Boron triflate: Lewis acidity

As mentioned several times Lewis acids are highly valuable catalysts but the most commonly used ones such as aluminium chloride and boron trifluoride are highly water sensitive and are not usually recovered at the end of a reaction, leading to a significant source of waste. In recent years there has been much research interest in lanthanide triflates (trifluoro-methanesulfonates) as water stable, recyclable Lewis acid catalysts. This unusual water stability opens up the possibility for either carrying out reactions in water or using water to extract and recover the catalyst from the reaction medium. 

The aldol reactions introduced thus far have been performed under basic conditions where enolate species are involved as the reactive intermediate. In contrast to the commonly accepted carbon-anion chemistry, Mukaiyama developed another practical method in which enol species can be used as the key intermediates. He is the first chemist to successfully demonstrate that acid-catalyzed aldol reactions using Lewis acid (such as TiCU) and silyl enol ether as a stable enol equivalent can work as well.17 Furthermore, he developed the boron tri-fluoromethane sulfonate (triflate)-mediated aldol reactions via the formation of formyl enol ethers. 

High polymers are generally obtained on treatment with Lewis acids at low temperatures in an inert solvent. Boron trifluoride and boron trifluoride etherate are the most widely used catalysts, but a small amount of water must be present, which is termed a promoter triethylaluminum and triisobutylaluminum are also useful initiators and are generally used with addition of water. Alkylating agents, such as ethyl triflate, triethyloxonium tetrafluoro-borate and hexafluorophosphite, and 2-methyl-l,3-dioxolenium perchlorate, are also effective initiators (76MI51301,72MI51304). 

Lewis acids, such as boron trifluoride etherate, zinc triflate <2000H(53)1421>. N-Unsubstituted tetrazoles 24 can be alkylated with primary aliphatic alcohols using the Mitsunobu protocol (Equation 26) <1996SC2687>. 

Imines may be activated by complexation with Lewis acids, but this also increases the acidity of a-hydrogen atoms. A combination of copper(i) halide and boron trifluoride etherate is a possible solution to the problem [6, 7]. Activation by trimethylsilyl triflate is also effective with aldimines (though not with ketimines) [7, 8]. 

With catalytic amounts of rare earth metal triflates, heterocarbonyl compounds, e.g. acylhy-drazones, are also successfully activated. From the latter and silyl enolates (Scheme 4), the coupling products are obtained directly or in a one-pot synthesis in the presence of 5 mol% of Sc(OTf)3 or Yb(OTf)3 in 45-96% yield. For example, compound 17 was isolated in 92 % yield and was subsequently cyclized with base to the corresponding pyrazolone (Scheme 4) [16]. In comparison with typical Lewis acids, such as SnCl4, (10% yield) and boron trifluoride etherate (42 % yield), Sc(OT03 proved to be superior. 

Electrc hilic reagents capable of Mnging about Pummerer rearrangement include inoiganic and organic acids, carboxylic anhydrides, acyl halides, isocyanates, caibodiimides, irimethylsilyl halides and triflate, sulfonyl and sulfenyl halides, phosphorus pentoxide and many typical Lewis acids such as boron... 

Methyl benzenesulfenate activated by Lewis acids, such as trimethylsilyl triflate or boron tri-fluoride-diethyl ether complex, induces arenealkene cyclization, presumably via an episulfonium... 

Although stereoselective formation of enolates from acyclic ketones with bases such as LDA is rather difficult, stereodefined boron enolates are more readily accessible. In the Mukaiyama method, an ethyl ketone is treated with a dialkylboron triflate and a tertiary amine, usually i-Pr2NEt. The resultant Z-(0) boron enolates (also known as enol borinates) are believed to be formed under kinetic control by deprotonation of the Lewis acid-complexed substrate. Brown and co-workers have shown that E- 0) boron enolates may be prepared by treatment of ethyl ketones with dicyclohexylboron chloride in the presence of Et3N. ... 

Taking into account the competitive hydrolysis of the silyl enol ether, this reaction is remarkable. The method was shown to be general and was extended to a variety of aldehydes and several a,j9-unsaturated carbonyl compounds giving uniformly 1,4-addition with aldehydes and a mixture of 1,4- and 1,2-adducts in the case of ketones [187]. Later, this aqueous version of the Mukaiya-ma reaction was shown to give near quantitative yields in the presence of a water-tolerant Lewis acid such as ytterbium triflate [188]. Keeping with the same concept,copper(II) triflate [189],indium(III) trichloride [190],tris(pentafluoro-phenyl)boron [191] and scandium(III) triflate in the presence of a surfactant [192] have proved to be active catalysts. 

These results are interpreted as shown in Figure 6.79. The formation of 6.92 takes place via a cyclic transition state Cj Si. Chelate 6.91 is disrupted in order to allow the coordination of the aldehyde to the boron atom. As usual, steric interactions are minimized in the favored transition state. In the presence of excess boron triflate or of another Lewis acid which can activate the aldehyde carbonyl group, the boron chelate is no longer disrupted. Two acyclic transition-state models, A Re and A Si, can be envisioned according to the nature of the Lewis acid [106], Open transition states are also proposed for the reactions of 6.91 with CF3CHO, which does not require electrophilic assistance [1261]. 

Indoles react with epoxides and aziridines in the presence of Lewis acids (see 20.4.1 for reaction of indolyl anions with such reactants) with opening of the three-membered ring and consequent 3-(2-hydroxyethylation) and 3-(2-aminoethylation) of the heterocycle. Both ytterbium triflate and phenylboronic acid are good catalysts for reaction with epoxides under high pressure silica gel is also an effective catalyst, but reactions are slow at normal pressure and temperature. Reaction with aziridines can be catalysed by zinc triflate or boron trifluoride. °... 

To improve the yields and therefore the scope of this aqueous aldolization, the use of lanthanide triflates as water-tolerant Lewis acids was recommended [9, 60]. After completion of the reaction nearly 100% of the catalyst is recovered from the aqueous layer and can be re-used quite easily. Other water-tolerant Lewis acids, including indium chloride [61] and tris(pentafluorophenyl) boron [62], were proposed as catalysts in the aqueous aldol reaction. 

Furthermore, a vast number of organometallic catalyzed reactions can be performed in a biphasic manner thus proving that also uncommon reactions may be worth to be investigated in liquid/liquid systems. For instance, Braddock describes the atom economic nitration of aromatics in a two-phase process [192], Nitration of aromatics leads usually to excessive acid waste streams and the classical Lewis acid catalysts such as boron trifluoride are destroyed in the aqueous quench after the reaction thus making any recycle impossible. In the method of Braddock the ytterbium triflate catalyst is solved in the aqueous phase and can be recycled by a simple evaporative process. Monflier and Mortreux [193] investigated the nickel catalyzed isomerization of olefins, for instance allylbenzene, in a two phase system yielding good yields of cis- and trans-methylstyrene. 

In an important experiment, Mukaiyama and coworkers enolized carbonyl compounds under much milder conditions (low temperatures) with dialkylboryl triflate and a sterically hindered tertiary amine base such as 2,6-lutidine (2,6-dimethylpyridine) or diisopropylethylamine (DPEA).95-97 Less-hindered bases led to formation of a stable borane-amide complex (Lewis acid-Lewis base) and prevented the reaction with the carbonyl compound. Masamune et al,98 and Evans et a/.99100 carried out a study to investigate the reasons for the selective enolate formation. They showed that it depends on the boron ligand, base, solvent and the group attached to the carbonyl moiety. Ketones give (Z)-enolates with often excellent selectivity, whereas r-butyl thiolates give selectively the ( )-enolates (equations 32 and 33).100 101 Evans suggests that reactions with 9-BBN triflate are often under thermodynamic control.15 In equation... 

The reaction of acetals and ketals with enol silyl ethers, usually catalyzed with the more traditional Lewis acids such as boron trifluoride etherate or titanium tetrachloride, can be carried out with trimethylsilyl triflate catalysis50. In the example shown in equation 42 the resulting allylsilane was annulated with titanium tetrachloride. 

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