Photochemical Syntheses: 6. The Formation of Heptandiones From Acetylacetone and Alkenes

Abstract

The irradiation of acetylacetone in the presence of oct-1-ene, cyclopentene, cyclohexene, and 1-methylcyclohexene g i~. e s substituted heptandiones. These diketones may then be cyclized with acid or base. Irradiation of isopropenyl acetate and acetylacetone gives, after cyclization, nz-5-xylenol. The mechanism of the reactio~l is discussed: it represents the first cycloaddition t o an isolated ethylenic linkage.Saturated ketones have absorption bands in the ultraviolet a t about 280 mp. This band is associated with an n -+ a* transition of a non-bonded electron on the oxygen atom into an anti-bonding n-orbital (2): a,P-unsaturated ketones have a similar band a t somewhat longer wavelength. Such excitation of saturated ketones in condensed phases leads to a number of transformations which follow on the formation of the original excited species. Amongst these are c~~clobutanol formation and fission between the a-and 6-carbon atoms: both these processes are initiated by y-hydrogen abstraction by the electron-deficient oxygenIn the presence of substrates which may give stabilized radicals by hydrogen abstraction another course may be followed, particularly when there is no such y-hydrogen available for intramolecular abstraction. The activated ketone, most probably as the triplet (4, and references cited therein), is converted into the ketyl, which then, in turn, may react with another ketyl or with some other radical derived from the substrate (see, for instance, refs. 5, 6, 7).The irradiation of acyclic a-diketones also appears to conform to this general pattern. In the absence of a reactive substrate cyclobutanol formation proceeds in high yield (8), whilst in the presence of benzylic (9) or allylic hydrogen (10) alkylation is observed.The irradiation of P-diketones, on the other hand, does not appear to have been studied up to the present. Acetylacetone, the simplest readily available acyclic P-diketone, exists in solution in hydrocarbons largely in the enolic form (I) (11). In this system ?-hydrogen abstractio~l is, presumably, strongly favored following n -+ .rrx excitation, because of the proximity of that hydrogen atom. However, in this special case the ketyl + so formed is readily transformed into (11), the tautonler of (I), and differing from it a t most in small internuclear distance^.^ Hydrogen abstraction from the solvent may thus be expected to be slow and might not occur provided some other mode of chemical behavior were available to the excited diketone molecule.Irradiation of acetylacetone in cyclohexene induced a reaction which could be followed by the disappearance of the en01 bands a t about 6.3 p. T h e product, obtained in high yield, appeared homogeneous to gas-liquid chromatographic (g.1.c.) analysis and, significantly, no trace of bicyclohexenyl could be detected. The latter is an inevitable 'Part of the fnaterial contained herein formed a preliminary com~nunication (1). 2For a reaiew of these and other transformations following n + T* excitation see reference 5 ' .3 1 % the present...

The photocycloaddition of naphthoates with acetylacetone; dramatic acid promotions of product quantum yields

“…The 'H NMR coupling patterns and their chemical shifts clearly indicated that both 2 and 3 were the isomeric products in the addition of acetyl and acetonyl groups on the 1,2-position of 1, with alternate regiochemistry. A 2+2 photocyloaddition followed by the cyclobutane ring opening was assumed, in analogy to the de Mayo reaction (2,13); that led us to assign the stereochemistry as shown, which was also consistent with the subsequent reaction. The NOE results of 2 also supported the assignment by showing that the methine proton (4.47 ppm) and the methoxy methyl are close to each other (19).…”

Chemistry of Succinylsuccinic Acid Derivatives. Part I. On the photodimerization of diethyl succinylsuccinate

“…The production of ~nti-tricyclo[4.4.0~~~.0~~~~]-1,4,7,10-tetrae~hoxycarbonyl 3,6,9,12-tetrahydroxy-dodeca-3,9-diene in this photoreaction is the first example of dimerization of chelated enols in the solid state2) and it reaffirms the anticipated mechanism of participation of chelated enols in 2+2 cycloadditions [4]. The topochemical control [5] of the reaction is demonstrated by the absence of dimerization in solution, by the analogous solid-state dimerization of dimethyl succinylsuccinate3), as expected from its similar crystal structure [6], by contrast with the photostability3) of 1,4-diethoxycarbonyl-2,5-diamino-cyclohexa-1,4-diene [7] under the same conditions, owing to its obviously different crystal packing.…”

The dual pathway in photocycloaddition of 1,3-diketonatoboron difluorides: excimer reactions

“…In eq. [4], the second term is converted to k,~,l(l + ktz,) at [0] = 1 M. The substitution of N values gives kpt = 0.16kdt for 3 and k,, = 3.8kd, for 6. These relations indicate that cyclic olefin 6 is more efficient than acyclic olefin 3 to convert triplex *(A,A,O) to the product.…”