John D. Spence

Syntheses, thermal reactivities, and computational studies of aryl-fused quinoxalenediynes: effect of extended benzannelation on Bergman cyclization energetics.

A series of [b]-fused 6,7-diethynylquinoxaline derivatives have been synthesized through an imine condensation strategy to examine the effect of extended benzannelation on the thermal reactivity of enediynes. Absorption and emission spectra of the highly conjugated quinoxalenediynes were red-shifted approximately 100-200 nm relative to those of 1,2-diethynylbenzene. Strong exotherms indicative of enediyne cyclization were observed by differential scanning calorimetry, while solution cyclizations in the presence of 1,4-cyclohexadiene confirmed C(1)-C(6) Bergman cyclization. To provide further insight into Bergman cyclization energetics, computational studies were performed to compare changes in the cyclization enthalpy barrier, reaction enthalpy, and barrier of retro-Bergman ring-opening. Extension of benzannelation from 1,2-diethynylbenzene to either 2,3-diethynylnaphthalene or the 6,7-diethynylquinoxalines had a minimal effect on the cyclization barrier. In comparison, the enthalpies of cyclization were increased upon linearly extended benzannelation, which resulted in reduced barriers to retro-Bergman ring-opening. In addition, the orientation of extended benzannelation was found to have a significant effect on the cyclization endothermicity. In particular, 5,6-diethynylquinoxaline exhibited a 6.9 kcal/mol decrease in cyclization enthalpy compared to 6,7-diethynylquinoxaline due to increased aromatic stabilization energy in the respective angularly versus linearly fused azaacene cyclized products.

Syntheses and Reactivity of Naphthalenyl-Substituted Arenediynes

A series of naphthalenyl-substituted arenediynes were prepared to examine photochemical reactivity. For naphthalen-1-ylethynyl arenediyne, 350 nm photolysis resulted in a tandem [2 + 2] photocycloaddition to afford cyclobutene adducts. For naphthalen-2-ylethynyl derivatives, electron-donating methoxy substituents were found to facilitate C(1)-C(6) Bergman cyclization at 300 nm. Theoretical calculations provided further insight into thermal and photochemical reactivity.

Synthesis of diazacalix[8]arene and triazacalix[12]arene methyl ethers via intramolecular aryl amination.

Azacalixarenes derived from p-tert-butylphenol are generated by an intramolecular aryl amination strategy as the ring-closing step. The reaction produces the first examples of larger p-tert-butylcalixarenes with regioselective substitution of bridging methylenes with nitrogen atoms.

Towards hydrocarbon analogues of the porphyrins: synthesis and spectroscopic characterization of the first dicarbaporphyrin†

Condensation of 3,4-diethylpyrrole with 1,3-diformylindane in the presence of catalytic HBr afforded, following oxidation with FeCl3, the first example of a dicarbaporphyrin in moderate yield; this novel bridged annulene system retains a strong diamagnetic ring current in 1H NMR spectroscopy as well as a porphyrin-like UV–VIS spectrum.

Facile oxidation of a carbaporphyrin at the internal carbon atom: synthesis of novel benzo[18]annulene ketals†

Treatment of benzocarbaporphyrin 2 with refluxing FeCl3 in CHCl3–alcohol mixtures leads to a remarkably selective oxidation at the interior carbon atom to produce dialkoxy products 5; these species show potentially valuable long wavelength absorptions in their UV–VIS spectra.

Synthesis and Bergman cyclization of a ?-extended porphyrenediyneElectronic Supplementary Information (ESI) available: experimental details for the synthesis of compounds 7 and 8; 1H and 13C NMR spectra of 7 and 8. See http://www.rsc.org/suppdata/cc/b3/b312001e

Condensation of a porphyrin-2,3-dione with a 1,2-diaminoarenediyne affords a β-extended porphyrinic-enediyne: upon thermal Bergman cyclization the quinoxaline spacer positioned between the macrocycle and the enediyne prevents tandem radical cyclization to a picenoporphyrin.

Effect of Extended Benzannelation Orientation on Bergman and Related Cyclizations of Isomeric Quinoxalenediynes

A combined computational and experimental study was conducted to examine the effect of extended benzannelation orientation on C1-C5 and C1-C6 cyclization of acyclic quinoxalenediynes. Calculations (mPW1PW91/cc-pVTZ//mPW1PW91/6-31G(d,p)) on terminal and phenylethynyl-substituted 5,6-diethynylquinoxaline and 6,7-diethynylquinoxaline showed C1-C6 Bergman cyclization as the favored thermodynamic reaction pathway, with larger C1-C6 preference for the angular quinoxalenediynes due to gain of a new aromatic sextet. Kinetic studies, as a function of 1,4-cyclohexadiene concentration, revealed retro-Bergman ring opening predominates over hydrogen atom abstraction (k-1 > k2) for 6,7-diethynylquinoxaline while 5,6-diethynylquinoxaline undergoes irreversible Bergman cyclization indicative of a large retro-Bergman ring opening barrier (k2 > k-1). The effect of extended linear versus angular benzannelation on reaction pathway shows in the contrasting photocyclizations of phenylethynyl derivatives. While angular 5,6-diethynylquinoxalines gave exclusive C1-C6 photocyclization, linear 6,7-diethynylquinoxaline afforded C1-C5 fulvene products. Computed singlet-triplet gaps and biradical stabilization energies indicated weak interaction between the nitrogen lone pair and proximal radical center in angular 5,6-diethynylquinoxalines. The overall data indicates extended angular benzannelation effectively renders Bergman cyclization irreversible due to favorable aromatic stabilization energy, while extended linear benzannelation results in increased retro-Bergman ring opening, allowing C1-C5 cyclization to become a competitive reaction channel.