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Lopez-Calahorra and co-workers proposed an alternative mechanism involving thiazolium dimer VI with the reaction occurring through nucleophilic attack of this species on benzaldehyde to yield VII, followed by formation of intermediate type VIII, lacking stabilization of the carbanion. See, for example
-
Lopez-Calahorra and co-workers proposed an alternative mechanism involving thiazolium dimer VI with the reaction occurring through nucleophilic attack of this species on benzaldehyde to yield VII, followed by formation of intermediate type VIII, lacking stabilization of the carbanion. See, for example: Lopez-Calahorra, F.; Rubires, R. Tetrahedron 1995, 51, 9713
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79953222851
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See Supporting Information for the details of the kinetic experiments and the corresponding graphs.
-
See Supporting Information for the details of the kinetic experiments and the corresponding graphs.
-
-
-
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40
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79953218273
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The same kinetic analysis was conducted on catalyst 3a showing little difference in the corresponding catalytic cycle.
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The same kinetic analysis was conducted on catalyst 3a showing little difference in the corresponding catalytic cycle.
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-
-
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41
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77956896366
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This is consistent with Berkessel's observation that the catalyst resting state in the triazolylidene-catalyzed benzoin in THF is a dioxolane resulting from the tetrahedral alkoxy intermediate analogous to II adding another molecule of aldehyde in a reversible process. Thus, if proton transfer to generate III is slow, the alkoxide would prefer adding to an aldehyde to avoid the charge on oxygen in the absence of solvation. See
-
This is consistent with Berkessel's observation that the catalyst resting state in the triazolylidene-catalyzed benzoin in THF is a dioxolane resulting from the tetrahedral alkoxy intermediate analogous to II adding another molecule of aldehyde in a reversible process. Thus, if proton transfer to generate III is slow, the alkoxide would prefer adding to an aldehyde to avoid the charge on oxygen in the absence of solvation. See: Berkessel, A.; Elfert, S.; Etzenbach-Effers, K.; Teles, J. H. Angew. Chem., Int. Ed. 2010, 49, 7120
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42
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33947293711
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A 1,2-proton shift is a symmetry forbidden transformation (, and references therein) but has been calculated to have an ∼29 kcal/mol barrier (thiazolylidine) and ∼51 kcal/mol barrier (cyanide) in the formation of the acyl anion equivalent from formaldehyde See:; J. Mol. Model. 2006, 12, 591
-
A 1,2-proton shift is a symmetry forbidden transformation (Kemp, D. S. J. Org. Chem. 1971, 36, 202 and references therein) but has been calculated to have an ∼29 kcal/mol barrier (thiazolylidine) and ∼51 kcal/mol barrier (cyanide) in the formation of the acyl anion equivalent from formaldehyde See: Goldfuss, B.; Schumacher, M. J. Mol. Model. 2006, 12, 591
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Kemp, D.S.1
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44
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79953185990
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4 is completely insoluble under these conditions.
-
4 is completely insoluble under these conditions.
-
-
-
-
45
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79953206159
-
-
That substrate 12 still undergoes reaction means that there must be another mechanism available for proton transfer, presumably either via Hypothesis II above or involving another molecule of catalyst or intermediate, as argued by Yates (see ref 18).
-
That substrate 12 still undergoes reaction means that there must be another mechanism available for proton transfer, presumably either via Hypothesis II above or involving another molecule of catalyst or intermediate, as argued by Yates (see ref 18).
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