Dimer Dimerisation

The GABAb receptors were the first G-protein-coupled receptors to be observed to form functional heterodimers (Bowery and Enna 2000) where two G-protein molecules come together to act as a dimer to enhance their combined response. A similar effect has recently also been described for dopamine and somatostatin receptors (Rocheville et al. 2000) and it is likely that this may occur with other G-protein-coupled receptors. The significance of this in terms of the pharmacology of the receptors is unclear, or indeed whether dimerisation affects mechanisms such as desensitisation. 

Catalyst systems of the type [NiL X + AlEt Xj (where L = PR and X = halide) afford highly active catalysts for olefm dimerisation. However, when complex 11 (Scheme 13.8) is treated with AlEt Cl in the presence of 1-butene, in toluene at 20°C the only products observed were decomposition products, 12,13,14 no butene dimers were obtained [22], At low temperatures (-15°C) and using the complex with 1,3-diiso-propylimidazolin-2-ylidene as the NHC ligand, small amounts of butene dimers were observed. It is apparent from these results that Ni-NHC complexes are capable of olefin dimerisation, however, decomposition of the catalyst via reductive elimination predominates. 

The use of chiral C2-symmetric trifluoromethanesulfonamides derived from (i )-1,1 -binaphthyl-2,2 -diamine in similar reactions to those described above has led to the formation of the expected alcohols with enantioselectivities of 43-54% ees. Better enantioselectivities were observed by Paquette et al, resulting from the use of chiral C2-symmetric VERDI (verbenone dimers) disulfonamides derived from the dimerisation of (+ )-verbenone. Stereoselectivity levels ranging from 72 to 98% ee were observed, depending on the structural characteristics of the aldehyde (Scheme 3.45). ... 

Succinic anhydride is dimerised to 1,6-dioxaspiro [4.4] nonane-2,7-dione by heating with sodium hydroxide. Modification of an existing procedure by adding further sodium hydroxide after the initial reaction led to a severe exothermic reaction after heating for some 30 h which fused the glass flask to the heating mantle, probably at a temperature approaching 550°C. The reason for this was not known [1], At elevated temperatures and under influence of alkali, succinic acid condenses decarboxylatively beyond the dimeric spiroacetal, sometimes explosively. Contamination of the anhydride with base is to be avoided [2],... 

When NH4PF6 was employed as the supporting electrolyte, no CO was produced but near-quantitative formation of H2 was observed. During the electrolysis an air-stable, green, sparingly soluble material was produced, which was isolated and characterised as the dimer, Bipy)Re[CO]3 2. It was fairly reasonable to assume that this was formed via dimerisation of the radical (Bipy)Re[CO]3. The authors postulated that more dimerisation occurs in the presence of a non-coordinating anion such as PF , rather than coordinating anions such as Cl- or CIO , due to the labilisation (and loss) of Cl and thus the exposure of the sixth coordination site and subsequent dimerisation. 

Dmbpy)Re[CO]3 which then dimerises. The calculated vco for the 5-coordinate (Dmbpy)Re[CO]3 species are 1984 cm-1, 1862 cm-1 and 1849 cm-1, in agreement with three of the four bands observed at — 1.4 V. The peaks of the dimer have been reported to be at 1975 cm-1, 1940cm-1 and 1860cm-1, hence the authors found it not unreasonable to associate the 1943 cm-1 band in Figure 3.60 with 1940cm-1 feature of the dimer on the basis that the 1943 cm -1 and 1975 cm -1 bands may well lie under the features attributed to the monomer. 

Several examples of the synthesis of furoxans by dimerisation of nitryl oxides are shown below. The treatment of oximes 302 with iV-bromosuccinimide (NBS) and then with triethylamine leads to the formation of nitrile oxides 303, as shown by the presence of a strong IR absorption band at around 2300 cm 1 typical of the CNO group stretching. Slow dimerization of nitrile oxides 303 took place at room temperature leading to the furoxans 304 in good yields (Scheme 75 and Table 4) <2002S1701>. 

The solid-state structures of many of these radicals comprise n -n bonded diamagnetic dimers indicative of the fact that AE > PE. The symmetry of the SOMO often permits a number of symmetry-allowed n -n dimerisation processes. Among these five-membered heterocyclic radicals, the 1,2,3,5-di-thiadiazolyl radicals 2 are perhaps the most extensively studied and four different modes of association have been reported (Figure 3). 

Theoretical calculations on the dithiazolyl radical 4 (R=CF3) have recently shown that n -n dimerisation was unfavourable but association of two such dimers via electrostatic interactions generated a thermodynamically stable tetramer consistent with single crystal X-ray studies. Thus while the value of [AE-P ] may favour (or disfavour) dimer formation, the van der Waals, dipole contributions and electrostatic interactions to the lattice enthalpy should not be underestimated in assessing the thermodynamic stability or instability of these... 

Section 12.1.2.4). The majority of derivatives of 2 and all reported derivatives of 11 tend to associate as dimers in the solid state (Figure 2). Theoretical calculations14 and solution JJV/vis and EPR measurements15 all point towards greater dimerisation enthalpies for the Se analogues. 

The dimerisation energy for derivatives of 2 (ca. 35 kJ mol-1) is considerable, particularly in relation to the strength of intermolecular forces and some persistence is required in order to isolate derivatives of 2 which do not form 7T —7r dimers in the solid state. A survey of the monomeric derivatives has been published recently.26 Since the spin density distribution in 2 is rather insensitive to chemical tuning, approaches to inhibit dimerisation rely exclusively on structural modifications, which affect the nature of the intermolecular forces. Inclusion of sterically demanding groups, such as 13, 14 and 15 has proved partially successful (in the case of the diradical 14 one ring is involved in formation of a dimer, while the other retains its open shell character). 

Only 5 derivatives have been isolated to date. While the non-fused system forms a 71 -ji dimer (<7S s 3.27 A),86 structural studies on the more delocalised derivatives indicate a reduced tendency to dimerise. However, rather than forming optimal n-stacked motifs, there is a clear tendency to adopt slipped n-stack structures which minimise orbital overlap. As a consequence although the Hubbard energy is low, conductivity measurements show poor conductivities (10 5 S cm-1) similar to those observed in either the isomeric 1,3,2-dithiazolyl (4) or 1,2,3,4-dithiadiazolylradicals (2).84... 

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],... 

The aggregation behaviour and tautomerism of three 0,0 -dihydroxy and one o-hydroxy-o -methoxy monoazo dyes have been studied by UV-visible spectroscopy [17]. Evidence of monomer-dimer equilibria was obtained for all four of these mordant dye structures. Intermolecular hydrogen bonding between the hydroxy groups and hydrophobic interaction between aryl nuclei contribute to the dimerisation effect. 

An aqueous dispersion of a disperse dye contains an equilibrium distribution of solid dye particles of various sizes. Dyeing takes place from a saturated solution, which is maintained in this state by the presence of undissolved particles of dye. As dyeing proceeds, the smallest insoluble particles dissolve at a rate appropriate to maintain this saturated solution. Only the smallest moieties present, single molecules and dimers, are capable of becoming absorbed by cellulose acetate or polyester fibres. A recent study of three representative Cl Disperse dyes, namely the nitrodiphenylamine Yellow 42 (3.49), the monoazo Red 118 (3.50) and the anthraquinone Violet 26 (3.51), demonstrated that aggregation of dye molecules dissolved in aqueous surfactant solutions does not proceed beyond dimerisation. The proportion present as dimers reached a maximum at a surfactant dye molar ratio of 2 5 for all three dyes, implying the formation of mixed dye-surfactant micelles [52]. 

If two identical molecules combine chemically a dimer is obtained. Acetylene for instance, is dimerised to vinylacetylene. If smaller molecules of a substance unite then a large molecule, a polymer, of high Molecular weight is obtained. The individual small molecule from which a polymer is formed is called a monomer. The chemical process for the formation, of a polymer is called polymerisation. This is exemplified in the following equation. 

Dimerisation and ester coordination restricts the number of sites available for alkoxides to two only, while maintaining a comparatively Lewis-acidic titanium centre, as needed for the reaction. In the dimer, the methine protons, alkoxide groups, and ester groups are inequivalent, but they show a rapid exchange on the H NMR timescale at room temperature, as the AG for the process is only 64 kl.mol. This process is much faster than the catalytic reaction, but due to the C2-symmetry of the tartaric esters the resulting structures of the dimers are the same. 

Above we have mentioned several heterogeneous applications such as the OCT process and SHOP. Neohexene (3,3-dimethyl-1-butene), an important intermediate in the synthesis of fine chemicals, is produced from the dimer of isobutene, which consists of a mixture of 2,4,4-trimethyl-2-pentene and 2,4,4-trimethyl- 1-pentene. Cross-metathesis of the former with ethene yields the desired product. The catalyst is a mixture of W03/Si02 for metathesis and MgO for isomerisation at 370 °C and 30 bar. The isobutene is recycled to the isobutene dimerisation unit [48],... 

Probably the most familiar radical reactions leading to 1,2-D systems are the so called acyloin condensation and the different variants of the "pinacol condensation". Both types of condensation involve an electron-transfer from a metal atom to a carbonyl compound (whether an ester or an aldehyde or a ketone) to give a radical anion which either dimerises directly, if the concentration of the species is very high, or more generally it reacts with the starting neutral carbonyl compound and then a second electron is transferred from the metal to the radical dimer species (for an alternative mechanism of the acyloin condensation, see Bloomfield, 1975 [29]). 

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