Photochemistry of cis-1-phenylcyclohexene. Proof of involvement of trans isomer in reaction processes

Exploiting Visible Light Triggered Formation of trans-Cyclohexene for the Contra-thermodynamic Protection of Alcohols

We report herein a method for the contra-thermodynamic protection and thermodynamic deprotection of alcohols in which all reagents are returned to their original state. This is accomplished by the use of visible light photochemical energy to drive the formation of a highly strained trans-(Z)-cyclohexene. At STP the product ethers contain more potential energy than the starting materials and, thus, can be catalytically returned to the starting materials, effectively realizing a protection-deprotection scheme paid for with an energy currency.

Photochemically Produced Aminocyclobutanes as Masked Dienes in Thermal Electrocyclic Cascade Reactions

Cyclobutane products of a triplet sensitized enamide-alkene intramolecular [2 + 2] photocycloaddition have been shown to undergo fragmentation under acidic conditions. This lability has been exploited by inducing a complexity-generating thermal electrocyclic cascade sequence involving the in situ formation of a cyclobutene, followed by electrocyclic ring opening, Diels-Alder cycloaddition, and subsequent lactamization. This combination of excited state photochemistry and thermal electrocyclic cascade reactions allows simple planar molecules to be rapidly transformed into sp³-rich scaffolds.

Mechanistic Studies of UV Assisted [4 + 2] Cycloadditions in Synthetic Efforts toward Vibsanin E

Quantum chemical DFT calculations at the B3LYP/6-31G(d) level have been used to study the stereochemical course of the photochemical cycloaddition of enone 9 with dienes. The observed products of this photochemically induced cycloaddition showed a stereoselectivity, which is opposite to what would be expected by FMO considerations. The quantum chemical calculations revealed that the unusual stereoselectivity of the reaction can be rationalized by assuming a stereospecific photochemical cis-trans isomerization of enone 9 to trans isomer 9a followed by a thermal Diels-Alder reaction of the diene onto the highly reactive trans enone. The photochemical reaction step involves the selective formation of a twisted triplet intermediate, which accounts for the selectivity of the reaction.

Conical Intersection Control of Heterocyclic Photochemical Bond Scission1,2

The photochemistry of the heterocycle 5,6-dihydro-1-methyl-5,5-diphenylpyridin-2(1H)-one (compound 1 in the text) leads to two competitive reactions (the reactions are depicted in the Introduction to the article). These arise from fission of bond a, between the nitrogen (N-6) and C-1, and bond b, between C-4, with the two phenyl substituents, and C-5, adjacent to the nitrogen. Scission of bond a alone leads to a zwitterionic intermediate which can be trapped by nucleophiles, while cleavage of bonds a and b together affords two fragments--a ketene and an imine. The ketene could be intercepted with nucleophiles and the imine trimerized. Computation reveals little weakening of bonds a and b. But as stretching begins, conical intersections are encountered, leading to ground-state products.

Novel Oxa-di-π-methane and Norrish Type I Reactions in the S2 (π,π*) Excited State of a Series of β,γ-Unsaturated Ketones

[reaction: see text] Beta,gamma-unsaturated methyl ketones with electron-withdrawing groups at the gamma-position of the ene moiety undergo ODPM rearrangements and Norrish type I reactions on direct irradiation at 254 nm. The results are consistent with the involvement of alkene S(2) (pi,pi*) as reactive excited states in these processes.

Reflection on medium effects on photochemical reactivity

In reexamining medium effects on photochemical reactions, we have emphasized those on unequilibrated excited species such as the Franck-Condon species. Despite recent advances in femtochemistry, such a discussion in molecular photochemistry is uncommon, and the problem remains challenging on account of the extremely short-lived excited species. However, in such cases, a small perturbation resulting from, for example, weak guest-host interactions could turn into a determining factor in dictating the course of a photochemical channel of deactivation. Examples of medium-directed diabatic processes have been examined with this idea in mind. A modified view on rhodopsin photoisomerization is presented along with the consideration that confinement does not necessarily lead to inhibition of reactions of the trapped substrate.

Regioselectivity of the tri-pi-methane rearrangement: mechanistic and exploratory organic photochemistry

The di-pi-methane rearrangement with two pi-groups attached to the central "methane carbon" of the reactant and which leads to a pi-substituted cyclopropane has been studied intensively. Our present research had the goal of elucidating the regioselectivity of the tri-pi-methane counterpart. The reactants with three pi groups attached to the central carbon mechanistically are capable of affording both di-pi-methane and tri-pi-methane photoproducts. In common with the di-pi-methane system, bridging of two of the pi-systems affords a cyclopropyldicarbinyl diradical intermediate that opens to an allyl carbinyl diradical. This diradical has the option of closing to a three-membered or a five-membered ring. It was found that the regioselectivity of the initial pi-pi-bridging step and the three-ring opening of the cyclopropyldicarbinyl diradical exhibit regioselectivity parallel to that of the di-pi counterpart. Both three-ring and five-ring photoproducts were formed with the ratio varying with conversion. Since the three-ring (i.e. di-pi-methane) photoproducts were found to ring expand to the five-ring (i.e. tri-pi-methane) products, kinetics were employed to determine to what extent the reaction proceeds in a two-step versus direct formation of the five-ring product. It was found that the direct route was the major one.

The diverted di-pi-methane rearrangement; mechanistic and exploratory organic photochemistry

[reaction: see text] An unusual diversion of the di-pi-methane rearrangement has been encountered. The reaction is characteristic of di-pi-methane systems having one vinyl group bearing one or two carbonyl groups and provides a synthesis of dihydrofurans.

Mechanism of the photoinduced addition of methanol to the double bond of 2,2,4,6-tetramethyl- and 1,2,2,4,6-pentamethyl-1,2-dihydroquinolines

The mechanism of the photoinduced Markovnikov addition of methanol to the double bond of 2,2,4,6-tetramethyl-1,2-dihydroquinoline (1) and 1,2,2,4,6-pentamethyl-1,2-dihydroquinoline (2) was studied by the flash photolysis technique over a wide range of acetic acid and KOH concentrations. The successive formation of two transients was observed in neutral solutions in the case of compound 1. These transients have lifetimes on ms time-scales and absorption spectra with maxima at 420 and 480 nm for the first and the second transient, respectively. The decay rate constants of these transients (k1 and k2) were measured over the temperature range from 10 to 45 degrees C and the activation energies E1 approximately 0 and E2 (31.4 +/- 3.1) kJ mol(-1) were determined. In MeOH acidified with acetic acid only the second transient (lambda(max) = 480 nm) was observed, and it is proposed that this is due to a carbocation. The deuterium kinetic isotope effects determined in MeOD are 2 and 1.3 for k1 and k2, respectively, indicating that the first reaction is proton transfer proceeding via the A-SE2 mechanism and the second reaction is the nucleophilic addition of the solvent to the carbocation. In alkaline media, only the first transient (lambda(max) = 420 nm) was observed with a lifetime of 250 ms at [KOH] > 2 x 10(-3) mol dm(-3). In the case of compound 2, the formation of a carbocation with lambda(max) = 490 mn and a lifetime of several ms is observed within 10 ns of excitation in neutral, acidic, and alkaline solutions. The carbocation is quenched with KOH with a rate constant of 7 x 10(6) mol(-1) dm(3) s(-1). The quantum yields of the reaction and of the fluorescence were measured and possible mechanisms discussed.

Biradical intermediates in the photoisomerization of dibenzodihydropentalenofurans to dibenzosemibullvalenes

The photoisomerization of a few substituted dibenzodihydropentalenofurans to the corresponding dibenzosemibullvalenes is reported. Steady-state photolysis of the dibenzodihydropentalenofurans 3a--d gave the corresponding dibenzosemibullvalenes 2a--d in nearly quantitative yields. The quantum yields of this photoisomerization were found to be in the range 0.17--0.26. Laser flash photolysis studies of the dibenzodihydropentalenofurans 3a-e showed transients, with absorption maxima around 410 nm and decaying by first-order kinetics. The lifetimes were in the range 14--30 micros in degassed benzene at 25 degrees C. These transients were readily quenched (trapped) by molecular oxygen, and the Stern-Völmer quenching constants were found to be in the range (2.45--3.17) x 10(9) M(-1) s(-1). As a representative example, the 1,3-biradical intermediate from 3e was trapped by molecular oxygen to give the corresponding endoperoxide 11e. The transients were weakly quenched by triplet/radical quenchers such as 2,2,6,6-tetramethylpiperinyl-1-oxy (TEMPO) and 4-hydroxy-2,2,6,6-tetramethylpiperinyl-1-oxy (HTEMPO), and the quenching constants are found to be in the range (1.09--3.19) x 10(6) M(-1) s(-1). The decay rates of the transients were found to be temperature dependent and obeyed the Arrhenius equation. For example, the activation energy of the transient from 3a was approximately 4.5 kcal mol(-1) and the Arrhenius preexponential factor log(A/s(-1)) for the decay of the transients was approximately 7.5. On the basis of our studies, these transients were assigned as the ground-state triplet biradicals, generated by the cleavage of the C--O bond of the starting dibenzodihydropentalenofurans.

Excited- and ground-state versions of the tri-pi-methane rearrangement: mechanistic and exploratory organic photochemistry

The di-pi-methane rearrangement with two pi-groups bonded to a single carbon leading to pi-substituted cyclopropanes is now well established. The present research had as its goal the exploration of molecular systems having three pi-moieties attached to an sp(3)-hybridized atom in a search for a tri-pi-methane rearrangement. Indeed, it was found that such systems do rearrange photochemically to afford cyclopentenes. However, it was also established that vinylcyclopropanes ring-expand to cyclopentenes on direct irradiation. Since both three-ring and five-ring photoproducts often are found to be produced, it was important to establish that the observed photochemistry was really the result of a true single-step tri-pi-methane rearrangement and not the consequence of two sequential rearrangements, first to form a vinyl cyclopropane which subsequently ring expanded to the cyclopentene. The general situation has three species-A, B, and C-corresponding to tri-pi-methane reactant A, vinylcyclopropane photoproduct B, and cyclopentene photoproduct C. Three rate constants are involved, k(1) for A --> B, k(2) for A --> C, and k(3) for B --> C. The kinetics were applied to two examples with provision to avoid differential light absorption; this utilized singlet sensitization. It was determined that direct formation of the cyclopentene photoproduct proceeds more rapidly than the ring-expansion route. In contrast to the di-pi-methane rearrangement, the tri-pi-methane reaction was found to be preferred by the singlet, while in these sterically congested systems, the triplet led to di-pi-methane reactivity. Finally, a ground-state counterpart of the reaction was obtained.

Conformational selectivity in the diels-alder cycloaddition: predictions for reactions of s-trans-1,3-butadiene

Diels-Alder cycloaddition of s-trans-1,3-butadiene (1) should yield trans-cyclohexene (7), just as reaction of the s-cis conformer gives cis-cyclohexene (9). Investigation of this long-overlooked process with Hartree-Fock, Moller-Plesset, CASSCF, and DFT methods yielded in every case a C(2)-symmetric concerted transition state. At the B3LYP/6-31G (+ZPVE) level, this structure is predicted to be 42.6 kcal/mol above reactants, while the overall reaction is endothermic by 16.7 kcal/mol. A stepwise diradical process has been studied by UBLYP/6-31G theory and found to have barriers of 35.5 and 17.7 kcal/mol for the two steps. Spin correction lowers these values to 30.1 and 13.0 kcal/mol. The barrier to pi-bond rotation in cis-cyclohexene (9) is predicted (B3LYP theory) to be 62.4 kcal/mol, with trans-cyclohexene (7) lying 53.3 kcal/mol above cis isomer 9. Results suggest that pi-bond isomerization and concerted reaction may provide competitive routes for Diels-Alder cycloreversion. It is concluded that full understanding of the Diels-Alder reaction requires consideration of both conformers of 1,3-butadiene.

The tri-pi-methane rearrangement: mechanistic and exploratory organic photochemistry(1)

The di-pi-methane rearrangement is firmly established as a mode of synthesizing three-membered-ring compounds. We now report the tri-pi-methane counterpart.