Noelia Fuentes

Ti‐Catalyzed Barbier‐Type Allylations and Related Reactions

Titanocene(III) complexes, easily generated in situ from commercial Ti(IV) precursors, catalyze Barbier-type allylations, intramolecular crotylations (cyclizations), and prenylations of a wide range of aldehydes and ketones. The reaction displays surprising and unprecedented mechanistic subtleties. In cyclizations a fast and irreversible addition of an allyl radical to a Ti(III)-coordinated carbonyl group seems to occur. Intermolecular additions to conjugated aldehydes proceed through a coupling of a Ti(IV)-bound ketyl radical with an allyl radical. Reactions of ketones with allylic halides take place by the classical addition of an allylic organometallic reagent. The radical coupling processes enable transformations such as the highly regioselective alpha-prenylation that are otherwise difficult to achieve. The mild reaction conditions and the possibility to employ titanocene complexes in only catalytic quantities are highly attractive features of our protocol. These unusual properties have been taken advantage of for the straightforward synthesis of the natural products rosiridol, shikalkin, and 12-hydroxysqualene.

Divergent Titanium‐Mediated Allylations with Modulation by Nickel or Palladium

Titanocene(III)-mediated radical processes are important tools for the formation of C C bonds under mild conditions, and are compatible with many functional groups. Moreover, titanium(III) complexes can be used substoichiometrically, which has allowed the development of enantioselective versions of these reactions. A serious limitation of these radical processes, however, derives from the fact that titanium(III)-mediated radical generation requires reactive substrates, such as allylic halides, which are often cumbersome in introduction and manipulation. Allylic carbonates and carboxylates, in contrast, are easily prepared and handled but are inert against titanocene(III) complexes. Nevertheless, it is known that nickel and palladium complexes can readily activate allylic carbonates and carboxylates I (Scheme 1) to form h-allylmetal complexes (II). On the basis of these results, we deemed that the combination of palladium or nickel derivatives with titanocene(III) complexes would facilitate the development of novel allylation processes using accessible allyl carbonates or carboxylates. In the case of palladium catalysis, it is known that the Oppolzer-type cyclization of organometallic species (II, M = Pd, Scheme 1) to cyclic derivatives (VI) is relatively slow at room temperature. Thus, reduction of II by a singleelectron-transfer reagent, such as [Cp2TiCl], [6] could lead to the allylic radical III, which might be eventually trapped by a second [Cp2TiCl] species to give an allylic titanium(IV) complex IV. Finally, nucleophilic attack of the organometallic derivative IV on an aldehyde or other electrophilic reagent would provide the corresponding allylation product V. On the other hand, nickel-catalyzed carbocyclizations, via intermediates such as II (M = Ni, Scheme 1) to cyclic derivatives VI, are relatively fast at room temperature. Once formed, VI might be reduced by [Cp2TiCl] to a primary radical VII, which could be trapped by a second [Cp2TiCl] species to give an alkyl titanium(IV) complex VIII. Hydrolysis of the organometallic derivative VIII would yield carbocycles IX. Thus, we anticipated that the use of palladium or nickel catalysts could modulate titanium(III) to drive allylation reactions with allyl carboxylates by two different pathways, either through intermolecular coupling with electrophilic reagents or to give carbocyclic derivatives by an intramolecular allylation. To check our hypothesis, we chose allylic carbonate E-1 as a model allylation reagent. Thus, reaction of decanal with carbonate E-1 and an excess of [Cp2TiCl] (2.0 equiv), [8] in the presence of PdCl2 (20 mol%) and triphenylphosphine, [9] gave the expected coupling product 2 as a single stereoisomer in 76% yield (Scheme 2). In contrast, when carbonate E-1 was treated with an excess of [Cp2TiCl] (2.0 equiv) in the presence of [NiCl2(PPh3)2] (20 mol%), a mixture of carbocycles 3 and 4 (4:1 ratio) was obtained in almost quantitative Scheme 1. Mechanism for palladium-catalyzed, titanocene-mediated allylation of carbonyl compounds and nickel-catalyzed, titanocenemediated carbocylization of allylic carboxylates. [M] = transition metal catalyst; Cp = cyclopentadienyl; E = electrophilic reagent (e.g. aldehyde).R = OEt, Me, or Ph.

Versatile Bottom-up Approach to Stapled π-Conjugated Helical Scaffolds: Synthesis and Chiroptical Properties of Cyclic o-Phenylene Ethynylene Oligomers†

Spring loaded: the smallest members of a family of carbon nanocoils (CNCs), adopting a fixed helical structure, have been synthesized by introduction of one or two staples in o-phenylene ethynylene oligomers. The chiroptical responses of the systems having enantiopure L-tartrate-derived staples confirmed the induced helicity. Theoretical studies suggest that these CNCs are pseudoelastic.

Development of a new dual polarity and viscosity probe based on the foldamer concept.

Small molecular probes able to act as sensors are of enormous interest thanks to their multiple applications. Here, we report on the development of a novel supramolecular dual viscosity and polarity probe based on the foldamer concept, which increases the resolution limits of traditional probes at low viscosity values (0-4 mPa·s). The applicability of this new probe has been tested with a supramolecular organogel.

Thermally Driven Nanofuses Based on Organometallic Rotors

Molecular design of chromium arenes are theoretically studied as a model for the development of novel thermally-driven molecular fuses. This study correlates the switching event with a partial disconnection of the molecule from the metallic electrode mediated by changes in the conformational states of the molecule directed by external stimuli. Moreover, the reversibility of the process (the reconnection to the metallic electrode) is also considered for these systems when a reversal voltage pulse (reset) is applied. The energetic requirements of the on and off states are correlated with temperature through the Arrhenius equation. To carry out this study we performed density functional theory (DFT) calculations.

Computational Study of a Nanofuse Based on Organic Molecules

Molecular electronics might be a solution for the challenges raised in the context of the Moores law for the following years. In this paper we present a computational study simulating the electronic behavior of a new generation of molecular switches with excellent geometrical characteristic and a good switching ratio over a wide range of voltage.