Substituent effects on the formation and nucleophile selectivity of ring‐substituted phenonium ions in aqueous solution†. (18th June 2013)

• Record Type: Journal Article
• Authors: Tsuji, Yutaka; Ogawa, Shin; Richard, John P.; Page, Michael I.
• Is Part Of: Journal of physical organic chemistry. Volume 26:Number 12(2013:Dec.)
• Page Start: 970
• Page End: 976
• Publication Date: 2013-06-18
• Subjects: Chemistry, Physical organic -- Periodicals; 547.1
• DOI: 10.1002/poc.3145
• Languages: English
• ISSNs: 0894-3230

Abstract

<abstract abstract-type="main"> <title> <x xml:space="preserve">Abstract</x> </title> <p>The reaction of 2‐(4‐methyphenyl)ethyl tosylate (<bold>Me-1-OTs</bold>) in 50/50 (v/v) trifluoroethanol/water at 25 °C is first order in the concentration of azide anion nucleophile. A carbon‐13 NMR analysis of the products of the reactions of Me‐1‐[α‐<sup>13</sup>C]OTs in 50/50 (v/v) trifluoroethanol/water at 25 °C shows the formation of Me‐1‐[β‐<sup>13</sup>C]OH, Me‐1‐[β‐<sup>13</sup>C]OCH<sub>2</sub>CF<sub>3</sub> and <bold>Me-1-[β-13C]N3</bold> from the trapping of a symmetrical 4‐methylphenonium ion reaction <named-content id="d25e704" content-type="reactionType" xlink:type="simple" xmlns:xlink="http://www.w3.org/1999/xlink">intermediate</named-content> Me‐2<sup>+</sup>. The formation of <bold>Me-1-[α-13C]N3</bold> by concerted bimolecular displacement of azide ion at Me‐1‐[α‐<sup>13</sup>C]OTs (<italic>k</italic><sub>N</sub> = 3.8 × 10<sup>−6</sup> M<sup>−1</sup> s<sup>−1</sup>) and of Me‐1‐[α‐<sup>13</sup>C]OH and Me‐1‐[α‐<sup>13</sup>C]OCH<sub>2</sub>CF<sub>3</sub> by concerted bimolecular displacement of solvent (<italic>k</italic><sub>solv</sub> = 1.8 × 10<sup>−8</sup> s<sup>−1</sup>) is also observed. An analysis of the rate and product data provides a value of <italic>k</italic><sub>az</sub>/<italic>k</italic><sub>s</sub> = 32 M<sup>−1</sup> for partitioning of Me‐2<sup>+</sup> between addition of azide ion and solvent that is nearly three‐fold smaller than <italic>k</italic><sub>az</sub>/<italic>k</italic><sub>s</sub> = 83 M<sup>−1</sup> reported in an earlier study on the partitioning of MeO‐2<sup>+</sup> [<italic>J. Org. Chem</italic>. 2011, <italic>76</italic>, 9568]. This change is attributed to a decrease in nucleophile selectivity with increasing electrophile reactivity for the activation‐limited addition of solvent and azide anion to X‐2<sup>+</sup>. These data set a limit of 1/<italic>k</italic><sub>s</sub> ≥ 10<sup>−7</sup> s for the lifetime of <bold>Me-2+</bold> in aqueous solution. Copyright © 2013 John Wiley & Sons, Ltd.</p> </abstract>