Lewis base activation, example

The BINAM-based selenophosphoramide (28) has been shown to catalyse the enantioselective carbosulfenylation of ( )-alkyl- and aryl-substituted alkenes (26) with phthalimide-SPh to afford tetrahydronaphthalenes (27) with complete anti-diastereoselectivity, and generally high enantioselectivity (42-94% ee), as a new example of the Lewis base activation of an electrophile." ... 

More recently, another dimeric phosphoramide 108 has been proposed. In this case, the silyl ketene acetals add to aldehydes with excellent diastereo- and enantioselectivities. A representative example is given in Scheme 10.25. ° ° The method uses Lewis base activation of Lewis acid (complex 110). 

The Staudinger reaction [92], a [2 + 2]-cycloaddition of a ketene and a nucleophilic imine, usually proceeds by an initial imine attack on the ketene thus forming a zwitterionic enolate which subsequently cyclizes. This reaction is an expedient route to p-lactams, the core of numerous antibiotics (e.g., penicillins) and other biologically active molecules [93]. In contrast, for Lewis-base catalyzed asymmetric reactions, nonnucleophilic imines are required (to suppress a noncatalyzed background reaction), bearing, for example, an N-Ts [94] or -Boc-substituent [95]. 

Recently, novel bifunctionalized zinc catalysts have been developed (compounds (N) and (P), Scheme 55). They have both Lewis-acid and Lewis-base centers in their complexes, and show remarkable catalytic activity in direct aldol reactions.233-236 A Zn11 chiral diamine complex effectively catalyzes Mannich-type reactions of acylhydrazones in aqueous media to afford the corresponding adducts in high yields and selectivities (Scheme 56).237 This is the first example of catalytic asymmetric Mannich-type reactions in aqueous media, and it is remarkable that this chiral Zn11 complex is stable in aqueous media. 

Lewis acid sites have empty orbitals able to accept electron density from the occupied orbitals of a Lewis base, in parallel with back-donation from the catalyst to the empty anti-bonding orbitals of the base [33]. This interaction leads to the formation of an activated acid-base adduct. In the case of alkanes activation may proceed by hydride abstraction [38]. Y and Beta are good examples of zeolites with Lewis acidity, often quite significant for catalysis [39, 40]. 

One section in this review will deal with silyl cations, another with hypervalent silicon compounds. The concept of hypervalent sihcon compounds belongs, strictly speaking, to the class of Lewis base catalysis. However, since a Lewis base forms in situ with a silicon containing reagent or SiCl an intermediate, which functions as a Lewis acid to activate substrates during the reaction, we would also present a few examples in this review. Since silicon is a semimetal we leave it up to the reader to decide whether silicon catalysts should be considered as organocatalysts. 

There is ample evidence that bonds do interact with other occupied orbitals and that cr orbitals interact with other empty orbitals. Evidence for the latter case is primarily in the form of chemical activation of C—H bonds by adjacent groups with low-lying empty or half-filled orbitals. The increased acidity of C—H bonds, that is, reactivity with Lewis bases, under these circumstances is discussed in Chapter 10. Several examples will serve to exemplify the interaction of a bonds with adjacent filled orbitals. 

Occasionally rearrangements from more stable to less stable carbocations occur, but only if (1) the energy difference between them is not too large or (2) the carbocation that rearranges has no other possible rapid reactions open to it.9 For example, in superacid medium, in the temperature range 0-40°C, the proton nmr spectrum of isopropyl cation indicates that the two types of protons are exchanging rapidly. The activation energy for the process was found to be 16 kcal mole-1. In addition to other processes, the equilibrium shown in Equation 6.7 apparently occurs.10 In the superacid medium, no Lewis base is available... 

Shibasaki et al. have reported an asymmetric nitroaldol reaction catalyzed by chiral lanthanum alkoxide 18 to produce an optically active 2-hydroxy-1-nitroalkane with moderate-to-high enantiomeric excesses (Scheme 8B1.10) [27]. Apparently this novel catalyst acts as Lewis base. The proposed reaction mechanism is shown in Scheme 8B1.11, where the first step of the reaction is the ligand exchange between binaphthol and nitromethane. This reaction is probably the first successful example of the catalytic asymmetric reaction promoted by a Lew i s base metal catalyst. Future application of this methodology is quite promising. 

The addition of small amounts of a polar solvent can markedly alter the copolymerization behavior of, for example, the diene-styrene pair. The solvation of the active centers manifests itself in two ways the incorporation of styrene is enhanced and the modes of diene addition other than 1,4 are increased 264,273). Even a relatively weak Lewis base such as diphenyl ether will bring about these dual changes in anionic copolymerizations, as the work of Aggarwal and co-workers has shown 260>. Alterations in polyisoprene microstructure and the extent of styrene incorporation were found for ether concentrations as low as 6 vol. % (diphenyl ether has been shown52) to cause partial dissociation of the poly(styryl)lithium dimers. The findings of Aggarwal and co-workers 260) are a clear demonstration that even at relatively low concentrations diphenyl ether does interact with these anionic centers and further serve to invalidate the repetitive claim 78,158-i60,i6i) tjjat diphenyl ether — at an ether/active center ratio of 150 — does not interact with carbon-lithium active centers. 

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