Publication Date:
2014-12-09
Description:
Previous studies of the C(3)-hydrogen/deuterium exchange reactions 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 increase in p K a N1 in the presence of a N -pentafluorophenyl substituent relative to other N -aryl substituents with hydrogens or methyl substituents rather than fluorines at the ortho -positions. To probe the presence of an apparent donor effect of a N -pentafluorophenyl substituent, which differs from the more common electron withdrawing effect of this group, we have studied the analogous deuterium exchange reactions of four triazolium salts with heteroatoms or heteroatom substituents in the 2-position and/or 6-position of the N -aryl ring. These include triazolium salts with N -2,4,6-tribromophenyl , N -2,6-dichlorophenyl , N -2-pyridyl and N -2-pyrimidinyl substituents. The log k ex – p D profiles for , and were found to show similar trends at lower p D s as for the previously studied N -pentafluorophenyl triazolium salt, hence supporting the presence an apparent donor effect on p K a N1 . Surprisingly, the log k ex – p D profile for N -pyridyl salt uniquely showed acid catalysis at lower p D s. We propose herein that this data is best explained by invoking an intramolecular general base role for the N -(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 ( k DO , M −1 s −1 ), which could be obtained from data at p D s 〉1.5, were used to provide estimates of C(3)-carbon acid p K a C3 values for the four triazolium salts . Copyright © 2014 John Wiley & Sons, Ltd. We have studied the C(3)-H/D exchange reactions in D 2 O of four triazolium salts with heteroatoms or heteroatom substituents in the 2-position and/or 6-position of the N -aryl ring. The log k ex – p D rate profiles in each case are consistent with a change in mechanism at p D 〈 1.5 as a result of N 1-deuteronation of the triazole ring. In addition, formal acid catalysis of deuterium exchange was uniquely observed for the N -pyridyltriazolium salt providing evidence for a possible intramolecular proton transfer reaction involving the pyridyl substituent.
Print ISSN:
0894-3230
Electronic ISSN:
1099-1395
Topics:
Chemistry and Pharmacology
,
Physics
Permalink