did not lead to the formation of the isomeric system 167. This outcome was rationalized in terms of the initial formation of an indoleninium ion, followed by rearrangement... 

Only one isomeric system of the eight possible was reported during the period covered by this review. 

Since the review by McEwan in 1992, there have been several advances for simple isomeric systems.23... 

Obviously, a great deal more information could be obtained if the isomeric ions could be probed spectroscopically. Vibrational states of the various isomers are not generally well known, but some structural information is available. Thus, the rotational structure of vibrational transitions may provide a better signature for particular isomers. Certainly, insufficient data are available about the potential surfaces of electronically excited states for electronic excitation to be used as a probe, e.g., as in the very sensitive laser induced fluorescence. At present, there are sensitivity limitations in the infrared region of the spectrum, but this may well be an avenue for the future. The study of isomeric systems and their potential surfaces has just begun ... 

BLISS [Butylene isomerization system] A process for isomerizing / -butenes to isobutene. Piloted by Texas Olefins Company in Houston, TX, 1990 to 1992. 

Figure 1. (Left) Potential energy surface for the LiNC/LiCN isomerizing system drawn as a contours plot. The minimum energy path connecting the two stable linear isomers, LiNC (0 = 180°) and LiCN (6 = 0), is shown as a dotted line. 

Table I. Geometries and energies for the stationary points of the potential energy surface (Essers, Tennyson, and Wormer, 1982) used in our 2D model of the LiNC/LiCN isomerizing system.

Pyrrolo[l,2-r]oxazole (F). This parent structure must be partially saturated. There are three possible isomeric systems H,3H-( ) 15, 3H,SH-( ) 16, 3H,lH-( ) 17. 

Their studies revealed that acylation occurs preferentially at N(l) leading to a 40% yield of l-(ethoxycarbonyl)-7-methyl-l/7-pyrrolotetrazole 38 along with traces (4.6% yield) of l,5-bis(ethoxycarbonyl)-7-methyl-l//-pyrrolotetra-zole 39. The 1/7-pyrrolotetrazole structures of 38 and 39 were established by comparison of their 13C NMR spectra with a model 1//-isomeric system 40. Additional support for the l//-isomeric system came from 13C NMR spectral comparisons with pyrrolotetrazoles 41 and 42 that are representatives of the 2H- and 3//-systems. These studies showed that the 13C NMR spectral data presented earlier by Dulcere and co-workers <1990JOC571> in a previous acylation study of the monoanion 37 was not compatible with what was believed to be the 5//-pyrrolotetrazole 43. 

A derivative of the isomeric system, 120, is formed if AA esters are condensed with 2-carbalkoxy-3-formylindole under reducing conditions (90JOC1390). 

Irradiation of the caged oxadiazabicyclo[2.23]nonadiene derivative 24 (X = CH) brings about its rearrangement to the isomeric system 25 and the formation of the 1-ethoxycatbonyl-l//-azepine (26) <99H(51)141>. The latter is also the product of irradiation 25, but the authors interpret their kinetic results to suggest that the azepine is not derived solely from 25, but is also formed directly from 24. Similar studies with 24 (X = N) give 1-ethoxycarbonyl-l// l,2-diazepine (Scheme 4). 

Chromium(II) complexes of bipyridyls, terpyridyl and the phenanthrolines have been discussed in Section 35.2.2.1. Complexes of the ligands 2-aminomethylpyridine (pic, 2-picolyl-amine) and 8-aminoquinoline (amq), which have one heterocyclic and one amino nitrogen donor atom, have been prepared by methods similar to those in Scheme 10. The bis(amine) complexes are typical high-spin, distorted octahedral complexes, and the mono(amine) complexes, from their antiferromagnetic behaviour and reflectance spectra, are six-coordinate, halide-bridged polymers (Table 15).103 No tris(amine) complexes could be prepared so the attempt to find spin isomeric systems in octahedral chromium(II) systems was unsuccessful ([Cr(en)3]X2 are high-spin and [Cr(bipy)3]X3 and [CrX2(bipy)2] low-spin). 

Fusion of a pyrazole ring with a thiazepine nucleus results in three possible isomeric systems, one of which has a nitrogen bridgehead. Whereas this last possibility allows only one fusion, that on the d edge,... 

The Ni(IV) and Pd(IV) species appear to be Lewis acids, since crystalline adducts have been obtained with 7r-bases and Lewis bases such as pyrene, phenanthrene, A Af-dimethylaniline halide ions, and SCN- (18, 42). It has been briefly mentioned (42) that the substituted complexes [B9H9C2(CH3)2]2Ni", n = 0, —1, —2, exhibit a series of molecular rearrangements to form isomeric systems. 

Photosensitized isomerization of fluoranthenylalkenes 94 has been studied in search of a one-way isomerization system [228-231]. 

The oxidative decarboxylation of a cyclohepta[c c ]dithiophene (40), or the elimination of the trimethylammonium group in the salt (41) gave the fused cycloheptatriene (42) <82CS53>. However, in contrast to the isomeric system (43), a tropylium ion could not be formed from compound (42) <81JCS(P1)2904>. 

The dibenzo heterocycle (183) has been prepared by methods analogous to those used for the isomeric system (181). Cyclodeoxygenation of o-nitrophenyl-liT-benzotriazole (182 R = N02) using triethyl phosphite gives the colourless crystalline heterocycle (183) which is also formed by thermolysis (180 °C) of the azide (182 R = N3) (67JA2618, 67JA2633). Monobenzo derivatives have been similarly prepared (67JA2633). 

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