4 research outputs found
Mechanistic Investigations of the Rhodium Catalyzed Propargylic CH Activation
Previously we reported the redox-neutral
atom economic rhodium
catalyzed coupling of terminal alkynes with carboxylic acids using
the DPEphos ligand. We herein present a thorough mechanistic investigation
applying various spectroscopic and spectrometric methods (NMR, <i>in situ</i>-IR, ESI-MS) in combination with DFT calculations.
Our findings show that in contrast to the originally proposed mechanism,
the catalytic cycle involves an intramolecular protonation and not
an oxidative insertion of rhodium in the OH bond of the carboxylic
acid. A σ-allyl complex was identified as the resting state
of the catalytic transformation and characterized by X-ray crystallographic
analysis. By means of ESI-MS investigations we were able to detect
a reactive intermediate of the catalytic cycle
Univalent Gallium Salts of Weakly Coordinating Anions: Effective Initiators/Catalysts for the Synthesis of Highly Reactive Polyisobutylene
The
scope of the univalent gallium salts [GaÂ(C<sub>6</sub>H<sub>5</sub>F)<sub>2</sub>]<sup>+</sup>[AlÂ(OR<sup>F</sup>)<sub>4</sub>]<sup>−</sup> and the new completely characterized [GaÂ(1,3,5-Me<sub>3</sub>C<sub>6</sub>H<sub>3</sub>)<sub>2</sub>]<sup>+</sup>[AlÂ(OR<sup>F</sup>)<sub>4</sub>]<sup>−</sup> (R<sup>F</sup> = CÂ(CF<sub>3</sub>)<sub>3</sub>) was investigated in terms of initiating or
catalyzing the synthesis of highly reactive polyÂ(2-methylpropylene)î—¸highly
reactive polyisobutylene (HR-PIB)î—¸in several solvents. A series
of polymerization reactions proved the high efficiency and quality
of the univalent gallium salts for the polymerization of isobutylene.
The best results were obtained using very low concentrations of [GaÂ(C<sub>6</sub>H<sub>5</sub>F)<sub>2</sub>]<sup>+</sup>[AlÂ(OR<sup>F</sup>)<sub>4</sub>]<sup>−</sup> (down to 0.007 mol%) while working
at reaction temperatures of up to ±0 °C and in the noncarcinogenic
and non-water hazardous solvent toluene. Under these conditions, HR-PIB
with an α-content of terminal olefinic double bonds up to 91
mol% and a molecular weight of 1000–2000 was obtained in good
yields. Upon changing [GaÂ(C<sub>6</sub>H<sub>5</sub>F)<sub>2</sub>]<sup>+</sup>[AlÂ(OR<sup>F</sup>)<sub>4</sub>]<sup>−</sup> for
the electron richer [GaÂ(1,3,5-Me<sub>3</sub>C<sub>6</sub>H<sub>3</sub>)<sub>2</sub>]<sup>+</sup>[AlÂ(OR<sup>F</sup>)<sub>4</sub>]<sup>−</sup>, polymerization temperatures could be increased to +10 °C.
The reactivity of the galliumÂ(I) cations therefore seems to be tunable
through ligand exchange reactions. Experimental results, density functional
theory calculations, and mass spectrometric investigations point toward
a coordinative polymerization mechanism
Univalent Gallium Salts of Weakly Coordinating Anions: Effective Initiators/Catalysts for the Synthesis of Highly Reactive Polyisobutylene
The
scope of the univalent gallium salts [GaÂ(C<sub>6</sub>H<sub>5</sub>F)<sub>2</sub>]<sup>+</sup>[AlÂ(OR<sup>F</sup>)<sub>4</sub>]<sup>−</sup> and the new completely characterized [GaÂ(1,3,5-Me<sub>3</sub>C<sub>6</sub>H<sub>3</sub>)<sub>2</sub>]<sup>+</sup>[AlÂ(OR<sup>F</sup>)<sub>4</sub>]<sup>−</sup> (R<sup>F</sup> = CÂ(CF<sub>3</sub>)<sub>3</sub>) was investigated in terms of initiating or
catalyzing the synthesis of highly reactive polyÂ(2-methylpropylene)î—¸highly
reactive polyisobutylene (HR-PIB)î—¸in several solvents. A series
of polymerization reactions proved the high efficiency and quality
of the univalent gallium salts for the polymerization of isobutylene.
The best results were obtained using very low concentrations of [GaÂ(C<sub>6</sub>H<sub>5</sub>F)<sub>2</sub>]<sup>+</sup>[AlÂ(OR<sup>F</sup>)<sub>4</sub>]<sup>−</sup> (down to 0.007 mol%) while working
at reaction temperatures of up to ±0 °C and in the noncarcinogenic
and non-water hazardous solvent toluene. Under these conditions, HR-PIB
with an α-content of terminal olefinic double bonds up to 91
mol% and a molecular weight of 1000–2000 was obtained in good
yields. Upon changing [GaÂ(C<sub>6</sub>H<sub>5</sub>F)<sub>2</sub>]<sup>+</sup>[AlÂ(OR<sup>F</sup>)<sub>4</sub>]<sup>−</sup> for
the electron richer [GaÂ(1,3,5-Me<sub>3</sub>C<sub>6</sub>H<sub>3</sub>)<sub>2</sub>]<sup>+</sup>[AlÂ(OR<sup>F</sup>)<sub>4</sub>]<sup>−</sup>, polymerization temperatures could be increased to +10 °C.
The reactivity of the galliumÂ(I) cations therefore seems to be tunable
through ligand exchange reactions. Experimental results, density functional
theory calculations, and mass spectrometric investigations point toward
a coordinative polymerization mechanism
Mechanism of the Ti<sup>III</sup>-Catalyzed Acyloin-Type Umpolung: A Catalyst-Controlled Radical Reaction
The titaniumÂ(III)-catalyzed
cross-coupling between ketones and
nitriles provides an efficient stereoselective synthesis of α-hydroxyketones.
A detailed mechanistic investigation of this reaction is presented,
which involves a combination of several methods such as EPR, ESI-MS,
X-ray, in situ IR kinetics, and DFT calculations. Our findings reveal
that C–C bond formation is turnover-limiting and occurs by
a catalyst-controlled radical combination involving two titaniumÂ(III)
species. The resting state is identified as a cationic titanocene-nitrile
complex and the beneficial effect of added Et<sub>3</sub>N·HCl
on yield and enantioselectivity is elucidated: chloride coordination
initiates the radical coupling. The results are fundamental for the
understanding of titaniumÂ(III)-catalysis and of relevance for other
metal-catalyzed radical reactions. Our conclusions might apply to
a number of reductive coupling reactions for which conventional mechanisms
were proposed before