Molecular photochemistry. XXXVI. Photochemistry of some .alpha.-oxopropiophenones. Evidence for an unusual cyclic transition state for hydrogen abstraction

Abstract

AFOSR-70-1848) for its generous support of this work.(2) (a) Exceptions to this rule generally involve seven-membered (or larger) cyclic transition states: see, for example, P. Yates and J. M.

Photochemical rearrangements of .alpha.-methylene ketones

“…We thank Professor McDonald, Kansas State University, for 60-MHz nmr spectra of mixtures of endo-and exo-29, with which the data reported here are in substantial agreement. and 3% 4-(l-cyclobuten-l-yl)-2-butanone (31). These last two compounds were separated by reinjection onto column C.…”

“…Caled for C8Hi20: C, 77.37; H, 9.74. Found: C, 77.25; H, 9.94. 4-(l-Cyclobuten-l-yl)-2-butanone (31). A solution of 75 mg of 30 in 75 ml of pentane was photolyzed following procedure B, except that the light source was 16 RPR-3000 A reactor lamps.…”

Photochemical transformations of small-ring carbonyl compounds. 79. Involvement of enol tautomers in the photoisomerization of 3-substituted isochromanones

“…During the course of our investigations we found that 2hydroxy-2-phenylindanone (24) also exhibits some interesting photochemistry. Irradiation of 24 in methanol for 7 h gave a mixture of l-methoxy-3-phenyl-l//-2-benzopyran (25) and ti-benzoyl-o-tolualdehyde dimethylacetal (26). The yield of these two products was found to be markedly dependent on the reaction conditions and the work-up procedure used.…”

“…(CDCh) 5.24 (s, 2 ), 1.7-3.0 (m, 9 H), and 0.2 (s, 1 H). Treatment of 40 mg of -benzoyl-o-tolualdehyde with methanol at room temperature for 3 h in the presence of a trace of acid afforded a quantitative yield of 1 -methoxy-3-phenyi-1 //-2-benzopyran (25). When this compound was allowed to stand in the presence of methanol for an additional 5 h it was partially converted to a-benzoyl-o-tolualdehyde dimethylacetal.…”

“…[2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19] The concept of internal photoactivated hydrogen transfer (photoenolization) was originally postulated to explain the apparent lack of bimolecular photoreduction of 2-alkyl-, 2-amino-, and 2hydroxybenzophenones. 25 Yang2 "4 and other workers5"8 have provided chemical and physical evidence which demonstrate that the photoenolization process is an intramolecular photochemical reaction analogous to the Norrish type II process.20 Enols generated in this fashion have relatively long lifetimes4•8 and can be trapped with dienophiles.2•15 The remarkable stability of these enols is probably due principally to the absence of catalytic ionic contaminants which are generally present in nonphotochemical reactions. Recently, a number of widely scattered and isolated reports have appeared which indicate that 1,3-tautomerization of certain ketones can also occur upon electronic excitation.21 26 In earlier reports from this laboratory,27 evidence was presented which demonstrated that the enol content can be an overriding factor in determining the excited-state behavior of a carbonyl group.…”