A new transition state force field has been developed for the AD reaction, purely from quantum
mechanical reference data. A new methodology was used for converting quantum mechanical normal modes
into a form suitable for parametrization. The force field has been thoroughly validated by comparison to structural
and energetic data, and by prediction of experimental enantioselectivities. Excellent agreement was observed,
frequently within a few percent of the experimental enantioselectivity. The interactions responsible for
enantioselectivity have been identified and compared to the Sharpless and Corey models.
Density functional calculations
(Becke3LYP/6-31G(d)) on the (3 + 2) transition structures of
tetroxide mediated dihydroxylations of chiral allylic ethers show that
the stereoselectivity is controlled by the “inside
alkoxy effect” (Stork/Houk-Jäger model). In the special
case of Z-disubstituted alkenes, 1,3-allylic strain
model) controls the stereoselectivity.
The (2+2) cycloadditions of SO3 to ethene, propene,
and 2-methylpropene were investigated with
Hartree−Fock, MP2, QCISD(T), and hybrid Hartree−Fock/density
functional theory (HF-DFT) methods.
Experimental data support a (2+2) mechanism for these reactions.
The thermally allowed (3+2) cycloadditions
of SO3 to ethene and propene were also examined. With
the exception of MP2 calculations, all levels of
theory predict that SO3 reacts by a concerted (2+2)
pathway. The transition structure has considerable
zwitterionic character, and there is a strong preference for
Markovnikov addition. The rearrangements of
sulfites to sultones are disfavored. The origin of the large
preference for the normally forbidden (2+2) process
is attributed to frontier orbital interactions, and is contrasted to
the (3+2) cycloaddition mechanism favored
Hexachlorocyclopentadiene undergoes Diels−Alder reactions with
dienophiles possessing allylic
chiral centers with moderate to excellent anti selectivity.
Ab initio calculations on model systems
elucidate the origin of this stereoselectivity which follow the
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