“…This method is profoundly dependent on reactive carbene and metal-carbenoid intermediates [[M]=CR 2 ], in particular the donor-acceptor Rh-carbenoids (9,10,12) derived from highly energetic diazoalkanes, which require stringent precautions for handling (13,14). The metal-carbenoids are typically generated from diazomethane (CH 2 N 2 ), TMSCHN 2 , PhCHN 2 , and a-diazocarbonyl compounds by decomposition in reaction with transition-metal [M] catalysts (8,15,16), and more recently under photoredox (17,18) and engineered enzymatic (19)(20)(21)(22) conditions. The classic Simmons-Smith and related reactions (23) also transform alkenes into cyclopropanes, yet, again, these reactions require a large excess of unstable, often difficult-to-access 1,1-and 1,2-dihaloalkanes and highly reactive Et 2 Zn or Zn/Cu reagents to generate an adequate quantity of zinc carbenoids for satisfactory reactivity.…”