Cell-permeable photoswitchable small molecules, termed optojasps, are introduced to optically control the dynamics of the actin cytoskeleton and cellular functions that depend on it. These light-dependent effectors were designed from the F-actinstabilizing marine depsipeptide jasplakinolide by functionalizing them with azobenzene photoswitches. As demonstrated, optojasps can be employed to control cell viability, cell motility, and cytoskeletal signaling with the high spatial and temporal resolution that light affords. Optojasps can be expected to find applications in diverse areas of cell biological research. They may also provide a template for photopharmacology targeting the ubiquitous actin cytoskeleton with precision control in the micrometer range.
Parahydrogen (p‐H2) induced polarization (PHIP) NMR spectroscopy showed that [CpXRu] complexes with greatly different electronic properties invariably engage propargyl alcohol derivatives into gem‐hydrogenation with formation of pianostool ruthenium carbenes; in so doing, less electron rich CpX rings lower the barriers, stabilize the resulting complexes and hence provide opportunities for harnessing genuine carbene reactivity. The chemical character of the resulting ruthenium complexes was studied by DFT‐assisted analysis of the chemical shift tensors determined by solid‐state 13C NMR spectroscopy. The combined experimental and computational data draw the portrait of a family of ruthenium carbenes that amalgamate purely electrophilic behavior with characteristics more befitting metathesis‐active Grubbs‐type catalysts.
The unusual geminal hydrogenation of ap ropargyl alcohol derivative with [Cp X RuCl] as the catalyst entails formation of pianostool ruthenium carbenes in the first place; these reactive intermediates can be intercepted with tethered alkenes to give either cyclopropanes or cyclic olefins as the result of aformal metathesis event. The course of the reaction is critically dependent on the substitution pattern of the alkene trap.Recent investigations into the trans-hydrogenation of internal alkynes with the aid of [Cp*Ru]-based catalysts (Cp* = pentamethylcyclopentadienyl) showed that the perplexing stereochemical outcome of this reaction can be reached along two competing pathways (Scheme 1). [1][2][3][4] Theroutes bifurcate
The
only recently discovered
gem
-hydrogenation
of internal alkynes is a fundamentally new transformation, in which
both H atoms of dihydrogen are transferred to the same C atom of a
triple bond while the other position transforms into a discrete metal
carbene complex. [Cp*RuCl]
4
is presently the catalyst of
choice: the resulting piano-stool ruthenium carbenes can engage a
tethered alkene into either cyclopropanation or metathesis, and a
prototypical example of such a reactive intermediate with an olefin
ligated to the ruthenium center has been isolated and characterized
by X-ray diffraction. It is the substitution pattern of the olefin
that determines whether metathesis or cyclopropanation takes place:
a systematic survey using alkenes of largely different character in
combination with a computational study of the mechanism at the local
coupled cluster level of theory allowed the preparative results to
be sorted and an intuitive model with predictive power to be proposed.
This model links the course of the reaction to the polarization of
the double bond as well as to the stability of the secondary carbene
complex formed, if metathesis were to take place. The first application
of “hydrogenative metathesis” to the total synthesis
of sinularones E and F concurred with this interpretation and allowed
the proposed structure of these marine natural products to be confirmed.
During this synthesis, it was found that
gem
-hydrogenation
also provides opportunities for C–H functionalization. Moreover,
silylated alkynes are shown to participate well in hydrogenative metathesis,
which opens a new entry into valuable allylsilane building blocks.
Crystallographic evidence suggests that the polarized [Ru–Cl]
bond of the catalyst interacts with the neighboring R
3
Si
group. Since attractive interligand Cl/R
3
Si contacts had
already previously been invoked to explain the outcome of various
ruthenium-catalyzed reactions, including
trans
-hydrosilylation,
the experimental confirmation provided herein has implications beyond
the present case.
Enynes with a tethered carbonyl substituent are converted into substituted furan derivatives upon hydrogenation using [Cp*RuCl]4 as the catalyst. Paradoxically, this transformation can occur along two distinct pathways, each of which proceeds via discrete pianostool ruthenium carbenes. In the first case, hydrogenation and carbene formation are synchronized (“gem‐hydrogenation”), whereas the second pathway comprises carbene formation by carbophilic activation of the triple bond, followed by hydrogenative catalyst recycling. Representative carbene intermediates of either route were characterized by X‐ray crystallography; the structural data prove that the attack of the carbonyl group on the electrophilic carbene center follows a Bürgi–Dunitz trajectory.
gem
-Hydrogenation of an internal alkyne with the
aid of [Cp*RuCl]
4
as the precatalyst is a highly unorthodox
transformation, in which one C atom of the triple bond is transformed
into a methylene group, whereas the second C atom gets converted into
a ruthenium carbene. In the case of 1,3-enynes bearing a propargylic
steering substituent as the substrates, the reaction occurs regioselectively,
giving rise to vinyl carbene complexes that adopt interconverting
η
1
/η
3
-binding modes in solution;
a prototypical example of such a reactive intermediate was characterized
in detail by spectroscopic means. Although both forms are similarly
stable, only the η
3
-vinyl carbene proved kinetically
competent to insert into primary, secondary, or tertiary C–H
bonds on the steering group itself or another suitably placed ether,
acetal, orthoester, or (sulfon)amide substituent. The ensuing net
hydrogenative C–H insertion reaction is highly enabling in
that it gives ready access to spirocyclic as well as bridged ring
systems of immediate relevance as building blocks for medicinal chemistry.
Moreover, the reaction scales well and lends itself to the formation
of partly or fully deuterated isotopologues. Labeling experiments
in combination with PHIP NMR spectroscopy (PHIP = parahydrogen induced
polarization) confirmed that the reactions are indeed triggered by
gem
-hydrogenation, whereas kinetic data provided valuable
insights into the very nature of the turnover-limiting transition
state of the actual C–H insertion step.
The unusual geminal hydrogenation of a propargyl alcohol derivative with [CpXRuCl] as the catalyst entails formation of pianostool ruthenium carbenes in the first place; these reactive intermediates can be intercepted with tethered alkenes to give either cyclopropanes or cyclic olefins as the result of a formal metathesis event. The course of the reaction is critically dependent on the substitution pattern of the alkene trap.
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