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

Jinqi Xu,Richard G Weiss,Mathew George

doi:10.1590/s0001-37652006000100005

Jinqi Xu, Richard G Weiss + Show 1 more

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https://doi.org/10.1590/s0001-37652006000100005

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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 degrees C) and melted (at 50 degrees 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.

Highlights

The rates and regio- and stereo-chemistries of radical pair combinations are known to be sensitive to local environmental effects (Porter and Krebs 1988, John 1993, Curran et al 1995, Turro 2002, Ramamurthy and Schanze 2003)
The high microviscosity and “templating” effects of the PE reaction cavities restrict radical pair movements (Gu et al 1999, 2000a, b, Gu and Weiss 2000, 2001a, b), enabling even prochiral radical pair B to retain partial orientational memory of its chiral radical pair A precursor (Xu and Weiss 2003, 2005a, b, c). These results suggest that the rates of tumbling of a 1phenylethyl radical and of radical pair combination are similar in these reaction cages, and both are at least comparable to the rates of relaxation of the polymeric chains constituting the PE cavity walls (Gu et al 1999, 2000a, b, Gu and Weiss 2000, 2001a, b, Xu and Weiss 2003, 2005a, b, c)
Radical pair A may become singlet prochiral radical pair B if the 2-phenylpropanoyl radical cleaves into carbon monoxide and a 1-phenylethyl radical more rapidly than it adds to its 1-naphthoxy partner or escapes from the cage

Summary

Introduction

The rates and regio- and stereo-chemistries of radical pair combinations are known to be sensitive to local environmental effects (Porter and Krebs 1988, John 1993, Curran et al 1995, Turro 2002, Ramamurthy and Schanze 2003). The chemistries of the combinations of chiral radical pair A and prochiral radical pair B have been investigated in relatively low polarity media such as n-alkanes with different viscosities (Xu and Weiss 2005a), polyethylene (PE) films with a wide range of crystallinities above their glass transition temperatures (Tgs) (Xu and Weiss 2005b), and the solid phases of nnonadecane (Xu and Weiss 2005c). As expected, both the regio- and stereo-selectivities decrease as temperature is raised and medium viscosity is lowered (Xu and Weiss 2003, 2005a, b, c)

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