What Is Peterson Olefination?
The Peterson olefination reaction is a reaction in which aldehydes, ketones, and α-silicon-based organometallic compounds generate alkenes. α-Silicon-based organometallic compounds can be α-silicon-based lithium reagents, α-silicon-based magnesium reagents, α-silicon-based sodium reagents, α-silicon-based potassium reagents, etc. The Peterson olefination reaction can be considered a Wittig reaction of silicon reagents.
α-Silicon-based organometallic compounds react with carbonyl compounds to generate the intermediate β-hydroxysilicide, which, under acid or base conditions, yields the corresponding alkene. In the reaction, the intermediate β-hydroxysilicide can generally be isolated; however, when the α-silicon-based organometallic compound is an α-silicon-based sodium or α-silicon-based potassium reagent, the intermediate will rapidly undergo a subsequent elimination reaction to generate an alkene.
β-hydroxysilicides can generate cis (Z) or trans (E) alkenes under different reaction conditions. Under basic conditions, β-hydroxysilicides undergo syn elimination. Under acidic conditions, β-hydroxysilicides undergo anti (anti) elimination.
- Reagents: α-Silyl carbanions (e.g., (trimethylsilyl)methyl lithium, Me3SiCH2Li).
- Reactants: Carbonyl compounds, primarily aldehydes and ketones.
- Products: Alkenes and a silanolate salt (e.g., Me3SiOLi).
- Reaction Type: Carbonyl olefination.
- Related Reactions: Wittig reaction, Horner–Wadsworth–Emmons (HWE), Tebbe/Methylenation, Julia and Julia–Kocienski olefinations.
Fig 1. Peterson olefination reaction and its mechanism. [1]
Mechanism of Peterson Olefination
Step 1. Carbanion generation / nucleophile preparation. Typical approaches include deprotonation of silylmethyl precursors (e.g., R3SiCH2X or silylmethane derivatives) with strong base (n-BuLi, LDA) or use of silyl-stabilized organometallics (silyl Grignards, silyl lithium reagents). Stabilizing groups on the α-carbon can change whether elimination occurs in situ.
Step 2. Nucleophilic addition to the carbonyl. The α-silyl carbanion adds to an aldehyde/ketone to give a β-hydroxysilane (isolable in many cases). Stereochemistry (erythro/ threo) of this adduct depends on the addition transition state and can often be influenced by temperature and solvent.
Step 3. Elimination to the alkene. Two mechanistic pathways are commonly invoked:
- Base-promoted (syn) elimination. A concerted syn elimination pathway through a pentacoordinate silicon transition state or silicate intermediate is usually proposed; this pathway is stereospecific (syn) and is promoted by strong alkoxides or other bases.
- Acid-promoted (anti) elimination. Protonation of the β-hydroxysilane (or conversion to a better leaving group) is followed by an anti-elimination that proceeds via an oxonium-like intermediate or a stepwise process; this pathway is anti-stereospecific.
Because elimination stereochemistry is condition-dependent (syn vs anti), the same β-hydroxysilane diastereomer can be converted to different alkene stereoisomers by choosing base vs acid or vice versa — a powerful selectivity handle.
Application Examples of Peterson Olefination
- Example 1: Alexander F. Tiniakos et al. developed a novel Peterson olefination reaction for the preparation of allyl dimethylsilane precursors, which are versatile synthetic intermediates. This method was applied to the synthesis of the C16–C30 fragment of dolabelide C. [2]
- Example 2: Lushun Wang et al. efficiently synthesized appropriately functionalized advanced C(6–28) fragments of the marine macrocyclic lactone compound iriomoteolide-2a. The C(19)–C(28) fragments were constructed using the Paterson aldol reaction and the Peterson olefination reaction. [3]
Fig 2. Synthetic examples via Peterson olefination reaction.
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References
- Jie Jack Li. Name Reactions-A Collection of Detailed Mechanisms and Synthetic Applications, Sixth Edition, 2021, 440-442.
- Tiniakos, Alexander F., et al. Organic Letters 21.3 (2018): 589-592.
- Wang, Lushun, et al. Organic Letters 20.8 (2018): 2213-2215.
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