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A straightforward synthesis of cyclopropenylidene-stabilized phosphenium cations 1 a-g through the reaction of [(iPr2N)2C 3+Cl]BF4 with secondary phosphines is described. Their donor ability was evalu

Evidence is presented indicating that the stereochemistry of the Stevens rearrangement of the axially chiral onium salts 1a-d and 5 is dramatically structure-dependent. Thus, the binaphthyl ammonium salts (S)-(+)-1a-c react with a strong base with exclusive (100% de) formation of the corresponding rearranged amines (R,3R)-(+)-2a-c, demonstrating a complete transfer of the (S) axial dissymmetry/asymmetry into (R) asymmetry of the newly formed carbon center. Exactly opposite stereochemistry was established in an earlier study by Mislow of the biphenyl analogue (S)-(+)-5, yielding the rearranged products (S,9S)-(+)-6 and (R,9S)-(-)-7 with exclusive (S) configuration at the carbon center. Rearrangement of the sulfonium salt 1d is found to be intermediate between the two extremes, yielding a mixture of diastereoisomeric products 2d and 3d, which differ in configuration at the asymmetric carbon center. A direct proof is thus provided that two stereochemically different pathways can participate in the Stevens rearrangement. An explanation is suggested in terms of competition between suprafacial (concerted) and antarafacial (nonconcerted) mechanism.

A photochemical synthesis of [5]helicene employing a copper-based sensitizer 7 has been developed that avoids the disadvantages associated with the traditional UV light mediated method. The visible light mediated synthesis uses common glassware and a simple household light bulb without the competing formation of [2 + 2] cycloadducts, regioisomers, or the overoxidation product benzo[ghi]perylene 3. Preliminary results show that the reaction time can be significantly reduced through the use of a continuous flow strategy.

A facile synthesis of carbo- and heterohelicenes was achieved via tandem cycloaromatization of bisacetal precursors, which were readily prepared through C-C bond formation by Suzuki-Miyaura coupling. This cyclization was efficiently realized by a catalytic amount of trifluoromethanesulfonic acid (TfOH) in a cation-stabilizing solvent, 1,1,1,3,3,3-hexafluoropropan-2-ol (HFIP), which readily allowed gram-scale syntheses of higher-order helicenes, double helical helicenes, and heterohelicenes.

Vicinally diiodinated polycyclic aromatic hydrocarbons (I2-PAHs) are accessible from the corresponding diborylated B2-PAHs through boron/iodine exchange. The B2-PAHs have been prepared via twofold electrophilic borylation reactions templated by a vicinally disilylated benzene. Our protocol is applicable to fluorenes, acenes, annulated acenes, oligoaryls, and even [5]helicene. Using B2-naphthalene as the example, we have shown that the reaction scope can, in principle, be expanded to include the synthesis of vicinally dibrominated and dihydroxylated PAHs. The usefulness of the building blocks provided by our method in the field of optoelectronic materials was demonstrated by the successful conversion of I2-fluoranthene to the analogous doubly alkynylated fluoranthene emitter.

Photocyclodehydrogenation is a key reaction to synthesize helicenes; however, because of overannulation, it is not applicable to the synthesis of [5]helicene. Introduction of a cyano group was found to remove the orbital degeneracy of the low-lying unoccupied MOs; consequently, the lowest excitation comprises a single transition involving the C2-antisymmetric MO. Therefore, the problematic overannulation can be effectively suppressed. Moreover, in combination with the Knoevenagel reaction, a one-pot synthesis of 5,10-dicyano[5]helicene with 67% yield was accomplished.

Readily available biphenyl derivatives with ortho oxirane moiety react in the presence of cationic chromiun(III) porphyrin catalyst to afford phenanthrenes. The reaction is considered to be triggered by activation of the oxirane moiety through coordination to the Lewis acidic cationic chromium to give aldehyde via 1,2-hydride shift, which reacts with arene through intramolecular electrophilic aromatic substitution and subsequent dehydration. The reaction allows constructing a variety of polycyclic aromatic and heteroaromatic compounds.

Ahead of the PAC: Polycyclic aromatic compounds (PACs) can be easily accessed by the combination of Suzuki-Miyaura cross-coupling and a [Rh 2(OAc)4]-catalyzed carbene reaction using easily available bis(N-tosylhydrazone)s as intermediates (see scheme; Ts=4-toluenesulfonyl). Copyright

Reaction of tris(dimethylaminocyclopropenium) substituted phosphine 1 with K2PtCl4 afforded the bench stable complex 3 which upon treatment with Ag[CB11H6Cl6] turned out to be an excellent catalyst for the transformation of a variety of ortho-biaryl substituted alkynes into polycyclic homo- and heteroarenes of different size, shape, and curvature through a 6-endo-dig cyclization. This constitutes the first example ever reported of using a P1-centered trication as ligand in catalysis. The strong π-acceptor character of 1 that derives from its three positive charges substantially increases the intrinsic π-acidity of Pt in complex 1?PtCl2 and dramatically enhances its ability to activate π-systems toward nucleophilic attack. As a consequence, a remarkable acceleration of the model transformation is observed when compared with other classical π-acceptor ligands such as P(OPh)3 or P(C 6F5)3. Moreover, the employment of 1 as ligand also expands the scope of this reaction to previously inaccessible substitution patterns. Kinetic studies and deuterium labeling experiments as well as density functional theory (DFT) calculations were performed in order to explain these findings.

A general synthetic methodology for the preparation of functionalized (hetero)helicenes has been developed. It employs the sequence of a double propargyl organometallics (Li, Mg, Ga/In) addition to a tolan-2,2′- dialdehyde-type intermediate, a cobalt-catalyzed/cobalt-mediated [2 + 2 + 2] cycloisomerization of a triyne intermediate, and a double silica gel-assisted acetic acid elimination to receive pentahelicene, 1,14-diazapentahelicene, and 3,12-dichloro-, 3,12-dichloro-7-trimethylsilyl-, and 3,12-di-tert- butylpentahelicene. 3,12-Dichloropentahelicene undergoes a Suzuki-Miyaura coupling with aryl boronic acids (or ester) under palladium catalysis to afford 3,12-diarylpentahelicenes.

(Chemical Equation Presented) Metathesis with a twist! A ring-closing-metathesis strategy has been developed for the preparation of various substituted [5]helicene motifs and [6]- and [7]helicenes. The two optimized protocols include one method that utilizes the Grubbs second-generation catalyst under microwave-irradiation conditions and another that employs a modified Grubbs-Hoveyda catalyst at 40°C in a sealed reaction vessel. (Mes = mesityl).

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The synthesis of substituted polyaromatic compounds that contain at least four benzene rings fused together in an angular fashion is described. Suzuki coupling of 1-bromo-3,4-dihydronaphthalene-2-carbaldehyde with a number of aromatic boronic acids affords products such as 1-(1,4-dimethoxy-3-methyl-2-naphthyl)-3,4-dihydronaphthalene-2-carbaldehyde. Exposure of these dihydronaphthalenes to potassium tert-butoxide and DMF at 80°C yields polyaromatic compounds such as 9,14-dimethoxynaphtho[1,2-a]anthracene.

Readily available biphenyl derivatives containing an alkyne unit at one of their ortho-positions are converted into substituted phenanthrenes on exposure to catalytic amounts of either PtCl2, AuCl, AuCl3, GaCl3 or InCl3 in toluene. This 6-endo-dig cyclization likely proceeds through initial π-complexation of the alkyne unit followed by interception of the resulting η2-metal species by the adjacent arene ring. The reaction is inherently modular, allowing for substantial structural variations and for the incorporation of substituents at any site of the phenanthrene product. Moreover, it is readily extended to the heterocyclic series as exemplified by the preparation of benzoindoles, benzocarbazoles, naphthothiophenes, as well as bridgehead nitrogen heterocycles such as pyrrolo[1,2-a]quinolines. Depending on the chosen catalyst, biaryls bearing halo-alkyne units can either be converted into the corresponding 10-halo-phenanthrenes or into the isomeric 9-halo-phenanthrenes; in the latter case, the concomitant 1,2-halide shift is best explained by assuming a metal vinylidene species as the reactive intermediate. The scope of this novel method for the preparation of polycyclic arenes is illustrated by the total synthesis of a series of polyoxygenated phenanthrenes that are close relatives of the anticancer agent combretastatin A-4, as well as by the total synthesis of the aporphine alkaloid O-methyl-dehydroisopiline and its naturally occurring symmetrical dimer.

A new and rapid approach to [5]helicenes is described. A central feature is the use of a sequential, non-reducing radical cyclisation reaction of (Z,Z)-1,4-bis(2-iodostyryl)benzene derivatives, viz. 3→1, mediated by tributyltin hydride.

An original approach to helicene frameworks exploiting atom economic isomerization of appropriate energy-rich aromatic cis,cis-dienetriynes has been developed. The new paradigm provides nonphotochemical syntheses of helicenes based on the easy, convergent, and modular assembly of key cis,cis-dienetriynes and their nickel(0)-catalyzed [2+2+2] cycloisomerization. The potential of the methodology is underlined by the syntheses of the parent [5]helicene (2), 7,8-dibutyl[5]helicene (23), [6]helicene (24), and [7]helicene (25). The approach can be adapted to prepare functionalized helicenes as exemplified by the eight-step synthesis of 7,8-dibutyl-2,3-dimethoxy[6]helicene (34). Density functional theory (DFT) calculations showed that bis[2-((1Z)-1-buten-3- ynyl)phenyl]acetylene (1) and isomeric [5]helicene (2) differ enormously in the Gibbs energy content (ΔG= -136.6 kcal.mol-1) to favor highly the devised intramolecular simultaneous construction of three aromatic rings.

Tetrahydro analogues of [5]-, [6]- and [7] helicene have been easily prepared by intramolecular [2+2+2] cycloisomerization of appropriate triynes under CpCo(CO)2/PPh3 or Ni(cod)2/PPh3 catalysis. This nonphotochemical methodology allows enantioselective synthesis of a helical skeleton employing the Ni(cod)2/(S)-(-)-MOP catalytic system. On reaction with DDQ, tetrahydro[5]helicene was transformed to [5]helicene.

Short synthetic sequences to carbohelicenes have been achieved under thermal conditions, without using photochemistry and high dilution. Couplings of aromatic bis(bromomethyl) moieties, in the presence of an excess of LiHMDS, are key reactions in the final ring closures to carbohelicenes. These optimized, quick and efficient reactions occur at 0°C within 10 min. and often provide [5]-helicene, [5]-helicene derivatives and [7]-helicene in ~ 75% yield. Preliminary data questioned the formation of carbenoid anions and carbenes in the so called 'carbenoid couplings'.

A two-step synthetic approach to penta- and hexahelicenes substituted at the terminal aromatic rings has been studied. This approach is based on the Diels-Alder reaction of 5,5',8,8'-tetramethyl-3,3',4,4'-tetrahydro-[1,1']- binaphthalene (2b), 3-vinyl-1,2-dihydronaphthalene (5a), 5,8-dimethyl-3- vinyl-1,2-dihydronaphthalene (5b) and 3-vinyl-1,2-dihydrophenanthrene (15) followed by the aromatization of the cycloadducts. This method is flexible, efficient and of wide application to the synthesis of several benzenoid and nonbenzenoid penta- and hexahelicenes. The racemization energy barriers of helicenes 3, 9, 10, 13, 14, 17 and 18 were computed with the semiempirical quantum method AM1. The computed values show that, when methyl groups are introduced at the inner position of the terminal aromatic rings, the racemization barriers increase markedly. A structure analysis of the reaction products by 1H- and 13CNMR spectroscopy is also presented.

Short syntheses of [5]-helicene have been accomplished under thermal conditions, without using photochemistry and high dilution. Key reactions comprised a Mcmurry coupling of a dialdehyde or a carbenoid-type coupling of aromatic bis(bromomethyl) moieties. The last coupling provide a 72% yield of [5]-helicene on a gram-scale (15 min O°C).

Bromine substituents on benzene rings direct photocyclizations away from their ortho positions.This principle prevents helicenes from photocyclizing and methoxyls from photoeliminating.It makes possible the photosynthesis of helicene and of a tetramethoxy-derivative of helicene.