Esters and xanthates

The pyrolyses of esters and xanthates involve six-membered cyclic transition states and these are presumably almost strainless and chair-shaped rather than planar and eclipsed. Thus, they are more realistically regarded as occurring from syn-clinal rather than a syn conformation of the eliminating fragments in the substrate. 

Comparison of diastereoisomeric pairs has been used to demonstrate the syn nature of pyrolytic elimations. Both acetates of 2-deutero-l, 2-diphenyl-ethanol undergo pyrolysis to give tra/i -stilbene but the product from the erythro substrate is labelled with deuterium (135) while that from the threo substrate only contains hydrogen (136), viz. 

The same acetates give products labelled vice versa when anti-elimination is induced with potassium t-butoxide. The great preference for trani-stilbene formation is caused by the necessity to avoid eclipsing of the phenyl groups in the transition states. Similar syn stereospecificity is demonstrated for pyrolysis of 2-butyl-3-phenylxanthates , the threo isomer yielding t/j-di-methylstyrene (137) while the erythro isomer gives the tran -olefin (138). 

In the rigid indanyl system, y/z-elimination from the /ra/w-2-methyl-l-acetate occurs at a much lower temperature than the decomposition of the 

Realising the significance and necessity of jyn -elimination in Ej processes. Barton showed that this concept correctly predicted the direction of elimination of xanthates and esters of many natural products, based on 5- and 6-membered alicyclic rings and he subsequently used this method to predict unknown configurations of certain compounds. 

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PRODUCTS OF PYROLYSIS OF cis- AND Ira i-2-S U BSTIT UTE D CYCLOHEXYL ESTERS AND XANTHATES... 

For four- or five-membered cyclic transition states, pyrolysis only leads to beta-elimination if the C -H and C -X bonds can eclipse so as to ensure the necessary proximity for reaction. The syn nature of the amine oxide decomposition has been demonstrated by elimination from the diastereoisomeric pairs of the 1,2-diphenylpropyl system. In the cyclohexyl series, an eclipsed transition state is only achieved if elimination proceeds through the boat conformer and consequently the pyrolysis of 1-methylcyclohexylamine oxide gives an almost quantitative yield of the less stable exo-olefin. This orientation contrasts markedly with that of the six-membered cyclic transition states of the corresponding esters and xanthates which yield predominantly the endo-olefin, syn-clinal stereochemistry being preferred. For all the other ring... 

The orientational behaviour of amine oxide pyrolyses has been adequately summarised by Cope and TrumbulE . As for the acetate decompositions, orientation in the simple alkyl systems is controlled primarily by statistical factors but departure from this influence is noted with the bulky r-butyl substituent and the acid strengthening beta phenyl substituents Eclipsing effects are greater in the planar five-membered transition states than in the puckered six systems and this is borne out by the greater preference for trans-olefin formation from amine oxides than esters and xanthates (152, cf. 149). 

An important group of alkene-forming reactions, some of which are useful in synthesis, are pyrolytic eliminations. Included in this group are the pyrolyses of carboxylic esters and xanthates, of amine oxides, sulfoxides and selenoxides. These reactions take place in a concerted manner, by way of a cyclic transition state and therefore proceed with syn stereochemistry, such that the hydrogen atom and the leaving group depart from the same side of the incipient double bond (in contrast to the eliminations discussed in Section 2.1) (2.14). 

Both the sulfite and alkaline (kraft) methods can be modified to produce high purity chemical ceUulose. These pulps, usuaUy in the form of "dissolving pulps," are not only mosdy free of lignin and hemiceUulose, but the molecular weight of the ceUulose is degraded. This increases solubUity in alkah and provides desired viscosity levels in solution. These dissolving pulps are used to make derivatives such as sodium ceUulose xanthate [9051 -13-2] via alkah ceUulose, and various esters and ethers (see Cellulose esters Cellulose ethers). 

Scheme 5.9 illustrates some of the conditions that have been developed for the reductive deoxygenation of alcohols. Entries 1 to 4 illustrate the most commonly used methods for generation of thiono esters and their reduction by tri-M-butylstannane. These include formation of thiono carbonates (Entry 1), xanthates (Entry 2), and thiono imidazolides (Entries 3 and 4). Entry 5 is an example of use of dimethyl phosphite as the hydrogen donor. Entry 6 uses r .s-(trimethylsilyl)silane as the hydrogen atom donor. 

The precise reaction conditions for optimal yields depend upon the specific reagents and both thermal290 and photochemical291 conditions have been developed. Phenyl thionocarbonates are easily prepared and are useful in radical generating reactions.292 A variety of other thiono esters, including xanthates and imidazolyl thiocarbonates also can be used.293... 

Chemically dextrans are similar to one another. The activation energy for acid hydrolysis is about 30-35 Kcal/mol (5j. The C-2 hydroxyls appear to be the most reactive in most Lewis base and acid-type reactions. A wide variety of esters and ethers have been described as well as carbonates and xanthates ( 7,8j. In alkaline solution, dextran forms a varying complex with a number of metal ions (9). 

The highest PGM recovery was achieved using collector PM443, which is an amine + ester-modified xanthate. Among the chromium slime depressants evaluated, modified mixtures of organic acids, RQ depressants and a low-molecular-weight polyacrylic acid + pyrophosphate mixture were there. The effect of different chromium depressants on chromium assays of the PGM concentrate are illustrated in Figure 18.7. 

Some ore deposits contain cuprite as the principal mineral. Typically, these deposits contain appreciable amounts of slimes and clay minerals. The laboratory studies conducted on these types of ore indicated that improved metallurgical results can be achieved using the sulphidization method with ester-modified xanthate [8]. 

See also ALKANETHIOLS, ALKENEBIS(SULFONIUM PERCHLORATES) allyl trifluo-ROMETHANESULFONATES, ARENEDIAZO ARYL SULFIDES BIS(ARENEDIAZO) sulfides, BIS(SULFURDIIMIDES DIAZONIUM SULFATES, DIAZONIUM SULFIDES AND DERIVATIVES METAL AMIDOSULFATES, METAL PHOSPHORUS TRISULFIDES METAL SULFATES, METAL SULFIDES, NON-METAL SULFIDES SULFONIC ACID ESTERS, SULFUR BLACK, SULFUR ESTERS THIOPHENOXIDES, XANTHATES ... 

Pyrolyses of esters (60) and xanthate esters (61), either in the gas phase or in solution, give 1,2-elimination (Equations 7.46 and 7.47). These reactions are... 

The conversion of alcohols to xanthate esters, and treatment of the latter with DFIT provides access to 1°- and 2°-alkyl fluorides, probably through intermediates of the type shown in Scheme 38 [108]. 

II. STRUCTURES OF XANTHATE ANIONS, ESTERS, AND OXIDATION PRODUCTS... 

Carbon disulfide is the dithio derivative of C02. It is only a weak electrophile. Actually, it is so unreactive that in many reactions it can be used as a solvent. Consequently, only good nucleophiles can add to the C—S double bond of carbon disulfide. For example, alkali metal alkoxides add to carbon disulfide forming alkali metal xan-thates A (Figure 7.4). If one were to protonate this compound this would provide compound B, which is a derivative of free dithiocarbonic acid. It is unstable in the condensed phase in pure form, just as free carbonic acid and the unsubstituted carbamic acid (Formula B in Figure 7.3) are unstable. Compound B would therefore decompose spontaneously into ROH and CS2. Stable derivatives of alkali metal xanthates A are their esters C. They are referred to as xanthic add esters or xanthates. They are obtained by an alkylation (almost always by a methylation) of the alkali metal xanthates A. You have already learned about synthesis applications of xanthic acid esters in Figures 1.32, 4.13, and 4.14. 

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