Abstract
We report some new observations in BürgiDunitz territory – structurestructure correlations for the addition of N nucleophiles to the CO group. The amino aldehyde 1 exists predominantly in polar solvents, and exclusively in the crystal, as the carbonyl‐addition structure 2, but appears to demand hydrogen‐bonding solvation as part of the process. In terms of CO bond lengthening, the new system lies somewhere nearly half‐way between an amino aldehyde and a stable quaternary ammonium aminal: we present the structure 2⋅Me of the N^+^COMe derivative of 2 as an example. We have investigated the effects on the cyclisation equilibrium in this system of hydration, which is important, and of the degree of methylation of the azaadamantane skeleton, which – unexpectedly – is not. In terms of the reverse process, i.e., breaking the CN^+^ bond: in each case, the bond is lengthened by (n~(O)~__σ__*~(CN)~) electron‐pair donation from the O‐atom (the generalised anomeric effect, as in 2⋅Me↔3, below) [4]. This effect is strongest when the O‐atom is negatively charged, reduced by hydrogen‐bonding, and minimised by alkylation.