What Is Shapiro Reaction?
The Shapiro reaction is a highly valuable chemical transformation that converts ketones or aldehydes into olefins via their corresponding p-toluenesulfonylhydrazones. This reaction employs strong organolithium bases (e.g., methyllithium or n-butyllithium) to facilitate the decomposition of these hydrazones, leading to the formation of vinyl lithium intermediates. These intermediates can subsequently be quenched with protons or other electrophiles to produce substituted alkene.
- Reagents: Strong organolithium bases (e.g., n-BuLi, MeLi); p-toluenesulfonylhydrazide; electrophiles (e.g., H2O, alkyl halides, carbonyl compounds) for quenching.
- Reactants: Ketones or aldehydes.
- Products: Alkenes (typically terminal or less substituted); vinyllithium intermediates (before quenching).
- Reaction Type: Elimination-addition.
- Related Reactions: Bamford–Stevens reaction, Wittig reaction, Julia olefination.
Fig 1. Shapiro reaction and its mechanism. [1]
Mechanism of Shapiro Reaction
1. Hydrazone formation. Carbonyl + p-toluenesulfonyl hydrazide (TsNHNH2) → tosylhydrazone (R2C=NNHTs) — typically isolated or used crude.
2. Double deprotonation. A strong base (e.g., 2.0–3.0 equiv n-BuLi, or other lithium amide/aryllithiums) deprotonates the hydrazone: first at the N–H and then at the carbon adjacent to the C=N, giving an anionic species.
3. Collapse to diazo/diazonium-type intermediate. The anion expels the tosylate (or rearranges) to give a diazoalkane or closely related intermediate.
4. Nitrogen extrusion / formation of vinyllithium. Loss of N2 produces a carbanionic species — effectively a vinyllithium (R–C=CLi), i.e., a metalated alkene equivalent.
5. Quench or electrophile capture. Protonation yields the alkene product; alternatively the vinyllithium can be trapped with electrophiles (alkylation, silylation, borylation, carbonyl addition, etc.).
Note that this mechanism is quite distinct from that of the Bamford-Stevens reaction, which proceeds through diazo compound intermediates that decompose via carbene or carbocation pathways, particularly when performed in protic solvents or with other bases.
Shapiro Reaction vs. Bamford–Stevens Reaction
The Shapiro reaction is closely related to the Bamford-Stevens reaction, where aldehydes or ketones are converted into aryl sulfonylhydrazones and then decomposed with alkaline agents to generate alkenes. The key distinction is summarized as bellow:
Table: Comparison between Shapiro Reaction and Bamford-Stevens Reaction
| Feature / Criterion | Shapiro Reaction | Bamford–Stevens Reaction |
| Starting material | Tosyl (or sulfonyl) hydrazone from ketone/aldehyde | Tosyl (or sulfonyl) hydrazone from ketone/aldehyde |
| Key reagents | Strong organolithium (e.g., n-BuLi, 2–3 equiv) | Alkoxide (NaOEt, t-BuOK) or thermal/acidic conditions |
| Solvent & typical temp. | Dry THF, −78 → 0 °C (inert atmosphere) | Alcohols or ethers for alkoxide variant (reflux possible); nonpolar/heat for carbene pathway |
| Reactive intermediate | Vinyllithium (metalated alkene) | Diazoalkane → either carbanion (→ alkene) or free carbene (in aprotic/thermal conditions) |
| Typical product / outcome | Alkene or electrophile-trapped vinyl derivative (high versatility) | Alkene (common) or carbene-derived products (insertions, cyclopropanation) depending on conditions |
| Best use-case | When you want a vinyl-metal handle for trapping/functionalization | Simple deoxygenation to alkenes or intentional carbene chemistry |
| Functional-group tolerance | Lower — sensitive to acidic/protic groups and electrophiles (due to organolithium) | Often milder towards some groups, but diazo/carbene reactivity can cause side reactions |
| Stereochemical control | E/Z depends on vinyllithium geometry and quench — variable | E/Z often variable; solvent and mechanism (carbanion vs carbene) influence outcome |
Application Examples of Shapiro Reaction
- Example 1: Patrick Pfaff et al. synthesized a novel vetiver odor molecule by CeCl3·2LiCl-mediated alkylation of (3-methylbut-1-en-2-yl)lithium generated from hydrazone 23 via the Shapiro reaction, and obtained the target compound. [2]
- Example 2: Qian Dou et al. reported a simple synthetic route for ursodeoxycholic acid (UDCA), using the Shapiro reaction as the key step, to synthesize UDCA from hyodeoxycholic acid (HDCA) with an overall yield of 26%. [3]
Fig 2. Synthetic examples via Shapiro reaction.
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References
- Jie Jack Li. Name Reactions-A Collection of Detailed Mechanisms and Synthetic Applications, Sixth Edition, 2021, 486-488.
- Pfaff, Patrick, et al. European Journal of Organic Chemistry 2019.15 (2019): 2643-2652.
- Dou, Qian, et al. Synthesis 48.04 (2016): 588-594.
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