8 research outputs found
Highly Versatile PdāThioetherāPhosphite Catalytic Systems for Asymmetric Allylic Alkylation, Amination, and Etherification Reactions
A Pdāfuranoside thioetherāphosphite
catalytic system
that can create new CāC, CāN, and CāO bonds in
several substrate types using a wide range of nucleophiles in high
yields and enantioselectivities has been identified. Of particular
note are the excellent enantioselectivities obtained in the etherification
of linear and cyclic substrates. The potential application of the
new Pdāthioetherāphosphite catalytic systems was also
demonstrated by the synthesis of the chiral carbo- and heterocycles
PHOX-Based Phosphite-Oxazoline Ligands for the Enantioselective Ir-Catalyzed Hydrogenation of Cyclic Ī²āEnamides
Simple
Ir-PHOX-based phosphite-oxazoline catalysts have been successfully
applied in the asymmetric hydrogenation of cyclic Ī²-enamides
providing better enantioselectivities than previous effective Ru and
Rh catalysts. This protocol allows the synthesis of 2-aminotetralines
and 3-aminochromanes, key structural units found in many therapeutic
agents and biologically active natural products, in high chemical
yields and enantioselectivities (eeās up to 99%). High enantioselectivities
have also been achieved in the hydrogenation of cyclic Ī±-enamides
Iridium-Catalyzed Asymmetric Hydrogenation with Simple Cyclohexane-Based P/S Ligands: <i>In Situ</i> HP-NMR and DFT Calculations for the Characterization of Reaction Intermediates
We
report a reduced but structurally valuable phosphite/phosphinite-thioether
ligand library for the Ir-hydrogenation of 40 minimally functionalized
alkenes, including relevant examples with poorly coordinative groups.
We found that enantiomeric excesses are mainly dependent on the substrate
structure and on some ligand parameters (i.e., the type of thioether/phosphorus
moieties and the configuration of the phosphite group), whereas the
substituents of the biaryl phosphite moiety had little impact. By
tuning the ligand parameters we were able to find highly selective
catalysts for a range of substrates (eeās up to 99%). These
phosphite/phosphinite-thioether ligands have a simple backbone and
thus yield simple NMR spectra that reduce signal overlap and facilitate
the identification of relevant intermediates. Therefore, by combining
HP-NMR spectroscopy and theoretical studies, we were also able to
identify the catalytically competent Ir-dihydride alkene species,
which made it possible to explain the enantioselectivity obtained
Iridium-Catalyzed Asymmetric Hydrogenation with Simple Cyclohexane-Based P/S Ligands: <i>In Situ</i> HP-NMR and DFT Calculations for the Characterization of Reaction Intermediates
We
report a reduced but structurally valuable phosphite/phosphinite-thioether
ligand library for the Ir-hydrogenation of 40 minimally functionalized
alkenes, including relevant examples with poorly coordinative groups.
We found that enantiomeric excesses are mainly dependent on the substrate
structure and on some ligand parameters (i.e., the type of thioether/phosphorus
moieties and the configuration of the phosphite group), whereas the
substituents of the biaryl phosphite moiety had little impact. By
tuning the ligand parameters we were able to find highly selective
catalysts for a range of substrates (eeās up to 99%). These
phosphite/phosphinite-thioether ligands have a simple backbone and
thus yield simple NMR spectra that reduce signal overlap and facilitate
the identification of relevant intermediates. Therefore, by combining
HP-NMR spectroscopy and theoretical studies, we were also able to
identify the catalytically competent Ir-dihydride alkene species,
which made it possible to explain the enantioselectivity obtained
Triazolylidene Iridium Complexes for Highly Efficient and Versatile Transfer Hydrogenation of Cī»O, Cī»N, and Cī»C Bonds and for Acceptorless Alcohol Oxidation
A set of iridiumĀ(I)
and iridiumĀ(III) complexes is reported with triazolylidene ligands
that contain pendant benzoxazole, thiazole, and methyl ether groups
as potentially chelating donor sites. The bonding mode of these groups
was identified by NMR spectroscopy and X-ray structure analysis. The
complexes were evaluated as catalyst precursors in transfer hydrogenation
and in acceptorless alcohol oxidation. High-valent iridiumĀ(III) complexes
were identified as the most active precursors for the oxidative alcohol
dehydrogenation, while a low-valent iridiumĀ(I) complex with a methyl
ether functionality was most active in reductive transfer hydrogenation.
This catalyst precursor is highly versatile and efficiently hydrogenates
ketones, aldehydes, imines, allylic alcohols, and most notably also
unpolarized olefins, a notoriously difficult substrate for transfer
hydrogenation. Turnover frequencies up to 260 h<sup>ā1</sup> were recorded for olefin hydrogenation, whereas hydrogen transfer
to ketones and aldehydes reached maximum turnover frequencies greater
than 2000 h<sup>ā1</sup>. Mechanistic investigations using
a combination of isotope labeling experiments, kinetic isotope effect
measurements, and Hammett parameter correlations indicate that the
turnover-limiting step is hydride transfer from the metal to the substrate
in transfer hydrogenation, while in alcohol dehydrogenation, the limiting
step is substrate coordination to the metal center
Asymmetric Catalyzed Allylic Substitution Using a Pd/PāS Catalyst Library with Exceptional High Substrate and Nucleophile Versatility: DFT and Pd-Ļ-allyl Key Intermediates Studies
A large library of
furanoside phosphite/phosphinite/phosphine-thioether
ligands <b>L1</b>ā<b>L17a</b>ā<b>l</b> has been applied in the Pd-catalyzed allylic substitution reactions
of several substrate types using a wide range of nucleophiles. These
ligands, which are prepared from inexpensive d-xylose, also
incorporate the advantages of the heterodonor, the robustness of the
thioether moiety, and the extra control provided by the flexibility
of the chiral pocket through the presence of a biaryl phosphite group
and a modular sugar backbone. By selecting the ligand components,
we have been able to identify catalytic systems that can create new
CāC, CāN, and CāO bonds in several substrate
types (hindered and unhindered) using a wide range of nucleophiles
in high yields and enantioselectivities (eeās up to >99%).
Of particular note are the excellent enantioselectivities obtained
in the etherification of linear and cyclic substrates, which represent
the first example of successful etherification of both substrate types.
The DFT computational study is in agreement with an early transition
state. Further studies on the Pd-Ļ-allyl intermediates provided
a deep understanding of the effect of ligand structure in the origin
of enantioselectivity
AminoāP Ligands from Iminosugars: New Readily Available and Modular Ligands for Enantioselective Pd-Catalyzed Allylic Substitutions
The construction
of a novel class of amino-phosphite/phosphinite/phosphine
ligands containing a protected pyrrolidine-3,4-diol moiety is presented.
These ligands are obtained from readily available sugars. They thus
contain the advantages of carbohydrates in terms of selection of the
stereogenic carbons, polyfunctional groups able to modulate the electronic
and steric properties, and the general good stability of carbohydrate
derivatives. They constitute a novel class of P,N-ligands that have
been used in the enantioselective allylic substitutions of acyclic
and cyclic substrates with varied electronic and steric requirements,
using different C- and N-nucleophiles, with high enantioselectivities.
Among the three groups of P,N-ligands (amino-P; P = phosphite, phosphinite,
and phosphine groups) the new amino-phosphite ligands give the widest
substrate and nucleophile scope, including the more challenging hindered
linear and cyclic substrates. In particular, for carbohydrate-derived
amino-phosphite ligands and linear substrates, high enantioselectivity
in the reactions requires an <i>R</i> configuration of the
binaphthyl moiety. However, for cyclic substrates both product enantiomers
can be reached by setting out the chirality of the binaphthyl phosphite
moiety. A detailed investigation of the appropriate Pd intermediates
is also presented
Conformational Preferences of a Tropos Biphenyl Phosphinooxazolineāa Ligand with Wide Substrate Scope
Excellent
enantioselectivities are observed in palladium-catalyzed
allylic substitutions of a wide range of substrate types and nucleophiles
using a bidentate ligand composed of oxazoline and chirally flexible
biaryl phosphite elements. This unusually wide substrate scope is
shown by experimental and theoretical studies of its Ī·<sup>3</sup>-allyl and Ī·<sup>2</sup>-olefin complexes not to be a result
of configurational interconversion of the biaryl unit, since the ligand
in all reactions adopts an <i>S</i><sub>a</sub>,<i>S</i> configuration on coordination to palladium, but rather
the ability of the ligand to adapt the size of the substrate-binding
pocket to the reacting substrate. This ability also serves as an explanation
to its excellent performance in other types of catalytic processes