Is epoxide formation syn or anti?
The epoxidation reaction is believed to occur in a single step with a transition state incorporating all of the bonding events shown in the equation. Consequently, epoxidations by peracids always have syn-stereoselectivity, and seldom give structural rearrangement.
What does mCPBA do to alkenes?
mCPBA forms epoxides when added to alkenes. One of the key features of this reaction is that the stereochemistry is always retained. That is, a cis alkene will give the cis-epoxide, and a trans alkene will give a trans epoxide. This is a prime example of a stereoselective reaction.
What is a dihydroxylation reaction?
Dihydroxylation is the process by which an alkene is converted into a vicinal diol. Although there are many routes to accomplish this oxidation, the most common and direct processes use a high-oxidation-state transition metal (typically osmium or manganese).
Is Dihydroxylation a syn or anti?
Alkenes can be dihydroxylated by two different stereochemical pathways: anti-dihydroxylation or syn-dihydroxylation. The opening of epoxides follows the anti-dihydroxylation mechanism, while potassium permanganate or osmium tetroxide produce the syn-dihydroxylated products.
What is anti hydroxylation reaction?
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.
How do you form mCPBA?
mCPBA can be prepared by reacting m-Chlorobenzoyl chloride with a basic solution of hydrogen peroxide, followed by acidification.
What reagent is commonly used to form epoxides from alkenes?
The peroxyacid reagent forms an acid as by-product, while the epoxide is formed. The most common peroxyacid used for the epoxidation of alkenes (like propene) is meta-chloroperoxybenzoic acid, or mCPBA.
How do you synthesis mCPBA?
mCPBA can be prepared by reacting m-Chlorobenzoyl chloride with a basic solution of hydrogen peroxide, followed by acidification. It is sold commercially as a shelf-stable mixture that is less than 72% mCPBA, with the balance made up of m-chlorobenzoic acid (10%) and water.
How do you do anti-dihydroxylation of epoxide?
The epoxide ring can be opened under acid conditions and this is how the anti-dihydroxylation is achieved. The acid serves as catalyst by converting the oxygen into a better leaving group alcohol:
What is the mechanism of anti dihydroxylation?
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.
What is the mechanism of epoxide formation?
Let’s look at the mechanism to form our epoxide. So we start with our percarboxylic acid here, which looks a lot like a carboxylic acid except it has an extra oxygen. And the bond between these two oxygen atoms is weak, so this bond is going to break in the mechanism.
How to do anti-dihydroxylation of alkene?
In this post, we will focus on the anti-dihydroxylation which is achieved by converting the alkene into an epoxide first. This is usually done with peroxyacids such as MCPBA ( meta -chloroperoxy benzoic acid) or Peroxyacetic acid: