Rhodium‐Catalyzed ortho‐Selective CF Bond Borylation of Polyfluoroarenes with BpinBpin

An ortho‐selective C? F bond borylation between N‐heterocycle‐substituted polyfluoroarenes and Bpin‐Bpin with simple and commercially available [Rh(cod)₂]BF₄ as a catalyst is now reported. The reaction proceeds under mild reaction conditions with high efficiency and broad substrate scope, even toward monofluoroarene, thus providing a facile access to a wide range of borylated fluoroarenes that are useful for photoelectronic materials. Preliminary mechanistic studies reveal that a Rh^III/V catalytic cycle via a key intermediate rhodium(III) hydride complex [(H)Rh^IIIL_n(Bpin)] may be involved in the reaction.     

Copper‐Catalyzed Asymmetric Three‐Component Borylstannation: Enantioselective Formation of CSn Bond

In summary, a first copper‐catalyzed synthesis of α‐aryl‐β‐borylstannane compounds was accomplished through three‐component borylstannation of aryl‐substituted alkenes. In the exploration of an asymmetric variant, chiral sulfinylphosphine ligands proved advantageous in controlling stereochemistry of B?Cu addition and in promoting transmetalation of enantioenriched alkyl?Cu species. The stereochemical outcome supported a sequential syn‐borylcupration and configuration‐retentive transmetalation mechanism. Moreover, α‐chiral β‐borylstannanes were easily transformed into a diverse array of secondary alkylstannanes and triarylethane with high enantiomeric purity. The applications of chiral sulfinylphosphine ligands to other tandem Cu?B addition reactions are currently under investigation in our group.     

PC‐Activated Bimetallic Rhodium Xantphos Complexes: Formation and Catalytic Dehydrocoupling of Amine–Boranes

{Rh(xantphos)}‐based phosphido dimers form by P? C activation of xantphos (4,5‐bis(diphenylphosphino)‐9,9‐dimethylxanthene) in the presence of amine–boranes. These dimers are active dehydrocoupling catalysts, forming polymeric [H₂BNMeH]_n from H₃B?NMeH₂ and dimeric [H₂BNMe₂]₂ from H₃B?NMe₂H at low catalyst loadings (0.1 mol %). Mechanistic investigations support a dimeric active species, suggesting that bimetallic catalysis may be possible in amine–borane dehydropolymerization.     

Revised Role of Selectfluor in Homogeneous Au‐Catalyzed Oxidative CO Bond Formations

The pairing of transition metal catalysis with the reagent Selectfluor (F‐TEDA–BF₄) has attracted considerable attention due to its utility in myriad C?C and C?heteroatom bond‐forming reactions. However, little mechanistic information is available for Selectfluor‐mediated transition metal‐catalyzed reactions and controversy surrounds the precise role of Selectfluor in these processes. We present herein a systematic investigation of homogeneous Au‐catalyzed oxidative C?O bond‐forming reactions using density functional theory calculations. Currently, Selectfluor is thought to serve as an external oxidant in Au^I/Au^III catalysis. However, our investigations suggest that these reactions follow a newly proposed mechanism in which Selectfluor functions as an electrophilic fluorinating reagent involved in a fluorination/defluorination cycle. We have also explored Selectfluor‐mediated gold‐catalyzed homocoupling reactions, which, when cyclopropyl propargylbenzoate is used as a substrate, lead to an unexpected byproduct.     

Iridium‐Catalyzed Synthesis of Diaryl Ethers by Means of Chemoselective CF Bond Activation and the Formation of BF Bonds

Transition‐metal‐catalyzed C? F activation, in comparison with C? H activation, is more difficult to achieve and therefore less fully understood, mainly because carbon–fluorine bonds are the strongest known single bonds to carbon and have been very difficult to cleave. Transition‐metal complexes are often more effective at cleaving stronger bonds, such as C(sp²)? X versus C(sp³)? X. Here, the iridium‐catalyzed C? F activation of fluorarenes was achieved through the use of bis(pinacolato)diboron with the formation of the B? F bond and self‐coupling. This strategy provides a convenient method with which to convert fluoride aromatic compounds into symmetrical diaryl ether compounds. Moreover, the chemoselective products of the C? F bond cleavage were obtained at high yields with the C? Br and C? Cl bonds remaining.     

Multicomponent Synthesis of Chiral Bidentate Unsymmetrical Unsaturated N‐Heterocyclic Carbenes: Copper‐Catalyzed Asymmetric CC Bond Formation

A multicomponent strategy was applied to the synthesis of chiral bidentate unsaturated hydroxyalkyl‐ and carboxyalkyl‐N‐heterocyclic carbene (NHC) precursors. The newly developed low‐cost chiral ligands derived from amino alcohols and amino acids were evaluated in copper‐catalyzed asymmetric conjugated addition and asymmetric allylic alkylation, which afforded the desired tertiary and quaternary carbon stereocenters with excellent regio‐ and enantioselectivities (up to 99:1 e.r.).     

Cross‐Aldol Reaction of Isatin with Acetone Catalyzed by Leucinol: A Mechanistic Investigation

Comprehensive mechanistic studies on the enantioselective aldol reaction between isatin ( 1 a ) and acetone, catalyzed by L ‐leucinol ( 3 a ), unraveled that isatin, apart from being a substrate, also plays an active catalytic role. Conversion of the intermediate oxazolidine 4 into the reactive syn‐enamine 6 , catalyzed by isatin, was identified as the rate‐determining step by both the calculations (ΔG^≠=26.1 kcal mol^?1 for the analogous L ‐alaninol, 3 b ) and the kinetic isotope effect (k_H/k_D=2.7 observed for the reaction using [D₆]acetone). The subsequent reaction of the syn‐enamine 6 with isatin produces (S)‐ 2 a (calculated ΔG^≠=11.6 kcal mol^?1). The calculations suggest that the overall stereochemistry is controlled by two key events: 1) the isatin‐catalyzed formation of the syn‐enamine 6 , which is thermodynamically favored over its anti‐rotamer 7 by 2.3 kcal mol^?1; and 2) the high preference of the syn‐enamine 6 to produce (S)‐ 2 a on reaction with isatin ( 1 a ) rather than its enantiomer (ΔΔG^≠=2.6 kcal mol^?1).     

Realistic Energy Surfaces for Real‐World Systems: An IMOMO CCSD(T):DFT Scheme for Rhodium‐Catalyzed Hydroformylation with the 6‐DPPon Ligand

The hydroformylation of terminal alkenes is one of the most important homogeneously catalyzed processes in industry, and the atomistic understanding of this reaction has attracted enormous interest in the past. Herein, the whole catalytic cycle for rhodium‐catalyzed hydroformylation with the 6‐diphenylphosphinopyridine‐(2H)‐1‐one (6‐DPPon) ligand 1 was studied. This catalytic transformation is challenging to describe computationally, since two requirements must be met: 1) changes in the hydrogen‐bond network must be modeled accurately and 2) bond‐formation/bond‐breaking processes in the coordination sphere of the rhodium center must be calculated accurately. Depending on the functionals used (BP86, B3LYP), the results were found to differ strongly. Therefore, the complete cycle was calculated by using highly accurate CCSD(T) computations for a PH₃ model ligand. By applying an integrated molecular orbital plus molecular orbital (IMOMO) method consisting of CCSD(T) as high level and DFT as low‐level method, excellent agreement between the two functionals was achieved. To further test the reliability of the calculations, the energetic‐span model was used to compare experimentally derived and computed activation barriers. The accuracy of the new IMOMO method apparently makes it possible to predict the catalytic potential of real‐world systems.     

Rhodium‐Catalyzed CS and CN Functionalization of Arenes: Combination of CH Activation and Hypervalent Iodine Chemistry

Rhodium‐catalyzed sulfonylation, thioetherification, thiocyanation, and other heterofunctionalizations of arenes bearing a heterocyclic directing group have been realized. The reaction proceeds by initial Rh^III‐catalyzed C?H hyperiodination of arene at room temperature followed by uncatalyzed nucleophilic functionalization. A diaryliodonium salt is isolated as an intermediate, which represents umpolung of the arene substrate, in contrast to previous studies that suggested umpolung of the coupling partner.     

Encapsulation of Metalloporphyrins in a Self‐Assembled Cubic M8L6 Cage: A New Molecular Flask for Cobalt–Porphyrin‐Catalysed Radical‐Type Reactions

The synthesis of a new, cubic M₈L₆ cage is described. This new assembly was characterised by using NMR spectroscopy, DOSY, TGA, MS, and molecular modelling techniques. Interestingly, the enlarged cavity size of this new supramolecular assembly allows the selective encapsulation of tetra(4‐pyridyl)metalloporphyrins (M^II(TPyP), M=Zn, Co). The obtained encapsulated cobalt–porphyrin embedded in the cubic zinc–porphyrin assembly is the first example of a catalytically active encapsulated transition‐metal complex in a cubic M₈L₆ cage. The substrate accessibility of this system was demonstrated through radical‐trapping experiments, and its catalytic activity was demonstrated in two different radical‐type transformations. The reactivity of the encapsulated Co^II(TPyP) complex is significantly increased compared to free Co^II(TPyP) and other cobalt–porphyrin complexes. The reactions catalysed by this system are the first examples of cobalt–porphyrin‐catalysed radical‐type transformations involving diazo compounds which occur inside a supramolecular cage.     

B(C6F5)3‐Catalyzed Hydrogenation of Oxime Ethers without Cleavage of the NO Bond

The hydrogenation of oximes and oxime ethers is usually hampered by N? O bond cleavage, hence affording amines rather than hydroxylamines. The boron Lewis acid B(C₆F₅)₃ is found to catalyze the chemoselective hydrogenation of oxime ethers at elevated or even room temperature under 100 bar dihydrogen pressure. The use of the triisopropylsilyl group as a protecting group allows for facile liberation of the free hydroxylamines.     

Evaporation‐Driven Self‐Assembly of Colloidal Silica Dispersion: New Insights on Janus Particles

The evaporation driven self‐assembly of novel colloidal silica Janus particles was evaluated by scanning electron microscopy in comparison to unfunctionalized silica particles. The cyclodextrin‐ and azobenzene‐modified compound was obtained utilizing Pickering emulsion approach, in which the particles were immobilized on solidified wax droplets and subsequently functionalized. Silica particles were modified with 3‐aminopropyl trimethoxysilane and afterward reacted with tosyl‐β‐CD or phenylazo(benzoic acid), respectively. Mesoscopic structures of the colloidal dispersions, as dried films from aqueous solution, have been investigated by scanning electron microscopy and dynamic light scattering. Interestingly, it has been observed that the Janus particles show a significantly different evaporation‐induced assembly than the unmodified particles.     

Gold‐Catalyzed C(sp3)H/C(sp)H Coupling/Cyclization/Oxidative Alkynylation Sequence: A Powerful Strategy for the Synthesis of 3‐Alkynyl Polysubstituted Furans

In sharp contrast to the gold‐catalyzed reactions of alkynes/allenes with nucleophiles, gold‐catalyzed oxidative cross‐couplings and especially C? H/C? H cross‐coupling have been under represented. By taking advantage of the unique redox property and carbophilic π acidity of gold, this work realizes the first gold‐catalyzed direct C(sp³)? H alkynylation of 1,3‐dicarbonyl compounds with terminal alkynes under mild reaction conditions, with subsequent cyclization and in situ oxidative alkynylation. A variety of terminal alkynes including aryl, heteroaryl, alkenyl, alkynyl, alkyl, and cyclopropyl alkynes all successfully participate in the domino reaction. The protocol offers a simple and region‐defined approach to 3‐alkynyl polysubstituted furans.     

Structural and Mechanistic Insights into s‐Block Bimetallic Catalysis: Sodium Magnesiate‐Catalyzed Guanylation of Amines

To advance the catalytic applications of s‐block mixed‐metal complexes, sodium magnesiate [NaMg(CH₂SiMe₃)₃] ( 1 ) is reported as an efficient precatalyst for the guanylation of a variety of anilines and secondary amines with carbodiimides. First examples of hydrophosphination of carbodiimides by using a Mg catalyst are also described. The catalytic ability of the mixed‐metal system is much greater than that of its homometallic components [NaCH₂SiMe₃] and [Mg(CH₂SiMe₃)₂]. Stoichiometric studies suggest that magnesiate amido and guanidinate complexes are intermediates in these catalytic routes. Reactivity and kinetic studies imply that these guanylation reactions occur via (tris)amide intermediates that react with carbodiiimides in insertion steps. The rate law for the guanylation of N,N′‐diisopropylcarbodiimide with 4‐tert‐butylaniline catalyzed by 1 is first order with respect to [amine], [carbodiimide], and [catalyst], and the reaction shows a large kinetic isotopic effect, which is consistent with an amine‐assisted rate‐determining carbodiimide insertion transition state. Studies to assess the effect of sodium in these transformations denote a secondary role with little involvement in the catalytic cycle.     

Extended Reaction Scope of Thiamine Diphosphate Dependent Cyclohexane‐1,2‐dione Hydrolase: From CC Bond Cleavage to CC Bond Ligation

ThDP‐dependent cyclohexane‐1,2‐dione hydrolase (CDH) catalyzes the C? C bond cleavage of cyclohexane‐1,2‐dione to 6‐oxohexanoate, and the asymmetric benzoin condensation between benzaldehyde and pyruvate. One of the two reactivities of CDH was selectively knocked down by mutation experiments. CDH‐H28A is much less able to catalyze the C? C bond formation, while the ability for C? C bond cleavage is still intact. The double variant CDH‐H28A/N484A shows the opposite behavior and catalyzes the addition of pyruvate to cyclohexane‐1,2‐dione, resulting in the formation of a tertiary alcohol. Several acyloins of tertiary alcohols are formed with 54–94 % enantiomeric excess. In addition to pyruvate, methyl pyruvate and butane‐2,3‐dione are alternative donor substrates for C? C bond formation. Thus, the very rare aldehyde–ketone cross‐benzoin reaction has been solved by design of an enzyme variant.     

The self‐assembled of cyclic D,L‐α‐peptide systems: Insights into the structure and energetics

Cyclic D,L ‐α‐peptides are able to self‐assemble to nanotubes, although the inherent reason of the stability of this kind of nanotube as well as the intrinsic driving force of self‐assembly of the cyclic D ,L ‐α‐peptides still remain elusive. In this work, using several computational approaches, we investigated the structural and energy characteristics of a series of cyclo[(‐L ‐Phe‐D ‐Ala‐)₄] and cyclo[(‐L ‐Ala‐D ‐Ala‐)₄] oligomers. The results reveal that the thermodynamic stability, cooperativity, and self‐assembly patterns of cyclic D ,L ‐α‐peptide nanotubes are mainly determined by the interactions between cross‐strand side chains instead of those between backbones. For cyclo[(‐L ‐Phe‐D ‐Ala‐)₄] oligomers, the steric interaction between cross‐strand side chains, especially the electrostatic repulsion between the phenyls in Phe residues, brings anticooperative effect into parallel stacking mode, which is responsible for the preference of self‐assembling nanotube in antiparallel vs. parallel stacking orientation. Based on our results, a novel self‐assembling mechanism is put forward—it is the L ‐L antiparallel dimer of cyclo[(‐L ‐Phe‐D ‐Ala‐)₄], instead of the commonly presumed monomer, that acts as the basic building block in self assembly. It explains why these cyclic peptides uniquely self‐assemble to form antiparallel nanotubes. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2010