[scubbabubba]

Could anybody explain how this works?
I understand how steric effects have a role in stereoselectivity but I was reading the Wikipedia page on the Henry reaction (https://en.wikipedia.org/wiki/Nitroaldol_reaction#cite_note-nitro-3) and there is a mention of stereoselectivity being governed also by "a transition state that minimizes dipole by orienting the nitro group and carbonyl oxygen anti (on opposite sides) each other".
I've tried looking this up but I can't find any mention of this on my textbook and I'm only finding articles I don't have access to that look at electronic control or steric control of stereoselectivity.

[pgk]

It’s not a mystery! Apart the steric factors, thermodynamic factors (like in this case) may also influence the stereoselectivity of a reaction, by favoring the stereospecific transition state that is more thermodynamically stable. Furthermore, stereoselective organometallic catalysts may interfere to both steric and thermodynamic factors and lead to the desired steroselectivity. Indicatively, see:
1). The Henry reaction: recent examples, Tetrahedron, (2001), 57, 915-945, (2001)
http://chemistry.mdma.ch/hiveboard/picproxie_docs/000448477-04082418194214626.pdf
2). Unveiling Reliable Catalysts for the Asymmetric Nitroaldol (Henry) Reaction, Angew.Chem., Int.Ed., 43, 5442-5444, (2004)
http://www.uwindsor.ca/people/jgreen/sites/uwindsor.ca.people.jgreen/files/acie-2005-43-5442-palomo.pdf
3). Salicylaldimine based copper (II) complex-potential catalyst for asymmetric Henry reaction, ARKIVOC, 2010(11), 370-379, (2010)
https://quod.lib.umich.edu/cgi/p/pod/dod-idx/salicylaldimine-based-copper-ii-complex-potential-catalyst.pdf?c=ark;idno=5550190.0011.b30;format=pdf
4). Organocatalytic Enantioselective Henry Reactions, Symmetry, 3, 220-245, (2011)
http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.698.1496&rep=rep1&type=pdf
5). Advances in Contemporary Research, Asymmetric Henry reaction catalysed by transition metal complexes: A short review, Indian Journal of Chemistry-Advances in Contemporary Research, 52B, 87-108, (2013)
http://nopr.niscair.res.in/bitstream/123456789/15621/1/IJCB%2052B%281%29%2087-108.pdf