Corey-House Synthesis: Preparation of Unsymmetrical Alkanes
The Corey-House Synthesis is a versatile organometallic coupling reaction used to synthesize alkanes. Unlike the Wurtz reaction, which is limited to symmetrical alkanes, the Corey-House synthesis is excellent for preparing unsymmetrical alkanes with good yields.
1. The General Concept
The reaction involves the coupling of an alkyl halide ($R'-X$) with a Lithium Dialkylcuprate (known as the Gilman Reagent, $R_2CuLi$).
Where:
- $R_2CuLi$: Gilman Reagent (acts as the source of nucleophile $R^-$).
- $R'-X$: Substrate Alkyl Halide (must be primary or methyl).
- $R-R'$: Desired Alkane Product.
2. Preparation of Gilman Reagent
The reagent is prepared in situ in two steps:
Step 1: Formation of Alkyllithium
An alkyl halide ($R-X$) reacts with Lithium metal in dry ether.
Step 2: Formation of Lithium Dialkylcuprate
The alkyllithium reacts with Cuprous Iodide ($CuI$).
3. Mechanism of Coupling
The Gilman reagent reacts with the second alkyl halide ($R'-X$). The reaction follows a pathway similar to the $S_N2$ mechanism, where the alkyl group from the cuprate attacks the alkyl halide.
Key Constraint
For the reaction to proceed efficiently, the substrate alkyl halide ($R'-X$) must be Methyl or Primary ($1^\circ$). If a secondary or tertiary alkyl halide is used as the substrate, Elimination dominates, producing alkenes instead of alkanes.
Example: Synthesis of Propane
Reaction between Lithium Dimethylcuprate and Ethyl Chloride.
$$ (CH_3)_2CuLi + CH_3CH_2Cl \rightarrow \underbrace{CH_3CH_2CH_3}_{\text{Propane}} + CH_3Cu + LiCl $$4. Advantages over Wurtz Reaction
| Feature | Wurtz Reaction | Corey-House Synthesis |
|---|---|---|
| Product Type | Only Symmetrical Alkanes (Even C) | Symmetrical & Unsymmetrical (Odd C) |
| Mixed Reactants | Gives mixture of 3 products (Poor yield) | Gives specific product (High yield) |
| Versatility | Limited to Alkyl halides | Reagent ($R$) can be Aryl, Vinyl, or Alkyl |
5. Important Notes
- The alkyl group ($R$) inside the Gilman reagent ($R_2CuLi$) can be $1^\circ$, $2^\circ$, $3^\circ$, aryl, or vinyl.
- The alkyl group ($R'$) of the substrate ($R'-X$) must be methyl or $1^\circ$.
- This method is ideal for synthesizing alkanes with odd numbers of carbon atoms (e.g., Propane, Pentane).
Corey-House Quiz
Test your concepts on Gilman reagents and alkane synthesis. 10 MCQs with explanations.
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