Rajendra Rathore

Oxidative C−C Bond Formation (Scholl Reaction) with DDQ as an Efficient and Easily Recyclable Oxidant

DDQ in the presence of an acid is known to oxidize a variety of aromatic donors to the corresponding cation radicals. Herein, we now demonstrate that the DDQ/H(+) system can be effectively utilized for the oxidative C-C bond formations or biaryl synthesis. The efficient preparation of a variety of polyaromatic hydrocarbons including graphitic hexa-peri-hexabenzocoronenes, ease of isolation of the clean products, and ready regeneration of DDQ from easily recovered reduced DDQ-H(2) advances the use of DDQ/H(+) for Scholl reactions.

Probing the arenium-ion (protontransfer) versus the cation-radical (electron transfer) mechanism of Scholl reaction using DDQ as oxidant.

DDQ/H(+) system readily oxidizes a variety of electron donors with oxidation potential as high as approximately 1.7 V to the corresponding cation radicals. A re-examination of the controversial arenium-ion versus cation-radical mechanisms for Scholl reaction using DDQ/H(+) together with commonly utilized FeCl(3) as oxidants led us to demonstrate that the reaction proceeds largely via a cation-radical mechanism. The critical experimental evidence in support of a cation-radical pathway for the Scholl reaction includes the following: (i) There is no reaction in Scholl precursors in a mixture of dichloromethane and various acids (10% v/v). (ii) The necessity to use powerful oxidants such as ferric chloride (FeCl(3)) or DDQ/H(+) for Scholl reactions is inconsistent with the arenium-ion mechanism in light of the fact that aromatization of the dihydro intermediates (formed via arenium-ion mechanism) can be easily accomplished with rather weak oxidants such as iodine or air. (iii) Various Scholl precursors with oxidation potentials <or=1.7 V vs SCE undergo ready oxidative C-C bond formation with DDQ/H(+) as oxidant, whereas Scholl precursors with oxidation potentials greater than >1.7 V vs SCE do not react. (iv) Finally, the feasibility of the dicationic intermediate, formed by loss of two electrons, has been demonstrated by its generation from a tetraphenylene derivative using DDQ/H(+) as an oxidant.

Synthesis, Optical, and Electronic Properties of Soluble Poly-p-phenylene Oligomers as Models for Molecular Wires

A homologous series of soluble poly-p-phenylenes containing up to eight phenylene moieties (PP(2)-PP(8)) with branched iso-alkyl (or bis-n-alkylmethyl) groups has been synthesized and the structure-property relationship with regards to various optical and electronic properties established. All electronic and optoelectronic properties of poly-p-phenylenes followed a 1/n relationship (where n is number of phenylene moieties) with the increasing number of phenylene moieties. The low-energy electronic transition of the PP(2)-PP(7) cation radicals, generated either by laser-flash photolysis or by chemical oxidation, varied as well according to the inverse (1/n) relationship. The observed inverse relationship of all measured electronic and optoelectronic properties against the increasing number of phenylene units in various PP(n)s, as well as X-ray crystallography of both neutral and a cation-radical salt of a representative tetra-p-phenylene oligomer allows us to demonstrate that the effective conjugation length in poly-p-phenylenes is, in part, controlled by the increasing number of interactions of ortho hydrogens which may prevent simultaneous planarization of the continuous arrays of a large number of phenylene moieties.

Cooperative interaction of hydronium ion with an ethereally fenced hexaarylbenzene-based receptor: an NMR and theoretical study.

Using (1)H and (13)C NMR and DFT calculations, the structure and interactions of the symmetric ethereally fenced hexaarylbenzene receptor 1 with hydronium ions were studied. Both 1 and its equimolecular complex 1.H(3)O(+) exhibit C(3v) symmetry. According to DFT, two similar optimal structures of the complex exist, the more stable one being 15.4 kJ/mol lower in energy. The equilibrium between 1 and 1.H(3)O(+) complexes is characterized by the stabilization constant K = 1.97 x 10(6) (i.e., the binding constant eta = 6.3) according to both proton and carbon NMR spectra. The exchange dynamics between 1 and the complex measured by the delay-varied CPMG sequence had to be corrected for the internal exchange processes in both 1 (conformation change) and the complex (vacillation between the two minima). After this correction, the correlation time of exchange was found to be 4.76 x 10(-5) s. Such relatively fast exchange can be explained only by it being mediated by the excess water molecules present in the system.

ortho-Phenylenes: unusual conjugated oligomers with a surprisingly long effective conjugation length.

ortho-Phenylenes represent a fundamental but relatively unexplored class of conjugated molecular architecture. We have developed a robust synthetic approach to monodisperse o-phenylene oligomers which we have demonstrated by synthesizing a homologous series up to the dodecamer. The o-phenylenes exhibit complex conformational behavior but are biased toward a specific 2-fold-symmetric conformation which we believe corresponds to a stacked helix. Surprisingly, the series exhibits long-range delocalization, as measured by bathochromic shifts in UV/vis spectra. Although the overall magnitude of the shifts is modest (but comparable to some other classes of conjugated materials), the effective conjugation length of the series is approximately eight repeat units. The oligomers also exhibit an unusual hypsochromic shift in their fluorescence spectra with increasing length. The origin of these trends is discussed in the context of conformational analysis and DFT calculations of the frontier molecular orbitals for the series.

Sequential Oxidative Transformation of Tetraarylethylenes to 9,10-Diarylphenanthrenes and Dibenzo[g,p]chrysenes using DDQ as an Oxidant

Tetraarylethylenes can be sequentially transformed into 9,10-diarylphenanthrenes and dibenzo[g,p]chrysenes using 1 and 2 equiv of DDQ, respectively, in CH(2)Cl(2) containing methanesulfonic acid, in excellent yields. Efficient access to substituted dibenzochrysenes from tetraarylethylenes establishes the versatility of this procedure over the existing multistep syntheses of dibenzochrysenes. Moreover, the ready regeneration of DDQ from easily recovered reduced DDQ-H(2) continues to advance the use of DDQ/H(+) for the oxidative C-C bond forming reactions.

Crossover from Single-Step Tunneling to Multistep Hopping for Molecular Triplet Energy Transfer

Tracking Triplet-State Transfers In devices such as organic transistors and photovoltaic cells, energy flow from donor to acceptor sites can occur via electrons that have been excited into higher electronic levels, which create a triplet state with two unpaired spins. At short distances between donor and acceptor, the transfer occurs through direct tunneling, but at longer distances, the electron “hops” in a multistep process. Vura-Weis et al. (p. 1547) used femtosecond transient absorption spectroscopy to observe this crossover in transfer mechanism directly in a series of molecules with varying bridge lengths between the donor and acceptor. Ultrafast absorption spectroscopy reveals a change in mechanism when sites for electron transfer become more distant. Triplet energy transfer (TT), a key process in molecular and organic electronics, generally occurs by either strongly distance-dependent single-step tunneling or weakly distance-dependent multistep hopping. We have synthesized a series of π-stacked molecules consisting of a benzophenone donor, one to three fluorene bridges, and a naphthalene acceptor, and studied the rate of TT from benzophenone to naphthalene across the fluorene bridge using femtosecond transient absorption spectroscopy. We show that the dominant TT mechanism switches from tunneling to wire-like hopping between bridge lengths 1 and 2. The crossover observed for TT can be determined by direct observation of the bridge-occupied state.

A Facile Synthesis of Elusive Alkoxy-Substituted Hexa-peri-hexabenzocoronene

Oxidative cyclodehydrogenation of hexakis(4-alkoxyphenyl)benzene produces a quantitative yield of an indenofluorene derivative rather than the expected alkoxy-substituted hexa-peri-hexabenzocoronene (HBC). The structure of the unexpected indenofluorene was established by X-ray crystallography. The mechanistic considerations for the formation of the indenofluorene derivative led us to devise an alternative synthesis of elusive alkoxy-substituted HBC-a potentially important, disk-shaped structure for the preparation of liquid crystalline materials for practical applications in the emerging areas of molecular electronics and nanotechnology.

A Convenient Method of Benzylic Oxidation with Pyridinium Chlorochromate

Abstract Oxidation of indans and tetralin derivatives to their corresponding indanones and tetralones is of considerable value i n organic synthesis and many methods have been reported for accomplishing this conversion. Traditionally these oxidations are performed with chromic acid in acetic acid and the yields in general are moderate. Recently Eisenbraun4 has studied indetail the benzylic oxidation with the Jones reagent and compared the selectivity and yield of oxidation with other chromium(V1) reagents like bipyridinim chlorochrmate, (BiPCC). It was found that there was no improvement in yield and in the case of BiPCC a molar ratio 16% 1 (oxidant: substrate) was employed to get

Key Role of End-Capping Groups in Optoelectronic Properties of Poly-p-phenylene Cation Radicals.

Poly-p-phenylenes (PPs) are prototype systems for understanding the charge transport in π-conjugated polymers. In a combined computational and experimental study, we demonstrate that the smooth evolution of redox and optoelectronic properties of PP cation radicals toward the polymeric limit can be significantly altered by electron-donating iso-alkyl and iso-alkoxy end-capping groups. A multiparabolic model (MPM) developed and validated here rationalizes this unexpected effect by interplay of the two modes of hole stabilization: due to the framework of equivalent p-phenylene units and due to the electron-donating end-capping groups. A symmetric, bell-shaped hole in unsubstituted PPs becomes either slightly skewed and shifted toward an end of the molecule in iso-alkyl-capped PPs or highly deformed and concentrated on a terminal unit in PPs with strongly electron-donating iso-alkoxy capping groups. The MPM shows that the observed linear 1/n evolution of the PP cation radical properties toward the polymer limit originates from the hole stabilization due to the growing chain of p-phenylene units, while shifting of the hole toward electron-donating end-capping groups leads to early breakdown of these 1/n dependencies. These insights, along with the readily applicable and flexible multistate parabolic model, can guide studies of complex donor–spacer–acceptor systems and doped molecular wires to aid the design of the next generation materials for long-range charge transport and photovoltaic applications.