Conformational dynamics of triple rotors: Tris(dimethylamino)methane, triisopropylamine, and related molecules

Journal of Organic Chemistry

Volumn 61, Issue 4, 1996, Pages 1290-1296

  Anderson, J Edgar ^a   Casarini, Daniele ^b   Lunazzi, Lodovico ^b  

^a UNIVERSITY COLLEGE LONDON   (United Kingdom)

^b UNIVERSITY OF BOLOGNA   (Italy)

Author keywords

[No Author keywords available]

Indexed keywords

EID: 0000826346     PISSN: 00223263     EISSN: None     Source Type: Journal    
DOI: 10.1021/jo951227t     Document Type: Article

References (43)

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11. Bushweller, C. H. In Acyclic Organonitrogen Stereodynamics; Lambert, J. B.,Takeuchi, Y., Eds.; VCH: New York, 1992; Chapt. 1, p 1. Nitrogen inversion is almost inevitably accompanied by a net measure of rotation about the C-N bonds to achieve a new local conformational minimum, thus the interconversion of 7′ and 7″ implies that one methyl group has rotated past a C-H bond during inversion. There are high-barrier rotation processes which are likely to be distinct from this nitrogen inversion/rotation, such as the 7′ to 7 interconversion when one N-methyl bond has to rotate past a C-Me bond. When, because of the molecular structure, both processes have to be slow before a dynamic effect is seen in the NMR, careful arguments have to be adduced to assign the experimental barrier to one or another process. To the extent that a transition state conformation with a flat nitrogen and simultaneous eclipsing along the significant C-N bond is implausible, one or another process is rate-determining and may in principle be assigned the experimentally measured barrier.
12. (a) Allinger, N. L.; Yuh, Y. H.; Lii, J.-H. J. Am. Chem. Soc. 1989, 111, 8551.
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14. MMX forcefield as in PCMODEL, Serena Software, Bloomington, IN. See also Gajewski, J. J.; Gilbert, K. K.; and McKelvey, J. Advances in Molecular Modelling; JAI Press: Greenwich, CT, 1992; Vol. 2.
15. An arrangement with two or more groups anti, [i.e. (a,a,-g) or (a,a,a)] is of relatively high energy and no minimum of this type was found in the MMX calculations. MM3 calculations too do not have the (a,a,a) arrangement as a minimum but located a shallow minimum 10 with H-C-N-lp torsion angles -128°, -151°, -103° which thus bears the description (a,a,-g) and is included in Table 1 for completeness sake, although all three groups are far removed from the 180° and 60° torsion angles expected for a "perfect" (a,a,-g) situation.
16. Anderson, J. E.; Casarini, D.; Lunazzi, L. J. Org.Chem. 1993, 58, 714.
17. -1 between 8 and 9, the proportion of the latter would be as low as 14% at -142°, if ΔS° is reasonably assumed to be negligible.
18. 2 group is not a minimum (as indicated by both MM3 and MMX calculations) and therefore cannot be populated. As a consequence it is immaterial whether the latter group is represented as planar or as locked into a unique pyramidal invertomer.
19. In practice a 90° rotation about the CH-N(1) bond (see Scheme 3) transforms 14 into 11 (-g,-g,-g of Table 1) and a subsequent rotation about the CH-N(1) bond also re-establish the original situation. Of course the process continues involving also the 90° rotation about the CH-N(3) bond. These motions (which are likely to occur in sequence and not simultaneously) are able to re-establish the same structure 14 having, meanwhile, exchanged b with both a and c without exchanging directly a with c, as experimentally observed.
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27. A second explanation is that the equilibrium involving 8, 9, and 8′ (as represented by 14 of Scheme 3) is still rapid in the solid but that the plane of symmetry present in 14 is removed by the lattice, as happens if adjacent molecules are not in the plane of symmetry nor on the axis perpendicular to the plane of symmetry through the central carbon atom.
28. 2, see: Forsyth, D. A.; Johnson, S. M. J. Am. Chem. Soc. 1994, 116, 11484).
29. 2, see: Forsyth, D. A.; Johnson, S. M. J. Am. Chem. Soc. 1994, 116, 11484).
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32. Anderson, J. E.; Casarini, D.; Lunazzi, L. J. Chem. Soc. Perkin Trans. 2 1991, 1431.
33. 15N chemical shifts of some amines were also attributed to the planarity of the nitrogen atom of triisopropylamine (3), see: Wong, T. C.; Collazo, L. R.; Guziec, F. S., Jr. Tetrahedron 1995, 51, 649.
34. 3 also yielded a C-N-C angle of 119.3°, see: Gaensslen, M.; Gross, U.; Oberhammer, H.; Rüdiger, S. Angew. Chem. Int. Ed. Engl. 1992, 31, 1467.
35. Bock et al. (ref 7) used for their MM calculations of triisopropylamine (3) the MM2 force field, as clearly stated twice in their references 4a and 4b. Thus the MM3 indication, which appears on Table 2 of ref 7, is likely to be a misprint. This would explain why the angle C-N-C of 3, obtained in ref 7 by means of their MM calculations (i.e. 117.9°), appears to be smaller than ours (i.e. 119.4°. nearly coincident with the experimental value of 119.2°) where MM3 calculations were really employed.
36. 16N shifts, as well as the JNH coupling constants, also agree in indicating a tetrahedral structure for the ion 5. (see: Wong et al. in ref 26).
37. Anderson, J. E.; Ijeh, A. I. Unpublished results.
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39. Kantlehner, W.; Speh, P.; Brauner, H. J. Synthesis 1983, 905. See also: Bredereck, H.; Effenberger. F.; Brendle, T.; Muffler, H. Chem. Ber. 1968, 101, 1885.
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