Preparation of conjugated dienes

Problem 14.1 Which of the following molecule,s contains conjugation Circle the conjugated part in each. 

Conjugated dienes are generally prepared by the methods previously discussed for alkene synthesis. The base-induced elimination of HX from an allylic halide is one such reaction. 

3-Butadiene, a substance used industrially to make polymers, is prepared by thermal cracking of butane over a chromium oxide/aluminum oxide catalyst, but this procedure is of little use in the laboratory. 

Other simple conjugated dienes used in poljnmer s5mthesis indue chloroprene (2-chloro-1,3-butadiene) and isoprene (2-methyl-1,3-butadiem Isoprene has been prepared industrially by several methods, including tl-acid-catalyzed double dehydration of 3-methyl-l,3-butanediol. 

Conjugated dienes are similar to other alkenes in much of their chemisti but there are also important differences. One such difference is stabiliii Conjugated dienes are somewhat more stable than nonconjugated diene 

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Scheme 14.34. Stereoselective preparation of conjugated dienes by the titanocene(ll)-promoted reaction of thioacetals with alkynes.

The preparation of conjugated diene complexes will be presented by groups. In addition, isomerization reactions, or degenerate rearrangements with activation energies >25 kcalmol-1, will be considered in this section. 

The tellurolate-promoted anfi -debromination by method G is extended to the preparation of conjugated dienes, starting from 1,2,3,4-tetrabromoalkanes and cycloalkanes, 1, 4-dibromo-2-aIkenes and aUylic dibromides. ... 

The preparation of conjugated dienes from pyridines is exemplified by the transformation of 2-picoline into the sex pheromone (669) of Lobesia botrana, a major pest of vineyards (Scheme 154) (80TL67). Thus, the lithio salt of 2-picoline was alkylated by 2-(5-chloropentyl-oxy)tetrahydropyran, the resulting pyridine (665) N-methylated, and the pyridinium salt reduced by sodium borohydride. Quaternization of the 1,2,3,6-tetrahydropyridine (666) and Hofmann elimination gave the (7 , 9Z)-undecadien-l-ol (667) as the sole isomer. Protection of the alcohol and treatment of the corresponding ammonium salt (668) of the amine with lithium dimethylcuprate gave pure (669) after hydrolysis, acetylation and HPLC purification. 

A GENERAL SYNTHETIC METHOD FOR THE PREPARATION OF CONJUGATED DIENES FROM OLEFINS USING BROMOMETHANESULFONYL BROMIDE 1,2-DIMETHYLENECYCLOHEXANE (Cyclohexane, 1,2-bis(methylene)-)... 

Sulfoxonium ylides also undergo 3,2-sigmatropic rearrangements. A useful preparation of conjugated dienes is based on a 3,2-rearrangement followed by spontaneous elimination of methanesulfenic acid (Scheme 59, equation a). In some cases, the reaction proceeds without isolation of the intermediate ylide (Scheme 59, equation b). 

The olefin synthesis has been extended to the preparation of conjugated dienes from the tosylhydrazoncs of /3-unsatnrated ketones.61 The reaction is particularly useful for preparation of 1,3-cyclohexadienes as shown by the preparation of (8). /3-lononc tosylhydrazone was converted into the triene (9). The method cannot be used to prepare allenes. Further experiments indicated that the eliminated a-hydro-gen must be part of a methylene or methyl group. Methyllithium was found to be superior to -butyllithium. 

There has been a further report (see Vol. 28, p. 8 for earlier) on the preparation of conjugated dienes of trans-configuration by treatment of sugar-derived allyl stannanes with ZnCh. Thus, for example, stannane 195 gave diene-aldehyde 196, and 197 gave 198. ... 

The stereo- and regioselective synthesis of conjugated alkadienes are of great importance in organic chemistry. A number of methods for the preparation of conjugated dienes and polyenes were developed uitilizing... 

The metal-based preparations of conjugated dienes represent the majority of the strategies employed to attain stereoselectively 1,3-dienes connected to aryl and/or alkyl appendages. The starting material can be stereodefined when double bonds or enynes are engaged. When alkynes or allenols are employed, two double bonds are generated in a stereoselective manner. 

The preparation of conjugated dienes firom stereodefined 1,3-enynes can be carried out differently when they are bromo- or alkyl-substituted. The control of the... 

The intermolecular coupling of aUcyne and aUcene is a straightforward solution for the preparation of conjugated dienes without prefunctionalization of reagents. Cheng recently described the use of Co(II) salts and ethylenebis(diphenylphosphine) (dppe)... 

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