BH is a lewis acid Six electrons in outer shell Coordinates to oxygen electron pairs in ethers Electrophilic H H—B H—B-0: H H Borane THF BH -THF complex C Thomson- Brooks Cole
BH3 is a Lewis Acid • Six electrons in outer shell • Coordinates to oxygen electron pairs in ethers
Hydroboration-Oxidation Forms an alcohol from an alkene Addition of H-BH2 (from BH3-THF complex) to three alkenes gives a trialkylborane Oxidation with alkaline hydrogen peroxide(h2o2)in water produces the alcohol derived from the alkene THF t bha H2O2 OH..O B(oh3 Cyclohexene Cyclohexanol (87%) Tricyclohexylborane Thomson· Brooks cole
• Addition of H-BH2 (from BH3 -THF complex) to three alkenes gives a trialkylborane • Oxidation with alkaline hydrogen peroxide(H2O2 ) in water produces the alcohol derived from the alkene Hydroboration-Oxidation Forms an Alcohol from an Alkene
Orientation in Hydration via hydroboration Regiochemistry is opposite to Markovnikov orientation OH is added to carbon with most hs H and oh add with syn stereochemistry, to the same face of the alkene(opposite of anti addition) H B H OH -OH CH BH3 H H THF HO CH CHa 1-Methylcyclopentene trans-2-Methyleyclopentanol Alkylborane intermediate (85%) Thomson· Brooks cole
Orientation in Hydration via Hydroboration • Regiochemistry is opposite to Markovnikov orientation – OH is added to carbon with most H’s • H and OH add with syn stereochemistry, to the same face of the alkene (opposite of anti addition)
Hydroboration, Electronic Effects Give non -markovnikov More stable carbocation is also consistent with steric preferences CH cha H B-H H2B Partial 3 cation CH35H3((more stable transition state) I-Methyleyclopenteney CH3 H CH3 H H H BH2 Partia|2° cation NOT formed (less stable transition state
Hydroboration, Electronic Effects Give Non-Markovnikov More stable carbocation is also consistent with steric preferences
Hydroboration-Oxygen Insertion ster p H,O2, OH- inserts OH in place of B Retains svn orientation R H,O,, OH R H CH CH OH B R R
Hydroboration - Oxygen Insertion Step • H2O2 , OHinserts OH in place of B • Retains syn orientation