The [1,2]-Brook Rearrangement of α-silyl carbinols is an intramolecular 1,2-anionic migration of a silyl group from carbon to oxygen in the presence of a catalytic amount of a base such as Et2NH, NaH or NaOH. The migratory aptitude is general over a range of homologues, and [1,n]-carbon to oxygen migrations are commonly referred to as Brook Rearrangements.
Mechanism of the Brook Rearrangement
The mechanism as described by Brook (Acc. Chem. Res. 1974, 7, 77. DOI) includes the formation of a cyclic pentavalent silicon species immediately following the deprotonation. Subsequent ring opening and irreversible, fast protonation of the carbanion by the starting alcohol or the conjugate base leads to the corresponding silyl ether:
The greater strength of the oxygen-silicon bond compared to the carbon-silicon bond provides the driving force for the conversion of silyl carbinols to the corresponding silyl ethers. An electron-withdrawing R group facilitates the kinetics of the carbanion formation.
In the presence of a strong base in stoichiometric amounts, the equilibrium between alkoxide and carbanion is relative to the stabilities of the corresponding anionic species:
Here, the presence of an electron withdrawing group R shifts the equilibrium to the right, whereas counterions that form strong ion pairs with oxygen such as lithium favor an oxygen to carbon silyl migration (retro-Brook Rearrangement). Destabilization of the alkoxides using polar solvents such as THF also shifts the equilibrium towards the silyl ethers.
The use of a stoichiometric amount of base and the control of the equilibrium enables tandem strategies to introduce electrophiles:
The use of acylsilanes makes even more sophisticated tactics possible:
For concrete reactions, please check the recent literature section at the end of this site and the review written by Moser (Tetrahedron 2001, 57, 2065. DOI), which explains the basics for tandem bond formation strategies and gives several additional, interesting examples.
Amide Enolate Additions to Acylsilanes: In Situ Generation of Unusual and Stereoselective Homoenolate Equivalents
R. B. Lettan, II, C. V. Galliford, C. C. Woodward, K. A. Scheidt, J. Am. Chem. Soc., 2009, 131, 8805-8814.