Lewis acids, transition metal catalysts

Many rhodium(II) complexes are excellent catalysts for metal-carbenoid-mediated enantioselective C-H insertion reactions [101]. In 2002, computational studies by Nakamura and co-workers suggested the dirhodium tetracarboxylate catalyzed diazo compounds insertion reaction to alkanes C-H bonds proceed through a three-centered hydride-transfer-like transition state (Fig. 25) [102]. Only one rhodium atom of the catalyst is involved in the formation of rhodium carbene intermediate, while the other rhodium atom served as a mobile ligand, which enhanced the electrophilicity of the first one and facilitate the cleavage of rhodium-carbon bond. In this case, the metal-metal bond constitutes a special example of Lewis acid activation of Lewis acidic transition-metal catalyst. 

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]. 

The addition of cyanotrialkylsilanes to carbonyl compounds, and especially the development of catalysts for this transformation continues to attract considerable attention owing to the role of cyanohydrin trialkysilyl ethers and cyanohydrins as versatile intermediates in organic synthesis [149]. A variety of catalysts has been developed for such reactions including Lewis acids, transition metal complexes, 18-crown-6 complexes of alkali metals, tetracyanoethylene, Lewis bases, and alkali earth bases. Trimethylcyanation of ketones using Lewis bases does not seem to have been mentioned in the literature. 

This chapter documents advances in the use of catalysis for regioselective transformations of carbohydrate derivatives. For this discussion, the catalysts are grouped into broad structural or functional classes (Lewis bases, Brpnsted acids, Lewis acids, transition metals). Processes that make use of enzyme catalysis (chemoenzymatic methods or metaboUc engineering) are not discussed here [8-10]. Emphasis is placed oti recent results that were not discussed in previous reviews on this and related topics [11—13]. 

Beyond the concept of combined acid catalysis, there are also many other bifunctional acid catalysts interacting with nucleophiles and electrophiles simultaneously, and thus benefiting through such cooperative effect. Selected examples on Lewis acid/hydrogen bonding cooperative catalysis and Lewis acid/transition-metal... 

Flowever, ionic liquids acting as transition metal catalysts are not necessarily based on classical Lewis acids. Dyson et al. recently reported the ionic liquid [BMIM][Co(CO)4] [38]. The system was obtained as an intense blue-green colored liquid by metathesis between [BMIM]C1 and Na[Co(CO)4]. The liquid was used as a catalyst in the debromination of 2-bromoketones to their corresponding ketones. 

Mikami M, Hatano M, Akiyama K (2005) Active Pd(II) Complexes as Either Lewis Acid Catalysts or Transition Metal Catalysts. 14 279-322 Minatti A, DOtz KH (2004) Chromium-Templated Benzannulation Reactions. 13 123-156 Miura M, Satoh T (2005) Catalytic Processes Involving b-Carbon Elimination. 14 1-20 Miura M, Satoh T (2005) Arylation Reactions via C-H Bond Cleavage. 14 55-84 Mizobe Y, see Hidai M (1999) 3 227-241... 

A direct addition of cydoethers to terminal alkynes has been discovered by Zhang and Li (Scheme 6.136) [271]. The best results were obtained when the reactions were run without additional solvent and in the absence of additives such as transition metal catalysts, Lewis acids, or radical initiators. Typically, the cycloether was used in large excess (200 molar equivalents) as solvent under sealed-vessel conditions. At a reaction temperature of 200 °C, moderate to good yields of the vinyl cycloether products (as mixtures of as and trans isomers) were obtained. The reaction is proposed to follow a radical pathway. 

Like so many other reactions, the ene reaction has been given new life by metal catalysis. The use of metals ranges from common Lewis acids, which simply lower the barrier of activation of the hetero-ene reactions to transition metal catalysts which are directly involved in the bond-breaking and -forming events, rendering reactions formal ene processes. This review is meant to serve as a guide to the vast amount of data that have accumulated in this area over the past decade (1994-2004). If a particular subject has been reviewed recently, the citation is provided and only work done since the time of that review is included here. Finally, the examples included within are meant to capture the essence of the field, the scope, limitations, and synthetic utility therefore, this review is not exhaustive. 

Because of the high nucleophilicity and reactivity of diazoalkanes, catalytic decomposition occurs readily, not only with a wide range of transition metal complexes but also with Brpnsted or Lewis acids. Well-established catalysts for diazodecomposition include zinc halides [638,639], palladium(II) acetate [640-642], rhodium(II) carboxylates [626,643] and copper(I) triflate [636]. Copper(II)... 

Suga et al. (197) reported the first stereocontrolled 1,3-dipolar cycloaddition reactions of carbonyl ylides with electron-deficient alkenes using a Lewis acid catalyst. Carbonyl ylides are highly reactive 1,3-dipoles and cannot be isolated. They are mainly generated through transition metal carbenoid intermediates derived in situ from diazo precursors by treatment with a transition metal catalyst. When methyl o-(diazoacetyl)benzoate is treated with A-methylmaleimide at reflux... 

The synthesis of functionalized zinc organometallics can be accomplished with a variety of methods that have been developed in recent years. The intrinsic moderate reactivity of organozinc reagents can be dramatically increased by the use of the appropriate transition metal catalyst or Lewis acid. Furthermore, the low ionic character of the carbon-zinc bond allows the preparation of a variety of chiral zinc organometallics with synthetically useful configurational stability. These properties make organozinc compounds ideal inteimediates for the synthesis of complex and polyfunctionalized organic molecules. 

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