CO sBr Catalytic System

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. 

The accelerated rate for alcoholysis with le, which was observed for the 10 % Pd/C catalytic system, was also seen with the Mn(CO)sBr catalyst. Reactions of le with primary, secondary or tertiary alcohols resulted in moderate yields of the corresponding silyl ketals after 2 h (Table 8 and 9). When mono-alkoxy silane from 3-hydroxy butyrate (lg) was treated with homoallyl alcohol in the presence of Mn(CO)sBr as the catalyst under the standard conditions, 76 % of the silyl ketal was obtained. These silyl ethers possess neighboring carbonyl groups that can participate in the reaction by forming a more reactive pentacoordinated silicon center upon addition of the silane to the metal center.. 

Another similarity between the two catalysts was the small electronic effect observed for 2-phenylethanol versus benzyl alcohol. The same 1.1 1 ratio in favor of 2-phenylethanol was observed when a competition experiment was performed between the two alcohols. 

---

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. 

---