proline - aminocatalysis - enamine catalysis - iminium ion catalysis
I vividly remember the day in 1999 when I set up an
experiment that today likely bores to tears each and every chemist
around the globe: the proline-catalyzed aldol reaction
of acetone with p -nitro benzaldehyde.
Not particularly much was expected from this experiment and I reasoned
that somebody must have tried this before. Neither would I have
predicted the highly successful outcome of this reaction,
[¹ ]
nor what would happen
in subsequent years, which brought literally hundreds of papers
suggesting to have taken inspiration from my initial observation.
[² ]
Key to the phenomenal
growth of our finding into an entire field was not so
much the discovery of the proline-catalyzed intermolecular aldol
reaction via an enamine intermediate or the subsequent amino acid
derivative catalyzed Diels-Alder reactions via an iminium
ion,
[³ ]
but rather the realization
that enamine catalysis and iminium ion catalysis
are truly generic carbonyl activation modes .
[4 ]
Once this had been fully appreciated,
the ‘aminocatalysis explosion’ took
off.
Some highlights of this development include the proline-catalyzed
Mannich reaction, the aldehyde cross-aldolization, enamine catalytic α-alkylations
and α-functionalizations, various Michael additions, SOMO
catalysis, ACDC, and of course the many wonderful organocascade processes
that make combined use of enamine and iminium ion catalysis (for
a small collection of proline-based catalytic transformations, see
Scheme
[¹ ]
).
[4 ]
In addition to the many new reactions
that continue to be explored and advanced, several new proline-based
catalysts have also been designed and discovered of which the prolinol
silyl ethers are probably the most versatile. But other successful
motifs have also been described, including highly active
and enantioselective proline amides, some remarkable fully synthetic
motifs, and cinchona-derived primary amines with complementary reactivity.
Benjamin List
Scheme 1 A small and incomplete
selection of highly enantioselective proline (derivative)
catalyzed reactions.
[² ]
[4 ]
[5 ]
So what are the challenges of proline-based aminocatalysis? I
think we now largely appreciate the scope of the catalysis
principle, and to some extend the requirements for designing effective
catalysts. I doubt though that there will be many more fundamentally
new reactions discovered soon, although I still see substantial
room for important research in the area. There are three main interrelated topics
that will likely be addressed: (1) mechanistic understanding, (2)
substrate scope enlargement of the various reactions, and (3) advanced
catalysts. While proline-based enamine catalysis has delivered truly
marvelous reactions, consider for example the scope and enantioselectivity
of the proline-catalyzed Mannich reaction, certain issues
remain to be solved. It may not sound so important, but a general aldehyde cross-aldol reaction
that furnishes syn instead of the common anti products would be considered a true
breakthrough by many in the field. Similarly, direct aldehyde α-alkylations
are still narrow in scope and yet could be extremely useful.
These are just two high-profile challenges, but there are many other
problems that remain to be solved. One issue concerns the development of
advanced catalysts. Activity and selectivity need to be constantly
refined and improved, and catalyst recyclability will become a more
important issue. It is clear that such advancements are likely only
to be achievable through further mechanistic understanding and it
is therefore gratifying to see that mechanistic studies are currently
a hot topic in organocatalysis as a whole.
This Cluster, edited by Hisashi Yamamoto and myself, presents
some recent work by leading experts in the field of proline-based
aminocatalysis that addresses some of the above issues. Isn’t
it amazing how much high quality research this field continues to
deliver? I am very grateful to all authors for their wonderful contributions
and can’t wait to see the advancements of the next decade….