7 research outputs found
High π‑Facial and <i>exo</i>-Selectivity for the Intramolecular Diels–Alder Cycloaddition of Dodeca-3,9,11-trien-5-one Precursors to 2-<i>epi</i>-Symbioimine and Related Compounds
An
unconstrained exocyclic stereogenic center and a removable trimethylsilyl
group are combined to induce high π-facial selectivity and near-exclusive <i>exo</i>-selectivity in the intramolecular Diels–Alder
cycloaddition of dodeca-3,9,11-trien-5-ones. This strategy provides
direct access to polysubstituted <i>trans</i>-1-decalones
related to the symbioimines in good yield and acceptable diastereoselectivity
Decarboxylative Generation of 2‑Azaallyl Anions: 2‑Iminoalcohols via a Decarboxylative Erlenmeyer Reaction
Condensation
between the tetrabutylammonium salt of 2,2-diphenylglycine
and aldehydes results in a decarboxylative Erlenmeyer reaction, affording
1,2-diaryl-2-iminoalcohols as a mixture of diastereomers in good yields.
The diastereomeric ratio shifts over time, with the <i>anti</i> diastereomer and the <i>syn</i> oxazolidine tautomer serving
as the kinetic and thermodynamic products, respectively. Addition
of Lewis acids can catalyze the rates of reaction and product equilibration.
The results highlight the stereochemical promiscuity of 1,2-diaryl-2-iminoalcohols
in the presence of Lewis acids and Brønsted bases
Exploring the Steric and Electronic Factors Governing the Regio- and Enantioselectivity of the Pd-Catalyzed Decarboxylative Generation and Allylation of 2‑Azaallyl Anions
The impact of the steric and electronic
factors in both the <i>para</i>-substituted benzaldimine
and 2,2-diarylglycine components
on the regioselectivity and enantioselectivity of the palladium-catalyzed
decarboxylative allylation of allyl 2,2-diarylglycinate aryl imines
was explored. These studies revealed that using 2,2-diÂ(2-methoxyphenyl)Âglycine
as the amino acid linchpin allowed for the exclusive synthesis of
the desired homoallylic benzophenone imine regioisomers, independent
of the nature of the imine moiety, in typically high yields. The resulting
enantiomeric ratios, however, are slightly decreased in comparison
to the transformations involving the corresponding allyl 2,2-diphenylglycinate
imines, but this is more than balanced out by the increases in yield
and regioselectivity. Overall, these studies suggest a general strategy
for the highly regioselective functionalization of 2-azaallyl anions
Palladium-Catalyzed Decarboxylative Generation and Asymmetric Allylation of α‑Imino Anions
A palladium-catalyzed
asymmetric decarboxylative allylic alkylation
of allyl 2,2-diphenylglycinate imines using (<i>S,S</i>)-<i>f</i>-binaphane as a chiral supporting ligand has been developed.
This transformation allows for decarboxylative generation and enantioselective
allylation of nonenolate α-imino (2-azaallyl anions) to afford
α-aryl homoallylic imines
Enantiodifferentiation in the Photoisomerization of (<i>Z</i>,<i>Z</i>)‑1,3-Cyclooctadiene in the Cavity of γ‑Cyclodextrin–Curcubit[6]uril-Wheeled [4]Rotaxanes with an Encapsulated Photosensitizer
A biphenyl photosensitizer axle was
implanted into the cavities
of native and capped Îł-cyclodextrins through rotaxanation using
a cucubit[6]Âuril-templated azide–alkyne 1,3-dipolar cycloaddition,
resulting in the construction of highly defined chiral binding/sensitizing
sites. The orientation and interaction of the axle and capping moieties
at the ground and excited states were interrogated by NMR, UV–vis,
circular dichroism, and fluorescence spectroscopic studies. In situ
photoisomerization of (<i>Z</i>,<i>Z</i>)-1,3-cyclooctadiene
sensitized in the cavity of these [4]Ârotaxanes afforded (<i>Z</i>,<i>E</i>)-1,3-cyclooctadiene in up to 15.3% ee, which
represents the highest level of enantiodifferentiation obtained to
date for this supramolecular photochirogenesis
Transition-Metal-Free Radical C(sp<sup>3</sup>)–C(sp<sup>2</sup>) and C(sp<sup>3</sup>)–C(sp<sup>3</sup>) Coupling Enabled by 2‑Azaallyls as Super-Electron-Donors and Coupling-Partners
The
past decade has witnessed the rapid development of radical
generation strategies and their applications in C–C bond-forming
reactions. Most of these processes require initiators, transition
metal catalysts, or organometallic reagents. Herein, we report the
discovery of a simple organic system (2-azaallyl anions) that enables
radical coupling reactions under transition-metal-free conditions.
Deprotonation of <i>N</i>-benzyl ketimines generates semistabilized
2-azaallyl anions that behave as “super-electron-donors”
(SEDs) and reduce aryl iodides and alkyl halides to aryl and alkyl
radicals. The SET process converts the 2-azaallyl anions into persistent
2-azaallyl radicals, which capture the aryl and alkyl radicals to
form C–C bonds. The radical coupling of aryl and alkyl radicals
with 2-azaallyl radicals makes possible the synthesis of functionalized
amine derivatives without the use of exogenous radical initiators
or transition metal catalysts. Radical clock studies and 2-azaallyl
anion coupling studies provide mechanistic insight for this unique
reactivity
Supramolecular Photochirogenesis Driven by Higher-Order Complexation: Enantiodifferentiating Photocyclodimerization of 2‑Anthracenecarboxylate to Slipped Cyclodimers via a 2:2 Complex with β‑Cyclodextrin
Chiral
slipped 5,8:9′,10′-cyclodimers were preferentially
produced over classical 9,10:9′,10′-cyclodimers upon
supramolecular photocyclodimerization of 2-anthracenecarboxylate (AC)
mediated by β-cyclodextrin (β-CD). This photochirogenic
route to the slipped cyclodimers, exclusively head-to-tail (HT) and
highly enantioselective, has long been overlooked in foregoing studies
but is dominant in reality and is absolutely supramolecularly activated
by 2:2 complexation of AC with β-CD. The intricate structural
and photophysical aspects of this higher-order complexation-triggered
process have been comprehensively elucidated, while the absolute configurations
of the slipped cyclodimers have been unambiguously assigned by comparing
the experimental and theoretical circular dichroism spectra. In the
2:2 complex, two ACs packed in a dual β-CD capsule are not fully
overlapped with each other but are only partially stacked in a slipped <i>anti</i>- or <i>syn</i>-HT manner. Hence, they do
not spontaneously cyclodimerize upon photoexcitation but instead emit
long-lived excimer fluorescence at wavelengths slightly longer than
the monomer fluorescence, indicating that the slipped excimer is neither
extremely reactive nor completely relaxed in conformation and energy.
Because of the slipped conformation of the AC pair in the soft capsule,
the subsequent photocyclodimerization becomes manipulable by various
internal or external factors, such as temperature, pressure, added
salt, and host modification, enabling us to exclusively obtain the
slipped cyclodimers with high regio- and enantioselectivities. In
this supramolecularly driven photochirogenesis, the dual β-CD
capsule functions as a chiral organophotocatalyst to trigger and accelerate
the nonclassical photochirogenic route to slipped cyclodimers by preorganizing
the conformation of the encapsulated AC pair, formally mimicking a
catalytic antibody