9 research outputs found
Site-Selective Hydrogenation of Electron-Poor Alkenes and Dienes Enabled by a Rh-Catalyzed Hydride Addition/Protonolysis Mechanism
The transition metal catalyzed hydrogenation of alkenes is a well-developed technology used on a lab scale as well as on large scales in the chemical industry. Site- and chemoselective mono-hydrogenations of polarized conjugated dienes remain challenging. Instead, stoichiometric main-group hydrides are used rather than H2. As part of an effort to develop a scalable route to prepare geranylacetone, we discovered that Rh(CO)2acac/xantphos based catalysts enable the selective monohydrogenation of electron-poor 1,3-dienes, enones, and other polyunsaturated substrates. D-labeling and DFT studies support a mechanism where a nucleophilic Rh(I)-hydride selectively adds to electron-poor alkenes and the resulting Rh-enolate undergoes subsequent inner-sphere protonation by alcohol solvent. The finding that (Ln)Rh(H)(CO) type catalysts can enable selective mono-hydrogenation of electron-poor (poly)enes provides a valuable tool in the design of related chemoselective reduction processes of unsaturated substrates
Study of Precatalyst Degradation Leading to the Discovery of a New Ru<sup>0</sup> Precatalyst for Hydrogenation and Dehydrogenation
The complex Ru-MACHO
(<b>1</b>) is a widely used precatalyst
for hydrogenation and dehydrogenation reactions under basic conditions.
In an attempt to identify the active catalyst form, <b>1</b> was reacted with a strong base. The formation of previously unreported
species was observed by NMR and mass spectrometry. This observation
indicated that complex <b>1</b> quickly degraded under basic
conditions when no substrate was present. X-ray crystallography enabled
the identification of three complexes as products of this degradation
of complex <b>1</b>. These complexes suggested degradation pathways
which included ligand cleavage and reassembly, along with reduction
of the ruthenium atom. One of the decomposition products, the Ru<sup>0</sup> complex [RuÂ(NÂ(CH<sub>2</sub>CH<sub>2</sub>PPh<sub>2</sub>)<sub>3</sub>)ÂCO] (<b>5</b>), was prepared independently and
studied. <b>5</b> was found to be active, entirely additive-free,
in the acceptorless dehydrogenation of aliphatic alcohols to esters.
The hydrogenation of esters catalyzed by <b>5</b> was also demonstrated
under base-free conditions with methanol as an additive. Protic substrates
were shown to add reversibly to complex <b>5</b>, generating
Ru<sup>II</sup>–hydrido species, thus presenting a rare example
of reversible oxidative addition from Ru<sup>0</sup> to Ru<sup>II</sup> and reductive elimination from Ru<sup>II</sup> to Ru<sup>0</sup>
Ru(II)-Triphos Catalyzed Amination of Alcohols with Ammonia via Ionic Species
An active and selective system for
the amination of primary alcohols
to primary amines with ammonia based on ruthenium and triphos as the
tridentate phosphine ligand was developed. On the basis of detailed
mechanistic studies, we propose that the active catalyst is, unlike
the previously reported systems on this reaction, a cationic ruthenium
complex. The experimental findings are supported by detailed density
functional theory (DFT) calculations on the catalytic cycle. Because
of the cationic nature of the active catalyst, strong anion and solvent
effects were observed in the catalytic amination reaction when using
the ruthenium triphos complexes. Therefore, a higher activity could
be achieved when the nonpolar solvent toluene is used in this amination
instead of tetrahydrofuran. Our findings can help to develop and optimize
the system systematically for an application to relevant target molecules
Selective Alkylation of Amines with Alcohols by Cp*–Iridium(III) Half-Sandwich Complexes
[Cp*Ir(Pro)Cl] (Pro = prolinato) was identified among a series of Cp*–iridium half-sandwich complexes as a highly reactive and selective catalyst for the alkylation of amines with alcohols. It is active under mild conditions in either toluene or water without the need for base or other additives, tolerates a wide range of alcohols and amines, and gives secondary amines in good to excellent isolated yields
Alcohol Amination with Aminoacidato Cp*Ir(III)-Complexes as Catalysts: Dissociation of the Chelating Ligand during Initiation
The use of aminoacidato Cp*IrÂ(III)-complexes
in catalytic alcohol amination reactions of primary and secondary
alcohols with amines permits to carry out these transformations at
very mild reaction conditions without the use of an additional base.
Herein we discuss the fate of the chelating aminoacidato ligands upon
initiation of Cp*IrÂ(III)-complexes from a mechanistic perspective.
Catalyst initiation has been followed by NMR using isotopically labeled <sup>13</sup>C,<sup>15</sup>N-glycinato complexes
Alcohol Amination with Aminoacidato Cp*Ir(III)-Complexes as Catalysts: Dissociation of the Chelating Ligand during Initiation
The use of aminoacidato Cp*IrÂ(III)-complexes
in catalytic alcohol amination reactions of primary and secondary
alcohols with amines permits to carry out these transformations at
very mild reaction conditions without the use of an additional base.
Herein we discuss the fate of the chelating aminoacidato ligands upon
initiation of Cp*IrÂ(III)-complexes from a mechanistic perspective.
Catalyst initiation has been followed by NMR using isotopically labeled <sup>13</sup>C,<sup>15</sup>N-glycinato complexes
Alcohol Amination with Ammonia Catalyzed by an Acridine-Based Ruthenium Pincer Complex: A Mechanistic Study
The mechanistic course
of the amination of alcohols with ammonia
catalyzed by a structurally modified congener of Milstein’s
well-defined acridine-based PNP-pincer Ru complex has been investigated
both experimentally and by DFT calculations. Several key Ru intermediates
have been isolated and characterized. The detailed analysis of a series
of possible catalytic pathways (e.g., with and without metal–ligand
cooperation, inner- and outer-sphere mechanisms) leads us to conclude
that the most favorable pathway for this catalyst does not require
metal–ligand cooperation
Iminosugar C‑Glycoside Analogues of α‑d‑GlcNAc-1-Phosphate: Synthesis and Bacterial Transglycosylase Inhibition
We
herein describe the first synthesis of iminosugar C-glycosides
of α-d-GlcNAc-1-phosphate in 10 steps starting from
unprotected d-GlcNAc. A diastereoselective intramolecular
iodoamination–cyclization as the key step was employed to construct
the central piperidine ring of the iminosugar and the C-glycosidic
structure of α-d-GlcNAc. Finally, the iminosugar phosphonate
and its elongated phosphate analogue were accessed. These phosphorus-containing
iminosugars were coupled efficiently with lipophilic monophosphates
to give lipid-linked pyrophosphate derivatives, which are lipid II
mimetics endowed with potent inhibitory properties toward bacterial
transglycosylases (TGase)