Philippe C. Gros

Bimetallic combinations for dehalogenative metalation involving organic compounds.

Organic Compounds David Tilly,† Floris Chevallier,† Florence Mongin,*,† and Philippe C. Gros*,‡,§ †Equipe Chimie et Photonique Molećulaires, Institut des Sciences Chimiques de Rennes, UMR 6226 CNRS-Universite ́ de Rennes 1, Bat̂iment 10A, Case 1003, Campus de Beaulieu, Avenue du Geńeŕal Leclerc, 35042 Rennes Ced́ex, France ‡HECRIN, Universite ́ de Lorraine, SRSMC UMR 7565, Boulevard des Aiguillettes, 54506 Vandoeuvre-les̀-Nancy, France HECRIN, CNRS, SRSMC UMR 7565, Boulevard des Aiguillettes, 54506 Vandoeuvre-les̀-Nancy, France

nBuLi/Lithium Aminoalkoxide Aggregates: New and Promising Lithiating Agents for Pyridine Derivatives

nBuLi/lithium aminoalkoxide aggregates have proven to be useful new lithiating agents. This review covers the current status of the authors’ works on the direct metallation of pyridine derivatives with these reagents, featuring the scope of their synthetic potential. (© Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002)

Combination of Cobalt and Iron Polypyridine Complexes for Improving the Charge Separation and Collection in Ru(terpyridine)2‐Sensitised Solar Cells

Mixtures of polypyridine Fe(II) and Co(II) complexes are used as electron mediators in Ru-thienyltpy-sensitised solar cells (tpy=terpyridine). The use of the metalorganic redox couples allows for improved charge-collection efficiency with respect to the classical iodide/iodine couple which, when associated to Ru-tpy(2) dyes, usually produces poor performance. The improved charge collection is explained by a combination of effective dye regeneration and decreased recombination with the oxidised electrolyte on the basis of data obtained by transient spectroscopy and photoelectrochemical measurements. The efficiency of the regeneration cascade is also critically dependent upon the ability of the Co(II) complex to intercept Fe(III) centres, as clearly indicated by chronocoulometry experiments.

Theoretical study of new ruthenium-based dyes for dye-sensitized solar cells.

Two relevant, recently reported, ruthenium-based complexes to be used as sensitizers in Grätzel photovoltaic cells are theoretically studied. The UV/vis absorption spectra have been computed within the time-dependent density functional theory formalism. The obtained excitation energies are compared with the experimental results, and the nature of the transition is analyzed in terms of the electronic density. A preliminary study on the performance of different functionals against the equation of motion coupled cluster is performed on a smaller model system.

Deprotonative Metalation of Chloro‐ and Bromopyridines Using Amido‐Based Bimetallic Species and Regioselectivity‐Computed CH Acidity Relationships

A series of chloro- and bromopyridines have been deprotometalated by using a range of 2,2,6,6-tetramethylpiperidino-based mixed lithium-metal combinations. Whereas lithium-zinc and lithium-cadmium bases afforded different mono- and diiodides after subsequent interception with iodine, complete regioselectivities were observed with the corresponding lithium-copper combination, as demonstrated by subsequent trapping with benzoyl chlorides. The obtained selectivities have been discussed in light of the CH acidities of the substrates, determined both in the gas phase and as a solution in THF by using the DFT B3LYP method.

Selective ring-opening of ω-epoxyalkyl (meth)acrylates. An efficient access to bifunctional monomers

Abstract It is shown that ω-epoxyalkyl (meth)acrylates constitute good precursors of bifunctional monomers by selective opening of the oxirane ring.

A theoretical study on nBuLi/lithium aminoalkoxide aggregation in hexane and THF.

Theoretical calculations on aggregation of nBuLi/lithium aminoalkoxide superbases, such as nBuLi/LiDMAE (LiDMAE = Me(2)N(CH(2))(2)OLi) and nBuLi/LiPM (LiPM = Li-N-methyl-2-pyrrolidine methoxide) in gas phase and solution are reported. The combination of equimolar amounts of each component in hexane induced unusual reactivity of the resulting superbase, which remains misunderstood. In order to elucidate the corresponding reaction mechanisms, it is imperative to get a deeper insight into the energetics of aggregation and the effect of the medium on equilibrium constants. In the present study, we compute and compare the stability of (nBuLi)(n), (LiPM)(n), and equimolecular mixed aggregates (nBuLi:LiPM)(n) in gas phase, hexane, and THF. Calculations have been carried out at the density functional theory level (B3LYP/6-31G(d)) using continuum and discrete continuum models of solvation. Higher-level calculations (MP2/aug-ccpVQZ) have been done in some cases for test purposes. Enthalpic and entropic contributions have been discussed and were shown to play an opposite role in hexane (or gas phase) and THF. The characteristics of LiPM and mixed nBuLi/LiPM solutions are found to be significantly different from those of nBuLi solutions. These calculations are in accordance with experimental data in both hexane and THF. Further comparison of theoretical and experimental results for gas-phase Li(+)-THF and Li(+)-DME complexes has enabled a discussion on computational errors for entropic contributions in THF. The value for the release of a THF solvent molecule is proposed to be DeltaS approximately 23 eu. These results provide new insights to the aggregation of organolithium compounds in solution and will be useful for the investigation of other systems.

Computed CH Acidity of Biaryl Compounds and Their Deprotonative Metalation by Using a Mixed Lithium/Zinc‐TMP Base

With the aim of synthesizing biaryl compounds, several aromatic iodides were prepared by the deprotonative metalation of methoxybenzenes, 3-substituted naphthalenes, isoquinoline, and methoxypyridines by using a mixed lithium/zinc-TMP (TMP=2,2,6,6-tetramethylpiperidino) base and subsequent iodolysis. The halides thus obtained, as well as commercial compounds, were cross-coupled under palladium catalysis (e.g., Suzuki coupling with 2,4-dimethoxy-5-pyrimidylboronic acid) to afford various representative biaryl compounds. Deprotometalation of the latter compounds was performed by using the lithium/zinc-TMP base and evaluated by subsequent iodolysis. The outcome of these reactions has been discussed in light of the CH acidities of these substrates, as determined in THF solution by using the DFT B3LYP method. Except for in the presence of decidedly lower pKa values, the regioselectivities of the deprotometalation reactions tend to be governed by nearby coordinating atoms rather than by site acidities. In particular, azine and diazine nitrogen atoms have been shown to be efficient in inducing the reactions with the lithium/zinc-TMP base at adjacent sites (e.g., by using 1-(2-methoxyphenyl)isoquinoline, 4-(2,5-dimethoxyphenyl)-3-methoxypyridine, or 5-(2,5-dimethoxyphenyl)-2,4-dimethoxypyrimidine as the substrate), a behavior that has already been observed upon treatment with lithium amides under kinetic conditions. Finally, the iodinated biaryl derivatives were involved in palladium-catalyzed reactions.

Metalation of pyridines with nBuLi-Li-aminoalkoxide mixed aggregates: The origin of chemoselectivity.

The reactivity of alkyllithium-lithium-aminoalkoxide unimetallic superbases has been investigated. These systems are used for deprotonative lithiation of pyridine derivatives in apolar non-coordinating media with excellent regio- and chemoselectivity, in deep contrast with alkyllithium. With the aim of getting a better understanding of the chemistry behind these promising reagents, we have carried out a joint experimental and theoretical study of the metalation of 2-chloropyridine with combinations of nBuLi and (S)-(-)-N-methyl-2-pyrrolidinylmethoxide (LiPM). Nucleophilic addition or alpha-lithiation has been observed, depending on conditions (solvent, temperature, stoichiometry), while ortho-metalation was not detected. Theoretical calculations using Density Functional Theory (B3LYP/6-31G(d) method) have then been carried out in gas phase at 195 K to characterize the relevant chemical species (reactive aggregates, transition structures) and estimate free energies of activation and relative reaction rates. Solvent effects in hexane have been neglected according to previous calculations. The effect of coordinating solvents such as THF has been qualitatively discussed. A major achievement of the present work has been to demonstrate that chemoselectivity crucially depends on aggregate type: dimers systematically lead to nucleophilic addition, while tetramers lead to alpha-lithiation. Besides, the calculations predict dimers to be more reactive than tetramers, yet they are much less stable, so that the observed selectivity results from the combination of both properties. A simple procedure to evaluate the basicity of an organlithium compound has been proposed. It has allowed us to show that the nBuLi-LiPM tetramer has a significantly larger basicity than its corresponding dimer, which is not at all the case for nBuLi aggregates, thus explaining differences in selectivity. Solvent and temperature effects on nBuLi-LiPM reactivity have been analyzed. By increasing the temperature in hexane, or changing the solvent from hexane to THF, dimer concentration is expected to rise, and likewise the weight of nucleophilic addition rises, in agreement with the experimental findings.

First Regioselective C-2 Lithiation of 3- and 4-Chloropyridines

We have shown that the BuLi/LiDMAE reagent promotes the clean and regioselective C2 lithiation of 3- and 4-chloropyridines, while other reagents such as LDA or BuLi/TMEDA lead to classical ortho lithiation products or mixtures of regioisomers. The method was successfully applied to the preparation of various reactive 2,3- and 2,4-disubstituted pyridines.