Ethers nomenclatural priority

The answer is both For the Li enolate, the usual rule makes OU of lower priority than oMe, so it s E, while the silyl enol ether (or silyl ketene acetal ) has OSi of higher priority than OMe, so it s Z. This is merely a nomenclature problem, but it would be irritating to have to reverse all our arguments for lithium enolates simply because lithium is of lower atomic number than carbon. So, for the sake of consistency, it is much better to avoid the use of Eand Z with enolates and instead use cis and trans, which then always refer to the relationship between the substituent and the anionic oxygen (bearing the metal). 

One special application comes when silyl enol ethers of allylic esters provide the starting material this is known as the Claisen-Ireland reaction. Esters normally form E-enolates 181 with LDA at low temperature. Because the E/Z nomenclature depends on the hierarchy of the substituents it is particularly ridiculous when applied to enol derivatives of esters. A lithium enolate (Li < C) would have the opposite stereochemical label from a silyl enol ether (Si > C) so a uniform scheme is adopted whereby the metal - O bond always has priority over the other. The mechanism 180 may remind you of the reasons for this - they are discussed in more detail in chapter 4. 

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