Selectivity_SUPP - H 3C H Selectivity in Organic Chemistry...

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1 Selectivity in Organic Chemistry H 3 C H O CHEMISTRY 215 TEXTBOOK SUPPLEMENT S 1.1 Overview of Selectivity A. Chemoselectivity When an organic compound contains more than one functional group, the question arises about whether a reaction will take place at one, the other, or both of those groups. For example, the two functional groups in 4-oxopentanoic acid, a ketone and an acid, are both capable of being reduced with lithium aluminum hydride. O OH O 4-oxopentanoic acid 1) LiAlH 4 2) acid workup OH OH On the other hand, the reaction between 4-oxopentanoic acid and sodium borohydride results in the reduction of the ketone and only the deprotonation of the acid. 1,4-pentanediol O OH O O OH NaBH 4 CH 3 OH O Na OH OH O H 3 O 4-hydroxypentanoic acid When a reagent, such as sodium borohydride, reacts exclusively or predominantly in the competition between two or more functional groups that could give the same reaction, then the reaction is called chemoselective . Sodium borohydride, then, is a more chemoselective reducing agent than lithium aluminum hydride.
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2 Throughout your study of organic chemistry, you have encountered many examples of chemoselectivity. Differences in electrophilic and nucleophilic character can both result in differences in chemoselectivity. Here are a few familiar examples of chemoselectivity. HO NH 2 O O O (CH 3 CH 2 ) 3 N 1.0 equivalent HO N H O O H 2 Pd-C O H O OCH 3 O PhMgBr 1.0 equivalent 1) 2) protonation Ph O OCH 3 OH CH 3 H 2 Pd-BaSO 4 quinoline H 3 C When there is no chemoselective method for carrying out a reaction where there could be a competition, chemists often use the protecting group strategy you saw employed in the synthesis of peptides. In the following example, the combination of unprotected alanine with unprotected phenylalanine could give four different dipeptides plus amounts of tri- and tetrapeptide byproducts. Although the protecting group strategy prevents this from happening, it adds extra experimental steps to the procedure. The search for chemoselective methods for carrying out reactions is of ongoing interest because it avoids the need for the time, effort, and additional chemical waste associated with using protecting groups. H 3 N O OH H CH 3 H 3 N O OH H CH 2 Ph + alanine (Ala) phenylalanine (Phe) Ala-Ala Ala-Phe Phe-Ala Phe-Phe four possible dipeptides formed with no predictable selectivity CH 3 OH cat. H 2 SO 4 H 2 N O OCH 3 H CH 3 Cl OCH 2 Ph O N H O OH H CH 2 Ph O PhH 2 CO DCC N H O H N H CH 2 Ph O PhH 2 CO O OCH 3 H 3 C H 1) H 2 /Pd-C 2) anhydrous HBr Phe-Ala only H H (CH 3 CH 2 ) 3 N 1) 2) adjust pH adjust pH 1) 2) +
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3 B. Regioselectivity Regioselectivity is the preference for one direction of bond-making, or bond-breaking, when there is a choice, in a chemical reaction. You first encountered this in the electrophilic addition reactions of strong and weak Bronsted acids to carbon-carbon pi bonds (eq. 1). Other examples include the selectivity for opening a bromonium ion intermediate with water (formation of halohydrins) (eq. 2), the preference for ortho, meta, and para substitution in electrophilic aromatic substitution reactions (eq. 3), and the
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