Diels-Alder reaction butadiene equivalent

Benzoquinone is an excellent dienophile in the Diels-Alder reaction. What product would you expect from reaction of benzoquinone with 1 equivalent of 1,3-butadiene Prom reaction with 2 equivalents of 1,3-butadiene ... 

The Diels-Alder reaction is the best known and most widely used pericyclic reaction. Two limiting mechanisms are possible (see Fig. 10.11) and have been vigorously debated. In the first, the addition takes place in concerted fashion with two equivalent new bonds forming in the transition state (bottom center, Fig. 10.11), while for the second reaction path the addition occurs stepwise (top row, Fig. 10.11). The stepwise path involves the formation of a single bond between the diene (butadiene in our example) and the dienophile (ethylene) and (most likely) a diradical intermediate, although zwitterion structures have also been proposed. In the last step, ring closure results with the formation of a second new carbon carbon bond. Either step may be rate determining. 

Ab initio and density functional theoretical studies of the 4 + 2-cycloaddition of 2-azabutadiene with formaldehyde predict a concerted reaction that agrees well with experimental evidence.184 The azadiene A-plienyl-l-aza-2-cyanobuta-l,3-diene reacts with electron-rich, electron-poor, and neutral dipolarophiles under mild thermal conditions.185 5,6-Diliydro-4//-1,2-oxazines have been shown to be usefiil as synthon equivalents of 2-cyano-l-azabuta-1,3-dienes.186 The intramolecular Diels-Alder reaction of 1-aza-l,3-butadienes (106) can be activated by a 2-cyano substituent (Scheme 37).187 Stereoselectivity in the hetero-Diels-Alder reactions of heterobutadienes, nitrosoalkenes, and heterodienophiles has been extensively reviewed.188 The azadiene l-(f-butyldimethylsilyloxy)-l-azabuta-1,3 -diene (107) reacts with halobenzo-quinones, naphthoquinones, and A-phcnylmalcimidc to yield low to good yields of various pyridine heterocycles (108) (Scheme 38).189 The 4 + 2-cycloaddition of homophthalic anhydride with A-(cinnamylidcnc)tritylaminc produces the 3,4-adduct whereas with A -(cinnamylidcnc)bcnzylidinc the 1,2-adduct is produced.190... 

In a pericyclic reaction, electron counting can be effected in several ways, all equivalent. For example, in the Diels-Alder reaction, one can count the number of conjugated atoms in butadiene and in ethylene, or the number of bonds made (two o and one n bonds) or broken (three n bonds) in the process. In all cases, a total of six intervening electrons are obtained. 

The surface equivalent of the Diels-Alder reaction was first predicted to be the most important reaction by Konecny and Doren [57,58] who used DFT calculations to investigate the adsorption of 1,3-cyclohexadiene on Si(100). They obtained binding energies of 54kcalmol 1 for the [4 4- 2] product and SQkcalmoP1 for the [2 + 2] product, and also a negligible barrier for the [4+2] process. Concomitantly to this study, Bent and co-workers [59] carried out an infrared spectroscopic investigation of 1,3-butadiene and... 

When the diene is acyclic, [4+4] cycloaddition remains the primary reaction pathway even when the result is a highly reactive and unstable trans-alkene product, e.g., 150 and 151 (Sch. 35). With 1,3-butadiene, these intermediates are intercepted by an additional equivalent of the diene, to give the 2 1 adducts 152 and 153. When a diene is used that cannot achieve an s-cis conformation such as 154, Diels-Alder reaction with [4+4] adducts 155 and 156 is impossible and these compounds relieve strain via Cope rearrangement to give cyclobutanes 157 and 158, respectively [98]. An intramolecular version of this reaction has been reported [99]. 

Hetero-Diels-Alder reactions of l-oxa-l,3-butadienes with vinyl ethers, which lead to 3,4-dihydro-2H-pyran derivatives, are synthetically equivalent to Michael type conjugate additions. Wada and coworkers presented the first examples of a catalytic asymmetric intermolecular hetero-Diels-Alder reaction by the use of ( )-2-oxo-l-phenylsulfonyl-3-alkenes 25 and vinyl ethers 26 (Table 3) [25]. 

According to Pearson, whether the bonds being broken in the reagents correspond (after symmetrization) to the bonds being formed in the product is an adequate and equivalent criterion for the reaction to be allowed or forbidden in the sense of Woodward-Hoffinann. Apart from simplicity, the efficiency of the rule of bond symmetry is also ensured by the fact that it is equally applicable to it- and cr-bonds. Hence, its predictions are equally applicable to both the full-face ethylene dimerization (Scheme 4a) or to the Diels-Alder reaction (Scheme 4b) and to addition reaction, e.g., the addition of molecular hydrogen to ethylene or butadiene. 

Figure 13.21 Ring current in the transition structure of the Diels-Alder reaction (ethy-leneH-butadiene cyclohexene), (a) the six basis orbitals exhibit a cyclic overlap that is topologically equivalent with the % system of delocalized electrons In benzene.

Diels-Alder Reactions.—Cyclopentenone is a notoriously poor dienophile, but its a-phenylthio-derivative (140) reacts cleanly with excess butadiene (catalysed or uncatalysed) to give (141) or (142) after appropriate desulphurizations. " Thus, in the latter case, (140) acts as a cyclopentynone equivalent. 

The diol 4 had never been described in the literature before we used it in the synthesis of 1. Initially, we tried to synthesize it using an enantioselective Diels-Alder reaction between a chiral fumarate equivalent 5 (Scheme 10.1) and butadiene 6, followed by double-bond dihydroxylation [41]. However, a more effective protocol for large-scale synthesis could finally be based on the procedure shown in Scheme 10.2, leading to enantiomericaUy pure (lS,2S)-cyclohex-4-ene-1,2-dicarboxylic acid 7 [42]. Here, a Bolm desymmetrization of tetrahydrophthahc anhydride 8 using quinine leads to monoester 9 in 89% ee. The monoester 9, in turn, can be epimerized to the trans isomer 10 which is hydrolyzed to obtain 7, after crystallization. This intermediate is the starting material for the synthesis of 4 and of other conformationally constrained DCCHD to be used as monosaccharide mimics [43, 44]. Our current best protocol for the large-scale synthesis of 7 is reported at the end of this chapter. 

Treating 1,3-butadiene with HBr at elevated temperature gives the 1,4-adduct, which can be treated with sodium hydroxide to give an Sn2 process in which Br is replaced with a hydroxyl group. Oxidation with PCC converts the alcohol into an aldehyde, which can then be treated with another equivalent of 1,3-butadiene to give a Diels-Alder reaction that affords the product. 

Fig. 4.8 Representation of all non-equivalent bonds of the Ng2 C6o compound. The activation energies (in kcal mop ) corresponding to the Diels-Alder cycloaddition reaction between 1,3-butadiene and all non-equivalent bonds for all considered noble gas endohedral compounds. Ng2 Cjo has been represented on the right. A grey scale has been used to represent the different noble gases endohedral compounds black color is used to represent the helium-based fullerene, light grey for neon, medium grey for argon, dark grey for krypton, and white for xenon...

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