The Pd-catalyzed
asymmetric [4+2] cycloaddition reaction of an aliphatic 1,4-dipole
with singly activated electron-deficient alkenes is realized for the
first time, enabled by using a newly developed benzylic substituted
P,N-ligand, affording tetrahydropyrans having three continuous chiral
centers in high yields with high diastereo- and enantioselectivities.
The rational transition states of the reaction as well as the role
of the benzylic chiral center are proposed.
The activated alkynes
have been used successfully for the first time as the dipolarophile
in the palladium-catalyzed asymmetric (3 + 2) cycloaddition, affording
highly functionalized cyclopentenes in good to high yields with high
chemoselectivities and good to high enantioselectivities. The introduction
of an additional carbonyl group at the α-position of the alkynyl
esters is the key to activating the carbon–carbon triple bond.
The reaction process was investigated, and an inverse process of Pd-catalyzed
(3 + 2) cycloaddition was observed.
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