Chapter 7.7: Hydration of Alkynes to Aldehydes and Ketones

keto-enol tauteomerism

a type of isomerism involving keto (from ketone) and enol tautomers

• Reaction of this sterically hindered dialkylborane with a terminal alkyne results in a single hydroboration and formation of an alkenylborane o Treatment of an alkenylborane with hydrogen peroxide in aqueous sodium hydroxide gives a product that corresponds to hydration of an alkyne; that is, it corresponds to addition of H to one carbon of the triple bond and OH to the other

constitutional isomers in equilibrium with each other that differ in the location of a hydrogen atom and a double bond relative to a heteroatom

o The “antler” groups provide steric discrimination so the H atom adds to the more substituted C atom

syn additon

o 1. (sia)2BH o 2. H2O2/HO-

 Step 1: Make a new bond between a nucleophile (pi bond) and an electrophile  Step 2: Make a new bond between a nucleophile and an electrophile  Step 3: Take a proton away  Step 4: Keto-enol tautomerism  Step 5: Add a proton  Step 6 and 7: Break a bond to give stable molecules or ions followed by keto-enol tautomerism

HgSo4/H2SO4

a compound containing a hydroxyl group bonded to a doubly bonded carbon atom

Non-Markovnikov; boron adds to the less substituted carbon

enol

o Borane adds to an internal alkyne producing a trialkenylborane o The alkenyl group reacts further with borane to undergo a second hydroboration o Treatment of an alkenylborane with hydrogen peroxide in aqueous sodium hydroxide gives a product that corresponds to hydration of an alkyne; that is, it corresponds to addition of H to one carbon of the triple bond and OH to the other

tautomers