Formation of dimers and trimers

Many reagents are able to chlorinate aromatic pyrazole derivatives chlorine-water, chlorine in carbon tetrachloride, hypochlorous acid, chlorine in acetic acid (one of the best experimental procedures), hydrochloric acid and hydrogen peroxide in acetic acid, sulfuryl chloride (another useful procedure), etc. iV-Unsubstituted pyrazoles are often used as silver salts. When methyl groups are present they are sometimes chlorinated yielding CCI3 groups. Formation of dimers and trimers (308 R = C1) has also been observed. 

The formation of dimers and trimers is a major issue in hydrogenations with iridium catalysts. In the context of developing an industrial process to produce (S)-metolachlor via an enantioselective imine hydrogenation (see Chapters 34 and 37), Blaser et al. investigated the causes of catalyst deactivation in the iri-dium/bisphosphine-catalyzed hydrogenation of DMA imine (Scheme 44.11) [84]. 

Interestingly, the formation of dimers and trimers by intermolecular aminolysis has also been observed in concentrated solution (>1%) of the mono-bactam aztreonam (5.8) [124],... 

It seems that formation of dimers and trimers from soybean oil is the slow step of polymerization, an evidence for step polymerization mechanism. Once formed, these dimers and trimers polymerize very quickly to a high molecular weight and exhibits chain reaction kinetics. This phenomenon is caused by the soybean oil molecule itself, a large molecule with low activity, due to mid chain double bond location. However, the formed dimers and trimers have more unsaturated carbon-carbon double bonds per molecule, and they are easily polymerized to high molecular weight polymers. We propose further investigation on the possible combination of two polymerization mechanisms for the polymerization of soybean oil in SCCO2. Kinetic study of soybean oil polymerization currently is carried out in our laboratory. 

The early investigation of thermal degradation of polystyrene has been carried out by Staudinger and Steinhofer in order to account for the formation of dimer and trimer and to determine the head-to-tail arrangement of the repeating units in this polymer. 

The various intersubunit binding domains of the oligomer would be expected to cleave under different conditions to yield trimers or dimers. Formation of dimers and trimers in the presence of SDS has been reported, but details of conditions were not described 140). In addition, the porcine enzyme in 33% dioxane retains 50% of its activity for several days and exhibits a molecular weight of 140,000 (5<9). Propellor shaped trimers have been seen in electron micrographs (136,137,139) and micrographs of tilted fields containing these particles reveal that they consist of three subunits, presumably formed by cleavage between layers of trimers (139). 

Dimer yields of 25% to 50% by weight, along with about 5% to 15% of the stereoisomeric trimers, can be achieved by the addition of Lewis acids such as AICI3, TiCl4, BF3, SnCh, etc. to the reaction mixture. Strong protic acids such as trifluoroacetic or sulfuric acid are also effective for the formation of dimers and trimers. 

The Raman spectra of the products of y-irradiation of solid Cm showed the formation of dimers and trimers. IR evidence has been reported for the formation of a fullerene trimer, Ciso." ... 

Scheme 16.2 Formation of dimers and trimers from l,3-bis(bromomethyl)benzene and 2,6-bis (bromomethyl)pyridine building blocks, respectively...

Michael Addition Reaction. Hf(OTf)4 catalyzed the 1,4-addition reactions of indole to enones in the presence of pyridine-diamine ligands. A 1 1 mixtureof Hf(OTf)4 and a pyridinediamine ligand formed a Hf(OTf)4-pyridinediamine complex, which catalyzed the 1,4-addition reaction very efficiently without the formation of dimer and trimer of indole. The stereochemistry of the P3Tidinediamine ligands was found to play an important role in the relative rates of reaction (eq 1) ... 

The intermediate formation of dimeric and trimeric EDOT (see Figure 8.3, formulae wifhouf sfereochemical aspects) is a slow equilibrium reaction, but does not influence the overall reaction rate. For the sake of clarity, in Figure 8.2 the trimer formation has been omitted. 

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