Triisopropylamine

Triisopropylamine is an organic chemical compound consisting of three isopropyl groups bound to a central nitrogen atom. As a hindered tertiary amine, it can be used as a non-nucleophilic base and as a stabilizer for polymers; however, its applications are limited by its relatively high cost and difficult synthesis.

Structure

In the early 1990s, theoretical studies and electron diffraction analysis of the 3D structure of the molecule, in the gas phase or in non-polar solvents, indicated that the bonds between the nitrogen atom and the three carbon atoms were nearly coplanar in the ground state, instead of forming a trigonal pyramid as in simpler amines. The average C-N-C angle was claimed to be 119.2°, much closer to the 120° of the flat configuration than to the 111.8° of trimethylamine. This peculiarity was attributed to steric hindrance by the bulky isopropyl radicals. However, in 1998 X-ray diffraction analysis of the crystallized solid showed that the C₃N core is actually pyramidal, with the N atom lying approximately 0.28 Å off the carbons' plane (whereas in trimethylamine the distance is about 0.45 Å). However the researchers could not rule out the crystal field effect as the cause of the asymmetry.

The C-C-C planes of the isopropyl groups are slightly tilted (about 5°) relative to the threefold symmetry axis of the C₃N core.

Even more hindered amines

Triisopropylamine is notable as being among the most sterically hindered amines currently known. To date, di(''tert''-alkyl)(''sec''-alkyl)amines like di-''tert''-butyl(isopropyl)amine (^''t''Bu₂ⁱPrN) and diadamantylcyclohexylamine (Ad₂CyN) have been prepared in low yield, as have a handful of tri-''tert''-alkylamines in which two of the ''tert''-alkyl groups are tied together in a five-membered ring. As a result of the extreme steric crowding, these tertiary amines were found to be unusually susceptible to β-elimination to give an alkene and a secondary amine, even at temperatures close to room temperature (as low as 40 °C). As a result of these and other synthetic difficulties, the authors of a 2018 study predict that the even more crowded tri''-tert-''butylamine (^''t''Bu₃N, unknown as of 2023) will likely remain a longstanding unsolved synthetic challenge, even though ''ab initio'' calculations indicate that it would be of reasonable stability if it could be prepared. Calculations indicate that tri-''tert''-butylamine should be planar at nitrogen. Though arguably a more hindered molecule, the structurally analogous tri-''tert''-alkyl carbinol 2,2,4,4-tetramethyl-3-''t''-butyl-pentane-3-ol (tri-''tert''-butylcarbinol, ^''t''Bu₃COH), has been known for decades (prepared by Bartlett and coworkers in 1945).

Preparation

Steric effects make triisopropylamine difficult to synthesise and unlike less hindered tertiary amines (such as triethylamine) it cannot be produced by the alkylation of ammonia with alcohol; attempts to do so stall at diisopropylamine. It can be prepared from diisopropylamine on the laboratory scale:

:

See also

* Tetra-''tert''-butylethylene

References

{{reflist

Non-nucleophilic bases
Diisopropylamino compounds
Tertiary amines