Allylation Flashcards
What is the general reaction for allylation?
aldehyde + allyl metal –> homoallylic alcohol
What type of nucleophiles are allyl metals?
Ambident i.e. can react through two different atoms. Choice of metal and reaction conditions will dictate which reaction pathway is adopted. Normally SE2’ desired.
Examples of type 1 allyl metal reagents
Allyl boranes; allyl boronates. Addition reaction is highly stereoselective so E/Z ratio of crotyl metal determines syn/anti ratio of products
Describe the crotylation reaction
Proceeds through a closed or cyclic (6-membered ring Zimmerman-Traxler) transition state)
Describe the outcomes of the crotylation reaction
E-crotyl boron reagents = anti product; Z-crotyl boron reagents = syn
What is the role of triethanolamine, N(CH2CH2OH)3?
Cleaves the 3 B-O bonds in addition product and forms stable boron complex N(CH2CH2O)3B where N lone pair can coordinate to B. Acts as Bronsted acid; acts as a Lewis base; cleaves boronate ester to liberate homoallylic alcohol; acts as ligand for B
Describe the trend in lewis acidity for boranes/boronates
Alkyl boranes more lewis acidic than alkyl boronates. Corresponding allyl boranes more reactive than allyl boronates
What is the result of the internal coordination of B to the carbonyl O?
Intramolecularises allyl reaction; increases electrophilicity of aldehyde; increases nucleophilicity of allylating reagent; ensures high degree of stereoselectivity
Why might simple protolytic work up be insufficient for releasing alcohol from B product?
Due to strength of B-O bond. By exploiting vacant p AO of neutral B species, an oxidative work up with peroxide provides a solution to this. Work up has no effect on stereochemistry which is set in through cyclic transition state
Name some reagents that would form an ate complex and stabilise BH3
Things that behave as Lewis bases. DMS; pyridine; THF; triphenylphosphine
