Jay S. Siegel

Proton-Catalyzed, Silane-Fueled Friedel-Crafts Coupling of Fluoroarenes

Putting the F in Friedel-Crafts The Friedel-Crafts class of reactions, among the oldest and most broadly applied in organic chemistry, form carbon-carbon bonds between aromatic rings and a variety of non-aromatic substituents, such as alkyl groups. Generally, a metal complex is used to activate chlorinated or brominated precursors of these substituents, but by using silicon-based reagents to activate a fluorinated precursor, Allemann et al. (p. 574) extend the reaction to coupling of two different aromatic sites, leading to efficient formation of elaborate polycyclic structures. The method relies on the unusual strength of silicon-fluorine bonds as a driving force. Silicon-fluorine bond formation expands the range of compounds that can be used in a reaction that forms carbon-carbon bonds. The venerable Friedel-Crafts reaction appends alkyl or acyl groups to aromatic rings through alkyl or acyl cation equivalents typically generated by Lewis acids. We show that phenyl cation equivalents, generated from otherwise unreactive aryl fluorides, allow extension of the Friedel-Crafts reaction to intramolecular aryl couplings. The enabling feature of this reaction is the exchange of carbon-fluorine for silicon-fluorine bond enthalpies; the reaction is activated by an intermediate silyl cation. Catalytic quantities of protons or silyl cations paired with weakly coordinating carborane counterions initiate the reactions, after which proton transfer in the final aromatization step regenerates the active silyl cation species by protodesilylation of a quaternary silane. The methodology allows the high-yield formation of a range of tailored polycyclic aromatic hydrocarbons and graphene fragments.

Multiethynyl Corannulenes: Synthesis, Structure, and Properties

Syntheses, crystal structures, ab initio density functional theory computations, and photophysical properties of 1,6-di-, 1,2,5,6-tetra-, and 1,3,5,7,9-pentaethynyl-substituted corannulenes (classes 3, 4, and 5, respectively) are reported. Classes 3 and 4 were prepared from the corresponding corannulenyl bromides and terminal alkynes in excellent yields (nine examples, with yields of 57-92%) using the Sonogarshira reaction. Class 5 was prepared from 1,3,5,7,9-pentacholorocorannulene and trimethylalkynylstannanes using a modification of Nolans procedure (8 examples, with yields of 45-93%). The molecular packing in crystals of 1,6-diphenylethynyl-2,5-dimethylcorannulene (3-Ph2) displays a polar columnar structure with all of the molecule bowls oriented in the same direction. Similarly, 1,2,5,6-tetrakis(3,5-dimethylphenylethynyl)corannulene [4-Ar(c)5] and 1,3,5,7,9-pentakis(3,5-dimethylphenylethynyl)corannulene [5-Ar(c)5] form columnar structures, but the bowls are oriented in opposing directions. Additionally, the number of attached alkynyl arms is correlated with an increase in bowl depth of the corrannulene nucleus. Most of the aryl derivatives displayed high-quantum-efficiency solution luminescence and variable emission wavelengths that were dependent on the nature of the substitution.

Induced-fit catalysis of corannulene bowl-to-bowl inversion

Stereoelectronic complementarity between the active site of an enzyme and the transition state of a reaction is one of the tenets of enzyme catalysis. This report illustrates the principles of enzyme catalysis (first proposed by Pauling and Jencks) through a well-defined model system that has been fully characterized crystallographically, computationally and kinetically. Catalysis of the bowl-to-bowl inversion processes that pertain to corannulene is achieved by combining ground-state destabilization and transition-state stabilization within the cavity of an extended tetracationic cyclophane. This synthetic receptor fulfils a role reminiscent of a catalytic antibody by stabilizing the planar transition state for the bowl-to-bowl inversion of (ethyl)corannulene (which accelerates this process by a factor of ten at room temperature) by an induced-fit mechanism first formulated by Koshland.

Molecular Spur Gears Comprising Triptycene Rotators and Bibenzimidazole-Based Stators

Dynamic gearing of molecular spur gears, the most common type of mechanical gear, is elucidated. Molecular design and conformational analysis show that derivatives of 4,4-bis(triptycen-9-ylethynyl)bibenzimidazole represent suitable constructs to investigate gearing behavior of collateral triptycene (Tp) groups. To test this design, DFT calculations (B97-D/Def2-TZVP) were employed and the results suggest that these molecules undergo geared rotation preferentially to gear slippage. Synthesis of derivatives was carried out, providing a series of molecular spur gears, including the first desymmetrized spur gear molecules, which were subsequently subjected to stereochemical analysis.

Wide‐Ranging Host Capability of a PdII‐Linked M2L4 Molecular Capsule with an Anthracene Shell

This article reports that an M2L4 molecular capsule is capable of encapsulating various neutral molecules in quantitative yields. The capsule was obtained as a single product by mixing a small number of components; two Pd(II) ions and four bent bispyridine ligands containing two anthracene panels. Detailed studies of the host capability of the Pd(II)-linked capsule revealed that spherical (e.g., paracyclophane, adamantanes, and fullerene C60), planar (e.g., pyrenes and triphenylene), and bowl-shaped molecules (e.g., corannulene) were encapsulated in the large spherical cavity, giving rise to 1:1 and 1:2 host-guest complexes, respectively. The volume of the encapsulated guest molecules ranged from 190 to 490 Å(3). Within the capsule, the planar guests adopt a stacked-dimer structure and the bowl-shaped guests formed an unprecedented concave-to-concave capsular structure, which are fully shielded by the anthracene shell. Competitive binding experiments of the capsule with a set of the planar guests established a preferential binding series for pyrenes≈phenanthrene>triphenylene. Furthermore, the capsule showed the selective formation of an unusual ternary complex in the case of triphenylene and corannulene.

C-F activation of fluorobenzene by silylium carboranes: evidence for incipient phenyl cation reactivity.

Author(s): Duttwyler, Simon; Douvris, Christos; Fackler, Nathanael LP; Tham, Fook S; Reed, Christopher A; Baldridge, Kim K; Siegel, Jay S | Abstract: Si(mply) rips it apart: C-F activation of fluorobenzene has been achieved using the extremely strong silyl Lewis acids [Et3Si(X)]+ (X=PhF or Et3SiH) and [(2,6-dixylyl-C6H 3)SiMe2] + paired with the anion CHB 11Cl11-. They abstract fluoride from unactivated fluorobenzene to give arylated products, consistent with phenyl-cation-like reactivity (see scheme).

Building 2D crystals from 5-fold-symmetric molecules

Concepts of close packing in monolayers of 5-fold-symmetric buckybowls are discussed. When the symmetry of lattice and molecular building blocks are incompatible, new strategies evolve. Corannulene forms a hexagonal lattice on Cu(111) by tilting away from the C(5) symmetry and aligning one hexagonal ring parallel to the surface. The chiral 5-fold-substituted chloro and methyl derivatives do not show this tilt and maintain the 5-fold symmetry as adsorbates. Consequently, a nonperfect tiling is observed. Their lattices are quasi-hexagonal: one in an antiparallel fashion with almost pm symmetry and the other with azimuthal and positional disorder on the hexagonal grid. Our results are in remarkable agreement with computational and mechanical modeling experiments of close packing of hard pentagonal discs in macroscopic two-dimensional systems and prove the validity of such modeling strategies.

Reversible phase transitions in a buckybowl monolayer.

Like penguins on ice, buckybowl molecules move closer together when cooled on a copper surface (see model of a corannulene molecule adsorbed on Cu(111)). Upon heating, the molecules spread out into the original crystal phase again. The lower density at room temperature can be explained by the increase in entropy owing to the excitation of bowl vibrations at the surface.

Structural, optical, and electrochemical properties of three-dimensional push-pull corannulenes.

Electrochemically active corannulene derivatives with various numbers of electron-donating 4-(N,N-dimethylamino)phenylethynyl (1-4) or electron-withdrawing cyanobutadienyl peripheral substitutents (5-8) were prepared. The latter derivatives resulted from formal [2 + 2] cycloaddition of cyanoolefins to 1-4 followed by retro-electrocyclization. Conformational properties were examined by variable-temperature NMR and X-ray diffraction and opto-electronic properties by electronic absorption/emission spectra and electrochemical measurements; these analyses were corroborated by dispersion-corrected density functional calculations at the level of B97-D/def2-TZVPP. In CH(2)Cl(2), 1-4 exhibit intramolecular charge-transfer (ICT) absorptions at 350-550 nm and green (λ(em) ~ 540 nm) or orange (600 nm) fluorescence with high quantum yields (56-98%) and are more readily reduced than corannulene by up to 490 mV. The variation of optical gap and redox potentials of 1-4 does not correlate with the number of substituents. Cyanobutadienyl corannulenes 5-8 show red-shifted ICT absorptions with end-absorptions approaching 800 nm. Intersubstituent interactions lead to distortions of the corannulene core and lower the molecular symmetry. NMR, X-ray, and computational studies on 5 and 8 with one cyanobutadienyl substituent suggested the formation of intermolecular corannulene dimers. Bowl-inversion barriers around ΔG(‡) = 10-11 kcal/mol were determined for these two molecules.

Poly(methylhydrosiloxane)-supported chiral imidazolinones: new versatile, highly efficient and recyclable organocatalysts for stereoselective Diels–Alder cycloaddition reactions

Poly(methylhydrosiloxane) (PMHS) supported organic catalysts promoted the Diels-Alder reaction of dienes with α,β-unsaturated aldehydes, also in pure water, in yields and enantiomeric excesses comparable to those observed with the non-supported catalysts (up to 93% ee). Recycling of the catalysts was performed with no loss of enantioselectivity for at least five reaction cycles.