28 research outputs found
B-Methylated Amine-Boranes:Substituent Redistribution, Catalytic Dehydrogenation, and Facile Metal-Free Hydrogen Transfer Reactions
Although
the dehydrogenation chemistry of amine-boranes substituted at nitrogen
has attracted considerable attention, much less is known about the
reactivity of their B-substituted analogues. When the B-methylated
amine-borane adducts, RRâČNH·BH<sub>2</sub>Me (<b>1a</b>: R = RâČ = H; <b>1b</b>: R = Me, RâČ = H; <b>1c</b>: R = RâČ = Me; <b>1d</b>: R = RâČ = <i>i</i>Pr), were heated to 70 °C in solution (THF or toluene),
redistribution reactions were observed involving the apparent scrambling
of the methyl and hydrogen substituents on boron to afford a mixture
of the species RRâČNH·BH<sub>3â<i>x</i></sub>Me<sub><i>x</i></sub> (<i>x</i> = 0â3).
These reactions were postulated to arise via amine-borane dissociation
followed by the reversible formation of diborane intermediates and
adduct reformation. Dehydrocoupling of <b>1a</b>â<b>1d</b> with RhÂ(I), IrÂ(III), and Ni(0) precatalysts in THF at 20
°C resulted in an array of products, including aminoborane RRâČNî»BHMe,
cyclic diborazane [RRâČNâBHMe]<sub>2</sub>, and borazine
[RNâBMe]<sub>3</sub> based on analysis by in situ <sup>11</sup>B NMR spectroscopy, with peak assignments further supported by density
functional theory (DFT) calculations. Significantly, very rapid, metal-free
hydrogen transfer between <b>1a</b> and the monomeric aminoborane, <i>i</i>Pr<sub>2</sub>Nî»BH<sub>2</sub>, to yield <i>i</i>Pr<sub>2</sub>NH·BH<sub>3</sub> (together with dehydrogenation
products derived from <b>1a</b>) was complete within only 10
min at 20 °C in THF, substantially faster than for the N-substituted
analogue MeNH<sub>2</sub>·BH<sub>3</sub>. DFT calculations revealed
that the hydrogen transfer proceeded via a concerted mechanism through
a cyclic six-membered transition state analogous to that previously
reported for the reaction of the <i>N</i>-dimethyl species
Me<sub>2</sub>NH·BH<sub>3</sub> and <i>i</i>Pr<sub>2</sub>Nî»BH<sub>2</sub>. However, as a result of the presence
of an electron donating methyl substituent on boron rather than on
nitrogen, the process was more thermodynamically favorable and the
activation energy barrier was reduced
Mechanistic Studies of the Dehydrocoupling and Dehydropolymerization of Amine-Boranes Using a [Rh(Xantphos)](+) Catalyst
A detailed catalytic, stoichiometric, and mechanistic study on the dehydrocoupling of H3B·NMe2H and dehydropolymerization of H3B·NMeH2 using the [Rh(Xantphos)](+) fragment is reported. At 0.2 mol % catalyst loadings, dehydrocoupling produces dimeric [H2B-NMe2]2 and poly(methylaminoborane) (M(n) = 22,700 g mol(-1), PDI = 2.1), respectively. The stoichiometric and catalytic kinetic data obtained suggest that similar mechanisms operate for both substrates, in which a key feature is an induction period that generates the active catalyst, proposed to be a Rh-amido-borane, that reversibly binds additional amine-borane so that saturation kinetics (Michaelis-Menten type steady-state approximation) operate during catalysis. B-N bond formation (with H3B·NMeH2) or elimination of amino-borane (with H3B·NMe2H) follows, in which N-H activation is proposed to be turnover limiting (KIE = 2.1 ± 0.2), with suggested mechanisms that only differ in that B-N bond formation (and the resulting propagation of a polymer chain) is favored for H3B·NMeH2 but not H3B·NMe2H. Importantly, for the dehydropolymerization of H3B·NMeH2, polymer formation follows a chain growth process from the metal (relatively high degrees of polymerization at low conversions, increased catalyst loadings lead to lower-molecular-weight polymer), which is not living, and control of polymer molecular weight can be also achieved by using H2 (M(n) = 2,800 g mol(-1), PDI = 1.8) or THF solvent (M(n) = 52,200 g mol(-1), PDI = 1.4). Hydrogen is suggested to act as a chain transfer agent in a similar way to the polymerization of ethene, leading to low-molecular-weight polymer, while THF acts to attenuate chain transfer and accordingly longer polymer chains are formed. In situ studies on the likely active species present data that support a Rh-amido-borane intermediate as the active catalyst. An alternative Rh(III) hydrido-boryl complex, which has been independently synthesized and structurally characterized, is discounted as an intermediate by kinetic studies. A mechanism for dehydropolymerization is suggested in which the putative amido-borane species dehydrogenates an additional H3B·NMeH2 to form the "real monomer" amino-borane H2BâNMeH that undergoes insertion into the Rh-amido bond to propagate the growing polymer chain from the metal. Such a process is directly analogous to the chain growth mechanism for single-site olefin polymerization
Revising the WHO verbal autopsy instrument to facilitate routine cause-of-death monitoring.
OBJECTIVE: Verbal autopsy (VA) is a systematic approach for determining causes of death (CoD) in populations without routine medical certification. It has mainly been used in research contexts and involved relatively lengthy interviews. Our objective here is to describe the process used to shorten, simplify, and standardise the VA process to make it feasible for application on a larger scale such as in routine civil registration and vital statistics (CRVS) systems. METHODS: A literature review of existing VA instruments was undertaken. The World Health Organization (WHO) then facilitated an international consultation process to review experiences with existing VA instruments, including those from WHO, the Demographic Evaluation of Populations and their Health in Developing Countries (INDEPTH) Network, InterVA, and the Population Health Metrics Research Consortium (PHMRC). In an expert meeting, consideration was given to formulating a workable VA CoD list [with mapping to the International Classification of Diseases and Related Health Problems, Tenth Revision (ICD-10) CoD] and to the viability and utility of existing VA interview questions, with a view to undertaking systematic simplification. FINDINGS: A revised VA CoD list was compiled enabling mapping of all ICD-10 CoD onto 62 VA cause categories, chosen on the grounds of public health significance as well as potential for ascertainment from VA. A set of 221 indicators for inclusion in the revised VA instrument was developed on the basis of accumulated experience, with appropriate skip patterns for various population sub-groups. The duration of a VA interview was reduced by about 40% with this new approach. CONCLUSIONS: The revised VA instrument resulting from this consultation process is presented here as a means of making it available for widespread use and evaluation. It is envisaged that this will be used in conjunction with automated models for assigning CoD from VA data, rather than involving physicians
Mechanisms of the Thermal and Catalytic Redistributions, Oligomerizations, and Polymerizations of Linear Diborazanes
Catalytic Redistribution and Polymerization of Diborazanes: Unexpected Observation of Metal-Free Hydrogen Transfer between Aminoboranes and Amine-Boranes
Amine-borane dehydrogenation chemistry: Metal-free hydrogen transfer, new catalysts and mechanisms, and the synthesis of polyaminoboranes
Distannoxane speciation during esterification catalysis: revealing insights provided by electrospray ionization mass spectrometry
Polyaminoborane main chain scission using N-heterocyclic carbenes; formation of donor-stabilised monomeric aminoboranes
Photogeneration of a Phosphonium Alkylidene Olefin Metathesis Catalyst
Treatment of ruthenium carbide (H<sub>2</sub>IMes)Â(Cl)<sub>2</sub>(PCy<sub>3</sub>)ÂRuC (<b>1</b>) with the photoacid generator
(PAG) [Ph<sub>3</sub>S]Â[OTf] (<b>3</b>) under 254 nm light results
in a highly efficient catalyst for ring-closing metathesis (RCM) and
ring-opening metathesis polymerization (ROMP) reactions. The reactions
proceed via formation of the ruthenium phosphonium alkylidene complex
[(H<sub>2</sub>IMes)Â(Cl)<sub>2</sub>Ruî»CÂ(H)ÂPCy<sub>3</sub>]Â[OTf]
as the active catalytic species. In the case of ROMP of cycloalkenes,
reactions do not require addition of PAG and protonation of <b>1</b> proceeds via allylic CâH bond activation of the substrate
under UV light