Raphaël Robiette

Insights into the Origin of Solid-State Photochromism and Thermochromism of N-Salicylideneanils: The Intriguing Case of Aminopyridines.

The relationships between the crystal structure and optical properties of switchable N-salicylideneanils have been revised and discussed on the basis of new experimental results and a computational approach. N-salicylidene-3-aminopyridine (L(3)) is a versatile thermo- and photochromic molecule. It also exhibits an infinitely slow thermal back relaxation (k = 9.9x10(-8) s(-1)) after photoswitching that is suitable for optical memories. Contrary to reports in the literature, N-salicylidene-4-aminopyridine (L(4)) is exclusively thermochromic. To explain these unexpected optical properties in the solid state, crystallography combined with UV-visible spectroscopic data was exploited. L(3) was also used as a ligand in new thermochromic coordination complexes [M(CH(3)OH)(2)(L(3))(2)(NCX)(2)], in which M(II) = Fe, Co, Ni, Cu or Mn and X = S or Se (1-6), which allowed the fine-tuning of the electron density in the photochromic moiety. The influence of the coordination through the nitrogen of the pyridine ring is also fully discussed.

Biologically active γ-lactams: synthesis and natural sources

The γ-lactam moiety is present in a large number of natural and non-natural biologically active compounds. The range of biological activities covered by these compounds is very broad. Functionalized γ-lactams are thus of high interest and have great potential in medicinal chemistry. This review provides a description of the title compounds by focusing on their synthesis, natural sources and biological activities.

Asymmetric Synthesis of 2,3-Dihydrobenzofurans by a [4+1] Annulation Between Ammonium Ylides and In Situ Generated o-Quinone Methides

Abstract A highly enantio‐ and diastereoselective [4+1] annulation between in situ generated ammonium ylides and o‐quinone methides for the synthesis of a variety of 2,3‐dihydrobenzofurans has been developed. The key factors controlling the reactivity and stereoselectivity were systematically investigated by experimental and computational means and the energy profiles obtained provide a deeper insight into the mechanistic details of this reaction.

Julia-Kocienski reaction-based 1,3-diene synthesis: aldehyde-dependent (E,E/E,Z)-selectivity.

A new modification of Julia-Kocienski olefination reaction based on the use of cation-specific chelating agents that yields 1,3-dienes with predictable (E/Z)-selectivity on newly created double bond was developed. The influence of the aldehyde structure on reaction (E/Z) selectivity is discussed and rationalized.

Chiral properties of tetrathiatriarylmethyl spin probes

We report that tetrathiatriarylmethyl (trityl) EPR probes are chiral molecules at room temperature, the two stereoisomers that differ in their helicity being configurationally stable enough to be separated and stored independently.

[4+2] Cycloaddition of 1-aminodienes and 2-substituted vinylphosphonates: application to asymmetric synthesis of 3-amino-5-phosphono-1-cyclohexene derivatives

N-Butadienylsuccinimide (1), iso-propyl N-butadienyl-(S)-pyroglutamate (5) and N-butadienyl-(R)-4-phenyloxazolidin-2-one (6) reacted with vinylphosphonates, vicinally-substituted (2) by electronwithdrawing groups (CO2Me, CN, COMe), to furnish [4+2] cycloadducts (3-4,7-10, and 11-14) in moderate to good yields (40-88%). The reactions were highly selective: regioselectivity of 95-100%, endoselectivity of 75-92% and facial selectivity of 80-95%. The major diastereoisomers were fully characterized by H-1 and C-13 NMR spectroscopy

Development of a Regioselective N-Methylation of (Benz)imidazoles Providing the More Sterically Hindered Isomer

An efficient and highly regioselective N-methylation of (NH)-(benz)imidazoles furnishing the sterically more hindered, less stable, and usually minor regioisomer has been developed. The methodology involves very mild reaction conditions and tolerates a wide range of functional groups.

Towards a General Understanding of Carbonyl-Stabilised Ammonium Ylide-Mediated Epoxidation Reactions

Abstract The key factors for carbonyl‐stabilised ammonium ylide‐mediated epoxidation reactions were systematically investigated by experimental and computational means and the hereby obtained energy profiles provide explanations for the observed experimental results. In addition, we were able to identify the first tertiary amine‐based chiral auxiliary that allows for high enantioselectivities and high yields for such epoxidation reactions.

Reactions of silyl-stabilised sulfur ylides with organoboranes: enantioselectivity, mechanism, and understanding

The reaction of trimethylsilyl-substituted sulfonium ylides with organoboranes (Ph(3)B, Et(3)B) has been studied and although homologated products were obtained in good yield (after oxidation to the corresponding alcohols), the enantiomeric excesses were low with our camphor-based chiral sulfide (up to 40% ee, cf. corresponding phenyl-substituted sulfonium ylides gave >95% ee). Cross-over experiments were conducted to ascertain the nature of this difference in selectivity. Thus, aryl- and silyl-substituted sulfonium ylides (1 equiv.) were (separately) reacted with Et(3)B (1.5 equiv.) followed by Ph(3)B (1.5 equiv.) The experiments were repeated changing the order of addition of the two boranes. It was found that the aryl-substituted sulfonium ylide only trapped the first borane that was added indicating that ate complex formation was non-reversible and so was the selectivity determining step. In contrast the silyl-substituted sulfonium ylide only trapped Ph(3)B (it is more reactive than Et(3)B) indicating that ate complex formation was reversible and so 1,2-migration was now the selectivity determining step. The reactions have been studied computationally and the experimental observations have been reproduced. They have further revealed that the cause of reversibility in the case of the silyl-substituted sulfonium ylides results from ate complex formation being less exothermic and a higher barrier to 1,2-migration.

Formation of four-membered heterocycles

This review focuses on new developments in [2 þ 2] cycloadditions leading to four-membered heterocycles (b-lactams, b-lactones, b-sultams, azetidines, etc.) which appeared between 1992 and 2011. A special attention is drawn to the mechanistic aspects of the reactions and their consequences on the stereochemical outcome of the cycloaddition processes. Accordingly, the subdivision in sections reflects this point of view: the entries are related to reactions of various reagents (ketenes, enolates, ynolates, isocyanates, etc.) with CQX bonds instead of a list of types of heterocycles. The accent is also put on asymmetric catalysis, which is of particular interest for the enantioselective synthesis of biologically active compounds or their precursors.