Off the Beaten Path: Almost Clean Formation of Indene from the ortho-Benzyne + Allyl Reaction.
ABSTRACT: Polycyclic aromatic hydrocarbons (PAHs) play an important role in chemistry both in the terrestrial setting and in the interstellar medium. Various, albeit often inefficient, chemical mechanisms have been proposed to explain PAH formation, but few yield polycyclic hydrocarbons cleanly. Alternative and quite promising pathways have been suggested to address these shortcomings with key starting reactants including resonance stabilized radicals (RSRs) and o-benzyne. Here we report on a combined experimental and theoretical study of the reaction allyl + o-benzyne. Indene was found to be the primary product and statistical modeling predicts only 0.1% phenylallene and 0.1% 3-phenyl-1-propyne as side products. The quantitative and likely barrierless formation of indene yields important insights into the role resonance stabilized radicals play in the formation of polycyclic hydrocarbons.
Project description:An intramolecular benzyne-diene [4 + 2] cycloaddition with broad substrate scope has been realized by using a cleavable silicon tether, allowing access to various polycyclic structures. 2-Bromo-6-(chlorodiisopropylsilyl)phenyl tosylate serves as an efficient platform for (1) rapid attachment of various arynophiles to the benzyne precursor via a Si-O bond and (2) facile generation of benzyne via halogen-metal exchange with Ph3MgLi.
Project description:A new thermal isomerization of polyynes is described. Benzyne intermediates substituted by a C(RR')OR'' substituent adjacent to one of the benzyne sp-hybridized carbons give rise to products in which the OR' moiety has migrated to the proximal benzyne carbon. This process likely proceeds via sequential formation of multiple reactive intermediates: an initial thermally generated benzyne, a strained benzoxetenonium ion, and an o-quinone methide. As some examples demonstrate, the overall transformation can be quite efficient. The mechanism of this novel reaction is further supported by experiments and DFT calculations.
Project description:The removal of two vicinal hydrogen atoms from an alkane to produce an alkene is a challenge for synthetic chemists. In nature, desaturases and acetylenases are adept at achieving this essential oxidative functionalization reaction, for example during the biosynthesis of unsaturated fatty acids, eicosanoids, gibberellins and carotenoids. Alkane-to-alkene conversion almost always involves one or more chemical intermediates in a multistep reaction pathway; these may be either isolable species (such as alcohols or alkyl halides) or reactive intermediates (such as carbocations, alkyl radicals, or ?-alkyl-metal species). Here we report a desaturation reaction of simple, unactivated alkanes that is mechanistically unique. We show that benzynes are capable of the concerted removal of two vicinal hydrogen atoms from a hydrocarbon. The discovery of this exothermic, net redox process was enabled by the simple thermal generation of reactive benzyne intermediates through the hexadehydro-Diels-Alder cycloisomerization reaction of triyne substrates. We are not aware of any single-step, bimolecular reaction in which two hydrogen atoms are simultaneously transferred from a saturated alkane. Computational studies indicate a preferred geometry with eclipsed vicinal C-H bonds in the alkane donor.
Project description:A new benzyne transformation is described that affords versatile biaryl structures without recourse to transition-metal catalysis or stoichiometric amounts of organometallic building blocks. Aryl sulfonamides add to benzyne upon fluoride activation, and then undergo an aryl Truce-Smiles rearrangement to afford biaryls with sulfur dioxide extrusion. The reaction proceeds under simple reaction conditions and has excellent scope for the synthesis of sterically hindered atropisomeric biaryl amines.
Project description:The arylation of bicyclic oxathiane glycosyl donors has been achieved using benzyne generated in situ from 1-aminobenzotriazole (1-ABT) and lead tetraacetate. Following sulfur arylation, glycosylation of acetate ions proceeded with high levels of stereoselectivity to afford ? -glycosyl acetates in a 'one-pot' reaction, even in the presence of alternative acceptor alcohols.
Project description:Reactions of indene and various substituted indoles with [(diimine)PtII(Me)(TFE)]+ cations have been studied (diimine = ArN = C(Me) - C(Me) = NAr; TFE = 2,2,2-trifluoroethanol). Indene displaces the TFE ligand from platinum to form a stable ? coordination complex that, upon heating, undergoes C-H activation with first order kinetics, ?H‡ = 29 kcal/mol, ?S‡ = 10 eu, and a kinetic isotope effect of 1.1 at 60 °C. Indoles also initially form coordination complexes through the C2=C3 olefin, but these undergo rearrangement to the corresponding N-bound complexes. The relative rates of initial coordination and rearrangement are affected by excess acid or methyl substitution on indole.
Project description:In the title compound, C(20)H(13)NO, the indene ring is disordered over two sites with an occupancy ratio of 0.557?(2):0.443?(2). Both disordered components of indene are nearly perpendicular to the naphthalene ring system, making dihedral angles of 90.9?(2) and 85.0?(5)°. The five-membered ring of the 1H-pyrrol-2(3H)-one adopts an envelope conformation with the spiro C atom at the flap position. Inter-molecular classical N-H?O and weak C-H?O hydrogen bonding is present in the crystal structure.
Project description:In situ generated benzyne reacts at room temperature with (triphos)Pt-CH3(+) to form a five-coordinate ?-complex (2) that is isolable and stable in solution. Thermolysis of 2 at 60 °C generates (triphos)Pt(o-tolyl)(+) (3), which is the product of formal migratory insertion of CH3(-) onto the coordinated benzyne. The reaction of 2 with the acid Ph2NH2(+) yields toluene at room temperature over the course of 8 h, while the same reaction with 3 only proceeds to 40% conversion over 2 days. These data indicate that the protonolysis of 2 does not proceed by CH3 migration onto benzyne to form 3 followed by protodemetalation. Instead, the data suggest either that protonation of 2 is first and is followed by H migration to yield a Pt(IV)Ph(Me) dication or that this latter species is generated by direct protonolysis of coordinated benzyne prior to reductive elimination of toluene.
Project description:The benzyne functionalization of chemical vapor deposition grown large area graphene and graphite was performed using a mixture of o-trimethylsilylphenyl triflate and cesium fluoride that react with the carbon surface. The reaction requires at least 2 days of treatment before the appearance of Raman and energy-dispersive X-ray spectral signatures that verify modification. Raman spectra of modified graphene and graphite show a rich structure of lines corresponding to C=C-C, C-H, and low frequency modes of surface-attached benzyne rings.
Project description:A synthetic route to racemic helicenes via a vinylacetylene mediated gas phase chemistry involving elementary reactions with aryl radicals is presented. In contrast to traditional synthetic routes involving solution chemistry and ionic reaction intermediates, the gas phase synthesis involves a targeted ring annulation involving free radical intermediates. Exploiting the simplest helicene as a benchmark, we show that the gas phase reaction of the 4-phenanthrenyl radical ([C14H9]•) with vinylacetylene (C4H4) yields -helicene (C18H12) along with atomic hydrogen via a low-barrier mechanism through a resonance-stabilized free radical intermediate (C18H13). This pathway may represent a versatile mechanism to build up even more complex polycyclic aromatic hydrocarbons such as - and -helicene via stepwise ring annulation through bimolecular gas phase reactions in circumstellar envelopes of carbon-rich stars, whereas secondary reactions involving hydrogen atom assisted isomerization of thermodynamically less stable isomers of -helicene might be important in combustion flames as well.