Roland Fröhlich

Reversible Metal-Free Carbon Dioxide Binding by Frustrated Lewis Pairs†

Frustrated Lewis pairs comprising phosphine and borane react to reversibly bind and release CO2, offering a rare example of metal-free CO2 sequestration. The mechanism of formation of CO2 derivatives by almost simultaneous P-C and O-B bond formation was characterized by quantum chemical calculations.

Evidence for an Equilibrium between an N‐heterocyclic Carbene and Its Dimer in Solution

Evidence for the Wanzlick equilibrium between carbene 1 and dibenzotetraazafulvalene (1)(2) at ambient temperature has been found (see scheme). Enetetramines of type (1)(2) can also be cleaved by coordinatively unsaturated transition metal compounds to form dicarbene complexes.

Self-Assembly of 2,8,14,20-Tetraisobutyl-5,11,17,23-tetrahydroxyresorc[4]arene

We report herein the observation of a hexameric structure of a hydroxyresorc[4]arene in the solid state, enclosing a large interior space. This artificial molecular container is stabilized only by hydrogen bonds. The tendency to form aggregates in solution is demonstrated mainly by means of ESI-MS methods.

Iridium Metal Complexes Containing N-Heterocyclic Carbene Ligands for Blue-Light-Emitting Electrochemical Cells

A new series of cationic blue-emitting, heteroleptic iridium(III)-based metal complexes were systematically synthesized using two 4,6-difluorophenylpyridine ligands as well as one methyl- or n-butyl-substituted bisimidazolium salt carbene-type ligand. In degassed CH(2)Cl(2), all complexes display highly efficient, blue phosphorescence (λ(max) ∼ 452 nm; emission quantum yield ∼ 0.30) at room temperature and also show blue emission in a thin film. The measured photophysical properties of the complexes have been rationalized with the help of quantum-chemical calculations. Because of the high solubility of the complexes, solution-processed devices, light-emitting electrochemical cells (LEECs), were made. The results showed that true blue emission and short turn-on time is achieved when an ionic conductor, tetrabutylammonium trifluoromethanesulfonate, was used as the matrix for the film containing the emitters. These iridium complexes and the described devices are the bluest materials ever reported and the first case of LEECs based on carbene ligands.

Reactions of an intramolecular frustrated Lewis pair with unsaturated substrates: evidence for a concerted olefin addition reaction.

The intramolecular frustrated Lewis pair (mesityl)(2)P-CH(2)-CH(2)-B(C(6)F(5))(2) was generated in situ by HB(C(6)F(5))(2) hydroboration of dimesitylvinylphosphine. The compound reacts with 1-pentyne by C-H bond cleavage. It undergoes a 1,2-addition to the carbonyl group of trans-cinnamic aldehyde to yield a zwitterionic six-membered heterocycle by B-O and P-C bond formation. The Lewis pair regioselectively adds to the electron-rich C=C double bond of ethyl vinyl ether, and it selectively undergoes an exo-cis-2,3-addition to norbornene. A combined experimental/theoretical study suggests that this reaction takes place in an asynchronous concerted fashion with the B-C bond being formed in slight preference to the P-C bond. The addition products were characterized by X-ray crystal structure analyses.

Enantioselective Haloetherification by Asymmetric Opening of meso-Halonium Ions

A new approach to enantioselective haloetherification reactions via desymmetrization of in situ-generated meso-halonium ions is described. The combination of N-haloamides as a halogen source and sodium salts of chiral phosphoric acids as catalysts can be used for the cyclization of symmetrical ene-diol substrates, yielding the haloetherification products under practical conditions in enantioenriched form.

IBiox[(−)-menthyl]: A Sterically Demanding Chiral NHC Ligand

An exceedingly sterically demanding, rigid, and chiral NHC ligand, IBiox[(-)-menthyl] (1), was prepared and structurally characterized. With a buried volume of approximately 50%, this ligand arguably represents one of the most sterically demanding monodentate ligands. The ability to use aryl chloride substrates in intramolecular palladium-catalyzed alpha-arylations reveals its unique reactivity. Moreover, C(2)-symmetric 1 allows the highly enantioselective formation of oxindoles with up to 99% ee.

Capture of NO by a Frustrated Lewis Pair: A New Type of Persistent N-Oxyl Radical**

Frustrated Lewis Pairs (FLPs) show a rapidly increasing spectrum of interesting chemical reactions. They have been reported to activate dihydrogen under mild conditions and to serve as metal free hydrogenation catalysts toward specific organic substrates. FLPs add to numerous unsaturated substrates such as alkenes and alkynes, carbonyl compounds, azides and even CO2 or N2O. [3,4] For instance, the intramolecular P/B-FLP 1 adds to nitrosobenzene to form the sixmembered heterocycle 2. Nitric oxide (NO) is an important messenger molecule and regulator in biological systems. We find that the reactive frustrated Lewis pair 1 cleanly and rapidly reacts with this essential small molecule to form the persistent heterocyclicNoxyl radical “P/B-FLP-NOC” (3). Compound 3 represents a novel type of N-oxyl radical related to the ubiquitous TEMPO radical (4) and its congeners (e.g. 5 (PINO) and 6, see Scheme 1). Herein we describe the synthesis, characterization, and O-based reactivity of this novel type of N-oxyl radical derived from a frustrated Lewis pair and nitric oxide. Treatment of a yellow solution of the FLP Mes2PCH2CH2B(C6F5)2 (1) [10] (in situ generated from Mes2PCH=CH2 and Piers borane [HB(C6F5)2] ) in fluorobenzene with 1 equiv NOgas gave rise to an intense green solution fromwhich blue crystals of the P/B-FLP-NOC product 3 were isolated in 58% yield by precipitation with pentane (Scheme 1). X-ray crystal structure analysis of 3 (Figure 1)

N,N-addition of frustrated Lewis pairs to nitric oxide: an easy entry to a unique family of aminoxyl radicals.

The intramolecular cyclohexylene-bridged P/B frustrated Lewis pair [Mes(2)P-C(6)H(10)-B(C(6)F(5))(2)] 1b reacts rapidly with NO to give the persistent FLP-NO aminoxyl radical 2b formed by P/B addition to the nitrogen atom of NO. This species was fully characterized by X-ray diffraction, EPR and UV/vis spectroscopies, C,H,N elemental analysis, and DFT calculations. The reactive oxygen-centered radical 2b undergoes a H-atom abstraction (HAA) reaction with 1,4-cyclohexadiene to give the diamagnetic FLP-NOH product 3b. FLP-NO 2b reacts with toluene at 70 °C in an HAA/radical capture sequence to give a 1:1 mixture of FLP-NOH 3b and FLP-NO-CH(2)Ph 4b, both characterized by X-ray diffraction. Structurally related FLPs [Mes(2)P-CHR(1)-CHR(2)-B(C(6)F(5))(2)] 1c, 1d, and 1e react analogously with NO to give the respective persistent FLP-NO radicals 2c, 2d, and 2e, respectively, which show similar HAA and O-functionalization reactions. The FLP-NO-CHMePh 6b derived from 1-bromoethylbenzene undergoes NO-C bond cleavage at 120 °C with an activation energy of E(a) = 35(2) kcal/mol. Species 6b induces the controlled nitroxide-mediated radical polymerization (NMP) of styrene at 130 °C to give polystyrene with a polydispersity index of 1.3. The FLP-NO systems represent a new family of aminoxyl radicals that are easily available by N,N-cycloaddition of C(2)-bridged intramolecular P/B frustrated Lewis pairs to nitric oxide.

Stereoselective Palladium‐Catalyzed Carboaminoxylations of Indoles with Arylboronic Acids and TEMPO

Indoles are not indolent: Various indoles react with arylboronic acids chemodivergently. C-H arylation of free indole and N-methylindole gives the corresponding C(2)-arylated indoles A whereas N-acylated, N-benzoylated, and N-Boc-protected indoles provide the corresponding arylcarboaminoxylated products B with excellent diastereoselectivity in good to excellent yields.