Head-to-tail cross-coupling

Alkyl-l,3-dienes. Palladium black, prepared from Pd(OAc)2 and B2H6, is the most effective catalyst for head-to-tail cross-coupling of 1-alkenyl iodides with 1-alkenyl-1,3,2-benzodioxaboroles to give mainly 2-alkyl-1,3-dienes (equation I).1... 

In addition to the head-to-head cross-coupling reactions, it has been discovered that the reaction of phenyl or 1-alkenyl iodides with (E)-l-alkenyl-l,3,2-benzo-dioxaboroles produces the head-to-tail cross-coupling products, 2-phenyl-1-alkenes or 2-alkyl-1,3-alkadienes, respectively (Eq. 107) The reaction is profoundly affected by catalytic quantities of palladium compounds (Pd black prepared by reduction of Pd(OAc)j with diborane is especially effective) in the presence of tri-ethylamine. 

Organopalladium(II) halides add mainly to the electron-deficient carbon of an unsymmetrical alkene [46] to give 15, which readily isomerizes to 16 via a sequence of elimination and re-addition of the hydridopalladium(U) iodide. Finally, the elimination of iodoborane with the aid of triethylamine gives the head-to-tail-cross-coupling product. A deuterium labeling study proves the addition-elimination mechanism where a -hydrogen transfers to the terminal carbon (Scheme 2-17) [48]. 

Miyaura, N. Suzuki, A. 1981. The palladium-catalyzed head-to-tail cross-coupling reaction of 1-alkenylboranes with phenyl or 1-alkenyl iodides. A novel synthesis of 2-phenyl-1-aUcenes or 2-alkyl-1,3-alkadienes via oiganoboranes. J. Organomet. Chem. 213 C53-C56. 

Palladium catalysts enable the dimerization of functional alkynes with selective head-to-tail coupling in the presence of bulky phosphine ligands, for both homocoupling of terminal alkynes or cross-coupling of mono and disubstituted alkynes [4, 6]. On the other hand, a palladium/imidazolium system gives linear (E)-enynes as the predominant products [7]. 

Scheme 2.1) followed by selective cross-coupling gives head-to-tail coupled poly(3-alkylthiophene)s. Substituents may be limited by the organolithium intermediates required to access the cross-coupling competent monomer. However, designing for post polymerization functionalization provides a partial solution [46-48]. Telechelic synthesis [43, 49] or isolable monomers, allowing polymerization in diverse solvents, are notable advantages. 

The geometry of both reactants is preserved during the palladium-catalysed cross-coupling of 1-alkenylboranes with 1-alkenyl bromides under strongly basic conditions even when each of the substrates has a Z-configuration (Scheme 67). Under more weakly basic conditions, reactions of this type give mainly head-to-tail coupled products (Scheme 68). °... 

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... 

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