General Methods.1 H-NMR spectra were recorded on Varian 400 (400 MHz) spectrometers. Chemical shifts are reported in ppm from TMS with the solvent resonance as the internal standard (deuterochloroform: 7.24 ppm). Data are reported as follows: chemical shift, multiplicity (s = singlet, d = doublet, t = triplet, q = quartet, sext = sextet, sept = septet, p = pseudo, b = broad, m = multiplet), coupling constants (Hz). 13 C-NMR spectra were recorded on a Varian 400 (100 MHz) spectrometers with complete proton decoupling. Chemical shifts are reported in ppm from TMS with the solvent as the internal standard (deuterochloroform: 77.0 ppm). Monodimensional NOE experiment (400 MHz, CDCl 3 , 25 °C) was performed by using a DPFGSE-NOE sequence, with a 50 Hz pulse and a mixing time of 1.5 s. Irradiation at the frequency of proton H 1 (5.65 ppm) showed strong positive NOE response of the H 4 frequency, confirming the sin-relationship. Weaker NOE effects were also observed for the equatorial H 2 and the axial H 3 protons. GC-MS spectra were taken by EI ionization at 70 eV on a Hewlett-Packard 5971 with GC injection. They are reported as: m/z (rel. intense). LC-electrospray ionization mass spectra were obtained with Agilent Technologies MSD1100 single-quadrupole mass spectrometer. Chromatographic purification was done with 240-400 mesh silica gel. Other anhydrous solvents were supplied by Sigma Aldrich in Sureseal® bottles and used without any further purification. Commercially available chemicals were purchased from Sigma Aldrich, Stream and TCI and used without any further purification. Melting points were determined with Bibby Stuart Scientific Melting Point Apparatus SMP 3 and are not corrected. Agilent Technologies LC/MSD Trap 1100 series (nebulizer: 15.0 PSI, dry Gas: 5.0 L/min, dry Temperature: 325 °C, capillary voltage positive scan: 4000 mA, capillary voltage negative scan: 3500 mA). Preparation of β-naphthols 1 [1] and N-allenyl amides [2] were accomplished following the reported procedures.
The catalytic dearomatization of naphthols offers a unique platform to rapidly access structurally complex and densely functionalized molecular architectures. Thermodynamically more favorable, with respect to the analogous phenol‐variant, the transformation of the phenolic ring into the corresponding naphthalenone (cited here as naphthyl dearomatization) has faced a considerable amount of attention over the past decade with constant improvements towards stereocontrol. A collection of the most representative and recent catalytic variants involving carbon–carbon as well as carbon–heteroatom bond forming events are presented in this Minireview article.
Fabrication of amphiphilic mesostructured
silica as a heterogeneous
catalyst is beneficial to facilitate an aqueous reaction due to its
highly dispersed nature in water. In this work, by taking advantage
of a self-templating assembled strategy, we construct two types of
mesostructured silicas as heterogeneous catalysts, chiral ruthenium/diamine-functionalized
and chiral cinchonine-based squaramide-functionalized heterogeneous
catalysts, through the use of amphiphilic poly(ethylene glycol) monomethyl
ether-modified hyperbranched polyethoxysiloxane as a silica precursor.
As presented in the study, the chiral ruthenium/diamine-functionalized
catalyst performs an asymmetric transfer hydrogenation of acyclic
α-trifluoromethylimines to chiral α-trifluoromethylamines
in water, whereas the chiral squaramide-functionalized catalyst enables
efficiently asymmetric Michael addition of acetylacetone to nitroalkenes
in brine. Both highly catalytic performances are attributed to the
combined multifunctionalities of well-defined single-site chiral active
species, highly dispersed catalytic centers, and practical phase-transferred
function. Furthermore, both catalysts can also be recovered easily
and reused repeatedly for at least seven times without loss of catalytic
activity. Such a feature makes this self-templating assembly attractive
for construction of various heterogeneous catalysts.
New chiral BINOL‐based phosphate counterions have been synthesized, fully characterized, and employed in the enantioselective gold‐catalyzed dearomatization of β‐naphthols with allenamides. A range of densely functionalized C1‐allylated naphthalenones were realized under mild conditions and high levels of chemo‐, regio‐ and stereoselectivity (ee up to 95 %).
Chiral organorhodium-functionalized hollow-shell-structured nanospheres were prepared by immobilization of a chiral N-sulfonylated diamine-based organorhodium complex within an ethylene-bridged organosilicate shell. Structural analysis and characterization reveal its well-defined single-site rhodium active center, and transmission electron microscopy images reveal a uniform dispersion of hollow-shell-structured nanospheres. As a heterogenous catalyst, it exhibits excellent catalytic activity and enantioselectivity in synthesis of chiral phthalides by a tandem reduction/lactonization of ethyl 2-acylarylcarboxylates in aqueous medium. The high catalytic performance is attributed to the synergistic effect of the high hydrophobicity and the confined chiral organorhodium catalytic nature. The organorhodium-functionalized nanospheres could be conveniently recovered and reused at least 10 times without loss of catalytic activity. This feature makes it an attractive catalyst in environmentally friendly organic reactions. The results of this study offer a new approach to immobilize chiral organometal functionalities within the hollow-shell-structured nanospheres to prepare materials with high activity in heterogeneous asymmetric catalysis.
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