Density
functional theory calculations have revealed the mechanism
and origin of regio- and stereoselectivity in [2,3]-sigmatropic rearrangements
of diazoesters with allylic iodides/sulfides via chiral bisoxazoline-Cu(I)
catalysts. Initially, the two catalytic systems share a similar process
involving the generation of Cu(I)-carbene and the ensuing nucleophilic
attack by allylic iodide/sulfide. Then, the rearrangements bifurcate
at the generated metal-bound ylide species. For the iodonium ylide
system, it prefers to undergo a Cu(I)-assisted five-membered envelope
transition state to give the [2,3]-rearrangement product. However,
for the sulfonium ylide system, it favors to form a free ylide that
further allows a five-membered electrophilic transition state to offer
the [2,3]-rearrangement product. The metal-bound ylide mechanism is
disfavored for this [2,3]-rearrangement of sulfur ylide due to the
severe substrate–ligand steric repulsions during the isomerization.
Meanwhile, the free sulfonium ylide can be regarded as a sulfonium
ylene with a CS bond owing to the strong electronegativity
of sulfur and is stable, which promotes this pathway. In contrast,
the free iodonium ylide is more like a zwitterion with a carbanion
and an iodine cation due to the low electronegativity of iodine and
is unstable, which requires the copper(I) center to stabilize the
rearrangement. The regioselectivity is derived from the electronic
effect of phenyl on the charge distribution over the allyl moiety.
The stereoselectivity is mainly controlled by substrate–ligand
steric interactions, wherein the favored pathway tolerates less steric
hindrance between the substitutes of carbene and allyl moieties and
the bulky groups on bisoxazoline ligand.