Synthesis of Conjugated Dienes

Recently, dialkenylchloroboranes were reported to undergo methylcopper-induced coupling to produce symmetrical (E, )-1,3-dienes in excellent yields and high stereochemical purity (Eq. 85) 37,143). In this reaction, it is essential to utilize 3 molar 

An alternative procedure where alkenyldibromoboranes, without isolation, are directly converted to dienes is reported (Eq. 86) 36a). 

A more milder procedure is, where sodium methoxyalkenyldialkylborates obtained by the simple treatment of alkenyldialkylboranes with sodium methoxide, react readily with cuprous bromide-methylsulfide at 0 °C to afford symmetrical conjugated dienes (Eq. 87) 144). Although the intermediacy of a copper(I) borate complex formed 

Conjugated (E, )-dienes can also be synthesized from ( -1 -alkenyl-1, 3,2-bcnzo-dioxaboroles by palladium catalyzed reactions (Eq. 88) 142145 and (Eq. 89)146) in high regio- and stereospecificity. 

Unsymmetrical (E, )-dienes are also available by the following procedure using 1-halo-l-alkynes (Eq. 90)31). 

StereodeSned conjugated dienes are of great importance in organic synthesis. A novel protocol involving an ionic liquid catalyzed rearrangement of 

---

The coupling of alkenylboranes with alkenyl halides is particularly useful for the stereoselective synthesis of conjugated dienes of the four possible double bond isomers[499]. The E and Z forms of vinylboron compounds can be prepared by hydroboration of alkynes and haloalkynes, and their reaction with ( ) or (Z)-vinyl iodides or bromides proceeds without isomerization, and the conjugated dienes of four possible isomeric forms can be prepared in high purity. 

In addition to the synthetic applications related to the stereoselective or stereospecific syntheses of various systems, especially natural products, described in the previous subsection, a number of general synthetic uses of the reversible [2,3]-sigmatropic rearrangement of allylic sulfoxides are presented below. Several investigators110-113 have employed the allylic sulfenate-to-sulfoxide equilibrium in combination with the syn elimination of the latter as a method for the synthesis of conjugated dienes. For example, Reich and coworkers110,111 have reported a detailed study on the conversion of allylic alcohols to 1,3-dienes by sequential sulfenate sulfoxide rearrangement and syn elimination of the sulfoxide. This method of mild and efficient 1,4-dehydration of allylic alcohols has also been shown to proceed with overall cis stereochemistry in cyclic systems, as illustrated by equation 25. The reaction of trans-46 proceeds almost instantaneously at room temperature, while that of the cis-alcohol is much slower. This method has been subsequently applied for the synthesis of several natural products, such as the stereoselective transformation of the allylic alcohol 48 into the sex pheromone of the Red Bollworm Moth (49)112 and the conversion of isocodeine (50) into 6-demethoxythebaine (51)113. 

Nickel-bpy and nickel-pyridine catalytic systems have been applied to numerous electroreductive reactions,202 such as synthesis of ketones by heterocoupling of acyl and benzyl halides,210,213 addition of aryl bromides to activated alkenes,212,214 synthesis of conjugated dienes, unsaturated esters, ketones, and nitriles by homo- and cross-coupling involving alkenyl halides,215 reductive polymerization of aromatic and heteroaromatic dibromides,216-221 or cleavage of the C-0 bond in allyl ethers.222... 

---