Intramolecular ring formation

If natural rubber is treated with proton donors a product is formed which has the same empirical formula. (CjHjj), and is soluble in hydrocarbon solvents but which has a higher density, is inelastic and whose unsaturation is only 51% that of natural rubber. It is believed that intramolecular ring formation occurs to give products containing the segments shown in Figure 30.5. Known as cyclised rubber it may be prepared by treating rubber, on a mill, in solvent or in a latex with materials such as sulphuric acid or stannic chloride. 

Nitrogen-Oxygen Bond Formation The anodic oxidation of vicinal dioximes gives 3,4-diphenylfurazan-2-oxide by intramolecular ring formation (Scheme 42) [59]. 

As demonstrated by various techniques, including cyclic voltammetry, ESR and NMR spectroscopy, instead of 1, which is an open shell system, the diamagnetic dimer 2 was isolated. The suggested mechanism for the formation of 2 involves, as first step, the acid-catalyzed cleavage of the MEM group followed by an intramolecular ring formation to an 1,3-oxazetidinium system. This intermediate then loses formaldehyde and CO to form the azafulleronium ion which is... 

In the first reported solid-phase Richter reaction, cinnolines 143 were prepared through a two step sequence of a Heck-type coupling to give intermediates 142 followed by intramolecular ring formation and release of ciimolines 143 <99TL6201>. 

Intramolecular ring formation is also found to occur by reaction of the C—H bond of an alkyl group attached directly to the phosphorus. For example, if /ran.s-PtCl2[P(f-Bu)2H-Pr]2 is heated in 2-methoxyethanol solution, the product is thought to be the complex with a five-membered chelate ring (XV) formed by reaction of the terminal methyl of the n-propyl group. 

No doubt, donor-functionalized ligands are becoming more and more popular in organolanthanide chemistry. The availability of donor-functionalities may be helpful not only in the fine-tuning of the above-mentioned spectator ligand environment. Intramolecular ring formation via dative bonds [183] can produce monomeric, homoleptic systems which show remarkable properties... 

It is proved that at short lengths of dimethylsiloxane unit, HFC reaction proceeds in two directions intramolecular ring formation giving polycyclic products and intermolecular formation of cyclo-linear copolymers. Formation of polycyclic products is proved by the direct synthesis. 

The theory of the effective molarity (EM) [10] Galli and Mandolini defined the EM as the reactant concentration at which the intramolecular and the miermolecular processes occur at the same rate (ki ,ra/fciM = 1). If the concentration of the reactants is small enough, the intramolecular ring formation is favored. For the method normally used, where the reactants are introduced slowly into a large volume of solvent, the rate of addition is decisive. 

A comparable effect was also found, for example, in the case of substituted diphenylsilanes. The chemical shift of diphenylmethylchlorosilane was found to be 10.6 ppm. Introduction of a methoxymethyl substituent at the ortho position of the phenyl group was accompanied by a high-field shift of about 7 ppm leading to a chemical shift of 3.4 ppm by intramolecular coordination [2]. A decision whether the influence on the electronic structure of silicon is due to intramolecular ring formation (Scheme 3 B) or hyperconjugation (Scheme 3 C) cannot be made based on the present data. 

Equation (17) shows an example using intramolecular ring formation by dehalogenation... 

The intramolecular ring formation of a sulfonamide with a tosyloxy group allows formation of the C-substituted compound (69) <88JA6266>. The precursor (68) was synthesized by peptide formation and subsequent reduction of the amide groups (Equation (28)). 

Cyclopolymerization is an intramolecular ring formation reaction. For example, as well as double bonds attached to the main chain [Equation (16-54a), rings in the main chain (Equations (16-54b) and (16-54c)] can also be formed in the polymerization of the acrylic anhydride, a 1-6 diene ... 

As the example of 2-vinyl butadiene has already shown, the free radical polymerization of dienes is particularly interesting. Because of the two bifunctional double bonds, 1,4-dienes, CH2=CH—R—CH=CH2, are normally tetrafunctional and therefore lead to branching and cross-linking even at relatively low yield. With certain initiators, however, 1,3-dienes such as CH2=CH—CH=CH2, butadiene, or CH2==C(CH3)—CH=CH2, isoprene, react bifunctionally to form unbranched 1,4-poly (dienes). Symmetric 1,6-dienes, and frequently also symmetric 1,5-dienes, likewise give more or less unbranched molecules. In contrast to the l,4-poly(dienes), however, the 1,6- and l,5-poly(dienes) only possess slight unsaturation. That is, intramolecular ring formation by a cyclopolymerization occurs, for example, in the 1,6-diene acrylic acid anhydride. 

The similar epoxidation of novolacs with ECH has been additionally studied by Oyanguren and Williams [244]. In this system, intramolecular ring formation has been measured. 

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