Do carbodiimides exhibit axial chirality or do the nitrogens invert rapidly enough, as in trisubstituted amines with different alkyl groups?
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Yes, carbodiimides exhibit axial chirality similar to allenes. While there are significant differences in the allene - carbodiimide systems (see below) there are also some similarities. For example, just like in amines and imines the barrier to racemization in generally carbodiimidesis low, typically in the 6-10 kcal/m range.
Anet did a study on 1,3-diisopropylcarbodimide ("Energy barrier of racemization in diisopropylcarbodiimide", Frank A. L. Anet, J. C. Jochims, C. H. Bradley J. Am. Chem. Soc., 1970, 92 (8), pp 2557–2558, DOI: 10.1021/ja00711a064).
In the chiral compound each isopropyl group would contain two diastereotopic methyl groups split by the isopropyl proton so we would expect to see 2 doublets. On the other hand, if racemization is rapid all methyl groups become equivalent and we would then expect to see only one doublet. Experimentally it turns out that around -150 C the doublet broadens into 2 doublets giving rise to a barrier height for racemization around 6.7 kcal/m.
Important Notes:
1) Just like with amines and imines, placing very electronegative groups on the carbodiimide nitrogens (fluorine, for example) raises the barrier to racemization. Incorporating the carbodiimide structure into a small ring (see here for an example) can also raise the barrier to enatiomerization. Both techniques have been used to permit isolation of chiral carbodiimides.
2) Carbodiimides are not linear, but rather the $\ce{N-C-N}$ angle is around 170°. This is attributed to the terminal nitrogens being more electronegative than the central carbon and consequently affecting the hybridization of the central carbon (e.g. changing it from $\ce{sp^1}$ to $\ce{sp^{x}}$ where 1 < x < 2)
3) Racemization in carbodiimides can be achieved by two different mechanisms
- by traditional nitrogen inversion involving a planar nitrogen atom, and/or
- by traditional cis-trans isomerization about the $\ce{C=N}$ double bond
both mechanisms are thought to play a role in carbodiimide racemization.
ron
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Could you elaborate note 3? How does racemization occur by planarization of nitrogen, as it is already planar? – EJC Jun 06 '16 at 21:35
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I just mean the traditional nitrogen inversion transition state where the lone pair exists in a p-orbital and the other 3 ligands attached to nitrogen all lie in the same plane as the nitrogen. In the case of an imine or carbodiimide, 2 of the ligands are the pi bond. Think of the pi bond using the bent-bond (2-membered ring) formalism. Now the planar transition state occurs when the 2 bent-bonds and the R group attached to nitrogen and the nitrogen all lie in the same plane. – ron Jun 06 '16 at 21:45
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But aren't they already in a plane? The nitrogen is sp2. – EJC Jun 06 '16 at 21:52
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No, the pi-bonds to nitrogen are in a different plane (orthogonal plane) than the plane containing the $\ce{N-R}$ bond. Again view the pi-bond as a two-membered ring, that might make the 2 planes easier to visualize. – ron Jun 06 '16 at 21:55
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@Marko Geez, you can call it linearisation here... – Mithoron Jun 06 '16 at 21:59
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@Mithoron, if it is just like linearisation, how is that mechanism different from traditional cis-trans isomerisation about the C=N double bond? – EJC Jun 06 '16 at 22:21
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@Ron Could you please draw both isomerisation process? – EJC Jun 06 '16 at 22:21
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See this old Ron's answer http://chemistry.stackexchange.com/a/26319/9961 – Mithoron Jun 06 '16 at 22:38
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I've edited the picture. The "circle" between the C and the N on the right represents the pi bond or 2-membered ring bent-bond description. Does that help clarify the 2 processes? – ron Jun 06 '16 at 22:49
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Now I see what you meant. It's much easier to understand with diagram. Thank you very much. – EJC Jun 06 '16 at 23:00
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1Glad it helped. – ron Jun 06 '16 at 23:01