 <unk> trapping with various <unk> is <unk> less common . In one study , the <unk> <unk> is paired with a Michael reaction using an <unk> catalyst to initiate nucleophilic <unk> addition of the <unk> to β @-@ <unk> . In this tandem reaction the <unk> catalyst is required for both <unk> : it acts as the Lewis acid in the <unk> <unk> and in the next step the <unk> group of <unk> first coordinates to <unk> in a ligand exchange with the <unk> <unk> oxygen atom before the actual Michael addition takes place to the opposite face of the R @-@ group . 
