Catalytic Activation of Unstrained, Nonactivated Ketones Mediated by Platinum(II): Multiple C-C Bond Cleavage and CO Extrusion

Article abstract of DOI:10.1021/acs.organomet.9b00650

The complexes [Pt(tolpy)Cl(L)] (tolpy = 2-(4-tolyl)pyridyl; L = dmso, dms, py, PPh₃, CO) are precursors for the catalytic cleavage of C-C bonds and extrusion of CO from a series of unactivated ketones such as cyclohexanone; deuterium labeling experiments demonstrate the involvement of a transfer hydrogen step in the mechanism.

Full text of DOI:10.1021/acs.organomet.9b00650

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Cite This: Organometallics XXXX, XXX, XXX−XXX
Catalytic Activation of Unstrained, Nonactivated Ketones Mediated
by Platinum(II): Multiple C−C Bond Cleavage and CO Extrusion
Julia P. Sarju,† Debashish Dey,† Javier Torroba,† Adrian C. Whitwood,† Kelly Redeker,‡
and Duncan W. Bruce*,†
†Department of Chemistry, University of York, Heslington, York YO10 5DD, U.K.
‡Department of Biology, University of York, Wentworth Way, York YO10 5DD, U.K.
S
* Supporting Information
ABSTRACT: The complexes [Pt(tolpy)Cl(L)] (tolpy = 2-
(4-tolyl)pyridyl; L = dmso, dms, py, PPh3, CO) are precursors
for the catalytic cleavage of C−C bonds and extrusion of CO
from a series of unactivated ketones such as cyclohexanone;
deuterium labeling experiments demonstrate the involvement
of a transfer hydrogen step in the mechanism.
Scheme 1. Examples of Metal-Mediated C−C Activation of
Ketonic Substrates with Strained10 or Tethered4,16,26
Substrates
atalytic decarbonylation reactions have the potential to be
Csynthetically useful, atom-economical transformations for
a commonly encountered functional group, potentially affording
compounds that might otherwise be difficult to access.1−6 Such
reactions are well-known and trivial with aldehydes, but ketones
present a significantly greater challenge.1 For example, Gozin et
al. and Ameen et al. highlighted the importance of transition-
metal-catalyzed C−C bond cleavage in petroleum refinement
and the processing of oxygen-rich bioderived fuels.7,8
Stoichiometric extrusion of a carbonyl fragment was first
reported in 1965 by Rusina and Vlcek when cyclohexanone was
heated with RhCl3 and PPh3, affording [RhCl(CO)(PPh3)2]
and unknown hydrocarbon products.9 Since then, most
examples of ketone decarbonylation have employed strained
substrates10−13 or those that allow tethering of the substrate to
the metal via a directing group (e.g., Scheme 1arylpyridine
example).14−19 These reactions, which are normally stoichio-
metric, are often mediated by coordinatively unsaturated
rhodium complexes,20,21 with forcing conditions, high temper-
atures, and long reaction times being required.22−24 However, in
2016 Lee et al. achieved ketone Cα−CCO bond activation of a
number of systems, including cyclohexanone, at room temper-
ature using a rhodium(II) porphyrin system generating Rh(II)-
acyl complexes. While this approach does not lead to
decarbonylation, the mild reaction conditions reflect a
significant improvement on the earlier results.25 More recently,
Dong and co-workers have developed an elegant catalytic
strategy for Cα−CCO bond activation in which the carbonyl is
activated by catalytic amounts of amine or carbene and the
thermodynamic driving force is provided by concomitant
formation of new C−C bonds.26
made that heating [Pt(tolpy)(Cl)(S-dmso)] (1; tolpy = 2-(4-
tolyl)pyridyl) in air with cyclohexanone afforded the known32
complex [Pt(tolpy)(Cl)(CO)] (2; Scheme 2), characterized by
νCO 2098 cm−1, δ(195Pt) −3947 ppm and by single-crystal X-ray
methods (see the Supporting Information for details). With 1
and cyclohexanone as the only reagents, the carbonyl ligand
evidently came from decarbonylation of cyclohexanone with
accompanying C−C bond cleavage. To the best of our
knowledge, this transformation has remained undeveloped
since the report of cyclohexanone decarbonylation by Rusina
and Vlcek.9 More than 50 years on from their report, our
As part of earlier studies on ortho-platinated 2,5-bis(4-
alkoxyphenyl)pyridines,27 PtIII dimers were formed unexpect-
edly via oxidation of the PtII complexes by free or bound dmso in
the presence of acid, consistent with related oxidations reported
previously by Kukushkin et al.28,29 and Alexandrova et al.30,31 In
further studies of this reaction, the surprising observation was
Received: October 1, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.organomet.9b00650
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Organometallics
Communication
with a neutral chelating ligand, were also evaluated and neither 9
nor 10 showed any evidence of decarbonylation. Complex 8 did,
however, show the formation of butane and the starting complex
was the only one that could be identified at the end of the
reaction. Palladium analogues of 1 and 7 were not active,
suffering extensive decomposition even at low reaction temper-
atures.
Scheme 2. Platinum-Catalyzed Activation of
Cyclohexanone
a
Returning to the organic products, the reaction stoichiometry
shows a significant imbalance in hydrogen content to the extent
of four hydrogen atoms:
a
The transformation involves the cleavage of the three C−C bonds
indicated. The X-ray single crystal structure of 2 is illustrated.
C6H10O (cyclohexanone)
observation provides an opportunity to study this trans-
formation in greater detail.
→ C4H10 + CH4 + CO (≡ C6H14O)
With no other reagents present, this hydrogen must arise from
the cyclohexanone substrate, suggesting the possibility of a
transfer hydrogenation process. While cyclohexanone is not
generally known for its utility in such chemistry, Izawa et al. did
show some precedent for such a reaction, reporting a palladium-
mediated oxidation/aromatization of cyclohexanone to phe-
nol.35
Therefore, to demonstrate a transfer hydrogenation step, the
reaction of cyclohexanone with complex 1 was repeated in the
presence of 1,3-cyclohexadiene, which is well-known as a
transfer hydrogenation donor.36 At the end of the reaction,
benzene (formed when cyclohexadiene acts as a transfer
hydrogenation substrate) was observed as a product by
electronic spectroscopy, strongly supporting the involvement
of transfer hydrogenation.
Further compelling evidence for the mediation of transfer
hydrogenation comes from the observation that, while acetone
does not react thermally with complex 1, when the reaction is
carried out in the presence of 1,3-cyclohexadiene, formation of 2
along with methane and CO is observed. Likewise it is found
that acetophenone will only react in the presence of the transfer
hydrogenation substrate. Furthermore, carrying out the reaction
with acetone-d6 led to the formation of CD3H, confirming that
1,3-cyclohexadiene is the source of the hydrogen atoms
incorporated in the product. This observation is significant, as
there is only a single previous report of the decarbonylation of
acetone.37
Thus, GC and GC-MS analysis of the headspace gas after
reaction of cyclohexanone with 1 confirmed the formation of
butane and methane, with trace amounts of other small
hydrocarbons (see Scheme 2), as detailed in the Supporting
Information. In fact, the reaction of cyclopentanone with 1
under the same conditions showed the formation of methane
and propane, thereby suggesting that both substrates are
activated via the same mechanism. Rusina and Vlcek9 did not
report an analysis of any organic products, and so their reaction
was repeated, revealing the formation of butane, cyclohexane,
cyclohexene, and benzene in the approximate ratio of 1:0.8:1:3,
implying a different mechanistic route in their case.
Given the known photochemical instability of cyclohexanone
and cyclopentanone in the gas phase,33 and reports of
photochemical C−C bond activations,34 control experiments
established that activation of cyclohexanone mediated by 1
proceeded in the absence of natural light to give the same
products. In addition, irradiation of a room-temperature
solution of 1 in cyclohexanone (λmax 254 nm) at room
temperature led to no observable reaction, confirming that the
decarbonylation is thermal in nature.
To investigate the reaction further, it was carried out in the
presence of the related complexes 2−6 (Figure 1) which, like 1,
have a neutral fourth ligand. Complex 6 was found to be inactive,
while 2−5 led to reactions, and important in these observations
is that 2 is active, showing that the process is catalytic. The
related dimer 7 was also active. The Pt complexes 8−10, each
An important observation, however, is that the activation of
these ketones by complex 1 is accompanied by the formation of
particulate Pt with average hydrodynamic radius 10−50 nm, as
determined by dynamic light scattering. Addition of mercury to
the reaction mixture38 led to a significant reduction in the
formation of butane, suggesting intervention by these platinum
nanoparticles. However, when the reaction is tracked by in situ
infrared spectroscopy, immediate formation of complex 2 is
observed, which would suggest that an early decarbonylation
step is homogeneous (a heterogeneous process would typically
require an induction period as the nanoparticles form). In
addition, it is observed that, when cyclohexanone is heated with
either 6 or 10, nanoparticles are formed but there is no
decarbonylation. This would also support the proposal of an
early, homogeneous decarbonylation step. Transfer hydro-
genation, however, may be mediated by the platinum nano-
particles.
Previous examples of C−C bond scission with coupled
decarbonylation (e.g., Scheme 1) have typically led to simple
extrusion of CO, and while no mechanistic detail was given, the
fact that the present reaction with cyclohexanone give a different
range of products would imply a different mechanism. Thus, it is
Figure 1. Complexes tested for the thermal decarbonylation of ketones.
B
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Communication
assumed that the ketone coordinates to Pt (either η1 through
oxygen or η2 through CO) following dissociation of the
neutral, monodentate ligand (L in Scheme 2), after which the
C−C bond α to the carbonyl is cleaved (analogy with the
formation of metal-acyl complexes in the RhII-porphyrin
systems).25 A second C−C cleavage β to CO would then
extrude an oxygenated C2 unit (which would readily fragment to
CO and methane) leaving a C4 moiety.
Support for these suggestions comes from deuterium cross-
labeling studies conducted using cyclohexanone-d10 with 1,3-
cyclohexadiene as the transfer hydrogen donor. Under these
conditions, the major C1 product was CD2H2 (64%), which
suggests a fairly selective reaction (consistent with a
homogeneous process), while butane was found as a mixture
of isotopomers, mainly C4D5H5 (32%), C4D6H4 (32%), and
C4D7H3 (28%), more consistent with a relatively unselective
heterogeneous process. Similarly, use of 2,2,6,6-tetradeuter-
iocyclohexanone and 1,3-cyclohexadiene gave 72% CD2H2
(again quite selective) and 89% C4H10. Some of the key
mechanistic observations are collected in Scheme 3.
Details of complex synthesis and of the different reactions
(PDF)
Accession Codes
CCDC 1956985 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge via
www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_
request@ccdc.cam.ac.uk, or by contacting The Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge
CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail for D.W.B.: duncan.bruce@york.ac.uk.
ORCID
Julia P. Sarju: 0000-0002-7087-4382
Adrian C. Whitwood: 0000-0002-5132-5468
Duncan W. Bruce: 0000-0002-1365-2222
Author Contributions
The manuscript was written through contributions of all
authors.
Scheme 3. Complexes Tested for the Thermal
Decarbonylation of Ketones
a
Funding
We thank the University of York (J.P.S.) and Comunidad
Auntonoma de La Rioja (Spain) (J.T.) for funding.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Iman Khazal is thanked for the preparation of platinum
complexes 8−10 used in this study, and Dr. Jason Lynam
(York) is thanked for very helpful discussions.
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* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.9b00650.
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