Catalysis 3, Diels-Alder

Kiindig EP, Saudan CM, Bemardinelli G (1999) A stable and recoverable chiral Ru Lewis acid synthesis, asymmetric Diels-Alder catalysis and stmcture of the Lewis acid methacro-lein complex. Angew Chem Int Ed 38 1220-1223... 

Scheme 8.4S. Simple Diels—Alder catalysis by zirconocenium triflate.

Scheme 8.46. Diels—Alder catalysis by cationic alkoxyzirconocene complexes.

Asymmetric Diels—Alder catalysis was more successful with dication-like versions of the Zr-EBTHI system, and using conformationally better defined acyl-oxazoline dieno-philes. The bis(triflate) [Zr(EBTHI)(OTf)2] (Scheme 8.47) induced high levels of ee (>90%) in the cycloaddition to cyclopentadiene at low temperatures, especially in the polar solvent 2-nitropropane [87]. 

Scheme 8.47. Asymmetric Diels-Alder catalysis by an ansa-metallocene triflate.

Scheme 8.48. Accelerated Diels—Alder catalysis with BARF counterions.

Cationic zirconocenes serve as useful reagents in such diverse fields as alkene polymerization, carbohydrate chemistry, asymmetric catalysis, and so on. Reagents that were originally developed for polymerization reactions (MAO, ansa-metallocenes, non-nucleophi-lic borate counterions) have now found use in organic synthesis and are being employed for carbometalation reactions, hydrogenation, and Diels—Alder catalysis. 

Diels-Alder catalysis.1 This radical cation can increase the endo-selectivity of Diels-Alder reactions when the dienophile is a styrene or electron-rich alkene. This endo-selectivity obtains even in intramolecular Diels-Alder reactions. Thus the triene 2, a mixture of (Z)- and (E)-isomers, cyclizes in the presence of 1 to 0° to the hydroindanes 3 and 4 in the ratio 97 3. Similar cyclization of (E)-2 results in 3 and 4 in the ratio 98 2 therefore, the catalyst can effect isomerization of (Z)-2 to (E)-2. Even higher stereoselectivity is observed when the styrene group of 2 is replaced by a vinyl sulfide group (SC6H5 in place of QHtOCT ). 

Diels-Alder catalysis. Water can accelerate the Diels-Alder reaction of cyclopentad iene with methyl vinyl ketone or acrylonitrile by a hydrophobic interaction. / -Cyclodextrin can increase this effect, possibly because the components can fit into the hydrophobic cavity.1... 

Fig. 6.31 Catalytically active dendritic bis(oxazoline) ligands 2-4 and reference compound 1 for Diels-Alder catalysis (according to Chow et al.)...

Diels-Alder catalysis,3 Several Rh(I) complexes are remarkably efficient catalysts for intramolecular cyclization of electronically neutral dienynes or trienes. The... 

Another important research direction is the mimieking of enzymes and the construction of selective catalysts. For these purposes, the polymer is imprinted with the desired reaetion-product or better, a molecule which resembles the transition state of the reaction adducts. If the educts bind specifically to the recognition site, they become confined into these micro-reactors and are supposed to react faster and more defined than outside the cavities [445]. Examples for reactions in the presence of such synthetic enzymes can be found in [452,453,454,455,456,457] (cf Figure 40c). First positive results have been reported, e.g. an synthetic esterase , increasing the rate of alkaline hydrolysis of substituted phenyl-(2-(4-carboxy-phenyl)-acetic esters for 80 times [488] and Diels-Alder catalysis fiuic-tional holes containing titanium lewis-acids [489]... 

Santora BP, Larsen AO, Gagne MR (1998) Toward the molecular imprinting of titanium lewis acids Demonstration of diels-alder catalysis. OrganometaUics 17 3138... 

Chiral (helical) Lewis acids for asymmetric Diels-Alder catalysis are prepared from titanium tetraisopropoxide 5 and a chiral binaphthol ligand 4 [13]. The titanium reagent 6 plays an important role as chiral template for the fixation of a,fi-unsaturated aldehydes and thereby for the enantioface recognition of substrates. The asymmetric Diels-Alder reaction, e. g., of cyclopentadiene 7 and acrolein 8, is effected in the presence of catalytic (P)-6 (10 mol%), producing the endo adduct 9 (R =R2=H) in 88% ee (Scheme 1). 

Diels-Alder catalysis.2 This cation radical enhances the reactivity of a neutral or electron-rich cw-1,3-diene in Diels-Alder reactions. Thus 1,3-cyclohexadiene undergoes Diels-Alder dimerization only at temperatures around 200°. The presence of 5-10 mole % of this salt effects dimerization even at —78°, with the usual endoj exo selectivity (5 1). It also permits facile condensation of 1,3-cyclohexadiene with a hindered dienophile such as 2,5-dimethyl-2,4-hexadiene (equation I) the dimer of the former diene is a minor product (20% yield). 

Hersh and co-workers reported that catalysis for the Diels-Alder reactions of cy-clopentadiene with a, ff-unsaturated enones was induced by (Me3P)(CO)3(NO)W-FSbFs. In order to probe the mechanism of the Diels-Alder catalysis, a single-crystal X-ray diffraction study of [(Me3P)(CO)3(NO)(acrolein)W] SbF was carried out, and CT-type coordination was found to be present in the solid state [25]. This structure provides clear evidence for the preferred s-trans conformation of the a,//-unsaturated aldehyde unit, which would be expected by theoretical studies (Fig. 1-7). 

The controversy on the existence of in vivo Diels-Alder reactions cannot be put to rest here, but the numerous examples of natural products containing cyclohexene groups and the catalytic effectivity of biological surroundings support the idea of in vivo Diels-Alder reactions. Apart from cell-free extracts, RNA-based mixtures of metals also show catalytic activity and it was demonstrated that this catalyst system can be quite effective as an artificial Diels-Alder-ase . We will show that water, the prime solvent of biosynthesis, also catalyses [4 -+- 2]-cycloadditions. Considering that biosyntheses are often of exceptional selectivity, it is clear that understanding biomimetic transfonna-tions in water as the solvent is an important goal of modem chemistry. The possibilities offered by and the reasons for Diels-Alder catalysis in water will be the main topic of this chapter. 

Although the choice of the anion (such as trifiate, TfO ) in the Cu° complex did not affect the overall conversion in Diels-Alder catalysis, the stereoselectivity of the reaction was considerably influenced by the choice of anion used. The authors used EPR to investigate a series of Cu catalysts bearing TfO, SbFs, Cl and Br anions in the presence and absence of the dienophile [133]. The profile of the EPR spectra were significantly different for the complexes bearing the halide anions compared to those bearing the bulkier TfO and SbFg" anions, and the spectra were also notably dependent on the presence of the dienophile. 

J. Chen, Q. Deng, R. Wang, K. Honk, D. HUvert, Shape complementarity, binding-site dynamics, and transition state stabilization a theoretical study of Diels-Alder catalysis by antibody 1E9, Chembiochem, 2000, 1, 255-261. 

Santora, B.P. Larsen, A.O. Gagne, M.R. Toward the molecular imprinting of titanium Lewis acids demonstration of Diels-Alder catalysis. Organometallics 1998, 17, 3138-3140. 

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