A critical common feature in these low-energy SR TS1 is the stabilising H-bonding interactions between the oxygen from the i PrO group and hydrogen from the aldehyde (Figure .B). In the reaction with Cat-B, the critical H-bond is absent in the most stable TS1 and the aldehyde adopts the reverse orientation, which leads to the R ketone as the major product. This leads to an intuitively-satisfying explanation for the reversal in enantioselectivity arising from a small change in catalyst structure.