Cyclopentadiene complexes with copper

Since Evans s initial report, several chiral Lewis acids with copper as the central metal have been reported. Davies et al. and Ghosh et al. independently developed a bis(oxazoline) ligand prepared from aminoindanol, and applied the copper complex of this ligand to the asymmetric Diels-Alder reaction. Davies varied the link between the two oxazolines and found that cyclopropyl is the best connector (see catalyst 26), giving the cycloadduct of acryloyloxazolidinone and cyclopentadiene in high optical purity (98.4% ee) [35] (Scheme 1.45). Ghosh et al., on the other hand, obtained the same cycloadduct in 99% ee by the use of unsubstituted ligand (see catalyst 27) [36] (Scheme 1.46, Table 1.19). 

It is reported that the intermediate species in the CujO-Icrt-BuNC reaction system can undergo reactions with cyclopentadiene or with indene. The products obtained are C5HjCu(CNBu ) and indenylCu(CNBu )3 132). The postulate that a copper(I) isocyanide complex is the reaction intermediate here is reasonable. 

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. 

As anticipated from the complexation experiments, reaction of 4.42 with cyclopentadiene in the presence of copper(II)nitrate or ytterbium triflate was extremely slow and comparable to the rate of the reaction in the absence of Lewis-acid catalyst. Apparently, Lewis-acid catalysis of Diels-Alder reactions of p-amino ketone dienophiles is not practicable. 

Fortunately, in the presence of excess copper(II)nitrate, the elimination reaction is an order of magnitude slower than the desired Diels-Alder reaction with cyclopentadiene, so that upon addition of an excess of cyclopentadiene and copper(II)nitrate, 4.51 is converted smoothly into copper complex 4.53. Removal of the copper ions by treatment with an aqueous EDTA solution afforded in 71% yield crude Diels-Alder adduct 4.54. Catalysis of the Diels-Alder reaction by nickel(II)nitrate is also... 

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. 

In 2005, Carretero et al. reported a second example of chiral catalysts based on S/P-coordination employed in the catalysis of the enantioselective Diels-Alder reaction, namely palladium complexes of chiral planar l-phosphino-2-sulfenylferrocenes (Fesulphos). This new family of chiral ligands afforded, in the presence of PdCl2, high enantioselectivities of up to 95% ee, in the asymmetric Diels-Alder reaction of cyclopentadiene with A-acryloyl-l,3-oxazolidin-2-one (Scheme 5.17). The S/P-bidentate character of the Fesulphos ligands has been proved by X-ray diffraction analysis of several metal complexes. When the reaction was performed in the presence of the corresponding copper-chelates, a lower and opposite enantioselectivity was obtained. This difference of results was explained by the geometry of the palladium (square-planar) and copper (tetrahedral) complexes. 

Evans and coworkers262 demonstrated the utility of bis(oxazolidine)copper(II) complexes 425 as Lewis acid catalysts in Diels-Alder reactions of iV-enoyl-l,3-oxazolidin-2-ones 423 with cyclopentadiene, which gave adducts 424 (equation 128, Table 25). Their best results were obtained using catalyst 425c. Surprisingly, only 30% ee was obtained in the reaction between cyclopentadiene and 17a when catalyzed by 425a. Similar results were obtained for the thiazolidine-2-thione analogs of the iV-enoyl-l,3-oxazolidin-2-ones. 

The cationic aqua complexes of the C2-symmetric trans-chelating tridentate ligand 447 proved also highly effective chiral catalysts. The complexes involving the metal(II) perchlorates of iron, cobalt, nickel, copper and zinc produced the main endo adduct of cyclopentadiene and N - aery loy 1-1,3 -oxazo I i din -2 -one with very high ee values281. 

In 1993, Evans and co-workers examined phe-box 6, /-pr-box 45, and bu-box 3 ligands in the Diels-Alder reaction of cyclopentadiene 68 and 3-acryloyl-l,3-oxazolidin-2-one 69 using a weak Lewis acid such as copper(II) triflate." The results are summarized in Table 9.9. The reaction was carried out between —50 and —78 °C for 3-18 h and achieved selectivities of up to 98 2 (endo/exo) with an endo ee of >98% (using bu-box 3). Interestingly, the enantiomer produced in these reactions was the (25) configuration, compared to the (2K) isomer obtained with iron(III) and magnesium(II) as reported by Corey. This observed stereochemistry was explained by the chelation model of the copper(II) complex 74 (Fig. 9.23)... 

The importance of the rigid aminoindanol backbone in asymmetric catalytic Diels-Alder reactions is a subject of continued interest.16 50 One example immobilized the copper-inda-box complex onto mesoporous silica in the context of continuous large-scale production of chiral compounds.16 Using 10 mol% of this catalyst (Figure 17.5), the Diels-Alder reaction between /V-acryoyloxazo-lidinone and cyclopentadiene proceeded in 99% yield, 17 1 endo. exo selectivity, and 78% ee of endo cycloadduct. The catalyst could easily be recovered and reused several times without significant loss of diastereoselectivity (15 1 endo. exo selectivity after the fifth reuse) or enantioselectivity (72% ee after the fifth reuse).16 The same remarkable reactivity was observed with a number of diene-dienophile partners. 

The complexes are isolated, characterized and used as chiral Lewis acids. Dissociation of the labile ligand liberates a single coordination site at the metal center. These Lewis acids catalyze enantioselective Diels-Alder reactions. For instance, reaction of methacrolein with cyclopentadiene in the presence of the cationic iron complex (L = acrolein) occurs with exo selectivity and an enantiomeric excess of the same order of magnitude as those obtained with the successful boron and copper catalysts (eq 3). ... 

The effect of the ligand bite angle on the enantioselectivity in the copper(n) catalyzed Diels-Alder reaction of cyclopentadiene with A-acryloyl-l,3-oxazoUdin-2-one was studied using spiro bis(oxazolidine) based complexes 431a-d (Table The data... 

Cationic copper(II) complex 37 derived from a chiral bis(imine) ligand has also been shown to be an effective catalyst for reactions between cyclopentadiene and acylated thiazolidine-2-thione dienophiles, albeit with slightly lower se-lectivities than for the bis(oxazoline) complex 31 (Scheme 30) [93]. The bis(2,6-dichlorophenylimine) was found to be optimal among a number of electron-rich and -poor aryl imines screened. The reaction exhibits a positive non-linear effect which suggests that the minor ligand enantiomer can be sequestered by the formation of a catalytically less active (l ,l )/(S,S)Cu(II) dimer. 

The bis-oxazoline-catalysed Diels-Alder reaction is not restricted to the use of oxazolidinone-based dienophiles. For instance the a-sulfenylacrylate (8.74) and oi-hydroxyenones such as (8.75) undergo enantioselective Diels-Alder reaction with cyclopentadiene in the presence of the copper complex of (8.67) and (8.68) respectively. 

Figure 18.2 Diels-Alder reaction of cyclopentadiene with aza-chalcone catalyzed by copper complexes in the presence of DNA [6c,d].

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