Rate Information from an Optically Active Ether

1 Rate Information from an Optically Active Ether 

More detailed kinetic information about the motions and reactions of the alkyl/aryloxy radical pairs can be obtained if the alkyl carbon atom making an ether bond is chiral and the alkyl aryl ether is optically active. Because the radical centers of the alkyl fragments derived from irradiation of such ethers are either planar or invert very rapidly, they exist initially with their aryloxy partners as prochiral pairs. Their ability to retain the memory of the chirality of their parent ethers and impart it to the rearrangement products depends on the ratio between the in-cage rates of radical tumbling and radical pair combination. 

The corresponding esters are much less informative because the centers of chirality in their acyl radicals are structurally protected from racemization like that experienced by translational or rotational motions of prochiral alkyl radicals. In addition, the decarbonylated radicals derived from them are formed long after their acyl precursors have moved to orientations with respect to their aryloxy partners that result in a loss of the memory of their host stereochemistry within a cage see above. Thus, of the Claisen-like photoproducts from irradiation of (7 )-lb, only the BzON (i.e., 3b) retains a measurable amount of optical activity even in the solid phases of long -aIkane. However, in polyethylene hlms, all of the Claisen products from irradiation of (7 )-lb—2-BN, 4-BN, and 3b—exhibit signihcant ee values. In the same media, the photo-Fries products from lb retain virtually all of the enantiomeric purity of the 

DYNAMICS OF RADICAL PAIR PROCESSES IN BUEK POLYMERS 

FIGURE 13.2 Plots of (a) 2BN/4BN, (b) ee%tote,i of 2BN, and (c) ee%totai of 4BN from irradiations of (i )-3b at 296 ( ) or 333K ( ) versus bulk viscosity (tj) of various -alkanes. The curves are visual aids only they are not based on a physical model. Reprinted with permission from Ref Copyright (2005) American Chemical Society. 

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