Homolytic versus heterolytic cleavage for the photochemistry of 1-naphthylmethyl derivatives

Abstract

Can. J. Chem. 63, 3140 (1985). The photochemical cleavage of the I-naphthylrnethyl derivatives, 1-7, has been examined in methanol solvent under both direct and sensitized conditions. The cornpetitition between hornolytic and heterolytic cleavage as a function of multiplicity and leaving group has been studied in detail. Only substrates 1, 2, 3, and 7 react on sensitization with xanthone but evidence is presented that the resulting reactivity of 1, 2, and 3 may not be triplet cnergy transfer but rather exciplex formation. A semi-quantitative scale for photofugacities of the leaving groups from the excited singlet states has been established. 3140 (1985). Utilisant le rnkthanol comrne solvant, on a Ctudit le clivage photochimique des dCrivCs du naphtyl-l mCthyle 1 h 7 tant dans conditions directes que sous I'influence de sensibilisateurs. On a soigneusernent CtudiC la compCtition entre les clivages hornolytique et hCtirolytique en fonction tant de la rnultiplicitC que du groupe labile. Seuls les substrats 1, 2, 3 et 7 reagissent lorsqu'on utilise le xanthone cornrne sensibilateur; toutefois, on prouve que la rCactivitC rksultante des dCrivCs 1, 2 et 3 n'est pas due h un transfert d'tnergie du triplet rnais plutBt h la formation d'un exciplex. On a Ctabli une Cchelle semi-quantitative des photofugacitCs des groupes labiles h partir des Ctats singulets cxcites.[Traduit par le journal] Introduction Since the initial reports by Zimmerman et nl. (1, 2) on the photocleavage of benzylic derivatives (ArCH2-X) in nucleophilic solvents, there has been considerable interest in this seemingly simple process. Much of the work has focused on the competition between homolytic versus heterolytic cleavage as a function of the multiplicity of the excited state (i.e. direct versus sensitized irradiations) and the nature of the photofugal

Substituent effects on homolytic versus heterolytic photocleavage of (1-naphthylmethyl)trimethylammonium chlorides

“…Triplet quenching of these substrates (ET = 56-62 kcal/mol, Table 1) should be close to the diffusion controlled rate (1.8 X 10'' M-I s-I in methanol (17a)) by 2,5-dimethylhexadiene (ET = 58.7 kcal/mol(17b)). However, singlet state quenching of naphthalene derivatives by this diene is also very rapid (18) and a value of 9.3 x lo9 M-' s-' has been reported previously for 6 (2). Stem-Volmer fluorescence intensity quenching plots for 6-10 are shown in Fig.…”

“…Reaction multiplicity; quantum yields arzd singlet reaction rates Before attempting quantum yield and rate studies for these salts, information on reaction multiplicity was necessary to insure that only the singlet was reacting. As reported previously (2), this is straightforward for 6 (X = H) since xanthone sensitization gives no reaction; direct irradiations must be singlet reactions. For the other substrates this experiment is more difficult since their ultraviolet spectra, Fig.…”

“…The synthetic routes are outlined: Scheme 1 for 7, Scheme 2 for 8, Scheme 3 for 9, and Scheme 4 for 10. The unsubstituted compound, 6, has been reported previously (2). Details and appropriate references are in the experimental section.…”

“…In fact, this process must be faster than step 12, which is the diffusional separation of the contact pair to give the free I-naphthylmethyl radical. Photolysis of 1-naphthylmethyl derivatives in methanol normally gives the dimer of the l-naphthylmethyl radical as a major product derived from homolytic cleavage (2,24). Only with trimethylamine as the leaving group is 1 methylnaphthalene the exclusive product.…”

“…Perhaps the best quantitative confirmation of the meta effect or, more precisely, changes in charge distribution upon excitation controlling the rate of heterolytic bond cleavage is the work of Seiler and Wirz (11,12) for the photohydrolysis of trifluoromethyl substituted 1-and 2-naphthols, eq. [2] being an example. In a reaction assumed to proceed by an initial ionic For personal use only.…”