Telomerization studies

The propagation step in this free radical polymerization has been demonstrated to involve radical attack on episulfide sulfur (as depicted above) by telomerization studies with cyclohexane. From the structures of the two major products, 20 and 21, cyclohexyl radical attacks at the sulfur atom by displacing fluoroalkyl radical. This new radical, a prototype of the growing polymer chain, then abstracts hydrogen from cyclohexane to form 20 or attacks another molecule of 6 (n = 1) at sulfur to give 21. 

Telomerization Reactions. Butadiene can react readily with a number of chain-transfer agents to undergo telomerization reactions. The more often studied reagents are carbon dioxide (167—178), water (179—181), ammonia (182), alcohols (183—185), amines (186), acetic acid (187), water and CO2 (188), ammonia and CO2 (189), epoxide and CO2 (190), mercaptans (191), and other systems (171). These reactions have been widely studied and used in making unsaturated lactones, alcohols, amines, ethers, esters, and many other compounds. 

From studies of the telomerization between 1-alkenes and cycloalkenes another interesting result emerged. The expected triads were observed, but within each triad the ratio of the products varied between 1 10 1 and 1 20 1 (88). This is not consistent with the scheme of H6risson and Chauvin. However, with the scheme of Calderon, this can be explained if... 

Halocarbons including carbon tetrachloride, chloroform, bromotrichloroincthane6 (Scheme 6.7) and carbon tetrabromide have been widely used for the production of tclomcrs and transfer to these compounds has been the subject of a large number of investigations." Representative data are shown in Table 6.4. Telomerization involving halocarbons has also been developed as a means of studying the kinetics and mechanism of radical additions.66... 

Dibromodichoromethane was studied as telogen in the reaction with vinyl chloride (ref. 7). The authors showed that the reaction occurred as telomerization (under usual conditions) the changes in monomer / telogen ratio (from 1.5 to 15) allowed to vary the adduct content from 80 to 30 %. 

Industrially, the perfluoroalkyl iodides by telomerization are mostly made by a batch system using peroxide initiators. However, the difficulty of mass production, and the production of hydrogen-containing byproducts in the process are disadvantageous [4]. In this study, a continuous process for the preparation of perfluoroalkyl iodides over nanosized metal catalysts in gas phase and the effects of the particle size on the catalytic activities of different the preparation methods and active metals were considered. 

Few studies have been carried out on the telomerization of carboxylic acids other than acetic acid. Carboxylic acids are expected to react similarly with butadiene. The exception is formic acid No telomerization takes place, as described before (33, 34), and it behaves as a reductant rather than a nucleophile, forming 1,6- and 1,7-octadienes and octatriene. 

At last, nucleolin might play a specific role in telomeric replication and maintenance, as suggested by two types of data. First, it binds telomeric repeat (TTAGGG)n in vitro (Ishikawa et al, 1993 Pollice et al, 2000), with a marked preference for the single-stranded form. Secondly, it interacts in vitro and in vivo with hTERT (Khurts et al, 2004), the protein catalytic component of human telom-erase. This interaction takes place both in the cytoplasm and in the nucleolus, where it could promote the assembly of hTERT with the RNA subunit hTERC. As a conclusion, many data regarding the involvement of nucleolin in DNA replication are indirect and an experimental demonstration through knockdown or knockout studies is still awaited. 

Brummendorf TH, Mak J, Sabo KM et al. Longitudinal studies of telomere length in feline blood cells implications for hematopoietic stem cell turnover in vivo. Exp.Hematol. 2002 30 1147 1152. 

The strong acidity of the proton at the C2 position of a [AMIM] ion has been well recognized 183). This cation can react with palladium complexes to form inactive l,3-dialkylimidazol-2-ylidene palladium complexes 200), as confirmed in a study of the conventional Pd(OAc)2/PPh3/base catalyst in ionic liquids for the telomerization of butadiene with methanol at 85°C 201). 

Selective modification of polyols such as ethylene glycol, 1,3-propylene glycol, or glycerol with butadiene (1) has been studied [7-10]. The monosubstituted compounds are preferred due to their potential applications as surfactants, PVC plasticizers, or even in cosmetics. The telomerization of 1 with ethylene glycol yields a complex mixture including linear and branched mono- and ditelomers, as well as 1,3,7-octatriene and vinyl cyclohexene (Fig. 2) [11]. 

In a similar approach, the telomerization of butadiene (1) with glycerol (3) was studied [7, 8]. Moreover, no additional solvent was necessary with this substrate. The corresponding monoether can be used as detergent or emulsifier while, after hydrogenation and sulfonation, the diether could be transformed to potential detergent molecule. 

To circumvent the formation of ditelomers and to attempt recycling of the catalysts, the telomerization of polyols was studied in the presence of water using water soluble catalysts such as Pd/TPPTS (TPPTS = tris(m-sulfonato-phenyl) phosphine trisodium salt) [9, 12, 16, 17]. Behr et al. studied the telomerization of ethylene glycol under biphasic conditions. Under such reaction conditions, 80% of mono-telomer are formed and only traces of ditelomer and butadiene dimers are detected (Fig. 4). This is attributed to the solubility of the monomer in the catalyst phase. However, the catalyst is unstable and decomposes rapidly, leading to almost inactive catalyst after three runs. This is due to TPPTS oxidation during the work-up of the reaction and can be avoided by addition of 2.5 equiv. ligand in the solution prior to each run. 

The telomerization of butadiene (1) with protected pentoses has been particularly studied using 2,3,4-tri-(9-acetyl-D-xylopyranose (5) [21] and 2,3,4-tri-O-benzyl-p-D-arabinofuranose (8) [22] as the telogens, in a 50-mL stainless steel autoclave... 

In order to improve the activity in the absence of co-solvent, the use of a surfactant was studied in the presence of TPPTS-based catalyst [55]. Monflier et al. reported the hydrodimerization of 1 in the presence of surfactants in order to improve butadiene mass transfer in pure water solution [56-58]. Such an additive used in very low amount avoided the presence of an organic co-solvent. It was shown in the case of hydrodimerization that neutral or cationic surfactants played a significant role in the process. Similar behaviors were reported for the telomerization of 1 with 21. While 30% conversion of 1 was achieved in pure water after 24 h reaction time at 50°C using 0.4 mol% of catalyst, the conversion reached 87% when polyether surfactant (POEA) was added to the reaction medium under similar reaction conditions (Table 12). It was found that the conversion is strongly affected by the nature of the surfactant (Table 13). 

While the telomerization reaction of butadiene has been known for 40 years, its application to polyols issued from the biomass is recent and continues to be under investigation. Most polyols are much less active than MeOH or H2O that have found industrial developments. Higher catalytic loading are required, but recent studies disclosed very active catalytic systems which are promising. 

The principal side reaction is telomerization, which is mainly formation of a cyclic tetramer, a 16-membered cyclic ether. A great deal of study has been given to suppression of this undesired by-product by variations in catalysts, promoters and other conditions. 

Fluorinated radicals play a significant role in synthetic organo-fluorine chemistry, for example, in electrophilic radical addition to alkenes, single-electron transfer reactions (SET), telomerization of fluoroalkenes with perfluoroalkyl iodides, polymerization to fluoropolymers and copolymers, and thermal, photochemical and radiation destruction of fluorocarbons. Furthermore, such free radicals are of interest for studying structures, reaction kinetics and ESR spectroscopic parameters.38... 

---