Cycloadditions and DA reactions Flashcards
What are the key features of a pericyclic reaction
Cyclic transitions states and all bond making and breaking takes place in concert - without the formation of an intermediate
What are the key features of a cycloaddition reaction
Two pi systems coming together to form sigma bonds at the end of each component, a ring is formed, the reaction is bimolecular
What characterises a DA reaction
Its a [4+2] reaction, between a diene and dienophile. Product is an unsaturated cyclohexene
Substituent effects on the dienophile on DA reactions
EWG on the dienophile make the reaction faster
Substituent effects on the diene on DA reactions
EDG on the diene speed up the rate of reaction
Diene requirements for a DA reaction
must be s-cis
Definition of regioselectivity
Preference for chemical bonding in one orientation over another. The choice of direction produces different regioisomers.
How is regioselectivity controlled?
matching up ‘charges’. Charges are determined by EWG and EDG
What is Inverse Electron Demand DA
A DA reaction in which the typical charge distribution is reversed and the major regioisomer is the one in which the ‘charges’ clash
How are substituents ‘pushed’ on the diene?
Those on the ‘inside’ of the s-cis structure are pushed away from the line of attack by the dienophile
How are substituents ‘pushed’ on the dienophile
Those underneath or above the diene are pushed away from the diene
What are the exo and endo approaches?
The extended and compressed transition structures. The endo approach utilises secondary orbital interactions where possible with atoms 2 and 3 of the diene
Which carbons does the dienophile interact with on the diene
1 and 4
What are intramolecular DA reactions governed by?
Steric consideration and secondary orbital interactions
Why would some intramolecular DA reactions not show selectivity?
Short carbon tethers result in limited freedom of movement which means often mixtures of products form as supposed to there being a preference for a single isomer
Types of approach in intramolecular DA
Cross over and eclipsed (generally speaking)
Under what conditions does retro-DA occur
Forcing conditions. The reaction is in equilibrium with the forwards DA favoured. Aromatisation or the formation of gaseous products can drive equilibrium in favour of the reverse mechanism
