Question

What are the mechanisms for the below reactions?

Carbon-Carbon Bond Forming
1) Alkynide nucleophiles
2) Cyanide nucleophiles
3) Carbene cyclopropanations
3a) with diazomethane
3b) with alpha elimination
3c) Simmons-Smith reaction

Answer 1

01. Alkynide nucleophiles:

02. Cyanide nucleophile:

Cyanide ion: The negatively charged carbon atom of cyanide ion is usually the site of its nucleophilic character. Use of cyanide ion as a nucleophile permits the extension of a carbon chain by carbon-carbon bond formation. The product is an alkyl cyanide,or nitrile.

03. With diazomethane:

The preparation of methylene starts with the yellow gas diazomethane, CH2N2. Diazomethane can be exposed to light, heat or copper to facilitate the loss of nitrogen gas and the formation of the simplest carbene methylene. carbene such as methlyene will react with an alkene which will break the double bond and result with a cyclopropane. The reaction will usually leave stereochemistry of the double bond unchanged. As stated before, carbenes are generally formed along with the main reaction; hence the starting material is diazomethane.

04. With alpha elimination:

α elimination (eliminations in which both the proton and the leaving group are located on the same atom) follow a mechanism akin to an E1cB β-elimination. A strong base removes an acidic proton adjacent to an electron withdrawing group to give a carbanion. Loss of a leaving group from the carbanion creates a carbene. One of the best known elimination reactions occurs when chloroform is treated with base, forming a dichlorocarbene.

05. Simmons Smith reaction:

Carbenoids are substances that form cyclopropanes like carbenes but are not technically carbenes. One common example is the stereospecific Simmon-Smith reaction which utilizes the carbenoid ICH2ZnI. The carbenoid is formed in situ via the mixing of a Zn-Cu couple with CH2I2.