Late transition metal Lewis acids

Lewis acids play key roles in a large number of reactions, and their use in organic synthesis continues to see rapid development, particularly in the field of asymmetric catalysis [1], Late transition metal Lewis acid catalysts have emerged as a new class of compounds within this area. They offer neutral and mild conditions that are of interest for the needs of modern chemistry and its focus on economically and ecologically friendly methods. 

In comparison with classic Lewis acids derived from main group halides (e.g., B, Al, Sn), f-elements, and early transition metal halides, late transition metal Lewis acids often are more inert to ubiquitous impurities such as water, offer higher stability, tunable properties by ligand modification, and a well-defined structure and coordination chemistry, thus allowing detailed studies of reaction mechanisms, and a rational basis for catalyst optimization. Among this new class of late transition metal Lewis acids, ruthenium complexes - the subject of this chapter - display remarkable properties... 

Relatively soft, late transition-metal Lewis acids are also suitable catalysts for electrophilic fluorination. Indeed, the cationic chiral mthenium(II)-(15,25)-A,/V -bis(o-(diphenyl-phosphino)benzylidene)cyclohexane-1,2-diamine complex is a powerful catalyst for the enantioselective fluorination of (3-keto esters (Scheme 44.25). ... 

While main group metals and early transition metals are hard Lewis acids favoring coordination of carbonyl groups, gold, one of the late transition metals, is classified as a soft metal that activates 7r-electron systems like olefins and... 

Negishi previously reported that a wide variety of Lewis-acidic compounds catalyzed hydrozirconation of alkenes such as 1-decene 108 with /-BuZrCp2Cl.126 It was found that the reaction of 1-decene with 1.1 molar equiv. of TIB A, in the presence of 2-5 mol% of chlorine-containing late transition metals, led to the formation of 1-iododecane 110 after treatment of the product with iodine (Table 9). 

A major limitation of such Group IVB metallocene catalysts is that they are air- and moisture-sensitive and not tolerant to heteroatom-containing monomers. In the case of heteroatom-containing monomers the unbonded electron pairs on the heteroatom, such as oxygen, preferentially coordinate to the Lewis acid metal center in place of the carbon-carbon double bond. Some so-called middle and late transition metal organometallics are more tolerant to the presence of such heteroatoms and can be used as effective cocatalysts. These include some palladium, iron, cobalt, and nickel initiators. 

This reaction impressively demonstrates that even alkyne activation, to date an exclusive domain of late transition metals such as Pd or Pt, can be performed by the cheap and nontoxic Lewis acid Bi(OTf)3 [90]. 

A review has appeared on the synthesis of enantiomerically enriched aziridines by the addition of nitrenes to alkenes and of carbenes to imines.45 A study of the metal-catalysed aziridination of imines by ethyl diazoacetate found that mam group complexes, early and late transition metal complexes, and rare-earth metal complexes can catalyse the reaction.46 The proposed mechanism did not involve carbene intermediates, the role of the metal being as a Lewis acid to complex the imine lone pah. Ruthenium porphyrins were found to be efficient catalysts for the cyclopropana-tion of styrenes 47 High diastereoselectivities in favour of the //-product were seen but the use of chiral porphyrins gave only low ees. 

The effectiveness of various substituted BINOL ligands 12-16 in the Zr(IV)-or Ti(IV)-catalyzed enantioselective addition of allyltributyltin to aldehydes was also investigated by Spada and Umani-Ronchi [21], The number of noteworthy examples of asymmetric allylation of carbonyl compounds utilizing optically active catalysts of late transition metal complexes has increased since 1999. Chiral bis(oxazolinyl)phenyl rhodium(III) complex 17, developed by Mo-toyama and Nishiyama, is an air-stable and water-tolerant asymmetric Lewis acid catalyst [23,24]. Condensation of allylic stannanes with aldehydes under the influence of this catalyst results in formation of nonracemic allylated adducts with up to 80% ee (Scheme 3). In the case of the 2-butenyl addition reac-... 

Carbostannylation sometimes proceeds with no activators, but the applicability of such reactions is severely limited to a few types of special substrates. The development of activation methods in the mid 1990s made carbostannylation a synthetically significant tool. Three major activators are now available. Thus, radical initiators, Lewis acids, and late transition metals were found to be effective as activators by Hosomi, Yamamoto, and Shirakawa-Hiyama, respectively. These activators are required only in a catalytic amount to promote the reaction. Scheme 5.7.2 summarizes the mechanism of the activation of carbon-tin bonds by these activators. The cleavage of carbon-tin bonds by a radical initiator gives a stannyl radical, which adds to an unsaturated bond. The resulting alkenyl or alkyl radical abstracts an R... 

The group of Busch also presented evidence that the Mn(n) complex of a cross-bridged cyclam ligand, 4,ll-dimethyl-l,4,8,ll-tetraazabicyclo[6.6.2]hexadecanc, denoted as Mn"(Me2EBC)CL, forms a Mn(IV) adduct with lodosylbenzene which is a new active intermediate in epoxidation reactions [600,609,610]. These examples with Mn, and in the previous section with Fe, are instances of late transition metals catalyzing epoxidation reactions by both the redox and the Lewis acid mechanisms (see Chapter 3). 

Late transition metal ions that can accommodate a two-electron rise in their oxidation state, like Cr(III), Mn(III), and Fe(III), and likely Ru(I), operate by a redox mechanism of epoxidation. They receive an oxygen atom from a TO to form an oxene species (MO) which then transfers the oxygen to an olefin by the intermediacy of a metallacycle, or a radical or cation species. Interestingly, these systems are not inhibited by water or alcohol as are the Lewis acid metals. 

Mid- to Late Transition Metal Catalysts also Perform Epoxidation Reactions by the Lewis Acid Mechanism... 

Key Words Lewis acid adducts, Radical oxidations, Epoxidation, Hydrogen peroxide, Bond dissociation energy, Catalyst durability, Methyltrioxorhenium, Cross-bridged cyclam, Mn(IV), Late transition metal. Propylene oxide. Titanium silicalite (TS-1) catalyst, Ethylanthrahydroquinone/H2 process, Polyoxometallates, Mn(IV) catalyst. Hydrogen abstraction. Rebound mechanism, Isotopic label, t-BuOOH, Peroxide adduct. 2008 Elsevier B.v. 

This listing illustrates the common perspective that "early transition metal ions" perform oxidations by the Lewis acid pathway while "late transition metal ions" produce epoxides by the rebound mechanism. That generality has... 

MID- TO LATE TRANSITION METAL CATALYSTS ALSO PERFORM EPOXIDATION REACTIONS BY THE LEWIS ACID MECHANISM... 

Donor sites D. Phosphines have been largely privileged here due to their high affinity for mid and late transition metals with which Lewis acids have been mainly associated so far. Practical reasons (easy synthesis and handling, P NMR probe)... 

Most commonly used chiral Lewis acids have been derived from main group and early transition series elements. An initial attempt at utilizing optically active catalysts of late transition metal complexes for the enantioselective addition of allyltributylstannane to aldehydes was made by Nuss and Rennels [30]. Employment of Rh(COD)[(-)-DIOP]BF4 (11) as a catalyst, however, resulted in only a small degree of asymmetric induction (17% ee). 

While copper and iron Lewis acids are the most prominent late transition metal Diels-Alder catalysts, there are reports on the use of other chiral complexes derived from ruthenium [97,98],rhodium [99],andzinc [100] in enantioselective cycloaddition reactions, with variable levels of success. As a comparison study, the reactions of a zinc(II)-bis(oxazoline) catalyst 41 and zinc(II)-pyridylbis(ox-azoline) catalyst 42 were evaluated side-by-side with their copper(II) counterparts (Scheme 34) [101]. The study concluded that zinc(II) Lewis acids catalyzed a few cycloadditions selectively, but, in contrast to the [Cu(f-Bubox)](SbFg)2 complex 31b (Sect. 3.2.1), enantioselectivity was not maintained over a range of temperatures or substitution patterns on the dienophile. An X-ray crystal structure of [Zn(Ph-box)] (01)2 revealed a tetrahedral metal center the absolute stereochemistry of the adduct was consistent with the reaction from that geometry and opposite that obtained with Cu(II) complex 31. 

Late-transition metal salts have been utilized as catalysts to promote Friedel-Crafts acylation of arenes and heteroarenes with anhydrides. A mismatch between their soft metal center and the hard carbonyl oxygen atoms of the products avoids the formation of a kinetically inert complex and results in catalytic turnovers. Although late-transition metal salts exhibit, a priori, rather poor Lewis acidity, sufficient reactivity can be gained by rendering them cationic. The acylation of variously substituted... 

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