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Journal articleMcCarthy S, Desaunay O, Lee Wei Jie A, et al., 2022,
Homogeneous gold catalysis using complexes recovered from waste electronic equipment, ACS Sustainable Chemistry and Engineering, Vol: 10, Pages: 15726-15734, ISSN: 2168-0485
Despite the greater awareness of elemental sustainability and the benefits of the circular economy concept, much waste electrical and electronic equipment (WEEE) is still destined for landfill. Effective methods for valorizing this waste within our society are therefore imperative. In this contribution, two gold(III) complexes obtained as recovery products from WEEE and their anion metathesis products were investigated as homogenous catalysts. These four recovery products were successfully applied as catalysts for the cyclization of propargylic amides and the condensation of acetylacetone with o-iodoaniline. Impressive activity was also observed in the gold-catalyzed reaction between electron-rich arenes (2-methylfuran, 1,3-dimethoxybenzene, and azulene) and α,β-unsaturated carbonyl compounds (methyl vinyl ketone and cyclohexenone). These recovered compounds were also shown to be effective catalysts for the oxidative cross-coupling reaction of aryl silanes and arenes. When employed as Lewis acid catalysts for carbonyl-containing substrates, the WEEE-derived gold complexes could also be recovered at the end of the reaction and reused without loss in catalytic activity, enhancing still further the sustainability of the process. This is the first direct application in homogeneous catalysis of gold recovery products sourced from e-waste.
Journal articleBraddock D, Duran-Corbera A, Nilforoushan M, et al., 2022,
(±)-Polysiphenol and other Analogues via Symmetrical Intermolecular Dimerizations: a Synthetic, Spectroscopic, Structural and Computational Study, Journal of Natural Products, Vol: 85, Pages: 2650-2655, ISSN: 0163-3864
We report an improved total synthesis of 4,5-dibromo-9,10-dihydrophenanthrene-2,3,6,7-tetraol, (±)-polysiphenol, via intermolecular McMurray dimerization of 5-bromovanillin and subsequent intramolecular oxidative coupling as the key steps. The synthetic route is applicable to 4,5-dichloro- and 4,5-difluoro-halologues (as well as a 4,5-dialkyl-analogue). Distinctive AA′BB′ multiplets in their 1H NMR spectra for the dimethylene bridges of the dibromo and dichloro compounds reveal them to be room-temperature stable atropisomers, while for the difluoro compound they present as a singlet. X-ray crystal structure determinations of their tetramethylated synthetic precursors show atropisomeric twist angles of 48°, 46°, and 32°, respectively, with the former representing the largest yet observed in any 4,5-disubstituted-9,10-dihydrophenanthrene. DFT computational studies reveal an unprecedented two-stage atropisomeric interconversion process involving time-independent asynchronous rotations of the dimethylene bridge and the biaryl axis for halologues containing chlorine or bromine, but a more synchronous rotation for the difluoro analogue.
Journal articleBraddock D, Limpaitoon N, Oliwa K, et al., 2022,
A stereoselective hydride transfer reaction with contributions from attractive dispersion force control, Chemical Communications, Vol: 58, Pages: 4981-4984, ISSN: 1359-7345
The experimentally determined stereochemical outcome of an unprecedented hydride transfer from a lithium alkoxide to an aldehyde is reported, as deconvoluted by the combined use of a single enantiomer alkoxide in conjunction with a deuterium label. The stereoselective outcome is consistent with a computationally predicted transition state model stabilised by contributions from attractive dispersion forces.
Journal articleBraddock D, Davies J, Lickiss P, 2022,
Methyltrimethoxysilane (MTM) as a reagent for direct amidation of carboxylic acids, Organic Letters, Vol: 24, Pages: 1175-1179, ISSN: 1523-7052
Methyltrimethoxysilane [MTM, CH3Si(OMe)3] has been demonstrated to be an effective, inexpensive, and safe reagent for the direct amidation of carboxylic acids with amines. Two simple workup procedures that provide the pure amide product without the need for further purification have been developed. The first employs an aqueous base-mediated annihilation of MTM. The second involves simple product crystallization from the reaction mixture providing a low process mass intensity direct amidation protocol.
Journal articleMcCarthy S, Braddock D, Wilton-Ely J, 2021,
Strategies for sustainable palladium catalysis, Coordination Chemistry Reviews, Vol: 442, Pages: 1-26, ISSN: 0010-8545
Palladium is established both as an indispensable metal in modern synthetic chemistry and as a key component in catalytic converters and electronic equipment. The unrelenting demand for this metal increases both the financial and environmental cost, through mining, of this metal, rendering consumption unsustainable in its current form. This review provides a critical survey of the main approaches being pursued to address this issue, including low-loading catalysis, recyclable catalysts and the use of palladium recovered from secondary sources. The literature discussed herein reveals that catalysts that operate at low loadings are an attractive, sustainable option for many transformations, even when compared to catalyst systems that can be reused multiple times. Both low-loading and reusable catalysts depend on a finite natural supply of palladium, making the development of catalysts based on metal recovered from end-of-life materials (urban mining) a promising field of research. The way in which low-loading is achieved in different catalyst designs influences the choice of reaction temperature and solvent, which has additional environmental implications. Such factors are discussed for a wide range of systems alongside reaction and substrate scope, supported by extensive supplementary information. The survey aims to provide an insight into the relative merits of the approaches being explored and how the field of palladiumbased catalysis might progress towards more sustainable use of this critical metal.
Journal articleMcCarthy S, Lee Wei Jie A, Braddock D, et al., 2021,
From waste to green applications: the use of recovered gold and palladium in catalysis, Molecules, Vol: 26, ISSN: 1420-3049
The direct use in catalysis of precious metal recovery products from industrial and consumer waste is a very promising recent area of investigation. It represents a more sustainable, environmentally benign, and profitable way of managing the low abundance of precious metals, as well as encouraging new ways of exploiting their catalytic properties. This review demonstrates the feasibility and sustainability of this innovative approach, inspired by circular economy models, and aims to stimulate further research and industrial processes based on the valorisation of secondary resources of these raw materials. The overview of the use of recovered gold and palladium in catalytic processes will be complemented by critical appraisal of the recovery and reuse approaches that have been proposed.
Journal articleDavies J, Braddock D, Lickiss P, 2021,
Silicon compounds as stoichiometric coupling reagents for direct amidation, Organic and Biomolecular Chemistry, Vol: 19, Pages: 6746-6760, ISSN: 1477-0520
Despite being one of the most frequently carried out chemical reactions in industry, there is currently no amidation protocol that is regarded as safe, high yielding, environmentally friendly and inexpensive. The direct amidation of a carboxylic acid with an amine is viewed as an inherently good solution for developing such a protocol. Since the 1960s, there has been a gradual development in the use of silicon reagents for direct amidation. This review covers the methods published to April 2021 for silicon reagent mediated direct amidation of a carboxylic acid with an amine. This review also covers the use of polymeric SiO2 to promote direct amidation.
Journal articleBahou K, Braddock D, Meyer A, et al., 2020,
Relay cross metathesis for the iterative construction of terpenoids and synthesis of a diterpene benzoate macrolide of biogenetic relevance to the bromophycolides, Organic Letters, Vol: 22, Pages: 3176-3179, ISSN: 1523-7052
We report a relay cross metathesis (ReXM) reaction for the construction of terpenoids in an iterative protocol. The protocol features the cross metathesis of a relay-actuated Δ6,7-functionalized C10-monoterpenoid alcohol with C10-monoterpenoid citral to form a C15-sesquiterpene. Subsequent functional group manipulation allows for the method to be repeated in an iterative fashion. The method is used for the synthesis of a diterpene-benzoate macrolide of biogenetic relevance to the bromophycolide family of natural products.
Journal articleBahou K, Braddock D, Meyer A, et al., 2020,
A relay strategy actuates pre-existing trisubstituted olefins in monoterpenoids for cross metathesis with trisubstituted alkenes, The Journal of Organic Chemistry, Vol: 85, Pages: 4906-4917, ISSN: 0022-3263
A retrosynthetic disconnection-reconnection analysis of epoxypolyenes – substrates that can undergo cyclization to podocarpane-type tricycles – reveals relay-actuated 6,7-functionalized monoterpenoid alcohols for ruthenium benzylidene catalyzed olefin cross metathesis with homoprenyl benzenes. Successful implementation of this approach provided several epoxypolyenes as expected (E:Z, ca. 2-3:1). The method is further generalized for the cross metathesis of pre-existing trisubstituted olefins in other relay-actuated 6,7-functionalized monoterpenoid alcohols with various other trisubstituted alkenes to form new trisubstituted olefins. Epoxypolyene cyclization of an enantiomerically pure, but geometrically impure, epoxypolyene substrate provides an enantiomerically pure, trans-fused, podocarpane-type tricycle (from the E-geometrical isomer).
Journal articleLo Q, Sale D, Braddock D, et al., 2019,
New insights into the reaction capabilities of ionic organic bases in cu-catalysed amination, European Journal of Organic Chemistry, Vol: 2019, Pages: 1944-1951, ISSN: 1099-0690
The application of ionic organic bases in the copper‐catalyzed amination reaction (Ullmann reaction) has been studied at room temperature, with sub‐mol‐% catalyst loadings, and with more challenging amines at elevated temperatures. The cation present in the base has been shown to have little effect on the reaction at standard catalyst and ancillary ligand loadings, whereas the choice of anion is crucial for good reactivity. A substrate scope carried out at room temperature with the best performing bases, TBAM and TBPM, showed both bases to be highly effective under these mild reaction conditions. Moreover, under sub‐mol % catalyst loadings and room temperature conditions, TBPM gave good to excellent yields for a number of different amines and functionalized aryl iodides (14 examples). However, reactions involving more challenging amines gave little or no yield. By using more forceful conditions (120 °C) moderate to excellent yields of cross‐coupled products containing more challenging amines was achievable using TBPM and to a lesser extent with TBAM. As part of this work a study on the stability of the organic bases at 120 °C was undertaken. TBAM is shown to decompose to give nBu3N and mono‐butylmalonate at higher temperatures, and this can be correlated to a decrease in performance in the coupling reaction. The phosphonium cations in TBPM did not undergo analogous reactivity but were shown instead to experience some degree of deprotonation at the α‐CH2 to generate phosphonium ylides. This however did not lead to a significantly degradation in the activity of the TBPM in the cross‐coupling reaction.
Journal articleBraddock DC, Lickiss PD, Rowley BC, et al., 2018,
Tetramethyl Orthosilicate (TMOS) as a Reagent for Direct Amidation of Carboxylic Acids, ORGANIC LETTERS, Vol: 20, Pages: 950-953, ISSN: 1523-7060
- Author Web Link
- Open Access Link
- Citations: 45
Journal articleLo QA, Sale D, Braddock DC, et al., 2018,
Mechanistic and Performance Studies on the Ligand-Promoted Ullmann Amination Reaction, ACS CATALYSIS, Vol: 8, Pages: 101-109, ISSN: 2155-5435
- Author Web Link
- Open Access Link
- Citations: 26
Journal articleBahou KA, Braddock DC, Meyer AG, et al., 2017,
Kinetic Benchmarking Reveals the Competence of Prenyl Groups in Ring-Closing Metathesis, ORGANIC LETTERS, Vol: 19, Pages: 5332-5335, ISSN: 1523-7060
- Author Web Link
- Open Access Link
- Citations: 8
Journal articleClarke J, Bonney KJ, Yaqoob M, et al., 2016,
Epimeric Face-Selective Oxidations and Diastereodivergent Transannular Oxonium Ion Formation Fragmentations: Computational Modeling and Total Syntheses of 12-Epoxyobtusallene IV, 12-Epoxyobtusallene II, Obtusallene X, Marilzabicycloallene C, and Marilzabicycloallene D, Journal of Organic Chemistry, Vol: 81, Pages: 9539-9552, ISSN: 0022-3263
The total syntheses of 12-epoxyobtusallene IV, 12-epoxyobtusallene II, obtusallene X, marilzabicycloallene C, and marilzabicycloallene D as halogenated C15-acetogenin 12-membered bicyclic and tricyclic ether bromoallene-containing marine metabolites from Laurencia species are described. Two enantiomerically pure C4-epimeric dioxabicyclo[8.2.1]tridecenes were synthesized by E-selective ring-closing metathesis where their absolute stereochemistry was previously set via catalytic asymmetric homoallylic epoxidation and elaborated via regioselective epoxide-ring opening and diastereoselective bromoetherification. Epimeric face-selective oxidation of their Δ12,13 olefins followed by bromoallene installation allowed access to the oppositely configured 12,13-epoxides of 12-epoxyobtusallene II and 12-epoxyobtusallene IV. Subsequent exploration of their putative biomimetic oxonium ion formation-fragmentations reactions revealed diastereodivergent pathways giving marilzabicycloallene C and obtusallene X, respectively. The original configurations of the substrates evidently control oxonium ion formation and their subsequent preferred mode of fragmentation by nucleophilic attack at C9 or C12. Quantum modeling of this stereoselectivity at the ωB97X-D/Def2-TZVPPD/SCRF = methanol level revealed that in addition to direction resulting from hydrogen bonding, the dipole moment of the ion-pair transition state is an important factor. Marilzabicycloallene D as a pentahalogenated 12-membered bicyclic ether bromoallene was synthesized by a face-selective chloronium ion initiated oxonium ion formation-fragmentation process followed by subsequent bromoallene installation.
- Open Access Link
- Citations: 14
Journal articleBraddock DC, Mahtey A, Rzepa HS, et al., 2016,
Stable bromoallene oxides, Chemical Communications, Vol: 52, Pages: 11219-11222, ISSN: 1364-548X
The first stable bromoallene oxides were obtained by the DMDO epoxidation of 1-bromo-1,3-di-tert-alkylallenes, producing the first crystalline allene oxide of any kind. The epoxidations are regioselective for the bromine-bearing Δ1,2 alkene, and also face selective producing single diastereomer E-olefin products.
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