Chem E2a-Lecture 6-Annotated-2013

13 neighboring group parcipaon when a molecule has

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Unformatted text preview: important element in biological reactions! Consider the coenzyme S-adenosyl methionine (abbreviated SAM): NH2 N CH3 H3N N S N N O CO 2 OH OH S AM •  In proteins, linkages between polypeptide chains or within the same chain are often formed through disulfide bridges, involving residues of the amino acid cysteine. 13 Neighboring Group Par:cipa:on •  When a molecule has a nucleophilic atom (often S or N) or electron pair and a leaving group nearby in the same molecule, it can undergo an intramolecular SN2 reaction that may then be followed by an intermolecular SN2 reaction! Relative Rate H2O Cl S 100 OH oC H2O Cl 100 S oC OH + HCl 1 + HCl 3200 •  Why is the second reaction much faster? 14 Reading: Section 21.2 Here is another example of neighboring group participation. Predict the product(s) that will be observed. S Et NaCN OTs S A.  a CN S B.  Et b Et 60% CN CN c Et E.  racemic mixture of (a) & (c) F.  racemic mixture of (b) & (d) 8% 2% b S 9% 5% ce m c ic m ra ce ixt d m ur e ic m o. ix .. tu re .. CN ra d 16% a D.  Et S C.  Neighboring Group Par:cipa:on •  Here is another example of neighboring group participation. Draw a curved-arrow mechanism and predict the product(s) that will be observed. S Et NaCN OTs 15 Reading: Section 21.2...
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This document was uploaded on 01/29/2014.

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