Photo-Fries rearrangements of 1-naphthyl (R)-2-phenylpropanoate in poly(vinyl acetate) and ethyl acetate: influence of medium polarity and polymer relaxation on motions of singlet radical pairs

Abstract

Both the regio-and stereo-chemistries of the photoreactions of 1-naphthyl (R)-2-phenylpropanoate have been investigated in poly(vinyl acetate) films in their glassy (at 5 • C) and melted (at 50 • C) states and in ethyl acetate. These results are compared with those from irradiations in polyethylene films and in n-hexane. The regioselectivity of the intermediate 1-naphthoxy/(R)-2-phenylpropanoyl radical pair combinations is much higher in both the melt and glassy states of poly(vinyl acetate) films than that in the melt state of completely amorphous polyethylene films, but the stereoselectivity of intermediate prochiral 1-naphthoxy/1-phenylethyl radical pair combinations is much lower in poly(vinyl acetate). The results emphasize the need to control the ratio between the rates of radical tumbling and translation, as well as the ratio between the rates of in-cage motions and cage-escape, if high stereo-and regio-selectivities of combination products are to be achieved. A mechanistic picture of how the radicals of the intermediate pairs are affected by and interact with the various media is advanced.

Dynamics of Radical Pair Processes in Bulk Polymers

“…47 Also, the k 2B /k 4B /k E ratios in PVAc-83 at 323K are 1/0.35/2.4. 76 The most dramatic difference between these ratios and those reported in Table 13.5 for irradiations in the polyethylene films is the much lower relative rate of formation of 4-BN. The most economical explanation for these observations is that translational motions of the 2-phenylpropanoyl and 1-phenylethyl radicals (as well as of their 1-naphthoxy partner) are impeded by attractive interactions of the radical centers with ester groups of the polymer.…”

“…They indicate that the greater reactivity of the benzyl radical (from 1a) than the 1-phenylethyl radical (from 1b) is less selective in its reactions with 1-naphthoxy and that the longer period for decarbonylation of a phenylacetyl radical (than a 2-phenylpropanoyl radical) places the benzyl radical farther than the 1-phenylethyl radical from the oxygen atom of their 1-naphthoxy partners at the moment of their birth. The k 2A and k 4A from 1b in PVAc-83 at 323 K (i.e., well above Tg) as calculated from Equations 13.10 and 13.11 are (0.8-1.3) Â 10 9 and (2.0-3.3) Â 10 7 s À1 , respectively; 76 the range limits are determined by assuming that either all or none of the NOL emanates from cage escape at the 1-phenylethyl/1-naphthoxy stage. 47 Although it is possible to make similar calculations from data in the glass phase at 278K, the validity of the numbers is questionable because the efficiency of the irradiations is very low, indicating that the photochemistry may be occurring in sites that do not represent the bulk.…”