Proton transfer reactions of N‐aryl triazolium salts: unusual ortho‐substituent effects1. (7th December 2014)

• Record Type: Journal Article
• Authors: Tucker, David E.; Quinn, Peter; Massey, Richard S.; Collett, Christopher J.; Jasiewicz, David J.; Bramley, Christopher R.; Smith, Andrew D.; O'Donoghue, AnnMarie C.; Moss, R. A.; Gudmundsdottir, A.
• Is Part Of: Journal of physical organic chemistry. Volume 28:Number 2(2015:Feb.)
• Page Start: 108
• Page End: 115
• Publication Date: 2014-12-07
• Subjects: Chemistry, Physical organic -- Periodicals; 547.1
• DOI: 10.1002/poc.3399
• Languages: English
• ISSNs: 0894-3230

Abstract

<abstract abstract-type="main"> <title> <x xml:space="preserve">Abstract</x> </title> <p>Previous studies of the C(3)‐hydrogen/deuterium <named-content content-type="reactionType" xlink:type="simple" xmlns:xlink="http://www.w3.org/1999/xlink">exchange reactions</named-content> of the triazolium ion conjugate acids of triazolyl N‐heterocyclic carbenes revealed a change of mechanism under acidic conditions with N1‐protonation to a dicationic salt. Interestingly, the data suggested an <italic>increase</italic> in p<italic>K</italic><sub>a</sub><sup>N1</sup> in the presence of a <italic>N</italic>‐pentafluorophenyl substituent relative to other <italic>N</italic>‐aryl substituents with hydrogens or methyl substituents rather than fluorines at the <italic>ortho</italic>‐positions. To probe the presence of an apparent donor effect of a <italic>N</italic>‐pentafluorophenyl substituent, which differs from the more common electron withdrawing effect of this group, we have studied the analogous deuterium <named-content content-type="reactionType" xlink:type="simple" xmlns:xlink="http://www.w3.org/1999/xlink">exchange reactions</named-content> of four triazolium salts with heteroatoms or heteroatom substituents in the 2‐position and/or 6‐position of the <italic>N</italic>‐aryl ring. These include triazolium salts with <italic>N</italic>‐2, 4, 6‐tribromophenyl <bold>11</bold>, <italic>N</italic>‐2, 6‐dichlorophenyl <bold>12</bold>, <italic>N</italic>‐2‐pyridyl <bold>13</bold> and <italic>N</italic>‐2‐pyrimidinyl <bold>14</bold> substituents. The log <italic>k</italic><sub>ex</sub> – p<italic>D</italic> profiles for <bold>11</bold>, <bold>12</bold> and <bold>14</bold> were found to show similar trends at lower p<italic>D</italic>s as for the previously studied <italic>N</italic>‐pentafluorophenyl triazolium salt, hence supporting the presence an apparent donor effect on p<italic>K</italic><sub>a</sub><sup>N1</sup>. Surprisingly, the log <italic>k</italic><sub>ex</sub> – p<italic>D</italic> profile for <italic>N</italic>‐pyridyl salt <bold>13</bold> uniquely showed <named-content content-type="reactionType chemicalTechnology" xlink:type="simple" xmlns:xlink="http://www.w3.org/1999/xlink">acid catalysis</named-content> at lower p<italic>D</italic>s. We propose herein that this data is best explained by invoking an <named-content content-type="reactionType" xlink:type="simple" xmlns:xlink="http://www.w3.org/1999/xlink">intramolecular</named-content> general base role for the <italic>N</italic>‐(2‐pyridyl) substituent in conjunction with N1‐protonation on the triazolium ring. Finally, the second order rate constants for deuteroxide ion catalysed C(3)‐H/D exchange (<italic>k</italic><sub>DO</sub>, M<sup>−1</sup> s<sup>−1</sup>), which could be obtained from data at p<italic>D</italic>s >1.5, were used to provide estimates of C(3)‐carbon acid p<italic>K</italic><sub>a</sub><sup>C3</sup> values for the four triazolium salts <bold>11</bold>, <bold>12</bold>, <bold>13</bold>, <bold>14</bold>. Copyright © 2014 John Wiley & Sons, Ltd.</p> </abstract>