This discussion is based on the paper by Y.D. Wu and D. Lai, J.Amer.Chem.Soc. 117 11327 (1995), a DFT study on the selectivity of this reaction. The structure below is not the outcome of their calculations; it is a hand-made reconstruction of the complex they suggest as a transition state that explains the experimental findings.
The reagents in this reaction are: the catalysts titanium
tetraisopropoxide, and enantiomerically pure diethyl tartarate (DET,
either RR or SS), tert-butyl
hydroperoxide (TBHP) as the oxygen source, and of
course the allylic
alcohol to be epoxidated. |
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Where are the components to be found? |
Now a possible explanation of the enantioselectivity of this
reaction:
The green coloured tartarate has the two ester groups in a kind
of axial position: one takes part in the complexation of a
titanium atom, and it is the other one that interacts with the
tert.butyl group of the peroxide. (With methylhydroperoxide the
reaction is not selective).
It pushes the tert.butyl in the direction of the blue
tartarate.
This in turn has its effect on the position of the allylic
system: the calculations show that its O-C bond
prefers to be gauche with respect to (in between) the peroxide oxygen and the Ti-O-Ti bridging oxygen.
Because of this one face of the double bond forms a spiro-type of transition state with the
oxygen to be transferred. This yields an enantiomeric epoxide:
All color functions are toggle buttons, and you can reset all atom colors and positions here.
With the other enantiomer of DET (imagine the ester groups and hydrogens interchanged) one should imagine that now the blue one participates in Ti-complexation. The positions of the t-butylperoxide and allyl system are mirrored, resulting in the opposite epoxide enantiomer.
Hens Borkent, CMBI, University of Nijmegen, The Netherlands