Cyclopentadienes and Cyclohexadienes

Cyclopentadienes and cyclohexadienes are versatile dienes that are commonly used to study mechanistic, regio- and stereochemical aspects of the Diels-Alder reaction and for synthetic purposes. 

The cycloaddition between 25 and 26 is a crucial step for the synthesis of methyl cA-dihydrojasmonate, a key constituent of the commercial jasmine fragrance Hedione 27. 

Optically active norbornene derivatives [26] have been prepared by cycloaddition of hexachlorocyclopentadiene with /-menthylacrylate and /-menthylallyl-ether (Equation 2.9). Low levels of enantiomeric excess have been obtained in the thermal processes, whereas Lewis acid catalyzed reactions (BF3, BBr3, AICI3, SnCU, DCM, 40-80 °C) gave better results. 

Furanones are a class of chiral dienophiles very reactive in thermal cycloadditions. For example, (5R)-5-(/-menthyloxy)-2-(5H)-furanone (28) underwent Diels Alder reaction with cyclopentadiene (21) with complete re-face-selectivity (Equation 2.10), affording a cycloadduct which was used as a key intermediate in the synthesis of dehydro aspidospermidine [27]. 

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By using unactivated K-10 montmorillonite in the absence of solvent, the endo-exo selectivity of the cycloadditions of acrolein and methyl vinyl ketone with cyclopentadiene and cyclohexadiene is low [8] (Table 4.2, entry 3), while highly reactive dienophiles such as 1,4-benzoquinone and N-phenyl... 

Raney-nickel was found to be selective in the hydrogenation of cyclopentadiene and cyclohexadiene and of their methyl and ethyl derivatives at 0-40 °C and 2-5 bar pressure137,138. The skeletal nickel proved to be selective in the semihydrogenation of conjugated polyenic compounds (equation 52)139. 

An unusual Diels-Alder cycloaddition involving the Cp=Cy bond has been described. The reaction took place by treatment of the electron-deficient allenylidene moiety in complex [RuCp(=C=C=CPh2)(CO)(P -Pr3)][BF4] (46) with a 20-fold excess of isoprene at room temperature affording the cycloadduct 90 (Scheme 33) [287]. This Diels-Alder cycloaddition in which the allenylidene moiety acts as a dienophile was completely regioselective, only the Cp=Cy bond of the allenylidene skeleton being implicated. Furthermore, it was also regioselective with regard to the orientation of the diene with the exclusive attack of C(l) and C(4) carbons at the Cp and Cy positions, respectively. Allenylidene 46 also underwent Diels-Alder reactions with cyclopentadiene and cyclohexadiene to afford the... 

Z)-4-[(5)-2,2-Dimethyl-l,3-dioxolan -ylmethylene]-2-phenyl-5(47/)-oxazolone 632 can react as a dienophile in diastereoselective Diels-Alder reactions. Thus, 632 undergoes a thermally induced Diels-Alder reaction with cyclic dienes, for example, cyclopentadiene and cyclohexadiene, to afford a mixture of the four... 

The influence of Lewis acids on the 4 + 2-cycloaddition of (2ft,2/ft)-A,iV/-fumaro-ylbis[fenchane-8,2-sultam] with cyclopentadiene and cyclohexadiene was investigated by IR studies of the sultam compexes with various Lewis acids.101 The first enantios-elective silicon Lewis acid catalyst (91) catalysed the Diels-Alder cycloaddition of methacrolein and cyclopentadiene with 94% ee.102 [A1C13 + 2THF] is a new and efficient catalytic system for the Diels-Alder cycloaddition of a,/9-unsaturated carbonyl compounds with dienes under solvent-free conditions.103 Dendritic copper(II) triflate catalysts with a 2,2 -bipyridine core (92) increased the chemical yields of Diels-Alder adducts.104... 

On the other hand, lower reaction temperature (0-30 °C) was indispensable to decrease the background reaction. Under these conditions, a wide range of other 0C, 3-unsaturated ketones and substituted 2-pyrones had been converted into bicyclic chiral compounds 17 in high yield, diastereomeric ratio, and enantiomeric excess (Table 10.8). Interestingly, theauthors noted that, incontrastto 2-pyrone, electron-rich dienes bearing neither a hydrogen-bond acceptor nor donor such as cyclopentadiene and cyclohexadiene were inactive for the Diels-Alder reaction with benzylideneace-tone catalyzed by lp and TFA. They propose that the activation of 2-pyrone by the multifunctional amine IP is also required for the D-A reaction to occur [30],... 

Molecular oxygen can add to the double bonds of olefins such as styrene, 1,1-diphenylethylene, dimethyl- and diethyl-ketene, vinyl acetate, cyclopentadiene and cyclohexadiene with formation of polymeric peroxy compounds. The course of the reaction corresponds to that of a mixed polymerization 1 r>2 3 4b d... 

Dienes cannot react with an unsaturated bond unless they are in a cisoid conformation the latter is fixed in some cyclic dienes such as cyclopentadiene and cyclohexadiene. but is equilibrated with other conformations in open-chain dienes, the transoid being usually more favoured, viz-... 

It is apparent from Table 6 that methyl and methoxy substituents enhance the reactivity of butadiene and anthracene, while chlorine atoms decrease that of butadiene and cyclopentadiene, towards electrophilic dienophiles. Phenyl substituents have very different effects in positions I and 2 [columns (i) and (iv)] here the substituent is large enough to exert a marked steric influence on the cisoid transo/c/equilibrium of the diene, so that the conformation of 2-phenyl-butadiene is favoured, and therefore the overall reactivity of 2-phenyl-butadiene is enhanced, while trans-1 -phenyl-butadiene is little affected. Cyclic unsubstituted dienes are more reactive than butadiene however, the difference between cyclopentadiene and cyclohexadiene is enormous the high reactivity of the former must be in part attributed to factors other than the cisoid conformation. 

The 4-I-2-cycloaddition of trimethylsilyl (TMS)-substituted cyclopentadiene with phosphorylated nitroso alkenes formed bicyclo 1,2-oxazines. However, as dienophiles, phosphorylated nitroso alkenes also react with cyclopentadiene and cyclohexadiene. The phosphoramidite-rhodium complex (84)-catalysed 4- -2-cycloaddition of a,fi-unsaturated imines with isocyanates produced pyrimidinones in good yields and with high enantioselectivities. ... 

The use of ketene equivalents in 4- -2-cycloaddition reactions for organic synthesis has been reviewed. " a-Carbonyl ketenes behave as dienes in Diels-Alder reactions with 4-aryl-2-methyl-2,3-dihydro-l,5-benzothiadiazepines to yield regiospecific cycloadducts. The reactions of diphenylketene with cyclic ( -cw)-l,3-dienes such as cyclopentadiene and cyclohexadiene initially yield the Diels-Alder adducts, which are converted into the Staudinger cyclobutanones by [3,3]-sigmatropic rearrangementsl-Benzyl-l,3-diazabuta-l,3-dienes react with ketenes to produce the 4 - - 2-cycloadducts, substituted l-benzyl-4-(benzylidenimino)-4-phenylazetidin-2-ones, which rearrange into the more stable 5,6-dihydro-3//-pyrimidin-4-ones. The formal 4 - - 2-cycloaddition reaction of ketenimines has been used to synthesize benzoimidazo[l,2-Z ]isoquinolines. ... 

As would be obvious from molecular models, cyclobutadiene would be highly strained and hence very reactive. There is no evidence of MA reacting with cyclobutadiene reported. However, Cava and Mitchell have successfully isolated the adduct of benzocyclobutadiene with A-phenyl-maleimide, using the latter as a trapping agent. The five- and six-membered dienes can easily achieve the necessary cisoid configuration and hence are very reactive. Cycloheptadiene forms an adduct easily but less readily than cyclopentadiene and cyclohexadiene. Individual cases are discussed below. 

The earliest studies on this type of polymerization can be traced back to research by Alder (1-3) and Staudinger (23, 24) on the oligomers obtained from cyclopentadiene and cyclohexadiene [Eq. (III-2)]. 

The dimerization of cyclopentadiene and cyclohexadiene with maleic anhydride affords only endo-forms. The dimerization of fiiran with maleic anhydride in xylene (400 K) gives 28% endo-fonn and 72% exo-form. 

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