Organocatalytic cyclopropanation of an enal: (computational) product stereochemical assignments.

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In the previous post, I investigated the mechanism of cyclopropanation of an enal using a benzylic chloride using a quantum chemistry based procedure. Here I take a look at the NMR spectra of the resulting cyclopropane products, with an evaluation of the original stereochemical assignments.[1]

Three products were identified, 4a-c (aryl=2,4-dinitro) with a fourth diastereomer undetected. The relative stereochemistries were assigned[1] on the basis of NMR coupling constants, using the empirical Karplus or Bothner-By relationships. Here I calculate the NMR couplings at the B3LYP+GD3BJ/Def2-TZVPP/SCRF=chloroform level for a comparison, using a methyl group rather than the full n-heptyl one shown above.

System, Data DOI

10.14469/hpc/4650

Gibbs Energy J1(a)-2(b) J1(a)-3(c)

J3(c)-2(b)

4a (1S,2R,3R) expt (R-prolinol) 4.9 9.0 7.5
4a calc -910.861653 4.6 9.9 8.3
-910.860816 4.4 10.7 7.9
-910.859908 4.9 10.9 7.7
-910.860299 5.2 8.1 8.1
4b (1R,2R,3R) expt 9.6 5.3 6.7
4b calc -910.859549 10.8 5.1 7.7
4c (1S,2S,3R) expt 5.4 5.4 9.9
4c calc -910.859820 4.2 5.5 10.4
4d (1R,2S,3R) expt n/a
4d calc -910.855965 10.3 9.4 9.6

The variation resulting from rotations about the substituents (the o-nitro and the carbaldehyde) as seen for 4a can be up to ~2 Hz. This could if needed be averaged by weighting with the Boltzmann populations. Even without this procedure one can see that for the three diastereomers where values were measured, the calculated couplings agree to 1 Hz or better. This provides confirmation of the original assignments. This quantum-based method can be used in cases where simple formulaic relationships may apply less well.


For four conformations, rotating the carbaldehyde and the o-nitro groups, as in red above.

References

  1. M. Meazza, A. Kowalczuk, S. Watkins, S. Holland, T.A. Logothetis, and R. Rios, "Organocatalytic Cyclopropanation of (E)-Dec-2-enal: Synthesis, Spectral Analysis and Mechanistic Understanding", Journal of Chemical Education, vol. 95, pp. 1832-1839, 2018. http://dx.doi.org/10.1021/acs.jchemed.7b00566

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