8.7 Epoxidation, Anti Dihydroxylation, and Syn Dihyroxylation
PRE-HEALTH PREP
Epoxidation
| EPOXIDATION | ||
|---|---|---|
| | | |
| Two Groups Added | | |
| Reagents Added | RCO3H (usually MCPBA --a peroxyacid) | |
| Regioselectivity | N/A | |
| Stereospecificity | Syn | |
| Other Characteristics | ||
Epoxidation Mechanism
Epoxidation of an alkene is carried out using a peroxyacid (a.k.a. peracid--MCPBA is the most common peroxyacid and is often specifically used). The mechanism consists of a single step and forms an epoxide as well as a carboxylic acid.
Anti Dihydroxylation
| ANTI DIHYDROXYLATION | ||
|---|---|---|
| | | |
| Two Groups Added | | |
| Reagents Added | 1. RCO3H (usually MCPBA --a peroxyacid) 2. H3O+ | |
| Regioselectivity | N/A | |
| Stereospecificity | Anti | |
| Other Characteristics | Intermediate is a 3-membered ring (an epoxide). | |
Anti dihydroxylation results in the anti-addition of two hydroxyl groups across an alkene. If this occurs on a cyclic diene as in cyclohexene below then anti-addition will result in a trans relationship between the two hydroxyl groups.
Anti Dihydroxylation Mechanism
The first step in anti dihydroxylation is epoxidation of the alkene using a peroxyacid (a.k.a. peracid--MCPBA is the most common peroxyacid and is often specifically used). The second step is acid-catalyzed ring opening of the epoxide intermediate with water as the nucleophile. The epoxide is first protonated followed by back-side attack by water. It is this back-side attack that is the ultimate reason that this reaction results in anti addition.
Step 1 - Epoxidation
Step 2 - Acid-Catalyzed Ring Opening of an Epoxide
Syn Dihydroxylation
| SYN DIHYDROXYLATION | ||
|---|---|---|
| | | |
| Two Groups Added | | |
| Reagents Added | 1. OsO4 2. NaHSO3 or Na2SO3 or 1. KMnO4 (cold, dilute), NaOH | |
| Regioselectivity | N/A | |
| Stereospecificity | Syn | |
| Other Characteristics | Intermediate is a cyclic osmate or manganate ester. | |