Brookharts iron catalyst showing disruption of the catalytic cycle by ethanol

We have utilized three different catalytic systems however during this investigation we have not thoroughly studied their mechanisms. The Rh2(OAc)4 system performed as expected, but the 10 % Pd/C and the Mn(CO)sBr catalytic systems gave us results that presented a need to consider their mechanism. 

We propose that the carbonyl oxygen of the (+)-ethyl lactate acts as a Lewis base forming an intramolecular pentacoordinated organosilicon compound. Pentacoordination from a 

In the case where the silyl ketal is made up of the 2-phenylethanol and (+)-ethyl lactate (3a), the more basic oxygen would be the one on the 2-phenylethanol moiety. However protonation and eventually dissociation of this alcohol leads back to the starting silyl ether. Protonation could also occur on the oxygen of the (+)-ethyl lactate moiety. Once the (+)-ethyl lactate is protonated it could dissociate resulting in newly formed silyl ether. This newly formed silyl ether can react with the dissociated (+)-ethyl lactate, or it can go on to react with another 2-phenethyl ethanol which is present in a higher concentration than the dissociated (+)-ethyl 

The second experiment was to test the effect of bases on the rate of exchange during the alcoholysis reaction. This was to determine whether the exchange process was taking place by simple proton catalysis. We treated 2-phenylethanol and le under the standard reaction conditions but this time 4 mol % potassium carbonate (K2CO3) was added. After the reaction was completed, 16 % of the exchanged product was observed. There was no difference in the extent of exchange by added base. The experiment was also conducted with triethylamine (EtsN) as the base however the reaction did not proceed. 

We concluded from these experiments that the exchange process must be occurring along the pathway to the product and the final product, once formed, is stable to the reaction conditions. Based on these results we have proposed a new mechanism which will be discussed further in the next section. We will also suggest experiments to test our new hypothesis. 

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