Chemoselective hydrosilylation of hydroxyketones.

A chemoselective method for the hydrosilylation of ketones has been developed, using the combination of triphenylsilane and a catalyst prepared from Ni(COD)2 and the simple N-heterocyclic carbene IMes. The most notable feature of this method is that free hydroxyls are largely unaffected, thus providing a simple one-step procedure for the conversion of hydroxyketones to mono-protected diols, wherein the protecting group is exclusively installed on the ketone-derived hydroxyl. The process is typically high yielding with both simple ketones and more complex hydroxyketone substrates.

Trapping intermediates in an [8 + 2] cycloaddition reaction with the help of DFT calculations.

DFT calculations predict that the [8 + 2] cycloaddition reaction between ketenes and 8-azaheptafulvenes occurs stepwise through antiaromatic zwitterionic intermediates. With adequate modifications of both the electronic properties of the ketene and the reaction conditions, these elusive intermediates have been successfully trapped and fully characterized (X-ray), thus confirming the predicted stepwise nature of the transformation.

The electronic structure and photochemistry of group 6 bimetallic (Fischer) carbene complexes: beyond the photocarbonylation reaction.

The UV spectra of Group 6 metal carbene complexes bearing a CpM(CO)(3) (Cp = cyclopentadienyl) moiety bonded to the carbene carbon atom exhibit a redshift of the absorption maxima at higher wavelengths with respect to the parent monometallic complexes. This redshift is partly due to a higher occupation on the p(z) atomic orbital of the carbene carbon atom. Time-dependent DFT calculations accurately assign this band to a metal-to-ligand charge-transfer transition, thus showing that the presence of a second metal center does not affect the nature of the transition. However, the photochemical reactivity of Group 6 metal carbene complexes bearing a CpM(CO)(3) moiety strongly depends on the nature of this metal fragment. A new photoslippage reaction leading to fulvenes occurs when Mn-derived products 11 a, 11 b, and 12 a are irradiated (both Cr and W derivatives), whereas Re-derived product 11 c behaves like standard Fischer complexes and yields the usual photocarbonylation products. A new photoreduction process occurring in the metallacyclopropanone intermediate is also observed for these complexes. Both computational and deuteration experiments support this unprecedented photoslippage process. The key to this differential photoreactivity seems to be the M-Cp back-donation, which hampers the slippage process for Re derivatives and favors the carbonylation reaction.

Electronic structure of alkoxychromium(0) carbene complexes: a joint TD-DFT/experimental study.

The joint computational (TD-DFT) experimental study of the UV-vis spectroscopy of alkoxychromium(0) carbene complexes accurately assigns the vertical transitions responsible for the observed spectra of these compounds. Both the LF and the MLCT band have a remarkably pi-pi* character, which has been demonstrated by the strong dependence of the absorptions with the donor/acceptor nature of the substituent in p-substituted styrylchromium(0) carbene complexes. The effect of the substituent is also related with the equilibrium geometry of the complexes and the occupations of the p atomic orbital of the carbene carbon atom. Additionally, the ferrocenyl moiety behaves in chromium(0) (Fischer) carbene complexes as a pi-donor group.