Iron-Catalyzed Hydroborylative Cyclization of 1,6-Enynes.

We report first Fe-catalyzed hydroborylative cyclization reaction. The process provides one C-C and one C-B bond in a single operation and shows a wide scope, allowing the formation of carbo- and heterocycles containing a homoallylic boryl unit that can be further functionalized. The reaction takes place in smooth conditions, with inexpensive catalytic system and full atom economy since HBpin is the borylation agent, in contrast to our previously reported Pd-catalyzed reaction. Both aryl and alkyl substituted alkynes are reactive, revealing a wide reaction scope. Mechanistic studies suggest the intermediacy of FeII -hydride active catalyst capable to react with the alkyne group prior to alkene insertion, and computational studies suggest the occurrence of barrierless σ-bond metathesis involving HBpin and Fe-C bonds along the catalytic cycle.

Influence of the number of anchoring groups on the electronic and mechanical properties of benzene-, anthracene- and pentacene-based molecular devices.

One of the central issues of molecular electronics (ME) is the study of the molecule-metal electrode contacts, and their implications for the conductivity, charge-transport mechanism, and mechanical stability. In fact, stochastic on/off switching (blinking) reported in STM experiments is a major problem of single-molecule devices, and challenges the stability and reliability of these systems. Surprisingly, the ambiguous STM results all originate from devices that bind to the metallic electrode through a one-atom connection. In the present work, DFT is employed to study and compare the properties of a set of simple acenes that bind to metallic electrodes with an increasing number of connections, in order to determine whether the increasing numbers of anchoring groups have a direct repercussion on the stability of these systems. The conductivities of the three polycyclic aromatic hydrocarbons are calculated, as well as their transmission spectra and current profiles. The thermal and mechanical stability of these systems is studied by pulling and pushing the metal-molecule connection. The results show that molecules with more than one connection per electrode exhibit greater electrical efficiency and current stability.

Nickel-catalyzed cross-coupling of alkyl zinc halides for the formation of C(sp(2))-C(sp(3)) bonds: scope and mechanism.

Optimal conditions for a general Ni-catalysed Negishi cross-coupling of alkyl zinc halides with aryl, heteroaryl and alkenyl halides have been determined. These conditions allow the reaction to take place smoothly, with low catalyst loading, and in the presence of a wide variety of functional groups to afford products in good yields at room temperature. DFT studies on the mechanism support the occurrence of a catalytic cycle involving transmetalation of the alkyl zinc halide to Ni(I) followed by oxidative addition of the haloarene and C-C reductive elimination.

Nickel-catalysed Negishi cross-coupling reactions: scope and mechanisms.

The Negishi cross-coupling of organozinc reagents is a valuable tool for the formation of C-C bonds in the presence of functional groups. Although this reaction is catalysed by Ni or Pd complexes, the latter have been much more developed. Nevertheless, Ni derivatives have become important in recent years, especially for their activity in the formation of alkyl-alkyl bonds. The mechanisms involved in Ni-catalysed reactions are different from the usual catalytic cycle involving oxidative addition-transmetalation-reductive elimination that is generally accepted for Pd-catalysed reactions. This tutorial review presents the more recent advances in Ni-catalysed Negishi cross-couplings for the formation of different kinds of C-C bonds and discusses their proposed reaction mechanisms.

Titanocene-catalyzed cascade cyclization of epoxypolyprenes: straightforward synthesis of terpenoids by free-radical chemistry.

The titanocene-catalyzed cascade cyclization of epoxypolyenes, which are easily prepared from commercially available polyprenoids, has proven to be a useful procedure for the synthesis of C(10), C(15), C(20), and C(30) terpenoids, including monocyclic, bicyclic, and tricyclic natural products. Both theoretical and experimental evidence suggests that this cyclization takes place in a nonconcerted fashion via discrete carbon-centered radicals. Nevertheless, the termination step of the process seems to be subjected to a kind of water-dependent control, which is unusual in free-radical chemistry. The catalytic cycle is based on the use of the novel combination Me(3)SiCl/2,4,6-collidine to regenerate the titanocene catalyst. In practice this procedure has several advantages: it takes place at room temperature under mild conditions compatible with different functional groups, uses inexpensive reagents, and its end step can easily be controlled to give exocyclic double bonds by simply excluding water from the medium.