6 research outputs found
Executing and Rationalizing the Synthesis of a Difluorinated Analogue of a Ring-Expanded Calystegine B<sub>2</sub>
A difluorinated analogue of a ring-expanded calystegine
B<sub>2</sub> and some <i>N</i>-protected species were prepared
via
microwave-mediated transannular ring-opening of an epoxyketone. The
diastereofacial selectivity of the epoxidation reaction, which delivers
the key intermediate, and the regioselectivity of the transannular
reactions were analyzed by density functional theory (DFT) methods.
The epoxidation stereoselectivity arises from simple steric control,
whereas the ring-closure reactions are subject to thermodynamic control
Executing and Rationalizing the Synthesis of a Difluorinated Analogue of a Ring-Expanded Calystegine B<sub>2</sub>
A difluorinated analogue of a ring-expanded calystegine
B<sub>2</sub> and some <i>N</i>-protected species were prepared
via
microwave-mediated transannular ring-opening of an epoxyketone. The
diastereofacial selectivity of the epoxidation reaction, which delivers
the key intermediate, and the regioselectivity of the transannular
reactions were analyzed by density functional theory (DFT) methods.
The epoxidation stereoselectivity arises from simple steric control,
whereas the ring-closure reactions are subject to thermodynamic control
Toward a Simulation Approach for Alkene Ring-closing Metathesis: Scope and Limitations of a Model for RCM
A published model for revealing solvent effects on the ring-closing metathesis (RCM) reaction of diethyl diallylmalonate <b>7</b> has been evaluated over a wider range of conditions, to assess its suitability for new applications. Unfortunately, the model is too flexible and the published rate constants do not agree with experimental studies in the literature. However, by fixing the values of important rate constants and restricting the concentration ranges studied, useful conclusions can be drawn about the relative rates of RCM of different substrates, precatalyst concentration can be simulated accurately and the effect of precatalyst loading can be anticipated. Progress has also been made toward applying the model to precatalyst evaluation, but further modifications to the model are necessary to achieve much broader aims
SuzukiâMiyaura Coupling Reactions of Iodo(difluoroenol) Derivatives, Fluorinated Building Blocks Accessible at Near-Ambient Temperatures
A recently developed method for the near-ambient generation
of
difluorovinylzinc reagents has facilitated the preparation of 1-(<i>N</i>,<i>N</i>-diethylcarbamoyloxy)-2,2-difluoro-1-iodoethene
and 2,2-difluoro-1-iodo-1-(2â˛-methoxyethoxymethoxy)Âethene.
The utility of these reagents has been investigated in SuzukiâMiyaura
couplings with a range of potassium trifluoroborate coupling partners,
with the scope of successful couplings proving wide. Deiodinated species
appeared as significant side products, but a solvent change from <i>i</i>-PrOH to <i>t</i>-BuOH suppressed the pathway
to these species and improved coupling yields
Olefin Metathesis by GrubbsâHoveyda Complexes: Computational and Experimental Studies of the Mechanism and Substrate-Dependent Kinetics
The potential energy surfaces for
the activation of GrubbsâHoveyda-type
precatalysts with the substrates ethene, propene, 1-hexene, and ethyl
vinyl ether (EVE) have been probed at the density functional theory
(DFT) (M06-L) level. The energetically favored pathway of the reaction
leading to a 14e Fischer carbene and styrene starts with an initiation
step in which the incoming substrate and outgoing alkene ligand are
both clearly associated with the ruthenium center. For these substrates,
with the exception of ethene, the rate determining step is predicted
to be the formation of the metallocyclobutane (MCB). We have taken
the initial reactant to be a weak van der Waals complex between substrate
and precatalyst. This model yields good agreement between the computed
activation parameters for both the parent GrubbsâHoveyda and
Grela complex with EVE substrate, and the experimental values, reported
here. The alternative model which takes the initial reactant to be
two isolated molecules requires an estimate of the entropy loss on
formation of the initial complex in solution which is difficult to
evaluate. Our estimate of this quantity yields a barrier for the rate
determining step for the interchange mechanism which is close to the
value we find for the alternative mechanism in which the rate determining
step is the initial dissociation of the precatalyst. The relative
energetics of these two mechanisms involving different initiation
steps but with similar activation barriers, could well be dependent
upon the precatalyst and substrate in line with the recent experimental
findings of Plenio and co-workers
Synergic Effects Between NâHeterocyclic Carbene and Chelating BenzylideneâEther Ligands Toward the Initiation Step of HoveydaâGrubbs Type Ru Complexes
Synergic effects between ancillary N-heterocyclic carbenes
[(1,3-bisÂ(2,4,6-trimethylphenyl)-1,3-imidazoline-2-ylidene
or 1,3-bisÂ(2,6-di<i>iso</i>propylphenyl)-1,3-imidazoline-2-ylidene]
and chelating benzylideneâether ligands were investigated by
studying initiation rates and kinetic profiles of HoveydaâGrubbs
(HG) type Ru complexes. A newly designed Ru-benzylidene-oxazinone
precatalyst <b>4</b> was compared with Grela and Blechert complexes
bearing modified <i>iso</i>propyloxy chelating leaving groups
and with the standard HG complex to understand how the ancillary and
the leaving ligands interact and influence the catalytic activity