CHEM 125A: FRESHMAN ORGANIC CHEMISTRY II - Lecture 14 - Professor J. Michael McBride
Epoxide Opening, Dipolar Cycloaddition, and Ozonolysis
The formation of epoxides and the regiospecificity of their acid- and
base-catalyzed ring openings underlines the importance of thinking
carefully about how textbooks draw curved arrows and may sometimes read
too much into fundamentally inadequate experimental data. The ozonolysis
of alkenes begins with several 1,3-dipolar cycloadditions that can be
understood in terms of matching HOMOs with LUMOs of the corresponding
symmetry. The process continues with acetal hydrolysis and either
reduction or oxidation to obtain the desired product. Mechanisms of
these typical reactions are analyzed. Although addition to the C=O
double bond is usually considered nucleophilic, it can have an important
electrophilic component that makes it mechanistically analogous to the
"electrophilic" additions to C=C being discussed in these lectures. The
use of metals to access orbitals of the proper symmetry is introduced
through alkene dihydroxylation via cycloaddition of OsO4.
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