Poor synthesis efficiency complexes

When mixed with Et2AlCl, the vanadium(III) complex (87) polymerizes propylene at —78 °C in a living manner.241,242 Poor initiator efficiency ( 4%) and low activities were improved by employing complex (88) activities of lOOgmmol h bar 1 were reported and the polymerization of propylene remained living (Mw/Mn= 1.2-1.4) up to 40 °C.243 244 The synthesis of end-functionalized PP and PP copolymers has also been achieved using these initiators. 

It was recognized early that efficient olefin cross metathesis could provide new methods for the synthesis of complex molecules. However, neither (la) nor (2a) were very effective at intermolecular cross metathesis owing to poor reaction selectivity (cross vs. intramolecular metathesis) and low E. Z ratios see (E) (Z) Isomers) The advent of more active and functional group tolerant olefin metathesis catalysts recently made cross metathesis a viable route for constructing a large variety of fimctionalized acyclic alkenes. 

A problem with employment of ASON in a larger clinical setting is their poor uptake and inappropriate intracellular compartmentalization, e.g., sequestration in endosomal or lysosomal complexes. In addition, there is a need for a very careful selection of the ASON-mRNA pair sequences that would most efficiently hybridize. To date, several computer programs are used to predict the secondary and tertiary structures of the target mRNA and, in turn, which of the mRNA sequences are most accessible to the ASON. However, even with this sophisticated techniques, the choice of base-pairing partners still usually includes a component of empiricism. Despite these principal limitations, it has become clear that ASON can penetrate into cells and mediate their specific inhibitory effect of the protein synthesis in various circumstances. 

In solid-phase carbohydrate synthesis, it is impossible to purify the synthetic intermediates, whereas glycosylation reactions, especially the two-phase reactions, usually give relatively poor efficiency and stereoselectivity. As a result, a significant number and quantity of side products will be formed together with the desired product on the polymer. After carbohydrate chains are cut off from the polymer, a complex mixture is obtained, and the final product purification becomes difficult. To deal with this problem, several innovative strategies have been explored... 

Due to poor reactivity, aryl amines normally require higher reaction temperatures than aliphatic amines to ensure good conversion. In early studies, phenathroline and its Cu(I)-complex were used in the arylation of aryl amine [10, 11], but they were only applicable to the synthesis of triarylamine from secondary aryl amines. L-Proline (LI) promoted Cul-catalyzed arylation of primary aryl amines took place at 90°C (Table 9.1, entry 1) [3]. However, only electron-rich anilines gave complete conversion, while electron-deficient anilines provided low yields. Fu found that this drawback could be overcome by heating at 110°C and using pipecolinic acid (L5) as a ligand (entry 2) [12]. Similar studies were reported by Liu and coworkers in which DMEDA (Lll) was found to be a better ligand (entry 3) [13], Recently, Buchwald reported that pyrrole-2-carboxylic acid (L6) [14] is an efficient promoter for the synthesis of diarylamines (entry 4) (Table 9.3). 

In this field of luminescent organometallic probes, rhenium and iridium complexes have played so far the major role, thus explaining their predominance in this chapter. The lower number of platinum, rhodium, and gold fluorescent complexes could be explained by demanding synthesis, low luminescence efficiency, poor operational stability, unsuitability for biological applications, or even simply by the lack of systematic studies. 

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