SbF6 counterion

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). 

Trityl hexachloroantimonate polymerizes styrene only at relatively high concentrations (>10-2 M) [145]. This is apparently due to rapid formation of the covalent chloro adducts, which are relatively inactive in the presence of only a small amount of Lewis acid. Moreover, the SbCls generated in this reaction can add to styrene to form a 1,2-dichloroadduct and inactive SbCI3. In contrast, polymerization is fast with trityl hexaflu-oroantimonate [145], demonstrating the SbF6 counterion does not decompose to SbFs and F-terminated chains. 

Polymerization of 5-membered cyclic ether, tetrahydrofuran with SbF6 counterion, is the best example of such a system. 

Cationic cobalt(m) complexes have successfully been applied to the asymmetric carbonyl-ene reaction.15 The yield and enantioselectivity were dependent, to a large extent, upon the counterion, with SbF6- giving the best results (16, Equation (9)). The conditions were general for a variety of alkenes, but only glyoxaldehydes were used as the carbonyl component. 

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. 

It was assumed that the triflate counterions of 269c are fully dissociated in solution, at least in the presence of the dicarbonyl substrate. However, an examination of the influence of the counterion revealed that SbF6 provides a much more active catalyst than the triflate counterpart (199). Whereas the triflate catalyst 269c requires 10 h for the reaction to proceed to completion at -78°C, the SbF6 catalyst 271c induces complete conversion to cycloadduct in 4 h under identical conditions, albeit with slightly eroded diastereoselectivity (96 4 vs 98 2) (200). Enantioselec-tivity is identical for the two catalysts (>98% ee, endo diastereomer). 

The impact of the counterion is also evident when using decreased catalyst loadings. With 1 mol% catalyst in the reaction of acryloylimide and cyclopentadiene, no reaction is observed at -50°C using 269c, whereas the SbF6 catalyst 271c induces this reaction at -78°C requiring 8 h to proceed to completion. Selectivities are excellent under these conditions (96 4 endo exo, 96% ee) (200). 

Moss and coworkers have determined18 X-ray structures for the tricyclopropylcyclo-propenium and 1,2-dicyclopropyl-3-phenylcyclopropenium ions 4a and 4b with SbF6 and BF4" counterions, respectively. In these structures it was found that all the cyclopropyl groups are orthogonal to the cyclopropenium residues. The phenyl group was in the same plane as the cyclopropenium moiety. 

Evans, and later j0rgensen, studied the counterion effect of these Cj-symmetric bis(oxa-zoline)/Cu(II) complexes, and found that the counterion structure dramatically affected the catalytic efficiency, and SbFg- was the best among the anions examined (SbF6>PF6>OTf>BF4) [27,28] (Eq. 8A.15). This cationic bis(oxazoline)/Cu(II) catalyst has been successfully applied to asymmetric synthesis of enr-A1 -tetrahydrocannabinol [29] and enf-shikimic acid [30]. 

Aromatic Diazonium Ions. In contrast to alkyldiazonium ions, aryldiazonium ions are well-studied.469 77 They were known as early as 1894. They are isolable as ionic salts with a variety of counterions such as BIT, PF6, SbCl6, SbF6, AsF6-, and C104. They undergo a variety of nucleophilic reactions and an excellent review is available on the subject.478... 

PF6, SbF6, C104-, FSO3 counterions. The X-ray crystal structure of nitronium ion is known with hydrosulfate anion.515 The most widely used tetrafluoroborate nitronium salt (N02+BF4 ) is prepared by treating a mixture of nitric acid and anhydrous hydrogen fluoride wit boron trifluoride516 [Eq. (4.145)]. 

Figure 5. a) The isopropyl cation coordinated to FHF" counterion, b) The isopropyl cation coordinated to SbF6. (Reproduced from reference 26. Copyright 1998 American Chemical Society.)... 

In the superacids, the resulting counterions are weak nucleophiles SbFf, or [SbF6-(SbF5)n]-, [FS03-(SbF5)n]-, etc. 

Table 5 Enthalpy of Carbenium-Oxonium Ion Equilibria in solution, CH2CI2, 25°C, AsF6", SbF6, PF6- Counterions...

Both oxonium and carboxonium active species are relatively strong electrophiles (stronger than in the case of tetrahydrofuran). Thus, to avoid the termination by interaction with counterion, the stable counterions of low nucleophilicity are required. It has been shown that only the most stable complex anions SbF6 and AsF6 provide the living active species, whereas BF4, SbCl6, and even PF6 anions cause termination [98],... 

Pask and Plesch recently confirmed that the trityl ion is incapable of initiating the polymerisation of isobutene (counterion SbFg ). They found however that (p-ClPh)3C SbFg" and Rh2HC SbF6 are active towards that monomer as predicted by thermodynamical considerations. 

Another important group of ligands for metal-catalysed reactions are based on oxazolines derived from amino acids. Evans30 has used the bis-oxazoline or box ligands in Cu(II)-catalysed Diels-Alder reactions. Ligand 133 is derived from the unnatural amino-acid /< r/-leucine and, complexed with Cu(OTf)2, catalyses Diels-Alder reactions of the same dienophile 125 with various dienes. The best counterions are triflate (OTf) and SbF6. 

The more highly chelating pybox ligands such as 135, also prepared from ferMeucine, catalyse Diels-Alder reactions even with aldehydes such as 136. Now the counterion really must be the very non-nucleophilic SbF6. In these examples diastereo- (endo/exo) and enantioselectivity are excellent.31... 

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