Lewis acid catalysis of photochemical reactions. 5. Selective isomerization of conjugated butenoic and dienoic esters

Journal Article · Wed May 28 04:00:00 EDT 1986 · J. Am. Chem. Soc.; (United States)

DOI:https://doi.org/10.1021/ja00271a034· OSTI ID:5658201

Lewis, F D; Howard, D K; Barancyk, S V; Oxman, J D

The effects of Lewis and Broensted acids upon the photoisomerization reactions of several conjugated butenoic and dienoic esters have been investigated. Lewis acids inhibit the photochemical deconjugation of ..cap alpha..,..beta..- to ..beta..,..gamma..-unsaturated butenoic esters and shift the photoequilibrium between E and Z isomers toward the Z isomer. As such, irradiation of E ..cap alpha..,..beta..-unsaturated esters in the presence of EtAlCl/sub 2/ provides a convenient method for the preparation of the thermodynamically less stable Z isomer. Irradiation of methyl (E,E)-2,4-hexadienoate and methyl (E,E)-5-phenyl-2,4-pentadienoate in the absence of added catalysts results in nonselective E,Z isomerization to give mixtures of all four stereoisomers in roughly comparable yields. In the presence of the Broensted acid trifluoroacetic acid, quantitative conversion of methyl 2,4-hexadienoates to methyl 3,4-hexadienoate is observed. The acid serves as a catalyst for the thermal 1,3-hydrogen shift of an allenic enol ester formed via a photochemical 1,5-hydrogen shift of the conjugated esters. Irradiation of the ground-state complexes of the conjugated esters with the Lewis acids EtAlCl/sub 2/ or BF/sub 3/ results in selective E,Z isomerization about the ..cap alpha..,..beta..-double bond in methyl 2,4-hexadienoate and the ..gamma..,delta-double bond in methyl 5-phenyl-24-pentadienoate. The mechanistic bases for the observed effects of Lewis acids are selective excitation of the more strongly absorbing E complex and more efficient isomerization of the excited E vs. Z complex.

• Research Organization: Northwestern Univ., Evanston, IL
• OSTI ID: 5658201
• Journal Information: J. Am. Chem. Soc.; (United States), Journal Name: J. Am. Chem. Soc.; (United States) Vol. 108:11; ISSN JACSA
• Country of Publication: United States
• Language: English

Related Subjects

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY
400301 -- Organic Chemistry-- Chemical & Physicochemical Properties-- (-1987)
400500* -- Photochemistry
ABSORPTION SPECTROSCOPY
BUTYRIC ACID
CARBOXYLIC ACIDS
CATALYSIS
CATALYSTS
CATALYTIC EFFECTS
CHEMICAL REACTIONS
CHEMICAL SHIFT
DATA
EFFICIENCY
ESTERS
EXPERIMENTAL DATA
HYDROGEN COMPOUNDS
INFORMATION
INORGANIC ACIDS
ISOMERIZATION
ISOMERS
LEWIS ACIDS
MAGNETIC RESONANCE
MONOCARBOXYLIC ACIDS
NUCLEAR MAGNETIC RESONANCE
NUMERICAL DATA
ORGANIC ACIDS
ORGANIC COMPOUNDS
PHOTOCHEMICAL REACTIONS
QUANTUM EFFICIENCY
RESONANCE
SPECTRA
SPECTROSCOPY
THERMODYNAMICS
ULTRAVIOLET SPECTRA