Crystallographic characterization
of RuX(CO)(η3-C3H5)(JOSIPHOS),
where X = Cl, Br, or I, reveals
a halide-dependent diastereomeric preference that defines metal-centered
stereogenicity and, therefrom, the enantioselectivity of C−C
coupling in ruthenium-catalyzed anti-diastereo- and
enantioselective C−C couplings of primary alcohols with 1-aryl-1-propynes
to form products of carbonyl anti-(α-aryl)allylation.
Computational studies reveal that a non-classical hydrogen bond between
iodide and the aldehyde formyl CH bond stabilizes the favored transition
state for carbonyl addition. An improved catalytic system enabling
previously unattainable transformations was developed that employs
an iodide-containing precatalyst, RuI(CO)3(η3-C3H5), in combination with trifluoroethanol,
as illustrated by the first enantioselective ruthenium-catalyzed C−C
couplings of ethanol to form higher alcohols.
The
use of alkynes as vinylmetal pronucleophiles in intermolecular
enantioselective metal-catalyzed carbonyl and imine reductive couplings
to form allylic alcohols and amines is surveyed. Related hydrogen
autotransfer processes, wherein alcohols or amines serve dually as
reductants and carbonyl or imine proelectrophiles, also are cataloged,
as are applications in target-oriented synthesis. These processes
represent an emerging alternative to the use of stoichiometric vinylmetal
reagents or Nozaki–Hiyama–Kishi (NHK) reactions in carbonyl
and imine alkenylation.
The
first enantioselective ruthenium-catalyzed carbonyl vinylations
via hydrogen autotransfer are described. Using a ruthenium-JOSIPHOS
catalyst, primary alcohols 2a–2m and
2-butyne 1a are converted to chiral allylic alcohols 3a–3m with excellent levels of absolute
stereocontrol. Notably, 1°,2°-1,3-diols participate in site-selective
C–C coupling, enabling asymmetric carbonyl vinylation beyond
premetalated reagents, exogenous reductants, or hydroxyl protecting
groups. Using 2-propanol as a reductant, aldehydes dehydro-2a, 2l participate in highly enantioselective
2-butyne-mediated vinylation under otherwise identical reaction conditions.
Regio-, stereo-, and site-selective vinylations mediated by 2-pentyne 1b to form adducts 3n, 3o, and epi-3o also are described. The tiglyl alcohol
motif obtained upon butyne-mediated vinylation, which is itself found
in diverse secondary metabolites, may be converted to commonly encountered
polyketide stereodiads, -triads, and -tetrads, as demonstrated by
the formation of adducts 4a–4d. The
collective mechanistic studies, including deuterium labeling experiments,
corroborate a catalytic cycle involving alcohol dehydrogenation to
form a transient aldehyde and a ruthenium hydride, which engages in
alkyne hydrometalation to form a nucleophilic vinylruthenium species
that enacts carbonyl addition. A stereochemical model for carbonyl
addition invoking formyl CH···I[Ru] and CH···OC[Ru]
hydrogen bonds is proposed based on prior calculations and crystallographic
data.
Though light-emitting diodes (LEDs) combined with various color conversion techniques have been widely explored for VLC (visible light communication), E-O (electro-optical) frequency responses of devices with quantum dots (QDs) embedded within the nanoholes have rarely been addressed. Here we propose LEDs with embedded photonic crystal (PhC) nanohole patterns and green light QDs for studying small-signal E-O frequency bandwidths and large signal on–off keying E-O responses. We observe that the E-O modulation quality of PhC LEDs with QDs is better than a conventional LED with QDs when the overall blue mixed with green light output signal is considered. However, the optical response of only QD converted green light shows a contradictory result. The slower E-O conversion response is attributed to multi-path green light generation from both radiative and nonradiative energy transfer processes for QDs coated on the PhC LEDs.
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