Mark Botoshansky

Oxygen Atom Transfer Reactions from Isolated (Oxo)manganese(V) Corroles to Sulfides

A series of five free-base corroles were metalated and brominated to form 10 manganese(III) corroles. Two of the free-base corroles and six manganese(III) corroles were analyzed by X-ray crystallography, including one complex that may be considered a transition-state analogue of oxygen atom transfer (OAT) from (oxo)manganese(V) to thioansisole. Oxidation by ozone allowed for isolation of the 10 corresponding (oxo)manganese(V) corroles, whose characterization by (1)H and (19)F NMR spectroscopy and electrochemistry revealed a low-spin and triply bound manganese-oxygen moiety. Mechanistic insight was obtained by investigating their reactivity regarding stoichiometric OAT to a series of p-thioanisoles, revealing a magnitude difference on the order of 5 between the β-pyrrole brominated (oxo)manganese(V) corroles relative to the nonbrominated analogues. The main conclusion is that the (oxo)manganese(V) corroles are legitimate OAT agents under conditions where proposed oxidant-coordinated reaction intermediates are irrelevant. Large negative Hammett ρ constants are obtained for the more reactive (oxo)manganese(V) corroles, consistent with expectation for such electrophilic species. The least reactive complexes display very little selectivity to the electron-richness of the sulfides, as well as a non-first-order dependence on the concentration of (oxo)manganese(V) corrole. This suggests that disproportionation of the original (oxo)manganese(V) corrole to (oxo)manganese(IV) and (oxo)manganese(VI) corroles, followed by substrate oxidation by the latter complex, gains importance when the direct OAT process becomes progressively less favorable.

Forming all-carbon quaternary stereogenic centres in acyclic systems from alkynes

The formation of all-carbon quaternary stereocentres in acyclic systems is one of the most difficult contemporary challenges in modern synthetic organic chemistry. Particularly challenging is the preparation of all-carbon quaternary stereocentres in aldol adducts; this difficulty is problematic because the aldol reaction represents one of the most valuable chemical transformations in organic synthesis. The main problem that limits the formation of these stereocentres is the absence of an efficient method of preparing stereodefined trisubstituted enolates in acyclic systems. Here we describe a different approach that involves the formation of two new stereogenic centres—including the all-carbon quaternary one—via a combined carbometalation–oxidation reaction of an organocuprate to give a stereodefined trisubstituted enolate. We use this method to generate a series of aldol and Mannich products from ynamides with excellent diastereomeric and enantiomeric ratios and moderate yields.

Cobalt Corrole Catalyst for Efficient Hydrogen Evolution Reaction from H2O under Ambient Conditions: Reactivity, Spectroscopy, and Density Functional Theory Calculations

The feasibility of a hydrogen-based economy relies very much on the availability of catalysts for the hydrogen evolution reaction (HER) that are not based on Pt or other noble elements. Significant breakthroughs have been achieved with certain first row transition metal complexes in terms of low overpotentials and large turnover rates, but the majority of reported work utilized purified and deoxygenated solvents (most commonly mixtures of organic solvents/acids). Realizing that the design of earth abundant metal catalysts that operate under truly ambient conditions remains an unresolved challenge, we have now developed an electronically tuned Co(III) corrole that can catalyze the HER from aqueous sulfuric acid at as low as -0.3 V vs NHE, with a turnover frequency of 600 s(-1) and ≫10(7) catalytic turnovers. Under aerobic conditions, using H2O from naturally available sources without any pretreatment, the same complex catalyzes the reduction of H(+) with a Faradaic Yield (FY) of 52%. Density functional theory (DFT) calculations indicate that the electron density on a putative hydride species is delocalized off from the H atom into the macrocycle. This makes the protonation of a [Co(III)-H](-) species the rate determining step (rds) for the HER consistent with the experimental data.

Pincer Click Ligands

Tridentate pincer-type ligands of the general form DCD (where D and D are groups containing coordinating atoms) have been used to spectacular effect in coordination, mechanistic, synthetic, and supramolecular chemistry, as well as in nanoscience and in the development of sensors and molecular switches. Most significantly, the realization that pincer ligands offer both a unique, highly protective environment for the coordinated metal center and opportunities to finetune the steric and electronic properties of the metal atom has generated extensive research into the use of these complexes as catalysts. As a result, many important and challenging catalytic processes based on such systems have been developed. It is generally accepted that the reactivity, selectivity, and catalytic performance of pincer-based systems rely to a great extent on the characteristics of the donor groups D in the carefully selected ligand. The optimization of tailor-made catalysts involves extensive experimental investigation, in which the laborious synthesis of the ligands is often a serious bottleneck. In particular, the synthesis of nonsymmetrically substituted DCD ligands (D and D are different groups) represents a considerable challenge, as their preparation usually includes a series of steps and separations that commonly result in low yields. Consequently, the development of efficient and powerful methods for the rapid synthesis of a wide variety of tailormade ligands is of high importance. Although several methods for the preparation of bidentate ligand libraries have been reported, a strategy for the building of a tridentate ligand library is, to the best of our knowledge, still unknown. Here, we report a conceptually new general approach for the efficient and facile preparation of a novel family of tridentate pincer ligands of the DCD type. The tridentate mode of coordination was shown by the preparation and structural characterization of transition-metal complexes of these new ligands. Palladium complexes of this readily prepared set of representative ligands proved to be highly efficient catalysts in the Heck reaction. Traditionally, pincer ligands are prepared by attaching donor atoms to a ligand backbone. We developed an entirely different synthetic route that allowed access to a broad range of tailor-made pincer ligands. In designing our approach, we considered a methodology which would allow the selective and facile incorporation of two complementary monomeric donor groups D and D by covalent assembly to afford a pincer-type system DCD. The resulting molecule must also have a potential carbanion between the donor groups so as to bind the metal center in a pincer-type mode (Scheme 1a).

Extended Calixpyrroles: meso-Substituted Calix[6]pyrroles

A 1,3,5-alternate conformation of the pyrrole rings is adopted by the calix[6]pyrrole 1 in the crystal (see picture). Compound 1 was synthesized in a two-step process and although it crystallizes in the form of the adduct 1⋅3CH3 COCH3 ⋅H2 O⋅CHCl3 , there are no solvent molecules in the cavity (cross section ca. 60Å2 ) of this macrocycle which possesses alternating dimethyl and diphenyl substitutents at the meso positions.

Iodinated aluminum(III) corroles with long-lived triplet excited states.

The first reported iodination of a corrole leads to selective functionalization of the four C-H bonds on one pole of the macrocycle. An aluminum(III) complex of the tetraiodinated corrole, which exhibits red fluorescence, possesses a long-lived triplet excited state.

Nitrenium ions as ligands for transition metals

Unlike N-heterocyclic carbenes (NHCs), which are now used ubiquitously in metal-based chemistry, the nitrogen-derived analogue (in which a carbon is replaced with the isoelectronic nitrogen cation, a nitrenium ion) has remained elusive as a ligand for metals. This is especially intriguing, because several other main-group analogues of NHCs have been prepared, and have been shown to coordinate with transition-metal complexes. Here, we describe the preparation of several N-heterocyclic nitrenium ions that are isoelectronic and isostructural to NHCs, and study their ligand properties. The formation of relatively strong nitrenium-metal bonds is unambiguously confirmed, in solution by selective (15)N-labelling experiments, and in the solid state by X-ray crystallography. Experimental and computational studies of the electronic properties of this novel type of ligand suggest that they are poor σ-donors and good π-acceptors.

Mono(imidazolin-2-iminato) Titanium Complexes for Ethylene Polymerization at Low Amounts of Methylaluminoxane

The polymerization of ethylene with titanium complexes bearing one bulky imidazolin-2-iminato ligand (L) in the presence of MAO and/or TTPB as cocatalysts have been explored. The complex LTiCl3 and its methylated forms were prepared to shed light on the nature of the active polymerization species. With some of these complexes, the best catalytic activity was obtained at an Al:Ti ratio of 8.

Uranium(IV) Imidazolin-2-iminato Complexes: A New Class of Actinide Complexes

Acid-base reactions between imidazolin-2-imines (Im(R)NH) and [U(NMeEt)4] selectively afforded uranium(IV) imidazolin-2-iminato complexes of the type [U(NIm(tBu))4] (1), [U(NIm(Mes))3(NMeEt)] (2), and [U(NIm(Dipp))2(NMeEt)2] (3) depending on the steric demand of the substituents in the 1 and 3 positions of the imidazole heterocycle (R = tBu, Mes, Dipp). This new class of actinide complexes displays short U-N bonds and nearly linear U-N-C bond angles, suggesting a U-N bond order higher than 1, as well as extraordinarily high catalytic activity in the polymerization of ε-caprolactone.

Combined Carbometalation–Zinc Homologation–Allylation Reactions as a New Approach for Alkoxyallylation of Aldehydes

The combined carbometalation reaction of ynol ethers followed by a zinc homologation and further allylation reactions lead to an efficient preparation of allylic vicinal diols. The stereochemical outcome of the reaction shows that the substituent of the aldehyde occupies a pseudoaxial position in a Zimmerman-Traxler transition state.