The past few decades have seen tremendous progress in the synthesis and operation of molecular systems capable of controlled mechanical movement. Here we review the use of molecular machines as catalysts for controlling chemical reactions. We highlight the various catalyst designs with a focus on how the mechanical motion is used to control catalysis with varying degrees of success. This review discusses the current challenges of designing effective catalysts, the scope and limitations of various systems, as well as future potential and aims for the field. Although it is difficult to predict which concepts will become most important as so much work is at the proof of concept level, it seems clear that molecular machines have the potential to significantly impact the field of catalysis.We present an overview of innovations in using molecular machines as catalysts and discuss the concepts and principles emerging from the field. It is apparent that selectivity is a key challenge. 14 Perfectly selective switching of devices between 'on/off' states or between distinct catalytic functions has proven difficult to achieve. As with all catalysis, product chemo-and stereo-selectivity is also challenging. In addition, molecular machines must deal with kinetic factors that may Molecular machine: a system in which a stimulus triggers the controlled motion of one molecular or submolecular component relative to another and potentially results in a net task (or work) being done. 7 Chemoselectivity:The preferential reaction of one functional group over another in a chemical reaction. 15 Stereoselectivity:The preferential formation of one stereoisomer over another in a chemical reaction. If the stereoisomers are enantiomers, enantioselectivity applies (quantified by enantiomeric excess, e.e., or enantiomeric ratio, e.r.), if they are diastereomers, diastereoselectivity applies (quantified by diasteriomeric ratio, d.r.). 16
The control of tetrahedral carbon stereocentres remains a focus of modern synthetic chemistry and is enabled by their configurational stability. By contrast, trisubstituted nitrogen1, phosphorus2 and sulfur compounds3 undergo pyramidal inversion, a fundamental and well-recognized stereochemical phenomenon that is widely exploited4. However, the stereochemistry of oxonium ions—compounds bearing three substituents on a positively charged oxygen atom—is poorly developed and there are few applications of oxonium ions in synthesis beyond their existence as reactive intermediates5,6. There are no examples of configurationally stable oxonium ions in which the oxygen atom is the sole stereogenic centre, probably owing to the low barrier to oxygen pyramidal inversion7 and the perception that all oxonium ions are highly reactive. Here we describe the design, synthesis and characterization of a helically chiral triaryloxonium ion in which inversion of the oxygen lone pair is prevented through geometric restriction to enable it to function as a determinant of configuration. A combined synthesis and quantum calculation approach delineates design principles that enable configurationally stable and room-temperature isolable salts to be generated. We show that the barrier to inversion is greater than 110 kJ mol−1 and outline processes for resolution. This constitutes, to our knowledge, the only example of a chiral non-racemic and configurationally stable molecule in which the oxygen atom is the sole stereogenic centre.
The pyramidal inversion of trisubstituted nitrogen, phosphorus and sulfur compounds and its impact on configurational stability is a fundamental and well-recognized stereochemical phenomenon that is widely exploited. In contrast, the chemistry of oxonium ions – compounds bearing three substituents on a positively charged oxygen atom – is poorly developed and there are few applications in synthesis beyond their existence as reactive intermediates. There are no examples of configurationally stable oxonium ions in which the oxygen atom is the sole stereogenic centre, likely due to the low barrier to oxygen pyramidal inversion, and the perception that all oxonium ions are highly reactive. Here we describe the design, synthesis and characterization of a helically chiral triaryloxonium ion in which inversion of the oxygen lone pair is prevented through geometric restriction to enable it to function as a determinant of configuration. A combined synthesis and quantum calculation approach delineate design principles that enable configurationally stable and room-temperature isolable salts to be generated. We show that the barrier to inversion is >110 kJ mol-1 and outline a process for resolution. This constitutes the only example of a chiral non-racemic and configurationally stable molecule in which the oxygen atom is the sole stereogenic centre.
Arynes are highly reactive and versatile intermediates for the functionalization of aromatic rings that are often generated using strong bases or fluoride sources, which in some cases can limit functional group tolerance. Here we demonstrate that triaryl oxonium ions can be transformed into arynes through treatment with solid potassium phosphate at room temperature. A substantial range of functional group-bearing arynes including 4,5-pyrimidynes may be generated and trapped by cycloaddition reactions in high yields. Other arynophiles including nitrones, alkenes, and azides are compatible with these conditions. Quantum computation in conjunction with an intramolecular kinetic isotope study is consistent with an E1cB-like mechanism of elimination to form the aryne. These investigations demonstrate that the oxonium ion is a powerful electron-withdrawing group and a particularly effective leaving group. We anticipate this study will stimulate further investigations into the synthetic utility of aryl oxonium ions.
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