Carbene generation — aprotic alkene synthesis
Bamford-Stevens reaction
How the Bamford-Stevens reaction 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: Tosylhydrazone, strong base (NaOMe, NaH, or NaOEt). Conditions: Aprotic solvent (diglyme, toluene), 100–200 °C. Bonds formed: C=C; N₂; TsNa; bonds broken: N–H; N–S; C=N; C–H (migrating). Stereochemistry: Mixture of E/Z; thermodynamic (more substituted) alkene favoured. Rate and selectivity: Diazo decomposition to carbene is rate-limiting.
At a glance
Step 1: Deprotonate; lose Ts⁻, then N₂
Base deprotonates a tosylhydrazone's remaining N–H, and in aprotic, non-nucleophilic solvent, loss of sulfinate and N₂ generates a free carbene rather than the vinyl anion seen in the aqueous Shapiro reaction.
Step 2: 1,2-Hydride shift (or C–H insertion)
This carbene can either insert directly into a nearby C–H bond, or — more commonly — undergo a rapid 1,2-hydride shift, collapsing to the thermodynamically more stable, more substituted alkene.
Step 3: Opposite regiochemistry to Shapiro
This gives the opposite regiochemical outcome from the Shapiro reaction on the identical starting hydrazone — protic vs. aprotic conditions alone flip the mechanism from carbanion-based to carbene-based, and with it, which alkene isomer dominates.
Abbreviations and terms
- Ts
- Tosyl. p-Toluenesulfonyl (p-CH₃C₆H₄SO₂–).
- n-BuLi
- n-Butyllithium. Very strong base/nucleophile (pKa of butane ≈ 50); deprotonates alkynes and does lithium–halogen exchange.
- NaOEt
- Sodium ethoxide. Alkoxide base matched to ethyl esters (avoids transesterification).
- NaOMe
- Sodium methoxide. Alkoxide base matched to methyl esters.
- NaH
- Sodium hydride. Insoluble, irreversible base: removes H⁺ and bubbles off H₂.
- TS
- Transition state. The highest-energy point on the pathway between two minima; it cannot be isolated.
- Δ
- Heat. Reaction is run at elevated temperature.
How it compares with related reactions
| Bamford-Stevens reaction | Horner-Wadsworth-Emmons (HWE) reaction | Julia-Kocienski olefination | |
|---|---|---|---|
| Reagents | Tosylhydrazone, strong base (NaOMe, NaH, or NaOEt) | Phosphonate ester (EtO)₂P(O)CH₂CO₂Et, NaH or NaOMe, aldehyde | 1-phenyl-1H-tetrazol-5-yl (PT) sulfone, KHMDS (or NaHMDS, LiHMDS), aldehyde |
| Conditions | Aprotic solvent (diglyme, toluene), 100–200 °C | THF or DME, 0–25 °C | THF or DME, –78 °C → 25 °C |
| Stereochemistry | Mixture of E/Z; thermodynamic (more substituted) alkene favoured | Strong E-selectivity (trans) | Highly E-selective (anti-adduct, then anti elimination) |
| Rate / selectivity | Diazo decomposition to carbene is rate-limiting | Aldehyde addition rate-limiting; water-soluble phosphate easily removed | One-pot; Smiles shift and elimination are fast after addition |
Test yourself
Try to answer each question before opening it.
Which reagents and conditions are typical for the Bamford-Stevens reaction?
Tosylhydrazone, strong base (NaOMe, NaH, or NaOEt). Conditions: Aprotic solvent (diglyme, toluene), 100–200 °C.
Which bonds are formed and which are broken?
Formed: C=C; N₂; TsNa. Broken: N–H; N–S; C=N; C–H (migrating).
What is the stereochemical outcome?
Mixture of E/Z; thermodynamic (more substituted) alkene favoured
What controls the rate or the selectivity?
Diazo decomposition to carbene is rate-limiting
Study guides that help with this reaction
- SN1, SN2, E1 or E2? How to decide which mechanism wins
- Nucleophiles, electrophiles and leaving groups: what makes a reaction go