last updated Tuesday, January 19, 2016

1. a. The ligand provides the asymmetric induction into the epoxidation reaction. The cyclohexane part in the catalyst introduces the asymmetric induction into the system because the bridge is asymmetric (C2-asymmetry) when the (R, R) or the (S, S)-enantiomer of the diamine is used. The meso form of 1,2-diaminocyclohexane and 1,2-diaminobenzene (=o-phenylenediamine) are achiral, which would lead to the formation of a racemic mixture. In addition, the ligand helps to stabilize a broad variety of oxidation states on the manganese atom. Neither Mn(III) nor Mn(V) compounds are particularly stable without a ligand. By using different groups on the benzene ring, the reactivity of the catalyst and the stereoselectivity can be tuned as well.

b. In most cases, the different oxidation states are much further apart for main group elements (Mg²⁺/Mg: E⁰= -2.37 V, Mn²⁺/Mn: E⁰= -1.18 V). Transition metals display a larger number of oxidation states, which are more readily accessible. Thus, it is significantly easier for transition metal to go through a redox cycle.

c. The active specie has a "Mn=O" function in which the Mn-atom has a formal oxidation state of +V (O=MnO₂N₂⁺). The structure of this specie is shown in the reader on page 39. The Mn-atom is located in the center of a distorted square pyramid. The basal plane is formed by the two nitrogen and the two oxygen atoms, while the oxo ligand forms the apex. The intermediate is formed by the reaction of hypochlorite with the catalyst.

2.a. The given amount of indene is equivalent to 3.50 mmol (=0.567 g/162.19 g/mol). He should use 0.157 mmol of the catalyst (=3.50 mmol * 0.045), which are 0.100 g (0.157 mmol*635.2 g/mol).

b. Most procedures in the literature employ dichloromethane or acetonitrile as a solvent for the epoxidation reaction because they are polar enough to dissolve the catalyst and the intermediate. Hexane is not suitable because the catalyst is almost insoluble in this solvent.

c. The reaction is a phase transfer reaction (biphasic). Thus, proper mass transfer between the layers is very critical, which is improved by vigorous stirring.

d.

The stationary phase is silica and the mobile phase is hexane. Lane 1 shows the alkene (Rf= ~0.3) while lane 2 shows a sample of the reaction mixture. Since the reaction is incomplete, the lane displays two spots (epoxide: Rf= ~0.1 (blue spot), alkene: Rf= ~0.3 (pink)). The Rf-values are generally very low because the alkene and the epoxide are polar due to the presence of several oxygen atoms (epoxide, ester).

e. The optimization is necessary because the crude epoxide is purified by flash chromatography. The epoxide should display an R_f=0.3-0.4 and the alkene an R_f=0.7-0.8.

f. Epoxides are very reactive. Even during the reaction, some of the epoxide already hydrolyzes. Once the epoxidation is completed, the hydrolysis becomes the dominant reaction in the system. In order to reduce the loss due to hydrolysis, the reaction mixture should be worked up immediately.

g. The crude epoxide is stored in a closed shell vial over a small amount of anhydrous sodium sulfate.