Aldehyde binding through reversible C-C coupling with the pincer ligand upon alcohol dehydrogenation by a PNP-ruthenium catalyst

Article abstract of DOI:10.1021/ja303121v

Primary alcohol dehydrogenation by a PNP-Ru(II) catalyst was probed by low-temperature NMR experiments. Facile dehydrogenation occurred at -30 °C, but the resulting aldehydes were not found in solution, as they were trapped by the catalyst through a new mode of metal-ligand cooperation involving Ru-O coordination and an unusual, highly reversible C-C coupling with the PNP pincer ligand.

Full text of DOI:10.1021/ja303121v

Communication
pubs.acs.org/JACS
Aldehyde Binding through Reversible C−C Coupling with the Pincer
Ligand upon Alcohol Dehydrogenation by a PNP−Ruthenium
Catalyst
Michael Montag, Jing Zhang,† and David Milstein*
Department of Organic Chemistry, Weizmann Institute of Science, Rehovot 76000, Israel
S
* Supporting Information
The couplings of alcohols into esters and alcohols and
ABSTRACT: Primary alcohol dehydrogenation by a
PNP−Ru(II) catalyst was probed by low-temperature
NMR experiments. Facile dehydrogenation occurred at
−30 °C, but the resulting aldehydes were not found in
solution, as they were trapped by the catalyst through a
new mode of metal−ligand cooperation involving Ru−O
coordination and an unusual, highly reversible C−C
coupling with the PNP pincer ligand.
amines into amides, which are prototypical examples of catalysis
by the PNP− and PNN−ruthenium complexes, both involve
the dehydrogenation of a primary alcohol as a key step. It is
widely accepted that this is initiated by addition of the hydroxyl
O−H bond to the dearomatized catalyst, giving a rearomatized,
coordinatively saturated alkoxo complex (Scheme 1, X =
OCH2R). The alkoxide ligand is purported to undergo
subsequent β-hydride elimination, with concomitant generation
of the corresponding aldehyde (OCHR) and formation of a
trans-dihydrido complex (X = H) bearing an aromatic pyridine
moiety. The latter complex ultimately eliminates H2, thereby
regenerating the dearomatized catalyst and completing the first,
common phase of the two coupling reactions.
It is believed that in the following stages the liberated
aldehyde reacts with an available alcohol or amine to afford the
corresponding hemiacetal or hemiaminal intermediate in either
a catalyzed or uncatalyzed reaction. These intermediates are
then dehydrogenated by the catalyst to yield the final coupling
products, i.e., ester or amide. This putative sequence of
reactions, from the initial O−H addition to the final
dehydrogenation step, is based on the known chemistry of
PNP− and PNN−ruthenium complexes, but this has largely
been observed under ambient or catalytic conditions (typically
>100 °C), under which the proposed reaction intermediates are
unobservable.7 This prompted us to examine the catalytic
coupling reactions at low temperatures in an attempt to gain
new insights into their underlying mechanisms. In this
communication, we describe the results of a low-temperature
NMR study of dehydrogenative alcohol coupling by the PNP−
Ru(II) catalyst 1 (Scheme 2),8 focusing on aldehyde formation
and release. Most intriguingly, this examination revealed that
the aldehyde obtained by alcohol dehydrogenation is not
simply released into solution, nor does it coordinate to the
ruthenium center in an η1-O or η2-C,O fashion. Instead, it is
effectively captured by the catalyst through a new mode of
metal−ligand cooperation involving an unusual reversible C−C
coupling with the olefinic pincer ligand arm.
ne of the significant challenges facing modern catalysis is
O
the design of environmentally benign processes that will
replace the existing waste-producing reactions, many of which
are of fundamental synthetic importance. Our group has
developed a number of pyridine-based PNP- and PNN-type
ruthenium pincer complexes that efficiently catalyze a range of
industrially important C−O and C−N coupling reactions,
requiring no activating agents and generating hydrogen as the
only byproduct.1,2 These reactions include the direct coupling
of alcohols into esters2a,b,h as well as the coupling of alcohols
and amines into amides.2c,g The reverse reactions, namely,
direct hydrogenation of various carbonyl-containing substrates,
can also be efficiently promoted by these catalysts.2h,3,4 These
transformations are linked to the unique ability of the PNP- and
PNN-based catalysts to activate chemical bonds by metal−
ligand cooperation involving reversible dearomatization of the
pyridine backbone (Scheme 1). In these systems, under both
Scheme 1. Activation of an X−H bond (X = H, O, N) by
Metal−Ligand Cooperation in PNP- or PNN-Based
Ruthenium Catalysts (L = NR2, PR2; R = alkyl, aryl)
As shown in Scheme 2, addition of ethanol (EtOH) or
benzyl alcohol (BnOH) to a toluene-d8 solution of complex 1
at −80 °C afforded the corresponding alkoxo complex 2a or 2b,
respectively. These reactions, which are compatible with the
above-mentioned mechanism, were found to be rapid but
catalytic and noncatalytic conditions,5 the activation of an X−H
bond (X = H, O, N) occurs at the dearomatized, coordinatively
unsaturated complex.6 The bond is added across the metal−
ligand framework, with the Xδ− fragment coordinating to the
metal center and Hδ+ binding to the olefinic pincer ligand arm,
thereby rearomatizing the pyridine moiety.
Received: April 3, 2012
Published: June 15, 2012
© 2012 American Chemical Society
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Communication
reaction generally requires a vacant coordination site cis to the
alkoxide ligand, but in complexes 2a and 2b the cis positions
are occupied by CO and the tris-chelating bisphosphine PNP
ligand, neither of which is labile, particularly at low temper-
atures. We believe that two alternative, nonclassical dehydro-
genation mechanisms are plausible in the PNP−Ru system. The
first was previously proposed by our group for a coordinatively
saturated Ir(III)−alkoxo complex16 and involves full dissocia-
tion of the anionic alkoxide, followed by hydride transfer from
the dissociated alkoxide to the metal center. This mechanism is
favored by excess alcohol and has also been invoked by
Ozerov17 and Goldberg18 to explain apparent β-hydride
elimination in other late-transition-metal alkoxo complexes.
The second mechanism, which was recently suggested for a
PNN−Ru(II) catalyst on the basis of computational results,7e
involves direct hydrogen transfer from the alcohol to the
dearomatized complex without prior formation of an alkoxo
complex, in a manner similar to Noyori’s bifunctional
mechanism.19 Both mechanisms can account for the low-
temperature reactivity of the present PNP−Ru system,20 and
their relevance is currently being examined experimentally.21
Regardless of the exact mechanism, alcohol dehydrogenation
by complex 1 was expected to afford the free aldehyde, as well
as the previously reported rearomatized trans-dihydrido
complex 3 (Scheme 2).5b Indeed, as the dehydrogenation of
EtOH or BnOH progressed at −30 °C, complex 3 was
observed. However, no significant amount of free aldehyde was
detected by NMR spectroscopy. Instead, the solution was
found to contain a new complex that was identified as 4
(Scheme 2), an aldehyde adduct of complex 1 formed by
addition of the aldehyde across the metal−ligand framework
through Ru−O coordination and, unexpectedly, C−C coupling
with the pincer ligand arm. Complexes 3 and 4 emerged in
solution simultaneously and in equimolar amounts and reached
high yields within 1 h at −30 °C.22
Scheme 2. Low-Temperature Reactions of the Dearomatized
PNP−Ru(II) Complex 1 with Alcohols
highly reversible (see below), and an excess of each alcohol was
required to drive the reaction forward (e.g., a 3-fold excess gave
∼90% yield).9 The alkoxo complexes exhibited nearly identical
31P{1H} NMR spectra at −80 °C, featuring a singlet at ∼85
ppm.10 This is consistent with two symmetric phosphine
moieties in each complex, indicative of a mirror plane along the
Ru−N axis and perpendicular to the pyridine ring. This mirror
1
symmetry was also reflected in the H NMR spectra of these
complexes and, together with the presence of only two pyridine
signals in the range 6.6−7.0 ppm,11 clearly demonstrated the
1
aromatic nature of the pyridine moiety. The H NMR spectra
of 2a and 2b also featured hydride signals at −16.19 ppm (t,
2
2JPH = 20.6 Hz) and −16.54 ppm (t, JPH = 20.9 Hz),
respectively, both of which are within the typical chemical shift
range for a hydride ligand trans to an alkoxide moiety in
structurally similar Ru(II) complexes.3c,12,13 The 1H and
13C{1H} NMR signals associated with the alkoxide ligand in
2a were unequivocally identified by using EtOH 13C-labeled at
the 1-position, and the corresponding signals for 2b were
identified by analogy.
The identity of 4 was verified through independent synthesis
by treating a toluene-d8 solution of 1 with pure aldehyde at −70
°C (Scheme 3). The resulting acetaldehyde (MeCHO) and
Scheme 3. Reversible Trapping of Aldehydes by Complex 1
The fact that alcohol addition to complex 1 is a reversible
process was apparent upon addition of just 1 equiv of alcohol,
which led to only partial conversion accompanied by
considerable line broadening in both the H and 31P NMR
1
signals of complexes 1 and 2. However, unequivocal evidence
for this reversibility, even in the presence of excess alcohol, was
obtained by the spin saturation transfer (SST) technique.14
Selective spin saturation of the methylene hydrogens on the
alkoxide ligand of complex 2 resulted in a substantial decrease
1
benzaldehyde (PhCHO) adducts, complexes 4a and 4b,
respectively, exhibited nearly identical 31P{1H} NMR spectra
at this temperature, each featuring two AB doublets at ∼105
and ∼111 ppm (2JPP ≈ 260 Hz),23 a signal pattern that is
consistent with two asymmetric phosphine moieties. Both
in the intensity of the methylene H NMR signal of the free
alcohol, clearly demonstrating that the two species were
engaged in a dynamic equilibrium. Facile reversibility of the
alcohol addition reaction was observed at temperatures as low
as −80 °C,15 and was found to play an important role in the
reactivity of the PNP−Ru(II) system as the temperature of the
solution was allowed to rise (see below).
When the reaction mixture containing either 2a or 2b was
warmed to −30 °C, the alkoxo complex disappeared as alcohol
dehydrogenation ensued. The very fact that this reaction
occurred well below ambient temperature brings into question
the relevance of the conventional alkoxide β-hydride elimi-
nation mechanism to the PNP−Ru system. This type of
1
complexes also gave rise to a hydride signal in the H NMR
spectrum, at −15.30 ppm (dd, 2JPH = 22.0 Hz, 2JPH = 12.4 Hz)
for 4a and −15.78 ppm (dd, 2JPH = 22.6 Hz, 2JPH = 13.2 Hz) for
4b. As in the case of alkoxo complexes 2a and 2b, these
chemical shifts are consistent with a hydride trans to an
alkoxide.3c,12,13 Furthermore, as in the alkoxo complexes, the
1H NMR spectra of 4a and 4b indicated an aromatic pyridine
moiety, with signals in the range 6.1−7.0 ppm. This was further
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Communication
corroborated by the 15N NMR chemical shifts of these
complexes (273 and 275 ppm, respectively), which are very
close to that of aromatic complex 3 (270 ppm) and significantly
downfield of that of dearomatized complex 1 (204 ppm).
Lastly, the most unusual structural aspect of 4a and 4b, namely,
the fact that the aldehyde spans the metal−ligand framework
with C−C coupling to the pincer ligand, was confirmed by
using aldehydes with 13C-labeled carbonyl groups. The large
13C−13C coupling constant between the labeled carbon and the
pincer-arm carbon (∼25 Hz for both complexes) verified the
existence of a C−C bond.
The appearance of complex 4 can be rationalized by taking
into account the high reversibility of the reaction 1 + alcohol ⇄
2, which means that even when excess alcohol is used, the
dearomatized complex is still present in solution. Therefore,
upon alcohol dehydrogenation, the eliminated aldehyde can be
trapped by 1 to yield 4. This was independently demonstrated
by treating a toluene-d8 solution of 2a or 2b at −70 °C with
pure MeCHO or PhCHO, respectively, and observing that
alcohol displacement took place in both cases to afford 4a or 4b
within minutes. The absence of any substantial amount of free
aldehyde during the low-temperature dehydrogenation of the
alcohols attests to the rapidity of aldehyde sequestration by 1,
which is decidedly faster than the dehydrogenation step itself.
This was clearly illustrated by the reaction of 1 with either
MeCHO or PhCHO at −70 °C (Scheme 3), which
quantitatively yielded complex 4a or 4b within minutes,9
even when a subequivalent amount of the aldehyde was added.
By contrast, the reaction of 1 with the corresponding alcohols
at the same temperature yielded the alkoxo complexes, with no
significant dehydrogenation taking place within several hours.
The high rate of aldehyde capture was also evident from the
fact that during alcohol dehydrogenation at −30 °C, complexes
3 and 4 emerged simultaneously and in equimolar amounts as
described above, even when excess alcohol was used.
The low-temperature capture of MeCHO by 1 exhibited
reversibility similar to that of PhCHO, being observable by SST
experiments above −50 °C. Nevertheless, unlike the PhCHO
system, when the solution containing 4a was warmed to room
temperature, the regeneration of complex 1 and free aldehyde
was accompanied by rapid side reactions that gave a mixture of
complexes and organic products. Cooling this mixture to −70
°C did not cleanly regenerate 4a.
In conclusion, we have described new mechanistic details of
the dehydrogenation of primary alcohols by a PNP−Ru(II)
catalyst (1). These findings, based on a low-temperature NMR
study, shed new light on a key step that is common to various
dehydrogenative coupling reactions involving alcohols that are
catalyzed by PNP- and PNN-based ruthenium complexes. 1
was found to easily activate primary alcohols at −80 °C,
yielding the corresponding alkoxo complexes in accordance
with conventional wisdom. However, facile alcohol dehydro-
genation was already observed at −30 °C, a temperature far
lower than in a typical catalytic process, thereby raising the
possibility that dehydrogenation does not occur by the
commonly invoked β-hydride elimination mechanism, since
coordinative unsaturation is unlikely. More intriguingly, it was
discovered that the aldehyde obtained during alcohol
dehydrogenation is efficiently trapped by complex 1 through
unusual C−C coupling with the PNP pincer ligand. This
unexpected reaction, which was found to be highly reversible,
constitutes a new mode of metal−ligand cooperation. Further
experimental work is underway to examine the role of the
aldehyde adduct complexes (4) in the catalytic alcohol coupling
cycle, as well as other aspects of this remarkable process.
ASSOCIATED CONTENT
* Supporting Information
■
S
Experimental procedures and NMR data for new compounds.
This material is available free of charge via the Internet at
http://pubs.acs.org.
Remarkably, as facile and efficient as it appeared, the low-
temperature trapping of aldehydes by 1 was found to be
reversible upon warming, as was most clearly exemplified with
PhCHO. When a toluene-d8 solution of 4b at −70 °C was
gradually warmed, aldehyde elimination took place beginning at
about −50 °C, resulting in an equilibrium mixture of 4b, 1, and
the free aldehyde.24 The reversible nature of the reaction at this
and higher temperatures was unequivocally demonstrated by
SST experiments. When the solution was warmed to room
temperature, complex 1 and the free aldehyde were fully
regenerated, leaving no detectable amounts of 4b, even when a
2-fold excess of the aldehyde was present. Cooling the solution
back to −70 °C gave 4b as the only observable complex. Since
aldehyde binding to 1 occurs through C−C coupling with the
olefinic pincer ligand arm, the above observations clearly show
that C−C bond cleavage in the PNP−Ru system is a fully
reversible, highly facile reaction. Such ligand-based C−C bond
lability is unprecedented in pincer systems and is relatively rare
in other metal complexes.25,26 The role of the metal center in
promoting this type of reactivity is illustrated by referring to 2-
(2-hydroxy-2-phenylethyl)pyridine, an analogue of the alde-
hyde-coupled ligand framework in 4b. This organic molecule
has been previously reported to undergo similar C−C cleavage
through a retro-ene-type reaction to afford 2-methylpyridine
and PhCHO.27 However, unlike the PNP−Ru system, this
reaction was irreversible and required heating at 170 °C
(diglyme solution), with a half-life of 14 h under these harsh
conditions.
AUTHOR INFORMATION
Corresponding Author
david.milstein@weizmann.ac.il
■
Present Address
†College of Chemistry and Molecular Science, Wuhan
University, Wuhan 430072, P. R. China.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This research was supported by the European Research Council
under the FP7 Framework (ERC 246837), by the Minerva
Foundation, and by the Kimmel Center for Molecular Design.
D.M. holds the Israel Matz Professorial Chair of Organic
Chemistry.
REFERENCES
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obtained in ∼70% yield within the same time period.
(23) The NMR spectra of 4a and 4b exhibited significant chemical
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bonding. The data presented here were measured in the absence of
alcohol.
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aldehyde below this temperature was inferred from SST data.
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result of H-bonding. The NMR data presented here correspond to
∼50 mM residual alcohol.
(11) By comparison, the dearomatized complex 1 gave rise to
1
“pyridine” H NMR signals at 5.5−6.6 ppm.
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(13) An attempt was made to corroborate the existence of a Ru−O
bond by NMR techniques, utilizing EtOH enriched with 10% 17O.
However, no 17O−1H coupling was observed for the hydride ligand,
nor was 17O−31P coupling observed for the phosphorus atoms.
Moreover, variable-temperature 17O NMR spectra of the complex
exhibited no observable signals due to line broadening.
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