Abstract
Several amines with three bulky alkyl groups at the nitrogen atom, which exceed the steric crowding of triisopropylamine significantly, were synthesized, mainly by treating N‐chlorodialkylamines with Grignard reagents. In six cases, namely tert‐butyldiisopropylamine, 1‐adamantyl‐tert‐butylisopropylamine, di‐1‐adamantylamines with an additional N‐cyclohexyl or N‐exo‐2‐norbonyl substituent, as well as 2,2,6,6‐tetramethylpiperidine derivatives with N‐cyclohexyl or N‐neopentyl groups, appropriate single crystals were generated that enabled X‐ray diffraction studies and analysis of the molecular structures. The four noncyclic amines adopt triskele‐like conformations, and the sum of the three C−N−C angles is always in the range of 351.1° to 352.4°. Consequently, these amines proved to be structurally significantly flatter than trialkylamines without steric congestion, which is also signalized by the smaller heights of the NC3 pyramids (0.241–0.259 Å). There is no clear correlation between the heights of these pyramids and the degree of the steric crowding in the new amines, presumably because steric repulsion is partly compensated by dispersion interaction. In the cases of the two heterocyclic amines, the steric stress is smaller, and the molecular structures include quite different conformations. Quantum chemical calculations led to precise gas‐phase structures of the sterically overcrowded trialkylamines exhibiting heights of the NC3 pyramids and preferred molecular conformers which are similar to those resulting from the X‐ray studies.
Highlights
The amine entity belongs to the most important functional groups in chemistry.[1]
Quantum chemical calculations led to precise gas-phase structures of the sterically overcrowded trialkylamines exhibiting heights of the NC3 pyramids and preferred molecular conformers which are similar to those resulting from the X-ray studies
We mainly prepared tertiary amines 8, in which steric distress surpasses that of the standard compound 5 distinctly, by treating N-chlorodialkylamines 7 with Grignard reagents in the presence of a major excess of tetramethylethylenediamine (TMEDA).[10a,15] The substrates 7 were available by chlorination of the corresponding secondary amines with the help of N-chlorosuccinimide.[16]
Summary
Hünig’s base 1[2] and 2,2,6,6-tetramethylpiperidine (2),[3] play a major role as Brønsted bases with low nucleophilicity or as precursors of persistent nitroxyl radicals, like 3,[4] which are used for spin labeling tools (Figure 1).[5]. A sum of the angles at the nitrogen atom of 357.68 was detected, which is quite close to 3608, an indication of a planar structure.[12] This outcome was claimed to be confirmed by NMR investigations.[13] low-temperature single-crystal X-ray diffraction of 5 led to the CÀNÀC angle of 116.2(1) and a sum of angles of 348.68; a height of the NC3 pyramid (nitrogen at the top) of 0.27–0.29 (depending on temperature) was determined.[14] The latter value is more than a half of the corresponding height in triethylamine (6), which amounts to 0.467 . In the six cases of amines 8 a and 8 e–i, generation of single crystals and X-ray diffraction were successful to analyze the molecular structures. Based on these results, quantum chemical calculations led to precise gas-phase structures of the title compounds. Subsequent treatment with tert-butylisopropylamine yielded the desired product 8 e.[19]
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