Guy C. Lloyd-Jones

Homogeneous catalytic dehydrocoupling/dehydrogenation of amine-borane adducts by early transition metal, group 4 metallocene complexes.

The efficient catalytic dehydrocoupling of a range of amine-borane adducts, RRNH x BH(3) (R = R = Me 1a; R = R = (i)Pr 1b; R = Me, R = CH(2)Ph 1c) by a series of group 4 metallocene type precatalysts has been demonstrated. A reduction in catalytic activity was detected upon descending the group and also on substitution of the cyclopentadienyl (Cp) ligands with sterically bulky or electron-donating substituents. Precatalysts Cp(2)TiCl(2)/2(n)BuLi and Cp(2)Ti(PMe(3))(2), which are believed to act as precursors to [Cp(2)Ti], were found to promote the transformation of 1a to [Me(2)N-BH(2)](2) (3a) in a homogeneous catalytic process. Mechanistic studies identified the linear dimer Me(2)NH-BH(2)-NMe(2)-BH(3) (2a) as a reaction intermediate, which subsequently undergoes further catalytic dehydrogenation to form cyclic dimer 3a. Synthesis of the (2)H-isotopologues of 1a allowed the extraction of phenomenological kinetic isotope effects for 1a --> 2a and 2a --> 3a from initial rate data, which permitted the proposal of a catalytic cycle with plausible intermediates. Support for the presence of an active Ti(II) catalyst was provided by the lack of reactivity of Ti(III) complexes TiCl(3) and Cp(2)TiCl or Ti(0) in the form of THF soluble colloids or bulk Ti powder toward 1a or 1b. Modeling of the rates of consumption of 1a and formation of 3a during catalysis by Cp(2)Ti(PMe(3))(2) supported this conclusion and allowed the proposal of a two cycle, four step reaction mechanism. The proposed first cycle generates 2a in a two step process. In the second cycle, interaction of 2a with the same catalyst then results in a catalytic dehydrogenative ring closing reaction to form 3a, also in a two step process.

Aryl Trifluoroborates in Suzuki–Miyaura Coupling: The Roles of Endogenous Aryl Boronic Acid and Fluoride

A wide range of organoboron reagents can be used as alternative reagents to boronic acids in Suzuki–Miyaura (SM) coupling reactions. The readily prepared, convenient to handle potassium trifluoroborates, RBF3K, which have been developed by the groups of Genet and Molander, are often the reagents of choice for these transformations. Although extensive optimization of the base, solvent, and temperature is required for each class of substrate, their utility in SM coupling reactions has led to their widespread commercial availability. Apart from a preliminary study in 2003, their mode of action has not been investigated in detail, and the origin of their efficacy 5a] remains to be elucidated. Herein, we report the SM coupling of aryl trifluoroborate 1 with aryl bromide 2 to generate biaryl 3 (Scheme 1). We show that endogenous aryl boronic acid 4 and fluoride, both arising from 1, play key roles in the coupling reaction, being involved at all stages: from catalyst activation and catalytic turnover, through to the inhibition of side reactions. Collectively, these phenomena result in the exceptional performance of the reagent in the SM coupling. The SM coupling of 1 with 2 was studied in a toluene/ water (3:1) biphasic solution, and in a tetrahydrofuran/ water (10:1) solution, both systems being commonly employed for the SM coupling of trifluoroborates. The reactions in toluene/water, failed to go to completion: turnover ceased after 6 hours, affording 55 % of the basecatalyzed protodeboronation product 6 and 32% of coupling product 3. In aqueous tetrahydrofuran (Scheme 1) the reaction proceeded much more efficiently (5.5 h; > 95% yield of 3), with few side products ( 0.1–2%), even when the reaction was performed in air. In contrast, reaction of the boronic acid (4) under identical conditions, gave 3 in variable yield, and afforded substantially more of side products 9/10 (2–40%), compared to trifluoroborate substrate 1. The performance of aryl boronic acid reagents can be improved by the addition of KF, whereas trifluoroborate reagents require aqueous solvent systems for SM coupling with standard substrates. This observation has led to suggestions that mixed borates, [RBF(3 n)(OH)n] , 13] are the true transmetalating species. 5a, 10,13b] Base titration of 1 in a solely aqueous medium (D2O) was monitored by F and B NMR spectroscopy. Trifluoroborate 1 underwent hydrolysis via boronic acid 4 to give boronate 5 ; the transformation required approximately three equivalents of K2CO3 or Cs2CO3, or six equivalents of KOH to proceed to completion. At ambient temperature, boronate 5 slowly gave rise to fluorobenzene 6 by protodeboronation; the process was substantially faster at 55 8C. Rapid equilibrium between 4 and 5 gave rise to time-averaged F NMR chemical shifts (p-F-Ar nuclei), from which analysis of DdF values versus [base] was used to establish the mol% of boronate 5 (e.g. Figure 1a). When the dibasic nature of M2CO3 was taken into account, there was no significant difference in the curve Scheme 1. SM coupling of trifluoroborate 1 with bromide 2 to generate biaryl 3 together with the three major side products arising from protodeboronation (6), homocoupling (9), and oxidation (10).

Arylsilanes: application to gold-catalyzed oxyarylation of alkenes.

Arylsilanes are efficient reagents for the gold-catalyzed oxyarylation of alkenes (21 examples, up to 85% isolated yield). Using commercially available Ph(3)PAuCl and readily prepared, benign arylsilanes, these two- and three-component reactions proceed smoothly in air. The oxidant, Selectfluor, not only facilitates entry to the Au(I/III) manifold but also provides a fluoride anion for silane activation, thereby avoiding the need for addition of a stoichiometric base.

The molecularity of the Newman-Kwart rearrangement

It was recently reported that the venerable Newman-Kwart rearrangement (1→2) proceeds via mixed first- and second-order kinetics. Prior to this, the rearrangement had been considered to proceed exclusively via an intramolecular O(Ar)→S(Ar) migration. A new bimolecular pathway, possibly involving an 8-membered cyclic transition state, was proposed to account for reaction rates that increased disproportionately with substrate concentration under microwave heating conditions. We report a reanalysis of the kinetics and molecularity of the rearrangement of N,N-dimethyl O-(p-nitrophenyl)thiocarbamate 1a in N,N-dimethylacetamide solvent. Using HPLC, isotopic labeling ((2)H, (18)O, (34)S), and ESI-ICRMS methods, we show that there is no evidence for a bimolecular pathway en route to 2a, with near-perfect exponential decay in 1a at concentrations ranging from 0.11 to 4.70 M. Instead, it is demonstrated that under the microwave heating conditions, a delayed negative feedback signal to the microwave power balancing loop results in oscillatory reaction overheating. Due to higher tan δ in the solute, the amplitude of this oscillation increases with the concentration of 1a, and this phenomenon best accounts for the kinetic behavior previously misinterpreted as being mixed first- and second-order in nature.

Use of Reactive Tracers To Determine Ambient OH Radical Concentrations: Application within the Indoor Environment

The hydroxyl radical (OH) plays a key role in determining indoor air quality. However, its highly reactive nature and low concentration indoors impede direct analysis. This paper describes the techniques used to indirectly quantify indoor OH, including the development of a new method based on the instantaneous release of chemical tracers into the air. This method was used to detect ambient OH in two indoor seminar rooms following tracer detection by gas chromatography-mass spectrometry (GCMS). The results from these tests add to the small number of experiments that have measured indoor OH which are discussed with regard to future directions within air quality research.

A Simple and Effective Co-Catalyst for Ring-Closing Enyne Metathesis Using Grubbs I type Catalysts: A Practical Alternative to Mori's Conditions

“Catch-and-release”: Simple alk-1-enes are effective at liberating buta-1,3-dienes from vinyl alkylidene ruthenium complexes, as well as undergoing alkene–alkylidene exchange with enyne substrates to regenerate the alk-1-ene. This ability to “catch-and-release” ruthenium alkylidenes allows alk-1-enes higher than ethylene (“Moris conditions”) to be used as a co-catalysts in terminal enyne metathesis with the Grubbs generation I complex (L=PCy3).

Interrogation of a dynamic multi-catalyst ensemble in asymmetric catalysis

The ‘Trost Standard Ligand’ (2) is a chiral diphosphine ligand that distinguishes itself by the high selectivity it induces in the Pd-catalysed reactions of allylic substrates that generate slim cyclic or small linear intermediates. However, a range of unusual features, including memory effects, inverse dependence of selectivity and rate on catalyst concentration, high sensitivity to counter-ion, particularly chloride, and decreasing enantioselectivities at lower temperatures, are often encountered, thus requiring considerable optimisation of reaction conditions to attain optimum selectivity. These features can be accounted for by a model involving a dynamic multi-catalyst ensemble. To gain evidence for this model, the manner in which diphosphine 2 interacts with Pd–allyl cations, and in particular the higher-order systems it generates, has been investigated by use of NMR, isotopic labelling, polarimetry, UV, neutron scattering, X-ray crystallography and molecular modeling. Ligand 2 coordinates to Pd–allyl cations to generate a mononuclear P,P-chelate. This is found to readily form non-chelate oligomers, present in a range of forms, including rings, for which high homochiral selectivity in oligomerisation is demonstrated by the technique of pseudoenantiomers. All of these species are in relatively rapid equilibrium, with half-lives for interconversion in the range 2–6 s. Higher-order aggregation is also detected, and thus at even moderate concentrations (>50 mM) large rod-like aggregates are formed.