847573-68-6Relevant academic research and scientific papers
Competing H2 versus Intramolecular C-H Activation at a Dinuclear Nickel Complex via Metal-Metal Cooperative Oxidative Addition
Dechert, Sebastian,Duan, Peng-Cheng,Meyer, Franc,Stevens, Hendrik
, p. 6717 - 6728 (2020)
Nickel(I) metalloradicals bear great potential for the reductive activation of challenging substrates but are often too unstable to be isolated. Similar chemistry may be enabled by nickel(II) hydrides that store the reducing equivalents in hydride bonds and reductively eliminate H2 upon substrate binding. Here we present a pyrazolate-based bis(β-diketiminato) ligand [LPh]3- with bulky m-terphenyl substituents that can host two Ni-H units in close proximity. Complexes [LPh(NiII-H)2]- (3) are prone to intramolecular reductive H2 elimination, and an equilibrium between 3 and orthometalated dinickel(II) monohydride complexes 2 is evidenced. 2 is shown to form via intramolecular metal-metal cooperative phenyl group C(sp2)-H oxidative addition to the dinickel(I) intermediate [LPhNiI2]- (4). While NiI species have been implicated in catalytic C-H functionalization, discrete activation of C-H bonds at NiI complexes has rarely been described. The reversible H2 and C-H reductive elimination/oxidative addition equilibrium smoothly unmasks the powerful 2-electron reductant 4 from either 2 or 3, which is demonstrated by reaction with benzaldehyde. A dramatic cation effect is observed for the rate of interconversion of 2 and 3 and also for subsequent thermally driven formation of a twice orthometalated dinickel(II) complex 6. X-ray crystallographic and NMR titration studies indicate distinct interaction of the Lewis acidic cation with 2 and 3. The present system allows for the unmasking of a highly reactive [LPhNiI2]- intermediate 4 either via elimination of H2 from dihydride 3 or via reductive C-H elimination from monohydride 2. The latter does not release any H2 byproduct and adds a distinct platform for metal-metal cooperative two-electron substrate reductions while circumventing the isolation of any unstable superreduced form of the bimetallic scaffold.
Pairwise H2/D2 Exchange and H2 Substitution at a Bimetallic Dinickel(II) Complex Featuring Two Terminal Hydrides
Manz, Dennis-Helmut,Duan, Peng-Cheng,Dechert, Sebastian,Demeshko, Serhiy,Oswald, Rainer,John, Michael,Mata, Ricardo A.,Meyer, Franc
, p. 16720 - 16731 (2017/11/28)
A compartmental ligand scaffold HL with two β-diketiminato binding sites spanned by a pyrazolate bridge gave a series of dinuclear nickel(II) dihydride complexes M[LNi2(H)2], M = Na (Na·2) and K (K·2), which were isolated after reacting the precursor complex [LNi2(μ-Br)] (1) with MHBEt3 (M = Na and K). Crystallographic characterization showed the two hydride ligands to be directed into the bimetallic pocket, closely interacting with the alkali metal cation. Treatment of K·2 with dibenzo(18-crown-6) led to the separated ion pair [LNi2(H)2][K(DB18C6)] (2[K(DB18C6)]). Reaction of Na·2 or K·2 with D2 was investigated by a suite of 1H and 2H NMR experiments, revealing an unusual pairwise H2/D2 exchange process that synchronously involves both Ni-H moieties without H/D scrambling. A mechanistic picture was provided by DFT calculations which suggested facile recombination of the two terminal hydrides within the bimetallic cleft, with a moderate enthalpic barrier of ～62 kJ/mol, to give H2 and an antiferromagnetically coupled [LNiI2]- species. This was confirmed by SQUID monitoring during H2 release from solid 2[K(DB18C6)]. Interaction with the Lewis acid cation (Na+ or K+) significantly stabilizes the dihydride core. Kinetic data for the M[L(Ni-H)2] → H2 transition derived from 2D 1H EXSY spectra confirmed first-order dependence of H2 release on M·2 concentration and a strong effect of the alkali metal cation M+. Treating [LNi2(D)2]- with phenylacetylene led to D2 and dinickel(II) complex 3- with a twice reduced styrene-1,2-diyl bridging unit in the bimetallic pocket. Complexes [LNiII2(H)2]- having two adjacent terminal hydrides thus represent a masked version of a highly reactive dinickel(I) core. Storing two reducing equivalents in adjacent metal hydrides that evolve H2 upon substrate binding is reminiscent of the proposed N2 binding step at the FeMo cofactor of nitrogenase, suggesting the use of the present bimetallic scaffold for reductive bioinspired activation of a range of inert small molecules.
Utilization of ultrasonic irradiation as green and effective one-pot protocol to prepare a novel series of bis-2-amino-1, 3, 4-oxa(thia)diazoles and bis-tetrazoles
Arafa, Wael Abdelgayed Ahmed,Abdel-Magied, Ahmed Fawzy
, p. 327 - 340 (2017/12/06)
In an effective and straightforward conversion, bis-semicarbazones and bis-thiosemicarbazones are transformed into a diversity of novel substituted bis-2-amino-1, 3, 4-oxadiazoles and bis-2-amino-1, 3, 4-thiadiazoles, respectively under ultrasonic irradiation. Bis-tetrazoles are obtained from the dialdehydes by sequential reaction with hydroxylamine hydrochloride, phosphorus pentoxide and sodium azide without isolation of the intermediates oximes and nitriles. All the reactions proceed cleanly and smoothly under mild conditions, with short reaction times and broad functional groups possibility. No side reactions were observed.
6,7-DIHYDRO-PYRAZOLO[1,5-A]PYRAZIN-4-YLAMINE DERIVATIVES USEFUL AS INHIBITORS OF BETA-SECRETASE (BACE)
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, (2012/09/21)
The present invention relates to novel 6,7-dihydro-pyrazolo[1,5-a]pyrazin-4-yl-amine derivatives as inhibitors of beta-secretase, also known as beta-site amyloid cleaving enzyme, BACE, BACEl, Asp2, or memapsin2. The invention is also directed to pharmaceu
