Lewis acid stoichiometric

The stannane is easy to purify and handle, being stable at 0 C for many months. Treatment of the three-carbon component with the O-silylated enolate of cyclohexanone in the presence of a number of Lewis acids (stoichiometric) effects conversion to the e/. v-tused octahy-dro-l 77-indene cycloadduct with stereoselectivities and yields dependent on the conditions used. With aluminum(IIl) chloride as Lewis acid, the stereoisomer with the methoxy substituent in the exo orientation was obtained essentially exclusively. 

Reactions of another class are catalyzed by Pd(II) compounds which act as Lewis acids, and are treated in Chapter 5 and partly in Chapter 4. From the above-mentioned explanation, the reactions catalyzed by Pd(0) and Pd(II) are clearly different mechanistically. In this book the stoichiometric and catalytic reactions are classified further according to reacting substrates. However, this classification has some problems, viz. it leads to separate treatment of some unit reactions in different chapters. The carbonylation of alkenes is an example. Oxidative carbonylation of alkenes is treated in Chapter 3 and hydrocar-bonylation in Chapter 4. 

According to Figure 3, hydroperoxides are reduced to alcohols, and the sulfide group is oxidized to protonic and Lewis acids by a series of stoichiometric reactions. The sulfinic acid (21), sulfonic acid (23), sulfur trioxide, and sulfuric acid are capable of catalyzing the decomposition of hydroperoxides to nonradical species. 

Dichloroethane is produced commercially from hydrogen chloride and vinyl chloride at 20—55°C ia the presence of an aluminum, ferric, or 2iac chloride catalyst (8,9). Selectivity is nearly stoichiometric to 1,1-dichloroethane. Small amounts of 1,1,3-tfichlorobutane may be produced. Unreacted vinyl chloride and HCl exit the top of the reactor, and can be recycled or sent to vent recovery systems. The reactor product contains the Lewis acid catalyst and must be separated before distillation. Spent catalyst may be removed from the reaction mixture by contacting with a hydrocarbon or paraffin oil, which precipitates the metal chloride catalyst iato the oil (10). Other iaert Hquids such as sdoxanes and perfluorohydrocarbons have also been used (11). 

Chlorination with Other Reagents. Chlorotoluenes can also be obtained in good yields by the reaction of toluene with stoichiometric proportions of certain Lewis acid chlorides such as inon(III) chloride, as the chlorinating agent (51). Generally, the product mixture contains /)-chlorotoluene as the principal component. Several modifications have been proposed to improve product yields (52,53). 

The main strategy for catalytic enantioselective cycloaddition reactions of carbonyl compounds is the use of a chiral Lewis acid catalyst. This approach is probably the most efficient and economic way to effect an enantioselective reaction, because it allows the direct formation of chiral compounds from achiral substrates under mild conditions and requires a sub-stoichiometric amount of chiral material. 

To achieve catalytic enantioselective aza Diels-Alder reactions, choice of metal is very important. It has been shown that lanthanide triflates are excellent catalysts for achiral aza Diels-Alder reactions [5]. Although stoichiometric amounts of Lewis acids are often required, a small amount of the triflate effectively catalyzes the reactions. On the basis of these findings chiral lanthanides were used in catalytic asymmetric aza Diels-Alder reactions. The chiral lanthanide Lewis acids were first developed to realize highly enantioselective Diels-Alder reactions of 2-oxazolidin-l-one with dienes [6]. 

The field of reaction chemistry in ionic liquids was initially confined to the use of chloroaluminate(III) ionic liquids. With the development of neutral ionic liquids in the mid-1990s, the range of reactions that can be performed has expanded rapidly. In this chapter, reactions in both chloroaluminate(III) ionic liquids and in similar Lewis acidic media are described. In addition, stoichiometric reactions, mostly in neutral ionic liquids, are discussed. Review articles by several authors are available, including Welton [1] (reaction chemistry in ionic liquids), Holbrey [2] (properties and phase behavior), Earle [3] (reaction chemistry in ionic liquids), Pagni [4] (reaction chemistry in molten salts), Rooney [5] (physical properties of ionic liquids), Seddon [6, 7] (chloroaluminate(III) ionic liquids and industrial applications), Wasserscheid [8] (catalysis in ionic liquids), Dupont [9] (catalysis in ionic liquids) and Sheldon [10] (catalysis in ionic liquids). 

Acidic chloroaluminate ionic liquids have already been described as both solvents and catalysts for reactions conventionally catalyzed by AICI3, such as catalytic Friedel-Crafts alkylation [35] or stoichiometric Friedel-Crafts acylation [36], in Section 5.1. In a very similar manner, Lewis-acidic transition metal complexes can form complex anions by reaction with organic halide salts. Seddon and co-workers, for example, patented a Friedel-Crafts acylation process based on an acidic chloro-ferrate ionic liquid catalyst [37]. 

As Lewis acids, titanium(IV) chloride39-377-378 or titanium(IV) isopropoxide in combination with titanium(IV) chloride can be used in stoichiometric amounts40 4l. but triphenylmelhyl perchlorate or chlorotriphenylmethane with tin(II) chloride offers a mild, catalytic alternative42 46. 

A variety of reagents can bring about these ring openings, including amines and anhydrides used in stoichiometric amounts, or Lewis acids such as SnCl4 and Lewis bases such as tertiary amines, used in catalytic amounts. 

Other than energy considerations, on which there is little comparative data, the most important green role for photochemistry is in improving atom economy. Although only a preliminary research result, an excellent example of this is the avoidance of the need for stoichiometric amounts of Lewis acid catalysts in the synthesis of some acylated aromatic compounds. Benzoquinone can be reacted with an aldehyde under a sunlamp to yield an acylhydroquinone in up to 88% yield. The alternative procedure would involve reaction of an acyl chloride with hydroquinone and a... 

In 2007, Womack et al. published the conversion of 2-aUcylcinnamyldehydes to 2-aLkylindanones via a catalytic intramolecular Friedel-Crafts reaction. In the presence of 5-10 mol% FeCls different in situ generated ( )-2-alkylcinnamaldehydes-derived dimethyl acetals cyclized to l-methoxy-2-aIkyl-7//-indenes in good to high yields (Scheme 6) [22]. The transformation corresponds to a formal intramolecular Friedel-Crafts acylation which is achieved with catalytic quantities of Lewis acid. This result is in strong contrast to traditional Friedel-Crafts acylations which require stoichiometric amounts of Lewis acid. 

The plausible mechanism is based on the proposal by Jana and coworkers (Scheme 14). In this case, the sp-hybridized vinyl cation can be attacked by halide, instead of water, to give the ElZ isomer of the alkenyl halide. Compared with the systems using stoichiometric Lewis acid and strong base to prepare substituted alkenyl halides, the present method would provide an excellent alternative due to the environmentally benign system and atom efficiency. 

Whereas the Prins-type cyclizations reported in this and the preceeding chapter were performed using stoichiometric amounts of Fe salts as Lewis acids, a breakthrough in the field of catalysis was reported in 2009 when the first iron-catalyzed Prins- and aza-Prins cyclization was reported. The catalytic system, which is obtained by combining catalytic amounts of an iron salt with trimethylsilyl halides as a halide source, is widely applicable and promotes the construction of substituted six-membered oxa- and aza-cycles (Scheme 33) [44]. 

Hersh et al. found that the cationic complex [CpFe(CO)2(THF)]BF4 (23) can accelerate the [4 + 2] cycloaddition of acrolein and cyclopentadiene [32]. However, the catalytic activity was higher than expected from rate constants determined in stoichiometric experiments, indicating that a Brpnsted or Lewis acid impurity might accelerate this process and generating doubts about the role of 23. 

Another major cause of waste is the use of mineral acids (H2SO4, H3PO4, etc.) and Lewis acids (AICI3, ZnCL), often in stoichiometric amounts, which cannot be recovered and recycled. A typical example is the HNO3/H2SO4 mixture used in aromatic nitrations. Consequently, there is a discernible trend towards the use of solid, recyclable Brpnsted and Lewis acids, e.g. zeolites, acidic clays, etc. (see later) as alternatives to conventional mineral and Lewis acids. 

Many standard reactions that are widely applied in the production of fine chemicals employ. strong mineral or Lewis acids, such as sulphuric acid and aluminium chloride, often in stoichiometric quantities. This generates waste streams containing large amounts of spent acid, which cannot easily be recovered and recycled. Replacement of these soluble mineral and Lewis acids by recyclable. solid acids, such as zeolites, acid clays, and related materials, would represent a major breakthrough, especially if they functioned in truly catalytic quantities. Consequently, the application of solid acids in fine chemicals synthesis is currently the focus of much attention (Downing et al., 1997). 

It is important to select stoichiometric co-reductants or co-oxidants for the reversible cycle of a catalyst. A metallic co-reductant is ultimately converted to the corresponding metal salt in a higher oxidation state, which may work as a Lewis acid. Taking these interactions into account, the requisite catalytic system can be attained through multi-component interactions. Stereoselectivity should also be controlled, from synthetic points of view. The stereoselective and/or stereospecific transformations depend on the intermediary structure. The potential interaction and structural control permit efficient and selective methods in synthetic radical reactions. This chapter describes the construction of the catalytic system for one-electron reduction reactions represented by the pinacol coupling reaction. 

Sc(OTf)3 catalyzes Friedel-Crafts acylation reactions effectively (Scheme 12).52 While more than stoichiometric amounts of a Lewis acid such as A1C13 or BF3-OEt2 are needed because of consumption of the Lewis acid by coordination to products, a catalyic amount of Sc(OTf)3 is enough to complete the reactions. 

Enantioselective protonation of silyl enol ethers using a SnCl4-BINOL system has been developed (Scheme 83). 45 This Lewis-acid-assisted chiral Bronsted acid (LBA) is a highly effective chiral proton donor. In further studies, combined use of a catalytic amount of SnCl4, a BINOL derivative, and a stoichiometric amount of an achiral proton source is found to be effective for the reaction.346 347... 

In this chapter Lewis-acid-mediated reactions have been summarized. While more than stoichiometric amounts of the Lewis acids were employed in conventional reactions, many efforts have been made to reduce the amounts of Lewis acid needed, and many truly catalytic reactions have been developed. Chiral Lewis-acid catalysis has been of great interest in the 1990s and early 2000s, and various combinations of metals and ligands have been investigated. Importantly, the established understanding that Lewis acids are easily hydrolyzed in water has been exploded. Water-compatible Lewis acids are stable in air and moisture, and are easily recovered and reused in many cases. These Lewis acids may solve recent environmental issues, and will be used further in many reactions in future. 

Besides the silyl enolate-mediated aldol reactions, organotin(IY) enolates are also versatile nucleophiles toward various aldehydes in the absence or presence of Lewis acid.60 However, this reaction requires a stoichiometric amount of the toxic trialkyl tin compound, which may limit its application. Yanagisawa et al.61 found that in the presence of one equivalent of methanol, the aldol reaction of an aldehyde with a cyclohexenol trichloroacetate proceeds readily at 20°C, providing the aldol product with more than 70% yield. They thus carried out the asymmetric version of this reaction using a BINAP silver(I) complex as chiral catalyst (Scheme 3-34). As shown in Table 3-8, the Sn(IY)-mediated aldol reaction results in a good diastereoselectivity (,anti/syn ratio) and also high enantioselectivity for the major component. 

A stoichiometric amount of BINOL-boron chiral Lewis acid 118 activates... 

The reactivity of acylzirconocene chlorides towards carbon electrophiles is very low, and no reaction takes place with aldehydes at ambient temperature. In the reaction described in Scheme 5.12, addition of a silver salt gave the expected product, albeit in low yield (22—34%). The yield was improved to 79% by the use of a stoichiometric amount of boron trifluoride etherate (BF3OEt2) (1 equivalent with respect to the acylzirconocene chloride) at 0 °C. Other Lewis acids, such as chlorotitanium derivatives, zinc chloride, aluminum trichloride, etc., are less efficient. Neither ketones nor acid chlorides react with acylzirconocene chlorides. In Table 5.1, BF3 OEt2-mediated reactions of acylzirconocene chlorides with aldehydes in CH2C12 are listed. 

---