The silyl ether can accept a nucleophile to form a trigonal bypyrimidal geometry, so I don't see why a great nucleophile like a thiolate could react. I would try changing the solvent and possibly running the reaction at a colder temperature. If you were running the rxn at room temperature, then I would try -5ยบ in MeOH or MeOH/Acetone, or DMF/water because if you can't get the reaction to go under kinetic conditions then it will not work.