Didier Bourissou

Stable Noncyclic Singlet Carbenes

2.2.1. Diaminoand Aminohydrazinocarbenes 3338 2.2.2. Aminothioand Aminooxycarbenes 3340 2.3. Phosphinoaryland Phosphinoalkylcarbenes 3341 2.4. Aminoaryland Aminoalkylcarbenes 3344 2.5. Aminophosphonio and Aminosilylcarbenes 3346 2.6. Aminophosphinocarbenes 3347 3. Reactivity 3348 3.1. Typical Carbene Behavior 3348 3.1.1. Coupling Reactions 3348 3.1.2. Cycloaddition Reactions 3352 3.1.3. Insertion Reactions 3355 3.1.4. Migration Reactions 3358 3.2. Basic/Nucleophilic Behavior 3359 3.2.1. Protonation of Carbenes 3359 3.2.2. Reactions with Group 14 Electrophiles 3360 3.2.3. Reactions with Group 13 Lewis Acids 3360 3.2.4. Reactions with Group 15 Electrophiles 3361 3.3. Electrophilic Behavior 3362 3.3.1. Reaction with 1,5-Diazabicyclo[4.3.0]non-5-ene (DBN) 3362

Spontaneous Oxidative Addition of σ‐SiSi Bonds at Gold

The past decade has witnessed tremendous development in homogeneous gold catalysis. Thanks to unique soft and tunable Lewis acidity, gold(I) and gold(III) complexes have been shown to efficiently activate most types of p-bonded substrates. Besides these synthetic achievements, mechanistic issues have attracted considerable attention, and much progress has been accomplished recently thanks to the characterization of several key intermediates. With the aim of further extending the scope of gold catalysis, major interest has been devoted over the last few years to gold-mediated processes involving two-electron redox cycles. The isolobal analogy between Au(I/III) and Pd(0/II) complexes is stimulating the search for “gold versions” of palladium-catalyzed transformations, and in particular cross-couplings. Accordingly, a variety of intraand intermolecular coupling reactions have been shown to be efficiently catalyzed by gold complexes in the presence of strong exogeneous oxidants. A few studies have also suggested the ability of gold complexes to promote crosscouplings in a similar way to palladium catalysts, but possible palladium contamination has recently cast doubt on the involvement of gold complexes in these transformations. These recent contributions as a whole open a new facet of gold catalysis, and at the same time, raise fundamental questions about the properties and behavior of gold complexes. In particular, better knowledge on the basic elementary steps involved in redox cycles (oxidative addition, transmetallation, migratory insertion, b elimination, reductive elimination) is highly desirable to help further developments into this emerging area of gold catalysis. In this regard, it is striking to note that, despite the stability and availability of both Au and Au complexes, oxidative addition reactions at gold remain very elusive. In fact, the reluctance of gold to undergo oxidative addition is presumed to be an intrinsic limitation to achieve Au/Au redox cycles akin to those typically operating with Pd. Rare evidence for oxidative addition at gold dates back to the pioneering contribution of Kochi on the reaction of highly polarized sp C X bonds with alkyl gold(I) complexes. Furthermore, Bachman et al. reported in 2008 the first and to date only example of activation of an apolar s bond with gold. Fluorinated disulfides were shown to oxidatively add and reductively eliminate at gold in a reversible fashion. Seeking to gain more insight into s-bond activation and oxidative addition at gold, we became interested in extrapolating the strategy we have been developing for some years to study unusual metal–ligand interactions. Our approach consists in the use of anchoring phosphine sites, and this has been applied to support the coordination of Lewis acids and most recently s-Si Si bonds. Accordingly, the coordination of the diphosphine–disilane ligand 1 to copper and silver has been explored (Scheme 1), leading to the first structural characterization of a s complex with a coinage metal. As a further extension of this work, we were intrigued by the coordination of 1 to gold, considering three possible scenarios: 1) Formation of a s-complex upon side-on coordination of the disilane moiety, as observed with copper; 2) no participation of the s-Si Si bond to the coordination, as observed with silver; or 3) oxidative addition of the s-Si Si bond to give a bis(silyl)gold(III) complex. Oxidative addition

Transition‐Metal‐Mediated Cleavage of Fluoro‐Silanes under Mild Conditions

Si-F bond cleavage of fluoro-silanes was achieved by transition-metal complexes under mild and neutral conditions. The Iridium-hydride complex [Ir(H)(CO)(PPh3 )3 ] was found to readily break the Si-F bond of the diphosphine- difluorosilane {(o-Ph2 P)C6 H4 }2 Si(F)2 to afford a silyl complex [{[o-(iPh2 P)C6 H4 ]2 (F)Si}Ir(CO)(PPh3 )] and HF. Density functional theory calculations disclose a reaction mechanism in which a hypervalent silicon species with a dative Ir→Si interaction plays a crucial role. The Ir→Si interaction changes the character of the H on the Ir from hydridic to protic, and makes the F on Si more anionic, leading to the formation of H(δ+) ⋅⋅⋅F(δ-) interaction. Then the Si-F and Ir-H bonds are readily broken to afford the silyl complex and HF through σ-bond metathesis. Furthermore, the analogous rhodium complex [Rh(H)(CO)(PPh3 )3 ] was found to promote the cleavage of the Si-F bond of the triphosphine-monofluorosilane {(o-Ph2 P)C6 H4 }3 Si(F) even at ambient temperature.

PLGAs bearing carboxylated side chains: novel matrix formers with improved properties for controlled drug delivery.

Novel PLGA derivatives bearing carboxylated side chains have been synthesized and used to encapsulate the fragile drug apomorphine HCl with a solid-in-oil-in-water solvent extraction/evaporation method. Blends of d,l-lactide and l-3-(2-Benzyloxycarbonyl)Ethyl-1,4-Dioxane-2,5-dione (BED) were co-polymerized at different ratios via ring-opening using benzyl alcohol as initiator. Optionally, the ester groups in the side chains as well as the terminal ester groups were hydrogenolyzed (leading to free COOH groups). For reasons of comparison, different types of conventional PLGAs were also synthesized and used for apomorphine HCl encapsulation. The polymers and microparticles were thoroughly characterized using SEC, (1)H NMR, DSC, SEM, X-ray and laser diffraction, Headspace-GC as well as in vitro drug release measurements in flow-through cells and agitated flasks. Importantly, microparticles based on the new polymers bearing carboxylic groups in the polymeric side chains: (i) allowed a significant reduction of the amount of residual solvent (dichloromethane), and (ii) provided different types of drug release patterns compared to microparticles based on conventional PLGAs (at least partially due to altered polymer degradation kinetics). Thus, they offer an interesting potential as novel matrix formers in controlled drug delivery systems, overcoming potential shortcomings of standard PLGAs.

The Chemistry of Phosphinocarbenes

Publisher Summary Carbenes are electron-deficient, two-coordinate carbon compounds that have two nonbonding electrons at one carbon. This chapter reviews the theoretical aspect, synthesis, structural features, reactivity, and ligand properties of phosphinocarbenes. The (phosphino) (silyl)- and (phosphino) (phosphonio) carbenes are the only stable carbenes that feature a π-donor and a π-type-withdrawing substituent. They have a singlet ground state with a planar environment at phosphorus and a short phosphorus–carbon bond distance. However, because of the reluctance of phosphorus to keep this planar geometry, these “push-pull” carbenes behave in a manner very close to that of the most of the transient carbenes. Their lack of reactivity toward transition-metal centers is probably due to the excessive steric hindrance about the carbene center. They are excellent ligands for metal centers, and some of the complexes prepared show promising catalytic activity. Multinuclear nuclear magnetic resonance (NMR) spectroscopy is the most informative technique for analyzing the structure and bonding of phosphinocarbenes. The reactivity of phosphinocarbenes is strongly dependent on the nature of the other carbene substituent. A few complexes featuring a phosphinocarbene ligand in a number of different coordination modes (A, B, and C) are described.

CCDC 1849390: Experimental Crystal Structure Determination

Related Article: Charlie Blons, Sonia Mallet‐Ladeira, Abderrahmane Amgoune, Didier Bourissou|2018|Angew.Chem.,Int.Ed.|57|11732|doi:10.1002/anie.201807106