Acetates and xanthates

The majority of orientational studies refer to the pyrolysis of acetates although in cases in which corresponding xanthates have been decomposed the product distribution is found to be almost identical. Of course, bearing in mind the considerable difference in reaction temperature, the two pyrolyses may not always lead to the same product proportions and the more reactant-like nature of the xanthate reactions may cause them to be less sensitive to thermodynamic effects than the pyrolyses of the acetates. 

The principal factors affecting orientation in acetate decompositions have been adequately summarised by DePuy and King Essentially three influences were recognised, these being termed statistical, steric and thermodynamic effects. Statistical control is observed in pyrolysis of simple aliphatic esters which under the elevated reaction temperatures experience little resistance to conformational rotation and the number of beta hydrogen atoms in each branch determines the direction of elimination (147)= 37o distortion in statistical control is imposed by the steric influence of a t-butyl substituent (148), and is also illustrated by the predominance of trans- over m-olefin formation (148, 149) due to eclipsing effects . The latter example, however, may also arise from thermodynamic influences which are more certainly demonstrated by preferential elimination towards a phenyl rather than an alkyl substituent (150) . The influence of substituents on olefin stability rather than beta hydrogen acidity seems more critical as elimination occurs more often towards a p-methoxyphenyl rather than a phenyl substituent (151 

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The cellulose molecule contains three hydroxyl groups which can react and leave the chain backbone intact. These alcohol groups can be esterified with acetic anhydride to form cellulose acetate. This polymer is spun into the fiber acetate rayon. Similarly, the alcohol groups in cellulose react with CS2 in the presence of strong base to produce cellulose xanthates. When extruded into fibers, this material is called viscose rayon, and when extruded into sheets, cellophane. In both the acetate and xanthate formation, some chain degradation also occurs, so the resulting polymer chains are shorter than those in the starting cellulose. 

A number of non-stereospecific acetate and xanthate pyrolyses have been observed " . /4 t/-elimination is rationalised by invoking neighbouring-group participation by an appropriately situated methyl ester group in one case . The array of products arising from one of the xanthate pyrolyses can be accounted for in terms of competing heterolytic and homolytic eliminations . The pyrolyses of isothiocyanates and thiocyanates bear a close resemblance to the ester eliminations " . 

These pyrolytic iyn-eliminations in acetates and xanthates proceed via a six-membered transition state in which three electron pairs are shifted at the same time. They can be easily explained by PMO approach (Figure 6.12). 

FIGURE 6.12 PMO analysis of pyrolytic syn eliminations in acetates and xanthates. 

The influence of intrinsic and extrinsic Cotton effects on the o.r.d. and c.d. spectra of acetate and xanthate derivatives of polysaccharides has been studied. "... 

Simple Mo xanthate complexes were obtained by the reaction between Mo(II) acetate and the xanthate ligands in inert atmosphere (550, 581a). The red crystalline compounds had the stoichiometry Mo2(EtXant)4 and reacted readily with donor molecules such as tetrahydrofuran (550) pyridine, 4-picoline, and Et3As (581a). A Mo(lI)-acetate type of dimeric structure was proposed (581a) for this molecule and was confirmed by a subsequent x-ray study (550). 

A thermographic material with only one heat-sensitive component is prepared by reaction of an organometallic compound such as phenylmercury(II) nitrate with a thiourea derivative.230 The product is ball milled and coated in a binder such as poly(vinyl acetate). Other organometallic compounds disclosed are those of bismuth, tin, gallium and germanium combined with a variety of sulfur-containing compounds such as thioamides and xanthates. 

However, when RO is an acetate, tosylate and xanthate, which are better leaving groups than chloride in gas-phase pyrolysis2 3 the process was found to occur as shown in equation 12. 

Cellulose is a fibrous, tough, water-insoluble, and crystalline substance. As a result of these characteristics, it is often converted to its derivatives in order to make it more useful. The most commonly used derivatives of cellulose are carbox-ymethylcellulose, methylcellulose, hydroxyethylceUulose, hydroxypropylcellulose, cellulose acetate, and cellulose xanthate (12). Among these derivatives, cellulose acetate and cellulose xanthate are cellulose esters, which are now widely used in the manufacturing of fibers, films, and in injection molding thermoplastics. 

A two-step synthesis of modified 2 -C-nucleoside precursor, ethyl [2-(5-methyl-2,4-dioxo-3,4-dihydro-2i/-pyrimidin-l-yl)-4-hydroxyl-5-hydroxymethyltetra-hydrofuran-3-yl]fluoro-acetate 172, from protected glycal 170 and xanthate has been developed following the same idea, and a diastereomeric 1 1 mixture of 2,3-trans product 171 was obtained in 57% yield (O Scheme 46). The use of triethylborane as a free-radical initiator was less successful and a longer reaction time was also required. Interestingly, introducing th)miine at C-1 in the presence of silver triflate at 0°C was highly stereoselective, and only a C, C2-trans linked product was detected. 

A number of synthetically useful elimination reactions proceed thermally, with no base or acid required. These elimination reactions proceed through a concerted retro-ene mechanism. (The mechanism is sometimes called E,.) The thermal elimination of acetic acid from alkyl acetates and the elimination of RSCOSH from alkyl xanthates (the Chugaev reaction) are retro-ene reactions. 

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). 

Thiocarbamate is synthesized by adopting chloro acetic acid and xanthate as raw materials. The reactions can be given as follows ... 

Butanol is used to produce plasticizer-type esters like phthalates, phosphates, sebacates, oleates, and stearates. Two important ester derivatives, n-butyl acetate and n-butyl acrylate, used in the coating are produced from n-butanol. Glycol ether derivatives like ethylene glycol monobutyl ether (EB) used in the coating industry is the product of the n-butanol reaction with ethylene oxide in the presence of an acidic catalyst. Other important derivatives of n-butanol include butyl amines and butyl esters used as herbicides, butyl xanthate ore flotation aids, butylated urea, and melamine-formaldehyde resins. 

Several carboxylates have been prepared, either during separations of An + ions or for thermal decomposition as a route to the dioxides. Most of the actinide carboxylates can be prepared as hydrates by dissolving the appropriate oxide and hydroxide in carboxylic acid. Formates, acetates, oxalates, xanthates, and carbamates are also known. 

Thermal )6-elimination reactions of acetates, benzoates, xanthates, sulfoxides, selenoxides, and N-oxides are also group transfer reactions. All these elimination reactions are yn-stereospecific and proceed through a cyclic six membered—or five membered—ring transition state of 6e process by intramolecular transfer of hydrogen atom, where all the participating orbitals have suprafacial interactions. These reactions are fundamentally retro-group transfer reactions. 

Marchessault, Sarko etal. [10, 11] have investigated the chiroptical properties of amylose xanthates extensively. The CD of amylose triacetate in trifluoroethanol shows, apart from the expected acetyl z -> tt transition at about 230 nm, an additional shortwave peak. This is again explained by assuming a helical conformation of the amylose chain. Dextran acetates and mycodextran acetates, on the other hand, do not show conformational effects. The considerable changes in the CD spectra of amylose triacetate films when the temperature increases from 30 to 190°C are particularly remarkable. As X-ray diffraction analysis shows, these effects are connected with crystallisation processes and transformation during annealing. 

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