Synthesis of 1,4-Dienes

The results of this process for the synthesis of dienes are summarized in Table 24.10 [1]. 

In an another method, dialkyl chloroboranes undergo methylcopper-induced coupling to produce symmetrical ( , )-1,3-dienes in excellent yields and high stereochemical purity [2, 3]. In this reaction it is essential to use 3 equiv of methylcopper in order to achieve the effective conversion of organoboranes into dienes. 

A more milder procedure is where sodium methoxyalkenyl dialkylborates obtained by the simple treatment of alkenyldialkylboranes with sodium methoxide react readily with cuprous bromide-methyl sulfide at 0 °C to afford symmetrical conjugated , -dienes (Chart 24.6 Table 24.11) [4]. However, if the temperature is lowered to -15 °C, the dark blue-black complex formed is stable and can be trapped by allylbromide, which afford a stereochemically defined synthesis of 

Diene Workup (time) R R Yield (%) Isomer ratio (21 22) 

A rather limited possibility is the dehydration of certain 1,2- or 1,3-diols to form conjugated dienes. Pinacol (2,3-dimethyl-2,3-butanediol) can be transformed to 

3- dimethyl-1,3-butadiene over alumina [79], and the treatment of a variety of 

3- diols on zeolites also leads to dienes [80,81]. 2,4-Pentadiene is produced in the highest yield (81 %) by reacting 2,4-pentanedioI over NaX at 548 K. The synthesis of 1,3-butadiene can be accomplished by the dehydration of 2,3-butanediol or 1,3-butanediol in the presence of a variety of clay catalysts [38]. 

The behavior of ditertiary diols can be exceptional. Partial dehydration of ter-pene 1,4- and 1,5-diols over A zeolites gives unsaturated alcohols selectively 

Similarly, readily available (lZ)-l-hexenyldicyclohexylborane 1061 affords (4Z)-1,4-nonadiene (Eq. 94)148), 

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HWE reaction has been used extensively for the synthesis of dienes and polyenes. Examples from recent literature are shown in Table 16 (dienes) and Table 17 (polyenes). HWE reaction also has been used for intramolecular cyclizations leading to polyene macrolides (Table 18). 

TABLE 25. Synthesis of dienes through a Stille coupling reaction... 

TABLE 9. Electroreductive synthesis of dienes from benzenoid compounds... 

II. SYNTHESIS OF DIENES AND POLYENES LABELLED WITH STABLE... 

The present volume deals with the properties of dienes, described in chapters on theory, structural chemistry, conformations, thermochemistry and acidity and in chapters dealing with UV and Raman spectra, with electronic effects and the chemistry of radical cations and cations derived from them. The synthesis of dienes and polyenes, and various reactions that they undergo with radicals, with oxidants, under electrochemical conditions, and their use in synthetic photochemistry are among the topics discussed. Systems such as radialenes, or the reactions of dienes under pressure, comprise special topics of these functional groups. 

To explore the validity of the ring formation strategy, we drew up plans and carried out an enantioselective synthesis of diene 77 (Scheme 19). Initially, based on previous studies on similar medium- and large-ring syntheses, we surmised that elevated temperatures would be necessary for effective macrocycliza-... 

See also page 463, Section 7 for the synthesis of dienes and polyenes. 

In a similar manner, alkynes can undergo sequential carbometallation reactions, previously illustrated in equation (32).136 It is possible to combine carbocupration of alkynes with the use of a Michael acceptor as electrophile when that electrophile is a vinylphosphonium salt, carbocupration can be coupled to Wittig alkenation to result in a stereospecific synthesis of dienes via a one-pot, four-component, four carbon-carbon bond-forming reaction sequence.302... 

Cross-metathesis of two different alkenes 11 and 42 usually produces a mixture of products 6 and 15. However, depending on the functional groups R1 and R2, the cross-product 6 is obtained with high selectivity rather than the homoproduct 15 from 11 and 42. Some terminal alkenes, such as allylstannane [16], acrylonitrile [17,18] and allylsilane [19], undergo clean cross-metathesis to give cross-products 6 as the main product, rather than homoproducts 15. Cross-metathesis of the cyclic alkenes 43 with terminal alkenes 42 can be used for the synthesis of dienes 44. 

Other alkenyl iodonium salts which furnished functionalized dienes had a cyclo-hexenyl, a 2-(tosyloxy)hexenyl or a tosyloxyvinyl moiety the unsaturated partners included propenal, methyl acrylate and styrene. The yields were consistently high (64-85%) and the stereoselectivity invariably trans. An alternative method for the synthesis of dienes involved alkenyl stannanes such as CH2=CHSnBu3, E-PhCH=CHCH2SnBu3 and CH2=CHCH2SnBu3 [38]. Several alkynylstannanes afforded similarly enynes in good yield [39]. 

Organozinc reagents have been coupled with halides in the synthesis of dienes and other conjugated systems to form new carbon-carbon bonds such reactions have been reviewed477-479. In particular, the reaction of alkynyl zinc reagents with vinyl halides has been used in natural product synthesis, usually catalyzed by palladium complex catalysts. 

The same palladium-catalyzed domino cyclization-anion capture sequence involving carbopalladation of allene (69), which was employed by Grigg et al. for the synthesis of diene 70 (Scheme 10) can yield 1,3,5-hexatriene 194, when starting from 2-bromo-l-ene-6-yne 193 instead of the aryl iodide 68 (Scheme 32) [52], Under the conditions of its formation, 194 immediately undergoes thermal 67r-electrocyclization to give the bicyclic product 195. 

Devise a synthesis of diene A from (Z)-2-bromostyrene as the only organic starting material. Use a Suzuki reaction in one step of the synthesis. 

Highly reactive metallic titanium, prepared from TiCb and potassium, reduces enol phosphates to alkenes, permitting regioselective synthesis of dienes from a,p-unsaturated ketones. ... 

In Hanessian s approach to avermectin discussed in Section 3.1.11.4.1, the Julia coupling was used for the trisubstituted alkene and the diene portion of the molecule. The sulfone (439) was deprotonated with Bu"Li and the aldehyde (440) added to it to obtain a 47% yield (77% based on recovered sulfone) of -hydroxy sulfones (equation 103). The alcohol was converted to the chloride and the reductive cleavage carried out with sodium amalgam in 35% yield. The desired diene was the only detectable isomer (441). From the examples cited, it is apparent that the synthesis of ( , )-dienes by the Julia coupling is an extremely successful process, in terms of both yield and selectivity. 

For a recent comparative study of a nonstabilized ylide reacted with an unsaturated aldehyde and a semistabilized ylide with a saturated aldehyde in the synthesis of dienes, see R. Ideses and A. Shani, Tetrahedron, 1989, 45, 3523. 

The McMurry reaction involving low-valent titanium species accomplishes coupling of two carbonyl groups to furnish aUcenes. The low-valent titanium species is generated either from TiCls/LAH , TiCls/Mg or TiCU/Zn-Cu. When one or both carbonyl substrates carry one or more additional double bonds, dienes or polyenes result from this reaction (equation 109) . The McMurry coupling reaction is remarkably selective and a wide variety of functionalities are tolerated. This reaction can be carried out in both inter- and intramolecular modes to furnish a variety of dienes and polyenes. Synthesis of dienes and polyenes where McMurry coupling has been a key reaction is given in Table 21 . 

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