Diels catalysts structure

Kobayashi and co-workers exploited the use of lanthanide in a variety of achiral reactions and extended them into several catalytic asymmetric reactions. Their work commenced with catalytic asymmetric Diels-Alder reactions [32], The reaction was performed with a chiral ytterbium catalyst prepared from Yb(OTf)3, binaphthol and a tertiary amine. The amine significantly influenced reaction selectivity. When triethyl-amine was used in the preparation of the catalyst, the desired product was obtained in moderate ee (33%) (Table 8, entry 1). After screening several reaction conditions, they found that, in general, bulky amines gave better results (entries 2-6). They suggested interesting explanations of this experimental result on the basis of investigations into catalyst structure. Consequently, the use of cw-l,2,6-trimethylpiperidine combined with 4 A molecular sieves (4A MS) was found to produce the best result (yield 77%, endolexo = 89/11, endo = 95% ee) (entry 6). 

The first example of appreciable enantioselectivity in a catalytic asymmetric Diels-Alder reaction was reported in 1979 by Koga (Equation 14) [73]. This was achieved by use of 14 mol % of an aluminum complex resulting from reaction of menthol and EtAlCl2, as formulated in the proposed catalyst structure 133. In the presence of this catalyst, cycloadduct 134 was obtained in 57 % ee and 69 % yield. This result provided an important proof of principle that catalytic enantioselective Diels-Alder reactions can indeed be achieved with chiral Lewis acids [73, 74). 

Yamamoto developed a remarkable boron-derived catalyst for enantioselec-tive Diels-Alder reactions which is easily assembled from monoacylated tartaric acid and borane. Spectroscopic data provided evidence that supports the proposed catalyst structure 144 depicted in Equation 16 [79, 80]. Such chiral (acyloxy)borane (CAB) catalysts have been employed in numerous cyclo-additions with unsaturated aldehydes to afford the corresponding products, such as 145, with high selectivity (98% ee, endo exo > 99 1) [80]. 

The monomer, norbomene (or bicyclo[2.2.l]hept-2-ene), is produced by the Diels-Alder addition of ethylene to cyclopentadiene. The monomer is polymerised by a ring-opening mechanism to give a linear polymer with a repeat unit containing both an in-chain five-membered ring and a double bond. Both cis-and trans- structures are obtainable according to the choice of catalyst used ... 

Corey et al. reported that the catalyst 19, prepared from trimethylaluminum and the bis-trifluorosulfonamide of stilbenediamine (stien), with generation of methane, is a suitable catalyst for the Diels-Alder reaction of 3-acryloyl, and 3-crotonoyl-l,3-oxazo-lidin-2-ones, giving the cycloadducts in high optical purity [28] (Scheme 1.35, Table 1.14). X-ray structure analysis of the catalyst and and NMR studies revealed that... 

In the 1,3-dipolar cycloaddition reactions of especially allyl anion type 1,3-dipoles with alkenes the formation of diastereomers has to be considered. In reactions of nitrones with a terminal alkene the nitrone can approach the alkene in an endo or an exo fashion giving rise to two different diastereomers. The nomenclature endo and exo is well known from the Diels-Alder reaction [3]. The endo isomer arises from the reaction in which the nitrogen atom of the dipole points in the same direction as the substituent of the alkene as outlined in Scheme 6.7. However, compared with the Diels-Alder reaction in which the endo transition state is stabilized by secondary 7t-orbital interactions, the actual interaction of the N-nitrone p -orbital with a vicinal p -orbital on the alkene, and thus the stabilization, is small [25]. The endojexo selectivity in the 1,3-dipolar cycloaddition reaction is therefore primarily controlled by the structure of the substrates or by a catalyst. 

The cationic aqua complexes prepared from traws-chelating tridentate ligand, R,R-DBFOX/Ph, and various transition metal(II) perchlorates induce absolute enantio-selectivity in the Diels-Alder reactions of cyclopentadiene with 3-alkenoyl-2-oxazoli-dinone dienophiles. Unlike other bisoxazoline type complex catalysts [38, 43-54], the J ,J -DBFOX/Ph complex of Ni(C104)2-6H20, which has an octahedral structure with three aqua ligands, is isolable and can be stored in air for months without loss of catalytic activity. Iron(II), cobalt(II), copper(II), and zinc(II) complexes are similarly active. 

Although the aqua nickel(II) complex A was confirmed to be the active catalyst in the Diels-Alder reaction, no information was available about the structure of complex catalyst in solution because of the paramagnetic character of the nickel(II) ion. Either isolation or characterization of the substrate complex, formed by the further complexation of 3-acryloyl-2-oxazolidinone on to the l ,J -DBFOX/ Ph-Ni(C104)2 complex catalyst, was unsuccessful. One possible solution to this problem could be the NMR study by use of the J ,J -DBFOX/Ph-zinc(II) complex (G and H, Scheme 7.9) [57]. 

The theoretical investigations of Lewis acid-catalyzed 1,3-dipolar cycloaddition reactions are also very limited and only papers dealing with cycloaddition reactions of nitrones with alkenes have been investigated. The Influence of the Lewis acid catalyst on these reactions are very similar to what has been calculated for the carbo- and hetero-Diels-Alder reactions. The FMOs are perturbed by the coordination of the substrate to the Lewis acid giving a more favorable reaction with a lower transition-state energy. Furthermore, a more asynchronous transition-structure for the cycloaddition step, compared to the uncatalyzed reaction, has also been found for this class of reactions. 

Ghosh et al. [70] reviewed a few years ago the utihty of C2-symmetric chiral bis(oxazoline)-metal complexes for catalytic asymmetric synthesis, and they reserved an important place for Diels-Alder and related transformations. Bis(oxazoline) copper(II)triflate derivatives have been indeed described by Evans et al. as effective catalysts for the asymmetric Diels-Alder reaction [71]. The bis(oxazoline) Ugand 54 allowed the Diels-Alder transformation of two-point binding N-acylimide dienophiles with good yields, good diastereos-electivities (in favor of the endo diastereoisomer) and excellent ee values (up to 99%) [72]. These substrates represent the standard test for new catalysts development. To widen the use of Lewis acidic chiral Cu(ll) complexes, Evans et al. prepared and tested bis(oxazoHnyl)pyridine (PyBOx, structure 55, Scheme 26) as ligand [73]. 

Bis(oxazohnes) figands have been so widely used for the Diels-Alder reaction between N-2-alkenoyl-l,3-oxazolidine-2-one and cyclopentadiene that Lipkowitz and Pradhan developed a QSAR (quantitative structure-activity relationship) using Comparative Molecular Field Analysis (CoMFA) for a set of 23 copper-catalysts containing mainly bis(oxazoline) figands. The generated... 

The above-described structures are the main representatives of the family of nitrogen ligands, which cover a wide spectrum of activity and efficiency for catalytic C - C bond formations. To a lesser extent, amines or imines, associated with copper salts, and metalloporphyrins led to good catalysts for cyclo-propanation. Interestingly, sulfinylimine ligands, with the chirality provided solely by the sulfoxide moieties, have been also used as copper-chelates for the asymmetric Diels-Alder reaction. Amide derivatives (or pyridylamides) also proved their efficiency for the Tsuji-Trost reaction. 

In 1994, Kiindig et al. described a related catalyst, in which both CO ligands have been replaced by a chiral F,f -chelate ligand providing evidence that structurally defined cationic Fe sandwich complexes are indeed efficient catalysts for Diels-Alder reactions [33]. 

Fig. 6.6. Relative energies of four possible transition structures for Diels-Alder reaction of 1,3-butadiene and propenal, with and without BF3 catalyst. Geometric parameters of the most stable transition structures (.endo-cis) are shown. Adapted from J. Am. Chem. Soc., 120, 2415 (1998), by permission of the American Chemical Society.

The antibody-catalyzed Diels-Alder reaction developed by Schultz utilized a Diel-Alderase enzyme-like catalyst evolved from an antibody-combining site (Eq. 12.13). The idea is that the generation of antibodies to a structure that mimics the transition state for the Diels-Alder reaction should result in an antibody-combining site that lowers the entropy of activation by binding both the diene and dienophile in a reactive conformation. 

Figure 39 The enantioselective polymer-supported catalysts (61) of chiral oxazaborolidinone with cross-linking structures for use in the Diels-Alder reaction of methacrolein with cyclopentadiene. (Adapted from ref. 85.)...

Theoretical calculations have also permitted one to understand the simultaneous increase of reactivity and selectivity in Lewis acid catalyzed Diels-Alder reactions101-130. This has been traditionally interpreted by frontier orbital considerations through the destabilization of the dienophile s LUMO and the increase in the asymmetry of molecular orbital coefficients produced by the catalyst. Birney and Houk101 have correctly reproduced, at the RHF/3-21G level, the lowering of the energy barrier and the increase in the endo selectivity for the reaction between acrolein and butadiene catalyzed by BH3. They have shown that the catalytic effect leads to a more asynchronous mechanism, in which the transition state structure presents a large zwitterionic character. Similar results have been recently obtained, at several ab initio levels, for the reaction between sulfur dioxide and isoprene1. ... 

In 1990, Choudary [139] reported that titanium-pillared montmorillonites modified with tartrates are very selective solid catalysts for the Sharpless epoxidation, as well as for the oxidation of aromatic sulfides [140], Unfortunately, this research has not been reproduced by other authors. Therefore, a more classical strategy to modify different metal oxides with histidine was used by Moriguchi et al. [141], The catalyst showed a modest e.s. for the solvolysis of activated amino acid esters. Starting from these discoveries, Morihara et al. [142] created in 1993 the so-called molecular footprints on the surface of an Al-doped silica gel using an amino acid derivative as chiral template molecule. After removal of the template, the catalyst showed low but significant e.s. for the hydrolysis of a structurally related anhydride. On the same fines, Cativiela and coworkers [143] treated silica or alumina with diethylaluminum chloride and menthol. The resulting modified material catalyzed Diels-Alder reaction between cyclopentadiene and methacrolein with modest e.s. (30% e.e.). As mentioned in the Introduction, all these catalysts are not yet practically important but rather they demonstrate that amorphous metal oxides can be modified successfully. 

While it is one of the most important and versatile transformations available to organic chemists, there is no unequivocal example of a biological counterpart. Hence, attempts to generate antibodies which could catalyse this reaction were seen as an important target. The major task in producing a Diels-Alderase antibody lies in the choice of a suitable haptenic structure, because the transition state for the reaction resembles product more closely than reactants (Fig. 12). The reaction product itself is an inappropriate hapten because it is likely to result in severe product inhibition of the catalyst, thereby preventing turnover. 

Generally, at least in theory, an important aspect of cation-radical polymerization, from a commercial viewpoint, is that either catalysts or monomer cation-radicals can be generated electrochem-ically. Such an approach deserves a special treatment. The scope of cation-radical polymerization appears to be very substantial. A variety of cation-radical pericyclic reaction types can potentially be applied, including cyclobutanation, Diels-Alder addition, and cyclopropanation. The monomers that are most effectively employed in the cation-radical context are diverse and distinct from those that are used in standard polymerization methods (i.e., vinyl monomers). Consequently, the obtained polymers are structurally distinct from those available by conventional methods although the molecular masses observed so far are still modest. Further development in this area would be promising. 

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