reactive intermediates - enamines - iminium ions - structure elucidation
Significance
The present work of the Seebach group addresses the isolation
and characterization of reactive intermediates in organocatalysis. In
this regard, enamine 1 and iminium tetrafluoroborates 2-4 were
prepared through conventional methods from 2-phenyl acetaldehyde
or cinnamaldehyde, respectively, and the corresponding secondary
amine (salt) catalyst. Single crystals of enamine 1 and
iminium ions 2 and 3 were
subjected to Röntgen structure analysis. The structure
of iminium ion 4 was elucidated by 2D NMR
spectroscopy. Moreover, the Röntgen and NMR structures
were extensively discussed in connection with other, structurally
related compounds.
Comment
The experimentally determined NMR and Röntgen structures
of 1-4 were
also compared with computed structures, and the similarity is striking
(for DFT calculations of iminium ions similar to 3 and 4 derived from crotonaldehyde, see: R.
Gordillo, H. Carter, K. N. Houk Adv. Synth. Catal. 2004, 346, 1175).
In fact, Seebach et al. confirmed structural effects predicted by
theory now experimentally. These results suggest that in future
synthetic organic chemists will more and more rely on computational
methods for finding the best (organo)catalyst instead of carrying
out tedious experimental optimizations. Similar studies might also
be desirable for other classes of organocatalysts such
as primary amine (salt) catalysts.
