Cyclodimerization acid-catalyzed

The DA dimer obtained in the aminium salt reaction differs from that obtained by the PET method and from that obtained using the aminium salt/hindered base method, but is the same as that obtained by the Bronsted acid catalyzed reaction of this diene. The addition of insoluble bases hke sodium carbonate is not sufficient to suppress the extremely facile, acid-catalyzed cyclodimerization of this particular diene, which yield a highly stabilized tetramethylallyl carbocation intermediate. Nevertheless, this results demonstrates the necessity for caution in assigning a cation radical mechanism to a cyclodimerization reaction observed under aminium salt conditions. 

Cyclodimerization of 1,3-butadiene over Cu-exchanged zeolites FAU and EMT was followed in situ by DRIFT in a study by Voskoboinikov et al. [906]. The reaction was Brqnsted acid-catalyzed. Even though the deactivation was rather Uttle, it could be also monitored in situ through the IR spectra showing the formation of polycyclic naphthenes, which then in part consecutively transformed into aromatics. 

Methyl 2, -pentadienoate self-dimerization as a potential route to sebacic acid precursors was extensively evaluated. Homogeneous metal catalyzed dimerization of compounds containing conjugated double bonds is known to give cyclic, branched, and linear dimers(43-45). At higher temperatures or in the presence of many zero-valent metal catalysts, cyclodimerization (Equation 12.) is the only observable reaction. 

One of the problems associated with thermal cyclodimerization of alkenes is the elevated temperatures required which often cause the strained cyclobutane derivatives formed to undergo ring opening, resulting in the formation of secondary thermolysis products. This deficiency can be overcome by the use of catalysts (metals Lewis or Bronsted acids) which convert less reactive alkenes to reactive intermediates (metalated alkenes, cations, radical cations) which undergo cycloaddilion more efficiently. Nevertheless, a number of these catalysts can also cause the decomposition of the cyclobutanes formed in the initial reaction. Such catalyzed alkene cycloadditions are limited specifically to allyl cations, strained alkenes such as methylenccyclo-propane and donor-acceptor-substituted alkenes. The milder reaction conditions of the catalyzed process permit the extension of the scope of [2 + 2] cycloadditions to include alkene combinations which would not otherwise react. 

However, the zeolite is not a unique substrate for this reaction, as is indicated in a recent patent (180), where it is shown that a Cu+-exchanged mont-morillonite clay and synthetic amorphous aluminosilicate will also catalyze butadiene cyclodimerization with high selectivities to VCH (>95%). Preexchange of these aluminosilicates with Cs+ ions was claimed to increase catalyst stability. This is most probably explained by a reduction in surface acidity resulting from the alkali metal ion exchange. 

When phosphane-free nickel complexes, such as bis(cycloocta-l,5-diene)nickel(0) or te-tracarbonylnickel, are employed in the codimerization reaction of acrylic esters, the codimer arising from [2-1-1] addition to the electron-deficient double bond is the main product. The exo-isomer is the only product in these cyclopropanation reactions. This is opposite to the carbene and carbenoid addition reactions to alkenes catalyzed by copper complexes (see previous section) where the thermodynamically less favored e Jo-isomers are formed. This finding indicates that the reaction proceeds via organonickel intermediates rather than carbenoids or carbenes. The introduction of alkyl substituents in the /I-position of the electron-deficient alkenes favors isomerization and/or homo-cyclodimerization of the cyclopropenes. Thus, with methyl crotonate and 3,3-diphenylcyclopropene only 16% of the corresponding ethenylcyc-lopropane was obtained. Methyl 3,3-dimethylacrylate does not react at all with 3,3-dimethyl-cyclopropene, so that the methylester of tra 5-chrysanthemic acid cannot be prepared in this way. This reactivity pattern can be rationalized in terms of a different tendency of the alkenes to coordinate to nickel(O). This tendency decreases in the order un-, mono- < di-< tri- < tet-... 

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