Iron-catalyzed α-C–H functionalization of π-bonds: cross-dehydrogenative coupling and mechanistic insights

The deprotonation of propargylic C–H bonds for subsequent functionalization typically requires stoichiometric metal alkyl or amide reagents. In addition to the undesirable generation of stoichiometric metallic waste, these conditions limit the functional group compatibility and versatility of this functionalization strategy and often result in regioisomeric mixtures. In this article, we report the use of dicarbonyl cyclopentadienyliron(ii) complexes for the generation of propargylic anion equivalents toward the direct electrophilic functionalization of propargylic C–H bonds under mild, catalytic conditions. This technology was applied to the direct conversion of C–H bonds to C–C bonds for the synthesis of several functionalized scaffolds through a one-pot cross dehydrogenative coupling reaction with tetrahydroisoquinoline and related privileged heterocyclic scaffolds. A series of NMR studies and deuterium-labelling experiments indicated that the deprotonation of the propargylic C–H bond was the rate-determining step when a Cp*Fe(CO)₂-based catalyst system was employed.

[Cp*Fe(CO)₂]⁺ facilitates the α-deprotonation of unsaturated C–C bond for propargylic and allylic C–H functionalization. Mechanistic studies reveal insights into the superior performance of the electron-rich and hindered ligand on iron.     

2.2]paracyclophane/dehydroannulene hybrids: probing the aromaticity of the dehydro[14]annulene framework

The synthesis of [2.2]paracyclophane/dehydro[14]annulene hybrids 1 and 2 is reported. Comparison of the proton NMR spectra of 1 and 2 with their open precursors and with related model compounds reveals the pronounced effect of macrocycle formation upon the cyclophane protons H15/H16, which lie above the shielding cone of the diatropic [14]annulene moiety. [structure: see text]     

"Three-dimensional hybridization" with polyvalent DNA-gold nanoparticle conjugates

We have determined the minimum number of base pairings necessary to stabilize DNA-Au NP aggregates as a function of salt concentration for particles between 15 and 150 nm in diameter. Significantly, we find that sequences containing a single base pair interaction are capable of effecting hybridization between 150 nm DNA-Au NPs. While traditional DNA hybridization involves two strands interacting in one dimension (1D, Z), we propose that hybridization in the context of an aggregate of polyvalent DNA-Au NP conjugates occurs in three dimensions (many oligonucleotides oriented perpendicular to the X, Y plane engage in base pairing), making nanoparticle assembly possible with three or fewer base pairings per DNA strand. These studies enabled us to compare the stability of duplex DNA free in solution and bound to the nanoparticle surface. We estimate that 4-8, 6-19, or 8-33 additional DNA bases must be added to free duplex DNA to achieve melting temperatures equivalent to hybridized systems formed from 15, 60, or 150 nm DNA-Au NPs, respectively. In addition, we estimate that the equilibrium binding constant (K(eq)) for 15 nm DNA-Au NPs (3 base pairs) is approximately 3 orders of magnitude higher than the K(eq) for the corresponding nanoparticle free system.     

Synthesis and properties of butyl-capped dehydrothieno[14]annulenes and their conversion into terthiophenes

A series of butyl end-capped dehydrothieno[14]annulenes (DTAs) has been prepared. These compounds were further transformed into electron-rich terthiophenes (3TPs) by reaction with sodium sulfide. The DTA and 3TP macrocycles were compared qualitatively using UV-vis spectroscopy, and the latter were found to have lower energy absorptions. The conformations of these molecules were also examined by DFT-YFLP computations.     

Fine-tuning of radiolysis induced emission by variable substitution of donor-/acceptor-substituted tetrakis(arylethynyl)benzenes

Emission from charge recombination between radical cations and anions of various tetrakis(arylethynyl)benzenes (TAEBs) was measured during pulse radiolysis in benzene (Bz). The formation of TAEB in the excited singlet state ((1)TAEB*) can be attributed to the charge recombination between TAEB (*+) and TAEB (*-), which is initially generated from the radiolytic reaction. It was found that the charge recombination between TAEB (*+) and TAEB (*-) gave (1)TAEB* as the emissive species but not excimers because of the large repulsion between substituents caused by the rotation around C-C single bonds. Since donor-/acceptor-substituted TAEBs possess three types of charge-transfer pathways (linear-conjugated, cross-conjugated, and "bent"-conjugated pathways between the donor and acceptor substituents through the ethynyl linkage), the emission spectra of (1)TAEBs* with intramolecular charge transfer (ICT) character depend on the substitution pattern and the various types of donor and acceptor groups during pulse radiolysis. Through control of the substitution pattern (e.g., the position of the nitrogen atom within the pyridine ring or the number of acceptors per arene ring of the regioisomeric donor-/acceptor-substituted TAEBs with donating N, N-dibutylamino and accepting pyridine unit (N1-9) and those with donating N, N-dibutylamino and accepting one (F1-3), two trifluoromethyl (F4-6), or perfluorinated arene (F7-9) units), fine-tuning of radiolysis induced emission color can be achieved.     

Structure-property relationships of fluorinated donor/acceptor tetrakis(phenylethynyl)benzenes and bis(dehydrobenzoannuleno)benzenes

Nine new bisdonor/bisacceptor-functionalized tetrakis(phenylethynyl)benzene (TPEB) and six new bis(dehydrobenzoannuleno)benzene (DBA) chromophores have been synthesized. The compounds consist of electron-donating dibutylaniline groups connected through a conjugated phenyl-acetylene scaffold to benzotrifluoride, bis(trifluoromethyl)phenyl, or pentafluorophenyl acceptor groups. In comparison to previously reported analogues utilizing nitrophenyl or benzonitrile acceptor groups, the weaker acceptor groups exhibit visibly fluorescent intramolecular charge transfer (ICT) behavior, moderately narrow optical band gaps, moderately high quantum yields, and strong fluorescence solvatochromism. In this series of molecules, the strongly inductive fluoro acceptor groups result in optical properties similar to the resonance acceptor analogues, making them promising candidates for optical materials device components. The data also support recent investigations that question the utility of using UV/vis spectroscopy alone as a qualitative measure of conjugation. The bisDBAs exhibit weaker ICT behavior and self-association in solution than their corresponding nitro analogues, but show greater stability to decomposition via polymerization and smaller optical band gaps than their acyclic analogues.     

Synthesis of 1-substituted benzo[c]isoxazol-3(1H)-imines via tandem nitroso-ene/intramolecular cyclizations of 2-nitrosobenzonitrile

Instead of reacting via the expected coarctate cyclization pathway, 2-nitrosobenzonitrile undergoes a tandem nitroso-ene/intramolecular cyclization to form benzo[c]isoxazol-3(1H)-imines in very good yields under neutral conditions and at moderate temperatures. Treatment of three of the imines with HBF4 results in dimerization/condensation to furnish unusual, delocalized cationic systems.     

Emission of discrete vortex rings by a vibrating grid in superfluid 3He-B: a precursor to quantum turbulence

We report a transition in the vorticity generated by a grid moving in the B phase of superfluid 3He at T相似文献