The emergence of alkene metathesis has been accompanied with countless synthetic applications
across the molecular sciences, from drug synthesis to polymer sciences.[1] The unique mechanism of this reaction, combined with the outstanding functional
group tolerance of modern catalytic systems, has been key to the extremely rapid development
of this reaction. Recently, researchers have recognized the potential of developing
new metathesis reactions that go beyond the traditional olefin metathesis paradigm,
developing several new, mechanistically distinct metathesis reactions which can be
used to break and reform other important types of triple, double and single bonds.[2]
In this SYNLETT Cluster, we are delighted to celebrate these new developments by highlighting
some new ‘metathesis reactions beyond olefins’ reported by leading authors in this
field of research. The diversity of bonds (e.g., C–S, C–O, C=O, M–X) involved in this
cluster as well as the diversity of applications considered (organic synthesis, organometallic
synthesis, polymer chemistry and supramolecular chemistry) clearly highlight the untapped
potential of this research area.
Recently, carbonyl olefin metathesis has emerged as a useful alternative to traditional
carbonyl olefination methods, which are limited by their low atom economy and waste
disposal. Lambert has summarized, in a timely review, the recent efforts to turn this
new metathesis reaction into a synthetically useful process.[3] In his review he also describes an ingenious organocatalytic strategy which takes
advantage of a simple hydrazine catalyst to realize this challenging transformation.
Another exciting manuscript in this area, written by Nguyen et al., describes the
use of iodonium species as a new catalyst for ring-closing carbonyl olefin metathesis,
thus adding an important new catalytic strategy to the toolbox.[4]
Metathesis reactions do not need to proceed between unsaturated bonds and can thus
be extended to single bonds. A classical yet powerful illustration of this concept
is salt metathesis, which can readily be used to exchange two M–X bonds to synthesize
new metal complexes. This concept is illustrated in the manuscript from Grela et
al., who used this strategy to access some new Ru carbene complexes.[5] Due to their abundance in organic molecules, the metathesis of C–S bonds has recently
emerged as an important frontier area. In this Cluster, Tobisu and Chatani report
on a ring-closing C–S metathesis, which proceeds in the presence of a suitable thiolate
initiator, in order to access synthetically useful dibenzothiophenes.[6] In another contribution, von Delius et al. have described trithioester exchange
and metathesis reactions under acidic activation.[7] This new reaction adds to the toolbox of dynamic covalent reactions available for
the thermodynamically controlled synthesis of supramolecular architectures. Reversibly
cleaving C–O bonds is another attractive target reaction, and Miller and Pemba have
contributed to this issue with a kinetic study of the acetal metathesis reaction they
developed previously for the synthesis of new polymeric materials.[8]
Finally, Naka and Naraoka report on a different yet similar type of reversible reaction,
a reaction which transfers a molecule of water between a primary amide and a nitrile
under palladium catalysis.[9] This shuttle catalysis reaction enables the synthesis of dicarboxamides using the
release of volatile acetonitrile as a driving force, providing a mild entry into selective
hydrolysis of nitriles.
