Homo-coupled dienes

Direct homo-coupling of vinyl halides is a simple way of generating 1,3-dienes. This transformation can be achieved employing various transition metal catalysts such as nickel(O) reagent in the presence of phosphine ligand202 or a nickel(O) reagent in the presence of potassium iodide and thiourea (equation 116)203. 

The homo-coupling reaction of 1,1-dimethylallene proceeds at -50 °C to give a symmetrically substituted diene in good yield (Scheme 16.99) [105], In sharp contrast, the reaction of a monosubstituted allene proceeds via a titanabicycle to give an unsymmetrical diene. 

Vinyl stannanes also undergo oxidative homo-coupling under transition metal catalysis to result in dienes (equation An intramolecular version of this method... 

In order to account for the higher reaction rates, the well-known Frontier Molecular Orbital Theory was used. For the thermal reaction, the smallest HOMO-LUMO energy difference for the couple diene-dienophile is calculated to be high (9.1 eV). In this case the magnitude of the frontier orbital coefficients agrees with the experimental regioselectivity (Scheme 3). 

Negishi and his co-workers have described a new synthetic route to 1,5-dienes (or 1,5-enynes) which is particularly suitable for the construction of head-to-tail coupled isoprene units. The key step involves a regio- and stereo-selective palladium-catalysed cross-coupling of homoallylic (or homopropargylic) organozinc compounds with alkenyl halides (e.g. Scheme 95) homo-coupling... 

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... 

Homo- and cross-coupling reactions of allyhc halides and reactions of conjugated dienes with dichloroalkanes using reactive barium are reviewed in Section 5.2. The next section covers methods of generating allylic barium reagents and reactions of these carbanions with a variety of electrophiles. The last section describes examples of other carbon-carbon bond forming reactions promoted by barium compounds. 

The Ni-catalyzed coupling is a superior method for the preparation of unsymmetrical 1,5-dienes, an allylic nucleophile being generated from fragmentation of a tertiary homo-allylic alcohol. ... 

Multi-component reactions open a direct access to poly-functional derivatives/ For instance, reactions of unsaturated substrates and aldehydes in the presence of EtaSiH opened a rapid access to allyllic, homo-allylic or co-unsatu-rated alcohols, and NHC-Ni complexes were recently reported to be efficient catalysts for such oxidative coupling. Mori and co-workers described the first example of such condensation in 2001. They showed that an in situ generated IPr/Ni 1 1 complex led to the stereoselective formation of (Z)-homoal-lylic silyl alcohols [eqn (10.33)], while classical Ni -PPhj catalyst produced only (Ej-coupling products. Of note, IPr/Ni 1 2 and IMes/Ni 1 2 were less efficient in this reaction. These first results were extended to silylated dienes. Interestingly, it was shown that a mixed complex [(IPr)Ni(PPh3)] stereoselectively produced (Z)-allylsilanes while [Ni(PPh3)4] led only to (Ej-isomeric products. 

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