5 research outputs found
Addition of Sn–O<sup><i>i</i></sup>Pr across a CC Bond: Unusual Insertion of an Alkene into a Main-Group-Metal–Alkoxide Bond
An example of unusual addition of
a main-group-metal alkoxide across
an alkene Cî—»C bond was demonstrated with a dimethylvinylsilyl-substituted
Sn-POSS complex (POSS = incompletely condensed polyhedral oligomeric
silsesquioxane). The structure of the pentacoordinated Sn chelate
product was confirmed by <sup>1</sup>H, <sup>13</sup>C, and <sup>119</sup>Sn NMR and ESI-MS
Microkinetic Modeling of Homogeneous and Gold Nanoparticle-Catalyzed Oxidation of Cyclooctene
Small
gold nanoparticles (AuNPs) have recently shown potential
to act as a catalyst for oxidation reactions mediated by free radicals,
with their role postulated to be facilitating hydrogen abstraction
by gold superoxo species and/or activation of hydroperoxides. Cyclooctene
oxidation using molecular oxygen as the oxidant at 373 K showed high
selectivity to the epoxide product, cyclooctene oxide, with either <i>tert</i>-butyl hydroperoxide as an initiator or in the presence
of small AuNPs (5–8 atoms). While previous studies have investigated
the mechanism leading to high epoxide selectivity using density functional
theory, a full microkinetic model was developed in this work using
automated network generation to determine the relative contributions
of elementary reactions and the role of AuNPs. A cycle of radical
addition of peroxy and alkoxy radicals with subsequent epoxidation
reactions can justify the observed activity and selectivity to epoxide
at multiple temperatures and initiator concentrations. The alcohol,
ketone, and hydroperoxide minor products are formed when the peroxy
and alkoxy radicals perform hydrogen abstraction or β-scission.
The overall selectivity is determined by the competition between the
addition and hydrogen abstraction reactions. With the underlying homogeneous
reaction model validated, the effect of the AuNPs was determined through
an expanded model that includes cycles for both hydrogen abstraction
by superoxo gold species and hydroperoxide decomposition via species
derived from AuNPs. The model suggests that hydrogen abstraction by
the superoxo gold species dominates at short times compared to homogeneous
initiation, increasing the activity during the induction period and
reducing the time before radical chain oxidation commences. On the
time scale of an hour, hydroperoxide decomposition is significantly
catalyzed by the presence of gold species, increasing the overall
rate of chain propagation
Generating and Stabilizing Co(I) in a Nanocage Environment
A discrete nanocage
of core–shell design, in which carboxylic
acid groups were tethered to the core and silanol to the shell interior,
was found to react with Co<sub>2</sub>(CO)<sub>8</sub> to form and
stabilize a CoÂ(I)–CO species. The singular CO stretching band
of this new Co species at 1958 cm<sup>–1</sup> and its magnetic
susceptibility were consistent with CoÂ(I) compounds. When exposed
to O<sub>2</sub>, it transformed from an EPR inactive to an EPR active
species indicative of oxidation of CoÂ(I) to CoÂ(II) with the formation
of H<sub>2</sub>O<sub>2</sub>. It could be oxidized also by organoazide
or water. Its residence in the nanocage interior was confirmed by
size selectivity in the oxidation process and the fact that the entrapped
Co species could not be accessed by an electrode
Supported Tetrahedral Oxo-Sn Catalyst: Single Site, Two Modes of Catalysis
Mild calcination in ozone of a (POSS)-Sn-(POSS)
complex grafted
on silica generated a heterogenized catalyst that mostly retained
the tetrahedral coordination of its homogeneous precursor, as evidenced
by spectroscopic characterizations using EXAFS, NMR, UV–vis,
and DRIFT. The Sn centers are accessible and uniform and can be quantified
by stoichiometric pyridine poisoning. This Sn-catalyst is active in
hydride transfer reactions as a typical solid Lewis acid. However,
the Sn centers can also create Brønsted acidity with alcohol
by binding the alcohol strongly as alkoxide and transferring the hydroxyl
H to the neighboring Sn–O–Si bond. The resulting acidic
silanol is active in epoxide ring opening and acetalization reactions
Acceptorless Dehydrogenative Coupling of Neat Alcohols Using Group VI Sulfide Catalysts
Group VI sulfides
were synthesized via coprecipitation of elemental
sulfur and metal hexacarbonyl and characterized with XRD, XPS, and
TEM. These materials were then demonstrated as active catalysts for
the acceptorless dehydrogenative coupling of neat ethanol to ethyl
acetate, rapidly reaching equilibrium conversion and up to 90% selectivity.
Other primary alcohols form the corresponding esters, while diols
formed the corresponding cyclic ethers and oligomers