Lesson 6: Alkenes - OsO4, O3, Carbenes, and Synthesis

Lecture 6: Alkenes: OsO4, O3, Carbenes, and Synthesis

I. OsO4 (Osmium Tetroxide) Reaction:

1. Diol Formation:

   • Mechanism: Osmium tetroxide (OsO4) reacts with alkenes to form cyclic osmate esters initially.

   • Formation of Osmium(VIII) Intermediate: The pi electrons of the double bond attack osmium tetroxide, leading to the formation of a cyclic osmate ester intermediate.

   • Hydrolysis: The cyclic osmate ester is then hydrolyzed in aqueous workup to yield a vicinal diol (glycol).

2. Stereochemistry:

   • Syn Addition: OsO4 adds to the double bond in a syn manner, meaning that both atoms of osmium and oxygen add to the same side of the double bond.

II. Ozone (O3) Reaction:

1. Ozonolysis:

   • Mechanism: Ozone (O3) reacts with alkenes in a process called ozonolysis.

   • Formation of Ozonide: Ozone initially adds to the double bond to form an ozonide intermediate, which is unstable and quickly decomposes.

   • Cleavage: The ozonide undergoes cleavage to yield carbonyl compounds (aldehydes or ketones) and smaller oxygen-containing molecules (such as formaldehyde, formic acid, or carbon dioxide).

2. Types of Ozonolysis:

   • Reductive Ozonolysis: In the presence of reducing agents (like zinc or dimethyl sulfide), ozonides are reduced to aldehydes or ketones.

   • Oxidative Ozonolysis: In the absence of reducing agents, ozonides are cleaved to yield carboxylic acids.

III. Carbenes (Simmons-Smith Reaction):

1. Formation of Carbenes:

   • Mechanism: Carbenes, which are reactive intermediates with a divalent carbon, are formed using diazo compounds (like diazomethane) in the presence of transition metals (like copper).

   • Insertion: Carbenes insert into the carbon-hydrogen bonds of alkenes, resulting in the formation of cyclopropane derivatives.

IV. Synthesis Applications:

1. Functional Group Transformations:

   • Alcohol Synthesis: Diols from OsO4 reactions can be further oxidized to yield carbonyl compounds or reduced to yield alcohols.

   • Aldehyde/Ketone Synthesis: Carbonyl compounds formed from ozonolysis can undergo further reactions to yield various organic molecules.

   • Cyclopropane Synthesis: Carbenes can be used to introduce cyclopropane rings into organic structures.