Linear dimers

Pd-cataly2ed reactions of butadiene are different from those catalyzed by other transition metal complexes. Unlike Ni(0) catalysts, neither the well known cyclodimerization nor cyclotrimerization to form COD or CDT[1,2] takes place with Pd(0) catalysts. Pd(0) complexes catalyze two important reactions of conjugated dienes[3,4]. The first type is linear dimerization. The most characteristic and useful reaction of butadiene catalyzed by Pd(0) is dimerization with incorporation of nucleophiles. The bis-rr-allylpalladium complex 3 is believed to be an intermediate of 1,3,7-octatriene (7j and telomers 5 and 6[5,6]. The complex 3 is the resonance form of 2,5-divinylpalladacyclopentane (1) and pallada-3,7-cyclononadiene (2) formed by the oxidative cyclization of butadiene. The second reaction characteristic of Pd is the co-cyclization of butadiene with C = 0 bonds of aldehydes[7-9] and CO jlO] and C = N bonds of Schiff bases[ll] and isocyanate[12] to form the six-membered heterocyclic compounds 9 with two vinyl groups. The cyclization is explained by the insertion of these unsaturated bonds into the complex 1 to generate 8 and its reductive elimination to give 9. 

The linear dimerization of substituted conjugated dienes is difficult, but the Pd-catalyzed intramolecular dimerization reaction of the 1,3,9,11-tetraene 13 gives the 3-propenylidene-4-allylpiperidine derivative 14, which has the 1,3,7-octatriene system. The corresponding 1,3,8,10-tetraene also affords the 3-pro-penylindene-4-allylcyclopentane derivative[18]. 

ThioglycoHc acid can be identified by its in spectmm or by gas chromatography. Most of the by-products and self-esterification products are also detected by liquid chromatography, eg, thiodiglycolic acid, dithiodiglycolic acid, linear dimers, and polymers. Iron content can be assayed by the red sensitive complex of 1,10-phenanthroline [66-71-7] and ferrous ion of a mineralised sample. Ferric ion turns an aqueous ammonia solution deep red-violet. 

Irradiation of an aqueous solution of thymine (111 R = Me) in the presence of air produces (irreversibly) at least four products (111 R = CH20H, CHO, CO2H or H), possibly via dithymine peroxide , a linear dimeric molecule (112), The significance of these reactions has been discussed (65MI21300). 

The selective, Ni-catalyzed, biphasic dimerization of 1-butene to linear octenes has been studied in the author s group. A catalytic system well loiown for its ability to form linear dimers from 1-butene in conventional organic solvents - namely the square-planar Ni-complex (q-4-cycloocten-l-yl](l,l,l,5,5,5,-hexafluoro-2,4-pen-tanedionato-0,0 )nickel [(H-COD)Ni(hfacac)] [103] - was therefore used in chloroaluminate ionic liquids. 

The use of pyrrole and N-methylpyrrole was found to be preferable. Through the addition of N-methylpyrrole, all cationic side reactions could be effectively suppressed, and only dimerization products produced by Ni-catalysis were obtained. In this case the dimer selectivity was as high as 98 %. Scheme 5.2-21 shows the catalytic system that allowed the first successful application of [(H-COD)Ni(hfacac)] in the biphasic linear dimerization of 1-butene. 

Scheme 5.2-21 Ni-calalyzed, biphasic, linear dimerization in a slightly acidic,...

Table 2. Singlet-triplet energy separations (AEsx) in M-H monomers, M—M bond dissociation energies (BDEs), SCF calculated bond lengths for the hypothetical, linear dimers HMMH (M = B, Al, Ga, In, or Tl) ...

Fig. 14. Optimized structures of covalently bonded linear dimer (BD), non-bonded dimer (LD), and parallel dimer (PD)...

In some cases, such as the isopropyl or benzyl compounds in Eq. (27), both the cyclic [2 + 2] dimer and the linear dimer were formed in appreciable amounts.86... 

Use of less sterically hindered examples of 5 in combination with MAO allows for active catalysts for the linear (head-to-head) dimerisation of a-olefins such as 1-butene, 1-hexene, 1-decene and Chevron Phillips C20-24 a-olefin mixture (Scheme 4) [47], The mechanism for dimerisation is thought to involve an initial 1,2-insertion into an iron-hydride bond followed by a 2,1-insertion of the second alkene and then chain transfer to give the dimers. Structurally related cobalt systems have also been shown to promote dimerisation albeit with lower activities [62], Oligomerisation of the a-olefms propene, 1-butene and 1-hexene has additionally been achieved with the CF3-containing iron and cobalt systems 5j and 6j yielding highly linear dimers [23],... 

There are two main types of reactions of conjugated dienes catalyzed by palladium complexes. The first type is the linear dimerization to form 1,3,7-octatriene (16) in the absence of a nucleophile ... 

Neglect of the formation of polymeric rings, however, is sometimes too crude an approximation. It may happen that the cyclisation constant C2 of the linear dimer is larger than the cyclisation constant Cl of the monomer. It may also happen that the concentration of the monomer is comparable to Ct but smaller than C2. When this is the case, the open chain dimer, once formed, will show a higher tendency to cyclise than to react with the monomer to give the linear trimer. Under the above conditions the stepwise polymerisation is truncated after the first step, and the system is described to a useful approximation by scheme (6). 

SeF4 also forms an adduct with S03 which has been investigated in the solid state, the molten state, and in solution (81). The results of infrared, Raman, and NMR studies and some conductometric and cryo-scopic measurements can best be interpreted consistently in terms of a polymeric fluorosulfate bridged structure for the solid and the molten compound. There is also evidence for either a cyclic or linear dimer in dilute solutions in nitrobenzene or HS03F (Fig. 4). 

Two higher molecular weight ion series (m/z 562—1134 and m/z 934— 1154) were also observed in the ESI—MS and FTICR—MS spectra, corresponding with highest agreement to linear dimer structures... 

Dimerization of conjugated dienes and trienes is generally accomplished at elevated temperatures or in the presence of metal catalysts. Linear dimerization of butadiene occurs readily at room temperature on nickel catalysts bearing aminophosphinite (AMP) ligands, and the reaction rate is reportedly twice that observed in other nickel systems employing either morpholine, ethanol or P-methyloxaphospholidines as modifiers62. 1,3-Pentadiene dimerizes in the presence of 1 mol% nickel catalyst to give a diastereomeric mixture of 4,5-dimethyl-l,3,6-octatriene as shown in equation 42. 

While linear dimerization of dienoic esters can also be accomplished with nickel-AMP systems, other functionalized dienes undergo little or no conversion. The reaction of methyl hexa-2,4-dienoate, 81, furnishes diastereomeric trienoic diesters (82) in high yields (equation 43). 

An alternative method is to examine the relative rates of formation of circles and linear dimers by the ligase as a function of DNA length. Early experiments043) using this method yielded a = 7.1 x 10 12 dyn-cm.recent study using what should be a superior protocol yields a static torsion constant in the range a = (5.9 0.6) x 10 12 dyn-cm.<221)... 

A plausible alternative mechanism involves as a first step the formation of a linear dimer, XB=NR—BX=NR, according to the left part of Eq. (21). This linear dimer will be more stable in entropy but less stable in energy than the corresponding cyclodimer. The second step would be addition of iminoborane to the open-chain dimer giving the borazine in either a concerted or a two-step mechanism. The intramolecular cyclization of the open-chain dimer, according to the right-hand side of Eq. (21), would compete addition of an iminoborane. We cannot definitely exclude a mechanism via open-chain dimers. 

By contrast, the linear dimer gives an unsymmetrically substituted urea (CF3S)2NC(0)NC0-t-H2O ------------ (CFsS)aNC(0)NH2+C02... 

Markovnikov orientation. However, the product is too labile for storage at room temperature and must be reacted further at low temperature (examples are substitutions with methylthiol that work also with 137 and 138 [33]). Upon warming to room temperature the product JV-(l-bromoethyl)pyrrolidinone releases HBr and forms the linear dimer ( )-l,l -(3-methyl-l-propene-l,3-diyl)-bis(2-pyrrolidinone) that is most easily obtained by this technique [58]. 

The first solid-state linear dimerization was observed with N-vinylpyrrolidi-none. It was first quantitatively converted to its Markovnikov HBr addition product (by application of HBr gas at -40 °C), which upon warming to room temperature lost HBr and formed ( )-l,r-(3-methyl-l-propene-l,3-diyl)bis-(2-pyrrolidinone), but the yield was less than 100% [58]. Interestingly, such head-to-tail dimerizations of alkenes lead to shrinking and that may create reactivity even if the crystal lattice does not allow for molecular migrations due... 

By choice of appropriate conditions it is sometimes possible to stop the reaction at the dimer stage for example, in the case of styrene this leads to an unsaturated linear dimer, or, by ring closure, to a cyclic dimer ... 

Methyl 2, -pentadienoate self-dimerization as a potential route to sebacic acid precursors was extensively evaluated. Homogeneous metal catalyzed dimerization of compounds containing conjugated double bonds is known to give cyclic, branched, and linear dimers(43-45). At higher temperatures or in the presence of many zero-valent metal catalysts, cyclodimerization (Equation 12.) is the only observable reaction. 

Dimerization(46,47) with a palladium acetate/tertiary phosphine complex (di-ace tate-bis--[ tri-phenyl phosphine] palladium) in the absence of air gives primarily the desired linear dimer with smaller amount of branched, cyclic, and heavy product (Equation 13.). The reaction product is a complex mixture of cis and trans geometric isomers which makes analysis difficult. In order to simplify this problem, the product analysis is made after hydrogenation over 5% palladium on carbon. 

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