Methyl xanthates, pyrolysis

Bornene has been prepared from the reaction of 2-bromo-bornane-3-carboxyIic acid with aqueous sodium bicarbonate,6 by pyrolysis of isoborneol methyl xanthate,7 and by the j8-elimination of hydrogen chloride from bornyl chloride with sodium alkoxides in various solvents.2... 

Alcohols such as PhjCHCHjOH rearrange on treatment with acid they can be dehydrated by heating their methyl xanthates (Tschugaev reaictkMi). The pyrolysis proceeds by a cyclic transition state. Outline the steps using PhjCHCHjOH. ... 

The potassium xanthate salts of some tertiary alcohols on pyrolysis yield alkenes. The overall yield in this case is often markedly better than those obtained from the corresponding 5-methyl xanthates (the classical Chugaev procedure). 

Thermal decomposition of methyl xanthates is similar to the pyrolysis of acetates for the formation of the double bond. Olefins are obtained from primary, secondary, and tertiary alcohols without extensive isomerization or structural rearrangement. The other products of the pyrolysis of the methyl xanthates are methyl mercaptan and carbon oxy-sulfide. The xanthates prepared from primary alcohols are more difficult to decompose than those prepared from secondary and tertiary alcohols. Over-all yields of 22-51% have been obtained for a number of tertiary alkyl derivatives of ethylene. Originally the xanthates were made by successive treatment of the alcohol with sodium or potassium, carbon disulfide, and methyl iodide. In a modification of this procedure sodium... 

The same products, but in different ratios, occur in the gas phase eliminations of HCl from bornyl and isobornyl chlorides and from the pyrolysis reaction of bornyl and isobornyl benzoates and methyl xanthates . The isobornyl reaction is appreciably faster than that of the bornyl ester ((iso-B/B) = 6.8 at 345 °C), but proceeds at a slightly slower rate than that of cyclohexyl acetate, (CH/iso-B) = 2.1 at 600 °K. By analogy with the nonclassical carbonium ion interpretation of the solvolysis rates of bornyl and isobornyl chlorides, the participation of nonclassical carbonium ion intimate ion-pairs, e.g. 

The Corey-Winter reaction is a valuable route for the synthesis of unsaturated sugars and complex cycloalkenes. Alkenes may also be obtained by pyrolysis of methyl xanthates... 

Chugaev reaction The thermal pyrolysis of a methyl xanthate to an alkene. 

Although other xanthates have been used, methyl xanthates (R3 = Me) are by far the most commonly employed in the Chugaev elimination. For the preparation of the xanthate, a variety of bases have been used including sodium hydride, sodium hydroxide, sodium amide, and sodium/potassium metal. In the case of pure chiral alcohol stereoisomers, epimerisation of the alcohol stereocentre, under the basic conditions, can occur (with the corresponding xanthates leading to different products). Purification of the xanthate, prior to pyrolysis, is often a problem and it is usual to pyrolyze the crude xanthate directly. Pyrolysis of the xanthate is often carried out by distillation. Depending on the pyrolysis temperature, pressure, and the boiling point of the olefin, the product will either distill with the other products (COS, thiol) or remain in the reaction flask. 

Cram demonstrated that the Chugaev reaction is a syn elimination by showing that the pyrolysis of the S-methyl xanthate of diastereomeric 3-phenyl-2-butanols is stereospecific, as shown in equations 10.72 and 10.73.As noted, the eliminations also produced 3-phenyl-l-butene (in 30-40% yield). 

For the related Chugaev reaction, which involves the pyrolysis of xanthate esters (0-alkyl-5 -methyl xanthates), conditions are much milder (100—200 °C) (Scheme 6.20). 

Scheme 8.69. The Chugaev reaction. The salt of an alcohol is allowed to react with carbon disulfide, producing the sodium salt of a dithiocarbonate derivative, which, on Sn2 reaction with methyl iodide (CH3I), produces the corresponding xanthate ester. Heating the xanthate (pyrolysis) results in loss of carbon oxysulfide (COS), methane thiol (thiomethane), and the corresponding alkene.

Alcohols can be dehydrated via xanthate esters at temperatures that are much lower than those required for acetate pyrolysis. The preparation of xanthate esters involves reaction of the alkoxide with carbon disulfide. The resulting salt is alkylated with methyl iodide. 

FIRST-ORDER RATE COEFFICIENTS FOR THE PYROLYSIS OF METHYL C/S AND TranS 2-P-TOLUENESULFONYLCYCLOHEXYL SULFITES AND XANTHATES... 

The loss of C(0)S and MeSH occur at lower temperatures than the carboxylic ester pyrolysis and thus is more suitable for the preparation of sensitive alkenes. For example, heating the xanthate derived by sequential treatment of c -2-benzylcyclopen-tanol with sodium in toluene followed by addition of excess carbon disulfide and methyl iodide furnishes, via a 5yn-elimination, 3-benzylcyclopentene in 90% yield... 

Chugaev discovered the formation of olefins from the pyrolysis of xanthates in 1899,5 in connection with his studies on the optical properties of xanthates6 and other compounds.7 He subsequently employed the reaction in his structural investigations of terpenes and steroids, demonstrating its utility as an olefin-forming reaction and as a tool for structural determination. For example, he converted cholesterol into methyl cholesteryl xanthate (4)... 

Pyrolysis Methyl 4,6-0-benzylidene-2-deoxy- a-D -glucopyranoside 3-xanthate Methyl 4,6-0-benzylidene-2,3-didehydro-2,3-ideoxy-a-D-crt/ Aro-heXoside XLV [103]... 

Poly(p-tnercaptostyrene) was prepared by the polymerization of p-vinylphenyl thioacetate 24,25) (III) which was synthesized from p-aminoacetophenone (IV) (Scheme I). Conversitsi of FV via a diazonium salt to xanthate (V), followed by reduction with NaBH4 introduced a thiophenol group (VI). Pyrolysis of the corresponding diacetate produced III in 10% overall yield This styrene derivative polymerized readily and saponification of the resultant homopolymer yielded an alkaU soluble polymer. Copolymerization with methyl methacrylate gave, upon saponification, another synthetic mercaptan-containing copolymer. 

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