Photolysis of the 1-naphthylmethyl ester of substituted phenylacetic acids: intramolecular charge transfer and rates of decarboxylation of arylacyloxy radicals

Abstract

. 70, 992 (1992). The photolysis of esters 6 and 8 in methanol leads to products resulting from both naphthylmethyl cations and radicals. The product distribution is nearly independent of X for the esters 6 except when X equals methoxy. A mechanism involving initial homolytic cleavage of the carbon-oxygen bond in the excited singlet state of the ester is proposed. Competition between electron transfer in the radical pair to form the ion pair and decarboxylation of the arylacyloxy radical allows calculations of the rates for this decarboxylation process. The p value versus u is close to zero. When X equals methoxy, intramolecular electron transfer occurs with the naphthalene ring serving as the acceptor and the methoxyaromatic as the donor. This exciplex fragments to carbon dioxide and 1-(I-naphthy1)-2-arylethane.Key words: acyloxy radical, decarboxylation, photolysis of benzylic esters.JAMES W. HILBORN et JAMES A. PINCOCK. Can. J. Chem. 70, 992 (1992). La photolyse des esters 6 et 8 dans le methanol conduit ii la formation de produits resultant ii la fois de cations et de radicaux naphtylmethyles. Pour les esters 6, la distribution des produits est independante de la nature de X, except6 lorsque X est un groupe methoxyle. On propose un mecanisme impliquant un clivage initial homolytique de la liaison carboneoxygkne dans 1'Ctat singulet excite de l'ester. Une competition entre le transfert d'electrons dans la paire de radicaux pour former la paire d'ions et la dtcarboxylation du radical arylacyloxy permet de calculer les vitesses de ce processus de decarboxylation. La valeur de p versus a s'approche de zero. Lorsque X est un groupe methoxyle, il se produit un transfert intramolCculaire d'tlectrons avec le noyau naphtalenique servant de accepteur et le fragment methoxyaromatique agissant comme donneur. Cet exciplexe se fragmente en dioxyde de carbone et I-(I-naphty1)-2-arylethane.

Photodecarboxylation of Substituted Naphthylmethyl Arylacetate Esters: Synthesis of Naphthylarylethanes

“…Nevertheless, overall conversions were reasonably high (see Table ), the exception being ester 1i . It should be noted that, although considered, the corresponding methylnaphthalene photoproducts were not observed in these reactions, as consistent with previous studies involving 1-naphthylmethyl acetate esters. ,, …”

“…However, the methylethers ( 3 – 6 , Figure ) were also obtained in significant amounts, courtesy of the respective ionic-derived minor pathway being operative (path A, Figure ). For the mechanism given in Figure , the effects on the product formation of inter- ( k ET ) and intramolecular ( k E ) charge/electron transfer, i.e., path A vs paths B and C, respectively, are understood . Nevertheless, overall conversions were reasonably high (see Table ), the exception being ester 1i .…”

“…This could potentially be achieved via the use of a nonpolar solvent, so as to minimize path A, or by tuning the ester substrates to stabilize intramolecular charge transfer (exciplex formation, path C) and thereby support the formation of the desired diarylethane product. For example, selecting naphthylmethyl phenylacetate esters would present the acyloxy group of the ester as a good donor that would stabilize a radical cation to promote path C. Indeed, when a single methoxy group is added to the phenylacyloxy group of the naphthylmethyl ester, the charge transfer pathway ( k E ) leads to the production of radical-derived products ( k CO 2 E ) . In methanol, this single substituent preferentially directed the dominant mechanism to proceed via the charge transfer (exciplex) to form the diarylethane but was insufficient to effect the sole formation of the exciplex.…”

“…The photochemistry promises to be relatively uncomplicated since it occurs exclusively via the singlet excited state. The products are rapidly formed and almost exclusively within the solvent cage, , so that the substrate esters can include substituents (i.e., nitriles) that normally would react with radicals diffusing through the solvent. For these ester substrates, this methodology eliminates the need for complex synthetic steps and provides good yields of the desired naphthylarylethanes.…”

Intramolecular electron transfer in the photochemistry of substituted 1-naphthylmethyl esters of benzoic acids

“…The hydrocarbon 4 is formed by in-cage coupling of the 1-naphthylmethyl radical with R which is formed by loss of carbon dioxide from the initially formed acyloxy radical. The product-yield results for a large number of esters where both substituents on the naphthalene ring (2) and the structure of the R group (3,4) were varied could best be explained by the mechanism outlined in Scheme 1. We have suggested that for these esters the excited singlet state (the triplet state for 1-naphthylmethyl esters is unreactive (5,6)) cleaves exclusively to the radical pair.…”