In the TCEP challenge, RX1 reacted fully to selenocarbamate and PQ-OH. G1 reacted similarly although more slowly to give the thiocarbamate and PQ-OH, potentially reflecting the desired difference of C/J cyclisation kinetics (section "Design", aspect 4). The 5rather than 6-exo-trig cyclisation is supported in two ways: (i) The RX1 cyclisation byproduct was found as the monomer, while the G1 byproduct was a monooxidised dimer matching interpretation as an easily-formed and highly stable linear diselenide. (ii) Equimolar selenolate alkylators almost entirely blocked PQ-OH release from RX1, but only slightly reduced PQ-OH release from G1. This latter observation crucially suggests that on-reductant cyclisation (which also proceeds from an intermediate with an engaged selenium) may be significant for G1 but not RX1: so matching design aspect 3. The controls ran as expected: SS00 reacted fully to thiocarbamate and PQ-OH, while C1 remained intact. For full discussion see Supporting Information section 5.