Development of Transition Metal-Catalyzed Migratory Transformations
Kazem Shiroodi, Roohollah
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The transition metal-catalyzed 1,3-transposition reactions of different functional groups are powerful tools in organic synthesis. The first efficient and regioselective 1,3-transposition reaction of ynones (alkynyl ketones) and diynones have been developed. It was found that by employing both interrupted- and extended conjugation factors, the equilibrium between regioisomeric ynones can be selectively shifted toward the more thermodynamically stable isomer. Thus, ynones and diynones in the presence of a π-philic gold catalyst undergo a regioselective and efficient 1,3-transposition reaction to give the rearranged ynones as sole products. Transition metal-catalyzed migratory cycloisomerization reaction offers mild and selective assembly of functionalized heterocycles with diverse substitution patterns. Two complementary regioselective copper- and gold-catalyzed cycloisomerization reactions of boron-containing alkynyl epoxides toward C2- and C3-borylated furans have been developed. It was found that employment of a copper catalyst produces the C3-borylated furan via an initial 1,2-boryl migration. However, the more efficient gold-catalyzed protocol allows regiodivergent synthesis of borylated furans. Thus, a phosphite gold complex with more basic counterion strongly favors a 1,2-boryl migration during the cycloisomerization process to produce C3-borylated furans. In contrast, employment of NHC gold complex with a non-coordinating counterion affords C2-borylated furans, exclusively. One of the remarkable examples of atom- and step-economical transformation for the assembly of complex structures is the transition metal-catalyzed rearrangement of easily accessible propargylic esters and phosphates. A stereocontrolled isomerization of α-halosubstituted propargylic phosphates into valuable highly functionalized 1,3-dienes has been developed. This methodology features a double 1,3-phosphatyloxy/1,3-halogen migration relay. Stereodivergency of this method is accomplished by the choice of catalyst to selectively synthesize either (Z)- or (E)-1,3-dienes. Thus, the (Z)-dienes could be obtained exclusively in the presence of a copper catalyst, whereas the use of a gold catalyst affords the stereoisomeric (E)-dienes predominantly.
furan, alkynyl ketone