<i>O</i>‑Carboxyanhydrides: Useful Tools for the Preparation of Well-Defined Functionalized Polyesters

Over the last ten years, <i>O</i>-carboxyanhydrides (OCA) have attracted increasing attention as ring-opening polymerization (ROP) monomers. They are readily available from α-hydroxyacids and are significantly more reactive than 1,4-dioxane-2,5-diones. Thus, softer catalysts and milder reaction conditions can be used, allowing for a better control of the polymerization. Most attractive are the functionalized OCA that enable the introduction of functional groups along the polyester backbone and thereby vary and finely tune their physicochemical properties. In this viewpoint, the achievements made over the last years are critically overviewed. Particular attention is paid to the different catalytic approaches that have been reported for the ROP of these heterocycles and to the comparison with lactide ROP. In addition, the most representative examples of functionalized polyesters and polymer conjugates prepared from OCA are discussed

Coordination of Phosphinoboranes R₂PB(C₆F₅)₂ to Platinum: An Alkene-Type Behavior

The paucity of boron-containing heteroalkene complexes prompted us to explore the coordination of phosphinoboranes. The complexes {[R₂PB­(C₆F₅)₂]­Pt­(PPh₃)₂} (R = Cy, <i>t</i>-Bu) were obtained by ethylene displacement. Spectroscopic and crystallographic data indicated symmetric side-on coordination of the phosphinoborane to Pt. Thorough analysis of the bonding situation by computational means revealed important similarities but also significant differences between the phosphinoborane and ethylene complexes

Coordination of Phosphinoboranes R₂PB(C₆F₅)₂ to Platinum: An Alkene-Type Behavior

The paucity of boron-containing heteroalkene complexes prompted us to explore the coordination of phosphinoboranes. The complexes {[R₂PB­(C₆F₅)₂]­Pt­(PPh₃)₂} (R = Cy, <i>t</i>-Bu) were obtained by ethylene displacement. Spectroscopic and crystallographic data indicated symmetric side-on coordination of the phosphinoborane to Pt. Thorough analysis of the bonding situation by computational means revealed important similarities but also significant differences between the phosphinoborane and ethylene complexes

Coordination of Phosphinoboranes R₂PB(C₆F₅)₂ to Platinum: An Alkene-Type Behavior

The paucity of boron-containing heteroalkene complexes prompted us to explore the coordination of phosphinoboranes. The complexes {[R₂PB­(C₆F₅)₂]­Pt­(PPh₃)₂} (R = Cy, <i>t</i>-Bu) were obtained by ethylene displacement. Spectroscopic and crystallographic data indicated symmetric side-on coordination of the phosphinoborane to Pt. Thorough analysis of the bonding situation by computational means revealed important similarities but also significant differences between the phosphinoborane and ethylene complexes

Evaluation of the σ‑Donation from Group 11 Metals (Cu, Ag, Au) to Silane, Germane, and Stannane Based on the Experimental/Theoretical Systematic Approach

Reactions of group 11 metal chlorides (CuCl, AgCl, AuCl) with {(<i>o</i>-Ph₂P)­C₆H₄}₃E­(F) (E = Si (<b>1</b>), Ge (<b>2</b>), Sn (<b>3</b>)) provide a complete series of metallasilatrane [{(<i>o</i>-Ph₂P)­C₆H₄}₃(F)­Si]­MCl (E = Cu (<b>4</b>), Ag (<b>5</b>), Au (<b>6</b>)), metallagermatrane [{(<i>o</i>-Ph₂P)­C₆H₄}₃(F)­Ge]­MCl (E = Cu (<b>7</b>), Ag (<b>8</b>), Au (<b>9</b>)), and metallastannatrane [{(<i>o</i>-Ph₂P)­C₆H₄}₃(F)­Sn]­MCl (E = Cu (<b>10</b>), Ag (<b>11</b>), Au (<b>12</b>)) complexes. Structural analyses clearly show the presence of M→E interactions in all of these complexes and establish the presence of periodicity; heavier group 14 elements E act as stronger electron acceptor ligands, and heavier group 11 metals exhibit higher donor ability toward ER₄. Density functional theory calculations fully support these trends and suggest that σ-acceptor ability of saturated (four-coordinate) heavier group 14 element compounds toward group 11 metals is related to σ*­(E–F) molecular orbital levels, which mainly depend on the deviation of the geometry around E from tetrahedral geometry to trigonal bipyramidal

Synthesis, Geometry, and Bonding Nature of Heptacoordinate Compounds of Silicon and Germanium Featuring Three Phosphine Donors

Structural studies were performed on heptacoordinate compounds of silicon {(<i>o</i>-Ph₂P)­C₆H₄}₃SiX (X = F (<b>1</b>), Cl (<b>3</b>), H (<b>5</b>)) and germanium {(<i>o</i>-Ph₂P)­C₆H₄}₃GeX (X = F (<b>2</b>), Cl (<b>4</b>), H (<b>6</b>), Me (<b>7</b>)) compounds featuring three phosphine donors. We found that <b>5</b>, <b>6</b>, and <b>7</b> have approximately a <i>C</i>₃ symmetry similar to Corriu’s compounds (heptacoordinate silane {(<i>o</i>-Me₂NCH₂)­C₆H₄}₃SiX (X = F (<b>8</b>), H) and germane {(<i>o</i>-Me₂NCH₂)­C₆H₄}₃GeX (X = H, F) with three nitrogen donors coordinating to the central Si/Ge <i>trans</i> to the <i>C</i>_<i>ipso</i> atoms). In contrast, the Si compounds <b>1</b> and <b>3</b> and the Ge compounds <b>2</b> and <b>4</b> have novel heptacoordinate geometries; the incorporation of such electronegative substituents as F and Cl results in the change of one phosphine donor from the position <i>trans</i> to the <i>C</i>_<i>ipso</i> atom to that <i>trans</i> to the X atom. Compounds <b>1</b>–<b>4</b> retain this unprecedented geometry in solution but show dynamic behavior. The structural modification observed upon changing the substituent at Si and Ge is rationalized by electrostatic and charge transfer interactions

Gold-Mediated Insertion of Oxygen into Silicon–Silicon Bond: An Original Au(I)/Au(III) Redox Sequence

The diphosphine–disilane <i>i</i>Pr₂(<i>o</i>-C₆H₄)­SiMe₂SiMe₂(<i>o</i>-C₆H₄)­P<i>i</i>Pr₂ reacts with AuCl­(SMe₂) via coordination of the two phosphines and oxidative addition of the σ-Si–Si bond. The ensuing bis­(silyl) gold­(III) complex has been unequivocally authenticated by NMR spectroscopy at −60 °C. Upon heating, it evolves cleanly to give a disiloxane gold­(I) complex that has been fully characterized, including by X-ray diffraction analysis. Oxidation of the disilane proceeds via an original Au­(I)/Au­(III) redox sequence. According to ¹⁸O labeling experiments, both water and dioxygen are competent oxygen sources. Oxidative addition of the σ-Si–Si bond to form a bis­(silyl) gold­(III) complex seems to be a prerequisite for the disilane → disiloxane conversion to occur

Ligand-Enabled Oxidative Fluorination of Gold(I) and Light-Induced Aryl–F Coupling at Gold(III)

MeDalphos Au(I) complexes featuring aryl, alkynyl, and alkyl groups readily react with electrophilic fluorinating reagents such as N-fluorobenzenesulfonimide and Selectfluor. The ensuing [(MeDalphos)Au(R)F]+ complexes have been isolated and characterized by multinuclear NMR spectroscopy as well as X-ray diffraction. They adopt a square-planar contra-thermodynamic structure, with F trans to N. DFT/IBO calculations show that the N lone pair of MeDalphos assists and directs the transfer of F+ to gold. The [(MeDalphos)Au(Ar)F]+ (Ar = Mes, 2,6-F2Ph) complexes smoothly engage in C–C cross-coupling with PhCCSiMe3 and Me3SiCN, providing direct evidence for the oxidative fluorination/transmetalation/reductive elimination sequence proposed for F+-promoted gold-catalyzed transformations. Moreover, direct reductive elimination to forge a C–F bond at Au(III) was explored and substantiated. Thermal means proved unsuccessful, leading mostly to decomposition, but irradiation with UV–visible light enabled efficient promotion of aryl–F coupling (up to 90% yield). The light-induced reductive elimination proceeds under mild conditions; it works even with the electron-deprived 2,6-difluorophenyl group, and it is not limited to the contra-thermodynamic form of the aryl Au(III) fluoride complexes

Gold-Mediated Insertion of Oxygen into Silicon–Silicon Bond: An Original Au(I)/Au(III) Redox Sequence

The diphosphine–disilane <i>i</i>Pr₂(<i>o</i>-C₆H₄)­SiMe₂SiMe₂(<i>o</i>-C₆H₄)­P<i>i</i>Pr₂ reacts with AuCl­(SMe₂) via coordination of the two phosphines and oxidative addition of the σ-Si–Si bond. The ensuing bis­(silyl) gold­(III) complex has been unequivocally authenticated by NMR spectroscopy at −60 °C. Upon heating, it evolves cleanly to give a disiloxane gold­(I) complex that has been fully characterized, including by X-ray diffraction analysis. Oxidation of the disilane proceeds via an original Au­(I)/Au­(III) redox sequence. According to ¹⁸O labeling experiments, both water and dioxygen are competent oxygen sources. Oxidative addition of the σ-Si–Si bond to form a bis­(silyl) gold­(III) complex seems to be a prerequisite for the disilane → disiloxane conversion to occur

Mild and Efficient Preparation of Block and Gradient Copolymers by Methanesulfonic Acid Catalyzed Ring-Opening Polymerization of Caprolactone and Trimethylene Carbonate

Polycaprolactone/polytrimethylene carbonate copolymers of different microstructures have been prepared in toluene solution under mild conditions by controlled ring-opening polymerization of ε-caprolactone and trimethylene carbonate with methanesulfonic acid as catalyst. Sequential addition of the monomers led to the formation of well-defined di- and tri-block copolymers, demonstrating the ability of the catalytic system to cross-propagate. Simultaneous copolymerization yielded gradient copolymers as a result of the different copolymerization reactivity ratios and absence of undesirable redistribution reactions. DSC analyses showed a noticeable impact of the copolymer microstructure on the thermal properties