The conversion of chemical energy to drive directional motion at the molecular level allows biological systems, ranging from subcellular components to whole organisms, to perform a myriad of dynamic ...functions and respond to changes in the environment. Directional movement has been demonstrated in artificial molecular systems, but the fundamental motif of unidirectional rotary motion along a single-bond rotary axle induced by metal-catalysed transformation of chemical fuels has not been realized, and the challenge is to couple the metal-centred redox processes to stepwise changes in conformation to arrive at a full unidirectional rotary cycle. Here, we present the design of an organopalladium-based motor and the experimental demonstration of a 360° unidirectional rotary cycle using simple chemical fuels. Exploiting fundamental reactivity principles in organometallic chemistry enables control of directional rotation and offers the potential of harnessing the wealth of opportunities offered by transition-metal-based catalytic conversions to drive motion and dynamic functions.
Non‐racemic chiral boronic esters are recognised as immensely valuable building blocks in modern organic synthesis. Their stereospecific transformation into a variety of functional groups—from amines ...and halides to arenes and alkynes—along with their air and moisture stability, has established them as an important target for asymmetric synthesis. Efforts towards the stereoselective synthesis of secondary and tertiary alkyl boronic esters have spanned over five decades and are underpinned by a wealth of reactivity platforms, drawing on the unique and varied reactivity of boron. This Review summarizes strategies for the asymmetric synthesis of alkyl boronic esters, from the seminal hydroboration methods of H. C. Brown to the current state of the art.
Non‐racemic chiral boronic esters are immensely valuable building blocks in modern organic synthesis. Their stereospecific transformation into a variety of functional groups, along with their air and moisture stability, has established them as an important target for asymmetric synthesis. Methods for their asymmetric synthesis now span a wealth of reactivity platforms.
We report the first enantioselective Rh-catalyzed Markovnikov hydroboration of unactivated terminal alkenes. Using a novel sp2–sp3 hybridized diboron reagent and water as a proton source, a broad ...range of alkenes undergo hydroboration to provide secondary boronic esters with high regio- and enantiocontrol.
The development of new chemical transformations based on catalytic functionalization of unactivated C-H bonds has the potential to simplify the synthesis of complex molecules dramatically. Transition ...metal catalysis has emerged as a powerful tool with which to convert these unreactive bonds into carbon-carbon and carbon-heteroatom bonds, but the selective transformation of aliphatic C-H bonds is still a challenge. The most successful approaches involve a 'directing group', which positions the metal catalyst near a particular C-H bond, so that the C-H functionalization step occurs via cyclometallation. Most directed aliphatic C-H activation processes proceed through a five-membered-ring cyclometallated intermediate. Considering the number of new reactions that have arisen from such intermediates, it seems likely that identification of distinct cyclometallation pathways would lead to the development of other useful chemical transformations. Here we report a palladium-catalysed C-H bond activation mode that proceeds through a four-membered-ring cyclopalladation pathway. The chemistry described here leads to the selective transformation of a methyl group that is adjacent to an unprotected secondary amine into a synthetically versatile nitrogen heterocycle. The scope of this previously unknown bond disconnection is highlighted through the development of C-H amination and carbonylation processes, leading to the synthesis of aziridines and β-lactams (respectively), and is suggestive of a generic C-H functionalization platform that could simplify the synthesis of aliphatic secondary amines, a class of small molecules that are particularly important features of many pharmaceutical agents.
Responsive systems have recently gained much interest in the scientific community in attempts to mimic dynamic functions in biological systems. One of the fascinating potential applications of ...responsive systems lies in catalysis. Inspired by nature, novel responsive catalytic systems have been built that show analogy with allosteric regulation of enzymes. The design of responsive catalytic systems allows control of catalytic activity and selectivity. In this Review, advances in the field over the last four decades are discussed and a comparison is made amongst the dynamic responsive systems based on the principles underlying their catalytic mechanisms. The catalyst systems are sorted according to the triggers used to achieve control of the catalytic activity and the distinct catalytic reactions illustrated.
Hunting for switches: Dynamic responsive catalysts have drawn chemists’ attention in recent years. However, numerous challenges remain. This Review surveys advances in the development of responsive systems for catalytic functions and gives a comparison of different systems based on the principles underlying catalysis and switching.
Copper catalysts enable the electrophilic carbofunctionalization of alkynes with vinyl- and diaryliodonium triflates. The new process forms highly substituted alkenyl triflates from a range of ...alkynes via a pathway that is opposite to classical carbometalation. The alkenyl triflate products can be elaborated through cross-coupling reactions to generate synthetically useful tetrasubstituted alkenes
Dam removal: Listening in Foley, M. M.; Bellmore, J. R.; O'Connor, J. E. ...
Water resources research,
July 2017, 2017-07-00, 20170701, Letnik:
53, Številka:
7
Journal Article
Recenzirano
Dam removal is widely used as an approach for river restoration in the United States. The increase in dam removals—particularly large dams—and associated dam‐removal studies over the last few decades ...motivated a working group at the USGS John Wesley Powell Center for Analysis and Synthesis to review and synthesize available studies of dam removals and their findings. Based on dam removals thus far, some general conclusions have emerged: (1) physical responses are typically fast, with the rate of sediment erosion largely dependent on sediment characteristics and dam‐removal strategy; (2) ecological responses to dam removal differ among the affected upstream, downstream, and reservoir reaches; (3) dam removal tends to quickly reestablish connectivity, restoring the movement of material and organisms between upstream and downstream river reaches; (4) geographic context, river history, and land use significantly influence river restoration trajectories and recovery potential because they control broader physical and ecological processes and conditions; and (5) quantitative modeling capability is improving, particularly for physical and broad‐scale ecological effects, and gives managers information needed to understand and predict long‐term effects of dam removal on riverine ecosystems. Although these studies collectively enhance our understanding of how riverine ecosystems respond to dam removal, knowledge gaps remain because most studies have been short (< 5 years) and do not adequately represent the diversity of dam types, watershed conditions, and dam‐removal methods in the U.S.
Key Points
Dam removal is an increasingly common approach to river restoration in the United States
Dam‐removal studies provide insights on key controls influencing the physical and ecological responses to dam removal
Although many aspects of physical and ecological systems react quickly to dam removal, overall response trajectories depend on how and where dams are removed and overall watershed conditions
The synthetically versatile pinacol boronic ester group (Bpin) is generally thought of as a bulky moiety because of the two adjacent quaternary sp3‐hydribized carbon atoms in its diol backbone. ...However, recent diastereoselective reactions reported in the literature have cast doubt on this perception. Reported herein is a detailed experimental and computational analysis of Bpin and structurally related boronic esters which allows determination of three different steric parameters for the Bpin group: the A‐value, ligand cone angle, and percent buried volume. All three parameters suggest that the Bpin moiety is remarkably small, with the planarity of the oxygen‐boron‐oxygen motif playing an important role in minimising steric interactions. Of the three steric parameters, percent buried volume provides the best correlation between steric size and diastereoselectivity in a Diels–Alder reaction.
The Bpin group is generally perceived to be a sterically bulky substituent, but the data reported here show that it has a surprisingly small A‐value (0.42 kcal mol−1; cf. Me is 1.7, OH is 0.87). The percent buried volume seems to give a more accurate indication of the size of the Bpin moiety, which is somewhere between a primary and secondary alkyl group.
PDF
