Metal-bisoxazoline complexes

The enantioselective alkylation of indoles catalyzed by C2-symmetric chiral bisoxazoline-metal complexes 90 encouraged many groups to develop superior asymmetric catalysts which are cheap, accessible, air-stable and water-tolerant. Other analogs of the bisoxazoline-metal complex 90 as chiral catalysts and new Michael acceptors have also been studied. The enantioselective alkylations of indole derivatives with of-hydroxy enones using Cu(II)-bis(oxazoline) catalysts 93 and 94 provided the adducts in good yields... 

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. 

Dagorne, S., Bellemin-Laponnaz, S. and Maisse-Francois, A. (2007) Metal complexes incorporating monoanionic bisoxazolinate ligands Synthesis, structures, reactivity and applications in asymmetric catalysis. Eur. J. Inorg. Chem., 913-925. 

Takacs, J.M., Hrvatin, P.M., Atkins, J.M., Reddy, D.S. and Clark, J.L. (2005) The selective formation of neutral, heteroleptic zinc(ll) complexes via self-discrimination of chiral bisoxazoline racemates and pseudoracemates. New J. Chem., 29, 263—265 Atkins, J.M., Moteki, S.A., DiMagno, S.G. and Takacs, J.M. (2006) Single enantiomer, chiral donor—acceptor metal complexes from bisoxazoline pseudoracemates. Org. Lett., 8 (13), 2759-2762. 

A variety of bisoxazolinato rare-earth metal complexes such as 30 have been studied with regard to their hydroamination/cyclization catalytic activity [149]. The precatalysts show similar enantioselectivity and only slightly reduced catalytic activity when prepared in situ from [La N(SiMe3)2 3] and the bisoxazoline ligand. In this ligand accelerated catalyst system the highest rates were observed for a 1 1 metal to ligand ratio. 

In 1999, An wander et al. synthesized the first rare-earth metal complexes containing bisoxazolinate ligands [132]. Yttrium and lanthanum were chosen as examples for small and large rare-earth metal centers to show the high scope of the synthesis. Both types of complexes, mono-bisoxazolinates and bis-bisoxazolinates. 

In addition to bis-bisoxazolinate complexes 177 and 178, chiral and nonchi-ral bis-bisoxazolinate rare-earth metal complexes were synthesized to investigate their catalytic activity for ROP of D,L-lactide and D,L-P-butyrolactone [134]. By using the same synthetic pathway as for compounds 177 and 178, bis-bisoxazolinate complexes 179-182 were obtained via the amine elimination reactions of 2 equiv of the corresponding bisoxazolines HL33-HL35 with 1 equiv of [Ln N(SiHMe2)2 3(THF)2] (Ln = Y, La) in benzene or toluene (Scheme 68). 

In 2003, Marks et al. investigated the catalytic activity of various chiral bisoxazolinate rare-earth metal complexes in intramolecular hydroamina-tion/cyclization reactions [135]. Several examples of bisoxazolines used in the... 

If less stringent requirements with regard to chemical resistance, abrasion, and hardness are placed on the cross-linking density, carboxyl-containing acrylates can also be cross-linked by salt formation (e.g., with diamines [2.44] or metal complexes [2.61]). This procedure is widely employed, particularly for aqueous dispersions. Cross-linking with bisoxazolines has also been reported [2.45]. Epoxy groups can be incorporated into the binder via glycidyl (meth)acrylate and cross-linked with dicar-boxylic acids [2.40]. 

This stabilization of a transition state related to tihe uncatalyzed process can also occur in reactions catalyzed by transition metal complexes, such as those catalyzed by Lewis acids. For example, Diels-Alder reactions catalyzed by transition metal complexes sometimes ocau" by mechanisms related to the concerted [4+2] mechanism of tiie imcatalyzed process. In tiiis case, the catalyst changes the electronic properties of the substrate bound to the Lewis add in a fashion that reduces the barrier for tihe [4+2] cycloaddition. Figure 14.3 shows the transition state proposed for enantioselective Etiels-Alder reactions catalyzed by copper complexes. The transition state structure is proposed on tiie basis of the calculated structure of the Lewis add complex formed between tiie copper-bisoxazoline fragment and the acrylate. 

The careful mechanistic studies that have been documented by Evans are a highly attractive aspect associated with these catalyst systems, and the work is well worth consulting for the practical and mechanistic insight it provides [152, 154, 155]. The investigations have permitted an understanding of the structural and coordination chemistry of the metal complexes, which is sure to have a positive impact on the evolution of enantioselective catalysts. For example, Evans has noted that the general addition of TMSOTf can lead to considerable acceleration in the aldol reaction. Thus, the addition reaction of pyruvate and trimethylsilyl tert-butyl thioketene acetal mediated by 2 mol % of copper(bisoxazoline) affords product in 97 % ee over the course of 14 hours when the same reaction is conducted with one equivalent of TMSOTf and Cu catalyst under otherwise identical conditions, reaction is observed to reach completion in 35 minutes, with no deterioration in enan-... 

In a carbonyl-ene reaction of ethyl glyoxylate with a-methylstyrene catalysed by copper triflate-bisoxazoline complexes, ees of up to 100% have been achieved, but a dramatic switchover in stereochemistry is seen for an apparently minor change in bisoxazoline structure.185 A change in the metal geometry is implicated. 

The catalytic enantioselective addition of aromatic C - H bonds to alkenes would provide a simple and attractive method for the formation of optically active aryl substituted compounds from easily available starting materials. The first catalytic, highly enantioselective Michael addition of indoles was reported by Jorgensen and coworkers. The reactions used a,fl-unsaturated a-ketoesters and alkylidene malonates as Michael acceptors catalyzed by the chiral bisoxazoline (BOX)-metal(II) complexes as described in Scheme 27 [98,99]. 

This process (hetero Diels-Alder reaction leading to a dihydropyran system) may be also conducted in an asymmetric version application of chiral transition-metal catalysts based on BINOL, BDMAP, bisoxazolines, etc. provides adducts in very high optical purity (ee up to 99%) [1,6], In a series of papers Jurczak reported recently a highly enantioselective cycloaddition of 1-methoxy-1,3-butadiene and butyl glyoxylate catalyzed with chiral salen complexes [21],... 

The catalytic activity of the chiral complexes [Ln(L)Z2] shown in Scheme 70 was investigated in NMR-scale intramolecular hydroamination/cyclization reactions [135]. The rate dependence on the ionic radii of the center metal was studied by using 5 mol% bisoxazoline L32 and [Ln N(SiMe3)2 3] as precatalysts and 2,2-dimethyl-4-penten-l-amine as substrate (Scheme 71). The reaction rate as well as the enantioselectivities increased with increasing radius of the center metal. Therefore, the lanthanum compound 184 was the most active catalyst among the investigated complexes. 

Particularly effective catalysts are the chiral copper(ll) bisoxazoline complexes 66 and 134 (3.96). Best results are obtained when the dienophile has two sites for co-ordination to the metal. For example, the catalyst chelates to the two carbonyl groups of acrylimide dienophiles (as in structure 135) and cycloaddition with a diene leads to the adduct in high yield and with high optical purity (3.97). ... 

The SbFe-derived complex 134 is approximately 20 times more reactive than its triflate analogue 66. The use of copper as the metal allows a well-defined catalyst with (distorted) square-planar geometry, and analysis of the catalyst-substrate complex 135 allows the prediction that the diene component will approach from the less hindered Re face of the dienophile (note that the dienophile adopts the s-cis conformation). The bisoxazoline ligand has C2-symmetry and this is beneficial as it reduces the number of competing diastereomeric transition states. 

Copper complexes of the bisoxazoline ligands have been shown to be excellent asymmetric catalysts not only for the formation of carbocyclic systems, but also for the hetero-Diels-Alder reaction. Chelation of the two carbonyl groups of a 1,2-dicarbonyl compound to the metal atom of the catalyst sets up the substrate for cycloaddition with a diene. Thus, the activated diene 20 reacts with methyl pyruvate in the presence of only 0.05 mol% of the catalyst 66 to give the adduct 138 with very high enantiomeric excess (3.99). 

The asymmetric synthesis of aziridines can be achieved by a number of methods. The best alkene substrates are typically a,3-unsaturated esters, styrenes or chromenes, with aziridination by PhI=NTs and a metal-chiral ligand complex. For example, aziridination of tert-butyl cinnamate 73 occurs highly enantioselec-tively with copper(I) triflate and a bisoxazoline ligand (5.77). 

Figure 8.12 Metal-bisoxazoline complexes catalysts for Friedel-Crafts arylations.

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