Protein modification has entered the limelight of chemical and biological sciences, since, by appending small molecules into proteins surfaces, fundamental biological and biophysical processes may be studied and even modulated in a physiological context. Herein we present a new strategy to modify the lysine's ε-amino group and the protein's N-terminal, based on the formation of stable iminoboronates in aqueous media. This functionality enables the stable and complete modification of these amine groups, which can be reversible upon the addition of fructose, dopamine, or glutathione. A detailed DFT study is also presented to rationalize the observed stability toward hydrolysis of the iminoboronate constructs.
A complete catalytic cycle for the cyclotrimerization of acetylene with the CpRuCl fragment has been proposed and discussed based on DFT/B3LYP calculations, which revealed a couple of uncommon intermediates. The first is a metallacyclopentatriene complex RuCp(Cl)(C(4)H(4)) (B), generated through oxidative coupling of two alkyne ligands. It adds another alkyne in eta(2) fashion to give an alkyne complex (C). No less than three successive intermediates could be located for the subsequent arene formation. The first, an unusual five- and four-membered bicyclic ring system (D), rearranges to a very unsymmetrical metallaheptatetraene complex (E), which in turn provides CpRuCl(eta(2)-C(6)H(6)) (F) via a reductive elimination step. The asymmetry of E, including Cp ring slippage, removes the symmetry-forbidden character from this final step. Completion of the cycle is achieved by an exothermic displacement (21.4 kcal mol(-)(1)) of the arene by two acetylene molecules regenerating A. In addition to acetylene, the reaction of B with ethylene and carbon disulfide, the latter taken as a model for a molecule lacking hydrogen atoms, has also been investigated, and several parallels noted. In the case of the coordinated alkene, facile C-C coupling to the alpha carbon of the metallacycle is feasible due to an agostic assistance, which tends to counterbalance the reduced degree of unsaturation. Carbon disulfide, on the other hand, does not coordinate to ruthenium, but a C=S bond adds instead directly to the Ru=C bond. The final products of the reactions of B with acetylene, ethylene, and carbon disulfide are, respectively, benzene, cyclohexadiene, and thiopyrane-2-thione, the activation energies being lower for acetylene.
Mn(i) PNP pincer hydride complexes were shown for the first time to catalyse the hydrogenation of CO2 to formate reaching quantitative yields and high TONs.
We
have prepared and structurally characterized a new class of
Fe(II) PNP pincer hydride complexes [Fe(PNP-iPr)(H)(CO)(L)]n (L = Br–, CH3CN, pyridine, PMe3, SCN–, CO, BH4–; n = 0, +1) based on
the 2,6-diaminopyridine scaffold where the PiPr2 moieties of the PNP ligand are connected to the pyridine
ring via NH and/or NMe spacers. Complexes [Fe(PNP-iPr)(H)(CO)(L)]n with labile ligands (L
= Br–, CH3CN, BH4–) and NH spacers are efficient catalysts for the hydrogenation of
both ketones and aldehydes to alcohols under mild conditions, while
those containing inert ligands (L = pyridine, PMe3, SCN–, CO) are catalytically inactive. Interestingly, complex
[Fe(PNPMe-iPr)(H)(CO)(Br)], featuring
NMe spacers, is an efficient catalyst for the chemoselective hydrogenation
of aldehydes. The first type of complexes involves deprotonation of
the PNP ligand as well as heterolytic dihydrogen cleavage via metal-alkoxide
cooperation, but no reversible aromatization/deprotonation of the
PNP ligand. In the case of the N-methylated complex the mechanism
remains unclear, but obviously does not allow bifunctional activation
of dihydrogen. The experimental results complemented by DFT calculations
strongly support an insertion of the C=O bond of the carbonyl
compound into the Fe–H bond.
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