Regioselectivity electrophile reactions

TTie condensation of fluormated carbonyl compounds is a versatile approach to the stereo- and regioselective construction of specifically tluonnated materials It is possible to use fluormated reactants in vanous oxidation states and to utilize them either as the nucleophilic or electrophilic reaction partners Stereocontrol in those processes using the fluormated carbonyl compounds m a nucleophilic fashion has lagged behind the achievement of higher yields... 

Regioselectivity in reactions of heterocyclic anions of allyl-type with electrophiles 99CRV665. 

The n orbital amplitudes of ethene are identical on both carbons. Unsymmetrical substitutions polarize the n orbital. Electron acceptors or electrophiles attack the carbon with the larger r amplitude. The polarization of frontier orbitals is important for regioselectivities of reactions. Here, mechanism of the n orbital polarization of ethene by methyl substitution [4] is described (Scheme 5). 

Construction of the carbazole framework was achieved by slightly modifying the reaction conditions previously reported for the racemic synthesis (641,642). The reaction of the (R)-arylamine 928 with the iron complex salt 602 in air provided by concomitant oxidative cyclization the tricarbonyliron-complexed 4b,8a-dihydro-9H-carbazole (931). Demetalation of the complex 931, followed by aromatization and regioselective electrophilic bromination, afforded the 6-bromocarbazole 927, which represents a crucial precursor for the synthesis of the 6-substituted carbazole... 

Few other electrophilic reactions are known. The trimethyl-3/f-pyrazoIe 105 is oxidized by Af-bromoacetamide (NBA) to the bromohydrin 168 and cyclized further to the epoxide 169 by base (Scheme 63).165 Peracid oxidation of 53 gives the mono ALoxide 75 regioselectively (Scheme 28).108... 

We assume that readers are familiar with facts concerning the similarities and differences between benzene and pyridine derivatives. Pyridine is a base (pKa = 5.19, affording a pH of 8.5 for a 20% aqueous solution. It has a dipole moment of 2.15 D, as seen in Table 3). The fact that the nitrogen heteroatom with a higher electronegativity than carbon causes a depletion of 7i-electrons in y and in a positions leads to regioselective electrophilic substitution in p positions and, conversely, facilitates nucleophilic attack mainly in a positions, e.g., affording a-picoline and lithium hydride from the reaction of pyridine with methyllithium. 

The high regioselectivity of reactions of the salts 231 with various electrophilic agents was demonstrated by alkylation of 5-(3-chloropropyl)-l //-tctrazolc 250 with trityl chloride resin in the presence of Et3N. The corresponding resin containing the Nz-isomer 251 was obtained (Equation 39) <2000JC019>. 

The last chapter introduced some good disconnections based on carbonyl compounds as both nucleophiles and electrophiles but avoided all questions of chemo- or regioselectivity. These reactions are so important that you need to understand how to control these issues. All the chief difficulties crop up in the synthesis of the conjugated enone 1. 

Kinetic Aspects of Ar-SE Reactions Reactivity and Regioselectivity in Reactions of Electrophiles with Substituted Benzenes... 

In contrast to the protonation, the regioselectivity of reactions between other electrophiles and allenyl enolates derived from 2-en-4-ynoates is independent of the steric and electronic properties of the reaction partners.7,7a... 

As pointed out by Stork and coworkers in their definitive 1963 paper3, the reaction with electrophilic alkenes is especially successful since reaction at nitrogen is reversible. Reaction at the /2-carbon is (usually) rendered irreversible by, in the case of cyclohexanone enamines, internal proton transfer of the axial C-/2 proton to the anionic centre of the initially formed zwitterionic intermediate (34), under conditions of stereoelectronic control (Scheme 22). When this intramolecular proton transfer cannot occur in aprotic solvents, or when the product produced in protic solvents is a stronger carbon acid than adduct 35 (i.e. when Z = COR, N02), then carbon alkylation is also reversible and surprising changes in the regioselectivity of reaction may be observed (vide infra see also Section VI.D and Chapter 26). Cyclobutanes (36) and, in the case of a,/ -unsaturated... 

The regioselectivity of reaction of 2-tetralone enamines depends on the amine moiety as well as the electrophilic reagent. For example, / -nitrostyrene reacts with the morpholine enamine mainly at Cl9 whereas the pyrrolidine enamine reacts exclusively at C3, in contrast to phenyl isocyanate which reacts with both enamines at (V11. / -Nitrostyrene reacts with the morpholine enamine of fraw.s-decalin-2-one at C3 from an axial direction,... 

VI. AMIDOESTERS. STEREOSELECTIVE, REGIOSELECTIVE AND ELECTROPHILIC REACTIONS AT THE a-CARBON... 

Similarly to derivatives phenylhydrazone, 1,4-dilithio oximes also react with an achiral ester to give a heterocyclic isoxazole ring102 (equation 33). When two of the a-carbons on an oxime of a ketone are able to undergo deprotonation, the question of the deprotonation site arises. It was found that the regioselectivity in the electrophilic reaction of 1,4-dilithioketoxime salts is especially high and the electrophile reacts with the carbon syn to the oximic oxygen. Kofron and Yeh proved that this is the case by... 

Regioselective electrophilic bromination of 7-methoxy-3-methylcarbazole followed by ether cleavage and introduction of the prenyl group in a nickel-mediated coupling reaction provides siamenol (five steps, 34 % overall yield) [75] (Scheme 15.22). 

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