School of Chemistry - Research Publications
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ItemCytochrome P450Blt Enables Versatile Peptide Cyclisation to Generate Histidine- and Tyrosine-Containing Crosslinked Tripeptide Building Blocks(WILEY-V C H VERLAG GMBH, 2022-09-12)We report our investigation of the utility of peptide crosslinking cytochrome P450 enzymes from biarylitide biosynthesis to generate a range of cyclic tripeptides from simple synthons. The crosslinked tripeptides produced by this P450 include both tyrosine-histidine (A-N-B) and tyrosine-tryptophan (A-O-B) crosslinked tripeptides, the latter a rare example of a phenolic crosslink to an indole moiety. Tripeptides are easily isolated following proteolytic removal of the leader peptide and can incorporate a wide range of amino acids in the residue inside the crosslinked tripeptide. Given the utility of peptide crosslinks in important natural products and the synthetic challenge that these can represent, P450 enzymes have the potential to play roles as important tools in the generation of high-value cyclic tripeptides for incorporation in synthesis, which can be yet further diversified using selective chemical techniques through specific handles contained within these tripeptides.
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ItemA photo-switchable molecular capsule: sequential photoinduced processes(ROYAL SOC CHEMISTRY, 2022-11-30)The metastable trilacunary heteropolyoxomolybdate [PMo9O31(py)3]3- - {PMo9}; py = pyridine) and the ditopic pyridyl bearing diarylethene (DAE) (C25H16N2F6S2) self-assemble via a facile ligand replacement methodology to yield the photo-active molecular capsule [(PMo9O31)2(DAE)3]6-. The spatial arrangement and conformation of the three DAE ligands are directed by the surface chemistry of the molecular metal oxide precursor with exclusive ligation of the photo-active antiparallel rotamer to the polyoxometalate (POM) while the integrity of the assembly in solution has been verified by a suite of spectroscopic techniques. Electrocyclisation of the three DAEs occurs sequentially and has been investigated using a combination of steady-state and time-resolved spectroscopies with the discovery of a photochemical cascade whereby rapid photoinduced ring closure is followed by electron transfer from the ring-closed DAE to the POM in the latent donor-acceptor system on subsequent excitation. This interpretation is also supported by computational and detailed spectroelectrochemical analysis. Ring-closing quantum yields were also determined using a custom quantum yield determination setup (QYDS), providing insight into the impact of POM coordination on these processes.
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ItemA Series of Crystallographically Characterized Linear and Branched σ-Alkane Complexes of Rhodium: From Propane to 3-Methylpentane(AMER CHEMICAL SOC, 2021-04-07)Using solid-state molecular organometallic (SMOM) techniques, in particular solid/gas single-crystal to single-crystal reactivity, a series of σ-alkane complexes of the general formula [Rh(Cy2PCH2CH2PCy2)(ηn:ηm-alkane)][BArF4] have been prepared (alkane = propane, 2-methylbutane, hexane, 3-methylpentane; ArF = 3,5-(CF3)2C6H3). These new complexes have been characterized using single crystal X-ray diffraction, solid-state NMR spectroscopy and DFT computational techniques and present a variety of Rh(I)···H-C binding motifs at the metal coordination site: 1,2-η2:η2 (2-methylbutane), 1,3-η2:η2 (propane), 2,4-η2:η2 (hexane), and 1,4-η1:η2 (3-methylpentane). For the linear alkanes propane and hexane, some additional Rh(I)···H-C interactions with the geminal C-H bonds are also evident. The stability of these complexes with respect to alkane loss in the solid state varies with the identity of the alkane: from propane that decomposes rapidly at 295 K to 2-methylbutane that is stable and instead undergoes an acceptorless dehydrogenation to form a bound alkene complex. In each case the alkane sits in a binding pocket defined by the {Rh(Cy2PCH2CH2PCy2)}+ fragment and the surrounding array of [BArF4]- anions. For the propane complex, a small alkane binding energy, driven in part by a lack of stabilizing short contacts with the surrounding anions, correlates with the fleeting stability of this species. 2-Methylbutane forms more short contacts within the binding pocket, and as a result the complex is considerably more stable. However, the complex of the larger 3-methylpentane ligand shows lower stability. Empirically, there therefore appears to be an optimal fit between the size and shape of the alkane and overall stability. Such observations are related to guest/host interactions in solution supramolecular chemistry and the holistic role of 1°, 2°, and 3° environments in metalloenzymes.
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ItemUsing electrospray ionization-tandem mass spectrometry to explore formation and gas-phase chemistry of silver nanoclusters generated from the reaction of silver salts with NaBH4in the presence of bis(diphenylarsino)methane(WILEY, 2021-04)Electrospray ionization-mass spectrometry (ESI-MS) of mixtures of AgBF4 or AgNO3 with the capping ligand bis(diphenylarsino)methane ((Ph2 As)2 CH2 = dpam) in a solution of acetonitrile revealed the formation of the following cations: [Ag(CH3 CN)(dpam)]+ , [Ag(dpam)2 ]+ , [Ag2 (Cl)(dpam)2 ]+ , and [Ag3 (Cl)2 (dpam)3 ]+ . Addition of NaBH4 to these solutions results in the formation of the cluster cations [Ag2 (BH4 )(dpam)2 ]+ , [Ag2 (BH4 )(dpam)3 ]+ , [Ag3 (H)(BH4 )(dpam)3 ]+ , [Ag3 (BH4 )2 (dpam)3 ]+ , [Ag3 (H)(Cl)(dpam)3 ]+ , and [Ag3 (I)(BH4 )(dpam)3 ]+ , as established by ESI-MS. Use of NaBD4 confirmed that borohydride is the source of the hydride in these clusters. An Orbitrap Fusion LUMOS mass spectrometer was used to explore the gas-phase unimolecular chemistry of selected clusters via multistage mass spectrometry (MSn ) experiments employing low-energy collision-induced dissociation (CID) and high-energy collision-induced dissociation (HCD) experiments. The borohydride containing clusters fragment via two competing pathways: (i) ligand loss and (ii) B-H bond activation involving BH3 loss. Density functional theory (DFT) calculations were used to calculate the energetics of the optimized structures for all precursor ions, fragment ions, and neutrals and to estimate the reaction endothermicities. Generally, there is reasonable agreement between the most abundant product ion formed and the predicted endothermicity of the associated reaction channel. The DFT calculations predicted that the novel dimer [Ag2 (BH4 )(dpam)2 ]+ has a paddlewheel structure in which the dpam and BH4 - ligands bridge both silver centers.
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ItemIridium-catalysed 3,5-bis-borylation of phthalonitrile enables access to a family ofC4hoctaarylphthalocyanines(ROYAL SOC CHEMISTRY, 2020-08-04)Ir-catalysed borylation of phthalonitrile produces both 4-(Bpin)phthalonitrile (1) and 3,5-bis(Bpin)phthalonitrile (2), which are potential divergent intermediates for the synthesis of functionalized phthalocyanines. To exemplify the utility of 2, we have prepared a series of 3,5-bis-arylphthalonitriles that in turn undergo sterically controlled regioselective cyclotetramization to give previously unknown C4h 1,3,8,10,15,17,22,24-octaarylphthalocyanines.
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ItemTolerant to air σ-alkane complexes by surface modification of single crystalline solid-state molecular organometallics using vapour-phase cationic polymerisation: SMOM@polymer(ROYAL SOC CHEMISTRY, 2020-04-21)Vapour-phase surface-initiated cationic polymerisation of ethylvinylether occurs at single-crystals of the σ-alkane complex [Rh(Cy2PCH2CH2PCy2)(NBA)][BArF4]. This new surface interface makes these normally very air sensitive materials tolerant to air, while also allowing for onward single-crystal to single-crystal reactivity at metal sites within the lattice.
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ItemSynthesis of Highly Fluorinated Arene Complexes of [Rh(Chelating Phosphine)]+ Cations, and their use in Synthesis and Catalysis(WILEY-V C H VERLAG GMBH, 2020-03-02)The synthesis of rhodium complexes with weakly binding highly fluorinated benzene ligands is described: 1,2,3-F3 C6 H3 , 1,2,3,4-F4 C6 H2 and 1,2,3,4,5-F5 C6 H are shown to bind with cationic [Rh(Cy2 P(CH2 )x PCy2 )]+ fragments (x=1, 2). Their structures and reactivity with alkenes, and use in catalysis for promoting the Tishchenko reaction of a simple aldehyde, are demonstrated. Key to the synthesis of these complexes is the highly concentrated reaction conditions and use of the [Al{OC(CF3 )3 }4 ]- anion.
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ItemNo Preview AvailableMechanistic Studies of the Palladium-Catalyzed Desulfinative Cross-Coupling of Aryl Bromides and (Hetero)Aryl Sulfinate Salts(AMER CHEMICAL SOC, 2020-02-19)Pyridine and related heterocyclic sulfinates have recently emerged as effective nucleophilic coupling partners in palladium-catalyzed cross-coupling reactions with (hetero)aryl halides. These sulfinate reagents are straightforward to prepare, stable to storage and coupling reaction conditions, and deliver efficient reactions, thus offering many advantages, compared to the corresponding boron-derived reagents. Despite the success of these reactions, there are only scant details of the reaction mechanism. In this study, we use structural and kinetic analysis to investigate the mechanism of these important coupling reactions in detail. We compare a pyridine-2-sulfinate with a carbocyclic sulfinate and establish different catalyst resting states, and turnover limiting steps, for the two classes of reagent. For the carbocyclic sulfinate, the aryl bromide oxidative addition complex is the resting state intermediate, and transmetalation is turnover-limiting. In contrast, for the pyridine sulfinate, a chelated Pd(II) sulfinate complex formed post-transmetalation is the resting-state intermediate, and loss of SO2 from this complex is turnover-limiting. We also investigated the role of the basic additive potassium carbonate, the use of which is crucial for efficient reactions, and deduced a dual function in which carbonate is responsible for the removal of free sulfur dioxide from the reaction medium, and the potassium cation plays a role in accelerating transmetalation. In addition, we show that sulfinate homocoupling is responsible for converting Pd(OAc)2 to a catalytically active Pd(0) complex. Together, these studies shed light on the challenges that must be overcome to deliver improved, lower temperature versions of these synthetically important processes.
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ItemNo Preview AvailableRoom Temperature Acceptorless Alkane Dehydrogenation from Molecular σ-Alkane Complexes(AMER CHEMICAL SOC, 2019-07-24)The non-oxidative catalytic dehydrogenation of light alkanes via C-H activation is a highly endothermic process that generally requires high temperatures and/or a sacrificial hydrogen acceptor to overcome unfavorable thermodynamics. This is complicated by alkanes being such poor ligands, meaning that binding at metal centers prior to C-H activation is disfavored. We demonstrate that by biasing the pre-equilibrium of alkane binding, by using solid-state molecular organometallic chemistry (SMOM-chem), well-defined isobutane and cyclohexane σ-complexes, [Rh(Cy2PCH2CH2PCy2)(η:η-(H3C)CH(CH3)2][BArF4] and [Rh(Cy2PCH2CH2PCy2)(η:η-C6H12)][BArF4] can be prepared by simple hydrogenation in a solid/gas single-crystal to single-crystal transformation of precursor alkene complexes. Solid-gas H/D exchange with D2 occurs at all C-H bonds in both alkane complexes, pointing to a variety of low energy fluxional processes that occur for the bound alkane ligands in the solid-state. These are probed by variable temperature solid-state nuclear magnetic resonance experiments and periodic density functional theory (DFT) calculations. These alkane σ-complexes undergo spontaneous acceptorless dehydrogenation at 298 K to reform the corresponding isobutene and cyclohexadiene complexes, by simple application of vacuum or Ar-flow to remove H2. These processes can be followed temporally, and modeled using classical chemical, or Johnson-Mehl-Avrami-Kologoromov, kinetics. When per-deuteration is coupled with dehydrogenation of cyclohexane to cyclohexadiene, this allows for two successive KIEs to be determined [kH/kD = 3.6(5) and 10.8(6)], showing that the rate-determining steps involve C-H activation. Periodic DFT calculations predict overall barriers of 20.6 and 24.4 kcal/mol for the two dehydrogenation steps, in good agreement with the values determined experimentally. The calculations also identify significant C-H bond elongation in both rate-limiting transition states and suggest that the large kH/kD for the second dehydrogenation results from a pre-equilibrium involving C-H oxidative cleavage and a subsequent rate-limiting β-H transfer step.
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ItemNo Preview AvailableDehydropolymerization of H3B•NMeH2 Using a [Rh(DPEphos)]+ Catalyst: The Promoting Effect of NMeH2(AMER CHEMICAL SOC, 2019-04)[Rh(κ2-PP-DPEphos){η2η2-H2B(NMe3)(CH2)2 tBu}][BArF 4] acts as an effective precatalyst for the dehydropolymerization of H3B·NMeH2 to form N-methylpolyaminoborane (H2BNMeH) n . Control of polymer molecular weight is achieved by variation of precatalyst loading (0.1-1 mol %, an inverse relationship) and use of the chain-modifying agent H2: with M n ranging between 5 500 and 34 900 g/mol and Đ between 1.5 and 1.8. H2 evolution studies (1,2-F2C6H4 solvent) reveal an induction period that gets longer with higher precatalyst loading and complex kinetics with a noninteger order in [Rh]TOTAL. Speciation studies at 10 mol % indicate the initial formation of the amino-borane bridged dimer, [Rh2(κ2-PP-DPEphos)2(μ-H)(μ-H2BN=HMe)][BArF 4], followed by the crystallographically characterized amidodiboryl complex [Rh2(cis-κ2-PP-DPEphos)2(σ,μ-(H2B)2NHMe)][BArF 4]. Adding ∼2 equiv of NMeH2 in tetrahydrofuran (THF) solution to the precatalyst removes this induction period, pseudo-first-order kinetics are observed, a half-order relationship to [Rh]TOTAL is revealed with regard to dehydrogenation, and polymer molecular weights are increased (e.g., M n = 40 000 g/mol). Speciation studies suggest that NMeH2 acts to form the precatalysts [Rh(κ2-DPEphos)(NMeH2)2][BArF 4] and [Rh(κ2-DPEphos)(H)2(NMeH2)2][BArF 4], which were independently synthesized and shown to follow very similar dehydrogenation kinetics, and produce polymers of molecular weight comparable with [Rh(κ2-PP-DPEphos){η2-H2B(NMe3)(CH2)2 tBu}][BArF 4], which has been doped with amine. This promoting effect of added amine in situ is shown to be general in other cationic Rh-based systems, and possible mechanistic scenarios are discussed.