The Corey-Winter olefin synthesis is a series of chemical reactions that convert 1,2-diols into alkenes. The process begins with the reaction of a diol with thiophosgene to form a cyclic thiocarbonate. This intermediate then undergoes deoxygenation in the presence of trimethyl phosphite, yielding the corresponding alkene. The reaction is named after the chemists Elias James Corey and Roland Arthur Edwin Winter, who first reported it.

The Corey-Winter olefin synthesis is a stereoselective method for converting vicinal diols into alkenes. Its key advantages include the mild formation of C=C bonds (avoiding rearrangement side reactions typical of acid-catalyzed dehydrations) and control over alkene geometry (cis-diols yield Z-alkenes, while trans-diols produce E-alkenes). This reaction is particularly valuable in the synthesis of natural products and pharmaceutical intermediates.

Reagents: Thiophosgene, 1,1'-thiocarbonyldiimidazole (TCDI), trimethyl phosphite, triphenylphosphine
Reactants: 1,2-diols
Products: Alkenes
Reaction type: Elimination

Experimental Tips:

1. The reaction is highly stereospecific and typically proceeds in good yields.

2. It is particularly useful for synthesizing highly substituted and sterically hindered alkenes.

3. While thiophosgene allows the reaction to proceed at lower temperatures (beneficial for poly-substituted diols), it is highly toxic (similar to phosgene). 1,1'-Thiocarbonyldiimidazole (TCDI) serves as a safer alternative.

Reaction Mechanism

The mechanism likely proceeds via two possible pathways:

1. First Pathway (Carbene Intermediate)

   • The diol reacts with thiophosgene to form a cyclic thiocarbonate.

   • Trimethyl phosphite attacks the sulfur atom in the ring, generating S=P(OMe)₃ (driven by the formation of a strong P=S double bond).

   • This step releases a carbene intermediate, which subsequently fragments to release CO₂, yielding the final alkene product.

2. Alternative Pathway (No Free Carbene)

   • A second molecule of trimethyl phosphite directly attacks the carbanion center, leading to S–C bond cleavage.

   • The resulting phosphorus-stabilized carbanion undergoes elimination to form the alkene and an acyl phosphite ester, which then decarboxylates to complete the transformation.

This reaction is widely used in organic synthesis due to its mild conditions and excellent stereocontrol, making it a powerful tool for constructing complex alkenes.

Corey-Winter Olefin Synthesis Reaction Mechanism

Original literature

• E. J. Corey and Roland A. E. Winter. J. Am. Chem. Soc. 1963, 85(17), 2677-2678.