Max Planck chemists make metal carbenes from bench-stable thianthrenium salts instead of diazo compounds

Ritter's group put the carbene fragment on thianthrene, a sulfide whose unusual electronic and steric properties make handing the carbene to the metal more favorable, according to the institute's account ^[4]. Substrate scope is how you test that claim. "Sulfonium ylides have been used in cyclopropane chemistry for almost 60 years, but their reactivity was limited to Michael acceptors," said Deepak Behera, first author of the Nature paper ^[6][13]. Internal, unactivated alkenes fall outside that class ^[5]. A cyclopropane on one of them is evidence that a metal carbene did the work, since the old ylide chemistry could not reach those substrates ^[6].

The safety case starts from the hazard it is meant to remove. Diazoalkanes can be toxic, unstable and potentially explosive ^[1]. The alkyl thianthrenium salts, by contrast, are readily prepared and can be isolated and stored ^[7]. The thermal result covers only the salts the team tested, and for those it found no detectable exotherm ^[8].

Scale came from solvent-free ball milling, which the team used to run cyclopropanation at decagram scale ^[9]. That means tens of grams ^[1]. "The combination of bench-stable reagents and solvent-free ball milling makes the carbene chemistry particularly attractive from a practical and scalability perspective," said Sagnik Chatterjee ^[10]. For a methods paper, tens of grams is a solid bench demonstration. The public account does not give yields, name the metal catalyst, or compare costs with diazo routes, so anyone weighing a switch will have to go to the paper itself ^[13].

The work beyond cyclopropanation bears directly on the substitution question. Diazo compounds have been the standard way to generate carbenes for decades ^[1], so every reaction class the salts can handle is another place a lab could stop using them. The group showed sigma-bond insertion and sigmatropic rearrangements as well ^[11]. "Our aim was to establish a general approach to carbene-transfer chemistry that allows diverse carbene fragments to participate in different classes of reactions," said Ritter, director at the Max-Planck-Institut für Kohlenforschung ^[12][2].

In my view the substitution case holds for the reactions shown. A lab that avoids diazoalkanes now has a storable precursor for cyclopropanation, including on unactivated alkenes, and for insertion and rearrangement chemistry ^[5][7][11]. "Stand in for" is a fair description for the salts tested, at bench scale ^[8][1]. Production use is a separate question, and the published evidence so far stops at tens of grams ^[1].