Orthometallation as the proton source is supported by non-integer integration of t-butyl p-tolyl sulfoxide that was observed in the 1H NMR spectrum of the crude reaction products in the conversion of 18 to 21 (See Supporting Information, Figure S29). The authors surmise that non-integer integration of the protons ortho to the sulfinyl group arises via formation of 20 followed by directed orthometallation by excess t-BuLi followed by deuteration. While this evidence is indirect, a similar phenomenon has been previously reported in the lithium-sulfoxide exchange of paracyclophanyl p-tolyl sulfoxides. The authors had experimental evidence for orthometallation of t-butyl p-tolyl sulfoxide as the proton source; however, these side-products were obtained in low to moderate yields, thus not excluding the other possible proton sources proposed herein. See: Parmar R. Coles MP. Hitchcock PB. Rowlands GJ.
Synthesis 2010; 4177
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