Lewis base activation

The isomorphic substituted aluminum atom within the zeolite framework has a negative charge that is compensated by a counterion. When the counterion is a proton, a Bronsted acid site is created. Moreover, framework oxygen atoms can give rise to weak Lewis base activity. Noble metal ions can be introduced by ion exchanging the cations after synthesis. Incorporation of metals like Ti, V, Fe, and Cr in the framework can provide the zeolite with activity for redox reactions. 

Lundgren, S. Wingstrand, E. Penhoat, M. Moberg, C. Dual Lewis acid-Lewis base activation in enantioselective cyanation of aldehydes using acetyl cyanide and cyanoformate as cyanide sources. J. Am. Chem. Soc. 2005,127, 11592-11593. 

The typical B-NMR signals of iminoboranes are essentially unaltered when iminoboranes are dissolved in liquids with Lewis base activity (e.g., tertiary amines, tetrahydrofuran) (19). I conclude that equilibria like Eq. (28) are shifted far to the left, even in the presence of an excess of the Lewis base D. 

Recently, Denmark reported, based on the Lewis base activation of Lewis acids concept, a Lewis base catalyzed selenolactonization [244]. 

In a related study Denmark and Fan [22] investigated chiral Lewis base-catalyzed enantioselective a-additions of isocyanides to aldehydes in a Passerini-type reaction (Scheme 9.14). The development of the reaction was based on the concept of Lewis base activation of a weak Lewis acid such as SiCl4 forming a trichlorosilyl-Lewis base adduct which is capable of activating aldehydes towards nucleophilic attack. 

Catalytic, enantioselective addition of silyl ketene acetals to aldehydes has been carried out using a variant of bifunctional catalysis Lewis base activation of Lewis acids.145 The weakly acidic SiCU has been activated with a strongly basic phor-phoramide (the latter chiral), to form a chiral Lewis acid in situ. It has also been extended to vinylogous aldol reactions of silyl dienol ethers derived from esters. 

Oraganosilicon compounds react with a wide variety of organic electrophiles in the presence of a palladium catalyst and a Lewis base activator such as a fluoride or hydroxy ion to give the corresponding coupled products. The reaction is applicable to synthesis of diynes, enynes, arylacetylenes, alkenylarenes, biaryls, allylarenes and alkylarenes in addition to 1,3-, 1,4- and 1,5-dienes with tolerance for various functional groups. 

Table 4.5 Dual Lewis acid-Lewis base activation in enantioselective cyanation of aldehydes using acetyl cyanide.

Table 4.6 D ual Lewis acid-Lewis base activation in the enantioselective [2 + 2] cycloaddition of TMS-ketene and ethyl glyoxalate.

Sacchi, M.C. Shan, C. Locatelli, P. Tritto, I. Stereochemical investigation of the insertion step in MgCl2-supported Ziegler-Natta catalysts. The Lewis base activation effect. Macromolecules 1990, 23, 383-386. 

Desymmetrization with halogen nucleophiles was effectively demonstrated with two mechanistically-divergent chiral catalysts. Denmark disclosed a Lewis-base activated delivery of chloride that was catalyzed by the enantiopuxe phos-phoramide 16. Binding of the phosphoramide was believed to induce dissociation of SiCl4 into the chiral phosphorus/silicon cation and chloride anion, which subsequently ring-opened the activated epoxide. The best enantioselectivity was observed with cfs-stilbene oxide, which was formed in 94% yield and 87% ee (Scheme 14) [28]. 

Catalyst (3.115) is often referred to as the CBS catalyst after the names of the original authors (Corey, Bakshi and Shibata). A catalytic cycle was proposed which explains the experimental observations. The oxazaborolidine interacts reversibly with borane, which then allows complexation of the ketone to give the key intermediate (3.122), as depicted in Figure 3.2. In this process the catalyst acts as both a Lewis acid and Lewis base activating the borane towards hydride delivery and the ketone towards reduction by interaction with the boron in the oxazaborolidine. This dual activation and enhanced steric bulk of the pyrrolidine moiety leads to... 

Independently, Scheldt and coworkers developed the new integration of two distinct Lewis base activation modes to achieve an enantioselective [4 + 3] heterocycloaddition. The reaction provided lactones 128 in moderate yield with excellent enantioselectivity (Scheme 20.54). 

Sakurai pioneered the field of nucleophilic activation of reagents 21.5 by employing stoichiometric amounts of CsF or dilithium catecholates for highly diastereoselective allylation of aldehydes with allyltrihalosilanes 21.5. Later, it was demonstrated that the common dipolar aprotic solvents possessing a strongly Lewis basic ojg gen promote the allylation reaction. Thus, Kobayashi revealed that DMF can act both as a solvent and a Lewis-base activator. Denmark further established that, in contrast to DMF that was required in large excess to promote the allylation of aldehydes, HMPA can be used in substoichiometric quantities. 

Aldol-type Reactions by Lewis-base Activation of Lewis Acids... 

Most recently, Sun and co-workers [31] reported the first direct enantioselective hydrosilylation of prochiral IH-indoles by combined Brpnsted acid/Lewis base activation. The key factor for this methodology is the addition of one equivalent of water to react with HSiCH to generate a strong Brpnsted acid, HCl. In this way the reaction proceeds through the generation of electrophilic indolenium ions by C3 protonation with the in situ-formed HCl, accompanied by subsequent chiral Lewis base-mediated enantioselective hydrosilylation with HSiCH (Scheme 15.7). 

With the synergistic effect of Lewis base and Lewis acid, Denmark and coworkers have established several successful organic catalyst systems based on the strategy of Lewis acid activated Lewis base or Lewis base activation of Lewis acid, in which chiral phosphoramides expressed excellent diastereo- and stereoselectivity for the aldol reaction of silyl enolates and aldehydes (Scheme 65) (311-313). 

Denmark has also applied his new method of Lewis base activation of Lewis acids to the Keck reaction. This approach uses pbosphoramide 96, which can be viewed as a chiral version of HMPA. Phosphoramide 96 is used to activate the weak Lewis acid SiCU and form the strong, cationic, chiral Lewis acid 97. One benefit of this method is that there is very little backgrormd reaction due to the achiral Lewis acid SiCL, because the... 

Denmark has described several chiral bis-phosphoramides as Lewis base activators of weak Lewis acid SiCL and demonstrated their ability to mediate an enantioselective Passerini-type reaction. Catalytic bis-phosphoramide 36 and stoichiometric SiCU, in the presence of a proton scavenger (Htinig s base), promoted the formation of a-hydroxyamides with remarkably good enantioselectivity and yield. A variety of aromatic and unsaturated aldehydes and aryl, alkyl, and functionalized alkyl isocyanides proved to be useful and effective starting materials. High yields of Passerini products were attributed to the production of imidoyl chloride 37, which reduced the prevalence of the nitrilium intermediate and prevented the addition of a second equivalent of isonitrile. 

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