Vinylcyclohexene Diels-Alder-dimerization

The thermally induced Diels-Alder dimerization reaction producing vinylcyclohexene is very difficult to prevent except by lowering the storage... 

The thermally induced Diels-Alder dimerization reaction producing vinylcyclohexene is very difficult to prevent except by lowering the storage temperature (246). Since the reaction rate increases about ninefold for every 20°C increase in temperature (Table 6), care must be taken to keep butadiene at a low temperature. Table 6. Effects of Temperature on Dimerization Rate of Butadiene ... 

Typically, little butadiene is dimerized to 4-vinylcyclohexene under actual reaction conditionsOO,31). The butadiene used in the process, however, can contain up to. 5 weight % Diels-Alder product, and at this level oxidative carbonylation can become a significant heavies forming reaction (Equation 9.). The major product from this reaction comes from l, -dicarbonylation of the 4-vinylcyclohexene exocyclic double bond. 

Polymerization and dimerization of dienes are competing reactions that occur simultaneously during the course of the polymerization. The dimerization of BD occurs by a Diels-Alder reaction which results in the formation of vinylcyclohexene (VCH), a non-conjugated diene (Scheme 14). 

The production of styrene from butadiene has also been examined. The dimerization of butadiene in a Diels-Alder reaction yields 4-vinylcyclohexene, which can be dehydrogenated oxidatively into styrene. This process has not yet achieved any commercial importance. 

Butadiene dimerizes by a Diels-Alder reaction thereby forming mainly 4-ethenylcyclohexene (4-vinylcyclohexene) (eq. (2)). In this dimerization reaction, butadiene acts as both diene and dienophile. The dimerized product is always present in butadiene unless freshly distilled. Although this reaction occurs spontaneously, selective catalysts based upon nitrosyliron compounds do exist (44). Vinylcyclohexene is a starting material for plasticizers and antioxidants. It should be noted that this reaction also gives smaller amoimts of 1,2-divinylcyclobutane and 1,5-cyclooctadiene (45). 

Another photocatalytic reaction of some interest is the cyclic dimerization of alkenes, which has been reported for a number of different metal carbonyls including Fe(CO)s, Cr(CO)6, Ni(CO)4 and several substituted analogs [83-85]. Although such dimerizations are catalyzed thermally by some of the same species, the stereochemistries of the products differ depending both upon which complex is utilized and whether the reaction is carried out thermally or photochemically. The dimerization of butadiene to 4-vinylcyclohexene photocatalyzed by the iron dicarbonyl Fe(CO>2(NO)2 has been studied by examining the intermediates formed when the photolysis was carried out in liq. Xe [85]. The key species was found to be the disubstituted species Fe(NO)2(Ti -butadiene)2 which was proposed to undergo isomerization to Fe(NO)2( n -butadiene)(ri2-butadiene) which can undergo an internal Diels-Alder cyclization to the vinyl cyclohexene. 

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