Some studies in organic photochemistry

A brief review of those aspects of current photochemical research pertinent to the present study is given. The work itself is in three sections.
Section I is concerned with the photochemistry of the system dimethyl maleate/dimethyl fumarate/acetone. Irradiation of this system produced the cis- and trans-2,2-dimethyl-3,4-dimethoxycarbonyloxetanes (I and II), as well as causing cis&leftrightarrow;trans isomerisation of dimethyl maleate/dimethyl fumarate. No tetramethoxycarbonylcyclobutane (III) was formed. A mechanistic study of the formation of oxetane was complicated by the simultaneous occurrence of the cis&leftrightarrow;trans isomerisation of maleate/fumarate. This was overcome by examining the photochemistry of the system at irradiation time zero, before any reaction had occurred. This procedure also eliminated other difficulties inherent in any quantitative photochemical study such as the effects of polymer film formation and of the photochemical instability of the reaction products. A method was developed to evaluate the rates of formation of the irradiation products with precision. Particular emphasis was placed on assessing the errors in these quantities. The approach was to follow the light-induced reactions by means of gas chromatography and to estimate the rates at zero irradiation time by extrapolation using a polynomial least squares fit computer programme. It was found important to allow for fluctuations in lamp-output with time and a photomonitor was constructed for this purpose.
Dilute acetone solutions of pure dimethyl maleate and of pure dimethyl fumarate were irradiated. Both quartz and borosilicate glass reactors were employed thus allowing variation in proportion of incident light absorbed by each of the two components. Irradiations of dimethyl fumarate in acetone using quartz apparatus were conducted at three different initial concentrations.
The significance of the data obtained was verified by carrying out a material balance check and by investigating the importance of any small amounts of dissolved oxygen remaining in solution after deoxygenation prior to irradiation.
From a correlation of the observed rates of formation at irradiation time zero with the light absorption properties of the solution irradiated, substantial evidence emerged that oxetanes were formed by the action of excited maleate and of excited fumarate on ground state acetone.
This is the first case reported where evidence exists that oxetanes are formed by a route other than the action of the (n,π*) excited state of the ketone on a carbon-carbon double bond. Other workers have restricted their attention to systems involving unsaturated hydrocarbons.(1)
The data presented also indicate that another excited state of maleate (and probably also of fumarate) exists which was active in the cis&leftrightarrow;trans isomerisation reaction but which did not form oxetane with acetone. An attempt is made to identify the oxetane-forming state with the (n,π*) state and the state active only in cis&leftrightarrow;trans isomerisation with the (π, π*) state of the unsaturated ester. The further implications of the concentration dependence of these reactions is also considered.
Oxygen perturbation methods were used to obtain information concerning the relative triplet energy levels of maleate, of fumarate and of acetone, but although singlet-»triplet absorption was observed, no information concerning the chemically significant 0,0 bands was obtained. Electron impact spectroscopic studies are currently being pursued to this end.
Section II is concerned with the photochemistry of acetoxime. Irradiation of acetoxime in cyclohexene for 9 hr. yielded the adducts N-acetyl-N-methylcyclohexylamine (IV) and N-acetyl-N-methylcyclohex-2-enyl- amine (V) in 2% yield based on acetoxime consumed, together with bicyclohex-2-enyl (VI) and much polymer. This is the first occasion that oxime/olefin photoadducts have been recorded. Two possible mechanisms were investigated. The first involved the photoisomerisation of acetoxime to N-methylacetamide followed by the photo-addition of N-methylacetamide to cyclohexene to give the observed products. The second was the photochemical formation of a nitrone (VII) which photoisomerised to the oxazirane (VIII) and subsequently rearranged thermally to yield the observed amides. It was shown that neither mechanism was likely. No stable products were isolated from propionaldoxime/cyclohexene irradiation mixtures.
Section III is concerned with irradiations involving aldehydes. Dimethyl maleate, methyl crotonate, cyclohexene, styrene, ethyl vinyl ketone, crotonaldehyde and benzene were irradiated with propionaldehyde. One irradiation with acetaldehyde was also conducted. Adducts were observed only from dimethyl maleate, methyl crotonate and cyclohexene (IX, X and XI). The other irradiations yielded products derived from propionaldehyde alone. Competition experiments showed that the active entity involved in these reactions was probably a nucleophilic species derived from the aldehyde. A very close correlation was found to exist between the light induced and peroxide induced reaction and this was taken as good evidence that the same acyl radical species was responsible for both reactions.