The bioorthogonality of tetrazole photoclick chemistry has been reassessed. Upon photolysis of a tetrazole, the highly reactive nitrile imine formed undergoes rapid nucleophilic reaction with a variety of nucleophiles present in a biological system, along with the expected cycloaddition with alkenes. The alternative use of the tetrazole photoclick reaction was thus explored: tetrazoles were incorporated into Bodipy and Acedan dyes, providing novel photo-crosslinkers with one- and two-photon fluorescence Turn-ON properties that may be developed into protein-detecting biosensors. Further introduction of these photo-activatable, fluorogenic moieties into staurosporine resulted in the corresponding probes capable of photoinduced, no-wash imaging of endogenous kinase activities in live mammalian cells.
Carbenes derived from five-membered heterocycles with different numbers of nitrogen atoms ranging from two to four lead formally either to normal N-heterocyclic or mesoionic carbenes with, in some cases, the same skeletal structure. The electronic structures of fourteen of these compounds were examined by means of DFT calculations at the B3LYP/aug-cc-pVTZ level. The examined parameters include the energies of the σ-lone pair at Ccarbene and the π-HOMO of the protonated form, which are correlated to the first and second proton affinities. The singlet-triplet energy gap was used as a measure of the stability of the N-heterocyclic carbene (NHC) towards dimerisation. Natural population analysis provided insight into the variation of the pπ population and the natural charge at Ccarbene with NHC structure. Additionally, the transition metalNHC bond in L-AuCl and L-TiCl4 and the nature of the orbital interactions between the NHC and the transition-metal fragment were analysed in detail by the extended transition state-natural orbitals for chemical valence (ETS-NOCV) approach at the BP86/TZ2P level. Similarities and differences between the NHCgold and the NHCtitanium bond are discussed, and trends in key bonding properties can be traced back to the variation of the electronic parameters of the NHC.
Ten palladium(II) complexes bearing a pyrazolin-5-ylidene
ligand
have been synthesized by oxidative addition and silver carbene transfer
pathways. The weakly bound acetonitrile ligand in the initially obtained trans-[PdBr2(MeCN)(Pyry)] complex (6, Pyry = 1-phenyl-2,3-dimethylpyrazolin-5-ylidene) could be replaced
by other donor ligands, and additional NHC ligands were introduced
either by silver carbene transfer reactions or via reaction with in situ generated free carbenes. Using our previously reported 13C NMR-based electronic parameter, the pyrazolin-5-ylidene
ligand is estimated to be among the most strongly donating ligands
on our scale so far. The complexes obtained were employed as catalysts
for the direct arylation of pentafluorobenzene with moderate to good
yields under optimized conditions.
A series
of six benzimidazolium salts with an alkyl–alkyl
thioether moiety in the side chain has been synthesized. While it
was impossible to obtain the platinum(II) complexes by direct reaction
between ligand precursors and basic platinum salts, the mild silver
carbene transfer reaction gave the desired complexes in all cases.
X-ray crystallography confirmed the expected κ2
C,S coordination mode of the benzimidazolin-2-ylidene
ligands, with a cis arrangement of the carbene and
the hemilabile thioether moieties in all complexes. Preliminary studies
of the catalytic activity of these complexes showed them to be active
catalysts for the intermolecular hydroamination of alkynes with sterically
hindered anilines in conjunction with silver triflate. Additionally,
the complexes catalyzed the hydrosilylation of alkenes with excellent
yields and good regioselectivity.
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