have been obtained by hydrodistillation and analyzed by GC-MS. The Piper leaf oils have been screened for antibacterial activity against Bacillus cereus, Staphylococcus aureus, and Escherichia coli; for Artemia salina (brine shrimp) lethality; for in-vitro cytotoxic activity against MCF-7 (human breast tumor) cells; and inhibition of the cysteine protease cruzain. A cluster analysis comparison with previously published Piper leaf oil compositions was carried out in order to discern the differences and similarities between the volatile chemical compositions of Piper species.
Copper N-heterocyclic carbenes (NHCs) are an emerging
area of focus for catalysis and other applications. Using a straightforward
methodology, a new and highly modifiable tetradentate copper(II) NHC
complex was generated and characterized using X-ray crystallography,
UV–vis and EPR spectroscopy, cyclic voltammetry, and ESI-MS.
This copper(II) NHC complex adopted a distorted 4-coordinate coordination
mode and demonstrates a unique absorption spectrum for a copper(II)
species, but more interestingly, its redox properties indicate that
it can readily access all three common copper oxidation states under
atmospheric conditions. The tetradentate copper(II) NHC complex was
used to catalytically generate new C–N bonds from a series
of phenylboronic acids and amines. Once this CEL methodology was refined,
moderate to high yields were achieved using catalytic amounts of the
copper(II) complex to couple phenylboronic acids to a series of aniline
derivatives. Substituted phenylboronic acids and anilines had minimal
impact on the catalytic capabilities of this copper complex; however,
there is some indication that steric interactions between catalyst
and substrates may have an impact on efficient catalysis. The straightforward
synthesis of this framework and the utilization of an inexpensive,
first-row transition metal center in this system highlight the usefulness
of copper(II) NHCs as catalyst for cross-coupling reactions.
Complexes of copper and 1,10-phenanthroline have been utilized for organic transformations over the last 50 years. In many cases these systems are impacted by reaction conditions and perform best under an inert atmosphere. Here we explore the role the 1,10-phenanthroline ligand plays on the electronic structure and redox properties of copper coordination complexes, and what benefit related ligands may provide to enhance copper-based coupling reactions. Copper(II) triflate complexes bearing 1,10-phenanthroline (phen), ([Cu(phen) 2 -(OTf )]OTf, 1) and oxidized derivatives of phen including [Cu(edhp) 2 ](OTf ) 2 (2), [Cu(pdo) 2 ](OTf ) 2 (3), [Cu(dafo) 2 ](OTf ) 2 (4) [a] Previously, we have reported a discrete Cu 2+ phen complex, namely [Cu(phen) 2 OTf ]OTf (1), [14] which shows a good reactivity performing C-and N-atom transfer reactions. [15] However, the phen ligand can be easily oxidized to functionalize the central ring of this ligand system. The C=C bond between C5 and C6 positions can be oxidized to form the 5,6-epoxyphen (L2) [16] and 5,6-phendione (L3). [17] L3 can be further reacted to form the 4,5-diazafluorenone (L4) by the loss of CO. [18] These ligands Full Paper
Arylboronic acids are commonly used in modern organic chemistry to form new C–C and C–heteroatom bonds. These activated organic synthons show reactivity with heteroatoms in a range of substrates under ambient oxidative conditions. This broad reactivity has limited their use in protic, renewable solvents like water, ethanol, and methanol. Here, we report our efforts to study and optimize the activation of arylboronic acids by a copper(II) N-heterocyclic carbene (NHC) complex in aqueous solution and in a range of alcohols to generate phenol and aryl ethers, respectively. The optimized reactivity showcases the ability to make targeted C–O bonds, but also identifies conditions where water and alcohol activation could be limiting for C–C and C–heteroatom bond-forming reactions. This copper(II) complex shows strong reactivity toward arylboronic acid activation in aqueous medium at ambient temperature. The relationship between product formation and temperature and catalyst loading are described. Additionally, the effects of buffer, pH, base, and co-solvent are explored with respect to phenol and ether generation reactions. Characterization of the new copper(II) NCN-pincer complex by X-ray crystallography, HR-MS, cyclic voltammetry, FT-IR and UV-Vis spectral studies is reported.
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