Intermolecular cleavage

Meanwhile other experiments have been carried out to isolate DNA molecules with either RNA- or DNA-phosphoesterase activity [69-71,142,143]. Since Mg +-dependent rather than Pb +-dependent cleavage is compatible with intracellular conditions and thus, more suitable for possible medical applications, deoxyribozymes were selected that used Mg + instead of Pb for cleavage [112]. One optimized deoxyribozyme that emerged in this selection showed a cleavage rate of 0.01 min and was also capable of intermolecular cleavage. 

The first and as yet only molecule with conjugated Si=Si double bonds is the tetrasilabuta-1,3-diene (tetrasila-1,3-diene) 139, which was prepared as follows. The disilene 13 is treated with excess lithium to give the putative disilenyllithium species 137. In the second step of the sequence mesityl bromide was added in the expectation that the bulk of the aryl group and the poor solubility of mesityllithium would favor halogenation over the competing transarylation. Indeed, the bromodisilene 138 does appear to be formed smoothly but, like 137, it has not yet been positively identified. Intermolecular cleavage of lithium bromide from the two intermediates then furnishes the tetrasilabuta-1,3-diene 139 in 60% yield (equation 32)130. 

The first is the treatment of a hydrazine with a halosilane. The silylhydrazines are formed by intermolecular cleavage of a hydrogen halid [Eq. (1)]. 

II,G,4), six- (Section II,G,5), and even seven-membered rings (Section II,G,6) are described and finally the syntheses of coupled ring systems (Section II,G,7). Two main methods of preparation are described one is the intermolecular cleavage of hydrogen chloride from hydrazine and dialkyl- or aryldichlorosilanes,55 another is based on the cyclization of acyclic fluorosilylhydrazines.153839,56 57... 

An alternative mechanism of oxidation of such alcohols [82IJC(B)42], which also agrees with the kinetics of reaction, includes the participation of the carboxyl function. It includes the formation of an intermediate acyl hypochlorite and the subsequent intramolecular or intermolecular cleavage to the final products (Scheme 90). 

In order to confirm this reaction route and at the same time to keep the reaction process as simple as possible, the symmetrically substituted tetraaryldisilene 9 [8] was prepared by the method of West et al. [9] and allowed to react with an excess of lithium. The observed color changes, first to dark green and then to brown-red, suggest the initial transfer of an electron to an aromatic residue [10], followed by extrusion of LiR and formation of the postulated disilenyllithium compound. In a second step of the reaction sequence, 10 was allowed to react with bromomesitylene in anticipation that the poor solubility of the aryllithium compound would favor the halogenation over the competing transarylation. It appears that the bromodisilene 11 is indeed formed and then reacts further with 10 by intermolecular cleavage of lithium bromide to furnish the isolated hexaaryltetrasilabuta-1,3-diene 12 (Scheme 5) [II]. 

The fracture behavior of WPC and its fracture mechanism were investigated by Jeong [104]. As it was outlined in the previous sections, the interfacial fracture, fiber pull out, and fiber breakage are the main fracture mechanisms of AAfPC. Additionally, intermolecular cleavage may occur with cracks running across fiber, interfacial area, and polymer. The fracture surface of WPC specimens, captured by SEM are illustrated in Figure 17.8. 

Intermolecular reactions with typical cycloaddition components are also possible. Phenyl isocyanate in ether converts triisopropyldiaziridinimine (182) to the 1,2,4-triazolidine under mild conditions. Labeling with a deuterated isopropyl group revealed that cycloaddition is not preceded by N—N cleavage, which should have resulted in deuterium randomization (77AG(E)109). 

Compound 14 can be dismantled in a productive fashion by ret-rosynthetic cleavage of the indicated bonds (see Scheme 4). The intermolecular attack of the amino group in 15 upon the keto function in 16 would be expected to result in the formation of the desired oxime ether after loss of a water molecule. A few functional group manipulations would then complete the synthesis of intermediate 14. A valuable structural feature of 15 is the C-2 oxygen substituent. Although this oxygen atom is not expressed in the natural product, it would certainly play an important role in our... 

Intramolecular and intermolecular 1,3-dipolar cycloadditions of aziridine-2-car-boxylic esters with alkenes and alkynes have been investigated [131, 132]. Upon heating, aziridine-2-carboxylates undergo C-2-C-3 bond cleavage to form azome-... 

As stated above, olefin metathesis is in principle reversible, because all steps of the catalytic cycle are reversible. In preparatively useful transformations, the equilibrium is shifted to one side. This is most commonly achieved by removal of a volatile alkene, mostly ethene, from the reaction mixture. An obvious and well-established way to classify olefin metathesis reactions is depicted in Scheme 2. Depending on the structure of the olefin, metathesis may occur either inter- or intramolecularly. Intermolecular metathesis of two alkenes is called cross metathesis (CM) (if the two alkenes are identical, as in the case of the Phillips triolefin process, the term self metathesis is sometimes used). The intermolecular metathesis of an a,co-diene leads to polymeric structures and ethene this mode of metathesis is called acyclic diene metathesis (ADMET). Intramolecular metathesis of these substrates gives cycloalkenes and ethene (ring-closing metathesis, RCM) the reverse reaction is the cleavage of a cyclo-... 

The molecules in crystalline chlorine, bromine and iodine are packed in a different manner, as shown in Fig. 11.1. The rather different distances between atoms of adjacent molecules are remarkable. If we take the van der Waals distance, such as observed in organic and inorganic molecular compounds, as reference, then some of the intermolecular contacts in the b-c plane are shorter, whereas they are longer to the molecules of the next plane. We thus observe a certain degree of association of the halogen molecules within the b-c plane (dotted in Fig. 11.1, top left). This association increases from chlorine to iodine. The weaker attractive forces between the planes show up in the plate-like habit of the crystals and in their easy cleavage parallel to the layers. Similar association tendencies are also observed for the heavier elements of the fifth and sixth main groups. 

Incubation of proteins in aqueous media at elevated temperatures significantly reduces the number of methylol adducts and intermolecular cross-links. Increasing pressure not only seems to accentuate this process significantly, but also appears to be accompanied by protein cleavage at aspartate residues.13... 

The ring cleavage of 3-aryl-2-substituted-2//-azirines by molybdenum hexacarbonyl has been described earlier in regard to the synthesis of pyrroles, pyrazoles and isoxazoles. In contrast to this behavior, analogous reactions of 2-unsubstituted derivatives lead to the formation of mixtures of 2,5-diarylpyrazines (139) and isomeric 3,6- and 1,6-dihydropyrazine derivatives (140,141) (Scheme 163).47,53 It is possible that the pyrazine products are formed by an intermolecular nitrene mechanism akin to the intramolecular processes described earlier (see Scheme 22 in Section IV,A,1). 

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