Organic Chemistry Final Exam Review Notes

Terms in this set (404)

• Hydroboration-oxidation is a two-step reaction sequence that converts an alkene into an alcohol.
• BH3 is a reactive gas that exists mostly as a dimer, diborane (B2H6).
• Borane is a strong Lewis acid that reacts readily with Lewis bases.
• For ease of handling in the laboratory, it is commonly used as a complex with tetrahydrofuran (THF).
• Because the alkylborane formed by the reaction with one equivalent of alkene still has two B-H bonds, it can react with two more equivalents of alkene to form a trialkylborane.
• We often draw hydroboration as if addition stopped after one equivalent of alkene reacts with BH3.
• Instead all three B-H bonds actually react with three equivalents of an alkene to form a trialkylborane
• Since only one B-H bond is needed for hydroboration, commercially available dialkylboranes having the general structure R2BH are sometimes used instead of BH3.
• A common example is 9-borabicyclo[3.3.1]nonane (9-BBN).
With unsymmetrical alkenes, the boron atom bonds to the less substituted carbon atom.
• This regioselectivity can be explained by considering steric factors.
• The larger boron atom bonds to the less sterically hindered, more accessible carbon atom.
• Since alkylboranes react rapidly with water and spontaneously burn when exposed to air, they are oxidized, without isolation, with basic hydrogen peroxide (H2O2, ¯OH).
• Oxidation replaces the C-B bond with a C-O bond, forming a new OH group with retention of configuration.
• The overall result of this two-step sequence is syn addition of the elements of H and OH to a double bond in an "anti-Markovnikov" fashion.