The Rubottom Oxidation

What the Rubottom oxidation does

You have a ketone. You want an -OH on the carbon next to the carbonyl — an alpha-hydroxy ketone, also called an acyloin. Direct oxidation of the alpha C-H fails: that carbon is not nucleophilic enough to react with a peracid, and the enol tautomer that would react exists in only trace concentrations at equilibrium. The Rubottom oxidation solves this in three moves: trap the enol, epoxidize it, then unmask the alcohol.

1. Make a silyl enol ether. Deprotonate the ketone with LDA (kinetic enolate) or treat with TMSOTf / a base, and quench the enolate oxygen with a silyl group. This freezes the ketone in its enol form as a stable, isolable, electron-rich alkene: R₂C=C(R)-OSiR₃.
2. Epoxidize with a peracid. mCPBA delivers an oxygen atom across the enol ether's C=C, forming a 2-silyloxy epoxide (a silyloxy oxirane).
3. Collapse and desilylate. The strained, electron-rich epoxide opens — the silyloxy oxygen's lone pair pushes in, the C-O epoxide bond breaks, and the system rearranges to an alpha-silyloxy ketone. Aqueous or fluoride workup cleaves the Si-O bond to give the free alpha-hydroxy ketone.

   O                     OSiR₃                   O   OH
   ||     1) LDA          |      mCPBA            ||  |
 R-C-CH₂R'  ---------->  R-C=CHR'  --------->    R-C-CHR'
          2) R₃SiCl      (silyl enol ether)   (alpha-hydroxy ketone)
                                                after Si-O cleavage

The genius of the sequence is that the silyl group plays two roles. First it traps the fleeting enol so there is a real, purifiable alkene to oxidize. Then, after epoxidation, that same silyloxy group is the electronic engine that opens the epoxide toward the correct carbon — steering the oxygen to end up on the alpha carbon and the carbonyl to re-form on the original ketone carbon.

The mechanism, arrow by arrow

Follow the electrons. Start from the isolated silyl enol ether.

1. Electrophilic epoxidation. mCPBA is a peracid: R-C(=O)-O-O-H. Its terminal (distal) oxygen is electrophilic. The pi electrons of the electron-rich enol ether C=C attack that distal oxygen in a concerted, butterfly-like transition state. The O-O bond breaks heterolytically, and the proton is delivered intramolecularly to the departing m-chlorobenzoate. Result: a 2-silyloxy epoxide plus m-chlorobenzoic acid.
2. Epoxide opening (the diagnostic step). The silyloxy epoxide is unusually reactive because an oxygen bearing lone pairs (-OSiR₃) sits directly on a strained ring carbon. Those lone pairs push into the C-O epoxide bond that is anti-periplanar / adjacent, ejecting the epoxide oxygen as an alkoxide and generating a silyl-stabilized oxocarbenium ion (R₃Si-Oₓ=C<). This is essentially a Meinwald-type rearrangement driven by the silyloxy group.
3. 1,4-Silyl migration / collapse. The alkoxide oxygen that just formed is now on the alpha carbon; the silyl group migrates from the original enol oxygen to this new alkoxide across the O-C-C-O framework (a fast, thermodynamically favorable 1,4-silyl shift), and the carbonyl reconstitutes on the original ketone carbon. The molecule is now an alpha-silyloxy ketone: R-C(=O)-CH(OSiR₃)-R'.
4. Desilylation. On workup, water/acid (for labile TMS) or fluoride (TBAF, for robust TBS) cleaves the strong Si-O bond. Fluoride is the classic driver because the Si-F bond (~135 kcal/mol) is one of the strongest single bonds in chemistry, making desilylation essentially irreversible. Out comes the free alpha-hydroxy ketone.

  silyl enol ether        2-silyloxy epoxide         alpha-silyloxy ketone
        OSiR₃                   O                          O
        |                    /   \                        ||
    R-C=CH-R'   + mCPBA  R-C ---- CH-R'   -->        R-C-CH(OSiR₃)-R'
                            |                        (Si migrates O->O,
                          OSiR₃                       C=O re-forms)
                                                          |
                                                    workup (H₋O or F₋)
                                                          v
                                                    R-C(=O)-CH(OH)-R'

A subtlety worth remembering: the regiochemistry is not ambiguous. Because the silyloxy group can only stabilize a positive charge on its own carbon, the epoxide always opens so that the carbonyl regenerates on the carbon that originally bore the -OSi group, and the -OH lands on the other (alpha) carbon. You cannot accidentally get a beta-hydroxy ketone from a normal Rubottom.

Reagents, conditions, and practical recipe

The reaction is bench-stable and forgiving if you buffer it. A representative procedure:

• Silylating step. Kinetic route: LDA (1.05 equiv), THF, −78 °C, then TMSCl or TBSCl (1.1 equiv). Thermodynamic route: TMSOTf / Et₃N, or Me₃SiCl / NaI / Et₃N. TBS enol ethers are more robust and survive silica; TMS enol ethers are cheaper but hydrolyze easily.
• Oxidant. m-Chloroperoxybenzoic acid (mCPBA), 1.0-1.2 equiv. Commercial mCPBA is ~70-77% pure (the rest is m-chlorobenzoic acid and water), so weigh accordingly. Alternatives: dimethyldioxirane (DMDO), magnesium monoperoxyphthalate (MMPP), or peroxyacetic acid.
• Solvent and temperature. Dichloromethane (or hexane/CH₂Cl₂) at −20 to 0 °C. Cold, slow addition suppresses acid-catalyzed side reactions.
• Buffer. Solid NaHCO₃ or K₂CO₃ suspended in the flask, or an aqueous NaHCO₃ wash, neutralizes the m-chlorobenzoic acid so it cannot prematurely cleave the silyl enol ether or the product.
• Workup / desilylation. For TMS: dilute HCl or simply aqueous workup often suffices. For TBS: TBAF in THF, or HF⋅pyridine, cleanly removes the silyl group to give the free acyloin.

A crucial detail people forget: the peracid oxidation and the desilylation are two separate events. It is entirely normal to isolate the alpha-silyloxy ketone as a stable compound and remove the silyl group in a deliberate second step, which is handy when the free alpha-hydroxyl would interfere with later chemistry.

Scope, selectivity, and stereochemistry

The Rubottom works on silyl enol ethers derived from ketones and aldehydes, cyclic and acyclic, and tolerates a wide range of alkyl and aryl substitution. Because the enol ether is generated regioselectively (kinetic LDA enolate vs thermodynamic enolate), you choose which alpha carbon gets hydroxylated before you ever add oxidant.

• Facial selectivity. mCPBA approaches the less hindered face of the C=C. On a fused ring or next to an existing stereocenter, this typically delivers the -OH to the convex/exo face with high diastereoselectivity (often > 9:1).
• Regioselectivity. Set by the enolate. A kinetic TMS enol ether puts the double bond toward the less substituted alpha carbon; the thermodynamic enol ether favors the more substituted one. This is the single most powerful control element in the whole method.
• Enantioselective variants. To make a single enantiomer of the acyloin, swap the achiral peracid for a chiral oxidant: a fructose-derived chiral dioxirane (Shi's ketone / Oxone), or a Davis N-sulfonyloxaziridine that hydroxylates the enolate face-selectively. These deliver enantioenriched alpha-hydroxy carbonyls directly.

Rubottom vs other alpha-oxygenation methods

	Rubottom oxidation	Davis oxaziridine	Enolate + O₂ / MoOPH
What you oxidize	Silyl enol ether (isolated alkene)	Lithium / potassium enolate	Lithium enolate
Oxidant	mCPBA (peracid)	N-Sulfonyloxaziridine	O₂ then reduction, or MoOPH
Key intermediate	2-Silyloxy epoxide	Direct O-transfer to enolate	Alpha-peroxy / Mo-peroxo
Enantiocontrol	Via chiral dioxirane (Shi) add-on	Built in with chiral oxaziridine	Poor without chiral auxiliary
Functional-group tolerance	High; acid byproduct needs buffering	High; very mild	Moderate; strong base needed
Typical scale	Milligram to multigram, routine	Research scale, pricier reagent	Research scale
Main drawback	Must prepare & isolate the enol ether	Oxaziridine cost / synthesis	Over-oxidation to diketone

Worked example: alpha-hydroxylating cyclohexanone

Turn cyclohexanone into 2-hydroxycyclohexanone (an acyloin) in two operations.

  cyclohexanone
       |  1) LDA (1.05 eq), THF, -78 C
       |  2) TMSCl (1.1 eq)
       v
  1-(trimethylsilyloxy)cyclohexene        <-- silyl enol ether
       |  mCPBA (1.1 eq), CH₂Cl₂, NaHCO₃, -20 C
       v
  2-(trimethylsilyloxy)cyclohexanone      <-- alpha-silyloxy ketone
       |  dilute HCl (aq) workup   [or TBAF if TBS]
       v
  2-hydroxycyclohexanone                  <-- product, ~70-85%

• Step 1. LDA at −78 °C forms the kinetic enolate; TMSCl traps the oxygen to give 1-(trimethylsilyloxy)cyclohexene, a distillable liquid.
• Step 2. mCPBA epoxidizes the ring double bond to the 1-silyloxy-1,2-epoxycyclohexane. Buffered with NaHCO₃, the epoxide silently rearranges to 2-(trimethylsilyloxy)cyclohexanone.
• Step 3. A brief acidic aqueous workup cleaves the labile TMS ether, delivering 2-hydroxycyclohexanone in roughly 70-85% yield over the oxidation/desilylation.

Note the alpha-hydroxy ketone here can further tautomerize/oxidize; keeping the conditions cold and buffered stops it from running on to 1,2-cyclohexanedione.

Where the Rubottom shows up in synthesis

• Acyloin and alpha-hydroxy carbonyl natural products. Many terpenoids, steroids, and polyketides carry an -OH adjacent to a carbonyl. The Rubottom is the reflexive disconnection: an alpha-hydroxy ketone comes from the parent ketone via its silyl enol ether.
• Corticosteroid side chains. The 17-alpha and 21-hydroxy patterns of cortisone-type steroids sit next to carbonyls; silyl enol ether epoxidation is one classical way to introduce those oxygens with facial control dictated by the rigid steroid backbone.
• Setting up 1,2-diols and diketones. An acyloin is one oxidation state away from a 1,2-diol (reduce) or a 1,2-diketone (oxidize), so the Rubottom is a common entry point to those motifs.
• Prostaglandin and polyol synthesis. Densely oxygenated targets use regioselective silyl enol ether formation plus Rubottom to place a single alpha-OH among many other functional groups without touching them.
• Fragment coupling in total synthesis. Because the silyl-protected product is stable, the alpha-OSi ketone can be carried through subsequent steps and unmasked only at the end, making the Rubottom a protecting-group-compatible way to stash an alpha-oxygen.

Limitations and side reactions

• Acid-sensitive substrates. The m-chlorobenzoic acid byproduct can protonate and hydrolyze the silyl enol ether before it is oxidized, or acid-catalyze rearrangement of the product. Always buffer (NaHCO₃/K₂CO₃) or use DMDO, which produces only acetone as byproduct.
• Competing Baeyer-Villiger. If the molecule already contains a ketone C=O in addition to the enol ether, mCPBA can insert an oxygen into that carbonyl (Baeyer-Villiger) as a competing pathway. Chemoselectivity favors the electron-rich enol ether, but forcing conditions erode it.
• Over-oxidation of the product. Acyloins with an alpha C-H can epimerize under the acidic conditions or oxidize further to 1,2-diketones. Keep it cold, buffered, and stop the reaction promptly.
• Enolizable/epimerizable stereocenters. The newly formed alpha stereocenter is alpha to a carbonyl, so it can racemize via re-enolization if the workup is basic or prolonged; enantioenriched acyloins need mild, prompt workup.
• Enol ether regiochemistry must be controlled. A poorly controlled enolate gives a mixture of silyl enol ethers and therefore a mixture of regioisomeric alpha-hydroxy ketones. Get the enolate right first.

Historical discovery

The transformation is named for George M. Rubottom, who published the mCPBA oxidation of trimethylsilyl enol ethers to alpha-hydroxy carbonyl compounds in 1974 (Rubottom, Vazquez, and Pelegrina, Tetrahedron Letters). Around the same time A. G. Brook and, shortly after, A. Hassner (Hassner, Reuss, and Pinnick, J. Org. Chem. 1975) independently described closely related silyl enol ether oxidation chemistry, and the reaction is occasionally credited to all three groups. Before this, installing an alpha-hydroxyl reliably was awkward — routes went through alpha-halo ketones and displacement, or through molecular-oxygen oxidation of enolates that were prone to over-oxidation. Rubottom's insight — trap the enol as a silyl ether, then epoxidize the resulting stable alkene — turned a finicky operation into a two-flask bench procedure, and it has been a standard tool of natural-product synthesis ever since.