Lewis base-catalyzed reactions, carbon nucleophiles

As a Carbon Nucleophile in Lewis Base-catalyzed Reactions. Allylation of alkyl iodides with allyltrimethylsilane proceeds in the presence of phosphazenium fluoride. Tetra-butylammonium triphenyldifluorosilicate (TBAT) is useful for allylation of aldehydes, ketones, imines, and alkyl halides with allyltrimethylsilane (eq 63). 55 Similarly, TBAHF2 is an effective catalyst for allylation of aldehydes. The homoallylamines are synthesized from allyltrimethylsilane and imines with a catalytic amount of TBAF (eq 64). The reactions of thioketones as well as sulfines with allyltrimethylsilane can be mediated by TBAF to give allylic sulfides and allyl sulfoxides, respectively. Besides fluoride ion, 2,8,9-triisopropyl-2,5,8,9-tetra-aza-1-phosphabi-cyclo[3.3.3]undecane promotes the allylation of aldehydes with allyltrimethylsilane as a Lewis base catalyst (eq 65). ... 

The Lewis base-catalyzed reactions oftrichlorosilyl-protected carbon nucleophiles are important as silicon Lewis acid-promoted reactions. This review, however, does not deal with these topics because of the limited space and the availabihty of recent reviews [2, 38-40]. Denmark and coworkers have recently succeeded in enantioselective reactions using SiCU and chiral Lewis base catalysts, which are detailed in this section. 

Shackelford and co-workers studied the 1,2-addition of 2,2-dinitropropanol, 2,2,2-trinitroethanol, and 2-fluoro-2,2-dinitroethanol across the double bonds of vinyl ethers. These reactions are Lewis acid catalyzed because of the weak nucleophilic character of alcohols which contain two or three electron-withdrawing groups on the carbon p to the hydroxy functionality. Base catalysis is precluded since alkaline conditions lead to deformylation with the formation of formaldehyde and the nitronate salt. 

Silicon-based Lewis acids have been known for some time, and the related chemistry in catalysis has recently been reviewed [24]. Most examples in the literature are mainly based on achiral species and will be discussed only briefly in this section. In general, a broad variety of reactions can be catalyzed with compounds like MejSiOTf, MejSiNTf or MOjSiClO. One advantage over some metal Lewis acids is that they are compatible with many carbon nucleophiles like silyl enol ethers, allyl organometallic reagents and cuprates. 

Whether the nucleophile attacks the carbon or the heteroatom attacks the electrophilic species, the rate-determining step is usually the one involving nucleophihc attack. It may be observed that many of these reactions can be catalyzed by both acids and bases.Bases catalyze the reaction by converting a reagent of the form YH to the more powerful nucleophile (see p. 490). Acids catalyze it by converting the substrate to an heteroatom-stabilized cation (formation of 3), thus making it more attractive to nucleophilic attack. Similar catalysis can also be found with metallic ions (e.g., Ag ) which act here as Lewis acids. We have mentioned before (p. 242) that ions of type 3 are comparatively stable carbocations because the positive charge is spread by resonance. 

Carbon dioxide is a linear molecule in which the oxygen atoms are weak Lewis (and Br0nsted) bases and the carbon is electrophilic. Reactions of carbon dioxide are dominated by nucleophilic attacks at the carbon, which result in bending of the O—C—O angle to about 120°. Figure 5.3 illustrates four very different nucleophilic reactions hydroxide attack on CO2 to form bicarbonate the initial addition of ammonia to CO2, which ultimately produces urea the binding of CO2 to a macrocyclic cobalt(I) complex, which catalyzes CO2 reduction and the addition of an electron to CO2 to yield the carbon dioxide radical ion. The first three also exemplify the reactivity of CO2 with respect to nucleophilic attack on the carbon. 

Abstract Diazo compounds continue both to challenge and to fascinate practitioners of chemical synthesis. The most strategically powerful and unique type of reactivity observed with these reagents is a formal insertion of the dmior-acceptor carbon into C-C or C-H bmids alpha to carbonyl groups. Although the reaction does not involve discrete carbon-metal bonds, it can be catalyzed by metal-based Lewis acids. This chapter investigates both classical and modem developments in diazoalkyl carbon insertion with a special emphasis on nonstabilized nucleophiles. 

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