Frédéric-Georges Fontaine

Metal-free catalytic C-H bond activation and borylation of heteroarenes

A metal-free catalyst born of frustration Boron (a Lewis acid) and nitrogen or phosphorus fragments (both Lewis bases) tend to pair up. Keeping them separated on opposite ends of the same molecule creates a “frustrated” Lewis pair. Such molecules can manifest powerful reactivity, such as scission of the hydrogen-hydrogen bond in H2. Légaré et al. now extend this reactivity to the cleavage of carbon-hydrogen bonds in heteroaromatic compounds such as furans and pyrroles (see the Perspective by Bose and Marder). Their frustrated Lewis pair complex catalyzed borylation of these compounds. The selectivity pattern of the reaction complemented that seen with the metal catalysts conventionally used. Science, this issue p. 513; see also p. 473 Boron and nitrogen centers cooperatively catalyze a reaction that has previously relied on transition metal catalysts. [Also see Perspective by Bose and Marder] Transition metal complexes are efficient catalysts for the C-H bond functionalization of heteroarenes to generate useful products for the pharmaceutical and agricultural industries. However, the costly need to remove potentially toxic trace metals from the end products has prompted great interest in developing metal-free catalysts that can mimic metallic systems. We demonstrated that the borane (1-TMP-2-BH2-C6H4)2 (TMP, 2,2,6,6-tetramethylpiperidine) can activate the C-H bonds of heteroarenes and catalyze the borylation of furans, pyrroles, and electron-rich thiophenes. The selectivities complement those observed with most transition metal catalysts reported for this transformation.

Reducing CO2 to methanol using frustrated Lewis pairs : on the mechanism of phosphine-borane mediated hydroboration of CO2

The full mechanism of the hydroboration of CO2 by the highly active ambiphilic organocatalyst 1-Bcat-2-PPh2-C6H4 (Bcat = catecholboryl) was determined using computational and experimental methods. The intramolecular Lewis pair was shown to be involved in every step of the stepwise reduction. In contrast to traditional frustrated Lewis pair systems, the lack of steric hindrance around the Lewis basic fragment allows activation of the reducing agent while moderate Lewis acidity/basicity at the active centers promotes catalysis by releasing the reduction products. Simultaneous activation of both the reducing agent and carbon dioxide is the key to efficient catalysis in every reduction step.

Transition‐Metal‐Free Catalytic Reduction of Carbon Dioxide

Metal-free systems, including frustrated Lewis pairs (FLPs) have been shown to bind CO2. By reducing the Lewis acidity and basicity of the ambiphilic system, it is possible to generate active catalysts for the deoxygenative hydroboration of carbon dioxide to methanol derivatives with conversion rates comparable to those of transition-metal-based catalysts.

[(IMes)2Pt(H)(ClBC5H4SiMe3)]: a Borabenzene–Platinum Adduct with an Unusual Pt-Cl-B Interaction†

Since the first report of a metallaboratrane by Hill et al. in 1999, there has been a quest for transition metal complexes containing a dative interaction with Group 13 Lewis acids. In a recent report, Braunschweig and co-workers demonstrated that [(PCy3)2Pt ] (Cy = cyclohexyl) interacts with alanes to form Lewis adducts [(PCy3)2Pt(AlX3)], [2] which are rare examples of well-characterized alane (M!AlR3) complexes. 4] However, the analogous reaction with haloboranes does not yield Lewis adducts, but instead forms platinum boryl complexes, a class of anionic boron-containing species of which numerous examples have been reported to date. The only compounds having an M!B dative interaction with boron are supported by ambiphilic ligands. The boron atom of bisor tris(methimazolyl)boranes coordinates selectively to the transition metal in metallaboratranes. In contrast, in addition to interacting with a transition metal, the Lewis acid moiety of phosphinoboranes prepared by Bourissou can interact with an anionic ligand within the coordination sphere, depending on the nature of the transition metal and the ambiphilic ligand. Such an interaction is seldom observed with Lewis acids, and is supported by ambiphilic ligands, with the exception of BF4 adducts and halocarboranes, in which the Lewis acidity of the boron atom can no longer be considered. Whereas borabenzene adducts are the subject of continuing interest, and in particular for their electronic properties, the coordination modes of these heterocyclic molecules with transition metals are quite limited. A common feature of borabenzenes and boratabenzenes, the analogue with anionic nucleophiles, is their h coordination by the aromatic ring in an analogous fashion to arene and cyclopentadienyl ligands. The exception is a phosphidoboratabenzene, which binds transition metals by the phosphine moiety. Using the efficient and general strategy of forming borabenzene organic adducts from boracyclohexadiene by the elimination of Me3SiCl, which was developed by Fu et al. (Scheme 1), [19]

Substantiating the Influence of Pore Surface Functionalities on the Stability of Grubbs Catalyst in Mesoporous SBA‐15 Silica

The influence of pore surface functionalities in mesoporous SBA-15 silica on the stability of a model olefin metathesis catalyst, namely Grubbs I, is substantiated. In particular, it is demonstrated that the nature of the interaction between the ruthenium complex and the surface is strongly depending on the presence of surface silanols. For this study, differently functionalized mesoporous SBA-15 silica materials were synthesized according to standard procedures and, subsequently, the Grubbs I catalyst was incorporated into these different host materials. All of the materials were thoroughly characterized by elemental analyses, nitrogen physisorption at -196 °C, thermogravimetric analyses, solid-state NMR spectroscopy, and infrared spectroscopy (ATR-IR). By such in-depth characterization of the materials, it became possible to achieve models for the surface/catalyst interactions as a function of surface functionalities in SBA-15; for example, in the case of purely siliceous silanol-rich SBA-15, octenyl-silane modified SBA-15, and silylated equivalents. It was evidenced that large portions of the chemisorbed species that are detected spectroscopically arise from interactions between the tricyclohexylphosphine and the surface silanols. A catalytic study using diethyldiallylmalonate in presence of the various functionalized silicas shows that the presence of surface silanols significantly decreases the longevity of the ring-closing metathesis catalyst, whereas the passivation of the surface by trimethylsilyl groups slows down the catalysis rate, but does not affect significantly the lifetime of the catalyst. This contribution thus provides new insights into the functionalization of SBA-15 materials and the role of surface interactions for the grafting of organometallic complexes.

Homogeneous asymmetric transfer hydrogenation of ketones using a ruthenium catalyst anchored on chitosan: natural chirality at work

A pivaloyl functionalized chitosan biopolymer was used as a polyligand for the [Ru(p-cymene)Cl2] moiety. The functionalized biopolymer containing a metal to saccharide ratio of 0.33 was used as a catalyst for the asymmetric transfer hydrogenation of ketones. The reactions gave yields up to 80% and enantiomeric excesses up to 72%, with the intrinsic chirality of chitosan being the only source of enantioselection.

Synthesis and solid-state characterization of platinum complexes with hexadentate amino- and iminophosphine ligands

Hexadentate ligands cis,cis-C(6)H(9)(N[double bond, length as m-dash]CHC(6)H(4)(PPh(2)))(3) () and cis,cis-C(6)H(9)(NHCH(2)C(6)H(4)(PPh(2)))(3) () were synthesized starting from cis,cis-1,3,5-triaminocyclohexane, and characterized using NMR spectroscopy and single-crystal X-ray diffraction. These ligands can bind both Pt(0) and Pt(II) metal centers using either or both of the soft phosphine moieties and the hard amine/imine moieties. In many cases the resulting complexes are negligibly soluble; hence, (31)P and (195)Pt solid-state NMR (SSNMR) spectroscopy was applied to analyse the bonding modes of the hexadentate ligands. The (195)Pt SSNMR spectroscopy of these complexes is particularly challenging, since (1)H-(195)Pt cross polarization is extremely inefficient, the (195)Pt longitudinal relaxation times are extremely long and the (195)Pt powder patterns are expected to be quite broad due to platinum chemical shift anisotropy. It is demonstrated that the ultra-wideline (195)Pt SSNMR spectra can be efficiently acquired with a combination of frequency-stepped piecewise acquisitions and cross-polarization/Carr-Purcell Meiboom-Gill (CP/CPMG) NMR experiments. The (195)Pt and (31)P SSNMR data are correlated to important structural features in both Pt(0) and Pt(II) species.

Spontaneous Reduction of a Hydroborane To Generate a B−B Single Bond by the Use of a Lewis Pair

The ansa-aminohydroborane 1-NMe2 -2-(BH2 )C6 H4 crystallizes in an unprecedented type of dimer containing a B-H bond activated by one FLP moiety. Upon mild heating and without the use of any catalyst, this molecule liberates one equivalent of hydrogen to generate a diborane molecule. The synthesis and structural characterization of these new compounds, as well as the kinetic monitoring of the reaction and the DFT investigation of its mechanism, are reported.

Selective recovery of rare earth elements using chelating ligands grafted on mesoporous surfaces

Nowadays, rare earth elements (REEs) and their compounds are critical for the rapidly growing advanced technology sectors and clean energy demands. However, their separation and purification still remain challenging. Among different extracting agents used for REE separation, the diglycolamide (DGA)-based materials have attracted increasing attention as one of the most effective extracting agents. In this contribution, a series of new and element-selective sorbents were generated through derivatisation of the diglycolamide ligand (DGA), grafted to mesoporous silica and tested for the separation of rare earth elements. It is shown that, by tuning the ligand bite angle and its environment, it is possible to improve the selectivity towards specific rare earth elements.

Ambiphilic molecules for trapping reactive intermediates: interrupted Nazarov reaction of allenyl vinyl ketones with Me2PCH2AlMe2

The addition of the ambiphilic molecule Me(2)AlCH(2)PMe(2) (1) to the allenyl vinyl ketone 2 gave a trapped Nazarov reaction product. Under kinetic control, the addition of the phosphine was on the methylated carbon, contrary to expected steric and electronic considerations. Computational data pointed to hydrogen bonding between the phosphine and the methyl group guiding the regiochemistry of this reaction. This product rearranged to provide the expected, regioisomeric Nazarov product. With additional 1 this compound yielded a Michael-addition product via a retro-Nazarov process.