VII. Silyl Ether Rearrangement

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Rearrangement takes place during radical cyclization involving some silyl ethers. The primary evidence for this rearrangement is the dependence of product ring size on the concentration of Bu₃SnH, the hydrogen-atom transfer in these reactions. When the reaction shown in eq 14 is conducted in dilute Bu₃SnH solution, the major product contains a six-membered ring,³⁵ but at high Bu₃SnH concentration reaction regioselectivity changes to give a product with a five‑membered ring.^36,37 This concentration dependence can be explained by the more rapidly formed, but less stable, radical 46 having sufficient time and energy, when the concentration of Bu₃SnH is low, to be converted into the more stable radical 47, either by a rearrangement that involves a cyclic transition state or by a fragmentation-addition sequence (Scheme 18).³⁷ At high Bu₃SnH concentration hydrogen-atom abstraction occurs before ring expansion can take place.

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