Double elimination, alkyne preparation

The double elimination of HHal from 1,1- and 1,2-dihalogeno-alkanes to give alkynes (terminal and internal) under very mild conditions is preparatively very simple in petroleum ether, using solid KOBu and catalytic amounts of 18-crown-6 polyether.Different transition-state structures within the E2 mechanism, as well as different initial-state solvation conditions, have been proposed to rationalize the effects of equimolar amounts of crown ether and base on the dehydrochlorination of (p-ClC6H4)2CH2CH(3 x)Ch (x = 1, 2, or 3). ... 

The two basic methods used to prepare alkynes are double elimination from 1,2-dihaloalkanes and alkylation of alkynyl anions. This section deals with the first method, which provides a synthetic route to alkynes from alkenes Section 13-5 addresses the second, which converts terminal alkynes into more complex, internal ones. 

As discussed in Section 11-6, alkenes can be prepared by E2 reactions of haloalkanes. Application of this principle to alkyne synthesis suggests that treatment of vicinal dihaloalkanes with two equivalents of strong base should result in double elimination to furnish a triple bond. 

Vinyl sulfoxides have been used as synthetic equivalents of alkynes in reactions with diazoalkanes to prepare pyrazoles. The initially obtained adducts subsequently eliminate or rearrange the sulfoxide moiety to achieve pyrazoles lacking the sulfur function. Thus, the adducts resulting by reaction of CH2N2 with the sulfinylated double bond of allenyl sulfoxides 213 are transformed through a sulfoxide-sulfenate rearrangement into hydroxymethyl pyrazoles 214 [168], whereas those obtained by reaction with sulfinyl coumarins 215 suffered sulfinyl elimination into the pyrazoles 216 [169]. In both cases l,H-pyrazoles were obtained as a consequence of a final tautomerization step (Scheme 101). These studies were carried out on racemic sulfoxides. 

Three zirconium/cycloheptadienyne complexes (231a-c) have been prepared by /3-hydrogen elimination from a mixture of cycloheptatrienyl complexes 269-271 (Scheme 33) and have been used as intermediates for the preparation of a zirconaazulene.87 The alkyne complexes are formed to the exclusion of the allene isomer 268. This is believed to be due to the proximity of the /3-vinyl hydrogen that is a result of both the shorter double bond and its forced coplanarity with the metal. Allene formation from 269 might be induced by blocking the vinyl position (see Sections IV,B and IV,C), but this has not been tested. 

Alkenes and alkynes are prepared by elimination reactions in which a carbon-carbon single bond is converted to a double or triple bond. In elimination reactions, atoms or groups are eliminated from adjacent carbons. Elimination once produces double bonds twice produces triple bonds. 

There are several types of reactions in which a halogen, a sulfonate ester, or another functional group is lost from the molecule, along with a hydrogen or sometimes another functional group to generate a carbon-carbon double bond. Both alkenes and alkynes can be formed. This section will examine methods for the formation of such molecules via elimination reactions.A recent monograph describes several different preparations of alkenes. 

Alkynes can be prepared by elimination reactions under conditions similar to those used to form alkenes. Because an alkyne has two n bonds, two molar equivalents of HX must be eliminated from the starting material. One such suitable reactant is a vicinal dihahde, a compound with halogen atoms on adjacent carbon atoms. We recall that such compounds result from the addition of a halogen to the double bond of an alkene (Section 6.5). A geminal dihahde, which has both halogens on the same carbon atom, can also be used to synthesize alkynes. Geminal dichlorides can be made by reaction of phosphorus pentachloride with an aldehyde or ketone. 

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