Sigmatropic 1,3-H-migration

By serendipity we found a novel base induced stereocontrolled sigmatropic 1,3-H-migration. Benzylation of the readily available alcohol 48 to benzyl ether 49 occurs under standard conditions at 25°C. By mere accident the student (G. Funk) raised the temperature of the mixture to 80 °C and left the reaction at that temperature for 14 h. 

Sigmatropic shifts are reversible reactions in which a o-bonded group (usually H) migrates from one point to another within a molecular system. Sigmatropic shifts may occur either thermally or photochemi-cally, but 1,3- and 1,7-shifts are the most commonly encountered for the photochemical reactions. 

Unlike [l,3]-sigmatropic migrations of H-atom, which cannot occur under thermal conditions, the [ 1,3] -sigmatropic migrations of carbon can occur under thermal conditions. This is because carbon has two-lobed p-orbital. Therefore, carbon can simultaneously interact with the migrating source and the migrating terminus using either one of its lobes or both of its lobes, as shown in Fig. 8.58. 

A sigmatropic migration of hydrogen takes place at elevated temperatures in 19a. This leads to the establishment of a dynamic equilibrium between the three isomers 19a, 19b, and 19c (Scheme 2.17). A small amount (about 6%) of 2-indenyl(diethyl)borane 19c apparently appears by a [1,5]-H shift in the Ao-indenyl intermediate 19e, which is formed from 19a via two consequent [1,3]-B shifts. 

---