last updated Friday, October 09, 2015

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 or 1,2-diaminobenzene is achiral would not lead to the formation of a racemic mixture. In addition, the ligand is able to stabilize a broad variety of oxidation states on the manganese atom. Note that neither Mn(III) nor Mn(V) compounds are particularly stable without a ligand helping to stabilize them. By using different groups on the benzene ring, the reactivity of the catalyst can be tuned as well.

b. Manganese is a transition metal while magnesium is a main group metal. In case of magnesium, there are only two oxidation states known, Mg⁰ and Mg²⁺. These oxidation states are electrochemically very far away from each other in case of magnesium (E⁰= -2.36 V). In case of mangenese, there are many more oxidation states, which are closer together (i.e., Mn²⁺ --> Mn, E⁰= -1.18 V). 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.

2.a. The given amount of indene is equivalent to 4.50 mmol (=0.523 g/116.16 g/mol). He should use 0.315 mmol of the catalyst (=4.50 mmol * 0.07), which are 0.200 g (0.315 mmol*635.2 g/mol).

b. Most procedures in the literature employ dichloromethane as a solvent for the reaction because it is polar enough to dissolve the catalyst and the intermediate. Ethyl acetate dissolves the catalyst, the alkene and the intermediates sufficiently well. Hexane or petroleum ether are not suitable because the catalyst is almost completely insoluble in these solvents.

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 the optimized solvent mixture. Lane 1 shows the alkene (Rf= ~0.8) while lane 2 shows a sample of the reaction mixture. Since the reaction is incomplete, the lane displays two spots (epoxide: Rf= ~0.4 (blue spot), alkene: Rf= ~0.8 (green)).

e. The optimization is necessary because the crude epoxide is purified by flash chromatography. The optimized mixture allows for a good separation of the product mixture on the column.

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.