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kilomentor | 19 July, 2008 13:23
If you have a compound that is even quite a poor electron pair donor and that compound will not crystallize or will not crystallize to produce good quality crystals, what substance would be the best choice to test as a hydrogen bond donor? If you have a troublesome impurity in your product that can be predicted to be a better Lewis acid than your desired product, what would you propose as a good candidate to form a separable co-crystal with it?
An answer may be extracted a paper by the late Margaret C. Etter, Acct. Chem. Res. 1990, 23, 120-126. This is not a paper that synthetic organic chemists or process development chemists are likely to read. The lead author was a crystallographer and solid-state chemist. What makes the question interesting for us is that solid stoichiometric compositions can be made from substances with as poor a Lewis basicity as aliphatic ethers. The compound that forms these complexes is symmetrical bis-N,N-(3-nitrophenyl)urea. The compound can be easily synthesized from 3-nitroaniline and any phosgene equivalent giving a solid with melting point 256-258 C. It can be crystallized from any of acetic acid, benzene, chloroform, dichloromethane, ethanol, 95% ethanol or ethylene glycol. Heating the complex will drive off the Lewis base if it is volatile under high vacuum. One of two polymorphs will form but the form is not important for making co-crystals.
The complexes with a donor can be formed in a suitable solvent that is evaporated or if the donor is a solid, such as triphenyl phosphine oxide, simply by grinding two solids together. Making complexes with a component that is only present as an impurity in a product mixture has not yet been tried. The trick could perhaps be used to remove a difficult impurity such as triphenylphosphine oxide, dicyclohexylurea, dimethyl sulfoxide, polyethylene glycol. Even if a slight excess of the dinitro-urea was needed to completely remove the impurity of concern from a crude product mixture. Subsequent removal of this urea in second treatment might be much simpler than getting rid of the original troublesome impurity.
From the list of solvents from which symmetrical bis N,N-(3-nitrophenyl)urea can be crystallized, it would appear that the compound is insoluble in hydrocarbons and these might be useful as anti-solvents to increase the yield of adducts.
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