8.7 Epoxidation, Anti Dihydroxylation, and Syn Dihyroxylation

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    Epoxidation

     

    EPOXIDATION
    Less Substituted
    More Substituted
    Two Groups Added
    O
    O
    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 dihyroxylation mechanism step 1

    Anti Dihydroxylation

     

    ANTI DIHYDROXYLATION
    Less Substituted
    More Substituted
    Two Groups Added
    OH
    OH
    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 dihyroxylation

    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

    anti dihyroxylation mechanism step 1

    Step 2 - Acid-Catalyzed Ring Opening of an Epoxide

    anti dihydroxylation step 2

    Syn Dihydroxylation

     

    SYN DIHYDROXYLATION
    Less Substituted
    More Substituted
    Two Groups Added
    OH
    OH
    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.