Bis triflate complex

Collins and co-workers have also reported on an enantioselective catalytic Diels—Alder cycloaddition, in which zirconocene and titanocene bis(triflate) complexes were used as catalysts [104], The influence of the solvent polarity on the observed levels of stereoselectivity is noteworthy. For example, as shown in Scheme 6.34, with 108 as the catalyst, whereas in CH2C12 (1 mol% catalyst) the endo product was formed with 30% ee (30 1 endoxxo, 88% yield), in CH3N02 solution (5 mol% catalyst) the enantioselectivity was increased to 89% (7 1 endoxxo, 85% yield). Extensive 1H and 19F NMR studies further indicated that a mixture of metallocene—dienophile complexes was present in both solutions (-6 1 in CH2C12 and -2 1 in CH3N02, as shown in Scheme 6.34), and that most probably it was the minor complex isomer that was more reactive and led to the observed major enantiomer. For example, whereas nOe experiments led to ca. 5 % enhancement of the CpH proton signals of the same ring when Hb in the minor complex was irradiated, no enhancements were observed upon irradiation of Ha in the major complex. 

Preparative Methods can be prepared immediately before use from the commercially available bis(oxazoline) ligand and Cu(PF6)2 or Cu(OTf)2 by simply combining the two reagents in an appropriate solvent and stirring until complexation is complete (>2 h). Addition of 2 equiv of water to the bis(triflate) complex affords a di-aquo derivative that is bench-stable for several months. 

Handling, Storage, and Precautions the reagent is most effective under anhydrous conditions. The hydrated bis(triflate) complex can be activated in the presence of molecular sieves. 

Aziridination. The bis(triflate) complex (2) catalyzes the az-iridination of styrene derivatives in the presence of [iV-(p-toluene-sulfonyl)imino]phenyliodinane (eq 1). Spectroscopic studies revealed that catalysts prepared from both Cu OTf and Cu (OTf)2 were identical under the reaction conditions, thus leading to the conclusion that the Cu(II) complex is the catalytically active species. 

Ene Reactions. Bis(triflate) complex 2 catalyzes the ene reaction of glyoxylate esters (eq 15). Catalyst turnover was not observed at low temperature. As is the case with Diels-Alder reactions, the related bis(hexafluoroantimonate) complex is a more efficient catalyst for this transformation. 

A chiral zirconium bis(triflate) complex has been found to catalyze asymmetric Diels-Alder reactions (Eq. 25) [25]. 

The optically pure, structurally characterized zirconocenium tert- butoxide complex l(S,S)-(EBIH)Zr(OBut)(THF)]BPh4 (967, Scheme 242) was prepared by treatment of the corresponding dimethyl complex (A,A)-(EBIH)ZrMe2 966 with /< 7/-butanol in toluene followed by in situ protonolysis with [HNEt3]BPh4 in THF. The complex catalyzes the asymmetric Diels-Alder reaction between cyclopentadiene and various dienophiles.732 The enantiopure zirconocene bis(triflate) complex (A, ST)-(EBIH)Zr(OTf)2 968 also efficiently catalyzes the asymmetric Diels-Alder reaction between cyclopentadiene and oxazolidinones.733,734... 

Alternatively, the cross-coupling between two alcohols to provide unsym-metrical ethers was catalyzed by 1 mol% of the bis-triflate complex 26b. Excellent conversions (generally>95%) as well as good selectivities for the unsymmetrical ether were generally obtained. An example using allyl alcohol is displayed in Equation (8.17). 

More recently, Di Bernardo et al. (1993) have determined, by the same FT-IR method, the affinity of the perchlorate ion for lanthanide bis(triflate) complexes in anhydrous acetonitrile ... 

The reported values of log mix are 1.6 0.1 for both Tb and Lu. Since the apparent formation constants of the tris(triflate) complexes are 2.6 and 2.3, respectively (cf table 2), the affinity of the perchlorate ion for the bis(triflate) complexes is about ten times lower than the one of the triflate itself... 

Kobayashi and co-workers reported similar enantioselectivity switch in the bi-nol-yterrbium(III) triflate complex-catalyzed cycloaddition reactions [69] between N-benzylidenebenzylamine N-oxide and 3-crotonoyl-2-oxazolidinone [70]. The reaction in the presence of MS 4 A showed an exclusively high enantioselectivity of 96% ee, while that in the absence of MS 4 A (-50% ee) or in the presence of pyridine N-oxide (-83% ee) had the opposite enantioselectivity (Scheme 7.24). This chirality switch happens generally for the combination of a wide variety of nitrones and dipolarophiles. 

A highly fluorinated bis(pyrazolyl)borate ligand, dihydridobis(3,5-bis(trifluoromethyl)pyrazo-lyl)borate has been synthesized. The zinc triflate complex was prepared from the potassium salt of the ligand and exhibits distorted tetrahedral coordination which allows comparison of the amount of distortion, in relation to the electronic properties of substituents, with the methylated analog.165... 

Bis(oxazolinyl)pyridine-Ce(IV) triflate complex 78 catalyzed the enantioselective 1,3-DC of acyclic nitrones with a, 3-unsaturated 2-acyl imidazoles. For example, C-phenyl 7V-benzyl nitrone reacted with 77 in the presence of 78 to give the adduct 79 with excellent diastereo-and enantioselectivity. Isoxazolidine 79 was then converted into P -hydroxy-P-amino acid derivatives by hydrogenation of the N-0 bond in the presence of Pd(OH)2/C and cleavage of the 2-acyl imidazole with MeOTf in MeCN <06OL3351>. 

Many chiral, enantiomerically pure zirconocenes are known [20], In order to induce an asymmetric reaction, chiral zirconocenes have to be prepared, of which the most common are [(EBTHI)ZrCl2] EBTHI = r 10-ethylene-l,2-bis(tetrahydroindenyl), see Scheme 8.47 for the corresponding bis(triflate) and Erker s [(NMI)2ZrCl2] (NMI = r 5-neomenthyhn-dene) [21] (see Scheme 8.37). The [(EBTHI)ZrCl2] complex is commercially available as a racemate or in enantiomerically pure form (for a resolution procedure, see the supplementary material of [22]), and the precursor [(EBI)ZrCl2] is available as a racemate. 

The isomerization of an O-silyl ketene acetal to a C-silyl ester is catalyzed by a cationic zirconocene—alkoxide complex [92], This catalysis was observed as a side reaction in the zirconocene-catalyzed Mukaiyama aldol reactions and has not yet found synthetic use. The solvent-free bis(triflate) [Cp2Zr(OTf)2] also catalyzes the reaction in nitromethane (no reaction in dichloromethane), but in this case there may be competitive catalysis by TMSOTf (cf. the above discussion of the catalysis of the Mukaiyama aldol reaction) [91] (Scheme 8.51). 

The nature of the counterion has had a profound impact on catalysis, as will be seen. Structurally, it was of considerable interest to delineate the factors that influence selectivity and to examine whether the counterion plays a role in the solid-state geometry of these catalysts. While the hexafluoroantimonate copper complexes of bis(oxazoline) 55c are completely dissociated in the solid state, analogous triflate complexes exhibit weak bonding to one counterion in the apical position (2.62 A from the metal), Fig. 23. Association of the triflates in the solid state was also noted for Complex 266d. The water molecules are distorted toward the phenyl substituents, similar to the SbF6 complex 265d. 

The analogous cationic pyridylbis(oxazoline)-copper complexes exhibit square pyramidal geometries in the solid state. As in the bis(oxazoline) series, the triflate is closer to the metal than the SbF6 counterion (2.36 and 2.49 A vs 2.90 A). A single molecule of water is bound to the copper center in the triflate complex 267b, whereas the SbF6 complex 268b accommodates two water molecules in the coordination sphere, Fig. 24 (197). 

Ghosh and co-workers have also demonstrated that the Cu(II)-bis(oxazoline) complexes of conformationally constrained inda-box ligands 9a and ent-9a are excellent catalysts for the enantioselective Diels-Alder reaction. Using copper(II) trrflate as the metal source, the reaction resulted in selectivities up to >99 1 endo/ exo ratio with endo ee up to 99% (2R isomer), as shown in Table 9.10 (Fig. 9.24). Of particular interest, Cu(II)-phe-box ligand 6-derived catalyst complex exhibited considerably lower enantioselectivity (30%)." Furthermore, they have shown that the use of Mg(II) as the chelating metal resulted in a reversal of stereochemistry [up to 98 2 endo/exo and 61% endo ee for the (25) isomer]. Davies also showed that the use of copper(II) triflate with his stmcturally related inda-box ligands 9b and 34a led to similar selectivities. 

Yet another use for bis(oxazoline) ligands is in the synthesis of (a-chloroalkyl) boronates. " " As shown in Figure 9.71, the alkylboronate 240 was converted to the a-chloro derivative 241 in 86% ee through the use of ytterbium(lll) triflate-complexed phe-box 6. 

Enantioselective amination of enolsilanes has also benehted from the use of bis(oxazoline) lignads. " For example, (Z)-l-phenyl-l-(trimethylsilyloxy)-l-pro-pene, 242 was condensed with 243 using copper(ll) triflate-complex bu-box ligand 3 to afford 244 in 95% yield (99% ee) as shown in Figure 9.72. 

Much work in the review period has concerned enantioselective substitution in five-membered heterocyclics. The enantioselective alkylation of some pyrroles by unsaturated 2-acylimidazoles catalysed by the bis(oxazolinyl)pyridine-scandium(in) triflate complex (31) has been reported.39 Compound (33) is formed in 98% yield and 94% ee from the 2-acylimidazole (32) and pyrrole at —40 °C. A series of enantiomer- ically pure aziridin-2-ylmethanols has been tested as catalysts in the alkylation of /V-mclhylpyrrolc and (V-methylindole by ,/l-unsalura(cd aldehydes.40 Enantiomeric excesses of up to 75% were observed for the alkylation of /V-mcthylpyrrole by ( >crotonaldehyde using (2.S ,3.S )-3-mclhylazirin-2-yl(diphenyl)methanol TFA salt as catalyst to form (34). 

Simple bis(oxazoline) ligands, especially azabis(oxazolines), can catalyse the addition of indoles to benzylidene malonates in up to 99% ee, provided that excess of the chiral ligand is avoided.166 The paradigm followed in many asymmetric catalytic reactions that an excess of the chiral ligand with respect to the metal should improve enantioselectivity because the background reaction catalysed by a free metal is suppressed, was shown not to be applicable here,166 which might call for revisiting some of the many copper(II)-bis(oxazoline)-catalysed processes known. Enantioselective additions of pyrroles and indoles to ,/9-unsaturated 2-acylimidazoles catalysed by the bis(oxazolinyl)pyridine-scandium(III) triflate complex have been accomplished.167... 

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