Head-to-tail coupling product

The rhodium-trimethylphosphine system is remarkable because it catalyzes the dimerization of aryl substituted acetylenes, yielding the scarce branched head-to-tail coupling product [12], The iridium species generated from [Ir(COD)Cl]2 and phosphine selectively yields linear ( ) or (Z) enynes from silylalkynes, depending on whether triaryl or tripropylphosphines, respectively, are used [16]. 

The dimerisation of phenylacetylene to yield exclusively the head-to-tail coupling product has been achieved using the diruthenium complex 12 as catalyst (Scheme 7a) [31]. This was the first complex to display such selectivity and remains the only ruthenium complex to do so. Complex 12 contains two bridging... 

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). °... 

Reductive activation of the quinone shown in Scheme 7.9 and incubation in methanol afforded a complex mixture of products consisting mainly of head-to-tail coupling at C-5 or C-7 (Scheme 7.10). Minor reactions involve transfer of H2 from the hydroquinone to the ene-imine (internal redox reaction) and methanol trapping. The structures of the dimers and trimers in Scheme 7.10 were derived from H-NMR,... 

Steric factors are also important in hydrodimerizations carried out in acidic media. Excessive steric hindrance about the 0-carbon in an a, 0-unsaturated carbonyl compound can retard tail-to-tail coupling, e.g., 2 130 - 131, and lead to products of head-to-head (and occasionally head-to-tail) coupling. Thus in the reduction of mesityl oxide at pH lg.4 there is also formed a small amount of ketone 140, apparently formed via head-to-head coupling of 130 and subsequent pinacol rearrangement of 139 134) ... 

Carbene 132 is implicated in the photolysis of 1 since the observed289 photodimerization to 9,10-dihydrophenanthrene and -anthracene is best explained by head-to-head and head-to-tail coupling of this species. Moreover, the fact that allene 134 is isolated289,290 as the major product from irradiation of diesters 31 (equation 35) is fully consistent with a photo-Wolff rearrangement of the carbene. The minor product here involves cyclization... 

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]. 

By using a more sterically hindered and electron-rich catalyst, (C5Me5)RuCl (COD), y,<5-unsaturated acetals and aldehydes have been synthesized from alkynes and allyl alcohol [41] (Eq. 29). It is noteworthy that in this case the branched isomer is the major product. This is due to the bulkiness of the C5Me5, with respect to the C5H5 ligand, which favors the head-to-tail coupling. 

Similar results were obtained by Cooper and by Mijs et ah (24) from a number of other substituted dimers. With lightly substituted dimers, however, Mijs observed that at low temperatures the initial products consist almost entirely of tetramers the tetramer, moreover, is that corresponding to the quinone ketal rearrangement, rather than to head-to-tail coupling (Reaction 20). 

Aniline and its derivatives may form three types of products (Scheme 3) If no para-substituent is present, tail-to-tail coupling results in the formation of benzidine derivatives (XIX) anilines and A-alkylanilines also can undergo head-to-tail coupling to give 4-ami-nodiphenylamines (XX) even in the presence of certain / flra-substituents, one of which is eliminated head-to-head coupling forming azo compounds (XXI) is possible with N-unsubstituted anilines. 

The general pattern of anodic behavior of para-substituted anilines (68) was established in aqueous acidic media by Bacon and Adams17,106. The postulated one-electron oxidation of the substrate to the radical cation 681 is followed by rapid head-to-tail coupling of 681 with the substrate 68 giving protonated 4 -substituted 4-aminodiphenylamine in the oxidized form (69) as the final main product. The product 69 shows reversible redox peaks at more cathodic potentials, supporting its identification beside the spectral and chemical analysis. The product formation is preceded by elimination of one para-substituent and, if it leaves as an anion (e.g. halide, methoxide or ethoxide ion), then the overall electrochemical process (equation 1) corresponds to a one-electron (two electrons per two reactant molecules) process. However, if it leaves as a neutral group (e.g. CO2 in the oxidation of p-aminobenzoic acid), a dimer is formed in the reduced form and the overall reaction is a two-electron process. 

On the other hand, in acidic solutions of pH < 4 the head-to-tail coupling of radical cations 90c+ is favored and the main product is the cyclic dimer 3-aminophenoxazone 100. It can be reduced (equation 13) giving additional voltammetric peaks and its identification was supported by the IR spectrum in a KBr pellet. 

Head-to-tail coupling of radical cations 90b 1 was also confirmed and the structure of the product formed at Ag and Au electrodes was investigated by SERS and surface-enhanced resonance Raman (SERRS) spectra141. The pair of cathodic and anodic peaks observed at 0.34 V vs. SCE on CV curves after a few cycles during the oxidation of... 

Employing a polyelectrolyte to bind to and preferentially align the aniline monomers before polymerization (e.g., by S2082-) has shown promise in facilitating the desired head-to-tail coupling of the aniline substrates. During polymerization, the anionic polyelectrolytes such as poly(styrenesulfonate) and poly(acrylate)86-88 also provide the required counterions for charge compensation in the doped PAn products. This can lead to water-soluble or water-dispersed ES products. 

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