22 research outputs found
Selective Isomerization–Hydroformylation Sequence: A Strategy to Valuable α‑Methyl-Branched Aldehydes from Terminal Olefins
For
the first time, an original selective isomerization-hydroformylation
sequence to convert terminal olefins bearing an anionic moiety to
α-methyl-branched aldehydes with unprecedented selectivities
is reported. This opens up new synthetic avenues to these valuable
building blocks from inexpensive and bioavailable substrates. The
catalytic system involves a suitable selective monoisomerization catalyst
and a selective supramolecular catalyst that preorganizes a substrate
molecule prior to the hydroformylation reaction via hydrogen bonding.
In principle, the strategy can be extended to other classes of substrates,
providing suitable catalysts for the hydroformylation of internal
alkenes
Precise Supramolecular Control of Selectivity in the Rh-Catalyzed Hydroformylation of Terminal and Internal Alkenes
In
this study, we report a series of DIMPhos ligands <b>L1</b>–<b>L3</b>, bidentate phosphorus ligands equipped with
an integral anion binding site (the DIM pocket). Coordination studies
show that these ligands bind to a rhodium center in a bidentate fashion.
Experiments under hydroformylation conditions confirm the formation
of the mononuclear hydridobiscarbonyl rhodium complexes that are generally
assumed to be active in hydroformylation. The metal complexes formed
still strongly bind the anionic species in the binding site of the
ligand, without affecting the metal coordination sphere. These bifunctional
properties of DIMPhos are further demonstrated by the crystal structure
of the rhodium complex with acetate anion bound in the binding site
of the ligand. The catalytic studies demonstrate that substrate preorganization
by binding in the DIM pocket of the ligand results in unprecedented
selectivities in hydroformylation of terminal and internal alkenes
functionalized with an anionic group. Remarkably, the selectivity
controlling anionic group can be even 10 bonds away from the reactive
double bond, demonstrating the potential of this supramolecular approach.
Control experiments confirm the crucial role of the anion binding
for the selectivity. DFT studies on the decisive intermediates reveal
that the anion binding in the DIM pocket restricts the rotational
freedom of the reactive double bound. As a consequence, the pathway
to the undesired product is strongly hindered, whereas that for the
desired product is lowered in energy. Detailed kinetic studies, together
with the in situ spectroscopic measurements and isotope-labeling studies,
support this mode of operation and reveal that these supramolecular
systems follow enzymatic-type Michaelis–Menten kinetics, with
competitive product inhibition
Self-Assembly of a Confined Rhodium Catalyst for Asymmetric Hydroformylation of Unfunctionalized Internal Alkenes
A chiral supramolecular ligand has been assembled and
applied to
the rhodium-catalyzed asymmetric hydroformylation of unfunctionalized
internal alkenes. Spatial confinement of the metal center within a
chiral pocket results in reversed regioselectivity and remarkable
enantioselectivities
Photo- and Thermal Isomerization of (TP)Fe(CO)Cl<sub>2</sub> [TP = Bis(2-diphenylphosphinophenyl)Âphenylphosphine]
The
title complex displayed structural flexibility via photo- and
thermal-isomerization reactions between three isomers: (<i>mer</i>-TP)ÂFeÂ(CO)ÂCl<sub>2</sub> (<b>A</b>), <i>unsym</i>-(<i>fac</i>-TP)ÂFeÂ(CO)ÂCl<sub>2</sub> (<b>B</b>),
and <i>sym</i>-(<i>fac-</i>TP)ÂFeÂ(CO)ÂCl<sub>2</sub> (<b>C</b>). Irradiation of <b>A</b> at RT with 525 nm
light selectively produces <b>B</b>, while at 0 °C isomer <b>C</b> is formed with the intermediacy of <b>B</b>. UV–vis
spectroscopy combined with TD-DFT calculations revealed the nature
of the photoisomerization process. Kinetics of the thermal isomerization
of <b>C</b> to <b>B</b> and <b>B</b> to <b>A</b> have been studied with <sup>31</sup>P NMR spectroscopy in CD<sub>2</sub>Cl<sub>2</sub>, and activation parameters were determined.
Isomers <b>A</b> and <b>B</b> have been isolated and crystallographically
characterized
Photo- and Thermal Isomerization of (TP)Fe(CO)Cl<sub>2</sub> [TP = Bis(2-diphenylphosphinophenyl)Âphenylphosphine]
The
title complex displayed structural flexibility via photo- and
thermal-isomerization reactions between three isomers: (<i>mer</i>-TP)ÂFeÂ(CO)ÂCl<sub>2</sub> (<b>A</b>), <i>unsym</i>-(<i>fac</i>-TP)ÂFeÂ(CO)ÂCl<sub>2</sub> (<b>B</b>),
and <i>sym</i>-(<i>fac-</i>TP)ÂFeÂ(CO)ÂCl<sub>2</sub> (<b>C</b>). Irradiation of <b>A</b> at RT with 525 nm
light selectively produces <b>B</b>, while at 0 °C isomer <b>C</b> is formed with the intermediacy of <b>B</b>. UV–vis
spectroscopy combined with TD-DFT calculations revealed the nature
of the photoisomerization process. Kinetics of the thermal isomerization
of <b>C</b> to <b>B</b> and <b>B</b> to <b>A</b> have been studied with <sup>31</sup>P NMR spectroscopy in CD<sub>2</sub>Cl<sub>2</sub>, and activation parameters were determined.
Isomers <b>A</b> and <b>B</b> have been isolated and crystallographically
characterized
“Cofactor”-Controlled Enantioselective Catalysis
We report an achiral bisphosphine rhodium complex equipped with a binding site for the recognition of chiral anion guests. Upon binding small chiral guests<i>cofactors</i>the rhodium complex becomes chiral and can thus be used for asymmetric catalysis. Screening of a library of cofactors revealed that the best cofactors lead to hydrogenation catalysts that form the products with high enantioselectivity (ee’s up to 99%). Interestingly, a competition experiment shows that even in a mixture of 12 cofactors high ee is obtained, indicating that the complex based on the best cofactor dominates the catalysis
Formation and Site-Selective Reactivity of a Nonsymmetric Dinuclear Iridium BisMETAMORPhos Complex
A flexible
diÂ(sulfonamidophosphine) ligand (<b>1</b><sup><b>H2</b></sup>, Ph<sub>2</sub>PN<sup>H</sup>SÂ(O)<sub>2</sub>N<sup>H</sup>PPh<sub>2</sub>) was synthesized from commercially available
sulfamide and chlorodiphenylphosphine. Coordination of this new bisMETAMORPhos <b>1</b><sup><b>H2</b></sup> with [IrÂ(Cp*)ÂClÂ(ÎĽ-Cl)]<sub>2</sub> instantly led to the formation of the P,P-coordinated bimetallic
complex <b>2</b> [IrÂ(Cp*)ÂClÂ(Îş<sup>1</sup>-<i>P</i><sub>1</sub>; Îş<sup>1</sup>-<i>P</i><sub>2</sub>-<b>1</b><sup><b>H2</b></sup>)ÂIrÂ(Cp*)ÂCl]. Reaction of <b>2</b> using excess NaOAc led to the formation of nonsymmetric
homodinuclear complex <b>3</b> [IrÂ(Cp*)ÂClÂ(Îş<sup>2</sup>-<i>P</i>,<i>O</i>; Îş<sup>3</sup>-<i>P</i>,<i>N</i>,<i>C</i>; ÎĽ-<b>1</b>)ÂIrÂ(Cp*)], which contains two distinctly different Ir<sup>III</sup> centers, with a <i>fac</i>-<i>P</i>,<i>N</i>,<i>C</i> and a <i>fac</i>-<i>P</i>,<i>O</i>,<i>Cl</i> coordination environment.
The ligand is overall trisanionic due to additional intramolecular
C–H activation of a flanking phenyl ring. Complex <b>3</b> reacts selectively at the IrÂ(<i>P</i>,<i>O</i>,<i>Cl</i>) center with a single equivalent of HCl or H<sub>2</sub> to generate complexes <b>4</b> and <b>5</b>,
respectively. These complexes are generated via heterolytic cleavage
of the H–Cl or H–H bond, which reprotonates the ligand
showing its bifunctional applicability. The Ir–C bond is found
to be inert under these conditions
Growth and Characterization of PDMS-Stamped Halide Perovskite Single Microcrystals
Recently, halide perovskites have
attracted considerable attention
for optoelectronic applications, but further progress in this field
requires a thorough understanding of the fundamental properties of
these materials. Studying perovskites in their single-crystalline
form provides a model system for building such an understanding. In
this work, a simple solution-processed method combined with PDMS (polydimethylÂsiloxane)
stamping was used to prepare thin single microcrystals of halide perovskites.
The method is general for a broad array of materials including CH<sub>3</sub>NH<sub>3</sub>PbBr<sub>3</sub>, CH<sub>3</sub>NH<sub>3</sub>PbCl<sub>3</sub>, CH<sub>3</sub>NH<sub>3</sub>PbÂ(Br<sub>0.5</sub>Cl<sub>0.5</sub>)<sub>3</sub>, CH<sub>3</sub>NH<sub>3</sub>PbÂ(Br<sub>0.75</sub>Cl<sub>0.25</sub>)<sub>3</sub>, CsPbBr<sub>3</sub>, Cs<sub>3</sub>Bi<sub>2</sub>Br<sub>9</sub>, and Cs<sub>3</sub>Bi<sub>2</sub>I<sub>9</sub>. Electron backscatter diffraction (EBSD) was used to
investigate the microstructure of the crystals. In order to characterize
the microcrystals of CH<sub>3</sub>NH<sub>3</sub>PbBr<sub>3</sub> electrically,
the crystals were grown on prefabricated electrodes creating single-crystal
devices contacted from the back. This back-contacted platform circumvents
the incompatibility between halide perovskites and the aqueous chemistry
used in standard microfabriation processes. It also allows <i>in situ</i> characterization of the perovskite crystal while
it operates as a microscopic solar cell
Co<sup>III</sup>–Carbene Radical Approach to Substituted 1<i>H</i>‑Indenes
A new
strategy for the catalytic synthesis of substituted 1<i>H</i>-indenes via metalloradical activation of <i>o</i>-cinnamyl <i>N</i>-tosyl hydrazones is presented, taking
advantage of the intrinsic reactivity of a Co<sup>III</sup> carbene
radical intermediate. The reaction uses readily available starting
materials and is operationally simple, thus representing a practical
method for the construction of functionalized 1<i>H</i>-indene
derivatives. The cheap and easy to prepare low spin cobaltÂ(II) complex
[Co<sup>II</sup>(MeTAA)] (MeTAA = tetramethyltetraaza[14]Âannulene)
proved to be the most active catalyst among those investigated, which
demonstrates catalytic carbene radical reactivity for a nonporphyrin
cobaltÂ(II) complex, and for the first time catalytic activity of [Co<sup>II</sup>(MeTAA)] in general. The methodology has been successfully
applied to a broad range of substrates, producing 1<i>H</i>-indenes in good to excellent yields. The metallo-radical catalyzed
indene synthesis in this paper represents a unique example of a net
(formal) intramolecular carbene insertion reaction into a vinylic
CÂ(sp<sup>2</sup>)–H bond, made possible by a controlled radical
ring-closure process of the carbene radical intermediate involved.
The mechanism was investigated computationally, and the results were
confirmed by a series of supporting experimental reactions. Density
functional theory calculations reveal a stepwise process involving
activation of the diazo compound leading to formation of a Co<sup>III</sup>-carbene radical, followed by radical ring-closure to produce
an indanyl/benzyl radical intermediate. Subsequent indene product
elimination involving a 1,2-hydrogen transfer step regenerates the
catalyst. Trapping experiments using 2,2,6,6-tetra-methylpiperidine-1-oxyl
(TEMPO) radical or dibenzoylperoxide (DBPO) confirm the involvement
of cobaltÂ(III) carbene radical intermediates. Electron paramagnetic
resonance spectroscopic spin-trapping experiments using phenyl <i>N</i>-<i>tert</i>-butylnitrone (PBN) reveal the radical
nature of the reaction
Highly Selective Asymmetric Rh-Catalyzed Hydroformylation of Heterocyclic Olefins
A small family of new chiral hybrid, diphosphorus ligands,
consisting
of phosphine-phosphoramidites <b>L1</b> and <b>L2</b> and
phosphine-phosphonites <b>L3a–c</b>, was synthesized
for the application in Rh-catalyzed asymmetric hydroformylation of
heterocyclic olefins. High-pressure (HP)-NMR and HP-IR spectroscopy
under 5–10 bar of syngas has been employed to characterize
the corresponding catalyst resting state with each ligand. Indole-based
ligands <b>L1</b> and <b>L2</b> led to selective <i>ea</i> coordination, while the xanthene derived system <b>L3c</b> gave predominant <i>ee</i> coordination. Application
of the small bite-angle ligands <b>L1</b> and <b>L2</b> in the highly selective asymmetric hydroformylation (AHF) of the
challenging substrate 2,3-dihydrofuran (<b>1</b>) yielded the
2-carbaldehyde (<b>3</b>) as the major regioisomer in up to
68% yield (with ligand <b>L2</b>) along with good ee’s
of up to 62%. This is the first example in which the asymmetric hydroformylation
of <b>1</b> is both regio- and enantioselective for isomer <b>3</b>. Interestingly, use of ligand <b>L3c</b> in the same
reaction completely changed the regioselectivity to 3-carbaldehyde
(<b>4</b>) with a remarkably high enantioselectivity of 91%.
Ligand <b>L3c</b> also performs very well in the Rh-catalyzed
asymmetric hydroformylation of other heterocyclic olefins. Highly
enantioselective conversion of the notoriously difficult substrate
2,5-dihydrofuran (<b>2</b>) is achieved using the same catalyst,
with up to 91% ee, concomitant with complete regioselectivity to the
3-carbaldehyde product (<b>4</b>) under mild reaction conditions.
Interestingly, the Rh-catalyst derived from <b>L3c</b> is thus
able to produce both enantiomers of 3-carbaldehyde <b>4</b>,
simply by changing the substrate from <b>1</b> to <b>2</b>. Furthermore, 85% ee was obtained in the hydroformylation of <i>N-</i>acetyl-3-pyrroline (<b>5</b>) with exceptionally
high regioselectivities for 3-carbaldehyde <b>8Ac</b> (>99%).
Similarly, an ee of 86% for derivative <b>8Boc</b> was accomplished
using the same catalyst system in the AHF of <i>N-</i>(<i>tert</i>-butoxycarbonyl)-3-pyrroline (<b>6</b>). These
results represent the highest ee’s reported to date in the
AHF of dihydrofurans (<b>1</b>, <b>2</b>) and 3-pyrrolines
(<b>5</b>, <b>6</b>)