Metal-Ligand cooperation on a diruthenium platform: Selective imine formation through acceptorless dehydrogenative coupling of alcohols with amines

Article abstract of DOI:10.1002/chem.201304403

Metal-metal singly-bonded diruthenium complexes, bridged by naphthyridine-functionalized N-heterocyclic carbene (NHC) ligands featuring a hydroxy appendage on the naphthyridine unit, are obtained in a single-pot reaction of [Ru₂(CH₃COO)₂(CO)₄] with 1-benzyl-3-(5,7-dimethyl-1,8-naphthyrid-2-yl)imidazolium bromide (BIN-HBr) or 1-isopropyl-3-(5,7-dimethyl-1,8-naphthyrid-2-yl)imidazolium bromide (PIN-HBr), TlBF₄, and substituted benzaldehyde containing an electron-withdrawing group. The modified NHC-naphthyridine-hydroxy ligand spans the diruthenium unit in which the NHC carbon and hydroxy oxygen occupy the axial sites. All the synthesized compounds catalyze acceptorless dehydrogenation of alcohols to the corresponding aldehydes in the presence of a catalytic amount of weak base 1,4-diazabicyclo[2.2.2]octane (DABCO). Further, acceptorless dehydrogenative coupling (ADHC) of the alcohol with amines affords the corresponding imine as the sole product. The substrate scope is examined with 1 (BIN, p-nitrobenzaldehyde). A similar complex [Ru₂(CO) ₄(CH₃COO)(3-PhBIN)][Br], that is devoid of a hydroxy arm, is significantly less effective for the same reaction. Neutral complex 1 a, obtained by deprotonation of the hydroxy arm in 1, is found to be active for the ADHC of alcohols and amines under base-free conditions. A combination of control experiments, deuterium labeling, kinetic Hammett studies, and DFT calculations support metal-hydroxyl/hydroxide and metal-metal cooperation for alcohol activation and dehydrogenation. The bridging acetate plays a crucial role in allowing β-hydride elimination to occur. The ligand architecture on the diruthenium core causes rapid aldehyde extrusion from the metal coordination sphere, which is responsible for exclusive imine formation. Ligand lends a hand: Metal-hydroxy/hydroxide and metal-metal cooperation is demonstrated for acceptorless dehydrogenation of alcohols to give aldehydes. The ligand architecture ensures rapid extrusion of the aldehyde from the metal core, resulting in the formation of the corresponding imine as the sole coupled product with amines (see scheme; DABCO=1,4-diazabicyclo[2.2.2]octane).

Full text of DOI:10.1002/chem.201304403

DOI: 10.1002/chem.201304403
Full Paper
&
Dehydrogenation
Metal–Ligand Cooperation on a Diruthenium Platform: Selective
Imine Formation through Acceptorless Dehydrogenative Coupling
of Alcohols with Amines
Biswajit Saha, S. M. Wahidur Rahaman, Prosenjit Daw, Gargi Sengupta, and
Jitendra K. Bera*[a]
Dedicated to Dr. Ganesh Pandey on the occasion of his 60th birthday
Abstract: Metalꢀmetal singly-bonded diruthenium com-
plexes, bridged by naphthyridine-functionalized N-hetero-
cyclic carbene (NHC) ligands featuring a hydroxy appendage
on the naphthyridine unit, are obtained in a single-pot reac-
tion of [Ru2(CH3COO)2(CO)4] with 1-benzyl-3-(5,7-dimethyl-
1,8-naphthyrid-2-yl)imidazolium bromide (BIN·HBr) or 1-iso-
propyl-3-(5,7-dimethyl-1,8-naphthyrid-2-yl)imidazolium bro-
mide (PIN·HBr), TlBF4, and substituted benzaldehyde contain-
ing an electron-withdrawing group. The modified NHC-
naphthyridine-hydroxy ligand spans the diruthenium unit in
which the NHC carbon and hydroxy oxygen occupy the axial
sites. All the synthesized compounds catalyze acceptorless
dehydrogenation of alcohols to the corresponding alde-
hydes in the presence of a catalytic amount of weak base
1,4-diazabicyclo[2.2.2]octane (DABCO). Further, acceptorless
dehydrogenative coupling (ADHC) of the alcohol with
amines affords the corresponding imine as the sole product.
The substrate scope is examined with 1 (BIN, p-nitrobenzal-
dehyde). A similar complex [Ru2(CO)4(CH3COO)(3-PhBIN)][Br],
that is devoid of a hydroxy arm, is significantly less effective
for the same reaction. Neutral complex 1a, obtained by de-
protonation of the hydroxy arm in 1, is found to be active
for the ADHC of alcohols and amines under base-free condi-
tions. A combination of control experiments, deuterium la-
beling, kinetic Hammett studies, and DFT calculations sup-
port metal–hydroxyl/hydroxide and metal–metal coopera-
tion for alcohol activation and dehydrogenation. The bridg-
ing acetate plays a crucial role in allowing b-hydride elimina-
tion to occur. The ligand architecture on the diruthenium
core causes rapid aldehyde extrusion from the metal coordi-
nation sphere, which is responsible for exclusive imine
formation.
Introduction
In metal-catalyzed reactions, the catalytic activity is mainly
metal-based, whereas the ligand plays important roles in mod-
ulating electron density on the metal and controlling the steric
environment around it. However, there is an emerging class of
catalysts in which the ligand actively participates in bond-
breaking and bond-making processes, causing a reversible
structural transformation of the catalyst during substrate acti-
vation and product formation.[1] Synergistic cooperation be-
tween metal and ligand is recognized to promote superior cat-
alytic activity both in natural and synthetic systems.[2]
The concept of metal–ligand cooperativity has been exploit-
ed to develop a new generation of bifunctional catalysts.
Noyori’s catalyst (Scheme 1, I), which utilizes metal–amine/
Scheme 1. Metal–ligand cooperating catalysts.
metal–amide interconversion for the hydrogenation of carbon-
yl and carbonyl-type compounds, inspired much activity in this
area.[3] The interplay between Lewis acidic Rh and coordinated
amide nitrogen in Grꢀtzmacher’s catalyst (II) is linked to dehy-
drogenative coupling of primary alcohols with water, metha-
nol, or amine.[4] Milstein’s elegant PNP–Ru (III) and PNN–Ru (IV)
systems display an interesting mode of cooperation involving
[a] B. Saha, S. M. Wahidur Rahaman, P. Daw, G. Sengupta, Prof. J. K. Bera
Department of Chemistry
Center for Environmental Science and Engineering
Indian Institute of Technology Kanpur, Kanpur 208016 (India)
Fax: (+91)512-2597436
E-mail: jbera@iitk.ac.in
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201304403.
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ligand aromatization/dearomatization, which is exploited for
catalytic dehydrogenative synthesis of esters, acetals, imines,
amines, and amides from alcohols and the reverse reactions.[5]
Aliphatic PNP pincer complexes catalyze transfer hydrogena-
tion of ketones and imines, hydrogenation of esters, and dehy-
drogenation of ammonia–boron adducts.[6] Shvo’s catalyst (V)
dehydrogenates secondary alcohols following a simultaneous
b-elimination and proton transfer to the metal.[7] Although the
original catalyst is a dinuclear complex, the active species con-
tains a single Ru.[8] Yamaguchi and Fujita reported an Ir catalyst
that contains hydroxy pyridine for acceptorless dehydrogena-
tion of secondary alcohols.[9] Morris et al. have performed de-
tailed mechanistic investigations of alcohol-assisted outer-
sphere hydrogenation of ketones catalyzed by a Ru–NHC com-
plex incorporating an amine appendage (VI).[10] Gelman intro-
duced a dibenzobarrelene-based PC(sp3)P pincer ligand (VII)
that features -CH2OH arms.[11] Intramolecular cooperation be-
tween the metal and the hydroxide functionality brings about
the acceptorless dehydrogenative coupling (ADHC) of alcohols
with amines.[12]
Scheme 4. CꢀC bond-forming reaction through aldol-type addition on
a [Ru2(CO)4] core.
and carbonyl compounds on
a
[Ru2(CO)4]2+ framework
(Scheme 4).[15] In this work, electron-deficient aromatic benzal-
dehydes were found to work well with naphthyridine-NHC li-
gands. Four new diruthenium(I) complexes are described bear-
ing ligands that offers a protonic arm at the site trans to the
RuꢀRu bond (Scheme 5). These compounds activate the alco-
hol and catalyze ADHC of alcohols with amines for exclusive
formation of the corresponding imine. The metal–ligand coop-
eration mechanism is also examined in this work.
Recent activities on bifunctional catalysis have fo-
cused on the discovery of new cooperation modes
and the fabrication of novel molecular platforms.
Carefully designed metal–ligand cooperation has the
potential to realize better performing catalysts and
enable the development of new reactions. We have
been exploring bimetallic reactivity with the inten-
tion of developing useful catalysts based on bimetal-
lic systems.[13] Towards this end, naphthyridine-func-
tionalized N-heterocyclic
carbene (NHC) ligands
Scheme 5. Syntheses of diruthenium–NHC complexes bearing a protonic arm at the axial
site (present work).
have been incorporated
on a metal–metal singly
bonded [RuI–RuI] plat-
Results and Discussion
form.[14] Site-directed anchoring of
the ligand afforded a bridged com-
plex VIII (Scheme 2) featuring an
accessible axial site that was ex-
ploited for carbene-transfer cataly-
sis to a variety of substrates.[14] We
proposed that the introduction of
a protonic arm (ꢀXH; X=O, N)
might result in cooperative action
Single-pot reaction between [Ru2(CO)4(CH3COO)2], 1-benzyl-3-
(5,7-dimethyl-1,8-naphthyrid-2-yl)imidazolium
bromide
(BIN·HBr) or 1-isopropyl-3-(5,7-dimethyl-1,8-naphthyrid-2-yl)imi-
dazolium bromide (PIN·HBr), TlBF4, and benzaldehydes contain-
ing an electron-withdrawing group in the para position, pro-
vided compounds 1–4. Metal coordination of the BIN or PIN
ligand is vital prior to CꢀC coupling. Isolation of [Ru2(CO)4-
(CH3COO)(3-PhBIN)](Br) (VIII) confirmed the bridging disposi-
tion of the ligand on the diruthenium(I) core.[14] TlBF4 removed
the axial bromide, and subsequent aldol-type CꢀC bond for-
mation with different aldehydes gave the desired compounds.
It must be noted here that only electron-deficient benzalde-
hydes containing an electron-withdrawing nitro, cyanide, or tri-
fluoromethyl group afforded coupled products; attempts to
use ketones or electron-rich aldehydes failed. These results are
in contrast to CꢀC coupled products obtained with a variety of
ketones and 2-methyl-naphthyridine on the [Ru2(CO)4]2+
core.[15] Natural population analysis charge (NPA) calculations
on [Ru2(CO)4(CH3COO)(3-PhBIN)]Br show that NHC-based li-
gands make the Ru centers sufficiently electron rich,[14] but fails
to activate ketones or electron-rich aldehydes efficiently. An-
other plausible explanation involves the trans NHC unit, which
Scheme 2. Napthyridine–NHC
bridged diruthenium(I)
complex.
with the metal at the axial site, as illustrated in Scheme 3.
Functionalization at the ortho-position of the naphthyridine
unit is a difficult exercise. This was, however, previously ach-
ieved through the use of an aldol-type CꢀC bond-formation re-
action between the naphthyridine ortho-methyl substituent
Scheme 3. Proposed metal–ligand interplay at axial site of a diruthenium
platform.
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Figure 1. Molecular structure (40% probability thermal ellipsoids) of 1 with
important atoms labeled. Hydrogen atoms are omitted for clarity. Selected
bond lengths [ꢂ] and angles [8]: Ru1ꢀRu2 2.667(1), Ru2ꢀC20 2.061(5), Ru1ꢀ
C2 1.833(6), Ru1ꢀO5 2.134(3), Ru1ꢀN4 2.192(4), Ru1ꢀO9 2.338(4), Ru2ꢀC1
1.843(5). C4-Ru1-Ru2 95.3(2), O5-Ru1-Ru2 82.9(1), N4-Ru1-Ru2 85.3(2), O9-
Ru1-Ru2 152.8(1), C20-Ru2-O6 83.1(2), C20-Ru2-N3 77.3(2), C20-Ru2-Ru1
157.7(2), O6-Ru2-Ru1 84.2(9), C5-O6-Ru2 121.6(3), C31-O9-Ru1 121.8(3).
Figure 2. Simulated (dotted line) and experimental (solid line) mass distribu-
tions for [1ꢀBF4]+.
a stoichiometric amount of oxidant, thereby providing a non-
polluting route to aldehydes and related products.[17] The ac-
cessible protonic arm at the axial site of the diruthenium–NHC
complexes prompted us to study possible bifunctional accept-
orless dehydrogenation of alcohols to aldehydes. Reaction of
1 mmol benzyl alcohol and 1 mol% catalyst 1 in toluene at
reflux for 24 h in the presence of 5 mol% KOH afforded 98%
conversion into benzaldehyde. More importantly, addition of
1.2 mmol benzylamine and 4 ꢂ molecular sieves to this reac-
tion afforded N-benzylidene benzylamine as a single product
in high yield (>90%). First we screened a range of bases in
the model reaction involving 1 mmol benzyl alcohol and
1.2 mmol benzylamine. Interestingly, conversions were very
similar for a range of bases, including KOH, KOtBu, NaH, 1,8-
diazabicyclo[5.4.0]undec-7-ene (DBU) and 1,4-diazabicyclo-
[2.2.2]octane (DABCO) (Table 1). Only 5 mol% base was suffi-
cient to afford maximum conversion with 1 mol% catalyst.
Subsequent reactions were performed with DABCO as base.
Molecular sieves were essential for imine formation; perform-
ing the reaction in their absence gave a mixture of products
(aldehyde, amine and imine). The reaction was not efficient at
room temperature and best results were obtained in toluene
at reflux (1108C). Compounds 2–4 were also employed for the
model reaction under the optimized conditions and they af-
forded similar conversions (92–95%; Table 2). Remaining stud-
ies were carried out with 1 because of its easy accessibility as
a pure, crystalline product.
prohibits strong axial binding of the aldehyde and means that
the carbonyl carbon is not electrophilic enough to be attacked
by the methyl carbon.
The X-ray structure of [Ru2(L1)(CO)4(CH3COO)][BF4], in which
L1 =2-[7-(3-benzyl-1H-imidazol-1-ylidene)-5-methyl-1,8-naph-
thyridin-2-yl]-1-(4-nitrophenyl)ethanol (1; Figure 1) revealed the
formation of a modified naphthyridine-NHC ligand L1 through
CꢀC bond formation between BIN and p-nitrobenzaldehyde.
The diruthenium unit is spanned by the ligand and is addition-
ally bridged by an acetate unit. Two carbonyl groups are ori-
ented cis to each ruthenium. The RuꢀRu distance 2.667(1) ꢂ is
similar to that previously reported for the naphthyridine-NHC
bridged diruthenium complex Ru2(CO)4(CH3COO)(3-PhBIN)Br
(VIII; 2.691(1) ꢂ).[14] The carbene carbon is axially coordinated
and the second axial site is occupied by a weakly bound OH
group, with a RuꢀO distance of 2.338(4) ꢂ. The strong trans
effect of the carbene carbon leads to a longer RuꢀO distance
compared with [Ru2(CO)4(L’)2][BF4]2 (IX; L’=2-methyl-1-(1,8-
naphthyridin-2-yl)propan-2-ol) (2.244(6) ꢂ).[15] Similar com-
pounds were achieved for p-nitrobenzaldehyde (2; Figure S1 in
the Supporting Information), p-cyanobenzaldehyde (3; Fig-
ure S3), and p-trifluoromethylbenzaldehyde (4; Figure S5) with
PIN as NHC ligand. All four compounds feature a common hy-
droxy arm at the axial site of the Ru2(CO)4 core.
The 1H NMR spectrum of 1 reveals an interesting pattern
(Figure S7). The methylene ’CH2’ protons show two apparent
AB quartet signals centered at d=4.03 and 4.35 ppm with
2J(H,H) values 14.4 and 14.2 Hz, respectively. The appearance of
the AB pattern is due to a comparable chemical shift difference
The substrate scope was then examined under the opti-
mized conditions (catalyst 1 (1 mol%), alcohol/amine (1:1.2
mol), DABCO (5 mol%), 4 ꢂ molecular sieves, toluene, reflux,
24 h). Electron-rich p-methoxybenzyl alcohol gave excellent
conversions into the corresponding imines with benzylamine
(96%; Table 3, entry 1), p-methylbenzylamine (94%; entry 2),
cyclohexylamine (94%; entry 3), and hexylamine (91%; entry 4).
Substrates p-methylbenzyl alcohol and benzyl alcohol afforded
good yields with a range of amines (92–71%; entries 6–15).
Electron-deficient p-nitrobenzyl alcohol gave relatively lower
yields compared with electron-rich alcohols (entries 16 and 17).
Reaction of 2-phenylethanol with benzylamine or hexylamine
provided 86 and 82% imine (entries 18 and 19). Long-chain al-
1
and coupling constant. The remaining H signals are consistent
with its solid-state structure. The carbene carbon is observed
at d=173 ppm in the 13C NMR spectrum, and the ESI-MS ex-
hibits the expected signal at m/z 842, which is assigned for
[1ꢀBF4]+ (Figure 2).
Catalysts that perform acceptorless dehydrogenation (AD)
are of significant importance.[16] The process involves extraction
of hydrogen from unreactive alcohol without the need for
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Table 1. Screening of base for imine formation catalyzed by 1.[a]
Table 3. (Continued)
Entry Substrate
Substrate
Product
Yield
[%][b]
Entry
Base
Conversion [%][b]
1
2
3
4
5
KOH
KOtBu
NaH
DBU
DABCO
95
95
94
92
92
7
8
9
87
85
85
[a] Reaction conditions: benzyl alcohol (1 mmol), benzylamine (1.2 mmol),
base (5 mol%), catalyst 1 (1 mol%), toluene, 4 ꢂ molecular sieves, reflux,
24 h. [b] Conversions determined by GC analysis.
10
79
11
12
90
87
Table 2. Catalyst screening.[a]
13
14
87
84
Entry
Catalyst
Base
Conversion [%][b]
1
2
3
4
5
6
7
8
1
2
3
4
1a
1a
VIII
IX
DABCO
DABCO
DABCO
DABCO
DABCO
–
92
92
91
91
93
66
55
68, 86[d]
15
71
16
17
81
71
[c]
DABCO
DABCO
[a] Reaction conditions (unless mentioned otherwise): benzyl alcohol
(1 mmol), benzylamine (1.2 mmol), DABCO (5 mol%), catalyst (1 mol%),
toluene, 4 ꢂ molecular sieves, reflux, 24 h. [b] Conversions determined by
GC analysis. [c] Without any base. [d] After 48 h at 1308C in p-xylene as
solvent.
18
19
86
82
20
21
22
23
24
25
88
84
83
72
80
72
Table 3. Imine formation from alcohol and amine by catalyst 1.[a]
Entry Substrate
Substrate
Product
Yield
[%][b]
[a] Reaction conditions: alcohol (1 mmol), amine (1.2 mmol), DABCO
(5 mol%), catalyst 1 (1 mol%), toluene, 4 ꢂ molecular sieves, reflux, 24 h.
[b] Yield determined by GC-MS analysis with nonane as internal standard.
1
2
96
94
cohols octanol and hexanol afforded lower yields than benzyl
alcohol when treated with benzylamine and hexylamine (en-
tries 20–23). With 2-ethoxyethanol, benzylamine and hexyla-
mine provided 80 and 72% imine, respectively. Aniline convert-
ed into the corresponding imines when reacted with p-me-
thoxybenzyl alcohol (81%), p-methylbenzyl alcohol (79%), or
benzyl alcohol (71%) (entries 5, 10 and 15).
3
4
94
91
5
6
81
92
The Ru–NHC complex [RuCl2(IiPr)(p-cymene)] (IiPr=1,3-diiso-
propylimidazol-2-ylidene) catalyzes imine formation from alco-
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hols and amines with DABCO,[18] but gave amide in the pres-
ence of strong base KOtBu (15 mol%) and additive PCy3·HBF4
(5 mol%).[19] Bifunctional catalyst 1, however, gave only imine,
irrespective of the nature of the base. Furthermore, 1 also af-
forded an imine in the case of aniline, for which the Ru–NHC
complex failed to give a clean product. Gelman’s bifunctional
catalyst (VII) showed catalytic activity for imine formation ex-
ploiting hydroxy/hydroxide cooperation.[12]
We propose a bifunctional mechanism to account for the
conversion of alcohol into aldehyde. Because neutral 1a exhib-
its substantial activity in the absence of base, it is assumed
that the active catalytic species is the deprotonated form of 1.
The alcohol is activated in a bifunctional manner to form axial
diruthenium-alkoxide, causing the hydroxy arm to open up
(Scheme 7). Subsequent b-hydride elimination of alkoxide pro-
duces the aldehyde and the [Ru–Ru]-H intermediate. In the ab-
sence of amine, the hydride intermediate has been identified
by a characteristic signal at d=ꢀ7.37 ppm in the 1H NMR spec-
trum.[21] The active catalyst is regenerated with the liberation
of hydrogen and the extruded aldehyde reacts with amine to
give the imine as the final product.[22]
Diruthenium(I)
catalyst
[Ru2(CO)4(CH3COO)(3-PhBIN)][Br]
(VIII), which is devoid of a hydroxy appendage, dehydrogenat-
ed benzyl alcohol less efficiently than 1 under identical condi-
tions (59 vs. 98%). Accordingly, lower imine conversion (55%)
was observed for VIII than for 1 (92%) for the model reaction
(Table 2, entry 7). This result implies that the hydroxy append-
age promotes alcohol dehydrogenation. We further employed
catalyst IX, which contains two naphthyridine-based ligands,
but this afforded lower imine conversion (68%). The conver-
sion improved to 86% upon prolonged heating for 48 h at ele-
vated temperature (1308C).
The use of base was essential for this reaction; in the ab-
sence of base, catalyst 1 gave less than 5% conversion for the
reaction between benzyl alcohol and benzylamine, and un-
reacted alcohol and amine were recovered from the reaction
mixture. Obtaining imines from alcohols and amines under
base-free conditions is a challenge met by very few catalysts.[5c]
A neutral complex 1a, which exhibited high solubility in
common organic solvents, was synthesized by reacting 1 with
1
NaH in tetrahydrofuran (Scheme 6), and the H NMR spectrum
Scheme 7. Proposed mechanism for imine formation.
To investigate the mechanism in more detail, we carried out
studies with deuterated alcohol. A model imination reaction in
[D8]toluene did not afford deuterated product, thus ruling out
the possibility of isotope scrambling from solvent. Reaction of
a,a-[D2]-benzyl alcohol with benzylamine gave deuterated N-
benzylidene benzylamine as the major product (93:7 D/H ob-
served by GC-MS analysis; Scheme 8). The conventional metal-
Scheme 6. Synthesis of compound 1a.
of this complex showed a downfield shift of the methylene
protons by Dd=0.16 and 0.28 ppm compared with 1. The re-
maining proton signals in the NMR spectrum were similar to
those of 1. We could also achieve conversion of 1a into 1 in
the presence of alcohol.[20] Compound 1a catalyzed the model
ADHC reaction in the absence of any base with 55% conver-
sion. p-Methoxybenzyl alcohol afforded 68 and 66% imine
products with benzylamine and p-methylbenzylamine, respec-
tively, under base-free conditions, and the reaction between p-
methylbenzyl alcohol and benzylamine gave the correspond-
ing imine in 60% conversion. Thus, the bimetallic Ru system
exhibited moderate activity under acceptorless and base-free
conditions for imine formation, although the yields were lower
than those obtained with Milstein’s PNP–Ru catalyst (III).[5c] The
conversion improved upon the addition of base, finally match-
ing the activity of 1.
Scheme 8. Imination with a,a-[D2]-benzyl alcohol by catalyst 1.
based mechanism involves a RuII-dihydride species, which un-
dergoes reductive elimination followed by oxidative addition
of alcohol to
a
RuII-hydride-alkoxide species (see
Scheme S1).[18,23] Such a process necessarily leads to hydrogen
scrambling in the product. Madsen et al. observed 42% hydro-
gen incorporation for the catalyst [RuCl2(IiPr)(p-cymene)],
which lacks functional attributes for metal–ligand coopera-
tion.[18] The absence of significant isotope scrambling is indica-
tive of a bifunctional mechanism that involves a Ru-monohy-
1
dride intermediate.[24] The H NMR spectrum reveals only one
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signal corresponding to the metal-hydride.[25] The dehydrogen-
ation step involves the metal-hydride and the accessible ligand
proton. The presence of the generated hydrogen gas was con-
firmed by GC (thermal detector) techniques.[26]
relationship was obtained for the plot of ln(c0/c) of substituted
benzyl alcohols against the same values for benzyl alcohol
(Table S1), confirming first-order dependence of the reaction
with respect to alcohol. The slopes of the straight lines gave
relative reactivities (kX/kH), which were plotted against all possi-
One important aspect of catalyst 1 is that it selectively af-
fords imines without any trace of amide or other side-prod-
ucts. It is recognized that the ability of the metal unit to bind
the intermediate product aldehyde determines the product
identity.[5c,27] The amine attacks the metal-coordinated alde-
hyde and gives a hemiaminal that undergoes dehydration to
afford an imine or, alternatively, an amide is obtained that is
generated through a second dehydrogenation process.[27] The
NHC-based ligand architecture on the diruthenium core en-
sures rapid extrusion of aldehyde from the metal coordination
sphere, which, on reaction with amine, produces imine and
water. A reaction of benzyl alcohol with catalyst 1 did not
afford any trace of benzyl benzoate (GC-MS analysis) after 24 h,
supporting the assertion that the aldehyde is rapidly extruded
from the metal core.[23]
ble s values (s+, sꢀ, s.) of a particular substituent.[19a,29]
A
straight line was successfully generated only with s+ with
a negative slope (1=ꢀ0.703) (Figure S9), which suggests the
generation of a positive charge at the benzylic position of the
alcohol, thus supporting the proposition of b-hydride elimina-
tion for the conversion of alcohol into aldehyde.
To gain further mechanistic insight on the b-hydride elimina-
tion and the role of acetate, we carried out DFT calculations at
the B3LYP level of theory. A simplified system was chosen
whereby the benzyl group at the imidazolyl nitrogen was re-
placed by a methyl group and methanol was considered as
the substrate. The optimized structure of the active catalytic
species (Scheme 9, A) showed axial coordination of the hydrox-
ide (Ru2ꢀO2=2.133 ꢂ; aRu1-Ru2-O2=155.48). Methanol addi-
tion resulted in an axial [Ru–Ru]-OCH3 species B (Ru2ꢀO3=
2.148 ꢂ) forcing the arm to open up as the hydroxy. The bridg-
ing acetate then moved away to h1-bound mode, paving the
way for b-elimination. Because we looked for an axial hydride
prior to dehydrogenation, species C was optimized whereby
the methoxide migrated from the axial to the equatorial site
(Ru2ꢀO3=2.044 ꢂ), keeping the axial site open (Ru2···H1=
3.076 ꢂ), and it was calculated to be marginally higher
(0.82 kcalmolꢀ1) in energy than B. The methoxide then
changes its orientation to generate an agostic complex D that
involves CꢀH interaction with the metal (Ru2···H2=2.196 ꢂ).
Subsequent b-hydride elimination leads to an axial [Ru–Ru]-H
species E via transition state TS-DE, corresponding to a free
energy of activation of 16.11 kcalmolꢀ1. The TS-DE that con-
nects D and E was calculated to have a single imaginary fre-
quency of ꢀ394.21 cmꢀ1, and involved movement of one
The key step in the acceptorless alcohol dehydrogenation
reaction is b-hydride elimination, which requires a vacant site
cis to the alkoxide group.[28] The bridging acetate in 1 possibly
moves away from m2 to h1 mode, providing a pathway for the
elimination to occur. This is apparent because complex IX,
which features hydroxy arms but does not contain acetate,
gave lower yields under identical reaction conditions. The con-
version improved on prolonged heating at higher temperature
(Table 2, entry 8). It is assumed that one of the ligands under-
goes demetallation at high temperature, paving the way for
the reaction to occur.
Kinetic Hammett studies were carried out to investigate the
key b-hydride elimination step. Benzyl alcohol was allowed to
compete with para-substituted benzyl alcohols (X=OMe, Me,
F, NO2) in reaction with benzylamine (Scheme S2). The progress
of the reaction was monitored by GC-MS analysis and a linear
Scheme 9. Computed reaction pathway for alcohol dehydrogenation catalyzed by 1.
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methyl proton H2 towards Ru2 and methyl carbon C1 towards
methoxide oxygen O3. The RuꢀH distance in E was calculated
to be 1.688 ꢂ, and the Ru2ꢀO3 (2.219 ꢂ) and Ru2ꢀC1 (2.248 ꢂ)
distances revealed a side-on interaction with the aldehyde. A
more stable intermediate (F; 3.26 kcalmolꢀ1) was also comput-
ed whereby the aldehyde binds through the carbonyl oxygen
(Ru2ꢀO3=2.185 ꢂ; C1ꢀO3=1.159 ꢂ). The acetate reverts to
the bridging mode, expelling aldehyde from the metal coordi-
nation sphere (G), in a process that is energetically favorable
by 8.97 kcalmolꢀ1. The active catalyst A is regenerated by liber-
ation of hydrogen. Thus, catalyst 1 displays both Ru–hydroxy/
hydroxide and RuꢀRu cooperation for alcohol dehydrogena-
tion and subsequent selective imine formation.
Materials
Solvents were dried by conventional methods, distilled under nitro-
gen, and deoxygenated prior to use. RuCl3·nH2O (39% Ru) was pur-
chased from Arora Matthey, India. The compounds Ru2(CO)4-
(CH3COO)2, BIN·HBr, and PIN·HBr were synthesized by following the
literature procedures.[13,14,30] Benzyl alcohol-a,a-[D]2 was purchased
from Sigma-Aldrich and used without further purification.
Synthesis of 1: The ligand precursor BIN·HBr (69 mg, 0.17 mmol)
was added to an acetonitrile solution of Ru2(CH3COO)2(CO)4
(75 mg, 0.17 mmol) and the mixture was stirred at 608C for 48 h.
TlBF4 (49 mg, 0.17 mmol) was then added and stirred for an addi-
tional 30 min. The resulting mixture was filtered through a small
pad of Celite. The clear red solution was completely evaporated
under reduced pressure. The solid mass was dissolved in 15 mL of
dichloromethane. p-Nitrobenzaldehyde (40 mg, 0.26 mmol) was
added to this solution, which was stirred at room temperature for
12 h. The solvent was evaporated to dryness, and 10 mL of hexane
was added with stirring to induce precipitation. The red solid was
isolated, washed with hexane, and dried in vacuo. Crystals suitable
for X-ray diffraction were grown by layering petroleum ether over
a concentrated dichloromethane solution of compound 1 inside an
8 mm o.d. vacuum-sealed glass tube. Yield: 126 mg (78%). M.p.
>2508C; 1H NMR (500 MHz, CD3CN, 294 K): d=8.96 (d, J=9.3 Hz,
1H, NP), 8.49 (d, J=9.2 Hz, 1H, NP), 8.11 (d, J=2.2 Hz, 1H, Imdz),
8.09 (d, J=2.1 Hz, 1H, Imdz), 7.64 (s, 1H, NP), 7.46–7.33 (m, 9H,
2Ph), 5.62 (s, 2H, CH2benzyl), 5.60–5.58 (m, 1H, CHCHOH), 4.35 (dd, J1 =
14.2 Hz, J2 =2 Hz, 1H, CH2), 4.03 (dd, J1 =14.4 Hz, J2 =2 Hz, 1H,
CH2), 2.83 (s, 3H, CH3NP), 2.55 ppm (s, 3H, CH3OAc); 13C NMR
(125.8 MHz, CD3CN, 296.2 K): d=210.2 (CO), 201.8 (CO), 196.6 (CO),
191.6 (CO), 181.8 (OCOOAc), 172.9 (NCNIm), 156.9 (NCNNP), 155.3
(NNPCNIm), 152.3 (CCNNP), 152.3 (CCCNP), 141.2 (CCCNP), 140.8 (CCCNP),
135.6 (CCNPh), 135.3 (CCCNP), 130.5 (NCCIm), 129.2 (CCCPh), 129.0
(CCCPh), 128.8 (CCCPh), 128.5 (CCCPh), 127.9 (CCCPh), 127.6 (CCCbenzyl),
127.3 (CCCbenzyl), 126.9 (CCCbenzyl), 126.6 (CCCbenzyl), 124.4 (CCCbenzyl),
121.4 (CCCbenzyl), 119.5(CCCNP), 117.4 (NImCC), 55.6 (CCO), 49.6
(CCOOAc), 29.8 (CH3NP), 28.1 ppm (CH2NP); IR (KBr): n=2071 (CO),
2025 (CO), 1951 (CO), 1425 (OAc), 1060 cmꢀ1 (BF4); ESI-MS: m/z=
842 corresponding to [1ꢀBF4]+; elemental analysis calcd (%) for
C33H26N5O9BF4Ru2: C 42.82, H 2.83, N 7.57; found: C 42.71, H 2.72, N
7.58.
Conclusion
Catalyst 1 exhibits both metal–metal and metal–ligand cooper-
ation for the dehydrogenative coupling reaction between alco-
hols and amines. Alcohol addition at the axial site followed by
b-hydride elimination produces aldehyde and hydrogen. Deu-
terium labeling and kinetic experiments confirm bifunctional
alcohol activation and dehydrogenation in the catalytic cycle.
A similar diruthenium(I)–NHC complex without the hydroxy ap-
pendage was shown to be a poor catalyst. These results under-
score the metal–ligand cooperation in the activity of catalyst 1.
Furthermore, the bridging acetate on the diruthenium unit
plays an important role in allowing effective b-elimination to
occur. The NHC-based ligand architecture promotes aldehyde
extrusion from the metal coordination sphere, and the alde-
hyde then reacts with amine to give the corresponding imine
with the loss of a water molecule. The inability of the aldehyde
to reside on the dimetal core ensures no side-products such as
amides are produced. The present work demonstrates metal–
hydroxy/hydroxide cooperation that is reminiscent of Gelman’s
system but on a [RuIꢀRuI] platform.[12]
Synthesis of 1a: NaH (5 mg, 2 mmol) was added to [Ru2(L1)(CO)4-
(CH3COO)][BF4] (100 mg, 1 mmol) dissolved in 10 mL of THF and
the mixture was stirred for 6 h at room temperature. The THF was
removed and the sample evaporated to dryness, after which it was
redissolved in 15 mL of dichloromethane. The resulting mixture
was filtered through a small pad of Celite. The resulting filtrate was
concentrated under reduced pressure and petroleum ether was
added to induce precipitation. The violet-colored precipitate was
washed with petroleum ether and dried under vacuum. Yield:
Experimental Section
General Procedures
All reactions with metal complexes were carried out under an at-
mosphere of purified nitrogen by using standard Schlenk vessel
and vacuum line techniques. Infrared spectra were recorded in the
range 4000–400 cmꢀ1 on a Perkin Elmer Spectrum Two with KBr
1
90 mg (90%). M.p. >2508C; H NMR (500 MHz, CD3CN, 294 K): d=
8.96 (d, J=9.3 Hz, 1H, NP), 8.49 (d, J=9.2 Hz, 1H, NP), 8.11 (d, J=
2.2 Hz, 1H, Im), 8.09 (d, J=2.1 Hz, 1H, Im), 7.64 (s, 1H, NP), 7.46–
7.33 (m, 9H, 2Ph), 5.62 (s, 2H, CH2benzyl), 5.60–5.58 (m, 1H, CHCHOH),
4.49 (dd, J1 =14.0 Hz, J2 =2 Hz, 1H, CH2), 4.1 (dd, J1 =14.3 Hz, J2 =
2 Hz, 1H, CH2), 2.83 (s, 3H, CH3NP), 2.55 ppm (s, 3H, CH3OAc);
13C NMR (125.8 MHz, CD3CN, 296.2 K): d=210.2 (CO), 201.8 (CO),
196.6 (CO), 191.6 (CO), 181.3 (OCOOAc), 172.1 (NCNIm), 156.9 (NCNNP),
155.3 (NNPCNIm), 152.3 (CCNNP), 152.3 (CCCNP), 141.2 (CCCNP), 140.8
(CCCNP), 135.6 (CCNPh), 135.3 (CCCNP), 130.5 (NCCIm), 129.2 (CCCPh),
129.0 (CCCPh), 128.8 (CCCPh), 128.5 (CCCPh), 127.9 (CCCPh), 127.6
(CCCbenzyl), 127.3 (CCCbenzyl), 126.9 (CCCbenzyl), 126.6 (CCCbenzyl), 124.4
(CCCbenzyl), 121.4 (CCCbenzyl), 119.5 (CCCNP), 117.4 (NImCC), 55.6 (CCO),
49.4 (CCOOAc), 29.8 (CH3NP), 28.1 ppm (CH2NP); IR (KBr): n=2071 (CO),
1
pellets. H NMR spectra were obtained on a JEOL JNM–LA 400 and
1
500 MHz spectrometer. H NMR chemical shifts were referenced to
the residual hydrogen signal of the deuterated solvents. ESI-MS
were recorded on a Waters Micromass Quattro Micro triple quadru-
pole mass spectrometer. Elemental analyses were performed on
a Thermoquest EA1110 CHNS/O analyzer. GC-MS experiments were
performed on an Agilent 7890A GC and 5975C MS system. The re-
crystallized compounds were powdered, washed several times
with dry diethyl ether or hexane, and dried in vacuo for at least
48 h prior to elemental analyses. Melting points were measured in
open capillaries on a JSGW melting point apparatus and are
uncorrected.
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2025 (CO), 1951 (CO), 1425 cmꢀ1 (OAc); elemental analysis calcd
(%) for C33H25N5O9Ru2: C 47.31, H 3.01, N 8.36; found: C 47.26, H
2.98, N 8.32.
NP), 7.65 (d, J=5.28 Hz, 2H, Ph), 7.57 (d, J=1.1 Hz, 2H, Ph), 7.46
(d, J=8.2 Hz, 2H, Ph), 7.32 (d, J=8.4 Hz, 2H, Ph), 5.19–5.14 (m, 1H,
CHCHOH), 4.06 (dd, J1 =13.9 Hz, J2 =4.6 Hz, 1H, CH2), 3.71 (dd, J1 =
14.0 Hz, J2 =3.2 Hz, 1H, CH2), 2.43 (s, 3H, Me-NP), 2.75–2.76 (m, 1H,
CHiPr), 1.75 (d, J=6.6 Hz, 3H, CH3iPr), 1.71 ppm (d, J=6.7 Hz, 3H,
CH3iPr); 13C NMR (99.5 MHz, CD3CN, 296.2 K): d=208.2 (CO), 203.9
(CO), 202.5 (CO), 184.2 (CO), 178.3 (NCNIm), 165.4 (NCNNP), 163.8
(OCO), 156.1 (NNPCNIm), 155.2 (CCNNP), 151.4 (CCCNP), 145.2 (CCCNP),
141.8 (CCCNP), 135.7 (CCNPh), 129.7 (CCCNP), 128.2 (NCCIm), 126.4
(CCCPh), 125.2 (CCCPh), 125.2 (CCCPh), 125.1 (CCCPh), 122.6 (CCCPh),
122.5 (CCCPh), 120.6 (CCCPh), 118.5 (CCCPh), 118.5 (CCCPh), 117.4
(CCCPh), 112.7 (CCCPh), 54.5 (FCC), 48.9 (NCCiPr), 23.2 (CCCiPr),
21.5 ppm (CCCiPr); 19F NMR (372.5 MHz, CD3CN, 294 K): d=ꢀ63.0
(CF3C6H4COO), ꢀ63.5 (CF3L4), ꢀ151.7 (BF4); IR (KBr): n=2074 (CO),
Synthesis of 2: Compound 2 was synthesized following a similar
procedure employed for the synthesis of 1 by using PIN·HBr
(60 mg, 0.17 mmol), Ru2(CO)4(CH3COO)2 (75 mg, 0.17 mmol), TlBF4
(49 mg, 0.17 mmol), and p-nitrobenzaldehyde (40 mg, 0.26 mmol).
Crystals suitable for X-ray diffraction were grown by layering
hexane over a concentrated dichloromethane solution of 2 inside
an 8 mm o.d. vacuum-sealed glass tube. Yield: 113 mg (74%). M.p.
>2508C; 1H NMR (400 MHz, CD3CN, 294 K): d=8.94 (d, J=9.2 Hz,
1H, NP), 8.16 (d, J=9.2 Hz, 1H, NP), 8.10 (d, J=2.3 Hz, 1H, Im), 8.0
(d, J=8.7 Hz, 2H, Ph), 7.90 (d, J=8.3 Hz, 2H, Ph), 7.79 (s, 1H, NP),
7.57 (d, J=2.4 Hz, 1H, Im), 5.03–5.09 (m, 1H, CHCHOH), 4.35 (dd, J1 =
14.2 Hz, J2 =2 Hz, 1H, CH2), 4.01 (dd, J1 =14.2 Hz, J2 =2 Hz, 1H,
CH2), 2.82 (s, 3H, Me-NP), 2.75–2.76 (m, 1H, CHiPr), 2.46 (s, 3H,
CH3OAc), 1.64 (s, 3H, CH3iPr), 1.63 ppm (s, 3H, CH3iPr); 13C NMR
(99.5 MHz, CD3CN, 296.2 K): d=211.1 (CO), 206.5 (CO), 202.7 (CO),
191.9 (CO), 189.4 (NCNNP), 184.6 (NNPCNIm), 180.4 (OCOOAc), 179.4
(NCNIm), 162.4 (CCNNP), 156.3 (CCCNP), 153.2 (CCCNP), 142.2 (CCCNP),
141.2 (CCNPh), 139.1 (CCCNP), 133.2 (NCCIm), 129.8 (CCCPh), 127.0
(CCCPh), 125.1 (CCCPh), 123.0 (CCCPh), 121.5 (CCCPh), 120.4 (CCCNP),
119.2 (NImCC), 54.4 (CCO), 49.6 (CCOOAc), 48.9 (NCCiPr), 29.8 (CH3NP),
28.1 (CH2NP), 23.2 (CCCiPr), 18.5 ppm (CCCiPr); IR (KBr): n=2071 (CO),
2025 (CO), 1951 (CO), 1426 (OAc), 1061 cmꢀ1 (BF4); ESI-MS: m/z=
792 corresponding to [2ꢀBF4]+; elemental analysis calcd (%) for
C31H28N5O9BF4Ru2: C 39.74, H 2.87, N 7.99; found: C 39.69, H 2.81, N
7.84.
2033 (CO), 1984 (CO), 1325 (CF3C6H4COO), 1066 cmꢀ1 (BF4 ); ESI-MS:
–
m/z=945 corresponding to [4ꢀBF4]+; elemental analysis calcd (%)
for C36H27N4O7BF10Ru2: C 41.96, H 2.64, N 5.44; found: C 41.90, H
2.61, N 5.38.
General procedure for the catalytic reaction
Complex 1 (9.26 mg, 0.01 mmol), DABCO (5.6 mg, 0.05 mmol), alco-
hol (1 mmol), amine (1.2 mmol), nonane (0.2 mmol), 4ꢂ molecular
sieves (100 mg), and toluene (3 mL) were placed in an oven-dried
reaction vessel. The reaction mixture was heated at reflux at 1108C
with stirring for 24 h. It was cooled to room temperature and the
solvent was removed in vacuo. The residue was purified by silica
gel column chromatography (hexane/Et2O 10:0!9:1 with 5%
Et3N) to afford the imine.
Synthesis of 3: Compound 3 was synthesized following a similar
procedure employed for the synthesis of 1 by using PIN·HBr
(60 mg, 0.17 mmol), Ru2(CO)4(CH3COO)2 (75 mg, 0.17 mmol), TlBF4
(49 mg, 0.17 mmol), and p-cyanobenzaldehyde (34 mg, 0.26 mmol).
Crystals suitable for X-ray diffraction were grown by layering
hexane over a concentrated dichloromethane solution of 3 inside
an 8 mm o.d. vacuum-sealed glass tube. Yield: 107 mg (72%). M.p.
>2508C; 1H NMR (400 MHz, CD3CN, 294 K): d=8.94 (d, J=9.1 Hz,
1H, NP), 8.53 (d, J=9.4 Hz, 1H, NP), 8.20 (d, J=2.7 Hz, 1H, Im), 8.0
(d, J=8.7 Hz, 2H, Ph), 7.98 (d, J=8.5 Hz, 2H, Ph), 7.90 (s, 1H, NP),
7.50 (d, J=2.4 Hz, 1H, Im), 5.15–5.06 (m, 1H, CHCHOH), 4.34 (dd, J1 =
14.1 Hz, J2 =2 Hz, 1H, CH2), 4.03 (dd, J1 =14.2 Hz, J2 =2 Hz, 1H,
CH2), 2.83 (s, 3H, Me-NP), 2.74–2.72 (m, 1H, CHiPr), 2.46 (s, 3H,
CH3OAc), 1.66 (s, 3H, CH3iPr), 1.61 ppm (s, 3H, CH3iPr); 13C NMR
(99.5 MHz, CD3CN, 296.2 K): d=210.1 (CO), 206.1 (CO), 201.2 (CO),
192.0 (CO), 187.2 (NCNNP), 184.1 (NNPCNIm), 179.1 (NCNIm), 162.0
(CCNNP), 156.7 (CCCNP), 153.0 (CCCNP), 142.7 (CCCNP), 141.4 (CCNPh),
139.8 (CCCNP), 133.2 (NCCIm), 129.2 (CCCPh), 127.5 (CCCPh), 124.7
(CCCPh), 123.0 (CCCPh), 121.1 (CCCPh), 120.0 (CCCNP), 119.2 (NImCC),
55.4 (CCO), 54.1 (OCOOAc), 48.9 (CCOOAc), 48.2 (NCCiPr), 29.6 (CH3NP),
28.0 (CH2NP), 22.9 (CCCiPr), 18.9 ppm (CCCiPr); IR (KBr): n=2067 (CO),
2025 (CO), 1947 (CO), 1422 (OAc), 1060 cmꢀ1 (BF4); ESI-MS: m/z=
772 corresponding to [3ꢀBF4]+; elemental analysis calcd (%) for
C30H26N5O7BF4Ru2: C 42.02, H 3.06, N 8.17; found: C 41.99, H 3.01, N
8.09.
X-ray data collection and refinement
Single-crystal X-ray studies were performed on a CCD Bruker
SMART APEX diffractometer equipped with an Oxford Instruments
low-temperature attachment. All the data were collected at
100(2) K using graphite-monochromated MoKa radiation (la =
0.71073 ꢂ). The frames were indexed, integrated, and scaled by
using the SMART and SAINT software packages[31] and the data
were corrected for absorption by using the SADABS program.[32]
The structures were solved and refined with the SHELX suite of
programs.[33] All hydrogen atoms were included in the final stages
of the refinement and were refined with a typical riding model.
Structure solution and refinement details for compounds 1–4 are
provided in the Supporting Information. Anisotropic treatment of
these three atoms resulted nonpositive definite displacement ten-
sors and were therefore subjected to isotropic refinement. The
“SQUEEZE” option in the PLATON program[34] was used to remove
a disordered solvent molecule from the overall intensity data of
compounds 1 and 4. Pertinent crystallographic data for com-
pounds 1–4 are summarized in Table S2 in the Supporting Informa-
tion. The crystallographic figures used in this manuscript have
been generated using Diamond 3.1e software.[35] CCDC-949508 (1),
949509 (2), 949510 (3), and 949511 (4) contain the supplementary
crystallographic data for this paper. These data can be obtained
free of charge from The Cambridge Crystallographic Data Centre
via www.ccdc.cam.ac.uk/data_request/cif.
Synthesis of 4: Compound 4 was synthesized following a similar
procedure employed for the synthesis of 1 by using PIN·HBr ligand
precursor (60 mg, 0.17 mmol), Ru2(CO)4(CH3COO)2 (75 mg,
0.17 mmol), TlBF4 (49 mg, 0.17 mmol), and p-trifluromethylbenzal-
dehyde (45 mg, 0.26 mmol). Crystals suitable for X-ray diffraction
were grown by layering hexane over a concentrated dichlorome-
thane solution of 4 inside an 8 mm o.d. vacuum-sealed glass tube.
Yield: 129 mg (72%). M.p. >2508C; 1H NMR (400 MHz, CD3CN,
294 K): d=8.80 (d, J=5.5 Hz, 1H, NP), 8.09 (d, J=9.2 Hz, 1H, NP),
8.08 (d, J=2.3 Hz, 1H, Im), 8.06 (d, J=2.4 Hz, 1H, Im), 7.67 (s, 1H,
Computational study
Calculations were performed by using density functional theory
(DFT) with Becke’s three-parameter hybrid exchange functional[36]
and the Lee–Yang–Parr correlation functional (B3LYP).[37] Geometry-
optimized structures were characterized fully by analytical frequen-
cy calculations as minima on the potential energy surface. The
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Acknowledgements
This work was financially supported by the Department of Sci-
ence and Technology (DST), India. J.K.B. thanks DST for the
Swarnajayanti fellowship. B.S. and P.D. (SPMF) thank CSIR India,
and S.M.W.R. and G.S. thank IITK for fellowships.
Keywords: bridging
ligands
·
carbene
ligands
·
dehydrogenation · imines · ruthenium
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Received: November 10, 2013
Published online on && &&, 0000
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FULL PAPER
Ligand lends a hand: Metal–hydroxy/
hydroxide and metal–metal cooperation
is demonstrated for acceptorless dehy-
drogenation of alcohols to give alde-
hydes. The ligand architecture ensures
rapid extrusion of the aldehyde from
the metal core, resulting in the forma-
tion of the corresponding imine as the
sole coupled product with amines (see
scheme; DABCO=1,4-diazabicyclo-
[2.2.2]octane).
&
Dehydrogenation
B. Saha, S. M. Wahidur Rahaman, P. Daw,
G. Sengupta, J. K. Bera*
&& – &&
Metal–Ligand Cooperation on
a Diruthenium Platform: Selective
Imine Formation through Acceptorless
Dehydrogenative Coupling of Alcohols
with Amines
Chem. Eur. J. 2014, 20, 1 – 11
www.chemeurj.org
11
ꢁ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
&
These are not the final page numbers! ÞÞ