Stereocontrolled Synthesis Flashcards
Give an overview of organo-boron reagents.
Give an overview of organo-silicon reagents.
Give an overview of organo-sulfur reagents.
- Sulfur can exist in a number of oxidation states.
What is the Julia reaction?
Draw a reaction scheme for the formation of the product.
The formation of an E-alkene from a sulfone.
The E-alkene is always formed in a Julia reaction. Why?
An anion intermediate is formed which lives long enough for rotation about the C-C bond. Elimination takes place from the least sterically hindered anti-periplanar alignment to give the E-alkene.
What is the Kocienski modification and why was it developed?
Conditions of the Julia reaction are harsh therefore this method was developed, using a tetrazole functionalised sulfone.
What is the Peterson reaction?
Name two ways that the starting material can be made.
Formation of E or Z alkenes using a beta-hydroxy silane.
A beta-hydroxyl silane can be formed from:
- Vinyl silanes
- Metal-halogen exchange
Show how beta-hydroxyl silanes can be formed from vinyl silanes.
Show how beta-hydroxyl silanes can be formed from metal-halogen exchange.
Beta-hydroxy silanes can be converted into alkenes by acid or base in the Peterson reaction.
Draw the mechanism for the acid-promoted reaction.
Beta-hydroxy silanes can be converted into alkenes by acid or base in the Peterson reaction.
Draw the mechanism for the base-promoted reaction.
Draw the mechanism for the conversion of an E-vinyl silane / Z-vinyl silane to an alkene via electrophilic substitution.
Substitution occurs with retention of double bond geometry.
Draw the mechanism for electrophilic additions to allyl silanes.
This forms a beta-carbocation which undergoes loss of Me3Si+.
The electrophile attacks ANTI to silicon.
Draw the reaction between an allyl silane and an aldehyde via electrophilic addition.
There is very little stereocontrol at the new hydroxyl stereocentre.
What can be used in place of an allyl silane when adding to an aldehyde to allow for stereocontrol?
Allyl boranes.
As the lone pairs on the carbonyl oxygen can donate into the empty p-orbital on boron, a closed cyclic transition is possible and this leads to excellent levels of stereocontrol.