Diels-Alder reaction 1,3-cyclohexadiene system

A second example from the same group is the synthesis of an elaborate diethynyltriphenylene derivative (Scheme 7 Table 8,entries 12,13) [58].Zn/Pd-promoted homocoupling of a 4-iodo-l,2-dialkoxybenzene furnishes the desired tetraalkoxybiphenyl, an electron-rich aromatic system. Iron trichloride-catalyzed Friedel-Crafts arylation of the biphenyl derivative with dimethoxy-benzene furnishes an unsymmetrical triphenylene derivative. Deprotection, oxidation, and subsequent Diels-Alder reaction with cyclohexadiene is followed by catalytic hydrogenation and reoxidation. TMS-CC-Li attack on the quinone delivers the alkyne modules, treatment with SnCl2 aromatizes the six-mem-bered ring, while KOH in MeOH removes the TMS groups cleanly to give the elaborate monomer. 

The SC results strongly suggest that the Diels-Alder reaction between butadiene and ethene and the ring-opening of cyclohexadiene pass through aromatic conformations, while in the case of the 1,3-dipolar cycloaddition of fulminic acid to ethyne, the reacting system remains distinctly nonaromatic throughout the course of the reaction. 

It is frequent but not invariable that where a longer conjugated system has a geometrically accessible and symmetry-allowed transition structure like that in 5.90, the longer system is used. Thus, the [8+2] and [6+4] cycloadditions on pp. 15 16, and the [14+2] cycloaddition on p. 44 take place rather than perfectly reasonable Diels Alder reactions, and the 8-electron electrocyclic reactions of 4.51 and 4.54 takes place rather than disrotatory hexatriene-to-cyclohexadiene reactions. This kind of selectivity is called periselectivity. 

A triplex has been proposed as an intermediate in some photoreactions. Yang and co-workers [54] found that addition of 1,3-dienes to anthracene excimers leads to different products than does its adition to monomeric excited arenes. Lewis and co-workers [55, 56] found that stilbene excimers can be intercepted by dimethyl fumarate to give an oxetane through a presumed triplex. More recently, Schuster and co-workers [57,58] studied the Triplex Diels-Alder reaction of 1,3-dienes with enol, alkene, and acetylenic dienophiles. Take 9,10-dicyanonaphthalene (DCN)/indene (IN)/l,3-cyclohexadiene (CHD) system as an example ... 

The uncatalyzed Diels-Alder reaction is well known to be highly stereospecific, preferentially occurring via syn addition to both the diene and dienophilic components. Stereochemical studies of the cation radical Diels-Alder reaction have confirmed an analogous stereospecificity in two distinctly different systems. The initial study was carried out using the cycloaddition of the three geometric isomers of 2,4-hexadiene as dienophilic components and 1,3-cyclohexadiene as the diene component [39]. Each of the three isomers of the acyclic diene was found to add stereo-specifically to cyclohexadiene. In a more recent study, the cis and trans isomers of 1,2-diaryloxyethenes were found to add stereospecifically to 1,3-cyclopentadiene (Scheme 17) and also to 2,3-dimethyl-l,3-butadiene [46]. 

The above methods of synthesis have already used existent ring systems like difunctionalized benzene or cyclohexadienes as starting compounds. In an alternative approach one can generate benzene rings as part of the polymerization reaction, for example, by repetitive Diels-Alder reactions [100] or by valence isomerization [101]. Typical examples of the former include the combination of bisdienes, such as 54 and bisdienophiles, such as 55, whereby the formation of the benzene ring proceeds by extrusion of carbon monoxide from the original Diels-Alder products (scheme 9). The reaction thereby produces a mixture of para- and meto-linked polyphenylenes. 

A less-common activation parameter, the volume of activation (AF ), has been determined for a few Diels-Alder reactions carried out under pressure in liquid phase. The processes are cyclopentadiene dimerisation , isoprene dimerisation , addition of 2,3-dimethylbutadiene to butyl acrylate , addition of cyclopentadiene to dimethyl acetylene dicarboxylate , and addition of maleic anhydride to 1,3-cyclohexadiene, /rans-l-methoxybutadiene and isoprene . Activation volumes are negative, i.e. the reacting systems contract on passing from the initial to the transition state. In some cases the transition state appears to be even smaller than the adduct, independently of the solvent . Some of these experimental results gave rise to controversial interpretations however, the most recent ones favour a concerted four-center mechanism for the reaction. 

Zeolites can also be used to enhance the rate and selectivity of Diels-Alder reactions, particularly when used in conjunction with metal salts [159]. Ce-exchanged zeolite Y in conjunction with zinc bromide is an efficient catalyst system for reactions of a range of dienophiles with either cyclohexadiene (e.g. equation 4.40) or cyclopentadiene. 

The same problem was encountered during the study of a Diels-Alder reaction between p-benzoquinone (25) and cyclohexadiene (26) in the presence of capsule 242 (Scheme 9.3) [27, 28]. This system is a closed-shell capsule of approximately spherical shape, composed of two concave building blocks. When a mixture of the reactants was added to the capsule, only encapsulation of the quinone was observed. Cyclohexadiene was apparently encapsulated only in very small-yet sufficient-amounts, because the encapsulated product 27 of the Diels-Alder reaction was formed slowly over time. A 200-fold rate acceleration was observed in the presence of the capsule over the control reaction. Product inhibition prevented real turnover and therefore this capsule did not truly act as a catalyst True catalysis was subsequently observed when cyclohexadiene 26 was replaced by the stericaUy more-demanding thiophene dioxide derivative 28. In this case the product 29 of the Diels-Alder reaction with benzoquinone is expelled from the cavity as a result of its lowered affinity for the capsule [29]. 

That 1,2-cyclohexadiene (161, n = 3) is formed in a related system, from the reaction of 1-bromocyclohexene with potassium t-butoxide in dimethyl sulphoxide, is shown by trapping it by the highly reactive 1,3-diphenylisobenzofuran (162). The Diels-Alder adduct (163), differs from that of cyclohexyne (164) which was obtained from 1,2-dibromo-cyclohexene and Mg in the presence of (162). The product (164) did not isomerize to (163) (Wittig and Fritze, 1966). 

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