Trans-Concerted Addition to Alkynes: the case of Ynamide Silylzincation.

An original concerted antarafacial mechanism for the addition of diorganosilyl-zinc reagents across the C-C triple bond in ynamides is computationally investigated using DFT calculations. This concerted mechanism, leading to a trans-product in only one step, results in the formation of a Si-C and a Zn-C σ-bonds on opposite sides of the π-system. We demonstrate that the mechanism going through a η2-vinyl intermediate and the proposal of a radical chain pathway are energetically unsustainable. The retained concerted antarafacial pathway is tested on experimental selectivities: the regioselectivity, in favor of the silyl ß-addition in ynamide, and stereoselectivity, which is cis- with (Me2PhSi)2Zn but trans- with [(Me3Si)3Si]2Zn, are well reproduced by DFT calculations. The regio- and stereoselectivity are discussed using the activation strain model and a chemical bonding analysis.

Dynamic Kinetic Resolution Processes Based on the Switchable Configurational Instability of Allenyl Copper Reagents.

The configurational instability of allenyl copper reagents is unveiled. An experimental study highlights the crucial role of Li+ and of the reaction temperature in the control of the configurational stability of allenyl copper reagents. A judicious choice of the reaction conditions allows efficient dynamic kinetic resolution processes and gives a one-pot access to homopropargylic alcohols or amines bearing up to four contiguous defined stereogenic centers.

Intertwined Analytical, Experimental and Theoretical Studies on the Formation and Structure of a Copper Dienolate.

The reaction of a silyl dienolate, a Cu(II) salt and TBAT yielding the corresponding copper dienolate is addressed. A combined NMR and cyclic voltammetry analysis first highlight the role of TBAT in the Cu(II) to Cu(I) reduction and the structure of the precatalytic species. From these first results a second set of NMR and theoretical studies enable the determination of the structure and the mechanism of formation of the copper dienolate catalytic species. Finally, we showed that that the copper catalyst promote the E/Z s-cis/s-trans equilibration of the silyl dienolate precursor through a copper dienolate intermediate. All of these results unveil some peculiarities of the catalytic and asymmetric vinylogous Mukaiyama reaction.

Triggering Electron Transfer in Co(I) Dimers: Computational Evidences for a Reversible Disproportionation Mechanism.

An inner-sphere disproportionation mechanism of the Co(I) precursor CoCl(PPh3 )3 is described through a Density Functional Theory study. The essential role of oleylamine in this process is unravelled. A detailed analysis of the electronic structure of Cobalt dimers of the general formula Co2 Cl2 Ln (L=NH3 and PH3 ) demonstrates that electron transfer is triggered by asymetric coordination of amine and phosphine to stabilize a mixed-valence Co(II)-Co(0) dimer. This is consistent with the HSAB statement that both amine and phosphine ligands are required to stabilize the reaction products, respectively Co(II) and Co(0) centers. We propose a quasi-athermic multi-step disproportionation mechanism with low activation barriers where the electron transfer goes through simple ligand exchanges between Co.

Ligand effects on coordination properties of organolithium compounds: insights from computational experiments on a "weakened" Li.

A simple numerical experiment is presented which allows tuning the lithium electrophilicity, a parameter strongly affected by the solvent and additives coordination. A series of coordination of Li+ to carbanions or polydentate nucleophiles is examined showing the potential and the limits of this approach. The results suggest that such a simple trick can be remarkably helpful to model and decipher the effects of solvation on the structure and properties of lithiated organometallic species.

Dearomatization of 3-Nitroindoles with Highly γ-Functionalized Allenoates in Formal (3+2) Cycloadditions.

3-Nitroindoles are easily reacted with highly substituted γ-allenoates in the presence of a commercially available phosphine catalyst. For instance, allenoates derived from biomolecules such as amino and deoxycholic acids are combined for the first time with 3-nitroindole. The corresponding dearomatized (3+2) tricyclic cycloadducts are obtained as α-regioisomers exclusively. DFT computations shed light on this multi-step reaction mechanism and on the selectivities observed in the sequence.

Mechanism of Enolate Transfer between Si and Cu.

Exchange of X (F, Cl, OMe) and a substituted enolate chain between SiMe3 and various CuI complexes was examined. Reaction mechanisms pass through a cyclic transition state in which the reaction coordinate is associated with rotation of the SiMe3 moiety. The dependence of the thermodynamic and kinetic features on the nature of the active and ancillary ligands was examined. Formation of copper enolate is shown to be favored when stabilized enolates are used. Replacement of F by Cl reverses the preference of the reaction. This is associated with the small difference between the Cu-Cl and Si-Cl bond energies, in contrast to other Si-X bonds, which are systematically stronger than their Cu-X analogues.

Vinyl (Thio)Ethers versus Enamines: A DFT Insight into Their Divergent Reactivities toward Nitroalkenes.

Despite their apparent similarities, vinyl (thio)ethers and enamines display divergent reactivities toward nitroalkenes. Whereas [4+2] cycloadduct derivatives are generally obtained in the first case, a variety of adducts are observed in the second, and depend on the substrates and reaction conditions. Herein, a rationalization of this versatility is proposed through a theoretical study, in which the interactions between these electron-rich alkenes and nitroethylene are compared in conjugate addition, [4+2] and [2+2] cycloadditions, and ene reaction processes. The conjugate zwitterionic adduct is shown to play a key role. With vinyl (thio)ethers, its formation is disfavored, whereas, for enamines, it appears as a stabilized intermediate, leading to the thermodynamically favored formal ene adduct. This adduct is, however, kinetically disfavored, which opens up the way to control the selectivity of the whole process.

Asymmetric Sequential Cu-Catalyzed 1,6/1,4-Conjugate Additions of Hard Nucleophiles to Cyclic Dienones: Determination of Absolute Configurations and Origins of Enantioselectivity.

The first stereocontrolled Cu-catalyzed sequential 1,6/1,4-asymmetric conjugate addition (ACA) of C-metalated hard nucleophiles to cyclic dienones is reported. The use of DiPPAM (diphenylphosphinoazomethinylate) followed by a phosphoramidite as the stereoinducing ligands facilitated both high ee values for the 1,6-ACA and high de values for the 1,4-ACA reaction components, which thus gave enantioenriched 1,3-dialkylated moieties. The absolute configurations were determined by using vibrational circular dichroism (VCD) and optical rotatory dispersion (ORD) spectroscopy, in combination with DFT calculations and X-ray analysis. Interestingly, DFT calculations for the mechanism of enantioselective 1,6-addition by using an unprecedented Cu-Zn bimetallic catalytic system confirmed this attribution. Lastly, exploring intramolecular cyclization avenues for enantioenriched 1,3-dialkylated products provided access to the challenging drimane skeleton.

Selectivities of multicomponent [4 + 2]/[3 + 2] cycloadditions of 3-nitroindole with substituted alkenes: a DFT analysis.

The chemo-, regio-, and stereoselectivities of multicomponent [4 + 2]/[3 + 2] domino cycloaddition reactions involving nitroindole derivatives with vinylethers and acrylates are studied computationnally and compared to experimental results. In this process, the nitroarene first reacts as an electron-deficient heterodiene with the electron-rich alkene following an inverse electron-demand [4 + 2] process, leading to a nitronate intermediate in a fully selective way. This intermediate exclusively interacts, in a second step, with the electron-deficient alkene and undergoes a chemo- and regioselective [3 + 2] cycloaddition. The density functional theory calculations reported in this Article fully account for the selectivities observed experimentally. Electronic displacements along the reaction path are examined using a topological analysis of the electron-localization function (ELF). The first [4 + 2] reaction follows a classical concerted, although asynchronous process, which is reliably described by the frontier molecular-orbital (FMO) model. In contrast, the electronic displacements observed during the second [3 + 2] step are unexpected, involving an electron donation by the electron-deficient reaction partner.

Stereoselective and catalytic access to ß-enaminones: an entry to pyrimidines.

We describe herein a highly stereoselective access to Cbz-protected ß-enaminones 2 based on the NaOH catalyzed rearrangement of propargylic hydroxylamines 1. The synthetic potential of these ß-enaminones is illustrated in an original synthesis of pyrimidines.

Revisiting the holo- and hemidirected structural transition within the [Pb(CO)n]2+ model series using first-principles molecular dynamics.

In this article, we resort to first-principles molecular dynamic simulations to examine the thermal effects on the structure of [Pb(CO)(n)](2+) complexes. Values of n are chosen to sample structures where hemidirected (n = 2, 4 and 6) or holodirected (n = 7 and 8) structures are found when using static approaches. In all cases, highly flexible structures are observed. In particular, hemidirectional distortions are characterized using geometrical and topological analysis. The octacarbonyl complex exhibits the decoordination of one of its carbonyl ligands at 300 K.

Diastereodivergent behavior of alkyl versus cyano allenylcuprates toward aldehydes: a key role for lithium.

The stereodivergent behavior of allenyl(cyano)- and allenyl(alkyl)cuprates toward aldehydes, providing a selective preparation of both syn- and anti-homopropargylic alcohols, is described. This study, which combines both experimental and theoretical support, shows that the copper nontransferred "dummy ligand" controls the localization of the lithium cation with respect to the allenylcuprate moiety. As a consequence, Li(+) acts as a Lewis acid activator but also controls the diastereoselectivity during the addition of allenylcuprates onto aldehydes. The combined high selectivity, efficiency, and versatility of these cuprate compounds opens the way to new one-pot synthetic procedures, as illustrated by the combined Klein rearrangement/transmetalation methodology described herein.

Ph2P(BH3)Li: from ditopicity to dual reactivity.

A multinuclear NMR study shows that the deprotonation of diphenylphosphine-borane by n-BuLi in THF leads to a disolvated lithium phosphido-borane Ph(2)P(BH(3))Li of which Li(+) is connected to the hydrides on the boron and two THF molecules rather than to the phosphorus. This entity behaves as both a phosphination and a reducing agent, depending on the kinetic or thermodynamic control imposed to the reaction medium. Density functional theory computations show that H(2)P(BH(3))Li exhibits a ditopic character (the lithium cation can be in the vicinity of the hydride or of the phosphorus). It explains its dual reactivity (H- or P-addition), both routes going through somewhat similar six-membered transition states with low activation barriers.

Spin-driven activation of dioxygen in various metalloenzymes and their inspired models.

Although potentially powerful, molecular oxygen is an inert oxidant due to the triplet nature of its ground state. Therefore, many enzymesse various metal cations (M) to produce singlet active species M(n) O(2) . In this communication we investigate the topology of the Electron Localization Function (ELF) within five biomimetic complexes which are representative of the strategies followed by metalloenzymes to activate O(2) . Thanks to its coupling to the constrained DFT methods the ELF analysis reveals the tight connection between the spin state of the adduct and the spatial organization of the oxygen lone pairs. We suggest that enzymes could resort to spin state control to tune the regioselectivity of substrate oxidations.

MeLi + LiCl in THF: one heterodimer and no tetramers.

The structure of the aggregates formed when mixing methyllithium and lithium chloride in THF has been studied by multinuclear magnetic resonance at 170 K. The data suggest that only one new entity is observed, that is the dimer [(MeLi)(LiCl)], in equilibrium (K approximately 0.6) with [MeLi](4) and [LiCl](2). NMR diffusion measurements lead to the conclusion that this dimer is trisolvated in THF at 170 K, a solvation scheme in agreement with DFT computations.

Dioxygen activation by mononuclear copper enzymes: insights from a tripodal ligand mimicking their Cu(M) coordination sphere.

A mononuclear cuprous complex is proposed as a novel in silico model for the Cu(M) active site of noncoupled copper monooxygenases. To the best of our knowledge, it is one of the first biomimicking models that allows one to recover the intimate structural features of the enzymatic oxygenated adducts and to gain clear-cut insights relevant to dioxygen activation by these enzymes.

Shuffling lithiated mixed aggregates: NMR and Car-Parrinello molecular dynamics reveal an unexpected associative pathway.

The exchange of Me(6)Li aggregated to a lithium amide by (7)LiCl leads to a specific isotope distribution whose microscopic origin is assigned to an edge-to-edge interaction between the R(2)NLi-MeLi aggregate and (LiCl)(2) by NMR and Car-Parrinello molecular dynamics.

Benzofurans as efficient dienophiles in normal electron demand [4 + 2] cycloadditions.

Dearomatization of electron-poor benzofurans is possible through involvement of the aromatic 2,3-carbon-carbon double bond as dienophile in normal electron demand [4 + 2] cycloadditions. The tricyclic heterocycles thereby produced bear a quaternary center at the cis ring junction, a feature of many alkaloids such as morphine, galanthamine, or lunaridine. The products arising from the reaction have been shown to depend on different factors among which the type of the electron-withdrawing substituent of the benzofuran, the nature of the reacting diene, and the method of activation. In the presence of all-carbon dienes, the reaction yields the expected Diels-Alder adducts. When thermal activation is insufficient, a biactivation associating zinc chloride catalysis and high pressure is required to generate the cycloadducts in good yields and high stereoselectivities, for instance, when cyclohexadiene is involved in the process. The use of more functionalized dienes, such as those bearing alkoxy or silyloxy substituents, also shows the limits of the thermal activation, and hyperbaric conditions are, in this case, well-suited. The involvement of Danishefsky's diene induces a competition in the site of reactivity. The aromatic 2,3-carbon-carbon double bond is unambiguously the most reactive dienophile, and the 3-carbonyl unit becomes a competitive site of reactivity with benzofurans bearing substituents prone to heterocyloaddition, in particular under Lewis acid activation. The sequential involvement of both the aromatic double bond and the carbonyl moiety as dienophiles is then possible by using an excess of diene under high-pressure activation. In line with the experimental results, DFT computations suggest that the Diels-Alder process involving the aromatic double bond is preferred over the hetero-Diels-Alder route through an asynchronous concerted transition state. However, Lewis acid catalysis appears to favor the heterocycloaddition pathway through a stepwise mechanism in some cases.