Elliott Bay

Photochemistry of iminium salts : Electron transfer mechanisms for singlet quenching and photoaddition of n-electron donating alcohols and ethers

Abstract Several aspects of past and current studies in the area of iminium salt photochemistry are discussed. Investigations of olefin-iminium salt photoaddition and photocyclization reactions are reviewed and conclusions about electron-transfer pathways for fluorescence quenching and reaction are discussed. The results of recent studies of alcohol and ether photoaddition to 2-phenyl-1-pyrrolinium salts are presented. These C-C bond forming processes occur in moderate yields to produce β-amino alcohol or ether products. In addition, alcohols and ethers serve as efficient quenchers of pyrrolinium salt fluorescence. Rate constants for quenching appear to be dependent upon both the oxidation potential of the alcohols and ethers and the availability of C-H bond α to oxygen. This data along with deuterium isotope effects on quenching combine to suggest a common mechanism for both fluorescence quenching and photoaddition. The nature of this mechanism is tested using the comparative quenching effeciencies of the tertiary alcohols t -butyl alcohol and 1,2,2-trimethyl-1-cyclopropanol. The latter alcohol having a weak C-C bond adjacent to the hydroxyl function quenches the fluorescence of 2-phenyl-1-pyrrolium salts at a rate two orders of magnitude greater than for t-butyl alcohol. The observations made are interpreted in terms of a sequential electron-proton transfer mechanism for quenching and photoaddition. Lastly, the relationship of iminium salt photochemical studies to other investigations of electron-transfer photochemistry is discussed.

Photochemistry of iminium salts in the presence of N-electron donating alcohols and ethers

Abstract Irradiation of 2-phenyl and 2-isobutenyl-1-pyrrolinium salts in solutions of alcohols and ethers containing α-hydrogens leads to production of addition products. In addition, alcohols and ethers having low ionization potentials and α-hydrogens serve as efficient quenchers of fluorescence from 2-phenyl-1-pyrrolinium salts. Deuterium isotope effects on fluorescence quenching rate constants appear to implicate electron transfer mechanisms in photoaddition and quenching pathways.

The di-π-methane rearrangement : The effect of 1,4-diene structure and conformation on the reaction efficiency and stereochemistry

Abstract The photochemistry of the stereoisomeric 1,3-dimethyl-3(2-phenylethenyl)cyclohexenes has been explored Direct irradiation of the cis- and trans-β-styrylcyclohexenes leads to di-π-methane rearrangement, producing the endo- and exo-3,7-dimethyl-8-phenybicyclo[5.1.0]oct-2-enes, and cis -trans isomerization, interconverting the 1,4-diene containing substrates. Triplet sensitized photolysis of both substituted cyclohexenes leads exclusively to cis-trans isomerization. Results from low conversion direct irradiations of the cis- and trans-β-styrylcyclohexenes indicate that the singlet rearrangements are stereospecific, and lead to formation of the 8-exo and 8-endo-phenylbicyclic octenes, respectively. The relationship between di-π-methane structure and triplet reaction efficiency, and the effect of conformation on the rearrangement stereochemistry are discussed.

Conformational investigations of 6-EXO and 6-ENDO- methylbicyclo[3.1.0] hexan-3-one

Abstract R band (26.5–40 GHz) microwave spectra have been recorded for the common isotopic species of 6- endo -methylbicyclo[3.1.0]hexan-3-one and 6- exo -methylbicyclo[3.1.0]hexan-3-one. These spectra are shown to be consistent with a ground vibrational state boat conformation having five membered ring dihedral angles of 7.2 ± 0.3° and 20.5 ± 1.4° respectively. This information is correlated with product quantum yields obtained on photolysis of these molecules.

The microwave spectrum of 2,4-dioxabicyclo[3.1.0]hexan-3-one

Abstract The R band (26.5–40 GHz) microwave spectrum of 2,4-dioxabicyclo[3.1.0]hexan-3-one is reported. Rotational constants for the ground vibrational state of the common 12 C 4 1 H 4 16 O 3 and 13 C 1 , 13 C 6 isotopically substituted species (the latter observed in natural abundance) have been evaluated. In addition rotational constants of the V B = 1 to V B = 5 quanta associated with the bending vibration of the five membered ring have been determined. A partial r s structure has been calculated: r( C 1 − C 5 ) = 1.497± 0.016 A , r( C 1 − C 6 ) = r( C 6 − C 5 ) = 1.522 ± 0.015 A , ∠ C 6 C 1 C 5 = ∠ C 1 C 5 C 6 = 60°32′ ± 1°36′, ∠ C 1 C 6 C 5 = 58°′ ± 1°47′ . With certain assumed molecular information a least squares fit yields the following parameters: β = 68.5 ± 0.02°, r( C 1 O 2 = 1.408 ± 0.004 A , ∠ C 5 C 1 O 2 = 105.8 ± 0.02°, ∠ C 1 O 2 C 3 = 108.10 ± 0.03° , ∠ O 2 C 3 O 4 = 112.8 ± 0.02° .