Head to Tail Coupling

The anodic polymerization of aniline can occur by a radical cation coupling mechanism analogous to that shown in Scheme 1, with coupling occurring between the N of one molecule and the para-position of another (Structure 4)21,22 However, a variety of other mechanisms have also been proposed,21 and it is likely that their relative rates depend upon the conditions (solvent, potential, pH, etc.) employed. The links between monomers are therefore not exclusively between the N and para-position (head-to-tail coupling). Head-head (-N=N-) and tail-tail (para-para) coupling occur more often as the pH is increased.71... 

Alkylation reactions by the iminium methide species are well known in the mitomycin and mitosene literature 4,49,51-53 and are largely responsible for the cytotoxicity/antitumor activity of these compounds. As illustrated in Scheme 7.8, the electron-rich hydroquinone intermediate can also be attacked by the iminium ion resulting in either head-to-head or head-to-tail coupling. The head-to-head coupling illustrated in Scheme 7.8 is followed by a loss of formaldehyde to afford the coupled hydroquinone species that oxidizes to the head-to-head dimer upon aerobic workup. Analogous dimerization processes have been documented in the indole literature, 54-56 while the head-to-tail mechanism is unreported. In order to... 

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,... 

The head-to-tail-coupling reactions described above are potentially useful in the design of dynamic combinatorial libraries. Features of these reactions include the rapid and reversible formation of carbon-carbon bonds, multifunctional ene-imine building blocks, and formation of stereo centers upon ene-imine linkage. Support for template-directed synthesis utilizing ene-imine building blocks is the formation of a poly ene-imine species that could recognize 3 -GGA-5 sequences of DNA.48 It is noteworthy that some polyene-imines are helical and could form a triple helix with DNA. 

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) ... 

R.S. Loewe, S.M. Khersonsky, and R.D. McCullough, A simple method to prepare head-to-tail coupled, regioregular poly(3-alkylthiophenes) using Grignard metathesis, Adv. Mater., 11 250-253, 1999. 

R.S. Loewe, P.C. Ewbank, J. Liu, L. Zhai, and R.D. McCullough, Regioregular, head-to-tail coupled poly(3-alkylthiophenes) made easy but the GRIM method investigation of the reaction and the origin of regioselectivity, Macromolecules, 34 4324-4333, 2001. 

The exact formula was derived by Schulz [80] and the asymptotic equation is due to Zimm [81]. Schulz derived the distribution for/chains that are coupled together, where it has no influence whether one has a head to tail coupling of most probable distributions or whether these distributions are coupled onto a star center. Zimm recognized that the asymptotic form can be efficiently used to describe the distributions of fractions. The index o refers to the primary (most probable) chain distribution and p is the extent of reaction for this primary chain. 

Fig. (2). Rule of Ruzicka head to tail coupling of two isoprene units, giving myrcene...

To explain this unusual result, two basically different mechanisms were proposed (24). The first, the uncoupled electron mechanism assumed that oligomeric phenoxy radicals combined by a head-to-tail coupling reaction. This implied then that a resonance structure could be written... 

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]. 

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]. 

As was the case with the radical addition reactions discussed in Chapter 21, the addition always occurs so as to produce the more stable radical. Because most other substituents also stabilize a radical on the carbon to which they are attached, vinyl polymers typically result from head-to-tail coupling of their monomer units. 

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. 

A second rearrangement can place the third ring at the head of the sequence if rearrangement proceeds until one of the terminal rings becomes the cyclohexadienone ring, enolization yields a polymeric phenol identical in every way, except for the order of succession of the rings, with that which would be produced by the direct head-to-tail coupling of aryloxy radicals (Reaction 13). The formation of a polymeric phenol... 

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). 

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