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John C. Walton

Bio: John C. Walton is an academic researcher from University of St Andrews. The author has contributed to research in topics: Radical & Electron paramagnetic resonance. The author has an hindex of 40, co-authored 379 publications receiving 6261 citations. Previous affiliations of John C. Walton include National Institute for Nanotechnology & National Research Council.


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TL;DR: Metal-free radical chain sequences based around cyclohexadiene derivatives are being developed, and tetrathiafulvalenes mimic metals and allow a crossover from homolytic to ionic chemistry.
Abstract: An urgent search is being made for generic reagents that promote free radical synthetic transformations as means of banishing the toxic threat of organotin hydrides. Although second-generation tin reagents are beguiling, organosilanes and a range of thiocarbonyl compounds are more intrinsically benign. Metal-free radical chain sequences based around cyclohexadiene derivatives are being developed (see reaction scheme), and tetrathiafulvalenes mimic metals and allow a crossover from homolytic to ionic chemistry. Z=alkene.

280 citations

Journal ArticleDOI
TL;DR: These are the first NHC-boranes with boron-sulfur bonds, and their structures have been secured by spectroscopic and crystallographic means and Spectral analysis suggests that it is a pi-radical analogous to the benzyl radical.
Abstract: Reduction of xanthates by N-heterocyclic carbene boranes (NHC-boranes) has been suggested to occur by a radical chain mechanism involving heretofore unknown NHC-boryl radicals. In support of this suggestion, both the expected borane dithiocarbonate product and an unexpected borane xanthate product have now been isolated. These are the first NHC-boranes with boron-sulfur bonds, and their structures have been secured by spectroscopic and crystallographic means. The first rate constants for H-atom transfer from an NHC borane complex were determined by using the ring opening of a substituted cyclobutylcarbinyl radical as a clock reaction. The rate constant for reaction of the NHC-borane with a secondary alkyl radical at ambient temperature is 4 x 10(4) M(-1) s(-1), and the Arrhenius function displayed an entropic term (log A term) that was typical for a bimolecular reaction. The B-H bond dissociation energy of an NHC-borane complex has been estimated at 88 kcal/mol. The putative NHC-boryl radical in these transformations has been detected by EPR spectroscopy. Spectral analysis suggests that it is a pi-radical, analogous to the benzyl radical.

221 citations

Journal ArticleDOI
TL;DR: A convenient system for classifying free-radical cascades is described that is useful for evaluating and comparing cascades and aids the design of synthetic routes to polycyclic structures.
Abstract: Cascade, domino, or tandem processes, that link together two or more transformations in one pot, are increasing in popularity because they lead to improvements in synthetic efficiency and decreases in environmental impact. Not only do these cascades contain choice mechanistic gems but they also deliver compact and elegant syntheses of complex natural products. Longer cascades require more functional groups precisely configured within carefully designed initial molecular architectures. Such "purposeful" molecules can be thought of as chemical algorithms.This article surveys the phenomenal range of unimolecular free-radical cascades. A convenient system for classifying free-radical cascades is described that is useful for evaluating and comparing cascades and aids the design of synthetic routes to polycyclic structures.Double cyclization cascades lead to cyclopentylcyclopentane or bicyclo[3.3.0]octane derivatives. Precursors that contain a ring as a template have been used to control stereochemistry in syntheses of triquinanes and many related compounds. Of the cascades containing ring-cleavage steps, the most useful are the ring expansions which have opened up new synthetic routes to medium ring polycycles.The key design features of three-stage unimolecular free-radical cascades that yielded steroid structures, are linear arrays of radical acceptor units associated with methyl groups distributed every fifth C-atom in the precursor polyenes. Ring cleavage is the reverse of cyclization. In special, symmetrical structures, therefore, this led to sequences that were reversible, thus launching endlessly repeating cascades supported by delightfully fluxional structures. The science of "programming" organic molecules to achieve particular target structures is maturing rapidly. Coordination and classification of the welter of information in this area is intended to facilitate design and hence to extend the range and complexity of attainable structures.

196 citations

Journal ArticleDOI
TL;DR: Spectroscopic data and DFT computations demonstrated that the NHC-BH(2)* radicals are planar pi-delocalized species, which is well suited for their reactivity to be tuned by inclusion of different NHC ring units and by appropriate placement of diverse substituents.
Abstract: N-Heterocyclic carbene boranes (NHC−boranes) are a new “clean” class of reagents suitable for reductive radical chain transformations. Their structures are well suited for their reactivity to be tuned by inclusion of different NHC ring units and by appropriate placement of diverse substituents. EPR spectra were obtained for the boron-centered radicals generated on removal of one of the BH3 hydrogen atoms. This spectroscopic data, coupled with DFT computations, demonstrated that the NHC−BH2• radicals are planar π-delocalized species. tert-Butoxyl radicals abstracted hydrogen atoms from NHC−boranes more than 3 orders of magnitude faster than did C-centered radicals, although the rate decreased markedly for sterically shielded NHC−BH3 centers. Combinations of two NHC−boryl radicals afforded 1,2-bis-NHC−diboranes at rates which also depended strongly on steric shielding. The termination rate increased to the diffusion-controlled limit for sterically unhindered NHC−boryls. Bromine atoms were rapidly transferre...

183 citations

Journal ArticleDOI
TL;DR: In this article, the self-initiated oxidation of β-carotene with molecular oxygen was shown to depend on the oxygen partial pressure, and the main products in the early stage of the oxidation were shown to be 5,6-epoxy-β-Carotene.

158 citations


Cited by
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[...]

08 Dec 2001-BMJ
TL;DR: There is, I think, something ethereal about i —the square root of minus one, which seems an odd beast at that time—an intruder hovering on the edge of reality.
Abstract: There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality. Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

33,785 citations

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TL;DR: The origins and basis of green chemistry chart a course for achieving environmental and economic prosperity inherent in a sustainable world.
Abstract: Over the course of the past decade, green chemistry has demonstrated how fundamental scientific methodologies can protect human health and the environment in an economically beneficial manner. Significant progress is being made in several key research areas, such as catalysis, the design of safer chemicals and environmentally benign solvents, and the development of renewable feedstocks. Current and future chemists are being trained to design products and processes with an increased awareness for environmental impact. Outreach activities within the green chemistry community highlight the potential for chemistry to solve many of the global environmental challenges we now face. The origins and basis of green chemistry chart a course for achieving environmental and economic prosperity inherent in a sustainable world.

1,830 citations

Journal ArticleDOI
TL;DR: The power of cascade reactions in total synthesis is illustrated in the construction of complex molecules and underscore their future potential in chemical synthesis.
Abstract: The design and implementation of cascade reactions is a challenging facet of organic chemistry, yet one that can impart striking novelty, elegance, and efficiency to synthetic strategies. The application of cascade reactions to natural products synthesis represents a particularly demanding task, but the results can be both stunning and instructive. This Review highlights selected examples of cascade reactions in total synthesis, with particular emphasis on recent applications therein. The examples discussed herein illustrate the power of these processes in the construction of complex molecules and underscore their future potential in chemical synthesis.

1,762 citations

Journal ArticleDOI
01 Jul 1996-Lipids
TL;DR: Some possible reasons for the observed differences between the tocopherols (α-, β-, γ-, and δ-) in relation to their interactions with the important chemical species involved in lipid peroxidation, specifically trace metal ions, singlet oxygen, nitrogen oxides, and antioxidant synergists are highlighted.
Abstract: This article is a review of the fundamental chemistry of the tocopherols and tocotrienols relevant to their antioxidant action. Despite the general agreement that α-tocopherol is the most efficient antioxidant and vitamin E homologuein vivo, there was always a considerable discrepancy in its “absolute” and “relative” antioxidant effectivenessin vitro, especially when compared to γ-tocopherol. Many chemical, physical, biochemical, physicochemical, and other factors seem responsible for the observed discrepancy between the relative antioxidant potencies of the tocopherolsin vivo andin vitro. This paper aims at highlighting some possible reasons for the observed differences between the tocopherols (α-, β-, γ-, and δ-) in relation to their interactions with the important chemical species involved in lipid peroxidation, specifically trace metal ions, singlet oxygen, nitrogen oxides, and antioxidant synergists. Although literature reports related to the chemistry of the tocotrienols are quite meager, they also were included in the discussion in virtue of their structural and functional resemblance to the tocopherols.

1,726 citations