Substrate regioselectivity

Substrate Regioselectivity (% branched linear) Enantioselectivity (% ee for branched alcohol)... 

Allyl carbonates with a sp -substituent (alkenyl, aryl) are privileged substrates regioselectivities of b/1 >98 2 are obtained, using ligand LI in conjunction with procedures (d) or (e) or ligand L2 in conjunction with procedures (b), (d) or (e). 

Unlike all-carbon aryl halide substrates, regioselective Stille coupling of 2,3-dihalofurans has been observed. Taking advantage of C(2) activation, regioselective Stille coupling was achieved using 2,3-dibromofuran 27 to afford 72 [35]. 

For other polysubstituted arenes or heteroarenes, deprotonation at different sites can compete and yield product mixtures. The first reaction in Scheme 5.46 is an example of kinetically controlled carbanion formation, which shows that for some substrates regioselective metalations might be achieved by careful control of the reaction conditions. 

Hydroxylation primarily occurs at the y-position of the prohne ring to form y-hydroxyproline (6), although fl-hydroxyprohne is also formed in smaller amounts. These two products apparently result from two closely related enzymes exhibiting distinct substrate regioselectivity. 

The substrates being metabolized at different rates may share various types of relationships. They may be chemically dissimilar or similar (e.g., analogs), in which case the term of substrate selectivity is used in a narrow sense. Alternatively, the substrates may be isomers such as positional isomers (regioiso-mers) or stereoisomers, resulting in substrate regioselectivity or substrate stereoselectivity. Substrate enantioselectivity is a particular case of the latter (see Section 5.1.2). 

In some respects achieving the product specificities is the most important goal. Substrate specificity is critical in the soup of components found in a biological cell, but chemical reactions can usually be performed on pure substrates where substrate specificity of a catalyst is irrelevant. However, the ability of an enzyme to achieve selective reactions on a particular section of the substrate (regioselectivity) and with specific stereochemical consequences (stereoselectivity) are features to be admired and imitated in chemical systems. 

Activation energy of metabolism reactions with eflective atomic descriptors Training molecules not described (394 metabolic sites for aliphatic hydroxylation, 27 for A-dealky-lation, 9 for O-dealkylation, 85 for aromatic hydroxylation). N = 46 CYP3A4 substrates. Regioselectivity predictions 60% correct. 205... 

Thus, the data discussed in this chapter indicate broad possibilities for the use of macrocycles in catalysis and as nanoreactors. The use of macrocyclic receptors on calixarene platform as components of catalytic systems results in a significant increase of catalytic activity, as well as the attainment of substrate regioselectivity. 

Regioselectivity in FGIs is dominated by the pattern of substituents and by steric effects in the substrate as well as by the choice of appropriate reagents. 

In what may be an example of tme cluster catalysis, [HRU3 (CO) ] shows good catalytic activity and high regioselectivity using propylene as substrate (24,25). Solvent, CO partial pressure, and temperature are important variables. In monoglyme, at 80°C and starting partial pressures for C H, ... 

In view of the overall increased reactivity of furan compared with thiophene it would be anticipated that furan would be less regioselective in its reactions with electrophiles than thiophene. Possible reasons for the high regioselectivity of furan in electrophilic substitution reactions include complex formation between substrates and reagents and the ability of heteroatoms to assist in the stabilization of cationic intermediates (80CHE1195). 

Perfluoroalkylation of substituted benzenes and heterocyclic substrates has been accomplished through thermolysis of perfluoroalkyl iodides in the presence of the appropriate aromatic compound Isomeric mixtures are often obtained W-Methylpyrrole [143] and furan [148] yield only the a-substituted products (equation 128) Imidazoles are perfluoroalkylated under LTV irradiation [149] (equation 129). 4-Perfluoroalkylimidazoles are obtained regioselectively by SET reactions of an imidazole anion with fluoroalkyl iodides or bromides under mild conditions [150] (equation 130) (for the SET mechanism, see equation 57)... 

The relative ease of preparation of condensed thiazole derivatives is a consequence of facile thiazole ring closure, and therefore also benzothiazole amines with an amino group on the benzene ring (except for the weakly regioselective nitration of benzothiazoles) are very easily accessible and useful substrates for the Gould-Jacobs reaction. 

Regioselective nucleophilic substitution at the 5 position is proved to occur when 1-hydroxytryptophan and -tryptamine derivatives are treated with 85% HCOOH (99H1157). Truly amazing is the fact that only substrates carrying a C—C—N structure in the side chain at the 3 position can undergo this regioselective substitution. 

Depending on the substrate and the other reaction parameters, very higli re-gioselectivilies towards either a or y suhstilution can he obtained. In cetLain cases, the regioselectivity can easily he switclied between the two modes by changing the reaction conditions [11]. Compared to, for example, palladiumiO)-catalyzed allylic substitution reactions, the possibility of switching between S j2 and S j2 selectivity... 

Hie reactions were regioselective iu all cases, widi yia ratios of 90 10. Hie maximum ee, 8796, was obtained by treatment of a substrate coutaiuiug die elec-iron-wididrawing R substituent 4-CF widi diueopentylziuc. Cbaugiug die... 

Cuprates react rapidly witli allylic halides for acetates) [17, 23], propargyl halides for acetates) [ 106-108], and vinyloxiraties, often witli S 2 regioselectivity iSclieme 10.9) [17]. Hie reaction takes place witli emit stereodieniistry iwitli respect to tlie leaving group), while syn substitution occurs when an allylic carbamate is employed as tlie substrate [109]. 

With an unsymmetrical ketone as CH-acidic substrate, two regioisomeric products can be formed. A regioselective reaction may in such cases be achieved by employing a preformed iminium salt instead of formaldehyde and ammonia. An iminium salt reagent—the Eschenmoser salt—has also found application in Mannich reactions. ... 

---