Inter-Reaction Interactions Types

The following typical features of conjugated reactions may be distinguished  

The principle of independent proceeding of elementary chemical reactions is not fulfilled. 

The links between reactions are set via general intermediate compounds. 

Conjugated reactions proceed in open systems only. 

As indicated in Chapter 1, principally, the reactions may be conjugated, when one of them slows another one down, and somewhat inhibits it. The mechanism of slowing down the secondary reaction may have different origins (e.g. if the target reaction is of the catalytic type, and intermediate products (IP) poisoning the catalyst are formed in the primary reaction). Another possible case is realized when IP of both reactions recombine or disproportionate (active sites are eliminated). Such negative effects of chemical reactions allow us to 

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Both thiourea (tu) and thiosemicarbazide (tsc) are bifunctional ligands, containing a DD face in addition to one or two co-ordination sites. The reaction of [Zn(tu)4]2+ with a dicarboxylate normally occurs with displacement of thiourea to give coordination polymers of the type [Zn(tu)2(p-dicarboxylate)] in which the chains are cross-linked by DD-AA interactions [157]. The fumarate derivative contains identical inter-plane hydrogen bonding to that observed in (NEt4)2 [fumarate] 2tu [158], in which the zinc atom has formally been replaced by two tetraethylammonium cations. 

When two identical activated alkene functions are included in the same molecule, inter-molecular coupling has to compete with intramolecular hydrocyclization. In most cases the intramolecular reaction, which corresponds to an overall two-electron process, takes precedence. Few mechanistic studies of intramolecular couplings have been reported. The main question is whether the coupling takes place at the mono-radical anion stage in an RS-type reaction (one unit reduced, the other not reduced), or at the bis(radical anion) stage in an RR-type reaction (both units reduced). The last case implies weak electronic interaction between the electrophores. 

In presence of O2, the tertiary radicals produce peroxy radicals, which in turn yield the hydroperoxides via inter- and intramolecular H-atom abstractions). Hydroperoxides may undergo further photolytlc decomposition or decompose as a result of energy transfer from the exclmer(iJl), presumably by an exchange Interaction, to yield chain-end carbonyl compounds with concomitant chain scission. Acetophenone, which has recently been observed as a reaction product, could conceivably be formed from the carbonyl compound by a Norrlsh type II reaction. 

This contribution will be divided according to activation mechanisms used to achieve the targeted transformation and the reaction type itself. However, some caution is necessary. As already shown in the case of the proline-catalyzed inter-molecular aldol reaction (Scheme 4), 12 can be considered to act as a bifunctional catalyst. Therefore, a strict classification according to just one single activation mechanism will not always be possible and very often activation modes like e.g. enamine formation are accompanied with additional interactions, such as e.g. hydrogen bonding. 

Insertion into the C—C bond. According to ab initio calculations with an extended basis set of the 6-3IG type and the correlation energy taken into account [66], the energy profile of insertion of methylene into the single C—C bond is, in the main, determined by steric factors. So the activation barrier for insertion into the C—C bond of ethane is very high (46 kcal/mol). At the same time, a similar reaction with cyclopropane must proceed with hardly any activation (a mere 2 kcal/mol). Upon insertion into a strained cycle C—C bond whose electron density exhibits a maximum off the bond line, strong electron interactions become possible at longer inter-nuclear distances, so the authors of Ref [66] hold that such an insertion should proceed with no activation barrier. Table 8.4 contains calculated data on reactions of carbene insertion into some other bonds of the X—H type. 

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