Rearrangement and participation reactions occurring during the bromination of 7-trimethylsilylnitronorbornenes. Evidence for a discrete carbocation β to silicon

Abstract

Transannular neighbouring group participation by the endo-substituent occurs during bromination at the double bonds of methyl 3-endo-nitro-7-anti-trimethylsilylbicyclo[2.2.1]hept-5-ene-2-exo-carboxylate (12), the corresponding exo-nitro-endo-carboxylate isomer (15), and the exo-nitro-endo-methanol analogue (16). Silicon-assisted Wagner–Meerwein rearrangement occurs as a minor reaction for (12), but becomes the major pathway in the bromination of 5-endo-nitro-7-anti-trimethylsilylbicyclo[2.2.1 ]hept-2-ene (2) and 3-endo-nitro-7-anti-trimethylsilylbicyclo[2.2.1 ]hept-5-en-2-exo-ylmethanol (3). In the latter case, both 7-anti-bromo-3-exo-nitrobicyclo[2.2.1]hept-5-en-2-endo-ylmethanol (10) and a tricyclic ether (11) are formed. The structure of (11) was confirmed by X-ray crystallography. Evidence is presented for the involvement of a discrete carbocation β to silicon in this, rearrangement.