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Adrian C Whitwood

Publications and source records attributed to Adrian C Whitwood.

18 recordsLinked to original sources
PubMed2006

Parahydrogen studies of H2 addition to Ir(I) complexes containing chiral phosphine-thioether ligands: implications for catalysis.

Ir(CO)[CpFe{eta5-C5H3(PPh2)CH2SR}]Cl [R = Ph and (t)Bu], containing a kappa2:P,S ligand, undergoes H2 addition across the S-Ir-CO axis under kinetic control to form two distinct diastereoisomeric products, which then rearrange via S dissociation in a process that can be hijacked for useful catalysis, but ultimately form a single diastereoisomer of the thermodynamic product where the hydride ligands are trans to chloride and phosphine.

Journal Article↗
PubMed2006

Syntheses of copper(I)cis-1,3,5-tri-iminocyclohexane complexes.

Copper(I) complexes of the ligand cis-1,3,5-tris(cinnamylideneamino)cyclohexane (L) have been prepared from a versatile precursor complex, [Cu(I)(L)NCMe]BF4, which incorporates a labile acetonitrile ligand that can be exchanged to give a range of new Cu(L)X complexes (where X = Cl, Br, NO2, SPh). 1H NMR spectra and X-ray structures of the Cl, Br and NO2 complexes show L coordinated in a symmetric fashion about the copper centre. The complexes have been further characterised using UV/Visible spectroscopy and cyclic voltammetry. CuLCl shows an electrochemically reversible Cu(I/II) redox couple at 0.51 V (vs. Ag/AgCl) while the CuLNO2 complex shows an analogous quasi-reversible wave at 0.41 V (vs. Ag/AgCl).

Copper↗
PubMed2005

C-F Bond activation at Ni(0) and simple reactions of square planar Ni(II) fluoride complexes.

The reaction of Ni(COD)(2)(COD = 1,5-cyclooctadiene) with triethylphosphine and pentafluoropyridine in hexane has been shown previously to yield trans-[NiF(2-C(5)NF(4))(PEt(3))(2)](1a) with a preference for reaction at the 2-position of the heteroaromatic. The corresponding reaction with 2,3,5,6-tetrafluoropyridine was shown to yield trans-[NiF(2-C(5)NF(3)H)(PEt(3))(2)](1b). In this paper, we show that reaction of Ni(COD)(2) with triethylphosphine and pentafluoropyridine in THF yields a mixture of 1a and 1b. Competition reactions of Ni(COD)(2) with triethylphosphine in the presence of mixtures of heteroaromatics in hexane reveal a kinetic preference of k(pentafluoropyridine):k(2,3,5,6-tetrafluoropyridine)= 5.4:1. Treatment of 1a and 1b with Me(3)SiN(3) affords trans-[Ni(N(3))(2-C(5)NF(4))(PEt(3))(2)](2a) and trans-[Ni(N(3))(2-C(5)NHF(3))(PEt(3))(2)](2b), respectively. The complex trans-[Ni(NCO)(2-C(5)NHF(3))(PEt(3))(2)](3b) is obtained on reaction of with Me(3)SiNCO and by photolysis of under CO, while trans-[Ni(eta(1)-C [triple bond CPh)(2-C(5)NF(4))(PEt(3))(2)](4a) is obtained by reaction of phenylacetylene with 1a. Addition of KCN, KI and NaOAc to complex 1a affords trans-[Ni(X)(2-C(5)NF(4))(PEt(3))(2)](5a X = CN, 6a X = I, 7a X = OAc), respectively. The PEt(3) groups of complex are readily replaced by addition of 1,2-bis(dicyclohexylphosphino)ethane (dcpe) to produce [NiF(2-C(5)F(4)N)(dcpe)](8a). Addition of dcpe to trans-[Ni(OTf)(2-C(5)F(4)N)(PEt(3))(2)](10a), however, yields the salt [Ni(2-C(5)F(4)N)(dcpe)(PEt(3))](OTf)(9a) by substitution of only one PEt(3) and displacement of the triflate ligand. The structures of 2b, 4a, 7a and 8a were determined by X-ray crystallography. The influence of different ancillary ligands on the bond lengths and angles of square-planar nickel structures with polyfluoropyridyl ligands is analysed.

Journal Article↗
PubMed2005

A new polymorph and two inclusion compounds of 9,9'-spirobifluorene.

Two new inclusion compounds of 9,9'-spirobifluorene (SBF) incorporating benzene [(I), C25H16.C6H6] and biphenyl [(II), C25H16.C12H10], and a new polymorph of SBF [(III), C25H16] are reported. All three exhibit C-H...pi(arene) hydrogen bonds between adjacent SBF molecules. Compound (II) also contains biphenyl C-H to SBF pi-arene interactions. Collectively, hydrogen bonding gives rise to a chain and a layered motif in compounds (I) and (II), respectively.

Journal Article↗
PubMed2005

Detection of intermediates in cobalt-catalyzed hydroformylation using para-hydrogen-induced polarization.

para-Hydrogen-induced polarization methods are shown to enable the in situ detection of linear and branched monophosphine-containing intermediates during hydroformylation when Co(eta3-C3H5)(CO)2(PCy3) is the catalyst precursor. The NMR signal characteristics of the alkyl arms of these species provide direct evidence for the rapid interconversion of linear and branched cobalt alkyls prior to the CO insertion step. The observation of additional para-hydrogen-enhanced signals for the corresponding linear and branched aldehydes enables the reactions selectivity to be rapidly monitored as a function of H2 and CO pressure or reaction temperature.

Journal Article↗
PubMed2005

Experimental and computational studies of structure and bonding in parent and reduced forms of the azo dye Orange II.

The structure and bonding of the azo dye Orange II (Acid Orange 7) in parent and reduced forms have been studied using NMR, infrared, Raman, UV-visible, and electron paramagnetic resonance (EPR) spectroscopy, allied with density functional theory (DFT) calculations on three hydrazone models (no sulfonate, anionic sulfonate, and protonated sulfonate) and one azo model (protonated sulfonate). The calculated structures of the three hydrazone models are similar to each other and that of the model without a sulfonate group (Solvent Yellow 14) closely matches its reported crystal structure. The 1H and 13C NMR resonances of Orange II, assigned directly from 1D and 2D experimental data, indicate that it is present as > or = 95% hydrazone in aqueous solution, and as a ca. 70:30 hydrazone:azo mixture in dimethyl sulfoxide at 300 K. Overall, the experimental data from Orange II are matched well by calculations on the hydrazone model with a protonated sulfonate group; the IR, Raman, and UV-visible spectra of Orange II are assigned to specific vibrational modes and electronic transitions calculated for this model. The EPR spectrum obtained on one-electron reduction of Orange II by the 2-hydroxy-2-propyl radical (*CMe2OH) at pH 4 is attributed to the hydrazyl radical produced on protonation of the radical anion. Calculations on reduced forms of the model dyes support this assignment, with electron spin density on the two nitrogen atoms and the naphthyl ring; in addition, they provide estimates of the structures, vibrational spectra, and electronic transitions of the radicals.

Azo Compounds↗
PubMed2004

A combined parahydrogen and theoretical study of H2 activation by 16-electron d8 ruthenium(0) complexes and their subsequent catalytic behaviour.

The photochemical reaction of Ru(CO)(3)(L)(2), where L = PPh(3), PMe(3), PCy(3) and P(p-tolyl)(3) with parahydrogen (p-H(2)) has been studied by in-situ NMR spectroscopy and shown to result in two competing processes. The first of these involves loss of CO and results in the formation of the cis-cis-trans-L isomer of Ru(CO)(2)(L)(2)(H)(2), while in the second, a single photon induces loss of both CO and L and leads to the formation of cis-cis-cis Ru(CO)(2)(L)(2)(H)(2) and Ru(CO)(2)(L)(solvent)(H)(2) where solvent = toluene, THF and pyridine (py). In the case of L = PPh(3), cis-cis-trans-L Ru(CO)(2)(L)(2)(H)(2) is shown to be an effective hydrogenation catalyst with rate limiting phosphine dissociation proceeding at a rate of 2.2 s(-1) in pyridine at 355 K. Theoretical calculations and experimental observations show that H(2) addition to the Ru(CO)(2)(L)(2) proceeds to form cis-cis-trans-L Ru(CO)(2)(L)(2)(H)(2) as the major product via addition over the pi-accepting OC-Ru-CO axis.

Journal Article↗
PubMed2004

A versatile, non-biomimetic route to the preussomerins: syntheses of (+/-)-preussomerins F, K and L.

The first total syntheses of the title fungal metabolites preussomerins F, K and L are described and their structures confirmed thereby. The syntheses were achieved following a versatile, unified, non-biomimetic approach, which is easily extendable to prepare other known and novel members of this family. Key steps include the functionalisation of a 2-arylacetal anion, tandem one-pot Friedel-Crafts cyclisation-deprotection, regioselective substrate-directable hydrogenation and reductive-opening of epoxides.

Epoxy Compounds↗
PubMed2004

Synthesis and characterisation of cis-dioxomolybdenum(VI) complexes of N-substituted 3-hydroxy-2-pyridinones.

The butoxy- and methoxy-derivatives (HLc, and HLb, respectively) of 1-[(ethoxycarbonyl)methyl]-3-hydroxy-2-(1H)-pyridinone, HLa, were synthesized by transesterification and the mononuclear complexes [MoO2(L(a-c))2] were obtained by the reaction of the ligands with molybdenyl acetylacetonate. The three complexes were fully characterized by (1)H and (13)C NMR spectroscopy, mass spectrometry and elemental analyses. A variable temperature NMR study showed that the N-substituents do not have a significant influence on the racemisation process of the complexes. The comparison of the crystal structure of [MoO2(Lb)2] with [MoO2(La)2] confirmed that an additional methylene group does not affect the immediate coordination environment of the molybdenum centre. The methoxy-derivative HLb was also found to produce a dinuclear complex of composition [(MoO2(Lb)O)2], the structure of which was determined by X-ray diffraction.

Journal Article↗
PubMed2004

New perspectives in hydroformylation: a para-hydrogen study.

NMR studies on the reaction of Ir(CO)(PPh(3))(2)(eta(3)-C(3)H(5)) with para-H(2) and CO enable the complete mapping of the hydroformylation mechanism for an iridium monohydride catalyst via the detection of species which include iridium acyl and alkyl dihydride intermediates.

Journal Article↗
PubMed2004

A high-throughput approach to lanthanide complexes and their rapid screening in the ring opening polymerisation of caprolactone.

Libraries of lanthanide complexes supported by nitrogen and oxygen containing ligands have been synthesised using a high-throughput approach. The complexes were employed in the ring-opening polymerisation of epsilon-caprolactone, in some cases giving polycaprolactone of controlled molecular weight and narrow polydispersity. The libraries, based on twenty-one ligands and eight lanthanide reagents, were developed in order to determine the best combination of lanthanide metal and ligand. They were prepared via transamination reactions of [Ln[N(SiMe(3))(2)](3)] complexes with tetradentate dianionic ligands containing oxygen and nitrogen donors. 1H NMR spectroscopy was used to screen polymerisation activity. The steric demand of the ligand has a significant effect on the polymerisation process, as do the type of nitrogen donor and the size of the central Ln(3+) ion. Ligands containing aryl rings with bulky substituents such as tert-pentyl groups afforded species capable of performing controlled polymerisation of caprolactone, whereas less bulky groups such as methyl were not effective. Yttrium and mid-sized lanthanides such as samarium showed increased activity compared with the larger lanthanides, lanthanum and praseodymium, and the smaller lanthanides like ytterbium. X-ray crystal structures of a sterically demanding chelating amine-bis((2-hydroxyaryl)methyl) ligand and a chloride bridged dinuclear gadolinium complex are reported. The centrosymmetric molecule contains gadolinium in distorted capped trigonal prismatic environments bonded to two amine, two phenolate, one THF and two chloride donors.

Journal Article↗
PubMed2003

Synthesis of sparteine-like chiral diamines and evaluation in the enantioselective lithiation-substitution of N-(tert-butoxycarbonyl)pyrrolidine.

Three chiral diamines were synthesised and evaluated as sparteine surrogates in the lithiation-substitution of N-(tert-butoxycarbonyl)pyrrolidine. The synthesis and attempted resolution of sparteine-like diamines [(1S*,2R*,8R*)-10-methyl-6,10-diazatricyclo[6.3.1.0(2,6)]dodecane and (1S*,2R*,9R*)-11-methyl-7,11-diazatricyclo[7.3.1.0(2,7)]tridecane] (via inclusion complex formation) are reported. Unfortunately, it was only possible to resolve the diazatricyclo[7.3.1.0(2,7)]tridecane compound. An alternative route to (1R,2S,9S)-11-methyl-7,11-diazatricyclo[7.3.1.0(2,7)]tridecane starting from the natural product, (-)-cytisine, is described. This simple three-step route furnished gram-quantities of a (+)-sparteine surrogate. X-Ray crystallography of an intermediate in the route, (1R,5S,12S)-3-methoxycarbonyldecahydro-1,5-methanopyrido[1,2-a][1,5]diazocin-8-one, enabled the stereochemistry of all of the tricyclic diamines described in this paper to be unequivocally established. Two other diamines, starting from (S)-proline and resolved 2-piperidine ethanol, were prepared using standard methods. These diamines lacked the bispidine framework of (-)-sparteine and were found to impart vastly inferior enantioselectivity. It was concluded that, for the asymmetric lithiation substitution of N-Boc pyrrolidine, a rigid bispidine framework and only three of the four rings of (-)-sparteine are needed for high enantioselectivity. Furthermore, it is shown that diamine (1R,2S,9S)-11-methyl-7,11-diazatricyclo[7.3.1.0(2,7)]tridecane is the first successful (+)-sparteine surrogate.

Journal Article↗
PubMed2003

EPR and modelling studies of hydrogen-abstraction reactions relevant to polyolefin cross-linking and grafting chemistry.

EPR spectroscopy has been employed to study directly the selectivity of hydrogen-atom abstraction by some alkoxyl radicals from a variety of linear and branched alkanes, as well as linear alkenes, chosen as models for low molecular-weight polyolefin cross-linking systems. In situ thermal and photolytic approaches, as well as spin-trapping, have been employed to provide information relating to an accessible temperature range of 233-453 K. in part to mimic conditions relevant to melt processing of polyolefins. Rate constants (in the range 3 x 10(3)-3.7 x 10(5) dm3 mol-1 s-1 per hydrogen) have been determined for C-H abstraction at room temperature. Radical selectivity is largely governed by enthalpic effects (modelled via bond dissociation energy calculations and kinetic analysis). Direct evidence has been obtained for lack of reactivity, as a result of unfavourable steric interactions, for the secondary and tertiary C-H bonds in 2,4-dimethylpentane and 2,4,6-trimethylheptane, models for polypropylene. This has been rationalized via free-energy calculations using DFT.

Journal Article↗
PubMed2002

Observation of protein-derived (BSA) oxygen-centered radicals by EPR spin-trapping techniques.

Electron paramagnetic resonance (EPR) spin-trapping experiments, employing the novel spin-trap DEPMPO, provide evidence for the formation of protein-peroxyl radicals from the reaction of bovine serum albumin (BSA) or lysozyme with HO. in the presence of O2. Spin-trapping leads to the detection of anisotropic spectra of partially immobilized protein-peroxyl spin-adducts; positive identification is based on a novel spectrum simulation approach (through which broadened anisotropic spectra are simulated and compared with experiment) and by comparison of results with those obtained when MeO2. is trapped and the adduct frozen in a solid matrix.

Animals↗