Selective Isomerization–Hydroformylation Sequence: A Strategy to Valuable α‑Methyl-Branched Aldehydes from Terminal Olefins

For the first time, an original selective isomerization-hydroformylation sequence to convert terminal olefins bearing an anionic moiety to α-methyl-branched aldehydes with unprecedented selectivities is reported. This opens up new synthetic avenues to these valuable building blocks from inexpensive and bioavailable substrates. The catalytic system involves a suitable selective monoisomerization catalyst and a selective supramolecular catalyst that preorganizes a substrate molecule prior to the hydroformylation reaction via hydrogen bonding. In principle, the strategy can be extended to other classes of substrates, providing suitable catalysts for the hydroformylation of internal alkenes

Precise Supramolecular Control of Selectivity in the Rh-Catalyzed Hydroformylation of Terminal and Internal Alkenes

In this study, we report a series of DIMPhos ligands <b>L1</b>–<b>L3</b>, bidentate phosphorus ligands equipped with an integral anion binding site (the DIM pocket). Coordination studies show that these ligands bind to a rhodium center in a bidentate fashion. Experiments under hydroformylation conditions confirm the formation of the mononuclear hydridobiscarbonyl rhodium complexes that are generally assumed to be active in hydroformylation. The metal complexes formed still strongly bind the anionic species in the binding site of the ligand, without affecting the metal coordination sphere. These bifunctional properties of DIMPhos are further demonstrated by the crystal structure of the rhodium complex with acetate anion bound in the binding site of the ligand. The catalytic studies demonstrate that substrate preorganization by binding in the DIM pocket of the ligand results in unprecedented selectivities in hydroformylation of terminal and internal alkenes functionalized with an anionic group. Remarkably, the selectivity controlling anionic group can be even 10 bonds away from the reactive double bond, demonstrating the potential of this supramolecular approach. Control experiments confirm the crucial role of the anion binding for the selectivity. DFT studies on the decisive intermediates reveal that the anion binding in the DIM pocket restricts the rotational freedom of the reactive double bound. As a consequence, the pathway to the undesired product is strongly hindered, whereas that for the desired product is lowered in energy. Detailed kinetic studies, together with the in situ spectroscopic measurements and isotope-labeling studies, support this mode of operation and reveal that these supramolecular systems follow enzymatic-type Michaelis–Menten kinetics, with competitive product inhibition

Self-Assembly of a Confined Rhodium Catalyst for Asymmetric Hydroformylation of Unfunctionalized Internal Alkenes

A chiral supramolecular ligand has been assembled and applied to the rhodium-catalyzed asymmetric hydroformylation of unfunctionalized internal alkenes. Spatial confinement of the metal center within a chiral pocket results in reversed regioselectivity and remarkable enantioselectivities

Photo- and Thermal Isomerization of (TP)Fe(CO)Cl₂ [TP = Bis(2-diphenylphosphinophenyl)­phenylphosphine]

The title complex displayed structural flexibility via photo- and thermal-isomerization reactions between three isomers: (<i>mer</i>-TP)­Fe­(CO)­Cl₂ (<b>A</b>), <i>unsym</i>-(<i>fac</i>-TP)­Fe­(CO)­Cl₂ (<b>B</b>), and <i>sym</i>-(<i>fac-</i>TP)­Fe­(CO)­Cl₂ (<b>C</b>). Irradiation of <b>A</b> at RT with 525 nm light selectively produces <b>B</b>, while at 0 °C isomer <b>C</b> is formed with the intermediacy of <b>B</b>. UV–vis spectroscopy combined with TD-DFT calculations revealed the nature of the photoisomerization process. Kinetics of the thermal isomerization of <b>C</b> to <b>B</b> and <b>B</b> to <b>A</b> have been studied with ³¹P NMR spectroscopy in CD₂Cl₂, and activation parameters were determined. Isomers <b>A</b> and <b>B</b> have been isolated and crystallographically characterized

Photo- and Thermal Isomerization of (TP)Fe(CO)Cl₂ [TP = Bis(2-diphenylphosphinophenyl)­phenylphosphine]

The title complex displayed structural flexibility via photo- and thermal-isomerization reactions between three isomers: (<i>mer</i>-TP)­Fe­(CO)­Cl₂ (<b>A</b>), <i>unsym</i>-(<i>fac</i>-TP)­Fe­(CO)­Cl₂ (<b>B</b>), and <i>sym</i>-(<i>fac-</i>TP)­Fe­(CO)­Cl₂ (<b>C</b>). Irradiation of <b>A</b> at RT with 525 nm light selectively produces <b>B</b>, while at 0 °C isomer <b>C</b> is formed with the intermediacy of <b>B</b>. UV–vis spectroscopy combined with TD-DFT calculations revealed the nature of the photoisomerization process. Kinetics of the thermal isomerization of <b>C</b> to <b>B</b> and <b>B</b> to <b>A</b> have been studied with ³¹P NMR spectroscopy in CD₂Cl₂, and activation parameters were determined. Isomers <b>A</b> and <b>B</b> have been isolated and crystallographically characterized

“Cofactor”-Controlled Enantioselective Catalysis

We report an achiral bisphosphine rhodium complex equipped with a binding site for the recognition of chiral anion guests. Upon binding small chiral guests<i>cofactors</i>the rhodium complex becomes chiral and can thus be used for asymmetric catalysis. Screening of a library of cofactors revealed that the best cofactors lead to hydrogenation catalysts that form the products with high enantioselectivity (ee’s up to 99%). Interestingly, a competition experiment shows that even in a mixture of 12 cofactors high ee is obtained, indicating that the complex based on the best cofactor dominates the catalysis

Formation and Site-Selective Reactivity of a Nonsymmetric Dinuclear Iridium BisMETAMORPhos Complex

A flexible di­(sulfonamidophosphine) ligand (<b>1</b>^<b>H2</b>, Ph₂PN^HS­(O)₂N^HPPh₂) was synthesized from commercially available sulfamide and chlorodiphenylphosphine. Coordination of this new bisMETAMORPhos <b>1</b>^<b>H2</b> with [Ir­(Cp*)­Cl­(μ-Cl)]₂ instantly led to the formation of the P,P-coordinated bimetallic complex <b>2</b> [Ir­(Cp*)­Cl­(κ¹-<i>P</i>₁; κ¹-<i>P</i>₂-<b>1</b>^<b>H2</b>)­Ir­(Cp*)­Cl]. Reaction of <b>2</b> using excess NaOAc led to the formation of nonsymmetric homodinuclear complex <b>3</b> [Ir­(Cp*)­Cl­(κ²-<i>P</i>,<i>O</i>; κ³-<i>P</i>,<i>N</i>,<i>C</i>; μ-<b>1</b>)­Ir­(Cp*)], which contains two distinctly different Ir^III centers, with a <i>fac</i>-<i>P</i>,<i>N</i>,<i>C</i> and a <i>fac</i>-<i>P</i>,<i>O</i>,<i>Cl</i> coordination environment. The ligand is overall trisanionic due to additional intramolecular C–H activation of a flanking phenyl ring. Complex <b>3</b> reacts selectively at the Ir­(<i>P</i>,<i>O</i>,<i>Cl</i>) center with a single equivalent of HCl or H₂ to generate complexes <b>4</b> and <b>5</b>, respectively. These complexes are generated via heterolytic cleavage of the H–Cl or H–H bond, which reprotonates the ligand showing its bifunctional applicability. The Ir–C bond is found to be inert under these conditions

Growth and Characterization of PDMS-Stamped Halide Perovskite Single Microcrystals

Recently, halide perovskites have attracted considerable attention for optoelectronic applications, but further progress in this field requires a thorough understanding of the fundamental properties of these materials. Studying perovskites in their single-crystalline form provides a model system for building such an understanding. In this work, a simple solution-processed method combined with PDMS (polydimethyl­siloxane) stamping was used to prepare thin single microcrystals of halide perovskites. The method is general for a broad array of materials including CH₃NH₃PbBr₃, CH₃NH₃PbCl₃, CH₃NH₃Pb­(Br_0.5Cl_0.5)₃, CH₃NH₃Pb­(Br_0.75Cl_0.25)₃, CsPbBr₃, Cs₃Bi₂Br₉, and Cs₃Bi₂I₉. Electron backscatter diffraction (EBSD) was used to investigate the microstructure of the crystals. In order to characterize the microcrystals of CH₃NH₃PbBr₃ electrically, the crystals were grown on prefabricated electrodes creating single-crystal devices contacted from the back. This back-contacted platform circumvents the incompatibility between halide perovskites and the aqueous chemistry used in standard microfabriation processes. It also allows <i>in situ</i> characterization of the perovskite crystal while it operates as a microscopic solar cell

Co^III–Carbene Radical Approach to Substituted 1<i>H</i>‑Indenes

A new strategy for the catalytic synthesis of substituted 1<i>H</i>-indenes via metalloradical activation of <i>o</i>-cinnamyl <i>N</i>-tosyl hydrazones is presented, taking advantage of the intrinsic reactivity of a Co^III carbene radical intermediate. The reaction uses readily available starting materials and is operationally simple, thus representing a practical method for the construction of functionalized 1<i>H</i>-indene derivatives. The cheap and easy to prepare low spin cobalt­(II) complex [Co^II(MeTAA)] (MeTAA = tetramethyltetraaza[14]­annulene) proved to be the most active catalyst among those investigated, which demonstrates catalytic carbene radical reactivity for a nonporphyrin cobalt­(II) complex, and for the first time catalytic activity of [Co^II(MeTAA)] in general. The methodology has been successfully applied to a broad range of substrates, producing 1<i>H</i>-indenes in good to excellent yields. The metallo-radical catalyzed indene synthesis in this paper represents a unique example of a net (formal) intramolecular carbene insertion reaction into a vinylic C­(sp²)–H bond, made possible by a controlled radical ring-closure process of the carbene radical intermediate involved. The mechanism was investigated computationally, and the results were confirmed by a series of supporting experimental reactions. Density functional theory calculations reveal a stepwise process involving activation of the diazo compound leading to formation of a Co^III-carbene radical, followed by radical ring-closure to produce an indanyl/benzyl radical intermediate. Subsequent indene product elimination involving a 1,2-hydrogen transfer step regenerates the catalyst. Trapping experiments using 2,2,6,6-tetra-methylpiperidine-1-oxyl (TEMPO) radical or dibenzoylperoxide (DBPO) confirm the involvement of cobalt­(III) carbene radical intermediates. Electron paramagnetic resonance spectroscopic spin-trapping experiments using phenyl <i>N</i>-<i>tert</i>-butylnitrone (PBN) reveal the radical nature of the reaction

Highly Selective Asymmetric Rh-Catalyzed Hydroformylation of Heterocyclic Olefins

A small family of new chiral hybrid, diphosphorus ligands, consisting of phosphine-phosphoramidites <b>L1</b> and <b>L2</b> and phosphine-phosphonites <b>L3a–c</b>, was synthesized for the application in Rh-catalyzed asymmetric hydroformylation of heterocyclic olefins. High-pressure (HP)-NMR and HP-IR spectroscopy under 5–10 bar of syngas has been employed to characterize the corresponding catalyst resting state with each ligand. Indole-based ligands <b>L1</b> and <b>L2</b> led to selective <i>ea</i> coordination, while the xanthene derived system <b>L3c</b> gave predominant <i>ee</i> coordination. Application of the small bite-angle ligands <b>L1</b> and <b>L2</b> in the highly selective asymmetric hydroformylation (AHF) of the challenging substrate 2,3-dihydrofuran (<b>1</b>) yielded the 2-carbaldehyde (<b>3</b>) as the major regioisomer in up to 68% yield (with ligand <b>L2</b>) along with good ee’s of up to 62%. This is the first example in which the asymmetric hydroformylation of <b>1</b> is both regio- and enantioselective for isomer <b>3</b>. Interestingly, use of ligand <b>L3c</b> in the same reaction completely changed the regioselectivity to 3-carbaldehyde (<b>4</b>) with a remarkably high enantioselectivity of 91%. Ligand <b>L3c</b> also performs very well in the Rh-catalyzed asymmetric hydroformylation of other heterocyclic olefins. Highly enantioselective conversion of the notoriously difficult substrate 2,5-dihydrofuran (<b>2</b>) is achieved using the same catalyst, with up to 91% ee, concomitant with complete regioselectivity to the 3-carbaldehyde product (<b>4</b>) under mild reaction conditions. Interestingly, the Rh-catalyst derived from <b>L3c</b> is thus able to produce both enantiomers of 3-carbaldehyde <b>4</b>, simply by changing the substrate from <b>1</b> to <b>2</b>. Furthermore, 85% ee was obtained in the hydroformylation of <i>N-</i>acetyl-3-pyrroline (<b>5</b>) with exceptionally high regioselectivities for 3-carbaldehyde <b>8Ac</b> (>99%). Similarly, an ee of 86% for derivative <b>8Boc</b> was accomplished using the same catalyst system in the AHF of <i>N-</i>(<i>tert</i>-butoxycarbonyl)-3-pyrroline (<b>6</b>). These results represent the highest ee’s reported to date in the AHF of dihydrofurans (<b>1</b>, <b>2</b>) and 3-pyrrolines (<b>5</b>, <b>6</b>)