Electrophilic trapping reactions

Metal-Catalyzed Tandem Conjugate Addition/Electrophilic Trapping Reactions... 

In this area, Krische and co-workers have developed a family of catalytic transformations based on the use of enones as latent enolates. Nucleophilic activation of the enone is induced via carbometallation, nucleophilic organocatalysis or hydrometallation. The following examples illustrate some aspects of these new catalytic conjugate addition/electrophilic trapping reactions. 

The following series of allylic and 0-stannyl ketyl reactions is intended to provide insight into the reactivity and potential of these interesting synthetic intermediates. Various cyclizations, strained ring scissions, and electrophile trapping reactions will be demonstrated on diverse substrates such as ketones, aldehydes, enones and a-ketocyclopropanes. All of the chemistry described herein utilizes mild and neutral reaction conditions to accomplish a wide range of synthetic transformations. 

More recently, bisoxazolines were studied as ligands in [2,3]-Wittig rearrangements (up to 89% ee) [46] these ligands were also effective in a catalytic maimer [47]. Bisoxazoline 30 provides remarkably high enantioselectivity in the benzyhc lithiation-electrophile trapping reactions of benzyl methyl ether, iso-chroman and phthalan (up to 97% ee) (e.g.,Scheme 15) [48]. 

Directed orrAo-metallalions were also applied to quinoxalines. 2-Methoxyquinoxaline 255 was metallated and then trapped with numerous electrophiles <01T4489>. However, the yields of desired products 256 were low due to substantial dimerization which limited the efficiency of the electrophilic trapping reaction. 

In summary, selective halogen/metal exchange allows the consecutive introduction of three different functionalities by electrophilic trapping reactions in the quinohne positions 2, 3, and 4. 

Use of LTMP as base [52] in situ with Me3SiCl allows straightforward access to a variety of synthetically useful a, 3-epoxysilanes 232 at near ambient temperature directly from (enantiopure) terminal epoxides 231 (Scheme 5.55) [81]. This reaction relies on the fact that the hindered lithium amide LTMP is compatible with Me3SiCl under the reaction conditions and that the electrophile trapping of the nonstabilized lithiated epoxide intermediate must be very rapid, since the latter are usually thermally very labile. 

Sulfonylaziridine 243 was halogenated in carbon tetrahalides in the presence of KOH as base [86] (Scheme 5.61). Although other examples of electrophile trapping of sulfonyl- and phosphonyl-stabilized metalated aziridines exist, the reactions were not stereoselective [87]. 

Direct deprotonation/electrophile trapping of simple aziridines is also possible. Treatment of a range of N-Bus-protected terminal aziridines 265 with LTMP in the presence ofMe3SiCl in THF at-78 °C stereospecifically gave trans-a, 3-aziridinylsi-lanes 266 (Scheme 5.67) [96]. By increasing the reaction temperature (to 0 °C) it was also possible to a-silylate a (3-disubstituted aziridine one should note that attempted silylation of the analogous epoxide did not provide any of the desired product [81],... 

Extension of these processes to provide enantio-enriched products was successfully applied after desymmetrization of the starting materials. An example is shown below (Reaction 76), where silane-mediated xanthate deoxygenation-rearrangement-electrophile trapping afforded the conversion of (+)-94 to (+)-95 in 56% yield. ... 

Under suitable conditions, this can be a useful preparative method for cyclopropanes another preparative trapping reaction of CC12 is its electrophilic attack on phenols in the Reimer-Tiemann reaction (p. 290). 

As previously mentioned, allenes can only be obtained by 1,6-addition to acceptor-substituted enynes when the intermediate allenyl enolate reacts regioselectively with an electrophile at C-2 (or at the enolate oxygen atom to give an allenyl ketene acetal see Scheme 4.2). The regioselectivity of the simplest trapping reaction, the protonation, depends on the steric and electronic properties of the substrate, as well as the proton source. Whereas the allenyl enolates obtained from alkynyl enones 22 always provide conjugated dienones 23 by protonation at G-4 (possibly... 

Scheme 4.3. Regioselectivity of trapping reactions of acyclic allenyl enolates with different electrophiles.

Scheme 20). By trapping reactions of monolithiated 56, which is obtained by reaction of acetylene and n-butyllithium in THF at —78°C, with several electrophiles in yields up to 98%, it could be shown that the reaction gives monolithioacetylene... 

Tris(4-hthiophenyl)methane (76), a l,9,9 -trilithiated compound, was obtained by Oda and coworkers . The starting tribromo compound 75 reacts with three equivalents of w-butyUithium in THF at low temperatures. Trapping reactions with various electrophiles resulted in yields between 59 and 90% (Scheme 27). 

Although early works have reported variable yields and substrate dependence, a marked tendency to stereospecificity has been noticed. For example, the reaction of the trans epoxysilane 176 (Scheme 74) with lithium reagents results, after Si-Li exchange, electrophilic trapping of the resulting lithiooxirane by RLi and Li20 elimination, in the stereoselective formation of an E) alkene in good yield. ... 

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