19 research outputs found
<i>para</i>-Selective Alkylation of Sulfonylarenes by Cooperative Nickel/Aluminum Catalysis
A method
for the <i>para</i>-selective alkylation of a variety of
arenesulfonamides
and aromatic sulfones with 1-alkenes by cooperative nickel/aluminum
catalysis has been developed. Taking advantage of the sulfornyl functionality
serving as a removable <i>ortho</i>-directing group, the
reaction can be applied to facile access to 1,3-dialkyl-substitued
benzenes
Arylboration of Alkenes by Cooperative Palladium/Copper Catalysis
Arylboration of vinylarenes and methyl
crotonate with aryl halides
and bis(pinacolato)diboron by cooperative Pd/Cu catalysis has been
developed, giving 2-boryl-1,1-diarylethanes and an α-aryl-β-boryl
ester in a regioselective manner. The reaction is compatible with
a variety of functionalities and amenable to be scaled-up to a gram
scale with no detriment to the yield. A short synthesis of the biologically
active compound CDP840 was performed using the present reaction as
a key step
Reductive Denitration of Nitroarenes
The Pd-catalyzed
reductive denitration of nitroarenes has been
achieved via a direct cleavage of the C–NO<sub>2</sub> bonds.
The catalytic conditions reported exhibit a broad substrate scope
and good functional-group compatibility. Notably, the use of inexpensive
propan-2-ol as a mild reductant suppresses the competitive formation
of anilines, which are normally formed by other conventional reductions.
Mechanistic studies have revealed that alcohols serve as efficient
hydride donors in this reaction, possibly through β-hydride
elimination from palladium alkoxides
Aromatic C–H σ‑Bond Activation by Ni<sup>0</sup>, Pd<sup>0</sup>, and Pt<sup>0</sup> Alkene Complexes: Concerted Oxidative Addition to Metal vs Ligand-to-Ligand H Transfer Mechanism
C–H σ-bond
activation of arene (represented here by
benzene) by the Ni<sup>0</sup> propene complex Ni<sup>0</sup>(IMes)(C<sub>3</sub>H<sub>6</sub>) (IMes = 1,3-dimesitylimidazol-2-ylidene), which
is an important elementary step in Ni-catalyzed hydroarylation of
unactivated alkene with arene, was investigated by DFT calculations.
In the Ni<sup>0</sup> complex, the C–H activation occurs through
a ligand-to-ligand H transfer mechanism to yield Ni<sup>II</sup>(IMes)(C<sub>3</sub>H<sub>7</sub>)(Ph) (C<sub>3</sub>H<sub>7</sub> = propyl; Ph
= phenyl). In Pd<sup>0</sup> and Pt<sup>0</sup> analogues, the activation
occurs through concerted oxidative addition of the C–H bond
to the metal. Analysis of the electron redistribution during the C–H
activation highlights the difference between the two mechanisms. In
the ligand-to-ligand H transfer, charge transfer (CT) occurs from
the metal to the benzene. However, the atomic population of the transferring
H remains almost constant, suggesting that different CT simultaneously
occurs from the transferring H to the LUMO of propene. The electron
redistribution contrasts significantly with that found for Pd<sup>0</sup> and Pt<sup>0</sup>, in which CT occurs only from the metal
to the benzene. Preference for ligand-to-ligand H transfer over concerted
oxidative addition in the Ni<sup>0</sup> complex is shown to be due
to the smaller atomic radius of Ni in comparison to those of Pd and
Pt and the smaller Ni<sup>II</sup>–H bond energy relative to
the Pd<sup>II</sup>–H and Pt<sup>II</sup>–H energies.
Interestingly, the bulky ligand accelerates the ligand-to-ligand H
transfer in the Ni<sup>0</sup> complex by decreasing the distance
between the coordinated benzene and alkene substrates. Thus, the Gibbs
activation energy (Δ<i>G</i>°<sup>⧧</sup>) decreases in the case of cyclic-alkylaminocarbene with bulky substituents
(CACC-K3), while the Δ<i>G</i>°<sup>⧧</sup> values are similar in X-Phos, IMes, and nonsubstituted cyclic alkylaminocarbene
(CAAC-K0). An electron-withdrawing substituent on the arene accelerates
the C–H activation by favoring the metal to arene CT
How to Control Inversion vs Retention Transmetalation between Pd<sup>II</sup>–Phenyl and Cu<sup>I</sup>–Alkyl Complexes: Theoretical Insight
Transmetalation between Pd(Br)(Ph<sup>A</sup>)(PCyp<sub>3</sub>)<sub>2</sub> (Ph = phenyl, Cyp = cyclopentyl)
and Cu(C<sup>a</sup>HMePh<sup>B</sup>)(NHC) (NHC = 1,3-bis(2,6-diisopropylphenyl)-imidazolidin-2-ylidene)
is an important elementary step in recently reported catalytic cross-coupling
reaction by Pd/Cu cooperative system. DFT study discloses that the
transmetalation occurs with inversion of the stereochemistry of the
C<sup>a</sup>HMePh<sup>B</sup> group. In its transition state, the
C<sup>a</sup>HMePh<sup>B</sup> group has almost planar structure around
the C<sup>a</sup> atom. That planar geometry is stabilized by conjugation
between the π* orbital of the Ph<sup>B</sup> and the 2p orbital
of the C<sup>a</sup>. Another important factor is activation entropy
(Δ<i>S</i>°<sup>‡</sup>); retention transmetalation
occurs through Br-bridging transition state, which is less flexible
than that of the inversion transmetalation because of the Br-bridging
structure, leading to a smaller activation entropy in the retention
transition state than in the inversion transition state. For C<sup>a</sup>HMeEt group, transmetalation occurs in a retention manner.
In the planar C<sup>a</sup>HMeEt group of the inversion transition
state, the C<sup>a</sup> 2p orbital cannot find a conjugation partner
because of the absence of π-electron system in the C<sup>a</sup>HMeEt. Transmetalation of C<sup>a</sup>HMe(CHCH<sub>2</sub>) occurs in a retention manner because the vinyl π* is less
effective for the conjugation with the C<sup>a</sup> 2p because of
its higher orbital energy than the Ph π*. The introduction of
electron-withdrawing substituent on the Ph<sup>B</sup> is favorable
for inversion transmetalation. These results suggest that the stereochemistry
of the C<sup>a</sup> atom in transmetalation can be controlled by
electronic effect of the C<sup>a</sup>HMeR (R = phenyl, vinyl, or
alkyl) and sizes of the substituent and ligand
How to Control Inversion vs Retention Transmetalation between Pd<sup>II</sup>–Phenyl and Cu<sup>I</sup>–Alkyl Complexes: Theoretical Insight
Transmetalation between Pd(Br)(Ph<sup>A</sup>)(PCyp<sub>3</sub>)<sub>2</sub> (Ph = phenyl, Cyp = cyclopentyl)
and Cu(C<sup>a</sup>HMePh<sup>B</sup>)(NHC) (NHC = 1,3-bis(2,6-diisopropylphenyl)-imidazolidin-2-ylidene)
is an important elementary step in recently reported catalytic cross-coupling
reaction by Pd/Cu cooperative system. DFT study discloses that the
transmetalation occurs with inversion of the stereochemistry of the
C<sup>a</sup>HMePh<sup>B</sup> group. In its transition state, the
C<sup>a</sup>HMePh<sup>B</sup> group has almost planar structure around
the C<sup>a</sup> atom. That planar geometry is stabilized by conjugation
between the π* orbital of the Ph<sup>B</sup> and the 2p orbital
of the C<sup>a</sup>. Another important factor is activation entropy
(Δ<i>S</i>°<sup>‡</sup>); retention transmetalation
occurs through Br-bridging transition state, which is less flexible
than that of the inversion transmetalation because of the Br-bridging
structure, leading to a smaller activation entropy in the retention
transition state than in the inversion transition state. For C<sup>a</sup>HMeEt group, transmetalation occurs in a retention manner.
In the planar C<sup>a</sup>HMeEt group of the inversion transition
state, the C<sup>a</sup> 2p orbital cannot find a conjugation partner
because of the absence of π-electron system in the C<sup>a</sup>HMeEt. Transmetalation of C<sup>a</sup>HMe(CHCH<sub>2</sub>) occurs in a retention manner because the vinyl π* is less
effective for the conjugation with the C<sup>a</sup> 2p because of
its higher orbital energy than the Ph π*. The introduction of
electron-withdrawing substituent on the Ph<sup>B</sup> is favorable
for inversion transmetalation. These results suggest that the stereochemistry
of the C<sup>a</sup> atom in transmetalation can be controlled by
electronic effect of the C<sup>a</sup>HMeR (R = phenyl, vinyl, or
alkyl) and sizes of the substituent and ligand
Intramolecular Aminocyanation of Alkenes by Cooperative Palladium/Boron Catalysis
A cooperative palladium/triorganoboron
catalyst to accomplish the
intramolecular aminocyanation of alkenes through the cleavage of N–CN
bonds is reported. 4,5-Bis(diphenylphosphino)-9,9-dimethylxanthene
(Xantphos) is found to be crucial as a ligand for palladium to effectively
catalyze the transformation with high chemo- and regioselectivity.
A range of substituted indolines and pyrrolidines with both tetra-
or trisubstituted carbon and cyano functionalities are readily furnished
by the newly developed cyanofunctionalization reaction. A preliminary
example of enantioselective aminocyanation is also described
Contemporary Literature of the Fantastic in China Tom Mieville's Projection
У статті наведено погляди Чайни Тома М’євіля щодо завдання фантастичної літератури з огляду на аналіз письменником різновекторних розгалужень антастики: «високого» фентезі зразка Дж. Р. Р. Толкіна та «бульварної» фантастики (pulp fiction) зразка Г. Ф. Лавкрафта. Зроблено висновок про творче кредо письменника, відповідно до якого фантастична література покликана представляти реалістичну проблематику, виражену в химерних формах.The article presents an overview of contemporary trends in Anglophone fantastic literature explored through China Miéville’s ideas on fantasy and weird fiction. It aims to show the driving force behind the generic transformations of the fantastic which are rooted in the objective need for the genre to evolve and individual creative needs of the writers who practice it. The objective reason is identified to be the repetitiveness of the fantastic canon which renders it ill-suited for accommodating visions, anxieties, and
desires of the modern world. The individual reasons lie in the inability of the classical fantastic canon to express the author’s intentions for their writing. Maintaining dialogue between the fantasy and weird fiction tradition, China Mieville’s writing attempts to marry the realism of concern with the weird of expression in the secondary-world setting. The major points of fantasy criticism are overcome in the writer’s texts by abstaining from moralization, demythologizing heroism and allowing the plot to follow its own dynamics
instead of the prescribed set of genre protocols which allow for a more realistic narrative. The deployment of weird fiction strategies for genre blending frees the literature of the fantastic from the genre constraints and adds to the atmosphere of estrangement essential to rethinking social and aesthetic conventions. Thus, China Mieville’s writing
reflects the revitalization of the literature of the fantastic and facilitates in its making
Aromatic C–H σ‑Bond Activation by Ni<sup>0</sup>, Pd<sup>0</sup>, and Pt<sup>0</sup> Alkene Complexes: Concerted Oxidative Addition to Metal vs Ligand-to-Ligand H Transfer Mechanism
C–H σ-bond
activation of arene (represented here by
benzene) by the Ni<sup>0</sup> propene complex Ni<sup>0</sup>(IMes)(C<sub>3</sub>H<sub>6</sub>) (IMes = 1,3-dimesitylimidazol-2-ylidene), which
is an important elementary step in Ni-catalyzed hydroarylation of
unactivated alkene with arene, was investigated by DFT calculations.
In the Ni<sup>0</sup> complex, the C–H activation occurs through
a ligand-to-ligand H transfer mechanism to yield Ni<sup>II</sup>(IMes)(C<sub>3</sub>H<sub>7</sub>)(Ph) (C<sub>3</sub>H<sub>7</sub> = propyl; Ph
= phenyl). In Pd<sup>0</sup> and Pt<sup>0</sup> analogues, the activation
occurs through concerted oxidative addition of the C–H bond
to the metal. Analysis of the electron redistribution during the C–H
activation highlights the difference between the two mechanisms. In
the ligand-to-ligand H transfer, charge transfer (CT) occurs from
the metal to the benzene. However, the atomic population of the transferring
H remains almost constant, suggesting that different CT simultaneously
occurs from the transferring H to the LUMO of propene. The electron
redistribution contrasts significantly with that found for Pd<sup>0</sup> and Pt<sup>0</sup>, in which CT occurs only from the metal
to the benzene. Preference for ligand-to-ligand H transfer over concerted
oxidative addition in the Ni<sup>0</sup> complex is shown to be due
to the smaller atomic radius of Ni in comparison to those of Pd and
Pt and the smaller Ni<sup>II</sup>–H bond energy relative to
the Pd<sup>II</sup>–H and Pt<sup>II</sup>–H energies.
Interestingly, the bulky ligand accelerates the ligand-to-ligand H
transfer in the Ni<sup>0</sup> complex by decreasing the distance
between the coordinated benzene and alkene substrates. Thus, the Gibbs
activation energy (Δ<i>G</i>°<sup>⧧</sup>) decreases in the case of cyclic-alkylaminocarbene with bulky substituents
(CACC-K3), while the Δ<i>G</i>°<sup>⧧</sup> values are similar in X-Phos, IMes, and nonsubstituted cyclic alkylaminocarbene
(CAAC-K0). An electron-withdrawing substituent on the arene accelerates
the C–H activation by favoring the metal to arene CT
Theoretical Study of Nickel-Catalyzed Selective Alkenylation of Pyridine: Reaction Mechanism and Crucial Roles of Lewis Acid and Ligands in Determining the Selectivity
Selective
alkenylation of pyridine is challenging in synthetic
organic chemistry due to the poor reactivity and regioselectivity
of the aromatic ring. We theoretically investigated Ni-catalyzed selective
alkenylation of pyridine with DFT. The first step is coordination
of the pyridine–AlMe<sub>3</sub> adduct with the active species
Ni<sup>(0)</sup>(NHC)(C<sub>2</sub>H<sub>2</sub>) <b>1</b> in
an η<sup>2</sup>-fashion to form an intermediate <b>Int1</b>. After the isomerization of <b>Int1</b>, the oxidative addition
of the C–H bond of pyridine across the nickel–acetylene
moiety occurs via a transition state <b>TS2</b> to form a Ni<sup>(II)</sup>(NHC) pyridyl vinyl intermediate <b>Int3</b>. This
oxidative addition is rate-determining. The next step is C–C
bond formation between pyridyl and vinyl groups leading to the formation
of vinyl-pyridine (<b>P1</b>). One of the points at issue in
this type of functionalization is how to control the regioselectivity.
With the use of Ni(NHC)/AlMe<sub>3</sub> catalyst, the C<sup>4</sup>- and C<sup>3</sup>-alkenylated products (Δ<i>G</i>°<sup>⧧</sup> = 17.4 and 21.5 kcal mol<sup>–1</sup>, respectively) are formed preferably to the C<sup>2</sup> one (Δ<i>G</i>°<sup>⧧</sup> = 22.0 kcal mol<sup>–1</sup>). The higher selectivity of the C<sup>4</sup>-alkenylation over
the C<sup>3</sup> and the C<sup>2</sup> ones is attributed to the
small steric repulsion between NHC and AlMe<sub>3</sub> in the C<sup>4</sup>-alkenylation. Interestingly, with Ni(P(<i>i</i>-Pr)<sub>3</sub>)/AlMe<sub>3</sub> catalyst, the C<sup>2</sup>-alkenylation
occurs more easily than the C<sup>3</sup> and C<sup>4</sup> ones.
This regioselectivity arises
from the smaller steric repulsion induced by P(<i>i</i>-Pr)<sub>3</sub> than by bulky NHC. It is notable that AlMe<sub>3</sub> accelerates
the alkenylation by inducing the strong CT from Ni to pyridine–AlMe<sub>3</sub>. In the absence of AlMe<sub>3</sub>, pyridine strongly coordinates
with the Ni atom through the N atom, which increases Gibbs activation
energy (Δ<i>G</i>°<sup>⧧</sup> = ∼27
kcal mol<sup>–1</sup>) of the C–H bond activation. In
other words, AlMe<sub>3</sub> plays two important roles, acceleration
of the reaction and enhancement of the regioselectivity for the C<sup>4</sup>-alkenylation