Controlled Catalytic Chain Transfer Polymerization of Isobutylene in the Presence of <i>tert</i>-Butanol as Exo-Enhancer

Catalytic chain transfer polymerization (CCTP) of isobutylene in the presence of alcohol as an exo-enhancer with <i>tert</i>-butyl chloride/ethylaluminum dichloride (EADC)·bis­(2-chloroethyl) ether (CEE) has been investigated in hexanes at 0 °C. Increasing exo-olefin content was observed with increasing steric bulkiness of the alkyl group of the alcohol, i.e., <i>tert</i>-butyl > isopropyl > methyl. Here, we report that <i>tert-</i>butanol (<i>t</i>-BuOH) is an excellent exo-enhancer compared to other <i>tert</i>-alcohols such as <i>tert-</i>amyl alcohol (AmOH), 2-methyl-2-pentanol (MPOH), and 3-ethyl-3-pentanol (EPOH). The aromatic <i>tert</i>-alcohol cumyl alcohol was not an exo-enhancer but acted as an initiator. In the reaction of EADC.CEE and <i>t</i>-BuOH, <i>t</i>-butoxy­aluminum dichloride (<i>t</i>-BuOAlCl₂) was formed, which is the real exo-enhancer and is not stable at room temperature. Molecular weights were virtually unchanged in the presence <i>t</i>-BuOAlCl₂ with [<i>t</i>-BuOAlCl₂]:[EADC.CEE] < 0.5, and exo-olefin content increased ∼15% relative to polymerization in the absence of <i>t</i>-BuOAlCl₂. This is presumably due to stabilization of the cation by <i>t</i>-BuOAlCl₂ which slows isomerization of the PIB⁺. Stabilization of the cation was confirmed by ¹H NMR and UV–vis spectroscopy at 0 °C by adding <i>t</i>-BuOAlCl₂ to the diphenylmethyl cation, a representative stable cation. The rate constant of chain transfer (<i>k</i>_tr) was determined to be 2 × 10⁸ L mol^–1 s^–1 at 0 °C, which is not affected by <i>t</i>-BuOAlCl₂. Addition of an exo-enhancer is especially important for polymerization at CSTR conditions at low steady state monomer concentrations. This is the first report identifying the role of alcohols in CCTP and opens new vistas in the synthesis of highly reactive polyisobutylene

α‑Dicationic Chelating Phosphines: Synthesis and Application to the Hydroarylation of Dienes

A series of new P^P-chelating ligands constituted by a dicationic −[P­(H₂Im)₂]⁺² unit (H₂Im = 1,3-dimethyl-4,5-dihydroimidazol-2-ylidene) and a −PPh₂ group connected through structurally different backbones have been synthesized. Evaluation of their reactivity toward different metal centers provides evidence that the dicationic fragment, otherwise reluctant to coordinate metals, readily participates in the formation of chelates when embedded into such a scaffold. Moreover, it significantly enhances the Lewis acidity of the metals to which it coordinates. This property has been used to develop a Rh catalyst that efficiently triggers the hydroarylation of dienes with electron-rich aromatic molecules. Kinetic studies and deuterium-labeling experiments, as well as density functional theory calculations, were performed in order to rationalize these findings

Reductive Elimination of C₆F₅–C₆F₅ from Pd(II) Complexes: Influence of α‑Dicationic Chelating Phosphines

We report the synthesis and characterization through NMR and X-ray techniques of a series of [Pd­(C₆F₅)₂(P^∧P′)] complexes constituted by diphosphine chelating ligands of different nature and evaluate the rates for the challenging reductive elimination of C₆F₅–C₆F₅. By virtue of their very weak donor properties, dicationic ancillary ligands effectively promote the desired transformation. Density functional theory (DFT) calculations were performed to rationalize these findings. The Pd(0)-complexes formed after the elimination step could not be isolated because the Pd(0) center has a tremendous tendency to insert into one of the P–C⁺ bonds of the α-cationic ligands rendering Pd­(II)-phosphinidene complexes. The same behavior was observed for Ni(0) species

Changing management direction in Campo-Ma’an

The hydrogenation of internal alkynes with [Cp*Ru]-based catalysts is distinguished by an unorthodox stereochemical course in that <i>E</i>-alkenes are formed by <i>trans</i>-delivery of the two H atoms of H₂. A combined experimental and computational study now provides a comprehensive mechanistic picture: a metallacyclopropene (η²-vinyl complex) is primarily formed, which either evolves into the <i>E</i>-alkene via a concerted process or reacts to give a half-sandwich ruthenium carbene; in this case, one of the C atoms of the starting alkyne is converted into a methylene group. This transformation represents a formal <i>gem</i>-hydrogenation of a π-bond, which has hardly any precedent. The barriers for <i>trans</i>-hydrogenation and <i>gem</i>-hydrogenation are similar: whereas DFT predicts a preference for <i>trans</i>-hydrogenation, CCSD­(T) finds <i>gem</i>-hydrogenation slightly more facile. The carbene, once formed, will bind a second H₂ molecule and evolve to the desired <i>E</i>-alkene, a positional alkene isomer or the corresponding alkane; this associative pathway explains why double bond isomerization and over-reduction compete with <i>trans</i>-hydrogenation. The computed scenario concurs with <i>para</i>-hydrogen-induced polarization transfer (PHIP) NMR data, which confirm direct <i>trans</i>-delivery of H₂, the formation of carbene intermediates by <i>gem</i>-hydrogenation, and their evolution into product and side products alike. Propargylic −OR (R = H, Me) groups exert a strong directing and stabilizing effect, such that several carbene intermediates could be isolated and characterized by X-ray diffraction. The gathered information spurred significant preparative advances: specifically, highly selective <i>trans</i>-hydrogenations of propargylic alcohols are reported, which are compatible with many other reducible functional groups. Moreover, the ability to generate metal carbenes by <i>gem</i>-hydrogenation paved the way for noncanonical hydrogenative cyclopropanations, ring expansions, and cycloadditions

Half-Sandwich Ruthenium Carbene Complexes Link <i>trans</i>-Hydrogenation and <i>gem</i>-Hydrogenation of Internal Alkynes

The hydrogenation of internal alkynes with [Cp*Ru]-based catalysts is distinguished by an unorthodox stereochemical course in that <i>E</i>-alkenes are formed by <i>trans</i>-delivery of the two H atoms of H₂. A combined experimental and computational study now provides a comprehensive mechanistic picture: a metallacyclopropene (η²-vinyl complex) is primarily formed, which either evolves into the <i>E</i>-alkene via a concerted process or reacts to give a half-sandwich ruthenium carbene; in this case, one of the C atoms of the starting alkyne is converted into a methylene group. This transformation represents a formal <i>gem</i>-hydrogenation of a π-bond, which has hardly any precedent. The barriers for <i>trans</i>-hydrogenation and <i>gem</i>-hydrogenation are similar: whereas DFT predicts a preference for <i>trans</i>-hydrogenation, CCSD­(T) finds <i>gem</i>-hydrogenation slightly more facile. The carbene, once formed, will bind a second H₂ molecule and evolve to the desired <i>E</i>-alkene, a positional alkene isomer or the corresponding alkane; this associative pathway explains why double bond isomerization and over-reduction compete with <i>trans</i>-hydrogenation. The computed scenario concurs with <i>para</i>-hydrogen-induced polarization transfer (PHIP) NMR data, which confirm direct <i>trans</i>-delivery of H₂, the formation of carbene intermediates by <i>gem</i>-hydrogenation, and their evolution into product and side products alike. Propargylic −OR (R = H, Me) groups exert a strong directing and stabilizing effect, such that several carbene intermediates could be isolated and characterized by X-ray diffraction. The gathered information spurred significant preparative advances: specifically, highly selective <i>trans</i>-hydrogenations of propargylic alcohols are reported, which are compatible with many other reducible functional groups. Moreover, the ability to generate metal carbenes by <i>gem</i>-hydrogenation paved the way for noncanonical hydrogenative cyclopropanations, ring expansions, and cycloadditions