Scheme 4.

Effect of solvent polarity on the silyl ether trapping using an internal clock reaction. The ratio of intramolecular dihydrogen transfer (6c to 9, panel A) to silyl ether trapping (6c to 7c) processes shows a dependence on solvent polarity (results tabulated in panel C). DFT calculations on a truncated analog led to the identification of the concerted TS 8-TS (panel B, non-participating aliphatic hydrogen atoms removed for clarity). ¹H NMR spectrum of the crude product mixture from a typical experiment (panel D), this one for the reaction performed in 1,2-dichloroethane, resulting in a 6:1 ratio of products 7c:9.