Polar substrate reactions

Chlorobutyl rubber is prepared by chlorination of butyl rubber (chlorine content is about 1 wt%). This is a substitution reaction produced at the allylic position, so little carbon-carbon double unsaturation is lost. Therefore, chlorobutyl rubber has enhanced reactivity of the carbon-carbon double bonds and supplies additional reactive sites for cross-linking. Furthermore, enhanced adhesion is obtained to polar substrates and it can be blended with other, more unsaturated elastomers. 

The telomerization of butadiene with ethylene glycol was chosen as an example for a reaction of a polar and a non-polar substrate to a semipolar product (Scheme 1). 

Previous studies have shown that the rate of the O2 ene reaction with alkenes shows neghgible dependence on solvent polarity . A small variation in the distribution of the ene products by changing solvent was reported earlier . However, no mechanistic explanation was offered to account for the observed solvent effects. It is rather difficult to rationahze these results based on any of the currently proposed mechanisms of singlet oxygen ene reactions. Nevertheless, product distribution depends substantially on solvent polarity and reaction temperature only in substrates where both ene and dioxetane products are produced ° . 

Derivatization with achiral reagents is required for nonvolatile or highly polar substrates such as a-amino acids. Appropriate derivatization leads to improved resolvability by the introduction of suitable functional groups capable of hydrogen bonding or coordination. If derivatization is necessary, it has to be ensured that the chemical reaction does not lead to any racemization of the sample. 

These reactions are most common for polar double bond as reactants (carbonyls and imines) than for non-polar substrates (alkene and alkyne). Hence, hydrogen-transfer processes are a very interesting option in order to perform polar double bond hydrogenation since they allow mild conditions, high selectivity,... 

On the other hand, a pure Eley-Rideal mechanism, in which the aromatic compound in the liquid phase reacts with the adsorbed acylating agent was first proposed by Venuto et alP1,22] and more recently by others.[23] However, for acylation reactions of polar substrates (anisole, veratrole), chemisorption of the latter must be taken into account in the kinetic law. A modification, the modified Eley-Rideal mechanism, has been proposed 114,24-26 an adsorbed molecule of acylating agent should react with a nonadsorbed aromatic substrate, within the porous volume of the catalyst. However, the substrate is also competitively adsorbed on the active sites of the zeolite, acting somehow as a poison of the acid sites. That is what we checked through different kinetic studies of various aromatic electrophilic substitution reactions.[24-26]... 

Ionic liquids are immiscible with many organic solvents and compounds, which lends themselves to biphasic or multiphasic catalytic reactions. Most are also immiscible with fluorous phases and some are immiscible with water. In the ideal biphasic process involving ionic liquids, a soluble polar substrate is converted to a less polar - and thus insoluble -product, which will then form a separate phase. Attaching fluorous groups to the ionic liquid cation can reverse the solubility properties. 

A certain kind of radical transfer can be modelled by the transfer of a hydrogen atom from an alkane molecule to a small alkyl radical. This reaction was studied in detail in the gas phase. With hydrocarbon partners, heats of reaction are a fairly safe measure of the relative rate of transfer, as the pre-exponential Arrhenius factors remain approximately constant for a series of transfers to a given radical. Tabulated thermodynamic data indicate, however, [31, 32] that the correlation between the heat of reaction and the transfer rate is not valid for reactions of a radical with polar substrates [32, 33], In condensed phases, transfer reactions have not been sufficiently studied. Polymerizations themselves are the source of the most valuable, though incomplete, information. 

If this were the case, then the C-C coupling could in principle be possible but kinetically uncompetitive for nucleophilic radicals. If the C-C coupling were relatively slow for all radicals, then electrophilic radicals ought to be able to perform other radical cyclizations prior to this trapping. Support for Uiis idea came from substrate 68, where the intermediate electrophilic radical undergoes cyclization to form a new nucleophilic radical 69 which in turn undergoes radical-polar crossover reaction affording lactone 70. 

As mentioned earlier, research on creating hybrid catalytic materials has been an area of interest for some time, and thus, there are a variety of methods that are used to immobilize catalytic species onto support materials. Some common examples are physisorption of the catalyst to the surface, electrostatic catalyst/surface interactions, and encapsulation of the catalyst into the pores of microporous materials [2], These methods can suffer from leaching in many solvents, competitive binding with charged or polar substrates, and limited usable substrate size and diffusion due to the support s small pore size, respectively. The method that offers the most promise of stability of attachment as well as flexibility in synthesis is covalent reaction between the catalyst and the support. This is the method employed in our research. By approaching the tethering process in a controlled and defined way, the surface catalytic species can be more uniform and behave more similarly to very well-defined homogeneous catalysts. 

The protic-dipolar aprotic solvent effect on rates of 8 2 reactions of different charge type is summarized in Table 15. Polar substrates and nucleophiles, which are not H-bohd donors, and anions, which are strong H-bond acceptors, have been deliberately chosen to illustrate the solvent effect. 

The polar displacement reaction involves attack of nucleophile (N) and electrophile (E) on the substrate (S). 

---