Dimerization reactions water reagent

The conventional approach to the understanding of chemical events is to model a system with the reagents (reactants) as isolated participants. On occasion, recognition is made of the presence of a dimer or a hydrate as a functioning member of a chemical interaction (reaction). But these are exceptions. The role of the solvent (e.g. water in a biological reaction) is usually neglected because it is poorly understood (Testa, 1984). 

Over the past ten years, absolute rate data have been reported on the kinetics of several bimolecular silene reactions in solution, including both head-to-tail and head-to-head dimerization the [l,2]-addition reactions of nucleophilic reagents such as water, aliphatic alcohols, alkoxysilanes, carboxylic acids and amines and the ene-addition, [2 + 2]-cycloaddition and/or [4 + 2]-cycloaddition of ketones, aldehydes, esters, alkenes, dienes and oxygen. The normal outcomes of these reactions are summarized in Scheme 1. 

The salt is insoluble in water and in solvents such as hexane or toluene and may, therefore, be separated from dicobalt octacarbonyl which is readily soluble in hydrocarbon solvents. Thus, two analyses and the requisite simple calculations permit the estimation of both [Co(CO)4]2 and Co(CO)4-. A convenient apparatus for the determination has been described (Orchin and Wender, 41). Alternately, the concentration of dicobalt octacarbonyl may be measured by adding pyridine to the solution containing the carbonyls. All the dimer is converted to the anion with the evolution of carbon monoxide according to Equation (2). After gas evolution has ceased, a solution of iodine in pyridine may be added. This reagent is similar to aqueous iodine and liberates all the carbon monoxide in the anion originally present as well as that formed from the dimer by reaction with pyridine. 

Chiral epoxides are important intermediates in organic synthesis. A benchmark classic in the area of asymmetric catalytic oxidation is the Sharpless epoxidation of allylic alcohols in which a complex of titanium and tartrate salt is the active catalyst [273]. Its success is due to its ease of execution and the ready availability of reagents. A wide variety of primary allylic alcohols are epoxidized in >90% optical yield and 70-90% chemical yield using tert-butyl hydroperoxide as the oxygen donor and titanium-isopropoxide-diethyltartrate (DET) as the catalyst (Fig. 4.97). In order for this reaction to be catalytic, the exclusion of water is absolutely essential. This is achieved by adding 3 A or 4 A molecular sieves. The catalytic cycle is identical to that for titanium epoxidations discussed above (see Fig. 4.20) and the actual catalytic species is believed to be a 2 2 titanium(IV) tartrate dimer (see Fig. 4.98). The key step is the preferential transfer of oxygen from a coordinated alkylperoxo moiety to one enantioface of a coordinated allylic alcohol. For further information the reader is referred to the many reviews that have been written on this reaction [274, 275]. 

Although 3,4-dichloro-l,2,5-thiadiazole-l,1-dioxide (76) is related structurally to 3,4-dichlorothiophene-1,1-dioxide their chemical properties are quite different. The thiophene compound is a reactive diene and readily dimerizes via an auto Diels-Alder reaction. Furthermore, its chlorine atoms are unreactive toward weak nucleophilic reagents like water and alcohol. The thiadiazole compound, on the other hand, shows no tendency to dimerize and is highly reactive toward water and alcohol. 

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