Symmetrical compounds

A summary of the studies perfonned on symmetrical compounds having a diacetylene group at the centre is given in [94]. Most of the materials studied in the context of LB films have been diyonic acids (figure C2.4.8). 

Lastly, we isolated a tetrabromo compound 21 (ref. 10) which is derived from solvent. IH and NMR spectra of 21 indicates the formation of a highly symmetrical compound whose configuration is not known. Possible configurations are given on scheme 10. On an independent reaction we treated decalin with bromine at high temperature and obtained 21 in high yield. 

Application of the bromine substitution reaction allows the synthesis of aminoamides, alkoxyamides of simple alcohols and sugars, depsipeptides and (NH) pseudopeptides, C2 symmetric compounds. 

When the two partial sequences stemming from the central NH group are chemically and stereochemically identical, Ci symmetric products result. We consider such compoimds as candidates for biological smdies, as well as models of C2 chiral auxiliaries for asymmetric induction (ref. 23), or ligands for metal ions. Syntheses of C2 symmetric compounds requiring the use of chiral auxiliaries have been recently reported (ref. 24). 

The intermediate corresponding to 64 in the case of 58 is a symmetrical compound, and the three-membered ring can be opened with equal probability on either side of the carbonyl, accounting for the results with In the general case, 64 is not symmetrical and should open on the side that gives the more stable carbanion. This accounts for the fact that 60 and 61 give the same product. The intermediate in both cases is 63, which always opens to give the carbanion stabilized by resonance. The cyciopropanone intermediate (64) has been isolated in the case where R2=R5 f.Bu and R =R =H,... 

In both examples the reaction is shifted to the right side, because the un-symmetrical compound is highly associated and poorly soluble. If the diazastannylene and dicyclopentadienyltin are mixed in 1 1 ratio, an adduct is formed which is unstable and thus cannot be isolated it decomposes to the original stannylenes (Eq. (38)) 87). 

Any spin system that contains fluorine substituents that are chemically equivalent, but not magnetically equivalent is, by definition, second order. Such spectra can appear deceptively simple, or more commonly they can be amazingly complex. The fluorine and proton spectra of the simple, symmetrical compound, 1,1-difluoroethene exemplify the latter situation (Figures 2.5 and 2.6). 

Meso compound A symmetrical compound containing two chiral centres configured so that the chirality of one of the centres is equal and opposite to the other. Such internal compensation means that these compounds have no overall effect on polarised light (e.g., meso tartaric acid). 

In a somewhat related study by Hallberg and coworkers, C2-symmetric compounds based on a 1,2-dihydroxyethylene scaffold with elongated Pl/Pl side chains... 

The tetraphenylethanes described above are symmetrical compounds used to generate the same two radicals by dissociation, while pentaphenylethane (28) is an unsymmetrical derivative, giving two different radicals, triphenylmethyl and diphenylmethyl radicals [138]. The former cannot initiate radical polymerization, but the latter is available as an initiating radical to produce the polymer 28, which can function as the polymeric iniferter [106]. 

Dyes for cellulose acetate are relatively simple molecules, typified by Cl Disperse Red 15 (6.39 X = OH), Cl Disperse Violet 4 (6.39 X = NHCH3) and Cl Disperse Blue 3 (6.40), the last-named being manufactured from leucoquinizarin and the appropriate amines. The unsymmetrically substituted product inevitably contains significant amounts of the related symmetrical compounds. The widely used Cl Disperse Blue 3 is known to cause skin sensitisation when on nylon [17] and can also provoke an allergic reaction [18]. Bright red 2-alkoxy-l-amino-4-hydroxyanthraquinones, such as Cl Disperse Red 4 (6.41), can be obtained from l-amino-2,4-dibromoanthraquinone by hydrolysis to give l-amino-2-bromo-4-hydroxyanthraquinone (Cl Disperse Violet 17), which is then condensed with the appropriate alcohol. 

In addition, a number of unsymmetric dipnictenes, RP=AsR, RP=SbR,62 RSb=BiR,59 and RP=BiR,63 were reported. One example, the stibabismuthene RSb=BiR (R = Bbt), was formed by the elimination of HBr from RSbH2 and RBiBr2. The stibabismuthene is stable at ambient temperature in hydrocarbon solvents in the absence of air and light. When a solution in benzene was heated, the symmetrical compounds are formed (Scheme 7).59... 

In order to round-off our overview on the symmetric compounds of this class, we add the stilbene-like molecules TTPAE (20) (Tg = 111°C) [27], the bis-(styryl)anthracene (BSA, 21) [64], and DPVBi (22) [65] (Fig. 3.9). Compounds 20 and 21 are characterized by a rigid structure leading to a large electronic delocalization through the center of the molecules. Compound 22 is more flexible due to the biphenyl bond. The outer phenyl rings are twisted by steric repulsion, leading to a nonplanar structure of the stilbene units. 

In an aprotic solution, the mechanism of oxidation of diaryl disulfides was shown to be more complex than a direct cleavage of the S—S linkage [116,117,123], The occurrence of two consecutive reactions being of second kinetic order if potential-determined and of first order if current-determined, was established for the two-electron transfer steps. Dimerization of the cation radicals occurs on the ArS fragment, whose contribution to the HOMO is more important, and produces an intermediate disulfonium dication. The subsequent cleavage of the latter results in two ArS+ cations and a molecule of a disulfide (the same as a starting disulfide in the case of symmetrical compounds). This mechanism, EC2C1E (E = electrochemical, C = Chemical), has... 

The thermal decomposition of azoalkanes bearing geminal a-cyano and a-trimethylsiloxy groups has been the subject of a report. The symmetrical compound (107) decomposes near room temperature to afford entirely C—C dimers, whereas the unsymmetrical azoalkane (108) requires heating to 75 °C. A NMR product study of photolysed (107) in the presence of TEMPO showed that the fate of caged t-butyl-l-trimethylsiloxy-l-cyanoethyl radical pairs is disproportionation (17%), cage recombination (20%), and cage escape (63%). 

The Tfj-symmetrical hexaaddition pattern also represents an attractive core tecton for dendrimer chemistry [26, 31, 63-67]. Examples for such dendrimers, involving a core branching multiplicity of 12, are 38 and 39 [63, 64], Addition of six mesotropic cyanobiphenyl malonate addends produced the spherical thermotropic liquid crystal 40 [65], DSC and POM investigations revealed a smectic A phase between 80 and 133 °C. Interestingly, this spherical and highly symmetrical compound gives rise to liquid crystallinity despite the absence of molecular anisotropy. 

It will be noted that the symmetrical compound carbon tetrachloride differs markedly in the ratio from the unsymmetrical methyl alcohol. 

The synthesis of both symmetrical and non-symmetrical dithiophenylperfluorocy-clopentenes is facilitated by selective elimination of fluorine from perfluorcyclopen-tene, using a thiophenylithium (1.48), prepared from the corresponding bromo derivative by reaction with n-BuLi in THE The symmetrical product (1.51) is normally made without isolating the mono-substituted product (1.49), but the non-symmetrical compound (1.52) requires controlled production of (1.49) for reaction... 

The set of diacyl peroxides that have been investigated by X-ray diffraction and disclosed in the hterature in the last two decades is restricted to symmetric compounds with a mean 0-0 bond length 1.45 0.01 A (Table 14) and O—O—C... 

The symmetrical compounds (17, R = R = R ) are formed from an aldehyde and ammonium sulfide (2,29,35) or hydrogen sulfide and ammonia (33), Thialdine (17, R - R - R = Me) is an important aroma compound found in the volatiles of beef broth (48), pressure-cooked meat (49), and fried chicken (50), It is also obtained from acetaldehyde (2,29,37) or from B-mercaptoacetaldehyde and ammonium sulfide (37), In our experiments, it was synthesized as a white powder from a reaction of acetaldehyde with ammonium sulfide in 60% yield. 

Unsymmetrically 2.4.6-X -phosphorins with different substituents at the C—2 and C-6 positions can easily be distinguished from the symmetric compounds, since the C—3 and C—5 protons couple with each other, giving rise to an ABX splitting pattern (/h,h =1-2 Hz /h c-c-p = 6 Hz). For example. 

Desymmetrization of symmetric compounds involves breaking the symmetry in a substrate, i.e. elimination of one or several elements of symmetry (Scheme 4.2). Thus, it is an asymmetric synthetic transformation and can therefore give a quantitative yield. 

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