Stereoselectivity aromatic nucleophilic

It has been observed that in basic media aromatic hydrocarbons and benzylmethyl ethers bound to -Cr(CO)3 are easily nitrosated at the benzylic position. A review type article discusses the stereoselective manipulation of acetals derived from o-substituted benzaldehyde chromium tricarbonyls. The diastereoselective synthesis of a range of substituted cyclohexadienes has been reported from enandomerically pure (2-phenyl-4,S-dibydroxazole)chromium tiicaibonyl complexes. Aromatic nucleophilic substitution on halaogenoarene tricarbonylchromium complexes gives rise to a series of complexed aniline derivatives. The diastereoselective 1,4-addition of organocuprates to o-substituted-phenyI-(E)-enone chromium carbonyl conq)lexes provides a new method for remote stereocontrol at the 1,3, and S-positimis of the side chains. ... 

Campiani G, Nacci V, Corelli F, Anzini M (1991) Polycondensed heterocycles. Vll. A convenient synthesis of pyTrolo[l,2-a]quinoxaline derivatives by intramolecular aromatic nucleophilic displacement. Synth Commun 21(15-16) 1567-1576. doi 10.1080/00397919108021054 Chaudhuri G, Kundu NG (2000) A highly regio- and stereoselective synthesis of (Z)-... 

With Sulfur Nucleophiles N-Carboxy-protected aziridine-2-carboxylates react with thiols to give P-mercapto-ot-amino acid derivatives. The reaction is usually catalyzed by BF3 and the yields range from fair to excellent [15, 16, 108-111]. With N-unprotected 3-substituted aziridine-2-carboxylates, the ring-opening with thiols usually takes place with anti stereoselectivity, especially in the case of the C-3 aliphatic substituted substrates. In cases in which C-3 is aromatic, however, the stereoselectivity has been found to be a function of the substitution pattern on the aromatic ring 3-p-methoxy ph eri yl-su bs li In led aziridines 143a (Scheme 3.51) and... 

In recent years, the importance of aliphatic nitro compounds has greatly increased, due to the discovery of new selective transformations. These topics are discussed in the following chapters Stereoselective Henry reaction (chapter 3.3), Asymmetric Micheal additions (chapter 4.4), use of nitroalkenes as heterodienes in tandem [4+2]/[3+2] cycloadditions (chapter 8) and radical denitration (chapter 7.2). These reactions discovered in recent years constitute important tools in organic synthesis. They are discussed in more detail than the conventional reactions such as the Nef reaction, reduction to amines, synthesis of nitro sugars, alkylation and acylation (chapter 5). Concerning aromatic nitro chemistry, the preparation of substituted aromatic compounds via the SNAr reaction and nucleophilic aromatic substitution of hydrogen (VNS) are discussed (chapter 9). Preparation of heterocycles such as indoles, are covered (chapter 10). 

One aspect of asymmetric catalysis has become clear. Every part of the molecule seems to fulfill a role in the process, just as in enzymic catalysis. Whereas many of us have been used to simple acid or base catalysis, in which protonation or proton abstraction is the key step, bifunctional or even multifunctional catalysis is the rule in the processes discussed in this chapter.Thus it is not only the increase in nucleophilicity of the nucleophile by the quinine base (see Figures 6 and 19), nor only the increase in the electrophilicity of the electrophile caused by hydrogen bonding to the secondary alcohol function of the quinine, but also the many steric (i.e., van der Waals) interactions between the quinoline and quinuclidine portions of the molecule that exert the overall powerful guidance needed to effect high stereoselection. Important charge-transfer interactions between the quinoline portion of the molecule and aromatic substrates cannot be excluded. 

Electron transfer sensitization allows either the radical cation or the radical anion of an aromatic alkene to form as desired, which finally results in nucleophile addition with Markovnikov and anti-Markovnikov regiochemistry. In an apolar solvent, the tight radical ion pair undergoes a stereoselective reaction when the electron-accepting sensitizer is chiral (Figure 3.10). ... 

Nitrile oxides are widely used as dipoles in cycloaddition reactions for the synthesis of various heterocyclic rings. In order to promote reactions between nitrile oxides and less reactive carbon nucleophiles, Auricchio and coworkers studied the reactivity of nitrile oxides towards Lewis acids. They observed that, in the presence of gaseous BF3, nitrile oxides gave complexes in which the electrophilicity of the carbon atom was so enhanced that it could react with aromatic systems, stereoselectively yielding aryl oximes 65 and 66 (Scheme 35). ... 

The excellent ability of late transition metal complexes to activate alkynes to nucleophilic attack has made them effective catalysts in hydroamination reactions. The gold(l)-catalyzed cyclizations of trichloroacetimidates 438, derived from homopropargyl alcohols, furnished 2-(trichloromethyl)-5,6-dihydro-4f/-l,3-oxazines 439 under exceptionally mild conditions (Equation 48). This method was successfully applied to compounds possessing aliphatic and aromatic groups R. With R = Ph, cyclization resulted in formation of 439 with complete (Z)-stereoselectivity <2006OL3537>. 

Addition and cyclization reactions of imines, catalysed by IS r0 ns led acids, have been reviewed, including examples in water solvent and enantioselective cases.65 Another review examines stereoselective nucleophilic additions to the C=N bond of aromatic azines (60 references).66 (g)... 

Triethoxysilyl acetylene (51) allows a new organocatalytic approach toward the introduction of the alkyne moiety via a nucleophilic addition to aromatic aldehydes, ketones, and aldimines, with EtOK as catalyst (10 mol%). Although a catalytic asymmetric version has not yet been developed, the application of a chiral auxiliary, in the case of imines 53 (Scheme 7.9), led to an impressively high dia-stereoselectivity (20 1) [54], unparalleled by other acetylenic organometallics. 

The reaction of 1-iodonaphthalene with chiral-assisted imide enolate ion 3 provides an interesting example of the stereoselective coupling of an aromatic radical with a nucleophile. In this reaction, the diastereomeric isomers of the substitution compound are formed (43-64%), while the seledivity observed is highly dependent on the metal counterion used and its chelation properties (Scheme 10.11). All of the ions studied (Li, Na, K, Cs, Ti(IV)) are seledive however, the highest stereoseledivity... 

We talked a lot about regioselectivity two chapters ago, when you learned how to predict and explain which product(s) you get from electrophilic aromatic substitution reactions. The functional group is the aromatic ring where it reacts is the reaction s regioselectivity. Going back further, one of the first examples of regioselectivity you came across was nucleophilic addition to an unsaturated ketone. Addition can take place in a 1,2- or a 1,4-fashion—the question of which happens (where the unsaturated ketone reacts) is a question of regioselectivity, which we discussed in Chapters 10 and 23. We shall leave all discussion of stereoselectivity until Chapters 31-34. 

Arene(tricarbonyl)chromium complexes undergo a number of synthetically important transformations not usually observed for uncomplexed arenes. The chromium tricarbonyl moiety facilitates nucleophilic, electrophilic, and radical reactions at the benzylic position. Upon complexation, one side of the aromatic ring and adjacent functionalities is blocked by the metal carbonyl moiety and highly stereoselective reactions are usually observed even at relatively remote positions. In addition, the protons of the complexed aromatic ring have a substantially higher acidity and are readily removed and further substituted by electrophiles. Finally, the aromatic ring is activated toward addition reactions using a variety of nucleophiles. 

Stereoselective, conjngate allylation of more remote positions nsing aUyl trimethylsilane and TiCU is possible. Further transformation by nucleophilic addition and ringclosing metathesis affords fused polycychc ring systems (Scheme 88). Stabilized anions and organocopper see Copper OrganometaUic Chemistry) reagents also participate in Michael-type additions to complexed Q, /3-unsaturated aromatic enones. 

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