The Corey-Fuchs Reaction

What the Corey-Fuchs reaction does

Terminal alkynes are prized building blocks — they slot straight into Sonogashira couplings, click chemistry, and metal-acetylide additions. But making one from a garden-variety aldehyde is not obvious. The Corey-Fuchs reaction solves it in a two-pot sequence that lengthens the carbon chain by exactly one atom and installs a triple bond at the new end:

    R-CHO   ──►   R-CH=CBr₂   ──►   R-C≡C-H
   aldehyde      1,1-dibromo-      terminal
                  alkene           alkyne

   step 1: CBr₄ (1.1 eq), PPh₃ (2.2 eq), CH₂Cl₂, 0 °C
   step 2: n-BuLi (2.0-2.5 eq), THF, −78 °C; then H₂O

The original aldehyde carbon becomes one of the two alkyne carbons; the extra carbon comes from carbon tetrabromide. So an n-carbon aldehyde becomes an (n+1)-carbon terminal alkyne. Two equivalents of n-BuLi later, you don't just have the alkyne — you have its lithium acetylide still in the flask, which you can trap with an electrophile to make an internal alkyne without ever isolating the terminal one.

The mechanism, arrow by arrow

The two steps run on completely different chemistry, so it helps to treat them separately.

Step 1 — the Ramirez dibromoolefination

This is a Wittig olefination in disguise. You need a dibromomethylene ylide, Br₂C=PPh₃, but that compound is not stable enough to bottle — so it is generated in situ:

1. Reduce the CBr₄. One equivalent of triphenylphosphine attacks a bromine of CBr₄, forming Ph₃P–Br⁺ and the tribromomethanide anion ⁻CBr₃.
2. Form the ylide. A second equivalent of PPh₃ (or the bromophosphonium) drives loss of Br⁻ from ⁻CBr₃ to give the resonance-stabilized ylide Br₂C=PPh₃. This is why the recipe calls for two equivalents of phosphine per equivalent of CBr₄ — one is sacrificial, ending up as Ph₃PBr₂.
3. Wittig olefination. The nucleophilic ylide carbon adds to the aldehyde carbonyl. The betaine collapses through a four-membered oxaphosphetane, which fragments to expel Ph₃P=O (the thermodynamic sink) and deliver the 1,1-dibromoalkene R–CH=CBr₂.

   CBr₄  +  PPh₃   →   Br₃C⁻  +  Ph₃P-Br⁺
   Br₃C⁻ +  PPh₃   →   Br₂C=PPh₃  +  Br⁻        (the ylide)
   Br₂C=PPh₃  +  R-CHO   →   R-CH=CBr₂  +  Ph₃P=O

Step 2 — elimination, carbenoid rearrangement, second exchange

The dibromoalkene is dissolved in cold THF and hit with n-BuLi. Three things happen in quick succession:

1. First equivalent: exchange and elimination. n-BuLi does a lithium-halogen exchange on one of the two vinylic bromides, replacing it with lithium to give a bromovinyllithium, R–CH=C(Br)Li. Lithium and bromine now sit on the same terminal carbon, so α-elimination of LiBr expels the second halide and unveils an alkylidene carbene (an unsaturated carbenoid), R–CH=C:.
2. Fritsch-Buttenberg-Wiechell rearrangement. The hydrogen on the former aldehyde carbon undergoes a 1,2-shift onto the adjacent carbene center. Deuterium-labeling studies confirm this migrating group is the vinylic hydrogen (a 1,2-hydride shift), and it is the bond reorganization that fuses the two carbons into the triple bond, delivering the terminal alkyne R–C≡C–H.
3. Second equivalent: deprotonation to the acetylide. The freshly minted terminal alkyne has an acidic sp C–H (pK_a ≈ 25). The second equivalent of n-BuLi removes it, parking the molecule as the lithium acetylide R–C≡C–Li — a stable, isolable-in-flask carbon nucleophile. (An equivalent way to draw the sequence stops the first stage at a 1-bromoalkyne R–C≡C–Br, which the second equivalent then converts to the same acetylide by lithium-halogen exchange; both routes reach R–C≡C–Li and both need two equivalents.)
4. Quench. Adding water (or dilute acid) protonates the acetylide back to the terminal alkyne R–C≡C–H. Adding an electrophile instead installs a new group and gives an internal alkyne.

   R-CH=CBr₂ + n-BuLi → [R-CH=CBrLi] → −LiBr → R-CH=C:  → FBW → R-C≡C-H   (1st eq.)
   R-C≡C-H   + n-BuLi → R-C≡C-Li + n-BuH                                    (2nd eq.)
   R-C≡C-Li  + H₂O    → R-C≡C-H  + LiOH                                     (quench)

The net bookkeeping: both C–Br bonds are broken, the vinylic hydrogen does a 1,2-shift, and a C≡C triple bond is born. The driving force is the strength of that triple bond plus the formation of LiBr and n-butane (n-BuH from the deprotonation).

Reagents, stoichiometry, and conditions

• Step 1 reagents. Carbon tetrabromide (1.0-1.1 equiv), triphenylphosphine (2.0-2.2 equiv), typically in dichloromethane at 0 °C. Some protocols add zinc dust (Corey-Fuchs-Ramirez with Zn) to reduce the phosphine load and improve yields on sensitive substrates; the Zn variant uses CBr₄, PPh₃, and Zn in a 1:1:1 ratio and is milder.
• Step 1 workup. The reaction is fast (minutes to an hour). Quench, filter off triphenylphosphine oxide (often by adding pentane to precipitate it), and chromatograph. Yields of the dibromoalkene are commonly 80-95%.
• Step 2 reagents. n-Butyllithium, 2.0-2.5 equivalents, in dry THF or Et₂O, at −78 °C. The low temperature is essential — the vinyllithium/carbenoid intermediates decompose if warmed prematurely.
• Step 2 quench. Water or saturated NH₄Cl for the terminal alkyne. For an internal alkyne, add MeI, an alkyl bromide/HMPA, a chloroformate, DMF (to make a propargylic aldehyde), CO₂ (to make a propiolic acid), or a chlorosilane before quenching.
• Atmosphere. Strictly anhydrous and inert (Ar or N₂). n-BuLi is pyrophoric; CBr₄ and the dibromoalkenes are light-sensitive and best stored cold and dark.

Scope, selectivity, and what survives

The dibromoolefination is quite general: aromatic, aliphatic, and α,β-unsaturated aldehydes all convert to their 1,1-dibromoalkenes. Because the ylide is only weakly basic and the reaction is fast at 0 °C, epimerizable α-stereocenters usually survive step 1 intact — a real advantage over strongly basic olefinations.

The metalation step is where selectivity matters:

• No stereochemistry to worry about at the alkyne. The product is a linear sp-hybridized triple bond, so there is no E/Z issue — unlike the Wittig, which forces you to think about geometry. The 1,1-dibromoalkene intermediate is symmetric about the two bromines, so its own geometry is irrelevant.
• Base sensitivity is the main limit. Two equivalents of n-BuLi is a strong, nucleophilic base. Substrates with acidic protons (unprotected OH, NH, enolizable ketones), or with esters and nitriles that n-BuLi will attack, are problematic unless protected. This is the classic reason to switch to the milder Bestmann-Ohira reagent.
• Chemoselectivity. The lithium-halogen exchange is fast and selective for the C(sp²)-Br and C(sp)-Br bonds; aryl bromides elsewhere in the molecule can also exchange, so watch for competing metalation sites.

Corey-Fuchs vs other aldehyde-to-alkyne methods

	Corey-Fuchs	Seyferth-Gilbert	Bestmann-Ohira
Pots / steps	Two pots (isolate dibromoalkene)	One pot	One pot
Step-1 reagent	CBr₄ + 2 PPh₃	(MeO)₂P(O)CHN₂ (Seyferth-Gilbert reagent)	Dimethyl (1-diazo-2-oxopropyl)phosphonate
Base / conditions	2 n-BuLi, −78 °C	KOtBu, −78 °C	K₂CO₃, MeOH, RT
Base strength	Very strong (n-BuLi)	Strong (alkoxide)	Mild (carbonate)
Epimerization risk	Higher at metalation	Moderate	Lowest
Works on aromatic aldehydes	Yes	Yes	Yes
Works on enolizable aldehydes	Yes (step 1); care at step 2	Modest	Excellent
Can trap to internal alkyne	Yes — acetylide in flask	No	No
Can stop at bromoalkyne	Yes (1 equiv n-BuLi)	No	No
Byproducts	Ph₃P=O, LiBr, n-butane (or n-BuBr)	N₂, dimethyl phosphate	N₂, methyl acetate, phosphate

Worked example: an aldehyde to an internal alkyne in one flask

Suppose you have a protected aldehyde on a fragment and you want to extend it to a propargylic building block. Take benzaldehyde as the illustration:

   Step 1:  PhCHO  +  CBr₄ (1.1 eq)  +  PPh₃ (2.2 eq)
            CH₂Cl₂, 0 °C, 30 min
            →  Ph-CH=CBr₂   (β,β-dibromostyrene, ~90%)

   Step 2:  Ph-CH=CBr₂  +  n-BuLi (2.2 eq)
            THF, −78 °C, 1 h        →  Ph-C≡C-Li  (in situ)

   Quench A (H₂O):        →  Ph-C≡C-H   (phenylacetylene)
   Quench B (MeI/HMPA):   →  Ph-C≡C-CH₃ (1-phenylpropyne)
   Quench C (CO₂, H⁺):    →  Ph-C≡C-COOH (phenylpropiolic acid)

• Why it is powerful. Quench A, B, and C all start from the same flask of lithium acetylide. You have effectively homologated an aldehyde and then decorated the new terminus in a single operation.
• Practical tip. For alkylations (quench B), add HMPA or DMPU as a cosolvent — the naked acetylide is a sluggish nucleophile toward primary halides without it.

Real-world applications

• Total synthesis workhorse. Corey-Fuchs is a default homologation in fragment coupling. E. J. Corey's group and many others have used it to convert an advanced aldehyde into an alkyne that is then closed by ring-closing enyne metathesis, Sonogashira coupling, or a metal-acetylide addition. It shows up in syntheses of polyketides, macrolides, and enediyne antibiotics.
• Enediyne and polyyne construction. The clean access to terminal and 1-halo alkynes makes Corey-Fuchs a staple for building conjugated ene-yne and poly-yne cores, including the reactive enediyne warheads of the calicheamicin/dynemicin antibiotic family.
• Bromoalkyne building blocks. Stopping at the 1-bromoalkyne (one equivalent of n-BuLi) delivers R-C≡C-Br directly, feeding Cadiot-Chodkiewicz couplings to unsymmetrical diynes and copper-free Sonogashira variants.
• Isotope labeling. Because the extra carbon comes from CBr₄, using ¹³C- or ¹⁴C-labeled carbon tetrabromide places a labeled atom precisely at the alkyne terminus — handy for tracer studies and mechanism work.

Limitations and side reactions

• Base-sensitive substrates. The n-BuLi step is the choke point. Enolizable ketones, esters, epoxides, and acidic N-H/O-H groups need protection or the diazo-phosphonate (Bestmann-Ohira) alternative.
• Phosphine oxide cleanup. Step 1 generates a full equivalent of Ph₃P=O plus Ph₃PBr₂, which can co-elute with nonpolar dibromoalkenes and complicate purification. Precipitation with pentane/ether and filtration through silica is the usual fix; the Zn-modified protocol cuts the phosphine burden.
• Cost and toxicity of CBr₄. Carbon tetrabromide is expensive, moisture- and light-sensitive, and a suspected toxin; scale-up favors catalytic or Zn-assisted variants.
• Warming decomposition. Let the metalation warm above roughly −40 to −30 °C before the rearrangement is complete and you get tars from carbenoid decomposition. Keep it cold until the acetylide has formed.
• Ketone endpoint mismatch. Ketone-derived 1,1-dibromoalkenes lack a vinylic hydrogen for the classic elimination, so the terminal-alkyne endpoint is an aldehyde-only trick.

Who discovered it, and when

The reaction was published in 1972 by Elias James Corey and Philip L. Fuchs in a short Tetrahedron Letters communication titled "A synthetic method for formyl → ethynyl conversion." Corey — who would win the 1990 Nobel Prize in Chemistry for the theory and methodology of organic synthesis — and Fuchs, then in the Corey group at Harvard and later a professor at Purdue, recognized that a 1,1-dibromoalkene could be driven to an alkyne by double lithium-halogen exchange and elimination.

The two halves of the reaction each have older roots the authors stitched together. The dibromoolefination was reported by Fernando Ramirez and co-workers in 1962 (the CBr₄/PPh₃ Wittig-type olefination), so step 1 is often called the Ramirez olefination. The 1,2-shift that forges the triple bond is the Fritsch-Buttenberg-Wiechell rearrangement, described independently by all three chemists in 1894. Corey and Fuchs' contribution was showing that the whole cascade could be run reliably at −78 °C and, crucially, that the intermediate acetylide could be trapped to make internal alkynes — turning a curiosity into a general homologation.