Hydrogen dimerisation

Breslow studied the dimerisation of cyclopentadiene and the reaction between substituted maleimides and 9-(hydroxymethyl)anthracene in alcohol-water mixtures. He successfully correlated the rate constant with the solubility of the starting materials for each Diels-Alder reaction. From these relations he estimated the change in solvent accessible surface between initial state and activated complex " . Again, Breslow completely neglects hydrogen bonding interactions, but since he only studied alcohol-water mixtures, the enforced hydrophobic interactions will dominate the behaviour. Recently, also Diels-Alder reactions in dilute salt solutions in aqueous ethanol have been studied and minor rate increases have been observed Lubineau has demonstrated that addition of sugars can induce an extra acceleration of the aqueous Diels-Alder reaction . Also the effect of surfactants on Diels-Alder reactions has been studied. This topic will be extensively reviewed in Chapter 4. 

The aggregation behaviour and tautomerism of three 0,0 -dihydroxy and one o-hydroxy-o -methoxy monoazo dyes have been studied by UV-visible spectroscopy [17]. Evidence of monomer-dimer equilibria was obtained for all four of these mordant dye structures. Intermolecular hydrogen bonding between the hydroxy groups and hydrophobic interaction between aryl nuclei contribute to the dimerisation effect. 

Using the catalyst shown in Figure 10.5 propene leads to dimerisation only. The difference in behaviour between ethene and propene is explained by the hydrogen at a tertiary carbon that is formed using propene, which undergoes /3-hydride elimination. 

A final type of oxidation reaction associated with the loss of hydrogen atoms is seen in the oxidative dimerisation of phenols. In general, the oxidation of phenols, in either the presence or absence of metal ions, is not a clean process, and many products, derived from... 

In other cases, e.g. 22, electro-reduction involving replacement of fluorine by hydrogen (23) [24,30] or dimerisations (24, 25) [31], have been observed [30] (Scheme 9). 

Bromination of ketone 3.17 gives 3.18 which can be converted to azide 3.19. Hydrogenation of 3.19 in the presence of hydrochloric acid affords aminoketone hydrochloride salt 3.20. Such aminoketones are often isolated as the corresponding salts because the free aminoketones are prone to dimerisation, having both nucleophilic and electrophilic centres. (For a common alternative preparation of aminoketones, see the Knorr pyrrole synthesis, Chapter 2.) Liberation of the free base of 3.20 in the presence of the acid chloride affords amide 3.21 which is cyclised to oxazole 3.22. Ester hydrolysis then affords the biologically-active carboxylic acid 3.23. 

Dimer 32 presumably originates by abstraction of hydrogen from the solvent by radicals formed in the reaction, giving 33, which undergoes radical dimerisation. Nonetheless, evidence that radicals are present in the reaction mixture is not evidence that they are an intermediate in the reaction pathway. 

The reduction of alkyl and aryl halides by Sml2 provides access to radicals that can undergo a range of follow-up reactions, including dimerisation, reduction to an anion (organosamarium) or hydrogen atom abstraction from solvent as shown in Scheme 3.1. 

An unexpected strange reaction of 312-pyrazole 64 giving 65 via a carbene dimerisation followed by [2+2]-cycloaddition and 66 via a hydrogen abstraction followed by cyclisation has been reported. No clear statement was made on the multiplicity of that reaction 77>. 

As with the multi-species components, it is not necessary for hydrogen bonding to be used in order to form dimeric capsules. This is highlighted by the recent synthesis of a molecule which assembles into dimers that are held together by n-interactions [137]. The meso-hexaphenyl calix[6]pyrrole 59 assembles into a dimeric capsule via the interactions between six phenyl groups arranged in a central belt around the capsule. The capsule is capable of holding two molecules of chloroform simultaneously, as seen in the solid state structure (Fig. 51). Similar phenyl interactions have been observed in ap-phenylcalix[5]arene which has been shown to dimerise and include C60 within its cavity (Fig. 52) [138]. 

Around the same time as this, Reinhoudt developed a calix[4]arene system with only two urea or thiourea functionalities attached on opposite faces 70-72 [167]. These less substituted systems display both inter- and intramolecular hydrogen bonding as a result of the calixarene adopting a pinched cone conformation (demonstrated by the use of NOESY NMR). In some spectra it is impossible for the connectivities to be made within a single molecule, so the only possibility left is that dimerisation occurs. As with the initial experiments of Rebek and Bohmer, the extent of hydrogen bonding was observed to be solvent dependent. Concentration dependant FTIR was also used, to observe the effects on the NH stretching vibrations, but no concentration dependence was observed. Of the three urea derivatives used, only 72 showed no evidence of dimerisation... 

Scheme 1.1. Some characteristic reactions of radicals. A and B are any two types of reactive radical. A = B reaction [1]+ is a dimerisation. The reverse [ 1 ] is bond homolysis and may be induced thermally or photochemically. Reaction [2] represents addition to an alkene derivative. Reaction [3], hydrogen atom transfer, is one of the most important displacement reactions of radicals.

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