Cyclopentadiene reaction with benzoquinone

The asymmetric Diels-Alder cycloadditions of enantiopure (5)-5-(/ -tolylsulfinyl)-1,4-benzoquinones with Dane s diene under thermal and Lewis acid conditions produce tetracyclic quinones after spontaneous elimination of the sulfinyl group.The Diels-Alder reaction of barrelene with o-benzoquinone produces tetracyclo[6.2.2.2 .0 ]tetradeca-9,ll,13-triene-4,5-dione. Under kinetic control, the Diels-Alder cycloaddition of 2,3-dicyano-p-benzoquinone (98) with cyclopentadiene in MeOH produces the single cycloadduct (99) (Scheme 38). ... 

A few other reactions have been reported with catalysts 4 and 143 these are shown in Sch. 17. Northcott and Valenta investigated the reaction of the exo-cyclic methyle-nedienophiles 159 and 161 and the benzoquinone 163 with cyclopentadiene [39]. The absolute configurations of the products were not determined for these three reactions but it was determined that eatalyst 4 and catalyst ent-143 gave the same major enantiomer of 164 but different major enantiomers of 160. 

Marchand and coworkers102 reported a difference in site selectivity between the thermodynamically and kinetically controlled Diels-Alder reactions of cyclopentadiene with 2,3-dicyano-p-benzoquinone (126) (equation 37). Under kinetic conditions, the more reactive double bond of 126 reacted with cyclopentadiene affording 127, whereas the less substituted double bond reacted under thermodynamic conditions affording 128. Both reactions proceeded with complete endo selectivity. These findings were in agreement with ab initio HF/3-21G calculations. 

Dipole-dipole interactions have been used to assess the conformational populations of 2-haloketones (Eliel et al., 1965). With respect to SS, however, there are few applications in which these and related effects are considered. It is interesting that dipole induction and London dispersion effects were used some thirty years ago to account for the high endo over exo preference in the Diels-Alder reaction (Wassermann, 1965). Although effects are small for any pair of atoms, there are many closely packed atoms in a Diels-Alder transition state. At a carbon-carbon distance of 2-0 a between the atoms to be bonded, the energy favoring endo addition is 2-7 for dipole induction and 3-4 kcal/mole for dispersion in the reaction of cyclopentadiene with p-benzoquinone (Wassermann, 1965). These nonbonding attractive energies cooperate with the secondary HMO effects discussed earlier to lead to an endo product. 

Similar reactions have been reported for acenaphthylenes, cyclopentenes, p-benzoquinone, nor-bornadiene, cyclopentadiene, 1,3-cyclohexadiene, and 1,5-cyclooctadiene <78CB3037>. The reaction of dimethyl-7-oxabicyclo[2.2. l]hepta-2,5-diene-2,3-dicarboxylate (120) with the 1,3-dithiolone (113) proceeds with the formation of the cycloadduct (121) (Scheme 28) pyrolysis or photolysis of this adduct (121) resulted in a double fragmentation to give the thiophene (122) and the furan (123) <75CC840>. 

Various approaches to the assembly of the 1,3-dienes that engage in the Diels-Alder cycloaddition reactions leading to substrates for DPM and ODPM processes have been reported. Thus, for example. Yen and Liao demonstrated, during the course of a total synthesis of the Lycopodium alkaloid magellanine, that oxidation of acetovanillone (109) rScheme 9.151 with diacetoxyiodobenzene (DAIB) in the presence of methanol afforded the o-benzoquinone monoketal 110. The latter conpound engaged in an in situ Diels-Alder reaction with added cyclopentadiene (111) and the resulting adduct 112 proved to be an excellent substrate for the ODPM rearrangement reaction. Thus, photolysis of 112 as a solution in acetone afforded the pivotal tetracyclic diketone 113 in 92% yield. 

A review of the cycloaddition reactions of o-benzoquinones as carbodiene, heterodiene, dienophile, or heterodienophile has been published. In the Diels-Alder reaction of furans with masked o-benzoquinones (145), the furans unexpectedly behaved as dienophiles to yield cycloadducts (146) (Scheme 56). Masked benzoquinones behave as dienes which undergo Diels-Alder reactions with electron-rich dienophiles such as enol ethers and thienol ethers.The asymmetric Diels-Alder reactions of 5-substituted and 5,6-disubstituted (S)-2-(p-tolylsulflnyl)-l,4-benzoquinones with cyclopentadiene and fran -piperylene show complete regio- and jr-facial selectivities. The hetero-Diels-Alder reactions of o-benzoquinones with tetracyclone produce cyclopenta[I ][l,4]benzodioxinone derivatives in high yield. 

The reversibility of the Diels-Alder reaction has been put to good use in the protection of olefins. Ardis [69] used the butadiene adduct to protect the olefin in his synthesis of vinylidene cyanide. Alder [70] was able to monoepoxidise various benzoquinones as their cyclopentadiene adducts, and Chapman [71] used the same method to obtain half-reduction of benzoquinone. Regeneration of the olefin is achieved thermally and Alder [70] noted that this was easier with the fulvene adduct than with the cyclopentadiene adduct. Sauer [72] observed that the retro-Diels-Alder reaction was easier with adducts involving furan, fulvene, and anthracene as the diene component, and he made the anhydride (8) using this method. 

Acids accelerate the Diels-Alder reaction. The rate of the catalytic reaction is proportional to the acid concentration and the product of the concentrations of the reactants. The activation energy of the catalytic reaction is low. For the reaction of cyclopentadiene with p-benzoquinone catalyzed by trichloroacetic acid, it is equal to zero, and the constant k = 6.8 P mol /s at 298 K. The reaction occurs, evidently, through the intermediate formation of the carbocation. 

The photo-induced exo selectivity was observed in other classic Diels-Alder reactions. Data relating to some exo adducts obtained by reacting cyclopentadiene or cyclohexadiene with 2-methyl-1,4-benzoquinone, 5-hydroxynaphtho-quinone, 4-cyclopentene-l,3-dione and maleic anhydride are given in Scheme 4.13. The presence and amount of EtsN plays a decisive role in reversing the endo selectivity. The possibility that the prevalence of exo adduct is due to isomerization of endo adduct under photolytic conditions was rejected by control experiments, at least for less reactive dienophiles. 

The potential activation of different Lewis acid catalysts and their load effect when used in combination with this solvent were explored, in order to determine the improvement of rates and selectivity to the endo and exo isomers. The list of Lewis acid catalysts included Li(OTf), Li(NTf2), Znl2, AICI3, BF3, HOTf, HNTf2, Ce(0Tf)4 5H20, Y(OTf)3, Sc(OTf)3, Sc(NTf2) and a blank without any Lewis acid. The reaction conditions were as follows 2.2 mmol of cyclopentadiene + 2.0 mmol of dienophile + 0.2 mol% of catalyst in 2 mL [hmim][BF4]. When no catalyst was added, the two ketones (R =Me-C=0 R2 = R3 = H and Ri=Et-C=0 R2 = R3 = H) showed modest activity ( 50% in 1 h) with endojexo selectivity = 85/15. Whereas acrolein showed modest activity (59% conversion in 2 h), methacrolein and crotonaldehyde were inert without a Lewis acid catalyst. Acrylonitrile and methyl acrylate underwent low conversions in 1 h (16-17%) whereas, N-phenylmaleimide, maleic anhydride and 2-methyl-1,4-benzoquinone showed complete reaction in 5 min with high endo isomer yields. 

After Buchi confirmed this finding in the 19S0s, numerous other investigators immediately recognized the rapidity with which complex molecular architectures could be constructed by this reaction. Cookson irradiated the Diels-Alder adduct (3) of cyclopentadiene and benzoquinone to produce the caged diketone (4) (equation 2) while Eaton utilized a photochemical cycloaddition of diene (5) in a synthesis of the platonic solid cubane (6) (equation 3). ... 

Titanium alcoholates of diols 2.50 generated by exchange with (/-PrO TiC are also potent catalysts, and the most efficient catalyst is derived from 2.50 (R = Me, R = Ar=Ph). These catalysts promote the diastereo- and enantioselective cycloaddition of cyclopentadiene or acyclic dienes with some a,P-unsaturated imides 7.68 (R = H, Rg=Me, MeOOC) [45, 778] or of acyclic dienes with substituted benzoquinones at -78°C [1572, 1573] (Figure 9.20). Reactions occur at 0°C in the presence of molecular sieves, and only catalytic amounts of these alcoholates are used. Intramolecular cycloadditions also give useful sdectivities under these conditions [1574]. However, the process for preparation of the catalyst varies according to the reaction [778, 1573], and this is veiy important to the observation of high selectivities. 

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