Pincer Ru and Os complexes as efficient catalysts for racemization and deuteration of alcohols

Article abstract of DOI:10.1039/c1dt10498e

The pincer complexes [MX(CNN)(PP)] (M = Ru, Os; X = Cl, OTf; HCNN = 1-(6-arylpyridin-2-yl)methanamine; PP = diphosphine) have proven to efficiently catalyze both racemization and deuteration of alcohols in the presence of a base. Chiral alcohols have been racemized at 30-50 °C using 1 mol% of Ru or Os pincer complexes and 5 mol% of KOtBu in 2-propanol. Primary and secondary alcohols are efficiently deuterated at the α position, with respect to the OH group, using 2-propanol-d₈ as solvent with Ru or Os pincer complexes and KOtBu at 30-50 °C. For secondary alcohols incorporation of deuterium at the β position has also been observed. In 2-propanol-d ₈ the pincer complexes catalyze the simultaneous deuteration and racemization of (S)-1-phenylethanol, the two processes being strictly correlated. For both reactions much the same activity has been observed with the Ru and Os complexes. The pincer complexes display a superior activity with respect to the related compounds [MCl₂(NN)(PP)] (NN = bidentate amine or pyridine ligand). The synthesis of the new complexes [MCl(CNN)(PP)] (M = Ru, 2, 4 and Os, 6, 7; PP = dppb, dppf) and [Ru(OTf)(CNN)(dppb)] (3) is also reported. The Royal Society of Chemistry 2011.

Full text of DOI:10.1039/c1dt10498e

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Pincers and other hemilabile ligands
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PERSPECTIVE:
Cleavage of unreactive bonds with pincer metal complexes
Martin Albrecht and Monika M. Lindner
Dalton Trans., 2011, DOI: 10.1039/C1DT10339C
ARTICLES:
Pincer Ru and Os complexes as efficient catalysts for racemization and deuteration of alcohols
Gianluca Bossi, Elisabetta Putignano, Pierluigi Rigo and Walter Baratta
Dalton Trans., 2011, DOI: 10.1039/C1DT10498E
Mechanistic analysis of trans C–N reductive elimination from a square-planar macrocyclic aryl-
copper(III) complex
Lauren M. Huffman and Shannon S. Stahl
Dalton Trans., 2011, DOI: 10.1039/C1DT10463B
CSC-pincer versus pseudo-pincer complexes of palladium(II): a comparative study on
complexation and catalytic activities of NHC complexes
Dan Yuan, Haoyun Tang, Linfei Xiao and Han Vinh Huynh
Dalton Trans., 2011, DOI: 10.1039/C1DT10269A
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PAPER
Pincer Ru and Os complexes as efficient catalysts for racemization and
deuteration of alcohols
Gianluca Bossi, Elisabetta Putignano, Pierluigi Rigo and Walter Baratta*
Received 24th March 2011, Accepted 3rd May 2011
DOI: 10.1039/c1dt10498e
The pincer complexes [MX(CNN)(PP)] (M = Ru, Os; X = Cl, OTf; HCNN = 1-(6-arylpyridin-
2-yl)methanamine; PP = diphosphine) have proven to efficiently catalyze both racemization and
deuteration of alcohols in the presence of a base. Chiral alcohols have been racemized at 30–50 ◦C using
1 mol% of Ru or Os pincer complexes and 5 mol% of KOtBu in 2-propanol. Primary and secondary
alcohols are efficiently deuterated at the a position, with respect to the OH group, using 2-propanol-d8
as solvent with Ru or Os pincer complexes and KOtBu at 30–50 ◦C. For secondary alcohols
incorporation of deuterium at the b position has also been observed. In 2-propanol-d8 the pincer
complexes catalyze the simultaneous deuteration and racemization of (S)-1-phenylethanol, the two
processes being strictly correlated. For both reactions much the same activity has been observed with
the Ru and Os complexes. The pincer complexes display a superior activity with respect to the related
compounds [MCl2(NN)(PP)] (NN = bidentate amine or pyridine ligand). The synthesis of the new
complexes [MCl(CNN)(PP)] (M = Ru, 2, 4 and Os, 6, 7; PP = dppb, dppf) and [Ru(OTf)(CNN)(dppb)]
(3) is also reported.
catalysts for transfer hydrogenation (TH)7 of carbonyl compounds
can also induce the activation of alcohols. Incidentally, the pincer
complexes [MCl(CNN)(PP)]8 (M = Ru, Os; HCNN = 1-(6-
Introduction
Activation of the C–H bond vicinal to the hydroxyl group is a
fundamental step for broadening the reactivity of alcohols. In this
arylpyridin-2-yl)methanamine; PP = diphosphine), displaying the
NH2 function,9 are among the most efficient catalysts for the TH
of aldehydes and ketones. These systems are also active in the
hydrogenation10 of carbonyl compounds8b,d and in the dehydro-
genation of alcohols.11 Kinetic and NMR studies show that under
catalytic conditions, the chloride [RuCl(CNN)(dppb)] (1) (dppb =
1,4-bis(diphenylphosphino)butane) reacts with NaOiPr in iPrOH,
affording the Ru isopropoxide which rapidly equilibrates (<1 s at
RT) with the corresponding Ru hydride and acetone (Scheme 1).12
In this rare example of directly observable oxidation-reduction
pathway,13 both the NH2 function and the solvent (2-propanol)
play an active role in facilitating the activation of the a C–
H bond of the substrate within a hydrogen bonding network.12
The same behavior has been observed for the analogous osmium
complexes.8d This reversible C–H bond cleavage mediated by
pincer complexes opens the way for a broader reactivity of alcohols
via carbonyl intermediates. Arends et al. have demonstrated that
1 is active in the racemization of 1-phenylethanol in toluene at
context, the racemization of alcohols mediated by transition metal
complexes is an important transformation that in association with
the kinetic resolution can allow the preparation of optically active
alcohols.1 Efficient dynamic kinetic resolution (DKR) has been
achieved through the combination of a lipase with a number of
ruthenium complexes,2 based on the pioneering work of Ba¨ckvall
on the Shvo catalyst.3 Conversely, the C–H bond activation is
a crucial process in the catalytic H/D exchange at the carbon
centers of alcohols for obtaining deuterium-labeled compounds
for pharmaceutical and analytical chemistry.4 Several systems,
including [RuCl2(PPh3)3] and supported Ru and Pd have been
found as efficient catalysts for the deuteration of alcohols using
D2O.5 In addition, the C–H bond activation at the a position
represents a key step for increasing the alcohol reactivity for C–
C and C–N coupling reactions which occur through borrowing
hydrogen.6 Most of these catalytic reactions entail the reversible
alcohol dehydrogenation with formation of the more reactive
carbonyl compound (aldehyde, ketone). Among the different
metals used for these processes, ruthenium has widely been
investigated and the search for new catalysts able to activate
alcohols under mild conditions remains a challenge. On the basis
of the microscopic reversibility, it is expected that highly active
◦
70 C.14 More recently, fast racemization of alcohols has been
reported by Feringa et al. with half sandwich NH ruthenacycle
catalysts15 and by Nolan and Bosson using Ru complexes with
N-heterocyclic carbenes.16
We describe here that the pincer Ru and Os complexes
[MX(CNN)(PP)] (X = Cl, OTf) 1–8 in the presence of the
base KOtBu in 2-propanol display high catalytic activity in the
racemization of secondary alcohols at 30–50 ◦C (Fig. 1).
Dipartimento di Chimica, Fisica e Ambiente, Universita` di Udine, Via
Cotonificio 108, I-33100, Udine, Italy. E-mail: inorg@uniud.it
8986 | Dalton Trans., 2011, 40, 8986–8995
This journal is
The Royal Society of Chemistry 2011
©

Scheme 1 Formation of a RuOiPr complex and equilibrium between RuOiPr vs. RuH/acetone.
Fig. 1 Ru and Os pincer complexes [MX(CNN)(PP)].
These catalysts are also highly efficient in the deuteration
Results and discussion
of primary and secondary alcohols in 2-propanol-d8 and their
activity has been compared with that of the related hydrogenation
and TH systems [MCl2(NN)(PP)] (9–14) (NN = bidentate amine
or pyridine ligand) (Fig. 2).
Synthesis of the pincer complexes 2, 3, 4, 6 and 7
The orthometalated ruthenium(II) complex 2 was easily obtained
in high yield by reaction of the precursor [RuCl2(PPh3)(dppb)] with
an equimolar amount of rac-1-(6-phenylpyridin-2-yl)ethanamine
(a) in 2-propanol at reflux in the presence of NEt3 in excess,
according to the procedure reported for 1 (Scheme 2).17
On account of the presence of two stereogenic centers on the Ru
1
and C–Me, four stereoisomers are expected. The 31P{ H} NMR
spectrum of 2 shows two doublets at d = 57.4 and 42.1 ppm with
2JPP = 38.7 Hz, whereas the 1H NMR spectrum displays a doublet
at d = 1.44 ppm for the methyl group, in agreement with the
formation of 2 as a one pair of enantiomers. This agrees with the
previous studies on the analogous complex bearing a tert-butyl
group adjacent to NH2, suggesting that in the racemate 2 the
methyl group points towards the chloride.17
Treatment of 2 with one equivalent of thallium triflate in CH2Cl2
at room temperature (4 h), afforded the derivative 3 (Scheme 2).18
Fig. 2 Ru and Os complexes [MCl2(NN)(PP)] (NN = bidentate amine or
pyridine ligand).
1
The 31P{ H} NMR spectrum shows two broad signals at d = 66.0
and 42.1 ppm for a single species, while the 19F NMR shows a
singlet at d = -77.6 ppm, slightly shifted downfield with respect to
the free triflate anion (d~ -80.0),19 suggesting a weak coordination
of the oxygen donor ligand.20 By difference to 2, complex 3 displays
a relatively high solubility in different organic solvents, such as
toluene and iPrOH.
To the best of our knowledge, no examples of Os catalysts
for racemization and deuteration of alcohols have previously
been described. The isolation of the new pincer complexes of
ruthenium (2–4) and osmium (6, 7), some of them containing
1,1¢-bis(diphenylphosphino)ferrocene (dppf), is also reported.
This journal is
The Royal Society of Chemistry 2011
Dalton Trans., 2011, 40, 8986–8995 | 8987
©

Scheme 2 Syntheses of the Ru complexes 2 and 3.
Scheme 3 Syntheses of the Ru complex 4 and the Os complexes 6 and 7.
The pincer derivative 4, containing a flexible ferrocene
diphosphine,21 has been prepared by reaction of [RuCl2(PPh3)n-
(dppf)]m with 1-[6-(4¢-methylphenyl)pyridin-2-yl]methanamine (b)
in the presence of NEt3 in 2-propanol at reflux tempera-
ture. The intermediate [RuCl2(PPh3)n(dppf)]m was obtained from
[RuCl2(PPh3)3] with dppf in dichloromethane at RT (2 h)
(Scheme 3).22
1
The 31P{ H} NMR spectrum of 4 shows two doublets at d =
2
62.4 and 45.1 ppm with JPP = 36.0 Hz. The osmium pincer 6,
Scheme 4 Racemization of chiral alcohols catalyzed by Ru and Os pincer
complexes.
analogous to the ruthenium 2, has been prepared by treatment
of [OsCl2(PPh3)3] with dppb in CH2Cl2 at RT (3 h) and reaction
with the ligand a and triethylamine (10 equiv.) in 2-propanol at
reflux temperature (3 h) (Scheme 3).8d Similarly, the ferrocenyl
derivative 7 has been obtained from [OsCl2(PPh3)3], dppf and the
The same result (0% ee) is obtained using a 2-propanol/toluene
mixture (1/1 in volume) (entry 2). Under these catalytic con-
ditions, no racemization occurs with 1 without base, indicating
that KOtBu is crucial for the generation of the catalytically active
ruthenium hydride species (entry 3). Under these experimental
conditions, no formation of acetophenone, i.e., the product of
dehydrogenation, was observed via GC analysis. Complex 2,
bearing a methyl group adjacent to the NH2 function, catalyzes the
complete racemization in 1 h at 30 ◦C, indicating that the methyl
substituent does not inhibit the catalysis (entry 4). The highly
soluble triflate 3 is apparently less efficient leading to 18% ee in 2 h
(entry 5). The complex 4, containing the ferrocenyl diphosphine,
displays a lower activity at 30 ◦C (26% ee in 2 h), whereas at
50 ◦C complete racemization is attained in 1 h (entries 6 and
7). Interestingly, the osmium pincer 5 is found extremely active,
leading to complete racemization in 1 h at 30 ◦C and within 40 min
at 50 ◦C (entries 8 and 9). The comparison of these data with those
of the corresponding ruthenium complex, indicates that osmium
is more active with respect to ruthenium for which the reaction
is complete in 2 h.23 With NaOiPr, 5 displays much the same
rate as with KOtBu (entry 10), while no reaction occurs in the
1
pincer b. The 31P{ H} NMR spectrum of 7 shows two doublets
at d = 4.6 and 0.7 ppm with 2JPP = 20.2 Hz. By contrast 6 shows
only a singlet at d = 0.28 ppm, indicating that the two P atoms
1
1
have nearly the same chemical shift. The H and 13C{ H} NMR
spectra of 6 and 7 are similar to those of the related ruthenium
complexes 2 and 4, respectively, in agreement with the formation
of six-coordinate amino complexes. It is to point out that the
five coordinate amido species, which could form from the amino
complexes by elimination of HCl, have not been observed.
Racemization
The pincer complexes 1–8 (Fig. 1) in the presence of KOtBu have
proven to efficiently catalyze the racemization of the optically
active secondary alcohols (Scheme 4).
The substrate (S)-1-phenylethanol 15 (about 0.3 M), which was
used as a model substrate, is promptly racemized by complex 1 (1
mol%) in 2-propanol with KOtBu (5 mol%) at 30 ◦C in 2 h (entry
1 of Table 1).
8988 | Dalton Trans., 2011, 40, 8986–8995
This journal is
The Royal Society of Chemistry 2011
©

Table 1 Racemization of 15 catalyzed by Ru and Os complexes (1.0
mol%) with KOtBu (5.0 mol%) in 2-propanol
Table 2 Racemization of 15 catalyzed by Ru and Os catalysts (1.0 mol%)
in the presence of base (5 mol%) in toluene
Entry
Catalyst
T/◦C
Time/h
ee (%)
Entry Catalyst Base
T/◦C Time/h ee (%)
Ketone (%)
1
1
1
1
2
3
4
4
5
5
5
5
6
7
30
30
30
30
30
30
50
30
50
30
30
30
30
2
2
2
1
2
2
1
1
0
0
99
0
18
26
0
0
0
0
99
0
1
2
3
3
NEt3
DBU
50
50
2
2
5
6
4
2
4
2
24
6
6
6
99
78
2
0
2a
3b
4
16
33
18
0
0
0
0
8
8
0
3
4
5
6
7
8
9
10
11
3
DBUa
DBU
KOtBu
DBU
K2CO3
DBU
70
70
30
70
30
70
90
70
70
2
5
6
7
8
4
99
10
99
99
86
0
5
5
5
9
40 min
1
2
2
2
4
7
10c
11d
12
13
7
DBUa
DBU
DBU
9
99
99
12
0
40
0
a Base 10 mol%.
14
15
16
18
19
20
21
17
22
7
50
30
90
30
90
30
90
30
30
2
2
0
96
0
99
99
99
10
38
99
9
with DBU (5 mol%) the ee were 78 and 2% after 2 and 5 h,
respectively (entry 2) (Table 2).
9
45 min
10
10
11
12
13
14
2
2
2
4
2
2
It is worth noting that using toluene instead of iPrOH leads
to concomitant formation of acetophenone (33% in 5 h) via
a dehydrogenation reaction. By increasing the amount of base
(DBU 10 mol%) racemization occurs within 6 h at 70 ◦C and
with lower formation of acetophenone (18%, entry 3), indicating
that both the base and the solvent affect the selectivity of the
racemization reaction. The ferrocenyl complex 4 with DBU is not
a The reaction was carried out in 2-propanol/toluene (1/1 in volume).
b Without base. c Base: NaOiPr (5.0 mol%). d Base: DBU (5.0 mol%).
◦
active even at 70 C◦(entry 4). The osmium 5 with KOtBu gives
presence of the DBU base (entry 11). For the methyl substituted
6, complete racemization is attained in 2 h (entry 12). Similarly to
the ruthenium 4, the ◦ferrocenyl osmium complex 7 shows a lower
rate, affording at 30 C complete racemization in 4h, whereas at
10% ee in 2 h at 30 C without dehydrogenation (entry 5), while
no reaction occurs with DBU at 70 ◦C (entry 6) or K2CO3 at
30 ◦C (4 h, entry 7). The ferrocenyl deri◦vative 7 in the presence
of DBU (5 mol%) leads to 86% ee at 70 C in 24 h (entry 8). By
increasing the amount of base (10 mol%), complete racemization
is achieved at 90 ◦C after 6 h (entry 9). Finally the [MCl2(NN)(PP)]
complexes 9 and 12 in the presence of DBU were found not
active at 70 ◦C (entries 10, 11). The comparison of the activity
of the pincer complexes in toluene vs. 2-propanol shows that
high racemization rate is achieved in the protic media and in the
presence of a strong base. While in 2-propanol (hydrogen donor)
quantitative racemization of 1-phenylethanol is achieved without
formation of side products, in toluene catalytic dehydrogenation
to acetophenone is also observed, depending on the temperature
and base used.
◦
50 C the reaction occurs in 2 h (entries 13 and 14). The higher
temperature requested for the dppf Ru and Os derivatives may
be related to the lower basicity of dppf, with respect to dppb,
which may hinder the chloride dissociation and the formation
of the catalytically active M–H species. The comparison of the
catalytic activity of the Noyori type complexes [MCl2(NN)(PP)],
bearing bidentate amine or pyridine ligands, shows that these
systems are less active with respect to the pincer complexes. The
cis 2-aminomethylpyridine Ru complex 9, which is related to 4, is
almost not active at 30 ◦C and affords racemization in 45 min at
90 ◦C (entries 15, 16). It is worth noting that the cis dipyridine
compound 10 displays no catalytic activity even at 90 ◦C (entries
18, 19). Regarding the trans diamine complexes, the ferrocenyl 11
is not active at 30 ◦C (entry 20), while 12 bearing dppb leads to 10%
ee after 4 h at 90 ◦C (entry 21). For osmium, the ampy derivative
13, which is related to the pincer 5, affords 38% ee at 30 ◦C (2 h),
while the diamine 14 is not active under these conditions (entry
22).
These results show that pincer ruthenium and osmium com-
plexes display much the same activity in the racemization reaction.
In addition, these compounds are more active with respect to the
derivatives [MCl2(NN)(PP)], where the order of activity is ampy
> diamine > dipyridine.
The racemization of 15 was also carried out in non protic
solvents, such as toluene in the presence of a weak base. This point
is of particular importance for applications in DKR in which the
enzymatic reaction is generally not compatible with strong bases.
By using the labile triflate 3 (1 mol%) and NEt3, in place of KOtBu,
Chiral aliphatic alcohols have also been racemized by the pincer
complexes in 2-propanol. With the ruthenium 1 (1 mol%) and
KOtBu (5 mol%), (S)-2-butanol 16 is completely converted into
the racemate in 2 h at 50 ◦C (entry 1) (Table 3).
A similar activity has been observed with 2 (entry 2), whereas the
ferrocenyl 4 affords 4% ee in 2 h (entry 3). The osmium derivatives
5 and 6 give the racemization in 4 and 2 h (entries 4, 5), respectively,
whereas 7 shows poor catalytic activity (64% ee in 2 h) (entry 6).
Finally, the substrate (R)-heptanol 17 is racemized at 50 ◦C with
the ruthenium complexes 1, 2 and 4 in 2 h (entries 7–9). Also
the osmium complexes 5 and 6 lead to complete racemization
in 2 h (entries 10 and 11), whereas 7 shows moderate activity
(48% ee, 2 h).
Deuteration
The pincer Ru and Os complexes in the presence of KOtBu have
also been found to be efficient catalysts for the deuteration of
◦
no racemization was observed at 50 C in 2 h (entry 1), whereas
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The Royal Society of Chemistry 2011
Dalton Trans., 2011, 40, 8986–8995 | 8989
©

Table 3 Racemization of 16 and 17 catalyzed by pincer Ru and Os
complexes (1.0 mol%) with KOtBu (5.0 mol%) in 2-propanol at 50 ◦
(entries 4, 5). Also the osmium derivatives 5 and 7 have been found
to be highly active in the catalytic H - D exchange at 30 ◦C, leading
to 96 (2 h) and 92% (4 h) of a deuteration, while the values for
the b incorporation are 90 and 84%, respectively (entries 6, 7),
the activity of 5 being comparable with that of 1. To show the
synthetic potential of this reaction, 1-phenylethanol-d5 (110 mg,
purity >98%) has easily been prepared in 87% yield by reaction of
18 (1.0 mmol) with 2-propanol-d8 (2 ¥ 1.5 mL), using 0.5 mol% of
5 in the presence of KOtBu (2.5 mol%) at 40 ◦C. Although D2O is
the cheapest source for obtaining deuterium-labeled compounds,
the use of 2-propanol-d8 in combination with pincer complexes
is a straightforward procedure for the deuteration of alcohols.
The comparison of the catalytic activity of the pincer complexes
with the derivatives [MCl2(NN)(PP)], shows that the latter are
less active as for the racemization reaction. The ampy complex 13
gives 40 and 41% of a and b exchange at 30 ◦C (entry 8), whereas
the◦diamine 12 is less active (entries 9 and 10). Conversely, at
60 C 12 catalyzes the H–D exchange affording 93 (a) and 89%
(b) of D content in 2 h (entry 10). With 1 at 30 ◦C, 2-heptanol 19
undergoes deuteration at the a position (50% in 24 h) with a lower
rate with respect to 18 (entry 11). At 70 ◦C almost quantitative D
incorporation has been attained for the a hydrogen (99%), as well
as the CH3 and CH2 b hydrogens in 4 h, as established by 1H and
13C NMR experiments (entry 12).
C
Entry
Substrate
Catalyst
Time/h
ee (%)
1
2
3
4
5
6
7
8
16
16
16
16
16
16
17
17
17
17
17
17
1
2
4
5
6
7
1
2
4
5
6
7
2
2
2
4
2
2
2
2
2
2
2
2
0
0
4
0
0
64
0
0
0
9
10
11
12
0
0
48
alcohols in 2-propanol-d8. For secondary alcohols fast deuterium
incorporation at the a and b positions to the hydroxyl group is
◦
easily achieved at 30–60 C within a few hours using 1 mol% of
catalyst. With complex 1, rac-1-phenylethanol 18 undergoes H–
◦
D exchange at CH–O (96%) and CH3 (80%) in 2 h at 30 C, as
established by NMR measurements (entry 1, Table 4).
Primary alcohols are efficiently deuterated at the a position
using the pincer complexes. Benzyl alcohol 20 easily incorporates
deuterium at 50 ◦C in 1 h (94%) with 1 (1 mol%) in 2-propanol-d8
(entry 1, Table 5) (Scheme 5).
High deuterium incorporation into the substrate is ac-
complished using 2-propanol-d8 in large excess (2-propanol-
d8/substrate = 40), which takes H in the a and b positions. As
a matter of fact, nearly all quantitative hydrogen exchange at the
a position of the substrate and 2-propanol-d8 has been inferred
1
from H NMR measurements. In addition, in the basic protic
media the substrate is present as ROD since the hydroxyl O–H/O–
D exchange occurs instantaneously. Without base, insignificant
deuteration at the a position (< 4%) is observed with no b
incorporation, indicating that the H-D exchange is catalyzed by
Ru. Using D2O in place of 2-propanol-d8 at 50 ◦C the deuteration is
much slower and leads unexpectedly to a higher D content at the b
position (94% in 24 h) with respect to the a position (54%) (entry
3). Similarly to 1, complex 2 catalyzes the deuteration of 18 at
30 ◦C in 2-propanol-d8, leading to 91 and 60% of D incorporation
at the a and b positions, while 4 gives 87 and 77% (4 h), respectively
Scheme 5 Deuteration of primary alcohols catalyzed by complex 1.
Table 4 Deuteration of secondary alcohols catalyzed by Ru and Os complexes (1 mol%) in the presence of KOtBu (5 mol%) in 2-propanol-d8
Entry
Substrate
Catalyst
T/◦C
Time/h
D content in a (%)
D content in b (%)a
1
18
18
18
18
18
18
18
18
18
18
19
19
1
30
30
50
30
30
30
30
30
30
60
30
70
2
2
24
2
4
2
4
2
2
96
4
80
0
2
—
1
3b
4
54
91
87
96
92
40
11
93
50
99
94
60
77
90
84
41
0
89
40
96c
2
5
6
7
8
4
5
7
13
12
12
1
9
10
11
12
2
24
4
1
a For the methyl group. b The reaction was carried out in D2O. c Complete methylene deuteration was inferred from 13C NMR.
8990 | Dalton Trans., 2011, 40, 8986–8995
This journal is
The Royal Society of Chemistry 2011
©

Table 5 Deuteration of primary alcohols catalyzed by 1 and 5 (1 mol%) with KOtBu (5 mol%) in 2-propanol-d8 at 50 ◦
C
Entry
1
Substrate
Catalysxt
Time/h
D content in a (%)
D content in b (%)
20
1
1
3
1
24
1
3
5
0.5
1
1
94
95
98
0
94
99
99
90
95
94
95
95
/
/
0
0
0
7
14
0
0
0
0
0
2
21
21
22
1
1
1
3a
4
5
6
22
23
5
1
2
4
a The reaction was carried out in D2O.
Interestingly, ethanol 21 is deuterated at the a position in 1 h
(98%), with no significant incorporation of D at the b position
(entry 2). No deuteration of 21 has been observed with 1 using
D2O, in place of 2-propanol-d8, after 1 day (entry 3). Similarly to
21, 1-propanol 22 undergoes incorporation of D at the a position,
namely 94 and 99% after 1 and 3 h, respectively (entry 4). The 1H
NMR measurements carried out after 3 and 5 h show moderate
deuteration at the b position (7 and 14%). High catalytic activity
has also been observed with the osmium 5 which leads to 95% of
a incorporation in 22 after 1 h. Finally, also cyclohexylmethanol
23 has been deuterated at the a position (95% 2 h), without D
incorporation at the b position (entry 6).
By employment of the osmium complex 5 the reaction is faster
leading to 0% ee* and nearly quantitative D incorporation at
the a and b positions in 1 h (entry 2). The osmium derivative 8
bearing PPh3 is significantly less active compared to the analogous
diphosphine complexes, affording 64% ee* with 36 and 39% of
deuterium content at the a and b positions after 1 h, whereas
complete racemization and deuteration is attained after 24 h (entry
3). The data of Table 6 show that
ee* + (%)aD = 100
within the experimental errors. From the definition of ee* and
24
(%)aD follows that
When a chiral alcohol is dissolved in 2-propanol-d8 the pincer
complexes catalyze simultaneously the racemization and deuter-
ation reactions. With complex 1 (1 mol%) at 30 ◦C and in the
presence of KOtBu (5 mol%), (S)-1-phenylethanol 15 undergoes
racemization in 2 h (3% ee*)24 with D incorporation at the a and
b positions of 95 and 70%, respectively.
ee* + (%)aD = 100 ¥ (SH + SD - RD)/(SH + SD + RD) + 100 ¥ (SD
+ RD)/(SH + SD + RD) = 100 ¥ (SH + 2SD)/(SH + SD + RD)
According to the first equation, this implies that SD = RD and
therefore the a deuteration occurs with racemization and not
through a C–H/D exchange with retention of the configuration
at the carbon atom, indicating that the two reactions are strictly
correlated and occur simultaneously.
The results of this study on the racemization and deuteration of
alcohols with the pincer Ru and Os complexes, are in agreement
with a mechanism involving the reversible formation of ketone
(aldehyde) through a hydrogen transfer reaction.25 In the proposed
catalytic cycle for the alcohol racemization, the isopropoxide
After 4 h, complete racemization and higher deuteration in both
positions (99 and 95%) has been observed (entry 1, Table 6).
Table 6 Racemization and deuteration of 15 catalyzed by 1, 5 and 8 (1 mol%) with KOtBu (5 mol%) in 2-propanol-d8 at 30 ◦
C
Entry
Catalyst
Time/h
ee* (%)a
D content in a (%)
D content in b (%)
1
2
3
1
5
8
2
4
1
2
1
24
3
0
0
0
64
0
95
99
97
99
36
97
70
95
97
98
39
97
a ee* = 100 ¥ (SH + SD - RD)/(SH + SD + RD)
This journal is
The Royal Society of Chemistry 2011
Dalton Trans., 2011, 40, 8986–8995 | 8991
©

Scheme 6 Proposed catalytic cycle for the racemization of alcohols catalyzed by Ru and Os pincer complexes in 2-propanol.
[M(OiPr)(CNN)(PP)], which is formed from [MX(CNN)(PP)] in
basic 2-propanol, is protonated by the chiral substrate leading to
the alkoxide [M(OC*HRMe)(CNN)(PP)] (24) (Scheme 6).
This species affords the hydride [MH(CNN)(PP)] (25) with
extrusion of the ketone through a b hydrogen elimination
reaction, assisted by the NH2 function and the 2-propanol
media.12a The non enantioselective reduction of the ketone affords
[M(OCHRMe)(CNN)(PP)] (26) which is protonated by the chiral
alcohol, affording the racemic alcohol and closing the cycle.
This mechanism resembles that proposed by Ba¨ckvall et al. for
cyclopentadienyl Ru systems,26 although at this stage it is not clear
whether in protic media the formed ketone remains close to the
Ru hydride or it is free, leading to reduction, but not at the same
Ru–H center.
Conversely, if the reaction is performed in 2-propanol-d8,
primary and secondary (chiral) alcohols are converted to carbonyl
compounds and reduced to the a deuterium-labeled (racemic)
alcohols by a metal deuteride (Scheme 7).
The catalytic pathway may be envisioned as composed by
two mirrored cycles, identical to that proposed for the al-
cohol racemization, namely cycle A involving M–H and cy-
cle B for M–D species. In the basic media, the precur-
sor [MX(CNN)(PP)] reacts with the alcohol substrate and 2-
propanol-d8 affording the alkoxides [M(OCHRMe)(CNN)(PP)]
(24) and [M(OiPr-d7)(CNN)(PP)] (27d), respectively. According to
the previous mechanism, 24 leads to the hydride [MH(CNN)(PP)]
(25) with elimination of the carbonyl compound, whereas 27d
gives the deuteride [MD(CNN)(PP)] (25d) and acetone-d6. The
Scheme 7 Proposed catalytic pathway for the deuteration of alcohols catalyzed by Ru and Os pincer complexes in 2-propanol-d8.
8992 | Dalton Trans., 2011, 40, 8986–8995 This journal is The Royal Society of Chemistry 2011
©

subsequent “cross” reactions of the free carbonyl compound with
25d and acetone-d6 with 25 give the corresponding alkoxides
[M(OCDRMe)(CNN)(PP)] (26d) and [M(OiPr-d6)(CNN)(PP)]
(27). Finally, protonation of these species leads to the a deuterated
alcohol substrate and the 2-propanol-d7, closing the cycles. In
basic 2-propanol-d8 the NH2 function of the catalyst is likely
converted into the ND2 moiety, due to the facile H/D exchange.17
Although in 2-propanol-d8 an alternative pathway for the Ru–D
formation, involving a Ru–H/D–O exchange, cannot be ruled out,
this process appears less likely, since use of D2O retards drastically
the deuteration. It is worth noting that these data show that the
a hydrogens are exchanged more rapidly compared to the b ones.
While primary alcohols give moderate or negligible incorporation
at the b position, secondary alcohols give fast deuteration at the
CH3, whereas the b CH2 moiety of the alkyl chain is slowly
deuterated. No exchange has been observed for g or aromatic
hydrogen atoms and for the CH3 of the tBuOH, formed from
purification. The compounds [MCl2(PPh3)3] (M = Ru,28 Os29),
[RuCl2(PPh3)(dppb)],22 1,8f 58d and 88d were prepared according
to the literature procedure. NMR measurements were recorded
on a Bruker AC 200 spectrometer and the chemical shifts, in ppm,
1
1
are relative to TMS for H and 13C{ H}, and 85% H3PO4 for
1
31P{ H}. Elemental analyses (C, H, N) were carried out with a
Carlo Erba 1106 elemental analyzer, whereas the GC analyses were
performed with a Varian GP-3380 gas chromatograph equipped
with a MEGADEX-ETTBDMS-b chiral column.
Preparation of the pincer complexes
Ru complex 2. [RuCl2(PPh3)(dppb)] (250 mg, 0.290 mmol),
NEt3 (405 mL, 2.90 mmol) and rac-1-(6-phenylpyridin-2-
yl)ethanamine (70 mg, 0.353 mmol) were suspended in 2-propanol
(4.0 mL) and the mixture was refluxed for 3 h, obtaining a yellow
precipitate. After filtration the product was washed with methanol
(3 ¥ 1 mL) and dried under reduced pressure (180 mg, 82%).
Found: C, 64.45; H, 5.21; N, 3.43. Calc. for C41H41ClN2P2Ru: C,
1
KOtBu, as inferred from H and 13C NMR measurements. This
agrees with the oxido-reductive cycle in which the b hydrogens
of the alcohols undergo in the basic media deuterium exchange
through the formation of the corresponding carbonyl compound,
namely at the CH close to the C O group via enolates. The
difference of the behavior of primary vs. secondary alcohols are in
agreement with the literature data,4a and may be ascribed to the
higher redox potential of the aldehydes compared to the ketones,27
affording a lower amount of free aldehyde vs. ketone in solution.
1
64.77; H, 5.44; N, 3.68. H NMR (200.1 MHz; CD2Cl2) d 8.12
(2H, dt, JHH = 8.1 and 1.7 Hz, aromatic protons), 7.84 (3H, dt, JHH
= 8.2 and 1.4 Hz, aromatic protons), 7.44–6.78 (17H, m, aromatic
protons), 6.60 (3H, dt, JHH = 7.6 and 1.8 Hz, aromatic protons),
5.93 (2H, t, JHH = 8.5 Hz, aromatic protons), 3.75 (1H, m, CHN),
3.47 (1H, m, NH2), 3.08 (2H, m, CH2P), 2.40–1.60 (7H, m, NH2
1
and CH2), 1.44 (3H, d, JHH = 6.6 Hz, CH3). 13C{ H} NMR (50.3
MHz; CD2Cl2) d 181.9 (dd, JCP = 16.3 and 7.6 Hz, CRu), 163.7 (s,
NCC), 158.3 (s, NCCH), 149.3–116.1 (m, aromatic carbons), 57.4
(d, JCP = 2.7 Hz, CHN), 32.9 (d, JCP = 24.5 Hz, CH2P), 30.3 (d,
JCP = 31.7 Hz, CH2P), 26.4 (s, CH2), 23.0 (s, CH3), 21.5 (s, CH2).
Conclusion
In summary, we have found that the pincer complexes
[MCl(CNN)(PP)] (M = Ru, Os; PP = diphosphine) in basic 2-
propanol are efficient catalysts for the racemization of secondary
alcohols in basic 2-propanol. These systems also efficiently
catalyze the deuteration of primary and secondary alcohols in
2-propanol-d8. In addition to the a incorporation of deuterium
with respect to the OH group, the secondary alcohols undergo
deuteration at the b position. Much the same activity has been
observed for the ruthenium and osmium pincer complexes, which
are superior with respect to the complexes [MCl2(NN)(PP)]
(NN = bidentate amine or pyridine ligand). The new derivatives
bearing the ferrocenyl diphosphine are active at a slightly higher
temperature with respect to the analogous complexes with dppb
showing an alkyl backbone. The studies on (S)-1-phenylethanol
reveal that the deuteration occurs with simultaneous racemization,
suggesting that both reactions involve an oxido-reductive process
through formation of the corresponding ketone. Work is in
progress to shed light on the mechanism of these transformations
and improve the design of more efficient catalysis for the activation
of alcohol C–H bonds.
1
31P{ H} NMR (81.0 MHz; CD2Cl2) d 57.4 (d, JPP = 38.7 Hz), 42.1
(d, JPP = 38.7 Hz).
Ru complex 3. Compound 2 (150 mg, 0.197 mol) was dis-
solved in dichloromethane (5 mL) and thallium triflate (73 mg,
0.207 mol) was added. The orange suspension was stirred at room
temperature for 4h, filtered on Celite to eliminate TlCl and the
solution was reduced to 1 mL. Addition of heptane afforded a
yellow precipitate, which was filtered, washed with heptane and
dried under reduced pressure (143 mg, 83%). Found: C, 57.45; H,
4.70; N, 3.11. Calc. for C42H41F3N2O3P2RuS: C, 57.73; H, 4.73; N,
1
3.21. H NMR (200.1 MHz; CD2Cl2) d 7.91 (2H, m, aromatic
protons), 7.70–6.71 (23H, m, aromatic protons), 5.98 (2H, m,
aromatic protons), 3.78 (1H, m, CHN), 3.47 (1H, m, NH2), 3.05
(2H, m, CH2P), 2.39–1.60 (7H, m, NH2 and CH2), 1.46 (3H, d, JHH
1
= 6.4 Hz, CH3). 13C{ H} NMR (50.3 MHz; CD2Cl2) d 179.1 (s,
CRu), 164.6 (s, NCC), 161.0 (s, NCCH), 150.6–116.9 (m, aromatic
carbons and CF3), 58.5 (d, JCP = 2.0 Hz, CHN), 31.6 (d, JCP = 34.9
Hz, CH2P), 31.2 (d, JCP = 24.8 Hz, CH2P), 26.3 (s, CH2), 23.9 (s,
1
CH3), 21.7 (s, CH2). 31P{ H} NMR (81.0 MHz; CD2Cl2) d 66.0
(br s), 42.1 (br s). 19F NMR (188.3 MHz; CD2Cl2) d -77.6 (s).
Experimental
Ru complex 4. [RuCl2(PPh3)3] (100 mg, 0.104 mmol) and
dppf (64 mg, 0.115 mmol) were dissolved in dichloromethane
(1.5 mL) and the solution was stirred for 2 h at room temperature.
The solvent was evaporated under reduced pressure and 1-[6-
(4¢-methylphenyl)pyridin-2-yl]methanamine (26 mg, 0.131 mmol)
dissolved in 2-propanol (3 mL) and NEt3 (145 mL, 1.04 mmol)
were added. The mixture was refluxed for 3 h obtaining a yellow
precipitate which was filtered, washed with 2-propanol (3 ¥ 1 mL),
General comments
All reactions were carried out under an argon atmosphere
using standard Schlenk techniques. The solvents were care-
fully dried by standard methods and distilled under argon
before use. The diphosphanes and all other chemicals were
purchased from Aldrich and Strem and used without further
This journal is
The Royal Society of Chemistry 2011
Dalton Trans., 2011, 40, 8986–8995 | 8993
©

heptane (3 ¥ 1 mL) and dried under reduced pressure (75 mg, 81%).
Found: C, 63.11; H, 4.41; N, 3.11. Calc. for C47H41ClFeN2P2Ru: C,
63.56; H, 4.65; N, 3.15. 1H NMR (200.1 MHz; CD2Cl2) d 8.47 (2H,
dt, JHH = 8.6 and 1.4 Hz, aromatic protons), 8.22 (1H, s, aromatic
proton), 7.96 (2H, dt, JHH = 8.1 and 1.8 Hz, aromatic protons),
7.72–6.62 (19H, m, aromatic protons), 6.42 (2H, dt, JHH = 8.4 and
1.3 Hz, aromatic protons), 5.24 (1H, m, C5H4), 4.84 (1H, m, C5H4),
4.33 (1H, m, C5H4), 4.21 (1H, m, C5H4), 4.17 (1H, m, C5H4), 4.09
(1H, m, NH2), 3.99 (1H, m, C5H4), 3.84 (1H, m, C5H4), 3.51 (2H,
m, CH2), 3.16 (1H, m, C5H4), 2.32 (3H, s, CH3), 2.04 (1H, m,
NCCH2), 150.5–115.9 (m, aromatic carbons), 88.6 (d, JCP = 46.5
Hz, ipso-C5H4), 87.2 (d, JCP = 58.2 Hz, ipso-C5H4), 77.2 (s; C5H4),
76.9 (s, C5H4), 76.8 (s, C5H4), 76.2 (d, JCP = 3.6 Hz, C5H4), 73.5 (d,
JCP = 7.3 Hz, C5H4), 73.2 (d, JCP = 5.0 Hz, C5H4), 69.5 (d, JCP
=
1
5.5 Hz, C5H4), 69.0 (s, C5H4), 53.9 (s, CH2), 21.5 (s, CH3). 31P{ H}
NMR (81.0 MHz; CD2Cl2) d 4.6 (d, JPP = 20.2 Hz), 0.7 (d, JPP
=
20.2 Hz).
General procedure for the racemization of alcohols. The catalyst
(1.6 mmol) and KOtBu (8.2 mmol) were dissolved in 2-propanol
(0.5 mL) and the chiral alcohol (0.164 mmol) was added under
argon. The solution was stirred at the desired temperature and
monitored over time by taking aliquots (25 mL) that were filtered
over a short pad of silica (eluent: diethyl ether) and analyzed by
chiral GC.
1
NH2). 13C{ H} NMR (50.3 MHz; CD2Cl2) d 182.7 (s, CRu), 163.7
(s, NCC), 157.1 (s, NCCH2), 150.6–115.9 (m, aromatic carbons),
77.4 (d, JCP = 13.4 Hz, C5H4), 76.9 (d, JCP = 7.8 Hz; C5H4), 75.7
(d, JCP = 2.9 Hz; C5H4), 73.9 (d, JCP = 6.9 Hz; C5H4), 73.3 (d, JCP
=
4.8 Hz, C5H4), 69.5 (d, JCP = 5.0 Hz, C5H4), 69.3 (d, JCP = 3.3 Hz,
C5H4), 68.9 (d, JCP = 5.2 Hz, C5H4), 51.6 (s, CH2), 21.6 (s, CH3).
General procedure for deuteration of alcohols. In a NMR tube
the catalyst (1.6 mmol) and KOtBu (8.2 mmol) were dissolved in
2-propanol-d8 (0.5 mL) and the substrate (0.164 mmol) was added
under argon. The solution was kept under the desired temperature
and the incorporation of D content was monitored over time
by NMR spectroscopy. In the case of (S)-1-phenylethanol, the
solution was also analyzed by chiral GC, according to the
above procedure for the racemization of alcohols (2-propanol-
d8/substrate = 40).
1
31P{ H} NMR (81.0 MHz; CD2Cl2) d 62.4 (d, JPP = 36.0 Hz), 45.1
(d, JPP = 36.0 Hz).
Os complex 6. [OsCl2(PPh3)3] (300 mg, 0.286 mmol) and dppb
(146 mg, 0.342 mmol) were dissolved in dichloromethane (4 mL)
and the solution was stirred for 3 h at room temperature. The
solvent was evaporated under reduced pressure and rac-1-(6-
phenylpyridin-2-yl)ethanamine (71 mg, 0.358 mmol) dissolved in
2-propanol (3 mL) and NEt3 (400 mL, 2.87 mmol) were added.
The mixture was refluxed for 3 h obtaining a red precipitate which
was filtered, washed with 2-propanol (3 ¥ 3 mL), heptane (3 ¥
1 mL) and dried under reduced pressure (218 mg, 90%). Found:
C, 57.95; H, 4.91; N, 3.43. Calc. for C41H41ClN2OsP2: C, 57.98;
Preparation of 1-phenylethanol-d5. The osmium complex 5 (4.3
mg, 5.0 mmol) and KOtBu (2.8 mg, 25 mmol) were dissolved in 2-
propanol-d8 (1.5 mL) and 1-phenylethanol (122 mg, 1.00 mmol)
◦
was added under argon. The solution was stirred at 40 C for 5
1
H, 4.87; N, 3.30. H NMR (200.1 MHz; CD2Cl2) d 7.98 (2H,
h and the solvent was evaporated under reduced pressure. After
addition of 2-propanol-d8 (1.5 mL) the solution was stirred for 5
m, aromatic protons), 7.70 (3H, m, aromatic protons), 7.46–6.76
(17H, m, aromatic protons), 6.61 (3H, m, aromatic protons), 5.93
(2H, t, JHH = 7.9 Hz, aromatic protons), 3.69 (1H, m, NH2), 3.49
(1H, m, CHNH2), 3.06 (2H, m, CH2P), 2.47 (2H, m, CH2P), 1.95–
1.80 (5H, m, CH2 and NH2), 1.55 (3H, d, JHH = 5.6 Hz, CH3).
◦
h (40 C). Diethyl ether (2 mL) was added and the solution was
filtered over a short pad of silica, which was washed with diethyl
ether (4 mL). The mixture of solvents was evaporated at about 45
◦C, affording 1-phenylethanol-d5 (110 mg, 87% yield) (0.5 mol%
of 5 and 2.5 mol% of KOtBu).
1
13C{ H} NMR (50.3 MHz; CD2Cl2) d 166.1 (s, NCC), 162.5 (dd,
JCP = 5.7 and 2.5 Hz, OsC), 159.3 (s, NCCH), 148.0–116.6 (m,
aromatic carbons), 60.3 (s, CHNH2), 35.3 (d, JCP = 5.0 Hz, CH2P),
30.9 (d, JCP = 5.1 Hz, CH2P), 26.5 (s, CH2), 23.0 (s, CH3), 21.2 (s,
Acknowledgements
1
CH2). 31P{ H} NMR (81.0 MHz; CD2Cl2) d 0.28 (s).
`
This work was supported by the Ministero dell’Universita e della
Ricerca (MIUR) and the Regione Friuli Venezia Giulia. We thank
Johnson-Matthey/Alfa Aesar for a generous loan of ruthenium
and Mr. P. Polese for carrying out the elemental analyses.
X
Os complex 7. [OsCl2(PPh3)3] (100 mg, 0.095 mmol) and dppf
(58 mg, 0.105 mmol) were dissolved in dichloromethane (1.5
mL) and the solution was stirred for 2 h at room temperature.
The solvent was evaporated under reduced pressure and 1-[6-
(4¢-methylphenyl)pyridin-2-yl]methanamine (24 mg, 0.121 mmol)
dissolved in 2-propanol (3 mL) and NEt3 (132 mL, 0.95 mmol) were
added. The mixture was refluxed for 3 h obtaining a red precipitate
which was filtered, washed with 2-propanol (3 ¥ 1 mL), heptane (3
¥ 1 mL) and dried under reduced pressure (72 mg, 78%). Found:
C, 57.42; H, 4.03; N, 2.88. Calc. for C47H41ClFeN2OsP2: C, 57.76;
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