Photochemical Addition of Benzaldehyde to 1-Alkynes the Formation of lndanone Derivatives

Abstract

Irradiation of a solution of benzaldehyde and a monosubstituted acetylene(n-butyl, t-butyl, n-propyl or phenylacetylene) in benzene with high presure mercury lamp gave 3-substituted 1-indanone (3-n-butyl (yield, 9.0%), 3-t-butyl (9.0%), 3rn-propyl (9.0%) or 3-phenylindanone (1, 4%o)) The reaction of benzaldehyde with phenylacetylene gave trans-ev-phenyleinnamaldehyde (16, 6%)and trans-benzalacetophenone (O.4%) in addition to the indanone (Table 1 and Equation 2) The following mechanism of the indanone, formation (Equation 5) was prpposed from the result of the reaction using deuterated starting compounds. (C6D5CHO3, n-C4H9C CD) or using an excess amount of benzaldehyde as a hydrogen donor. Additien of benzoyl radical (formed from the photochemical decomposition of benzaldehyde) to the monosubstituted acetylene gives a vinyl radical, which cyclizes to a cyclohexadienyl type intermediate by an intramolqcular, attack to the phenyl ring of benzoyl moiety. Then the therma11y allowed [1-5]sigmatropic rearrangement of hydrogen may give the enol of the 3-substituted 1-indanone. This mechanism was, confirmed by an independeht study in which benzaldehyde was allowed to react with n-butylacetylene in the presence of a radical initiator.The photochemieal formation of trans-a-phenylcinnamaldehyde and trans-benzalacetophenone sugested the interrnediacy of the, corresponding oxetene derivatives. The differenee of the reaction pathway between phenylacetylene and the monoalkyl-substituted acetylene was explained in terms of the electron donating nature of the substitdent on the acetylenic bond. Phenylacetylene may be able to trap the electrophilic excited-state of benzalde hyde, whereas the monealkyl-substituted acetylene does not.