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      Catalytic Enantioselective [2,3]-Rearrangements of Allylic Ammonium Ylides: A Mechanistic and Computational Study

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          Abstract

          A mechanistic study of the isothiourea-catalyzed enantioselective [2,3]-rearrangement of allylic ammonium ylides is described. Reaction kinetic analyses using 19F NMR and density functional theory computations have elucidated a reaction profile and allowed identification of the catalyst resting state and turnover-rate limiting step. A catalytically relevant catalyst–substrate adduct has been observed, and its constitution elucidated unambiguously by 13C and 15N isotopic labeling. Isotopic entrainment has shown the observed catalyst–substrate adduct to be a genuine intermediate on the productive cycle toward catalysis. The influence of HOBt as an additive upon the reaction, catalyst resting state, and turnover-rate limiting step has been examined. Crossover experiments have probed the reversibility of each of the proposed steps of the catalytic cycle. Computations were also used to elucidate the origins of stereocontrol, with a 1,5-S···O interaction and the catalyst stereodirecting group providing transition structure rigidification and enantioselectivity, while preference for cation−π interactions over C–H···π is responsible for diastereoselectivity.

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          Calculation of Equilibrium Constants for Isotopic Exchange Reactions

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            A Case Study of the Mechanism of Alcohol-Mediated Morita Baylis–Hillman Reactions. The Importance of Experimental Observations

            The mechanism of the Morita Baylis–Hillman reaction has been heavily studied in the literature, and a long series of computational studies have defined complete theoretical energy profiles in these reactions. We employ here a combination of mechanistic probes, including the observation of intermediates, the independent generation and partitioning of intermediates, thermodynamic and kinetic measurements on the main reaction and side reactions, isotopic incorporation from solvent, and kinetic isotope effects, to define the mechanism and an experimental mechanistic free-energy profile for a prototypical Morita Baylis–Hillman reaction in methanol. The results are then used to critically evaluate the ability of computations to predict the mechanism. The most notable prediction of the many computational studies, that of a proton-shuttle pathway, is refuted in favor of a simple but computationally intractable acid–base mechanism. Computational predictions vary vastly, and it is not clear that any significant accurate information that was not already apparent from experiment could have been garnered from computations. With care, entropy calculations are only a minor contributor to the larger computational error, while literature entropy-correction processes lead to absurd free-energy predictions. The computations aid in interpreting observations but fail utterly as a replacement for experiment.
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              A combined kinetic-quantum mechanical model for assessment of catalytic cycles: application to cross-coupling and Heck reactions.

              The efficiency of catalytic cycles is measured by their turnover frequency (TOF). The degree of TOF control determines which states contribute most to the rate of the cycle, and thus indicates the steps that have the highest impact on the cycle. A kinetic model developed by Christiansen (Christiansen, J. A. Adv. Catal. 1953, 5, 311) for catalytic cycles is implemented here in a form that utilizes state energies. This enables one to assess the efficiency of quantum mechanically computed catalytic cycles like the palladium-catalyzed cross-coupling and Heck reactions, to test alternative hypotheses, and to make some predictions. This implementation can also account for effects such as Sabatier's volcano curve for heterogeneous catalysis. The model leads to a dependence of the TOF for any cycle on the "corrected" energy span quantity, deltaE, whose precise expression depends on the location of the summit and trough of the cycle in the step sequence of the cycle. Thus, knowing the highest energy transition state, the most abundant reaction intermediate, and the reaction energy enables one to make quick predictions about relative efficiency of cycles. At the same time, the degree of TOF control determines which states contribute most to the rate of reaction, and thus indicates the values to be included in the calculation of the energetic span and the steps that may be tinkered with to improve the cycle.
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                Author and article information

                Journal
                J Am Chem Soc
                J. Am. Chem. Soc
                ja
                jacsat
                Journal of the American Chemical Society
                American Chemical Society
                0002-7863
                1520-5126
                23 February 2017
                29 March 2017
                : 139
                : 12
                : 4366-4375
                Affiliations
                []EaStCHEM, School of Chemistry, University of St Andrews , North Haugh, St. Andrews, KY16 9ST, U.K.
                []Department of Chemistry, Oregon State University , 153 Gilbert Hall, Corvallis, Oregon 97333, United States
                [§ ]EaStCHEM, School of Chemistry, University of Edinburgh , Joseph Black Building, David Brewster Road, Edinburgh, EH9 3FJ, U.K.
                Author notes
                Article
                10.1021/jacs.6b11851
                5374492
                28230365
                d8942f7c-1d33-406d-bd4f-fb5d4cbac857
                Copyright © 2017 American Chemical Society

                This is an open access article published under a Creative Commons Attribution (CC-BY) License, which permits unrestricted use, distribution and reproduction in any medium, provided the author and source are cited.

                History
                : 16 November 2016
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                ja6b11851
                ja-2016-11851w

                Chemistry
                Chemistry

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