"(16) Great care was taken to avoid the presence of transition metal impurities. All starting materials were synthesized without using any transition metal...reagent transfers were performed with one-way plastic spatulas, and new glassware and stirbars were used for the cyclization reactions. The starting materials and reagents were analyzed to the detection limit of 4 ppb by atomic absorption spectroscopy (AAS) or inductively coupled mass spectrometry (ICP-MS).
Data [for a representative intermediate] - Cu < 4ppb, Pd <4 ppb; Kcarb - Cu < 4 ppb, Pd < 4 ppb; DMEDA [ligand] - Cu 2.4 ppm, Pd < 4 ppb." [Emphasis mine]
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| Sand - Probably > 4ppb "active" metals! Source: 123RF |
I'll be honest with you, I've never tested for metals in my starting materials below ~1 ppm (Food for thought: here's an EMA document detailing allowable catalyst residue in human medicines). I'd wager that 99.9% of workaday bench chemists haven't, either.
Bolm's group endures this rigor because, well, they literally wrote the book on trace metal catalysis. Quite honestly, I'd bet that they felt a bit uneasy when they measured the DMEDA copper concentration; more than a few of these "metal-free" reactions proceed with vanishingly small amounts of catalyst.
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