Cycloaddition, of cyclopentadiene

Norbornadienes, norbornenones and their homologs have been prepared [23, 24] by cycloaddition of cyclopentadiene (21) and cyclohexadiene (22) with l-benzenesulfonyl-2-trimethylsilylacetylene (23) and l-ethoxy-2-carbomethox-yacetylene (24). Both were efficient dienophiles in the cycloaddition processes and dienophile 23 acted as an effective acetylene equivalent (Scheme 2.12). Norbornanes and their homologs can also be attained by Diels-Alder reaction... 

Clay-catalyzed asymmetric Diels-Alder reactions were investigated by using chiral acrylates [10]. Zn(II)- and Ti(IV)-K-10 montmorillonite, calcined at 55 °C, did not efficiently catalyze the cycloadditions of cyclopentadiene (1) with acrylates that incorporate large-size chiral auxiliaries such as cA-3-neopentoxyisobornyl acrylate (2) and (-)-menthyl acrylate (3, R = H) (Figure 4.1). This result was probably due to diffusion problems. 

Pagni and coworkers [18] have conducted in-depth investigations on the cycloadditions of cyclopentadiene with methyl acrylate on alumina of varing activity (200 300 400 800 °) showing that the diastereoselectivity of the... 

The importance of the relationship between the macrocycle cavity and the binding of two reagents is shown by the cycloadditions of cyclopentadiene with diethyl fumarate and ethyl acrylate in aqueous solution. The presence of jS-CD strongly accelerates the first cycloaddition, while it slows down the reaction rate of the second, probably because the cavity favors the binding of two molecules of either diene or dienophile [65c]. 

The cycloadditions of cyclopentadiene 1 and its spiro-derivatives 109 and 110 with quinones 52, 111 and 112 (Scheme 4.20), carried out in water at 30 °C in the presence of 0.5% mol. of cetyltrimethylammonium bromide (CTAB), gave the endo adduct in about 3 h with good yield [72b]. With respect to the thermal Diels-Alder reaction, the great reaction rate enhancement in micellar medium (Scheme 4.20) can be ascribed to the increased concentration of the reactants in the micellar pseudophase where they are also more ordered. 

The aqueous medium also has beneficial effects on the diastereoselectivity of the Diels-Alder reactions. The endo addition that occurs in the classical cycloadditions of cyclopentadiene with methyl vinyl ketone and methyl acrylate is more favored when the reaction is carried out in aqueous medium than when it is performed in organic solvents (Table 6.4) [2b, c]. 

The result of the cycloaddition of cyclopentadiene with methylbenzoquinone (Scheme 6.20) is also interesting. In 5.0m LP-DE, diasteroisomeric bis adducts are formed, while in the absence of LP-DE, only one 1 1 adduct is obtained quantitatively. 

The diastereoselectivity of the cycloaddition of cyclopentadiene with methyl acrylate in SC-CO2 at 40 °C and subcritical liquid CO2 at 22 °C is practically the same endojexo = 75 25 and 76 24 respectively) and is comparable to that found in hydrocarbon solvents (73 27 and 75 25 in heptane and cyclohexane, respectively). This shows that CO2, in these states, behaves like an apolar solvent with very low polarizability [82]. 

Secondary orbital interactions (SOI) (Fig. 2) [5] between the non-reacting centers have been proposed to determine selectivities. For example, cyclopentadiene undergoes a cycloaddition reaction with acrolein 1 at 25 °C to give a norbomene derivative (Fig. 2a) [6]. The endo adduct (74.4%) was preferred over the exo adduct (25.6%). This endo selectivity has been interpreted in terms of the in-phase relation between the HOMO of the diene at the 2-position and the LUMO at the carbonyl carbon in the case of the endo approach (Fig. 2c). An unfavorable SOI (Fig. 2d) has also been reported for the cycloaddition of cyclopentadiene and acetylenic aldehyde 2 and its derivatives (Fig. 2b) [7-9]. The exo-TS has been proposed to be favored over the endo- IS. 

Cross-linked polymers bearing IV-sulfonyl amino acids as chiral ligands were converted to polymer bound oxazaborolidine catalysts by treatment with borane or bromoborane. In the cycloaddition of cyclopentadiene with methacrolein, these catalysts afforded the same enantioselectivities as their non-polymeric counterparts238. 

Yamamoto and colleagues developed achiral boron catalysts 379 and 380a-b derived from monoacylated tartaric acid and BH3 -THF as shown for 379 in equation 112. The cycloaddition of cyclopentadiene to acrylic acid (381) afforded endo 382 with 78% ee and 93% yield when catalyst 379 was employed (equation 113)239. 

Yamamoto and colleagues showed that very high enantioselectivities and yields were obtained in the cycloadditions of cyclopentadiene with several a-substituted acrylic aldehydes using binaphthol catalyst 387 (equation 116). 

The high stereopreference was rationalized by considering complex 388 in which an attractive n-n donor-acceptor interaction favors co-ordination of the dienophile to the face of the boron center which is cis to the 2-hydroxyphenyl substituent. Hydrogen bonding of the hydroxyl proton of the 2-hydroxyphenyl group to an oxygen of the adjacent B—O bond played an important role in the asymmetric induction. Protection of this hydroxy functionality with a benzyl group caused reversal of enantioselectivity in the cycloaddition of cyclopentadiene with methacrolein (model 389)244. 

Reilly and Oh explored the asymmetric induction of chiral catalysts derived from bis(dichloroborane) 397 in the cycloaddition of cyclopentadiene with a-bromoacrolein and methacrolein. /V-Tosyltryplophan (394) and chiral diols 395 and 396 were employed as chiral ligands246,247. The application of chiral iV-tosyltryptophan afforded the best results (equation 118, Table 22). 

Recently, Yamamoto and coworkers249 reported the first examples of chiral induction in the cycloadditions of cyclopentadiene to propargylic aldehydes 402 using catalysts 380c, 387 and 393 (equation 119). The cycloadditions were stated to proceed via exo transition states and were accelerated by coordination of the Lewis acid to the carbonyl group. 

Davies and colleagues266 studied the use of copper(II) complexes of chiral bis(oxazoli-dine) 430 as catalysts in the cycloadditions of cyclopentadiene to substituted /V-acryloyl-l,3-oxazolidin-2-ones. They observed high endo and enantioselectivities. Again, the highest enantioselectivities were observed using SbEfi as the counterion, although differences were small this time ee values of 92 and 95% were obtained for the triflate and S h I Y> based catalysts, respectively. 

Ghosh and coworkers269 reported high enantioselectivities using catalyst 432 in the cycloadditions of cyclopentadiene to several Af-enoyl-1,3-oxazolidin-2-ones (equation 131). Recently, complex 425c was successfully applied in the cycloaddition of IV-acryloy 1-1,3-oxazolidin-2-one to furan (equation 132)270,271 and l-acetoxy-2-methyl-l,3-butadiene272. 

The strong dependence of the reaction rate on the catalyst concentration relative to control experiments in which the amino-hydrogen atoms of 7 were substituted by methyl groups demonstrate that hydrogen bonding represents the major interaction responsible for the observed accelerations. Diels-Alder reactions are also accelerated by hydrogen-bond donors. It was shown that a biphenylenediol 9 is able to catalyse [4 + 2]-cycloadditions of cyclopentadiene, 2,3-dimethylbutadiene and other simple dienes with various a,fi-unsaturated carbonyl compounds (Table 14)175. 

In subsequent studies, methyl vinyl ketone (2.0 mmole) was chosen as the dienophile so as to determine the combined effect of the ionic liquid (2 mL) and the Lewis acids (0.2 and 0.5 wt%) upon the yield and selectivity. Without the Lewis acid catalyst, this system demonstrated a 52% conversion of the cyclopentadiene (2.2 mmol) in 1 h with the endojexo selectivity being 85/15. The cerium triflate-catalyzed reaction was quantitative in 5 min and the endo. exo selectivity was very good for this experiment as well (94 6, endo. exo). Also with the scandium or yttrium salts tested, reactions came to completion in a short time with high stereo-selection. Cerium, scandium and yttrium triflates are strong Lewis acids known to be quite effective catalysts in the cycloadditions of cyclopentadiene with acyclic aldehydes, ketones, quinones and cycloalkenones. These compounds are expected to act as strong Lewis acids because of their hard character and the electron-withdrawing triflate group. On the other hand, reaction times of 1 hour were required for... 

Additional animations show the positive nature of the hydrogen being transferred during pyrolysis of ethyl formate and the fact that the two new carbon-carbon bonds are formed at dilferent rates during Diels-Alder cycloaddition of cyclopentadiene and acrylonitrile. 

Individual activation energies from BP, BLYP, EDFl and B3LYP density functional models are similar (and different from those of Hartree-Fock and local density models). They are both smaller and larger than standard values, but typically deviate by only a few kcal/mol. The most conspicuous exception is for Diels-Alder cycloaddition of cyclopentadiene and ethylene. Density functional models show activation energies around 20 kcaPmol, consistent with the experimental estimate for the reaction but significantly larger than the 9 kcal/mol value obtained from MP2/6-311+G calculations. Overall, density functional models appear to provide an acceptable account of activation energies, and are recommended for use. Results from 6-3IG and 6-311+G basis sets are very similar, and it is difficult to justify use of the latter. 

Table 9-4 Relative Activation Energies of Diels-Alder Cycloadditions of Cyclopentadiene and Electron-Deficient Dienophiles ... 

Consider, for example, endolexo selectivity in the Diels-Alder cycloaddition of cyclopentadiene and 2-butanone. In cyclopentadiene as a solvent, the observed endolexo product ratio is 80 20 (endo preferred), corresponding to a transition state energy difference on the order of 0.5 kcal/mol. With water as the solvent, this ratio increases to 95 5, corresponding to an energy difference on the order of 2 kcal/ mol. Hartree-Fock 6-3IG caleulations on the respective endo and exo transition states are largely in accord. Uncorrected for solvent, they show a very slight (0.3 kcal/mol) preference for endo in accord with the data in (non-polar) cyclopentadiene. This preference increases to 1.5 kcal/mol when the solvent is added (according to the Cramer/... 

Figures 15-1 and 15-2 provide evidence for the extent to which transition states for closely-related reactions are very similar. Figure 15-1 compares the transition state for pyrolysis of ethyl formate (leading to formic acid and ethylene) with that for pyrolysis of cyclohexyl formate (leading to formic acid and cyclohexene). Figure 15-2 compares the transition state for Diels-Alder cycloaddition of cyclopentadiene and acrylonitrile with both syn and anti transition states for cycloaddition of...

A second set of comparisons assesses the consequences of use of approximate reactant and transition-state geometries for relative activation energy calculations, that is, activation energies for a series of closely related reactions relative to the activation energy of one member of the series. Two different examples have been provided, both of which involve Diels-Alder chemistry. The first involves cycloadditions of cyclopentadiene and a series of electron-deficient dienophiles. Experimental activation energies (relative to Diels-Alder... 

As was mentioned, cycloaddition of unactivated hydrocarbons, namely, that of cyclopentadiene, has practical significance. 5-Vinyl-2-norbomene is produced by the cycloaddition of cyclopentadiene and 1,3-butadiene546,547 [Eq. (6.96)] under conditions where side reactions (polymerization, formation of tetrahydroindene) are minimal. The product is then isomerized to 5-ethylidene-2-norbomene, which is a widely used comonomer in the manufacture of an EPDM (ethylene-propylene-diene monomer) copolymer (see Section 13.2.6). The reaction of cyclopentadiene (or dicyclopentadiene, its precursor) with ethylene leads to norbomene548,549 [Eq. (6.97)] 550... 

Vinilydene to acetylene rearrangement 213 Cycloaddition of cyclopentadiene and ketenes 214... 

The intermolecular [4+2] cycloaddition of cyclopentadiene and iV-acetyl-2-azetine 23 occurs when they are heated in toluene in a sealed tube to give a good yield (83%) of the Diels-Alder adduct 317 (Equation 35). Similar high-yielding addition reactions occur with substituted cyclopentadienes and 1,3-diphenylisobenzofuran to give endo-adducts <1999TL443>. 

The reaction of methacrolein with cyclopentadiene catalyzed by a chiral menthoxyaluminum complex gives adducts with ee s of up to 72%, but with other dienophiles little, if any, induction was noted.9495 A chiral cyclic amido aluminum complex 2 catalyzes the cycloaddition of cyclopentadiene with the fran.v-crotyl derivative 3 in good yield and enantioselectivity (Scheme 26.2).47 This chiral catalyst can also be easily recovered. 

Diels-Alder [,4 + 2 cycloaddition of cyclopentadiene to methyl acrylate at 30°C. 

Clear-cut examples of effects of zeolite pore architecture on the selectivity of Diels Alder reactions are not easily found. For instance, 4-vinylcyclohexene is formed with high selectivity from butadiene over a Cu -Y zeolite however, the selectivity is intrinsically due to the properties of Cu1, which can be stabilized by the zeolite, and not to the framework as such (30-31). A simple NaY has been used in the cycloaddition of cyclopentadiene and non-activated dienophiles such as stilbene. With such large primary reactants, formation of secondary products can be impeded by transition state shape selectivity. An exemplary reaction is the condensation of cyclopentadiene and cis-cyclooctene (32) ... 

The chemistry of (l-alkynyl)carbene iron complexes is different from that of chromium and tungsten compounds. [4+2] cycloaddition of cyclopentadiene to a (l-alkynyl)carbene iron complex 11,m (R = SiMe3, r-Bu, c-QHn, n-Pr, Ph) affords (l-alkenyl)carbene Fe(CO)4 complexes 55, but these are readily isomerized at 50°C to Fe(CO)2 complexes 57 by insertion of carbon monoxide into the Fe = C bond of an intermediate Fe(CO)3... 

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