The Bamford-Stevens Reaction

What the Bamford-Stevens does

Start from a ketone or aldehyde. Condense its carbonyl with p-toluenesulfonyl hydrazide (TsNHNH₂) to make a tosylhydrazone, R₂C=N-NHTs. Then hit that tosylhydrazone with a base and heat. The oxygen is already gone — it left as water during the condensation — and now the base tears out the tosyl group and the nitrogens, leaving behind a carbon–carbon double bond where you used to have a carbonyl. Net transformation: a C=O becomes a C=C at the same carbon, using an adjacent α-hydrogen to complete the alkene.

It is one of the classic ways to make an olefin from a ketone without a phosphorus ylide. Crucially, the double bond forms at the position the carbonyl already occupies — you are not adding a new carbon (as a Wittig would), you are converting the sp² carbonyl carbon into an sp² alkene carbon and pulling in a neighbor.

    R₂C=O  +  H₂N-NH-Ts  ──(−H₂O)──→  R₂C=N-NH-Ts        (tosylhydrazone)

    R₂C=N-NH-Ts  ──base, Δ──→  alkene  +  N₂↑  +  Ts⁻

    e.g.  cyclohexanone tosylhydrazone  ──NaOMe, diglyme, 160 °C──→  cyclohexene  +  N₂  +  TsNa

The step-by-step mechanism

Four moves get you from tosylhydrazone to alkene. The first two are the same in every version; the divergence happens at the reactive intermediate.

1. Deprotonation. The base (methoxide, hydride, or the alkoxide of the solvent) removes the acidic N-H proton of the tosylhydrazone. The lone pair on that nitrogen now becomes an anion — the trigger for everything that follows.
2. Sulfinate expulsion. The nitrogen anion pushes its lone pair to form a new N=N π bond, and the N-S bond breaks: the tosyl group leaves as p-toluenesulfinate (Ts⁻, an aryl sulfinate anion). What is unmasked is a diazo compound, R₂C=N⁺=N⁻ (a diazoalkane). This is the pivotal intermediate — bright, reactive, and shared by both the carbene and the cationic route.
3. Loss of N₂. The diazo compound now sheds molecular nitrogen. How it does so depends on the medium:
   • Aprotic solvent (diglyme, THF): the diazoalkane loses N₂ directly to give a singlet carbene, R₂C: — a carbon with only six valence electrons and an empty p-orbital.
   • Protic solvent (ethylene glycol, an alcohol): the diazoalkane is first protonated at carbon to give an alkyl diazonium ion, R₂CH-N₂⁺, which then loses N₂ to give a carbocation, R₂CH⁺.
4. 1,2-hydride shift → alkene. The carbene inserts by shifting a hydrogen from the neighboring carbon into its empty orbital, closing a C=C double bond. The carbocation does the equivalent: a β-hydride departs to a base (E1-like), quenching the positive charge and forming the same double bond. Either way you land on an alkene, and the N₂ has bubbled away.

  1.  R₂C=N-NHTs  +  ⁻OMe   →   R₂C=N-N⁻Ts   +  MeOH          (deprotonation)
  2.  R₂C=N-N⁻Ts             →   R₂C=N⁺=N⁻   +  Ts⁻            (sulfinate leaves → DIAZO)

  APROTIC path:
  3a. R₂C=N₂                 →   R₂C:  (singlet carbene)  +  N₂↑
  4a. R₂C:  --1,2-H shift-->     R(H)C=CR'  (alkene)

  PROTIC path:
  3b. R₂C=N₂  +  H⁺          →   R₂CH-N₂⁺   →   R₂CH⁺  +  N₂↑   (carbocation)
  4b. R₂CH⁺  --lose β-H-->       R(H)C=CR'  (alkene, Zaitsev-favored)

Reagents, base, and conditions

The recipe is short but the choice of base and solvent decides the mechanism and the product distribution.

• Make the tosylhydrazone first. Ketone + TsNHNH₂ (1.0-1.1 equiv), catalytic acid, in ethanol or methanol, room temperature to reflux. The tosylhydrazone usually crystallizes out and is bench-stable — you can store it and decompose it later.
• Classic Bamford-Stevens base. One equivalent of a sodium or potassium alkoxide — sodium methoxide (NaOMe), sodium ethylene glycolate, or sodium hydride (NaH). The base need only deprotonate the N-H; it is not consumed catalytically in the cation route.
• Temperature. Thermal decomposition typically needs 100-160 °C. Cyclohexanone tosylhydrazone with NaOMe in diglyme runs cleanly around 160 °C. Photochemical variants (hν) let you decompose at or below room temperature and cleanly favor the carbene.
• Solvent = mechanism switch. Aprotic diglyme or THF → singlet carbene, fewer rearrangements. Protic ethylene glycol or an alcohol → carbocation, Zaitsev alkene, possible skeletal rearrangement. This single choice is the most important decision in setting up the reaction.
• Watch the gas. A full equivalent of N₂ is released; the visible bubbling is a practical progress indicator, and the venting is part of what makes the reaction irreversible.

Scope, regiochemistry, and stereochemistry

Because two different reactive intermediates are on offer, the Bamford-Stevens family gives you a lever on which alkene you get.

• Aprotic (carbene) → often the less-hindered or cis-biased alkene. The singlet carbene's 1,2-H shift is fast and relatively insensitive to substitution; with a choice of α-hydrogens it can favor the thermodynamically less obvious product and often shows a modest bias toward the (Z)-alkene in acyclic cases.
• Protic (carbocation) → the more-substituted Zaitsev alkene. The free cation loses whichever β-hydrogen gives the most stable, most-substituted double bond, exactly as in an E1.
• Rearrangement risk in the cation route. Because a real carbocation forms in protic media, 1,2-alkyl and hydride shifts to a more stable cation can scramble the carbon skeleton, giving ring-expanded or migrated products. The aprotic carbene route avoids most of this.
• You must have an α-hydrogen. To make the alkene the carbene needs a neighboring C-H to shift. With no α-hydrogen, the alkene pathway is blocked and the carbene instead inserts into C-H bonds, cyclopropanates, or ring-expands — a synthetically useful diversion, not a failure.
• Substrate range. Works on aldehydes and ketones, cyclic and acyclic, including strained and polycyclic systems where carbene C-H insertion is exploited on purpose.

Bamford-Stevens vs Shapiro (and vs Wittig)

	Bamford-Stevens (aprotic)	Bamford-Stevens (protic)	Shapiro
Base	NaOMe / NaH, 1 equiv	Na glycolate, 1 equiv	n-BuLi or MeLi, ≥ 2 equiv
Reactive intermediate	Singlet carbene R₂C:	Carbocation R₂CH⁺	Vinyllithium (via dianion)
Temperature	100-160 °C or hν	100-160 °C	−78 °C to 0 °C
Regiochemistry	Often less-substituted / mixed	Zaitsev (more substituted)	Anti-Zaitsev (less substituted)
Skeletal rearrangement	Rare	Common (cation shifts)	None
Product handle	Neutral alkene	Neutral alkene	Nucleophilic vinyllithium (trap with electrophile)
Where the C=C ends up	Between former carbonyl C and an α-carbon		Same — but with clean regio/stereocontrol
vs Wittig	Wittig instead places C=C where the oxygen was, joining the carbonyl C to a new ylide carbon

Worked example: cyclohexanone → cyclohexene

The textbook demonstration converts cyclohexanone into cyclohexene.

  step 1 (make tosylhydrazone):
     cyclohexanone  +  TsNHNH₂  ──EtOH, cat. H⁺, reflux──→  cyclohexanone tosylhydrazone  +  H₂O

  step 2 (Bamford-Stevens, aprotic):
     tosylhydrazone  ──NaOMe (1 eq), diglyme, ~160 °C──→  cyclohexene  +  N₂↑  +  TsNa

• What happens inside step 2. Methoxide removes the N-H; sulfinate (TsNa) leaves to give diazocyclohexane; the diazo loses N₂ to give cyclohexylidene carbene; a ring C-H shifts 1,2 into the carbene to close the ring double bond — cyclohexene.
• Byproducts. One equivalent of N₂ gas and one equivalent of sodium p-toluenesulfinate. The gas evolution is visible and drives the reaction forward.
• Why aprotic here. In diglyme there is no proton source, so you go through the clean carbene rather than a cyclohexyl cation that could ring-contract or eliminate ambiguously.

A famous synthetic showcase is the generation of strained and bridged alkenes and the use of the carbene for intramolecular C-H insertion — the Bamford-Stevens carbene is a workhorse for building rings that are hard to reach any other way. In its Shapiro guise, the same tosylhydrazone chemistry appears in terpenoid and prostaglandin syntheses to install a trisubstituted alkene with defined geometry.

Limitations and side reactions

• Carbene misbehavior. The singlet carbene is promiscuous: it can insert into unintended C-H bonds, cyclopropanate a nearby alkene, or do a Wolff-type rearrangement if a carbonyl is adjacent, all competing with the desired 1,2-H shift.
• Carbocation rearrangement (protic route). Skeletal shifts scramble products; if you need a single clean regioisomer, avoid protic conditions or switch to Shapiro.
• Harsh thermal conditions. The classic thermal decomposition wants 100-160 °C, which is unfriendly to thermally sensitive functionality. Photochemical decomposition or the low-temperature Shapiro variant are the workarounds.
• Regiochemical ambiguity. Unsymmetrical ketones with α-hydrogens on both sides can give alkene mixtures unless one route (carbene vs cation) is deliberately chosen.
• Requires making the hydrazone. You pay an extra step (and consume TsNHNH₂) to install the tosylhydrazone before you can eliminate.

Variants and relatives

• Shapiro reaction (1967). The most important offshoot: two equivalents of an alkyllithium (n-BuLi, MeLi) at low temperature form a tosylhydrazone dianion that loses both Ts and N₂ to give a vinyllithium, which quenches to the less-substituted alkene or can be trapped with electrophiles (D₂O, CO₂, aldehydes). Clean regio- and stereochemistry, no carbocation, no rearrangement.
• Catalytic / transition-metal Bamford-Stevens. Rh(II) or Cu catalysts intercept the diazo intermediate before it loses N₂, channeling it into cyclopropanation, C-H insertion, or metal-carbene cross-coupling — turning the tosylhydrazone into a bench-stable, safe surrogate for a dangerous free diazo compound.
• Tosylhydrazones as diazo surrogates. A whole modern field (Barluenga, Wang, and others) uses base-decomposition of tosylhydrazones in situ as a safe way to make metal carbenes for reductive coupling, olefination, and N-H/O-H insertion — all descendants of the original Bamford-Stevens deprotonation-then-sulfinate-loss cascade.
• Photochemical Bamford-Stevens. UV irradiation decomposes the diazo at room temperature, favoring the singlet carbene and sparing thermally fragile substrates.

Discovery: Bamford and Stevens, 1952

The reaction is named for William Randall Bamford and Thomas Stevens Stevens, who reported the base-induced decomposition of tosylhydrazones to olefins in the Journal of the Chemical Society in 1952. T. S. Stevens (1900-2000) was a Scottish physical-organic chemist better known for the earlier Stevens rearrangement of ammonium ylides; the tosylhydrazone decomposition became one of his most cited contributions. In 1967 Robert H. Shapiro introduced the strong-alkyllithium variant that bears his name and gives the complementary anti-Zaitsev vinyllithium — the two names now bracket the classic and the modern faces of tosylhydrazone chemistry.