Chapter 7.8: Reduction of Alkynes

Na°/NH3

 Stops a catalytic reduction after the addition of one mole of hydrogen

o Internal alkynes react with borane to give a trialkenylborane o Treating a trialkenylborane with a carboxylic acid results in stereoselective replacement of boron by hydrogen: a cis-alkenyl group bonded to a boron is converted to a cis-alkene

finely powdered palladium metal deposited on solid calcium carbonate that has been specially modified with lead salts. Its particular use is as a catalyst for the reduction of an alkyne to a cis alkene

o Treatment of an alkyne with H2 in the presence of a transition metal catalyst, most commonly palladium, platinum, or nickel, results in the addition of two moles of H2 to the alkyne and its conversion to an alkane

Lindlar catalyst

o Stereochemistry: anti  trans or E product

 Step 1: A one-electron reduction of the alkyne gives an alkenyl radical anion (an anion containing an unpaired electron on one carbon and a negative charge on an adjacent carbon)  Step 2: Add a proton  Step 3: A one-electron reduction of the alkenyl radical gives and alkenyl anion  Step 4: Add a proton