Counter-cation Lewis Acid Effects

The origin of the acceleration produced by BF3 in epoxide alkylation reactions has been examined theoretically [124]. A plausible pathway for BF3 participation in the epoxide-opening is shown in Fig. 10.5. An epoxide/BF3 complex CPI may encounter the cuprate cluster to form a ternary complex CP2, or such a complex may 

Me3SiCl also affects the stereoselectivity of 1,2-additions to carbonyl compounds [133]. With the aid of suitable activators, these mildly reactive reagents show selectivities unattainable by the conventional reagents, as illustrated below for MeaSiCl-dependent chemoselectivity (Eq. 10.13) [134]. 

Recent tlieotetical studies of reductive elimination ftoni Me- Cu-S in tlie presence of BF suggest tliat reaction rate of tlie conjugate addition can inctease if one of tlie Me groups is detadied ftoni tlie coppetflll) to bind witli a hot on atom rsdienie I0.I2)[I29], 

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The Ln(OTf)3 were found to be effective for the activation of formaldehyde water solution. The effect of ytterbium salts was also investigated (table 3) (Kobayashi and Hachiya 1994). While Yb salts with less nucleophilic counter anions such as OTL or CIO4 effectively catalyzed the reaction, only low yields of the product were obtained when Yb salts with more nucleophilic counter anions such as Cl-, OAc , NO3, and SO -were employed. The Yb salts with less nucleophilic counter anions are more cationic and the high Lewis acidity promotes the desired reaction. 

Elimination of Thiophenol from Thioacetals. Conversion of thioacetals to vinyl sulfides is accomplished under exceptionally mild conditions by treatment with (CuOTOa-CeHe (eq 45). The reaction involves an a-phenylthio carbocation intermediate. Three factors contribute to the effectiveness of this synthetic method the Lewis acidity of a copperfT) cation that is unencumbered by a strongly coordinated counter anion, the solubility of the copper(I) triflate-benzene complex, and the insolubility of CuSPh in the reaction mixture. An analogous elimination reaction provides an effective route to phenylthio enol ethers from ketones (eq 46). ... 

The discrimination of the face of the chiral carbenium ion is determined by the hindrance of the flanking groups. In the case of chiral Bronsted acids, the chiral counter ion, formed after effective protonation or partial donation of the proton, surrounds the created cationic intermediate. One face of the intermediate is effectively covered by the chiral counter ion and the nucleophile reacts with the less covered face. However, in many SNl-type transformations mediated by Lewis acids, a transition state in which a couple of protons interact with the phosphates is often invoked. This double interaction seems cmcial and only a partial success in the use of nucleophiles was achieved. Nevertheless, a series of successful S l-type reactions were discovered and are highUghted in this section. The chiral ion pairs between phosphate anions and iminiums [64] or metal ions were developed [65]. This powerful strategy is called asymmetric counteranion-directed catalysis (ACDC) and it has been applied successfully to several innovative transformations. The generation of carbenium ion in situ from alcohol, with the use of acids able to form a tight chiral ion pair with the phosphate anion, can be used in S il-type... 

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