4 research outputs found
Carbon Nanotubes as Activating Tyrosinase Supports for the Selective Synthesis of Catechols
A series
of redox catalysts based on the immobilization of tyrosinase
on multiwalled carbon nanotubes has been prepared by applying the
layer-by-layer principle. The oxidized nanotubes (ox-MWCNTs) were
treated with polyÂ(diallyl dimethylammonium chloride) (PDDA) and tyrosinase
to yield ox-MWCNTs/PDDA/tyrosinase <b>I</b>. Catalysts <b>II</b> and <b>III</b> have been prepared by increasing the
number of layers of PDDA and enzyme, while <b>IV</b> was obtained
by co-immobilization of tyrosinase with bovine serum albumin (ox-MWCNTs/PDDA/BSA-tyrosinase).
Attempts to covalently bind tyrosinase provided weakly active systems.
The coating of the enzyme based on the simple layer-by-layer principle
has afforded catalysts <b>IâIII</b>, with a range of
activity from 21 units/mg (multilayer, <b>II</b>) to 66 units/mg
(monolayer, <b>I</b>), the best system being catalyst <b>IV</b> (80 units/mg). The novel catalysts were fully characterized
by scanning electron microscopy and atomic force microscopy, showing
increased activity with respect to that of the native enzyme. These
catalysts were used in the selective synthesis of catechols by oxidation
of <i>meta</i>- and <i>para</i>-substituted phenols
in an organic solvent (CH<sub>2</sub>Cl<sub>2</sub>) as the reaction
medium. It is worth noting that immobilized tyrosinase was able to
catalyze the oxidation of very hindered phenol derivatives that are
slightly reactive with the native enzyme. The increased reactivity
can be ascribed to a stabilization of the immobilized tyrosinase.
The novel catalysts <b>I</b> and <b>IV</b> retained their
activity for five subsequent reactions, showing a higher stability
in organic solvent than under traditional buffer conditions
Amides in Bio-oil by Hydrothermal Liquefaction of Organic Wastes: A Mass Spectrometric Study of the Thermochemical Reaction Products of Binary Mixtures of Amino Acids and Fatty Acids
Among biofuels, the bio-oil produced
by hydrothermal liquefaction
of waste biomass can be considered an alternative to fossil fuels
in industry as well as transport and heating compartments. The bio-oil
complex composition is directly dependent upon the specific biomass
used as feedstock and the process used for the chemical conversion.
The coexistence of proteins and lipids can explain, in principle,
the high percentage of fatty acid amides found in the produced bio-oil.
In the present study, the amides in a sample of bio-oil have been
separated by gas chromatography and identified at first on the basis
of their electron impact (EI) mass spectra. To distinguish between <i>N</i>-alkyl isomers, standard amides have been synthesized and
analyzed. Because the most reasonable origin of fatty acid amides
in hydrothermal bio-oils is the condensation reaction between fatty
acids and the decarboxylation products of amino acids, a series of
model experiments have been carried out by reacting hexadecanoic acid,
at high temperature and pressure, with each of the 20 amino acids
constitutive of proteins, looking for the formation of fatty acid
amides. Remarkably, by such experiments, all of the amides present
in the bio-oil have been recognized as hydrothermal coupling compounds
of the decomposition products of amino acids with fatty acids, thus
allowing for their structural elucidation and, also important, confirming
their (bio)Âchemical origin
Ligand-Controlled Regioselectivity in the Hydrothiolation of Alkynes by Rhodium N-Heterocyclic Carbene Catalysts
RhâN-heterocyclic carbene compounds [RhÂ(ÎŒ-Cl)Â(IPr)Â(η<sup>2</sup>-olefin)]<sub>2</sub> and RhClÂ(IPr)Â(py)Â(η<sup>2</sup>-olefin) (IPr = 1,3-bisÂ(2,6-diisopropylphenyl)Âimidazol-2-carbene,
py = pyridine, olefin = cyclooctene or ethylene) are highly active
catalysts for alkyne hydrothiolation under mild conditions. A regioselectivity
switch from linear to 1-substituted vinyl sulfides was observed when
mononuclear RhClÂ(IPr)Â(py)Â(η<sup>2</sup>-olefin) catalysts were
used instead of dinuclear precursors. A complex interplay between
electronic and steric effects exerted by IPr, pyridine, and hydride
ligands accounts for the observed regioselectivity. Both IPr and pyridine
ligands stabilize formation of square-pyramidal thiolateâhydride
active species in which the encumbered and powerful electron-donor
IPr ligand directs coordination of pyridine trans to it, consequently
blocking access of the incoming alkyne in this position. Simultaneously,
the higher trans director hydride ligand paves the way to a cis thiolateâalkyne
disposition, favoring formation of 2,2-disubstituted metalâalkenyl
species and subsequently the Markovnikov vinyl sulfides via alkenylâhydride
reductive elimination. DFT calculations support a plausible reaction
pathway where migratory insertion of the alkyne into the rhodiumâthiolate
bond is the rate-determining step
Ligand-Controlled Regioselectivity in the Hydrothiolation of Alkynes by Rhodium N-Heterocyclic Carbene Catalysts
RhâN-heterocyclic carbene compounds [RhÂ(ÎŒ-Cl)Â(IPr)Â(η<sup>2</sup>-olefin)]<sub>2</sub> and RhClÂ(IPr)Â(py)Â(η<sup>2</sup>-olefin) (IPr = 1,3-bisÂ(2,6-diisopropylphenyl)Âimidazol-2-carbene,
py = pyridine, olefin = cyclooctene or ethylene) are highly active
catalysts for alkyne hydrothiolation under mild conditions. A regioselectivity
switch from linear to 1-substituted vinyl sulfides was observed when
mononuclear RhClÂ(IPr)Â(py)Â(η<sup>2</sup>-olefin) catalysts were
used instead of dinuclear precursors. A complex interplay between
electronic and steric effects exerted by IPr, pyridine, and hydride
ligands accounts for the observed regioselectivity. Both IPr and pyridine
ligands stabilize formation of square-pyramidal thiolateâhydride
active species in which the encumbered and powerful electron-donor
IPr ligand directs coordination of pyridine trans to it, consequently
blocking access of the incoming alkyne in this position. Simultaneously,
the higher trans director hydride ligand paves the way to a cis thiolateâalkyne
disposition, favoring formation of 2,2-disubstituted metalâalkenyl
species and subsequently the Markovnikov vinyl sulfides via alkenylâhydride
reductive elimination. DFT calculations support a plausible reaction
pathway where migratory insertion of the alkyne into the rhodiumâthiolate
bond is the rate-determining step