Prior manual work has shown that the condensation of
mesitaldehyde and pyrrole leading to tetramesitylporphyrin
depends sensitively on concentration and requires cocatalysis
involving BF3·O(Et)2 and ethanol or other protic species. We
have applied an automated microscale chemistry workstation
capable of parallel or adaptive experimentation to systematically
investigate these cocatalysis conditions. Examination of experimental grids spanning a 1000-fold range of the concentrations of BF3·O(Et)2 and ethanol identified the best cocatalysis
conditions for various mesitaldehyde and pyrrole concentrations. As the reaction concentration increased from 10 to 200
mM, the optimal yields were achieved with a parallel increase
in BF3·O(Et)2 concentration (from 3.3 to 56 mM), but the
amount of ethanol remained relatively constant. Catalysis
conditions were identified that afforded ∼30% yields for
reactants in the range of 10−73 mM, thereby enabling the
reaction to be performed at increased concentration without
loss in yield. Application of a catalyst searching protocol
identified ethylene glycol, 2-methoxyethanol, and methanol as
effective cocatalysts from a set of 12 candidates. Collectively,
these results show the utility of an automated chemistry
workstation in acquiring a comprehensive set of data concerning
the scope of cocatalysis in the reaction of mesitaldehyde and
pyrrole.
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