Conspectus
The most
important means for tuning and improving
a catalyst’s
properties is the delicate exchange of the ligand shell around the
central metal atom. Perhaps for no other organometallic-catalyzed
reaction is this statement more valid than for ruthenium-based olefin
metathesis. Indeed, even the simple exchange of an oxygen atom for
a sulfur atom in a chelated ruthenium benzylidene about a decade ago
resulted in the development of extremely stable, photoactive catalysts.
This Account presents our perspective on the development of dormant
olefin metathesis catalysts that can be activated by external stimuli
and, more specifically, the use of light as an attractive inducing
agent.
The insight gained from a deeper understanding of the
properties
of
cis
-dichlororuthenium benzylidenes opened the
doorway for the systematic development of new and efficient light-activated
olefin metathesis catalysts and catalytic chromatic-orthogonal synthetic
schemes. Following this, ways to disrupt the ligand-to-metal bond
to accelerate the isomerization process that produced the active precatalyst
were actively pursued. Thus, we summarize herein the original thermal
activation experiments and how they brought about the discoveries
of photoactivation in the sulfur-chelated benzylidene family of catalysts.
The specific wavelengths of light that were used to dissociate the
sulfur–ruthenium bond allowed us to develop noncommutative
catalytic chromatic-orthogonal processes and to combine other photochemical
reactions with photoinduced olefin metathesis, including using external
light-absorbing molecules as “sunscreens” to achieve
novel selectivities. Alteration of the ligand sphere, including modifications
of the N-heterocyclic carbene (NHC) ligand and the introduction of
cyclic alkyl amino carbene (CAAC) ligands, produced more efficient
light-induced activity and special chemical selectivity. The use of
electron-rich sulfoxides and, more prominently, phosphites as the
agents that induce latency widened the spectrum of light-induced olefin
metathesis reactions even further by expanding the colors of light
that may now be used to activate the catalysts, which can be used
in applications such as stereolithography and 3D printing of tough
metathesis-derived polymers.