Experimental investigations of a partial Ru-O bond during the metal-ligand bifunctional addition in noyori-type enantioselective ketone hydrogenation

Article abstract of DOI:10.1021/ja202732q

The transition state for the metal-ligand bifunctional addition step in Noyori's enantioselective ketone hydrogenation was investigated using intramolecular trapping experiments. The bifunctional addition between the Ru dihydride trans-[Ru((R)-BINAP)(H)₂((R,R)-dpen)] and the hydroxy ketone 4-HOCH₂C₆H₄(CO)CH₃ at -80 °C exclusively formed the corresponding secondary ruthenium alkoxide trans-[Ru((R)-BINAP)(H)(4-HOCH₂C₆H₄CH(CH ₃)O)((R,R)-dpen)]. Combined with the results of control experiments, this observation provides strong evidence for the formation of a partial Ru-O bond in the transition state.

Full text of DOI:10.1021/ja202732q

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pubs.acs.org/JACS
Experimental Investigations of a Partial RuꢀO Bond during the
MetalꢀLigand Bifunctional Addition in Noyori-Type Enantioselective
Ketone Hydrogenation
Satoshi Takebayashi, Nupur Dabral, Mark Miskolzie, and Steven H. Bergens*
Department of Chemistry, University of Alberta, Edmonton, Alberta, Canada T6G 2G2
S Supporting Information
b
ABSTRACT: The transition state for the metalꢀligand
bifunctional addition step in Noyori’s enantioselective ke-
tone hydrogenation was investigated using intramolecular
trapping experiments. The bifunctional addition between
the Ru dihydride trans-[Ru((R)-BINAP)(H)2((R,R)-dpen)]
and the hydroxy ketone 4-HOCH2C6H4(CO)CH3 at
ꢀ80 °C exclusively formed the corresponding secondary
ruthenium alkoxide trans-[Ru((R)-BINAP)(H)(4-HOCH2C6-
H4CH(CH3)O)((R,R)-dpen)]. Combined with the results
of control experiments, this observation provides strong
evidence for the formation of a partial RuꢀO bond in the
transition state.
The bifunctional addition between dihydride 1 and the ketone
has been studied mostly by means of density functional theory
calculations on model compounds such as trans-[Ru(PH3)2-
(H)2(ethylenediamine)]11 and direct experimental studies
of related compounds such as Ru(η6-p-cymene)(H)((S,S)-
TsDPEN) [TsDPEN = N-(p-toluenesulfonyl)-1,2-diphenyl-
ethylenediamine]9c,14 trans-[Ru((R)-BINAP)(H)2(H2NCMe2-
CMe2NH2)] [BINAP = 2,20-bis(diphenylphosphino)-1,
10-binaphthyl],9b,12a and [2,5-Ph2-3,4-Ar2(η5-C4COH)]Ru(CO)2H
(Ar = Ph, 4-MeC6H4).9a,15,16 The conclusions of these studies
support the bifunctional addition (eq 1).
mong the most important advances in enantioselective
A
catalysis is Noyori’s discovery of the carbonyl hydrogena-
tion catalyst system trans-[Ru(diphosphine)(Cl)2(diamine)] þ
base.1 Many bifunctional catalyst systems for asymmetric
hydrogenations2 and transfer hydrogenations3 have been
developed on the basis of this discovery. A wide variety of
ketones have been hydrogenated with remarkable activities and
enantioselectivities using these catalysts in academia1c,3b,4 and
industry,5 forming important bioactive compounds.1b,5 In recent
developments, less reactive carbonyl compounds, including
esters, imides, and amides, have also been hydrogenated using
these catalyst systems.6 Further, active iron-catalyzed carbonyl
hydrogenations and transfer hydrogenations are also being
developed.7,8
We recently reported the low-temperature, high-yield pre-
paration and study of the Noyori ketone hydrogenation catalyst
trans-[Ru((R)-BINAP)(H)2((R,R)-dpen)] (1a) (eq 2).12c We
discovered that the addition between 1a and 1 equiv of aceto-
phenone was complete upon mixing at ꢀ80 °C. Unexpectedly,
the products of the addition at ꢀ80 °C were not the correspond-
ing Ru amide Ru((R)-BINAP)(H)((R,R)-HNCHPhCHPhNH2)
(2a) and 1-phenylethanol. Instead, the corresponding Ru alk-
oxide 3 was formed without an observable intermediate
(eq 2).12d Alkoxides such as 3 undergo facile base-assisted
elimination of the alkoxide ligand at ꢀ80 °C to generate amide
2a. The amide rapidly and reversibly adds H2 to form 1a at
ꢀ80 °C. Ru alkoxides have been observed experimentally with
related systems,17,9b and computational studies have predicted
their presence as well.11aꢀc,f
The mechanisms of these hydrogenations are being studied by
several research groups using methods that include kinetics
of product formation,9 deuterium exchange studies,10 computa-
tional studies,11 and stoichiometric reactions of intermediates
and model compounds.12 The prominent feature of these studies
is the metalꢀligand bifunctional addition, first proposed by
Noyori et al.1b Specifically, the hydridic RuꢀH and the protic
NꢀH in catalysts such as 1 add to the carbon and oxygen,
respectively, of the carbonyl group via a six-membered pericyclic
transition state to form the product alcohol and Ru amide 2
(eq 1). This step accounts for the high activity and CdO/CdC
selectivity of 18-electron species such as 1 toward these hydro-
genations. Amide 2 then undergoes addition of dihydrogen to
regenerate 1. Alcohol-assisted variants of this mechanism have
been also proposed.13,11b
Scheme 1 illustrates the likely pathways for the formation of
alkoxide 3. The first is the conventional bifunctional addition to
form amide 2a and the alcohol, that then rapidly react to form 3
Received: March 25, 2011
Published: June 02, 2011
r
2011 American Chemical Society
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|

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Scheme 1. Possible Mechanisms for the Formation of 3
unambiguously identify the product(s) because of overlapping
and broad signals in the NMR spectra.
Unexpectedly, the reaction between the hydroxylactone trapp-
ing agent 4b and 1a resulted in exclusive rapid formation of Ru
1
alkoxide 7b and H2 (eq 3). 7b was characterized by H, 31P,
13
1
1
1Hꢀ C gHSQC, and Hꢀ H gCOSY NMR experiments. 7b
was produced through reversible formation of amide 2a from 1a
via the elimination of H2. As mentioned above, our previous
isotope-labeling studies showed that addition of H2 to 2a forms
1a in a rapid and reversible manner at ꢀ78 °C. Although the
addition is reversible, it strongly favors the dihydride. The
reactivity of the lactone carbonyl in 4b toward the addition with
1a is lower than that of ketone carbonyls.6f The relatively low
reactivity at the lactone carbonyl explains why this trapping agent
reacted with the small amount of amide 2a instead of undergoing
the expected bifunctional addition.
Figure 1. Structures of intramolecular trapping agents.
[pathway (a) in Scheme 1]. We note that the addition between
1a and acetophenone in the presence of excess 2-PrOH as an
intermolecular trap formed 3 exclusively.12d The result of this
intermolecular trapping experiment can be explained if 2a and
1-phenylethanol are formed within a solvent cage or with a
sufficiently strong hydrogen bond between the OH group of the
alcohol and the nitrogen of the amide ligand [pathway (b) or (c)
in Scheme 1]. Casey proved that a combination of pathways (b)
and (c) occurs in the hydrogenation of imines catalyzed by [2,
5-Ph2-3,4-(4-MeC6H4)2(η5-C4COH)]Ru(CO)2H.15
Conversely, the bifunctional addition reaction between di-
hydride 1a and ketone 4c was rapid upon thawing at ꢀ80 °C and
formed Ru alkoxides 5c and 7c in 43 and 57% yield, respectively
(eq 4). This reaction was carried out a total of four times with the
same result. Most importantly, no formation of trapped alkoxide 6c
was observed.
Previously, Casey and B€ackvall reported intramolecular trap-
ping experiments to elucidate the mechanism for hydrogena-
tion of imines catalyzed by [2,5-Ph2-3,4-Ar2(η5-C4COH)]Ru-
(CO)2H (Ar = Ph, 4-MeC6H4).15,16 We now report the first such
intramolecular trapping experiments for the Noyori ketone
hydrogenation to investigate the mechanism of the bifunctional
addition with this system.
We prepared solutions of Ru dihydride 1a for this study
by reacting mixtures of trans-[Ru((R)-BINAP)(H)(L)((R,R)-
dpen)]BF4 (L= η2-H2 or THF-d8), 0.7ꢀ1 equiv of KN
(Si(CH3)3)2, and H2 (∼2 atm) at ꢀ78 °C in THF-d8 in an
NMR tube.12c Less than 1 equiv of KN(Si(CH3)3)2 was used in
order to avoid the presence of excess base, that can catalyze the
elimination of the alkoxide ligand from the Ru alkoxide
products.18 As such base-assisted eliminations are rapid even at
ꢀ78 °C, they would erase the kinetic regiochemistry of the
bifunctional addition.12c For the intramolecular trapping experi-
ments, layers of frozen THF-d8 solutions of the intramol-
ecular trapping agent (top) and dihydride 1a (bottom) were
thawed and mixed under ∼2 atm H2 at ꢀ80 °C in a precooled
NMR probe.
A series of intramolecular trapping agents were prepared and
screened to find a suitable candidate (Figure 1). Consistent with
the high activity of 1a toward the reduction of acetophenone, the
trapping agent 4a rapidly underwent bifunctional addition with
Ru dihydride 1a upon thawing at ꢀ80 °C to form the corre-
sponding Ru alkoxide product(s). However, it was impossible to
The alkoxide 7c was formed via the same mechanism as dis-
cussed for the formation of 7b (eq 3). Thus, elimination of H2
from 1a formed Ru amide 2a, that rapidly reacted with the
alcohol group in the unreduced ketoneꢀalcohol 4c to form
alkoxide 7c. Despite the rapid nature of the addition of ketone
groups to 1a, more than 50% of 1a reacted through addition of
the primary alcohol trap in 4c with amide 2a. This bias toward
alcohol addition is explained by two factors. First, sterically less
hindered primary alcohols coordinate more strongly than sec-
ondary alcohols. Second, primary alcohols are more acidic than
secondary alcohols.19 The formation of 7c does not impact the
mechanism of formation of 5c, the net RuꢀH insertion product.
The identities of the Ru alkoxides 5c, 6c, and 7c were determined
1
1
13
unambiguously by means of H, 31P, Hꢀ C gHSQC, and
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1
1Hꢀ H gCOSY NMR experiments. Ketoneꢀprimary alkoxide
Scheme 2. Proposed Transition State with a Partial RuꢀO
7c was also prepared independently by the reaction between Ru
amide 2a and 4c (eq 5).
Bond
between the carbonyl carbon and the hydride on Ru has an
electrophilic component that is promoted by hydrogen bonding
between the NꢀH group and the carbonyl oxygen. Thus, the
interaction is similar to addition of CO2 to an 18-electron metal
hydride and alkyl. We note that Darensbourg has established a
precedent for partial metalꢀoxygen bond formation during
the electrophilic attack of CO2 on 18-electron metalꢀalkyl
complexes.20 Further, this pathway is one of those proposed
for the Ru(H)2(phosphine)4-catalyzed hydrogenation of CO2.21
For example, Perutz reported that addition between cis-[Ru-
(H)2(Me2PCH2CH2PMe2)2] and CO2 at ꢀ50 °C quickly
formed the corresponding Ru carboxylate products in toluene-
d8 without dissociation of the diphosphine ligands.21a It is
therefore reasonable to propose a transition state that contains
a partial RuꢀO bond during the bifunctional addition of ketones
to 1a (Scheme 2).
In a control competition experiment, Ru amide 2a and 1 equiv
of the diol product (ꢀ)-8 (97% ee) were reacted in THF-d8 at
ꢀ78 °C. The (ꢀ)-enantiomer was chosen in order to match the
major product expected to form upon addition of 1a and 4c at
ꢀ80 °C. We reported that the stoichiometric addition between
1a and acetophenone at ꢀ80 °C in THF-d8 forms (S)-(ꢀ)-
1-phenylethanol in 83% ee and that the optical rotation of the
major enantiomer from hydrogenation of 4c is (ꢀ).12d Surpris-
ingly, the reaction of amide 2a with diol (ꢀ)-8 at ꢀ80 °C in
THF-d8 exclusively formed 6c, the product of addition of the
primary alcohol group, which was the species that was not formed
by the bifunctional addition of 4c to 1a (eq 6).
A continuum between pathway (c) in Scheme 1 and the
pathway in Scheme 2 is likely the current best description of the
metalꢀligand bifunctional addition. The strength of the partial
RuꢀN double bond (Scheme 1) and RuꢀO bond (Scheme 2)
depends on the particulars (ligand structure, solvent, substrate,
etc.) of each hydrogenation system. A similar conclusion has
been drawn for the mechanisms of late transition metal-catalyzed
CꢀH bond activation, which has been proposed to proceed via a
continuum between σ-bond metathesis and an oxidative addition/
reductive elimination sequence.22
In conclusion, this paper presents the first intramolecular trapping
experiments to elucidate a reaction mechanism for the formation of
Ru alkoxides in Noyori carbonyl hydrogenations. The results prove
that the solvent cage mechanism [pathway (b) in Scheme 1] is not
operative during the addition between 1a and 4c under these
conditions. Thus, the current most probable mechanisms are those
involving the formation of a sufficiently strong hydrogen bond
between amide 2a and the product alcohol [(c) in Scheme 1]
and/or the concerted formation of Ru alkoxides (Scheme 2).
As discussed above, the high selectivity toward the formation
of the primary alkoxide is explained by the higher acidity and
lower steric crowding of the primary alcohol. Thus, there exists a
strong bias toward the formation of primary alkoxide 6c over
secondary alkoxide 5c in this reaction. Therefore, if amide 2a and
diol 8 were formed within a solvent cage during the bifunctional
addition between dihydride 1a and alcoholꢀketone 4c, primary
alkoxide 6c should have been formed form as a major product as a
result of molecular tumbling within the solvent cage. The
exclusive formation of secondary alkoxide 5c upon addition
between 1a and 4c is conclusive evidence that the reaction path
via the formation of the free amide and diol within a solvent cage
(Scheme 1b) does not operate during the addition of 4c to 1a
under these conditions.
On the basis of these intramolecular trapping experiments, the
closest fit among the pathways for the bifunctional addition
shown in Scheme 1 is (c), that proceeds via the formation of a
hydrogen bond between the amide nitrogen in 2a and the
secondary alcohol proton in the product that is sufficiently
strong to prevent the formation of the favored primary alkoxide
6c (Scheme 1c). Formation of such a hydrogen bond must be
associated with diminished π donation from the amide nitrogen
to Ru, thereby resulting in coordination unsaturation at Ru.
Further, this hydrogen bond results in a partial negative charge
on oxygen. Such coordination unsaturation would promote the
formation of a RuꢀO bond during the addition. The interaction
’ ASSOCIATED CONTENT
S
Supporting Information. Experimental procedures and
b
characterization of compounds. This material is available free of
charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
steve.bergens@ualberta.ca
’ ACKNOWLEDGMENT
This work was supported by the Natural Sciences and
Engineering Research Council of Canada (NSERC) and the
University of Alberta.
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