Iridium-catalyzed reductive carbon-carbon bond cleavage reaction on a curved pyridylcorannulene skeleton

Strain-induced carbon-carbon bond cleavage of bowl-shaped sumanenone

This study details a highly effective ring-opening reaction that involves acid-mediated carbon-carbon bond cleavage of the buckybowl, sumanenone. The reaction of the bowl-shaped sumanenone with AcOH and TfOH results in the formation of a planar carboxylic acid. The examination of reactivity in comparison to planar analogues, along with theoretical calculations, suggests that the release of curved strain is a crucial factor for the success of this reaction.

Reversible Boron-Insertion into Aromatic C-C Bonds

Formation of borabicyclo[3.2.0]heptadiene derivatives was achieved via boron-insertion into aromatic C-C bonds in the photo-promoted skeletal rearrangement reaction of triarylboranes bearing an ortho-phosphino substituent (ambiphilic phosphine-boranes). The borabicyclo[3.2.0]heptadiene derivatives were fully characterized by NMR and X-ray analyses. The dearomatized products were demonstrated to undergo the reverse reaction in the dark at room temperature, realizing photochemical and thermal interconversion between triarylboranes and boron-doped bicyclic systems. Experimental and theoretical studies revealed that sequential two electrocyclic reactions involving E/Z-isomerization of an alkene moiety proceed via a highly strained trans-borepin intermediate.

"Broken-hearted" carbon bowl via electron shuttle reaction: energetics and electron coupling

Unprecedented one-step C[double bond, length as m-dash]C bond cleavage leading to opening of the buckybowl (π-bowl), that could provide access to carbon-rich structures with previously inaccessible topologies, is reported; highlighting the possibility to implement drastically different synthetic routes to π-bowls in contrast to conventional ones applied for polycyclic aromatic hydrocarbons. Through theoretical modeling, we evaluated the mechanistic pathways feasible for π-bowl planarization and factors that could affect such a transformation including strain and released energies. Through employment of Marcus theory, optical spectroscopy, and crystallographic analysis, we estimated the possibility of charge transfer and electron coupling between "open" corannulene and a strong electron acceptor such as 7,7,8,8-tetracyanoquinodimethane. Alternative to a one-pot solid-state corannulene "unzipping" route, we reported a nine-step solution-based approach for preparation of novel planar "open" corannulene-based derivatives in which electronic structures and photophysical profiles were estimated through the energies and isosurfaces of the frontier natural transition orbitals.

Enantioselective Cleavage of Cyclobutanols Through Ir-Catalyzed C-C Bond Activation: Mechanistic and Synthetic Aspects

The Ir-catalyzed conversion of prochiral tert-cyclobutanols to β-methyl-substituted ketones proceeds under comparably mild conditions in toluene (45-110 °C) and is particularly suited for the enantioselective desymmetrization of β-oxy-substituted substrates to give products with a quaternary chirality center with up to 95 % ee using DTBM-SegPhos as a chiral ligand. Deuteration experiments and kinetic isotope effect measurements revealed major mechanistic differences to related RhI -catalyzed transformations. Supported by DFT calculations we propose the initial formation of an IrIII hydride intermediate, which then undergoes a β-C elimination (C-C bond activation) prior to reductive C-H elimination. The computational model also allows the prediction of the stereochemical outcome. The Ir-catalyzed cyclobutanol cleavage is broadly applicable but fails for substrates bearing strongly coordinating groups. The method is of particular value for the stereo-controlled synthesis of substituted chromanes related to the tocopherols and other natural products.

A Zn(II)-Coordination Polymer for the Instantaneous Cleavage of Csp3-Csp3 Bond and Simultaneous Reduction of Ketone to Alcohol

Two coordination polymers of Zn(II) and Cu(II) with n-butylmalonic acid have been achieved in this work. The crystallographic structural descriptions along with the sedimentary rock-type microstructural morphology of these two coordination polymers (CPs) have been explored. The reactivity of β-hydroxy ketones with these two CPs has also been investigated. The Zn(II)-CP shows a specific reactivity with β-hydroxy ketone at room temperature and in open air conditions. Through a microcolumn-based filtration technique, the Zn(II)-CP shows the capability to break the C_sp³-C_sp³ σ bonds of β-hydroxy ketone and simultaneously reduce the associated ketone to alcohol. Such conversion has been progressed without the use of any additional external reducing agent and any chemical workup or column chromatographic purification protocol. Other similar type CPs of Cu(II) and Mn(II) with n-butylmalonic acid completely failed to show similar reactivity with β-hydroxy ketone. On the basis of much experimental evidence, the most possible mechanistic pathway of the reactivity between β-hydroxy ketone and Zn(II)-CP has also been proposed through this work.

Total Synthesis of α-Tocopherol through Enantioselective Iridium-Catalyzed Fragmentation of a Spiro-Cyclobutanol Intermediate

A conceptionally new strategy for the asymmetric (2R-selective) synthesis of α-tocopherol (vitamin E) was developed. In the stereocontrolled key step, a prochiral spiro[chromane-2,3'-cyclobutanol] unit is effectively desymmetrized under C-C bond activation in an unprecedented iridium-catalyzed transformation using (S)-DTBM-SegPhos as a chiral ligand (e.r. 97:3). To complete the synthesis, the side chain was attached through Ru-catalyzed cross-metathesis employing an alkene derived from (R,R)-hexahydrofarnesol. To suppress epimerization during the final hydrogenation, PtO₂ had to be used as a catalyst instead of Pd/C. In an alternative approach (employing a propargyl-substituted spiro-cyclobutanol), the side chain was constructed prior to the Ir-catalyzed ring fragmentation (>99:1 d.r.) through enyne cross-metathesis (using an alkene derived from (R)-dihydrocitronellal) followed by Cr-catalyzed 1,4-hydrogenation and (diastereoselective) Pfaltz hydrogenation of the resulting triple-substituted olefin. The work demonstrates the potential of iridium catalysis for enantioselective C-C bond activation.

Regioselective Hydrogenation of a 60-Carbon Nanographene Molecule toward a Circumbiphenyl Core

Regioselective peripheral hydrogenation of a nanographene molecule with 60 contiguous sp² carbons provides unprecedented access to peralkylated circumbiphenyl (1). Conversion to the circumbiphenyl core structure was unambiguously validated by MALDI-TOF mass spectrometry, NMR, FT-IR, and Raman spectroscopy. UV–vis absorption spectra and DFT calculations demonstrated the significant change of the optoelectronic properties upon peripheral hydrogenation. Stimulated emission from 1, observed via ultrafast transient absorption measurements, indicates potential as an optical gain material.

N-Functionalized Ferrocenes: Subvalent Group XIV Element Chlorides and tert-Butyllithium-Induced C-C Bond Cleavage under Mild Conditions

The ferrocene derivative (η⁵ -Cp)Fe{η⁵ -C₅ H₃ -1-(ArNCH)-2-(CH₂ NMe₂ )} (1; Ar=2,6-iPr₂ C₆ H₃ )) reacts diastereoselectively with LiR by carbolithiation and subsequent hydrolysis to give (η⁵ -Cp)Fe{η⁵ -C₅ H₃ -1-(ArHNCHR)-2-(CH₂ NMe₂ )} (3: R=tBu; 4: R=Ph; 5: R=Me) in high yields. For R=tBu, the organolithium derivative (η⁵ -Cp)Fe{η⁵ -C₅ H₃ -1-(ArLiNCHR)-2-(CH₂ NMe₂ )} (2) was isolated. Compound 2 reacts with GeCl₂ ⋅dioxane and SnCl₂ to give the metallylene amide chlorides (η⁵ -Cp)Fe{η⁵ -C₅ H₃ -1-(ArMNCHtBu)-2-(CH₂ NMe₂ )} 6 (M=GeCl) and 7 (M=SnCl), respectively, which each contain three stereogenic centers. The potential of 7 as a ligand in transition-metal chemistry is demonstrated by formation of its complex (η⁵ -Cp)Fe{η⁵ -C₅ H₃ -1-(ArMNCHtBu)-2-(CH₂ NMe₂ )} [9, M= Sn(Cl)W(CO)₅ ]. Treatment of 3 with tert-butyllithium at room temperature causes an unprecedented carbon-carbon bond cleavage whereas under kinetic control, lithiation at the Cp-3 position takes place, which leads to the isolation of (η⁵ -Cp)Fe{η⁵ -C₅ H₃ -1-(ArHNCHtBu)-2-(CH₂ NMe₂ )-3-SiMe₃ } (10).

Efficient Benzimidazolidinone Synthesis via Rhodium-Catalyzed Double-Decarbonylative C-C Activation/Cycloaddition between Isatins and Isocyanates

The first decarbonylative cycloaddition of less-strained cyclic ketones (isatins) with isocyanates is reported. Initiated by C-C activation, this distinct [5-2+2] transformation provides a rapid entry to access various benzimidazolidinone derivatives, through which a wide range of isocyanates can be efficiently coupled with broad functional group tolerance. A modified one-pot process, combining Curtius rearrangement and C-C activation, was also achieved by using acyl azides as the starting materials. Detailed mechanistic study revealed a surprising double-decarbonylative reaction pathway. The novel reactivity discovered in this basic research is expected to shed light on developing new heterocycle formation methods through a C-C/isocyanate coupling.

Monoreduced 1,2-dihydrocorannulene versus the parent corannulene

The monoanion of dihydrogenated corannulene isolated in the form of its potassium salt, namely tris(diglyme-κ(3)O,O',O'')potassium hexacyclo[11.5.2.0(4,17).0(7,16).0(10,15).0(14,18)]icosa-1,3,5,7(16),8,10(15),11,13,17-nonaenide, [K(C6H14O3)3](C20H12), has been structurally characterized for the first time. The X-ray study confirms the previous NMR spectroscopic prediction that the two H atoms are attached to the same six-membered ring to form 1,2-dihydrocorannulene, thus destroying the aromaticity of only one arene ring of the corannulene core. The direct comparison of (C20H12)(-) with the parent corannulene anion, (C20H10)(-), is provided to illustrate the geometry perturbations caused by rim hydrogenation.