Enantioselective Catalysis by Chiral Isothioureas Merad, Jérémy; Pons, Jean-Marc; Chuzel, Olivier ...
European journal of organic chemistry,
December 2016, Letnik:
2016, Številka:
34
Journal Article
Recenzirano
This review covers recent developments relating to organocatalyzed transformations by chiral isothioureas (ITUs) since their original introduction by Birman in 2006. This class of nucleophilic ...heterocycles was first involved in anhydride activation in enantioselective acyl transfer reactions, but it was more recently shown that activation of other reagents was possible, considerably enlarging their number of catalytic enantioselective transformations. Four main modes of activation as Lewis bases can currently be listed: (1) acylisothiouronium intermediates involved in acyl transfer, (2) silylisothiouronium species involved in silyl transfer, (3) acylisothiouronium enolates involved in several concerted and formal pericyclic transformations, and (4) α,β‐unsaturated acylisothiouronium species involved in domino transformations. This review is organized according to these different modes of activation of chiral isothioureas.
Molecular Swiss Army Knives! This review covers the reactivity of chiral isothioureas, which emerged as nucleophilic organocatalysts in 2006. Multiple modes of activation rapidly came to be displayed by these heterocycles; this now makes them powerful catalysts in the toolbox of the organic chemist.
This review article describes the evolution of Woodward's mechanistic thinking, beginning in the late 1930s and early 1940s with his proposal of a charge‐transfer mechanism for the Diels–Alder ...reaction, eventually leading to the Woodward–Katz two‐stage concerted mechanism in 1959, and then to its mechanistic solution in terms of orbital symmetry control. Houk′s research in the Woodward labs, testing the predictions of this theory, is described. Subsequent modern calculations with quantum mechanics and molecular dynamics simulations have shown that Woodward indeed had perfectly described not only the cyclopentadiene dimerization mechanism, but a new class of transition states now known as ambimodal or bis‐pericyclic transition states. In recent years, the Houk group has found that ambimodal reactions are operative in the 6+4 cycloaddition. Molecular dynamics simulations of many Diels–Alder and ambimodal cycloadditions provide a time‐resolved picture of how these reactions occur. Lastly, Roald Hoffmann provides a Coda in which he describes his joy in “being taken along the journey” of the cycloaddition story from Woodward's youth to today's trajectory simulations.
The evolution of understanding of the Diels–Alder reaction mechanism is described, from R. B. Woodward's boyhood visions, to Ken Houk's experiences as a graduate student in Woodward's laboratory, to today's dynamical view of Diels–Alder and higher‐order cycloadditions. Roald Hoffmann's Coda gives his view of Woodward and the zig‐zag processes of scientific discovery.
A novel metal‐free methodology for the synthesis of diacyl/acyl N‐allylhydrazines based on PhI(OAc)2‐promoted 1,2‐diaza‐Cope rearrangement reaction of β,γ‐unsaturated hydrazones with acetate/H2O has ...been described. This cascade reaction works efficiently under mild conditions. A possible mechanism involving hydrazonyl radicals and N–I(III) intermediates of β,γ‐unsaturated hydrazones is proposed.
A novel metal‐free methodology for the synthesis of diacyl/acyl N‐allylhydrazines based on PhI(OAc)2‐promoted 1,2‐diaza‐Cope rearrangement reaction of β,γ‐unsaturated hydrazones with acetate/H2O has been described. This cascade reaction works efficiently and features good substrate tolerance under mild conditions. A possible mechanism involving hydrazonyl radicals and N–I(III) intermediates of β,γ‐unsaturated hydrazones is proposed.
Nitrogen‐substituted alkynes such as ynamines and ynamides are competent partners in 4+2 cycloaddition reactions. This microreview aims to discuss in a historical context the main achievements in the ...work on this class of reactions that leads to valuable nitrogen‐containing heterocycles in a single‐step fashion. In the last section, our own work in the field of inverse‐electron‐demand Diels–Alder reactions between ynamides and pyrimidines is discussed.
A brief overview of the 4+2 cycloadditions of N‐alkynes (ynamines and ynamides) is presented. This journey starts in the 1960s and highlights the potential of these pericyclic reactions in delivering complex nitrogen‐containing heterocycles in a single‐step fashion.
We report an unprecedented domino polycyclization from readily available 2,4‐dienals and cyclic α,β‐unsaturated imines that is initiated by an iso‐Nazarov reaction. This Brønsted acid promoted ...reaction enables the concomitant formation of four bonds, three cycles, and four contiguous stereogenic centers to yield elaborated structures in a single operation. A range of fused hexacyclic molecules is obtained in a highly diastereoselective manner.
Ring, ring, ring: An unprecedented domino polycyclization from readily available 2,4‐dienals and cyclic α,β‐unsaturated imines, initiated by an iso‐Nazarov reaction, is reported. This Brønsted acid promoted transformation affords original polycyclic scaffolds and enables the formation of three new cycles, four new bonds, and four contiguous stereocenters in a single operation and in a highly diastereoselective manner.
Complexity‐increasing Domino reactions comprising C−H allenylation, a Diels–Alder reaction, and a retro‐Diels–Alder reaction were realized by a versatile catalyst derived from earth‐abundant, ...non‐toxic manganese. The C−H activation/Diels–Alder/retro‐Diels–Alder alkyne annulation sequence provided step‐economical access to valuable indolone alkaloid derivatives through a facile organometallic C−H activation manifold with transformable pyridines.
One stone, three birds: C−H/C−N/C−C functionalizations were accomplished through an effective domino C−H activation/Diels–Alder/retro‐Diels–Alder manifold by using an earth‐abundant, non‐toxic manganese catalyst. This alkyne annulation sequence provides step‐economical access to valuable indolone alkaloid derivatives.
Vibrational strong coupling (VSC) has recently been shown to change the rate and chemoselectivity of ground‐state chemical reactions via the formation of light–matter hybrid polaritonic states. ...However, the observation that vibrational‐mode symmetry has a large influence on charge‐transfer reactions under VSC suggests that symmetry considerations could be used to control other types of chemical selectivity through VSC. Here, we show that VSC influences the stereoselectivity of the thermal electrocyclic ring opening of a cyclobutene derivative, a reaction which follows the Woodward–Hoffmann rules. The direction of the change in stereoselectivity depends on the vibrational mode that is coupled, as do changes in rate and reaction thermodynamics. These results on pericyclic reactions confirm that symmetry plays a key role in chemistry under VSC.
Vacuum‐field strong coupling with molecular vibrations imposes symmetry constraints which alter the Woodward–Hoffmann selectivity of cyclobutene ring opening under thermal conditions.
The development of methods to detect damage in macromolecular materials is of paramount importance to understand their mechanical failure and the structure–property relationships of polymers. ...Mechanofluorophores are useful and sensitive molecular motifs for this purpose. However, to date, tailoring of their optical properties remains challenging and correlating emission intensity to force induced material damage and the respective events on the molecular level is complicated by intrinsic limitations of fluorescence and its detection techniques. Now, this is tackled by developing the first stress‐sensing motif that relies on photon upconversion. By combining the Diels–Alder adduct of a π‐extended anthracene with the porphyrin‐based triplet sensitizer PtOEP in polymers, triplet–triplet annihilation photon upconversion of green to blue light is mechanochemically activated in solution as well as in the solid state.
Stress sensing with upconverted light is realized by combining latent Diels–Alder adducts of π‐extended anthracene and maleimide as mechanophores with a porphyrin triplet sensitizer in polymers. Triplet–triplet annihilation is activated by force and allows better volume coverage within the material and lower energetic excitation of the fluorophores compared to stress sensing by regular fluorescence.
Pericyclic reactions are powerful transformations for the construction of carbon-carbon and carbon-heteroatom bonds in organic synthesis. Their role in biosynthesis is increasingly apparent, and ...mechanisms by which pericyclases can catalyse reactions are of major interest
. 4+2 cycloadditions (Diels-Alder reactions) have been widely used in organic synthesis
for the formation of six-membered rings and are now well-established in biosynthesis
. 6+4 and other 'higher-order' cycloadditions were predicted
in 1965, and are now increasingly common in the laboratory despite challenges arising from the generation of a highly strained ten-membered ring system
. However, although enzyme-catalysed 6+4 cycloadditions have been proposed
, they have not been proven to occur. Here we demonstrate a group of enzymes that catalyse a pericyclic 6+4 cycloaddition, which is a crucial step in the biosynthesis of streptoseomycin-type natural products. This type of pericyclase catalyses 6+4 and 4+2 cycloadditions through a single ambimodal transition state, which is consistent with previous proposals
. The 6+4 product is transformed to a less stable 4+2 adduct via a facile Cope rearrangement, and the 4+2 adduct is converted into the natural product enzymatically. Crystal structures of three pericyclases, computational simulations of potential energies and molecular dynamics, and site-directed mutagenesis establish the mechanism of this transformation. This work shows how enzymes are able to catalyse concerted pericyclic reactions involving ambimodal transition states.
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