Unsymmetrical compounds

In the compounds considered so far the sulfur chains are terminated by the same kind of atom or group at both ends. However, mixed organic disulfides and also other types of unsymmetrical di- and polysulfides exist- 

Organic disulfur chlorides, R—S—S—Cl with R = o-nitrophenyl and 9-anthryl, were described as early as 1922. Harris (IJfi) has recently studied properties and reactions of the o-nitrophenyl compound, and Himel and Edmonds 144) in patents described reactions of the f-butyl analog other aliphatic disulfur chlorides were reported by Bohme and van Ham S3) and Feh6r and Kruse 66) in 1958. With thiols, thioacetate, xanthates, and dithiocarbamates these compounds give unsymmetrical trisulfides. Bohme and Clement 32) in 1952 prepared acyl-disulfur and acyl-polysulfur chlorides, and derivatives such as an unsymmetrical pentasulfide. 

Bohme and Zinner (34), who prepared the methyl, ethyl, and benzyl derivatives. With iodine, the trisulfides give symmetrical liexasulfides. Feh6r and Kruse (66) prepared phenyl-hydrogen tri- and tetrasulfide and n-butyl-hydrogen tetrasulfide and studied their Raman spectra. The compounds are sensitive to bases, and salts are apparently unstable, although White (226) in 1918 reported a stable sodium salt of 2-naphthyl-hydrogen disulfide. 

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Azoxy Compounds. Where the position of the azoxy oxygen atom is unknown or immaterial, the compound is named in accordance with azo rules, with the affix azo replaced by azoxy. When the position of the azoxy oxygen atom in an unsymmetrical compound is designated, a prefix NNO- or ONN- is used. When both the groups attached to the azoxy radical are cited in the name of the compound, the prefix NNO- specifies that the second of these two groups is attached directly... 

The Hantsch pyridine synthesis provides the final step in the preparation of all dihydrop-yridines. This reaction consists in essence in the condensation of an aromatic aldehyde with an excess of an acetoacetate ester and ammonia. Tlie need to produce unsymmetrically subsrituted dihydropyridines led to the development of modifications on the synthesis. (The chirality in unsymmetrical compounds leads to marked enhancement in potency.) Methyl acetoacetate foniis an aldol product (30) with aldehyde 29 conjugate addition of ethyl acetoacetate would complete assembly of the carbon skeleton. Ammonia would provide the heterocyclic atom. Thus, application of this modified reaction affords the mixed diester felodipine 31 [8]. 

Random incorporation of two different acetoacetates can also be avoided by converting one of the acetoacetates to a derivative which carries the future pyridine nitrogen. For example, treatment of ethyl acetoacetate with ammonia gives the corresponding P-aminocrotonate 32. The aldehyde (34) required for preparation of such an unsymmetrical compound is prepared by reaction of the product from direct metallation of 33 with dimethylformamide. Condensation of that aldehyde with methyl acetoacetate and the p-aminocrotonate from isopropyl acetoacetate leads to isradipine (35) [9]. The same aldehyde with ethyl acetoacetate and the P-aminocrotonate from ethyl acetoacetate gives darodipine (36) [10]. In much the same vein, condensation of the ben-zaldehyde 37 with methyl acetoacetate and its P-aminocrotonate derivative affords riodipine (38) [11]. 

Cross coupling of two different carboxylates (= mixed Kolbe electrolysis) is a method for synthesizing unsymmetrical compounds (Eq. 8). As, however, the intermediate radicals combine statistically, the mixed coupling product... 

Formation of the oxazole ring is not always the last step in synthesis of the brightener. Unsymmetrical compounds that contain both a benzoxazole group and an ethene linkage can be prepared by the anil synthesis [51], in which a compound possessing an activated methyl group reacts with a Schiff base. The preparation of brightener 11.31 is an illustration of this method (Scheme 11.12). 

The four-electron reduction of symmetrical (R = CN Scheme 78) [114] or unsymmetrical (R = C02Me) [115] 4-(o-nitrophenyl)-l,4-dihydropyridines leads to aminobenzonaphthyridine N-oxide when the electrolysis is performed in slightly acidic or slightly basic medium in higher basic medium, the reduction of the unsymmetrical compound gives a cyclic hy-droxamic acid. 

Data for unsymmetrical compounds are obtained by directly observing the phosphorus AB spectra but the signs of coupling are obtained by spin-tickling or by analysis of higher order spectra. 

For quadricovalent silicon, germanium, and tin (and also for atoms such as nitrogen in a substituted ammonium ion) the same tetrahedral orientation of bonds is expected, since 3a-3p, 4a-4p, and 5 -5p hybridization is the same as that for the 2s-2p system. Observed values of bond angles in unsymmetrical compounds of these substances are also included in Table 4-3. 

We have been able to demonstrate by synthesizing the unsymmetrical compound 87 and recording its temperature-dependent H NMR spectra that, in addition to the equilibrium between the enantiomers 86a and 86b, further ligand rearrangements need also to be considered. 

Equation 9.29) and later the second (Equation 9.30). One test is to measure the effect on decomposition rate of changing one of the R groups. If R—N=N—R and R—N=N—R, where R - is a more highly stabilized radical than R, are compared, a lower rate should be found for the unsymmetrical compound in a concerted decomposition in a stepwise one the better radical R- should come off first in either compound, and the rate should be unaffected. The results support the concerted process (Equation 9.28) for the symmetrical azo compounds with R- a well-stabilized radical such as < 2CCN, 2CH, and so forth, but leave... 

A number of patents exist for the application of fused oxazoloazines in fluorescent whiteners, e.g. the use of 4,4 -bis(oxazolopyridin-2-yl)stilbenes (298) as fluorescent whiteners for polyesters (70GEP2003575). The highly unsymmetrical compound (299) is a fluorescent whitener for polyamide and polyester fibers (75USP3873531). 

Thioesters of methylarsenic(III) acid and an equimolar amount of tetrachloro-o-benzoquinone give unsymmetrical compounds (XXIX) (73) ... 

Divalent tellurium derivatives in which tellurium is bonded to two group IV elements, are known as linear and cyclic compounds. Symmetrical compounds with two silicon, germanium, tin, or lead atoms bonded to tellurium and unsymmetrical compounds, with two different group IV elements linked to tellurium have been prepared. Some of the compounds have an obnoxious odor4. 

These compounds are stable in the dark under an inert atmosphere for several weeks when stored at — 20° They are sensitive to light and to air1. The decompositions are accompanied by deposition of tellurium. Bis[trialkylsilyl] tellurium compounds deposit SiTe films on heating at 450°2. Unsymmetrical compounds have a tendency to form symmetrical compounds3,4. 

Tellurium dithiobenzoate halides and piperidinium or sodium dithiocarboxylates condensed at 20° in dichloromethane or a dichloromethane/ethanol mixture. When the aryl group in the piperidinium (sodium) dithiocarboxylate was not the same as that in the tellurium compound, all three possible tellurium dithiocarboxylates were formed. Their presence in the recrystallized products was confirmed by mass spectrometry. However, all attempts to isolate the unsymmetrical compounds in pure form failed1. 

The reactions of alkoxy tellurium pentafluorides with alcohols in the presence of pyridine were used to prepare dialkoxy tellurium tetrafluorides with two different alkoxy groups in the molecule1. These unsymmetrical compounds are suspected to be predominantly in the cis-form. 

A detailed analysis of the NMR spectra of the product obtained from phenyl tellurium bromide and 4-methylphenyl magnesium bromide revealed that diphenyl tellurium and bis[4-methylphenyl] tellurium had been formed in addition to the expected unsymmetrical compound. Pure 4-methylphenyl phenyl tellurium was obtained with stoichiometrically insufficient amounts of bromine and Grignard reagent5. 

When the bromine-lithium exchange and chalcogenol-formation are carried out in sequence employing S, Te or Se, Te, naphtho[1,8-cr/][l, 2]chalcogcnotelluroles were obtained. The unsymmetric compounds were separated from the two symmetric compounds by column chromatography on silica gel with hexane as the mobile phase3 6. 

Additionally, results for some unsymmetrical compounds and the question of radical stabilization by more than one substituent will be discussed. Finally, some of the consequences of substitution and strain on structural parameters will be briefly addressed 9b). The syntheses of all compounds referred to in this article and the determination of their structures and their configurations have been published or will be reported elsewhere 9b). All compounds were obtained on at least a 100 mg scale and their purity was confirmed by standard analytical procedures. 

The cryptands were first prepared in 1969 and form a series of well-defined complexes (cryptates) with alkali and alkaline-earth cations. In this chapter the synthesis of the first cryptand, 8, a macrobicyclic ligand, will be described1,2. The schematic representation (Fig. 5.1) shows that one deals with a multi-step synthesis. The major drawback of this approach is the rather large number of synthetic steps, but the route offers the advantage of being able to construct unsymmetrical compounds (A B C). 

The cleavage of dimethylzinc by borohydride can also yield the unsymmetrical compound, MeZnBH. Its IR spectrum shows a five-coordinated zinc polymer with ZnHjBHjZn units ... 

The di-hydroxy ethane corresponding to ethylidene chloride, i.e., the unsymmetrical compound, is not known. It will be recalled that the method of preparing the ethylidene chloride is from acet-aldehyde by treatment with phosphorus penta-chloride ... 

If the reaction is written with the anhydride instead of the acid itself it will be seen that the action consists in two chlorine atoms of the phosphorus penta-chloride replacing one of the carbonyl oxygens of the acid. This replacement of one oxygen by two chlorines is, we know, the true reaction of phosphorus penta-chloride (p. 81). We shall find, also, that this reaction and the unsymmetrical compound formed are similar to reactions and compounds which we shall consider in the study of the benzene di-basic acid, phthalic acid. 

It will be recalled that in the case of succinyl chloride both of these forms are obtained, but mostly the symmetrical (p. 282). Now from phthalic anhydride only one phthalyl chloride is obtained. This chloride acts like the unsymmetrical succinyl chloride, not like the symmetrical. The positive proof, however, that phthalyl chloride is the unsymmetrical compound is the following Sodium amalgam reduces phthalyl chloride by replacement of the chlorine with hydrogen. The compound formed is known as phthalide. This phthalide takes up water as anhydrides do and the product is hydroxy-methyl benzoic acid. 

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