7 research outputs found
Outer-Sphere Electrophilic Fluorination of Organometallic Complexes
Organofluorine chemistry plays a
key role in materials science,
pharmaceuticals, agrochemicals, and medical imaging. However, the
formation of new carbonâfluorine bonds with controlled regiochemistry
and functional group tolerance is synthetically challenging. The use
of metal complexes to promote fluorination reactions is of great current
interest, but even state-of-the-art approaches are limited in their
substrate scope, often require activated substrates, or do not allow
access to desirable functionality, such as alkenyl CÂ(sp<sup>2</sup>)âF or chiral CÂ(sp<sup>3</sup>)âF centers. Here, we
report the formation of new alkenyl and alkyl CâF bonds in
the coordination sphere of ruthenium via an unprecedented outer-sphere
electrophilic fluorination mechanism. The organometallic species involved
are derived from nonactivated substrates (pyridine and terminal alkynes),
and CâF bond formation occurs with full regio- and diastereoselectivity.
The fluorinated ligands that are formed are retained at the metal,
which allows subsequent metal-mediated reactivity
Outer-Sphere Electrophilic Fluorination of Organometallic Complexes
Organofluorine chemistry plays a
key role in materials science,
pharmaceuticals, agrochemicals, and medical imaging. However, the
formation of new carbonâfluorine bonds with controlled regiochemistry
and functional group tolerance is synthetically challenging. The use
of metal complexes to promote fluorination reactions is of great current
interest, but even state-of-the-art approaches are limited in their
substrate scope, often require activated substrates, or do not allow
access to desirable functionality, such as alkenyl CÂ(sp<sup>2</sup>)âF or chiral CÂ(sp<sup>3</sup>)âF centers. Here, we
report the formation of new alkenyl and alkyl CâF bonds in
the coordination sphere of ruthenium via an unprecedented outer-sphere
electrophilic fluorination mechanism. The organometallic species involved
are derived from nonactivated substrates (pyridine and terminal alkynes),
and CâF bond formation occurs with full regio- and diastereoselectivity.
The fluorinated ligands that are formed are retained at the metal,
which allows subsequent metal-mediated reactivity
Interactions in WaterâIonic Liquid Mixtures: Comparing Protic and Aprotic Systems
The
sensitivity of ionic liquids (ILs) to water affects their physical
and chemical properties, even at relatively low concentrations, yet
the structural thermodynamics of protic ILâ (PILâ) water
systems at low water concentrations still remains unclear. Using the
rigorous KirkwoodâBuff theory of solutions, which can quantify
the interactions between species in ILâwater systems solely
from thermodynamic data, we have shown the following: (1) Between
analogous protic and aprotic ILs (AILs), the AIL cholinium bisÂ(trifluoromethanesulfonyl)Âimide
([Ch]Â[NTf<sub>2</sub>]) shows stronger interactions with water at
low water concentrations, with the analogous PIL <i>N</i>,<i>N</i>-dimethylethanolammonium bisÂ(trifluoromethanesulfonyl)Âimide
([DMEtA]Â[NTf<sub>2</sub>]) having stronger waterâion interactions
at higher water contents, despite waterâion interactions weakening
with increasing water content in both systems. (2) Water has little
effect on the average ionâion interactions in both protic and
aprotic ILs, aside from the AIL [Ch]Â[NTf<sub>2</sub>], which shows
a strengthening of ionâion interactions with increasing water
content. (3) Self-association of water in both PILâwater systems
leading to the presence of large aggregates of water in IL-rich compositions
has been inferred. Waterâwater interactions in [DMEtA]Â[NTf<sub>2</sub>] were found to be similar to those of dialkylimidazolium
AILs, whereas these interactions were much larger in the PIL <i>N,N</i>-dimethylethanolammonium propionate ([DMEtA]Â[Pr]), attributed
to the change in anionâwater interactions
Ruthenium-Mediated CâH Functionalization of Pyridine: The Role of Vinylidene and Pyridylidene Ligands
A combined experimental and theoretical study has demonstrated
that [RuÂ(η<sup>5</sup>-C<sub>5</sub>H<sub>5</sub>)Â(py)<sub>2</sub>(PPh<sub>3</sub>)]<sup>+</sup> is a key intermediate, and active
catalyst for, the formation of 2-substituted <i>E</i>-styrylpyridines
from pyridine and terminal alkynes HCîŒCR (R = Ph, C<sub>6</sub>H<sub>4</sub>-4-CF<sub>3</sub>) in a 100% atom efficient manner under
mild conditions. A catalyst deactivation pathway involving formation
of the pyridylidene-containing complex [RuÂ(η<sup>5</sup>-C<sub>5</sub>H<sub>5</sub>)Â(Îș<sup>3</sup>-<i>C</i><sub>3</sub>-C<sub>5</sub>H<sub>4</sub>NCHî»CHR)Â(PPh<sub>3</sub>)]<sup>+</sup> and subsequently a 1-ruthanaindolizine complex has
been identified. Mechanistic studies using <sup>13</sup>C- and D-labeling
and DFT calculations suggest that a vinylidene-containing intermediate
[RuÂ(η<sup>5</sup>-C<sub>5</sub>H<sub>5</sub>)Â(py)Â(î»Cî»CHR)Â(PPh<sub>3</sub>)]<sup>+</sup> is formed, which can then proceed to the pyridylidene-containing
deactivation product or the desired product depending on the reaction
conditions. Nucleophilic attack by free pyridine at the α-carbon
in this complex subsequently leads to formation of a CâH agostic
complex that is the branching point for the productive and unproductive
pathways. The formation of the desired products relies on CâH
bond cleavage from this agostic complex in the presence of free pyridine
to give the pyridyl complex [RuÂ(η<sup>5</sup>-C<sub>5</sub>H<sub>5</sub>)Â(C<sub>5</sub>H<sub>4</sub>N)Â(î»Cî»CHR)Â(PPh<sub>3</sub>)]. Migration of the pyridyl ligand (or its pyridylidene tautomer)
to the α-carbon of the vinylidene, followed by protonation,
results in the formation of the 2-styrylpyridine. These studies demonstrate
that pyridylidene ligands play an important role in both the productive
and nonproductive pathways in this catalyst system
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>â</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>â</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)îžhighly
reactive polyisobutylene (HR-PIB)îžin 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>â</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 1000â2000 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>â</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>â</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>â</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>â</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)îžhighly
reactive polyisobutylene (HR-PIB)îžin 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>â</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 1000â2000 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>â</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>â</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
Reactive-Atom Scattering from Liquid Crystals at the LiquidâVacuum Interface: [C<sub>12</sub>mim][BF<sub>4</sub>] and 4âCyano-4âČ-Octylbiphenyl (8CB)
Two complementary
approaches were used to study the liquidâvacuum
interface of the liquid-crystalline ionic liquid 1-dodecyl-3-methylÂimidazolium
tetrafluoroborate ([C<sub>12</sub>mim]Â[BF<sub>4</sub>]) in the smectic
A (SmA) and isotropic phases. O atoms with two distinct incident translational
energies were scattered from the surface of [C<sub>12</sub>mim]Â[BF<sub>4</sub>]. Angle-dependent time-of-flight distributions and OH yields,
respectively, were recorded from high- and low-energy O atoms. There
were no significant changes in the measurements using either approach,
nor the properties derived from them, accompanying the transition
from the SmA to the isotropic phase. This indicates that the surface
structure of [C<sub>12</sub>mim]Â[BF<sub>4</sub>] remains essentially
unchanged across the phase boundary, implying that the bulk order
and surface structure are not strongly correlated for this material.
This effect is ascribed to the strong propensity for the outer surfaces
of ionic liquids to be dominated by alkyl chains, over an underlying
layer rich in anions and cation head groups, whether or not the bulk
material is a liquid crystal. In a comparative study, the OH yield
from the surface of the liquid crystal, 8CB, was found to be affected
by the bulk order, showing a surprising step increase at the SmAânematic
transition temperature, whose origin is the subject of speculation