One of the commonly accepted models for stereoselection without any modification to the Henry reaction is shown below where stereoselectivity is governed by the size of the R groups in the model (ex. carbon chain) as well as a transition state that minimizes dipole by orienting the nitro group and carbonyl oxygen anti (on opposite sides) each other. The R groups play a role in the transition state of the Henry reaction in that the larger the R groups are on each of the substrates, the more they will want to orient themselves away from each other (commonly referred to as steric effects)