The following image was taken from wikkipedia's article about the Formose reaction. What is the mechanism of the first step of the reaction sequence? How do the two formaldehyde molecules condense?

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    $\begingroup$ Wikipedia says: 'The reaction is catalyzed by a base and a divalent metal such as calcium.' So I'd guess first complexation, the proton abstraction, then C-C bond formation. $\endgroup$ – Martin - マーチン Feb 14 '16 at 15:22
  • $\begingroup$ Would it help to know that 1. under standard conditions, formaldehyde is a gas, 2. the reaction is performed in aqueous solution, 3. in aqueous solution, formaldehyde mostly exists as a hydrate $\ce{H2C(OH)2}$? $\endgroup$ – Klaus-Dieter Warzecha Feb 14 '16 at 16:50
  • $\begingroup$ There are many articles on the topic. I am far too lazy to read them and write up an answer but here are some: Tetrahedron Lett., 1959, 1 (21), 22-26, J. Org. Chem., 2006, 71 (25), 9503–9505, J. Am. Chem. Soc., 2014, 136 (10), 3720–3723 $\endgroup$ – orthocresol Feb 14 '16 at 19:57
  • $\begingroup$ @orthocresol In none of these articles is the first reaction considered. $\endgroup$ – RBW Feb 14 '16 at 20:02
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    $\begingroup$ @Marko I find your question very interesting! I think that we can definitely rule out direct deprotonation of formaldehyde in aqueous alkaline medium. However, deprotonation of methanediol, formadehyde exists as a hydrate in aqueous solution, seems possible, and it might be further facilitated by the presence of larger cations. But till now, I haven't figured out how an umpolung, rather than a Cannizzaro reaction might take place under these conditions. $\endgroup$ – Klaus-Dieter Warzecha Feb 14 '16 at 21:39

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