Chiral copper complexes structure

Further papers of interest in this section concern the decomposition of alkyl diazoacetates in 2,5-dimethylhexa-2,4-diene in the presence of chiral copper complexes (cf. Vol. 8, p. 33), a revision of the 2-pyrazoline structures [viz. to (88)] described in an early chrysanthemumdicarboxylic acid synthesis, and... 

These authors further described the synthesis and resolution (by chiral HPLC) of a new C2-symmetric planar-chiral bipyridine ligand [43] (see structure 35 in Scheme 18). They obtained an X-ray crystal structure of the corresponding copper complex proving a bidentate complexation. This system led to high diastereo- (up to 94%) and enantioselectivity (up to 94%) in the... 

Figure 10. X-ray structure of 201, a chiral, copper(I) complex of styrene...

The mechanism of the enantioselective 1,4-addition of Grignard reagents to a,j3-unsaturated carbonyl compounds promoted by copper complexes of chiral ferrocenyl diphosphines has been explored through kinetic, spectroscopic, and electrochemical analysis.86 On the basis of these studies, a structure of the active catalyst is proposed. The roles of the solvent, copper halide, and the Grignard reagent have been examined. 

Metallomesogens have been shown to form helical supramolecular organisations in their mesophases [95]. Chiral oxazoline complexes with various metal ions and six alkyl chains did not show LC behaviour, but when mixed with trinitrofluorenone form achiral smectic A phases [96]. Furthermore, when a branch was included in the structure of the ligands (Fig. 12) the corresponding complexes with copper(II) and palladium(II) form columnar mesophases which have a helical organisation [97]. The presence of the stereogenic centre near the central metal ion in these complexes (Fig. 12) is enough to cause the parallel molecules to stack in a tilted manner with... 

Since the copper complexes, [Cu(NN)2]+ and [Cu(NN)(PR3)2]+ (NN = 1,10-phenanthroline, 2,2 -bipyridine, and their derivatives) were applied to stoichiometric and catalytic photoreduction of cobalt(III) complexes [8a,b,e,9a,d], one can expect to perform the asymmetric photoreduction system with the similar copper(l) complexes if the optically active center is introduced into the copper(I) complex. To construct such an asymmetric photoreaction system, we need chiral copper(I) complex. Copper complex, however, takes a four-coordinate structure. This means that the molecular asymmetry around the metal center cannot exist in the copper complex, unlike in six-coordinate octahedral ruthenium(II) complexes. Thus we need to synthesize some chiral ligand in the copper complexes. 

From Cu(OTf)2 or Cu(OTf) and the chiral Cj-symmetric tris(oxazoline) ligand 9, copper complexes are obtained that are capable of catalyzing the allylic oxidation of cyclopentene by /er/-butyl perbenzoate in up to 84 % ee [12]. Even today, for most oxidations with chiral or achiral ligand systems, the structures of the real active metal catalysts are unknown. Because of this it is difficult to give a scientific rationale for the selectivities and inductions observed. 

Enantioselection can be controlled much more effectively with the appropriate chiral copper, rhodium, and cobalt catalyst.The first major breakthrough in this area was achieved by copper complexes with chiral salicylaldimine ligands that were obtained from salicylaldehyde and amino alcohols derived from a-amino acids (Aratani catalysts ). With bulky diazo esters, both the diastereoselectivity (transicis ratio) and the enantioselectivity can be increased. These facts have been used, inter alia, for the diastereo- and enantioselective synthesis of chrysan-themic and permethrinic acids which are components of pyrethroid insecticides (Table 10). 0-Trimethylsilyl enols can also be cyclopropanated enantioselectively with alkyl diazoacetates in the presence of Aratani catalysts. In detailed studies,the influence of various parameters, such as metal ligands in the catalyst, catalyst concentration, solvent, and alkene structure, on the enantioselectivity has been recorded. Enantiomeric excesses of up to 88% were obtained with catalyst 7 (R = Bz = 2-MeOCgH4). 

Copper complexes of chiral Pybox (pyridine-2,6-bis(oxazoline))-type ligands have been found to catalyze the enantioselective alkynylation of imines [26]. Moreover, the resultant optically active propargylamines are important intermediates for the synthesis of a variety of nitrogen compounds [27], as well as being a common structural feature of many biologically active compounds and natural products. Portnoy prepared PS-supported chiral Pybox-copper complex 35 via a five-step solid-phase synthetic sequence [28]. Cu(l) complexes of the polymeric Pybox ligands were then used as catalysts for the asymmetric addition of phenylacetylene to imine 36, as shown in Scheme 3.11. tBu-Pybox gave the best enantioselectivity of 83% ee in the synthesis of 37. 

The first example of asymmetric organometallic catalysis outside the area of polymer chemistry was the cyclopropanation of alkenes as described by Nozaki, Noyori et al. in 1966 [17]. The chiral catalyst used was a salen-copper complex 3 (Scheme 3), giving a maximum enantioselectivity of 10% ee. These low but well-established values initiated further research in this area. Later, Aratani et al. initiated the tuning of the structure of the copper catalyst at Sumitomo [18]. They were able to reach quite high level of enantioselectivity with copper catalyst 4. For example, 2,2-dimethyl-cyclopropane carboxylic acid was obtained in 92% ee, and subsequently used in a process to prepare cilastatine. 

In 1965, Denny et al. for the first time reported a catalytic asymmetric Kharasch-Sosnovsky reaction by using Cu(II)-(a)-ethyl camphorate as a catalyst, though enantioselectivity was low (Scheme 9) [21]. A quarter of a century later, natural or synthetic amino acids were introduced as chiral auxiliaries and much improved enantioselectivity (up to 65% ee) was achieved (Scheme 10) [22]. Although no detailed information on the structures of these copper complexes has been obtained, the observed non-linear relationship between the ee of the chiral auxiliary and the ee of the product suggests that the copper-amino acid complex is not monomeric but instead is oligomeric (at least dimeric) species [22e]. 

Ito and Katsuki (1994, and preceding papers mentioned in reference 5) demonstrated that the cyclopropanation of styrene with diazoacetates as well as the ring expansion of oxetanes to tetrahydrofurans gives products of high asymmetry (in part >90< o ee) if copper complexes of chiral 2,2 -bipyrldine derivatives are used as catalysts. The structure of these complexes is similar to that of catalysts like 8.160 and complexes of 8.169. 

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 use of chiral copper Lewis acids in enantioselective aldol processes has seen rapid development over the past 10 years. In particular, copper-catalyzed variants of the Mukaiyama aldol reaction received considerable attention in the years leading up to the new millennium. Evans and coworkers first demonstrated Cu(II)/pybox complex (59) as an efficient catalyst for highly enantioselective addition of a variety of silylketene acetals to aldehydes capable ofbidentate coordination (Scheme 17.12) [17]. In reactions utilizing silylketene acetals (61) and (63) with an additional stereoelement, diastereoselectivities and enantioselectivities were also high. A square pyramidal model (65), which has been further supported by a crystal structure of the complex, with the a-alkoxy aldehyde bound in a bidentate fashion accounts for the observed selectivity. 

This bis-copper complex 252+.(BF4 )2 is a dark red crystalline solid (needles). The high resolution mass spectrum (FAB) of 252+.(BF4 )2 shows a molecular peak at 1816,74 (calculated molecular weight for 25 + = 1817.10). The NMR spectrum of 25 + is in complete agreement with its structure. Since 25 + contains a double helix on two copper(I) atoms, it is chiral. This was demonstrated in the presence of Pirkle s reagent.[33] In order to prove the knotted topology of 25 +, we had to consider the various possible compounds obtained in the cyclisation reaction. From a tetrafunctional double helicoidal precursor, several 2+2 connections are possible, with the most probable ones being indicated in Figure 11. 

The authors assumed that the catalytically active species might be a copper(I) complex originating from reduction by the silyl dienolate 214. As a consequence, the aldol reaction was performed with the chiral copper(I) complex [Cu(OfBu)-(S)-270], and identical results in terms of the stereochemical outcome were obtained. In addition, the reaction was followed by react IR. The study led to evidence of a copper(I) enolate as the active nucleophile, and the catalytic cycle also shown in Scheme 5.77 was proposed. The reaction of the copper(I) complex Cu(OiBu)-(S)-270 with silyl dienolate 214 represents the entry into the catalytic cycle. Under release of trimethylsilyl triflate, the copper enolate 272 forms, whose existence is indicated by in situ IR spectroscopy. Its exact structure remains unclear, but the description as O-bound tautomer is plausible. Upon reaction with the aldehyde, the copper aldolate 273 is generated, which is then silylated by means of the silyl dienol ether 214 to give the (isolable) silylated alcohol 274 from which the aldol product 271 is liberated during the acidic workup [132b]. 

---