Dihydrophenanthrene Open-Shell Singlet Diradicals and Their Roles in the Mallory Photocyclization Reaction.

A computational study (ωB97X-D/6-31G(d)) of the Mallory photocyclization reaction has revealed that the well-established dihydrophenanthrene (DHP) intermediates can adopt either closed-shell (CS) or open-shell-diradical (OS) singlet ground states. A detailed study of the properties of DHPs allowed their classifications as OS, borderline-OS, borderline-CS, or CS intermediates. The triplet electronic state and higher energy CS* isomer of all the OS singlet diradicals were computationally located, and the expected relationship between the diradical index, , and the triplet energy and the OS-CS* energy gaps was established.

Aromatic Endoperoxides.

The fundamental aspects of aromatic endoperoxide chemistry are reviewed including their synthesis and reactions. The discussion will focus on factors that will both enhance and prevent the formation of aromatic endoperoxides and on structural features that will provide control over their ability to release singlet oxygen. This approach recognizes the dual use of aromatic hydrocarbons as both precursors of endoperoxides and as valuable materials for incorporation in electronic and photonic devices.

Synthesis, Structure, and Photochemical Behavior of [5]Heli-viologen Isomers.

The syntheses of isomeric helical viologens that have potential applications in supramolecular chemistry and catalysis have been developed. The structures of the molecules and their solid-state packing motifs have been determined by X-ray crystallography. Computational studies demonstrate that the magnitude of their racemization barriers is primarily determined by the identity of the helical scaffold and is insensitive to the placement of the viologen functional group.

Photophysical and electrochemical characterization of a helical viologen, N,N'-dimethyl-5,10-diaza[5]helicene.

The first helical viologen (4,4'-bipyridinium salt) has been prepared and characterized. Its reduction to the radical cation at -0.22 V vs SCE makes it the most easily reduced redox-active helicene known.

Synthesis, characterization, photophysics and photochemistry of pyrylogen electron transfer sensitizers.

A series of new dicationic sensitizers that are hybrids of pyrylium salts and viologens has been synthesized. The electrochemical and photophysical properties of these "pyrylogen" sensitizers are reported in sufficient detail to allow rationale design of new photoinduced electron transfer reactions. The range of their reduction potentials (+0.

Hydrolytic stability of N-methyl-2,6-dimesityl-4,4'-pyrylogen bis-tetrafluoroborate.

The synthesis and characterization of a new mesityl ring-substituted pyrylogen with a substantially decreased rate of reaction with water is reported. Computational and experimental data are presented that suggest that addition of water to the pyrylium ring of this highly sterically shielded pyrylogen is reversible. On the other hand, experimental data suggest that the overall hydrolysis of this new sterically shielded pyrylogen, but not the parent pyrylogen, is irreversible.

Computational and experimental evidence for the first direct spectroscopic detection of the pyrylogen neutral redox partner.

The first synthesis of the two-electron reduction product of a pyrylogen is reported. The magnitude of the experimentally determined disproportionation constant for a pyrylogen radical cation was used to advantage in order to provide compelling evidence for formation of this reduction product. Computational studies were used to provide verification of these results and to provide additional insight into the pyrylogen redox system.

Syntheses and properties of the new electron transfer sensitizers 4,2'-pyrylogens.

The syntheses and characterizations of the 4,2'-regioisomers of the dicationic pyrylogen electron transfer sensitizers are reported. The electrochemical and photophysical properties of these sensitizers are compared to the previously reported 4,4'-pyrylogens.

Photooxygenation of 1,5-thiaselenocane.

A direct comparison of photooxygenation at sulfur and selenium centers is reported, and the reactivity of 1,5-thiaselenocane is compared to that of 1,5-dithiacyclooctane.

Pyrylogens: synthesis, structural, electrochemical, and photophysical characterization of a new class of electron transfer sensitizers.

The synthesis and photophysical properties of a new series of dicationic electron transfer sensitizers have been reported. These new materials, pyrylogens, are hybrids of pyrylium cations and Viologen dications. Electron transfer reactions of neutral organic substrates using these new sensitizers generate radical-cation/radical-cation pairs whose repulsive (repellent) interaction is designed to compete with energy wasting return electron transfer (RET) by enhancing diffusive separation and formation of solvent separated ions.

Role of sulfide radical cations in electron transfer promoted molecular oxygenations at sulfur.

The methylene blue, N-methylquinolinium tetrafluoroborate, and pyrylium-cation-sensitized photooxygenations of 5H, 7H-dibenzo[b,g] [1,5]dithiocin, 1, and 1,5-dithiacyclooctane, 2, have been investigated. The methylene blue sensitized reactions exhibit all of the characteristics of a singlet oxygen reaction including isotope effects for the formation of a hydroperoxysulfonium ylide and the ability of 1 and 2 to quench the time-resolved emission of singlet oxygen at 1270 nm. The product compositions in the N-methylquinolinium tetrafluoroborate and pyrylium-cation-sensitized reactions are dramatically different and are both different from that anticipated for the participation of singlet oxygen.

Experimental and computational studies of nuclear substituted 1,1'-dimethyl-2,2'-bipyridinium tetrafluoroborates.

The synthesis and spectral properties of a new 2,2'-bipyridinium ion, 1,1'-dimethyl-4,4'-(dimethylamino)-2,2'-bipyridinium bis(tetrafluoroborate) are reported. Rotation of the dimethylamino group is slow at room temperature on the 400 MHz 1H and 100 MHz 13C NMR time scales. Complete line shape fit of the dynamically broadened NMR spectra was used to determine the activation barriers for this process.

Photooxidations of alkenes in fluorinated constrained media: fluoro-organically modified NaY as improved reactors for singlet oxygen "Ene" reactions.

Creating a stationary fluorinated environment inside the zeolite cavity can increase the reactivity observed for intrazeolite photooxidation of alkenes. Exchanging the zeolite with fluorinated organic cations is a much more effective strategy than simply using a fluorinated solvent for slurry irradiations. Use of cations containing C-F bonds is also more efficient than use of deuterated cations for creation of a singlet oxygen friendly environment where the quenching processes are slowed down.

A comparison of intrazeolite and solution singlet oxygen Ene reactions of allylic alcohols.

The singlet oxygen ene reactions of four allylic alcohols and for comparison an allylic ether have been examined both in solution and in zeolite Y. Brønsted acid sites in the zeolite were shown to induce decomposition of several of the allylic alcohols. Treatment of the zeolites with pyridine removed these acid sites and allowed intrazeolite reactions of the allylic alcohols without interference from decomposition.

Natural bond orbital analyses of persulfoxide stabilization by remote functional groups. The conformationally induced electrostatic stabilization sulfide photooxygenation mechanism.

The conformationally induced electrostatic stabilization (CIES) sulfide photooxygenation mechanism was computationally examined using an ab initio model and extended to the study of new donor atoms. The MP2/6-31G(d) geometries and a natural population analysis of natural lone-pair orbitals on the donor atoms support the mechanism and reveal that oxygen and nitrogen donor groups are more stabilizing than sulfur.

Syntheses, characterizations, and properties of electronically perturbed 1,1'-dimethyl-2,2'-bipyridinium tetrafluoroborates.

[reaction: see text] The syntheses of three new 2,2'-bipyridinium tetrafluoroborate sensitizers are reported. Their preliminary electrochemical and photophysical properties are compared to the properties of the more widely used pyrylium cation sensitizers. In addition, the first examples of triplet-triplet absorption spectra of 2,2'-bipyridinium ions are presented.

Conformationally induced electrostatic stabilization of persulfoxides: a new suggestion for inhibition of physical quenching of singlet oxygen by remote functional groups.

1,5-Dithiacyclooctane is shown to chemically react more efficiently and to remove singlet oxygen from solution more rapidly than either thiane or 1,4-dithiane. These unusual characteristics of the 1,5-dithiacyclooctane reaction were explored using ab initio quantum chemical methods. A large number of persulfoxides, thiadioxiranes, and hydroperoxy sulfonium ylides were located and their structures analyzed.

Comparison of photoelectron transfer in solution and the solid state for two intervalence radical cations.

[structure: see text] The optical diffuse reflectance and solution spectra of two bis-hydrazine radical cationic intervalence compounds have been compared. The results are consistent with an ion-pairing increase and an "effective polarity" in these crystals that is not far from that of acetonitrile or other polar solvents.

Zeolite-promoted oxidations of 1,1-diarylethylenes.

The intrazeolite photooxygenations of four diarylethylenes have been examined. Several intermediates, including an epoxide, have been identified by comparison to independently synthesized samples. Aldehyde intermediates were shown to undergo intrazeolite Norrish type I cleavages in competition with a novel new photooxygenation/autoxidation reaction.

Isotope effects as mechanistic probes in solution and in intrazeolite photooxygenations. The formation of a hydroperoxysulfonium ylide.

The reactions of singlet oxygen with 2,2,6,6-tetradeuterio-1,4-dithiane have been examined in acetone and methanol, and in the interior of the zeolite NaY. The product isotope effects k(H)/k(D) have been measured for sulfoxide and, when possible, for sulfone formation. The results provide evidence for a hydroperoxysulfonium ylide intermediate in acetone, a hydrogen bonded or sulfurane intermediate in methanol, and an interesting equilibrium between two complexed forms of the substrate in NaY.

Mechanistic organic chemistry in a microreactor. Zeolite-controlled photooxidations of organic sulfides.

The intrazeolite and solution photooxygenations of a series of sulfides have been compared. The unusual zeolite environment enhances the rates of reaction, it suppresses the Pummerer rearrangements, and it has a dramatic effect on the sulfoxide/sulfone ratio. A detailed kinetic study utilizing trapping experiments and intramolecular competition provides evidence for cation complexation to a persulfoxide intermediate as the underlying phenomenon for the unique intrazeolite behavior.

A new experimental protocol for intrazeolite photooxidations. The first product-based estimate of an upper limit for the intrazeolite singlet oxygen lifetime.

The intrazeolite photooxidations of several alkenes have been examined in hexane and in perfluorohexane slurries. The ability of perfluorohexane to increase the affinity of alkenes for the interior of the zeolite was documented. The nearly identical reaction rates in the two slurry solvents are attributed to a leveling of the singlet oxygen lifetime by the zeolite framework.

Intrazeolite photooxidations of electron-poor alkenes.

The inability of the intrazeolite environment to influence the regiochemistry of addition of singlet oxygen to electron-poor olefins is reported. This divergent behavior with respect to electron-rich alkenes is rationalized with a multicomplexation model that emphasizes the importance of intrazeolite substrate-cation binding.

The first example of a singlet oxygen induced double bond migration during sulfide photooxidation. Experimental evidence for sulfone formation via a hydroperoxy sulfonium ylide.

The first example of the formation of a sulfone concomitant with double bond migration during photooxidation of a sulfide is reported. Evidence is presented which demonstrates that the double bond migration is not a result of a prior acid-catalyzed rearrangement of an unrearranged sulfone precursor. This unusual observation is used to argue that the sulfone is formed via rearrangement of a hydroperoxy sulfonium ylide intermediate.

Reactions of Bis(p-methoxyphenyl)trisulfane and Its Oxides with Dimethyldioxirane and (Trifluoromethyl)methyldioxirane.

The reactions of dimethyldioxirane and (trifluoromethyl)methyldioxirane with bis(p-methoxyphenyl)trisulfane, its 1-oxide, its 2-oxide, and its 1,1-dioxide derivatives have been investigated. The reactions were followed by careful monitoring of the methoxy region of the (1)H NMR spectra and where possible by doping with authentic samples of the products. The decomposition of labile intermediates and products was investigated.