Anticipated selective dimerization

SCHEME 10 Anticipated selective dimerization of C-2 symmetric substrate. 

Nonetheless, a number of electroorganic synthesis reactions are known whose outcome i.e., whose yield and selectivity, is decisively determined by the nature of the electrode so that heterogeneous acceleration of at least one of several competitive reactions of the electrogenerated reactive intermediates might be anticipated. A famous case is the Kolbe reaction, which is essentially the anodic dimerization of alkyl radicals that are generated at platinum anodes by anodic oxidation of the anions of carboxylic acids ... 

The agreement between calculated and observed values of Table 20 is comparable to that obtained in Table 17 for the select solvents, except for water and alcohols. The latter solvents self-associate through H bonds to form very polar clusters giving rise to much larger (b values than might be anticipated from the permanent dipole moments of the monomers. Evidently, the formation of cyclic acetic acid dimer does not produce such an effect so that this hydroxylic solvent has normal values of both 0(c ) and fi—very similar to those for the aprotic derivatives methyl and ethyl acetates. 

Using these classifications, the outcome of a CM reaction can be anticipated by referring to Table 2.10. Four outcomes can result (1) no CM, where no metathesis reaction occurs (2) nonselective CM where a mixture of homodimers and the desired CM product will be obtained (3) statistical CM, where the yield of the desired CM product will depend on the relative concentrations of each alkene that were added at the start of the reaction (4) and selective CM, where the desired product is obtained selectively. CM outcomes are therefore dependent on the rate of secondary metathesis processes for example, in the reaction of a Type I with a Type 11 (or 111) alkene, dimerization of the Type I alkene will occur but the dimeis are consumable, while dimerization of the Type II (or III) alkene will be much slower (Scheme 2.14). In this case, productive metathesis to yield the desired product should dominate. 

A catalyst for the selective cyclodimerization of butadiene to 4-vinylcyclo-hexene can be generated by the reduction of [Fe(NO)2Cl], with metal carbonyls or electrochemically. Addition of cyclic amines to the isoprene dimerization catalyst [NiCl2(PPhs)2]-NaBH4-H20 enhances the formation of cyclic dimers the product distribution has been correlated with an electronic and structural properties of the amine. [Ni P(n-OC6H4R)3 J (R = Ph or SiMes) catalyses the selective (95 %) dimerization of butadirae to cyclo-octa-1,5-diene the catalysts appear to operate via two active centres, probably the Ni and P atoms. When solutions of (44) are allowed to stand and then treated with HCl or Brj signficant amounts of [1,1,3,3- H4]- and [2,2,3,3- H4]-butane are produced in addition to the anticipated [1,1,4,4- H4]-isomer. This would seem to indicate that there is an equilibrium (14) and in keeping with this (44)... 

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