Rearrangement — [1,2]-silyl migration
Brook rearrangement
How the Brook rearrangement works, in 3 steps. Each step shows the structures and the curved arrows that move the electrons.
In short. A 3-step mechanism. Typical reagents: Acylsilane + R–Li (or other α-silyl alkoxide source); electrophile E⁺. Conditions: THF or Et₂O, –78 to 25 °C; catalytic base or solvent polarity promotes the shift. Bonds formed: Si–O; carbanion; bonds broken: C–Si. Stereochemistry: [1,2]-shift proceeds with retention at carbon (via pentacoordinate Si). Rate and selectivity: Fast when the carbanion is stabilised (aryl, vinyl, CN); Si–O ≈ 110 vs Si–C ≈ 76 kcal/mol.
At a glance
Step 1: α-Silyl alkoxide forms
A silicon-substituted alcohol (an α-silyl alkoxide, formed for example by a nucleophile adding to an acylsilane) has its alkoxide oxygen positioned near the C–Si bond.
Step 2: Silyl group migrates C → O
The silyl group migrates from carbon to oxygen — the reverse polarity of the more familiar Peterson elimination's silyl departure — driven by the very strong Si–O bond that forms at the expense of the weaker Si–C bond.
Step 3: Carbanion relay to an electrophile
The result is a new carbanion, now stabilized wherever the original carbon substituents allow, with silicon now bonded to oxygen instead of carbon — this anion can go on to react with further electrophiles, making the Brook rearrangement a useful carbanion-relay strategy in complex synthesis.
Abbreviations and terms
- R
- R group. Any carbon substituent (alkyl, aryl…) whose identity does not matter for the mechanism.
- E+
- Electrophile. An electron-poor species that accepts an electron pair.
- Ph
- Phenyl. –C₆H₅.
- THF
- Tetrahydrofuran. Cyclic ether solvent; its oxygen lone pairs solvate Li⁺/Mg²⁺ so organometallics stay dissolved and reactive.
- Et2O
- Diethyl ether. Low-boiling ether solvent, classic for Grignard reagents.
How it compares with related reactions
| Brook rearrangement | Bamberger rearrangement | Beckmann rearrangement | |
|---|---|---|---|
| Reagents | Acylsilane + R–Li (or other α-silyl alkoxide source); electrophile E⁺ | N-Phenylhydroxylamine, dilute H₂SO₄ (aq) | H₂SO₄, PCl₅, TsCl or PPA |
| Conditions | THF or Et₂O, –78 to 25 °C; catalytic base or solvent polarity promotes the shift | H₂O, 25–60 °C | Acid, 100–130 °C (industrial: oleum) |
| Stereochemistry | [1,2]-shift proceeds with retention at carbon (via pentacoordinate Si) | None; para-selective | Group anti to OH migrates, with retention |
| Rate / selectivity | Fast when the carbanion is stabilised (aryl, vinyl, CN); Si–O ≈ 110 vs Si–C ≈ 76 kcal/mol | Loss of water to give the nitrenium ion is rate-limiting | Migration is rate-limiting; leaving-group ability of N–OX matters |
Test yourself
Try to answer each question before opening it.
Which reagents and conditions are typical for the Brook rearrangement?
Acylsilane + R–Li (or other α-silyl alkoxide source); electrophile E⁺. Conditions: THF or Et₂O, –78 to 25 °C; catalytic base or solvent polarity promotes the shift.
Which bonds are formed and which are broken?
Formed: Si–O; carbanion. Broken: C–Si.
What is the stereochemical outcome?
[1,2]-shift proceeds with retention at carbon (via pentacoordinate Si)
What controls the rate or the selectivity?
Fast when the carbanion is stabilised (aryl, vinyl, CN); Si–O ≈ 110 vs Si–C ≈ 76 kcal/mol
Study guides that help with this reaction
- Pericyclic reactions explained: cycloadditions, electrocyclisations and sigmatropic shifts
- Stereochemistry in mechanisms: inversion, retention, syn and anti