10 research outputs found
Intramolecular hydrogen bond activation: Thiourea-organocatalyzed enantioselective 1,3-dipolar cycloaddition of salicylaldehyde-derived azomethine ylides with nitroalkenes
An organocatalytic strategy for the synthesis of tetrasubstituted pyrrolidines with monoactivated azomethine ylides in high enantiomeric excess and excellent exo/endo selectivity is presented. The key to success is the intramolecular activation via hydrogen bonding through an o-hydroxy group, which allows the dipolar cycloaddition to take place in the presence of azomethine ylides bearing only one activating group. The intramolecular hydrogen bond in the azomethine ylide and the intermolecular hydrogen bond with the catalyst have been demonstrated by DFT calculations and mechanistic proofs to be crucial for the reaction to proceedThe Spanish Government (CTQ2015-64561-R, CTQ2016-
76061-P) and the European Research Council (ERC-CG,
contract number 647550) are acknowledged. We acknowledge
the generous allocation of computing time at the CCC (UAM).
S.D.-T. gratefully acknowledges the “Ramón y Cajal” program
(RYC-2010-07019). Financial support from the Spanish
Ministry of Economy and Competitiveness, through the
“Maria de Maeztu” Program of Excellence in R&D (MDM-
2014-0377
Copper(I)-Catalyzed Formal Carboboration of Alkynes: Synthesis of Tri- and Tetrasubstituted Vinylboronates
The first copper-catalyzed formal carboboration of alkynes,
in
which a C–B bond and a C–C bond are created in a single
catalytic cycle, is presented. The reaction proceeds with high regioselectivity
and <i>syn</i>-stereoselectivity to form tri- and tetrasubstituted
vinylboronic esters from commercially available bis(pinacolato)diboron.
A subsequent cross-coupling reaction gives access to highly substituted
alkenes
Asymmetric Synthesis of 1,2-Diamines bearing Tetrasubstituted Centers from Nonstabilized Azomethine Ylides and <i>N</i>‑Sulfinylketimines under Brønsted Acid Catalysis
The first asymmetric cycloaddition
of nonstabilized azomethine
ylide and <i>N</i>-sulfinylimines is presented. In reactions
with aryl–alkyl and heteroaryl–alkyl ketimines, excellent
diastereoselectivities and good yields are obtained in all cases,
regardless of the electronic character of the substituents at the
aromatic rings. Moreover, the cycloadducts obtained can easily be
deprotected in acid media, giving access to free 1,2-diamines which
are prevalent in many natural and pharmaceutical products
Asymmetric Intramolecular Pauson–Khand Reaction Mediated by a Remote Sulfenyl or Sulfinyl Group
In this work, we report the use of the asymmetric intramolecular
Pauson–Khand reactions of 4-aryl-4-cyano-1,6-enynes for obtaining
enantiomerically enriched bicyclo[3.3.0]octenones, and the influence
of both the quaternary stereocenter and the sulfur functions located
at <i>ortho</i>-position of the aryl group, on their stereoselectivity
and reactivity. The sulfenyl derivatives bearing substituted or unsubstituted
triple bonds and mono- and disubstituted alkene moieties afford bicyclo[3.3.0]octenones
in high yields with complete diastereocontrol. These results are explained
by assuming the association of the lone electron pair at sulfur to
the Co–alkyne complexes
Asymmetric Synthesis of Rauhut–Currier type Products by a Regioselective Mukaiyama Reaction under Bifunctional Catalysis
The reactivity and the regioselective
functionalization of silyl–diene
enol ethers under a bifunctional organocatalyst provokes a dramatic
change in the regioselectivity, from the 1,5- to the 1,3-functionalization.
This variation makes possible the 1,3-addition of silyl–dienol
ethers to nitroalkenes, giving access to the synthesis of tri- and
tetrasubstituted double bonds in Rauhut–Currier type products.
The process takes place under smooth conditions, nonanionic conditions,
and with a high enantiomeric excess. A rational mechanistic pathway
is presented based on DFT and mechanistic experiments
Asymmetric Synthesis of Secondary and Tertiary Propargylic Alcohols by Umpolung of Acetylenic Sulfones and <i>ortho</i>-Sulfinyl Carbanions
The
generation of diastereomerically enriched secondary benzyl
propargyl alcohols by the asymmetric addition of <i>ortho</i>-sulfinylbenzyl carbanions to sulfonylacetylene derivatives via formation
of a Csp–Csp<sup>3</sup> bond is described. This reaction proceeds
through an unusual α-attack (<i>anti</i>-Michael addition)
of the <i>ortho</i>-sulfinylbenzyl carbanions, followed
by elimination of the arylsulfonyl moiety. The scope of this alkynylation
reaction is also discussed. Moreover, the development of a new approach
for the synthesis of optically active tertiary benzylpropargyl alcohols
is described, discussing the possible stereocourse of the reaction
so as the influence of the ether 18-crown-6 and steric importance
of acetylenic substituent
Asymmetric Synthesis of Secondary and Tertiary Propargylic Alcohols by Umpolung of Acetylenic Sulfones and <i>ortho</i>-Sulfinyl Carbanions
The
generation of diastereomerically enriched secondary benzyl
propargyl alcohols by the asymmetric addition of <i>ortho</i>-sulfinylbenzyl carbanions to sulfonylacetylene derivatives via formation
of a Csp–Csp<sup>3</sup> bond is described. This reaction proceeds
through an unusual α-attack (<i>anti</i>-Michael addition)
of the <i>ortho</i>-sulfinylbenzyl carbanions, followed
by elimination of the arylsulfonyl moiety. The scope of this alkynylation
reaction is also discussed. Moreover, the development of a new approach
for the synthesis of optically active tertiary benzylpropargyl alcohols
is described, discussing the possible stereocourse of the reaction
so as the influence of the ether 18-crown-6 and steric importance
of acetylenic substituent
Control of the Dual Reactivity (Iminium-Dienamine) of β‑Arylmethyl α,β-Unsaturated Aldehydes in Organocatalytic 1,3-Dipolar Cycloadditions with <i>N</i>‑Benzoyl <i>C,N</i>-Cyclic Azomethine Imines
1,3-Dipolar cycloadditions
of <i>C,N</i>-cyclic azomethine
imines with α,β-unsaturated aldehydes can be performed
with complete control of the regio-, <i>exo</i>-, and enantioselectivity
under aminocatalytic conditions. The so far never studied competence
of the iminium-dienamine reactivity inherent to β-alkyl α,β-unsaturated
aldehydes was studied, which was possible by allowing achievement
of complete control of the chemoselectivity in reactions of the β-arylmethyl
derivatives with azomethine imines by using different additives and
organocatalysts, whose role has been rationalized by DFT calculations
and chemical proofs. Thus, it has been possible to selectively obtain
the pyrazolidines resulting from both the attack to the C2–C3
(via iminium) and the C3–C4 (via dienamine) bonds at the starting
enals, which can be used as precursors of interesting tetrahydroisoquinolinic
compounds
A Bifunctional Photoaminocatalyst for the Alkylation of Aldehydes: Design, Analysis, and Mechanistic Studies
A bifunctional
photoaminocatalyst based on imidazolidinone and
thioxanthone is presented. The preparation of these catalysts proceeds
in a two-step synthesis that allows an easy tuning of the steric properties.
The photophysical and electrochemical data of the imidazolidinone
photocatalysts have been determined, indicating that the catalysts
can work under visible light conditions. To corroborate the experimental
observations, ground state geometry optimization and energy transition
studies of thioxanthone and bifunctional catalyst <b>4c</b> were
optimized by time-dependent density functional theory (TD DFT) calculations.
The alkylation of aldehydes with this photoaminocatalyst works with
high enantioselectivities and yields due to the stereoelectronic properties
of the catalyst. A rational mechanistic cycle based on different mechanistic
experiments, TD DFT calculations, and laser flash photolysis is presented
A Bifunctional Photoaminocatalyst for the Alkylation of Aldehydes: Design, Analysis, and Mechanistic Studies
A bifunctional
photoaminocatalyst based on imidazolidinone and
thioxanthone is presented. The preparation of these catalysts proceeds
in a two-step synthesis that allows an easy tuning of the steric properties.
The photophysical and electrochemical data of the imidazolidinone
photocatalysts have been determined, indicating that the catalysts
can work under visible light conditions. To corroborate the experimental
observations, ground state geometry optimization and energy transition
studies of thioxanthone and bifunctional catalyst <b>4c</b> were
optimized by time-dependent density functional theory (TD DFT) calculations.
The alkylation of aldehydes with this photoaminocatalyst works with
high enantioselectivities and yields due to the stereoelectronic properties
of the catalyst. A rational mechanistic cycle based on different mechanistic
experiments, TD DFT calculations, and laser flash photolysis is presented