Cyclopentadienes in water

Figure A3.8.2 The correlation fimction k( ) for particular case of the reaction of methyl vinyl ketone with cyclopentadiene in water. The leveling-off of this function to reach a constant value at the plateau time tp is clearly seen.

Figure 1.5. Chemical potential of the initial state, the transition state and the product of the Diels-Alder reaction between methyl vinyl ketone and cyclopentadiene in water as compared to 1-propanol The data are taken from r. 56.

The cycloaddition of glyoxylic acid with cyclopentadiene in water at pH 6 and 60 °C is slow and occurs with low yield and low diastereoselectivity [18] (Scheme 6.17). Proton (pH = 0.9) [18], copper salts [27] and Bi(OTf)3 [28] accelerate the reaction and increase the diastereoselectivity. The lactones 28 and 29 originate from endo and exo cycloadducts 27, respectively. The proposed rearrangement is depicted in Scheme 6.17 for the major endo adduct 30. A competitive ene reaction that originates 28 and 29 cannot be excluded [28]. 

Copper-complexes prepared with other type of N-chelating ligands have been also prepared and evaluated as catalysts for the Diels-Alder reaction. Eng-berts et al. [103] studied enantioselective Diels-Alder reaction of 3-phenyl-l-(2-pyridyl)-2-propen-l-one with cyclopentadiene in water (Scheme 39). By using coordinating chiral, commercially available a-amino-adds and their derivatives with copper salts as catalysts, they obtained the desired product with yields generally exceeding 90%. With L-abrine (72 in Scheme 39) as chiral moiety, an enantiomeric excess of 74% could be achieved. Moreover, the catalyst solution was reused with no loss of enantioselectivity. 

The Diels-Alder reaction between oxazolone and cyclopentadiene in water was investigated by Cativiela (Eq. 12.35).96 Although the reaction is very slow, the ( )-5(4H)-oxazolone reacted with cyclopentadiene in an aqueous medium for six days at room temperature to form the corresponding sprioxazolones in a 95% yield. The cycloadducts were then readily converted into amino-norbomane carboxylic acids. 

Analogous studies on the dimerization of cyclopentadiene in water revealed a stabilization of the transition structure relative to the initial structure as a result of a difference in... 

Engberts and co-workers (Otto et al., 1996) reported a detailed study of a Diels-Alder reaction that was catalyzed by Lewis acids in water. They presented the results of the effects of Co Ni, Cu and Zn ions as Lewis acid catalysts on the rate and endo-exo selectivity of the DA reaction between the bidentate dienophiles 3-phenyl-l-(2-pyridyl)-2-propen-l-ones and cyclopentadiene in water (see fig. 6.6). Relative to the uncatalyzed reaction in acetonitrile, catalysis by 0.010 M Cu(N03)2 in water accelerates the Diels-Alder reaction by a factor of 79,300. Water does not induce an enhanced endo-selectivity for this reaction. 

Analogous studies on the dimerization of cyclopentadiene in water revealed a stabilization of the transition structure relative to the initial structure as a result of a difference in solvation of 1.7 kcalmol. Unfortunately, at least to our knowledge, reliable experimental data for this process are not available. Recently, in a similar approach, the Gibbs enthalpies of hydration of the Diels-Alder reaction of cyclopentadiene with isoprene and methyl vinyl ketone were determined. Surprisingly, it was observed that water stabilized the transition structure of the cyclopentadiene - - isoprene reaction more than that of the... 

The cycloaddition of a,a -dibromo (or dichloro) ketones with furan (or cyclopen-tadiene) gave very good yields when the reaction was conducted in pure water with iron powder. Furthermore, in the presence of triethylamine as the base, monobro-mo (or chloro) ketones react to furan (or cyclopentadiene) in water to afford the corresponding cycloadducts in near-quantitative yields (Eq. 5). In both cases, 2-oxy-allyl cation, the formation of which is favored in water, was considered as the reactive intermediate [54]. 

In the same manner Engberts and co-workers [23] used the copper (I I) salt of SDS as the catalyst in a Diels-Alder reaction of 3-(p-substituted phenyl)-l-(2-pyridyl)-2-propen-l-ones with cyclopentadiene in water as medium, and observed a spectacular enhancement of the reaction rate. 

The pX of cyclopentadiene in water, in which it is slightly soluble, IS 16 [12]. Its acidity is thus comparable with that of... 

Water has been shown to be the polar solvent of choice for the intramolecular Diels-Alder cycloaddition of immonium ions, e.g. (77), to produce the single tricyclic amine (78) (Scheme 28). The activation free energy and transmission coefficient for the Diels-Alder reaction between methyl vinyl ketone and cyclopentadiene in water have been calculated using molecular-dynamics and reaction-flux simulations. Water has been shown to enhance significantly the Ni +- and Co -catalysed Diels-Alder reaction between 3-phenyl-l-(2-pyridyl)prop-2-en-l-ones and... 

Grieco et al. have also reported the use of C-acyl iminium ions as heterodienophiles in the aqueous Diels-Alder reaction with cyclopentadiene [8]. This work details the cyclocondensation of C-acyl A-alkyl iminium ions 18 and C-acyl A-unsubstituted iminium ions 19 with cyclopentadiene in water ... 

The same researchers found that lanthanide triflates could catalyze aldol reactions and allylations in aqueous media [10]. However, Wang et al. exploited the potential of lanthanide triflates to act as stable Lewis-acid catalysts in the aqueous imino Diels-Alder reaction [11]. This variant of the aqueous heterocycloaddition protocol also expands the scope of such reactions. The use of higher aldehydes in such reactions generally met with limited success under the conventional protocol. For example, under the standard conditions, the reaction of hexanal and benzylamine hydrochloride with cyclopentadiene in water is sluggish and affords only 4% of the Diels-Alder adducts 26 and 27 in a 2.7/1 ratio. In sharp contrast, the addition of various lanthanide(lll) triflates (0.25 M) to this reaction results in substantial increases in both the rate and yield of Diels-Alder adduct formation. In particular, use of praseodymium(iii) triflate results in a 68% yield of adducts 26 and 27 ... 

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