Catenation reaction

There are two distinct approaches to catenane synthesis the statistical approach, and approaches relying on self-assembly, so-called directed synthesis . The statistical approach relies on the small chance that macrocyclisation may occur while a linear precursor is threaded through a macrocyclic component. Because this is a rather unlikely eventuality, it naturally results in low yields of interlinked product and is chiefly of historical interest. It was this kind of statistical approach that resulted in the first synthesis of a [2] catenane by Wasserman in 1960 (10.64), from cyclisation of the long-chain diester 10.65 while threaded through the annulus of a deuterated C34 cycloalkane 10.66 (Scheme 10.11), 57 Although the overall yield of the catenation reaction was less than 1 %, the existence of the catenane was firmly established. The relatively polar [2] catenane product, along with other polar macrocyclisation reaction products and... 

Based on the chemistry discussed so far, two-step procedures for the assembly of [4]- and [5]-catenanes have also been developed. In part, the success of these syntheses reflects the use of the intermediate ring size crown, tris-/ -phenylene-[51]-crown-15 which provides two non-equivalent templating sites for the catenation reactions. The strategies employed for the construction of these extraordinary multi-linked systems are given in Figure 5.15. The respective products were characterised by means of FAB-MS and LSI-MS, with the fragmentation patterns... 

In this case, the role of the enzyme in cytotoxicity is suggested by the observation of a similar hierarchy in the potency of different drugs for cytotoxicity, DNA breakage, and inhibition of the DNA catenation reaction of the enzyme. 

First, it is found that in nicked DNA E. coli topoisomerase I binds close to the site of the nick and, when allowed to cleave the DNA, cuts the intact strand nearly opposite the nick. Second, an important feature of the catenation reaction is that at least one DNA partner must contain a nick or a single-stranded gap. If the enzyme binds at the nick and allows passage of DNA through the intact strand, one can conclude that the... 

Using the recombinant 5. shibatae enzyme, catenation-decatenation reactions of the DNA topoisomerase VI have been investigated. The maximum activity of the DNA topoisomerases VI is obtained in the presence of 5 mAf spermidine and of 10 mAf MgCl2. In the absence of NaCl, the enzyme tends to catenate a negatively supercoiled plasmid DNA. In the presence of 25 mMNaCI or more, this catenation reaction is eliminated. It has also been shown that a covalent complex between the recombinant enzyme and DNA can be trapped by rapid addition of a denaturing agent. As with all the type II DNA topoisomerases, the DNA topoisomerases VI thus introduce a transient double-strand break in DNA where the enzyme becomes covalently attached to the cleaved DNA. 

Nevertheless, the reaction conditions required by strategy A, as well as strategy B, for the nucleophilic displacement of the halide ions in fragment g are a barrier to increasing spectacularly the yield of the catenation reaction. Indeed, the stability of the precatenane species Cu(3.4) or Cu(3)2 in basic medium is rather limited, especially at high temperature. A new approach was developed later, which utilizes a... 

Metal polysulfido complexes have attracted much interest not only from the viewpoint of fundamental chemistry but also because of their potential for applications. Various types of metal polysulfido complexes have been reported as shown in Fig. 1. The diversity of the structures results from the nature of sulfur atoms which can adopt a variety of coordination environments (mainly two- and three-coordination) and form catenated structures with various chain lengths. On the other hand, transition metal polysulfides have attracted interest as catalysts and intermediates in enzymatic processes and in catalytic reactions of industrial importance such as the desulfurization of oil and coal. In addition, there has been much interest in the use of metal polysulfido complexes as precursors for metal-sulfur clusters. The chemistry of metal polysulfido complexes has been studied extensively, and many reviews have been published [1-10]. 

On the other hand, formates are less stable. This seems to be related to the H-CO-S atomic catenation in which 2 is an oxygen or chlorine atom. Nevertheless, with esters, the presence of a base as well as a high temperature and pressure are necessary. In the presence of sodium methoxide in a small quantity (0.5%), at 100°C and under 70 bar, methyl formate decomposes according to the reaction below the violent release of carbon oxide creates a dangerous overpressure in the apparatus ... 

In the case of silicon, the tendency for catenation allows halides having the formula Si2X6 to be prepared. The chloride can be prepared by the reaction... 

The catenation abihty of lead being considerably lower than that of germanium or tin, plumbanes with Pb—Pb bonds are quite difficult to prepare, even though a number of such compounds have been synthesized. As already pointed out, the main method for the preparation of this type of compound is the reaction of an appropriate Grignard reagent with PbCl2159, e.g. 

Metallocyclopentenes are frequently formed in photochemical reactions of the Group 14 metal alkyls or catenates in the presence of dimethylbutadiene. This class of compound also has an extensive photochemistry82. For example, photolysis of 51 (R = H or Me) produced the allylic alcohols 52 and 53 and, for R = H, 54. These alcohols could be dehydrated over AI2O3 to give the germole 55 along with other diene compounds. 

One-dimensional nanostructures of chalcogens and chalcogenides. An overview of solution-phase methods for generating one-dimensional nanostructures of chalcogens and chalcogenides has been presented by Mayers et al. (2004). Attention was especially focused on Se and Te because of their characteristic crystal structures and their catenation tendency. Basically, reactions were considered such as ... 

In the case of trichlorophenoxide such a model would give rise to an alternating 1,2-/1,4-sequence (which we call syndioregic by analogy with syndiotactic). In the case of 4-bromo-2,6-dichloro-phenoxide the much greater reactivity of the bromine relative to chlorine overides the factor which favoured ortho over para reactivity in the trichlorophenoxide case to such an extent that 1,4 catenation results. In the case of 2-bromo-4,6-dichlorophenoxide the 2-bromo position should be very reactive, but the radicals produced by such reaction (type 5) are not able to attack again at the ortho position of coordinated phenoxide. 

In recent years, dehydrocoupling reactions catalysed by early transition metal complexes have become an increasingly important method for generating catenated species of the p-block elements. In addition to producing cyclic oligomers, this approach is used to prepare linear oligomers and polymers such as polysilanes and polystannanes of the type H(MR2) H (M = Si, Sn) (see Section 10.1.4). ... 

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