Scope of Electrophile

Most allylic substitution reactions have been conducted witti derivatives of allylic alcohols, such as acetates, phosphates, and carbonates. These reactions occur with allyl electrophiles displaying a wide structural variation. The allylic electrophiles can be cyclic or acyclic, substituted with aliphatic or aromatic groups, substituted at one or both termini, and substituted or unsubstituted at the central carbon. As discussed in more detail below, these substituents affect the regioselectivity of the substitution process. 

The relative reactivity of the derivatives follows the trend allyl carbonate allyl phosphate allyl acetate (see Equations 20.5-20.7). This difference in reactivity allows for chemoselective substitutions of one allylic alcohol derivative over another. Other allylic electrophiles, such as allylic sulfonates, which undergo cleavage of the carbon-sulfur bond, allylic nitro compounds, which undergo cleavage of the C-N bond, - and allylic 

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The scope of electrophilic aromatic substitution is quite large both the aromatic com pound and the electrophilic reagent are capable of wide variation Indeed it is this breadth of scope that makes electrophilic aromatic substitution so important Elec trophilic aromatic substitution is the method by which substituted derivatives of benzene are prepared We can gam a feeling for these reactions by examining a few typical exam pies m which benzene is the substrate These examples are listed m Table 12 1 and each will be discussed m more detail m Sections 12 3 through 12 7 First however let us look at the general mechanism of electrophilic aromatic substitution... 

The scope of electrophiles was explored with malonates and p-ketoesters, providing chiral amine adducts in high yield and enantioselectivities (Scheme 57) [109]. Addition of cyclic P-ketoesters was also explored with hydrazines, providing cyclic and bicyclic chiral amines with quaternary centers in high enantiomeric ratios (Scheme 58). 

Akiyama and coworkers extended the scope of electrophiles applicable to asymmetric Brpnsted acid catalysis with chiral phosphoric acids to nitroalkenes (Scheme 57). The Friedel-Crafts alkylation of indoles 29 with aromatic and aliphatic nitroalkenes 142 in the presence of BINOL phosphate (7 )-3r (10 mol%, R = SiPhj) and 3-A molecular sieves provided Friedel-Crafts adducts 143 in high yields and enantioselectivities (57 to >99%, 88-94% ee) [81]. The use of molecular sieves turned out to be critical and significantly improved both the yields and enantioselectivities. 

The discussion in this chapter highlights the performance of O-substituted hydroxylamines and oximes as electrophilic amino transfer reagents. In conclusion, the scope of electrophilic amination will be greatly enhanced by developing new O-substituted hydroxylamines and oximes and further improvements in electrophilic amination would be of great synthetic interest. 

Os(NHi)5 2+ - The Pentaammineosmium(ll) Fragment Tab. 13. Scope of electrophilic additions for N-substituted aniline complexes. 

Among the methodological studies [9d, 9e, 20d], the rich chemistry of carbinolamine alk-oxide DMGs 59-63 (Scheme 22) [39], while not explored with regard to the scope of electrophile introduction, predicts access to highly functionalized systems. Similarly as yet unexplored but of foreseeable value for 3,5-disubstituted patterns is the use of the O-carbamate DMG, 64 —> 65 (Scheme 23) [40]. Thus, in a prototype sequence, metalation-electrophile... 

A wide variety of electrophiles can effect aromatic substitution. Usually, it is a substitution of some other group for hydrogen that is of interest, but this is not always the case. Eor example, both silicon and mercury substituents can be replaced by electrophiles. Scheme 9.1 lists some of the specific electrophiles that are capable of carrying out substitution of hydrogen. Some indication of the relative reactivity of the electrophiles is given as well. Many of these electrophiles are not treated in detail until Part B. Nevertheless, it is important to recognize the very broad scope of electrophilic aromatic substitution. 

To expand of the scope of electrophiles in the lithiation-substitution reaction, aldehydes were employed and a highly diastereoselective homoaldol methodology was developed [104]. After generation of lithiated 141 under the standard conditions, transmetallation with EtjAlCl or TiCl(Oi-Pr)3, and substitution with various aldehydes provided the homoaldol products with both high diastereo-and enantioselectivities (Scheme 48). 

The scope of electrophilic aromatic substitution is quite large both the aromatic compound and the electrophilic reagent are capable of wide variation. Indeed, it is this breadth of scope that makes electrophilic aromatic substitution so important. Electrophilic aromatic... 

Scope of Electrophilic Cleavage of Metal-Carbon and Metal-Hydride a[Pg.454]

The scope of electrophile used in the vinylation reaction can be extended to aryl bromides by employing 2-(di-f-butylphosphino)-biphenyl as a ligand for palladium and raising the temperature to 50 °C (eq 3). The optimized reaction conditions employ a slightly higher loading of Da in comparison to the reaction with aryl iodides. The additional amount of Da is needed to suppress a secondary Heck reaction that provides the symmetrical stUbene as a byproduct. A broad range of styrene derivatives can be prepared by this method as shown in Table 3. 

Scheme 18.12 Scope of electrophiles in the three component sulfone synthesis.

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