The importance of the N-H bond in Ru/TsDPEN complexes for asymmetric transfer hydrogenation of ketones and imines

Article abstract of DOI:10.1039/c1ob05208j

Ru(ii) complexes of TsDPEN containing two alkyl groups on the non-tosylated nitrogen atom are poor catalysts for asymmetric transfer hydrogenation of ketones and imines; this observation provides direct evidence for the importance of the N-H interaction i

Full text of DOI:10.1039/c1ob05208j

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PAPER
The importance of the N–H bond in Ru/TsDPEN complexes for asymmetric
transfer hydrogenation of ketones and imines†
Rina Soni,a Fung Kei Cheung,a Guy C. Clarkson,a Jose E. D. Martins,a Mark A. Grahamb and Martin Wills*a
Received 20th November 2011, Accepted 21st February 2011
DOI: 10.1039/c1ob05208j
Ru(II) complexes of TsDPEN containing two alkyl groups on the non-tosylated nitrogen atom are poor
catalysts for asymmetric transfer hydrogenation of ketones and imines; this observation provides direct
evidence for the importance of the N–H interaction in the transition state for ketone reduction.
which the two hydrogen atoms are transferred through the cyclic
Introduction
six-membered transition state depicted in Fig. 1.5,6a Reactions
Asymmetric transfer hydrogenation (ATH) pre-catalysts of gen-
conducted in water appear to be further assisted by an additional
eral structure [(arene)Ru(TsDPEN-H)Cl] 11–3 are formed in
the reaction between TsDPEN 2 and the ruthenium dimer
[(arene)RuCl2]2. During the catalytic cycle, the unsaturated species
3 is formed, and this reacts with a suitable hydrogen donor to form
hydride 4.4 Hydride 4 transfers two hydrogen atoms to a substrate
such as a ketone or an imine to form an alcohol or an amine,
respectively. For ketone reduction, there is convincing evidence
that this transfer takes place via an outer sphere mechanism in
hydrogen bond from the solvent.6b The transition states for the
corresponding imine reductions are less well understood,3c–f,7a
although there is evidence that the iminium salt, formed by
protonation, rather than the free imine, is reduced. This may
involve an ionic mechanism, as has been proposed for related
hydrogenation reactions of imines.7b,7c
Fig. 1 Involvement of N–H from TsDPEN in ATH ketone reduction
transition state using 1 and 5.
Definitive evidence for the importance of the N–H interaction,
however, would require the study of complexes in which this bond
is not present; such complexes would be predicted to be poor
catalysts for reduction.
It has been reported that the N-methylated and N¢,N¢-
dimethylated derivative of TsDPEN are poor catalysts for ATH
reactions when a mesitylene group is employed as the arene.2b If an
6
h -benzene ring is used, however,7a good results can be obtained
with N¢-monoalkylated TsDPENs. Complex 5, which is similar
to 1 but formed from TsDPEN derivatives containing one methyl
group on the basic amine, is highly active in ATH reactions, and
an X-ray structure of the related N¢-benzyl derivative indicated
that the favoured conformation allows the catalytically important
N–H bond to be correctly positioned to interact with the ketone
substrate (also illustrated in Fig. 1).
aDepartment of Chemistry, The University of Warwick, Coventry, UK CV4
7AL. E-mail: m.wills@warwick.ac.uk; Fax: +44 24 7652 3260; Tel: +44 24
7652 4112
bCancer & Infection Chemistry, AstraZeneca, Mereside, Alderley Park,
Macclesfield, Cheshire, UK SK10 4TG
Results and discussion
† Electronic supplementary information (ESI) available: Experimental
details, X-ray structure and NMR spectra. CCDC reference numbers
793888 and 793889. For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/c1ob05208j
In order to eliminate the possibility of involvement of a N–H
bond in the transition state, TsDPEN derivatives with two alkyl
substituents on the basic nitrogen are required. The reaction
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of [(benzene)RuCl2]2 with the N¢N¢-dimethyl-TsDPEN 68 gave
[(benzene)Ru(6-H)Cl] 7, which proved to be sufficiently stable to
be isolated and characterised by X-ray crystallography (Fig. 2).9
The use of a benzene ring in 7 is important; complexes containing
substituted arene rings proved to be less stable. This instability
has been observed by others in attempts to prepare derivatives of
complex 7.10
Scheme 1 Synthesis of catalyst (R,R)-8.
Fig. 2 X-ray crystallographic structure of (R,R)-7.
We also prepared samples of the N¢-alkylated derivatives 8 and
9 of the tethered catalyst 10.11,12 Complex 8 was prepared via the
reaction of 2 and aldehyde 11 to give 12, which was subjected to a
second reductive amination with formaldehyde to form N¢-methyl
derivative 13. The dimer 14, formed by complexation of 13, was
converted into monomer 8 using Et3N in IPA (Scheme 1). An
X-ray crystallographic analysis of 8 (Fig. 3)13 confirmed its
structure. Although the pattern of bond connectivity in 7 and
8 were those predicted, both complexes formed the opposite
diastereoisomer with respect to the configuration at the Ru atom
compared to other TsDPEN-derived complexes.4,7a,11 Complex 9
was prepared in an analogous manner to 8 (see ESI†).
Fig. 4 Time course of acetophenone 15 reduction using tethered catalysts
10 (N–H) and 8 (N-Me). FA/TEA = 5 : 2, [ketone] = 0.86 M, S/C = 100,
25 ◦C, S/C = 100. Followed by 1H-NMR (400 MHz).
Fig. 3 X-ray crystallographic structure of (R,R)-8.
Complexes 7–9 were employed in ATH reductions of ace-
tophenone 15 and cyclic imine 16 (Scheme 2) using formic
acid/triethylamine (5 : 2) (FA/TEA) as the reducing agent. Each
proved to be sluggish relative to the complexes which contain
an N–H bond. Using 1 mol% of catalyst (R,R)-7, acetophenone
15 was reduced in only 1.7% conversion after 6 days (alcohol of
46% ee (R) was formed), whilst the reduction of 16 gave a better
conversion of 91% after 5 days (18% ee (S)). In the case of imine
reduction, the addition of a cosolvent slowed the reaction further.
Fig. 4 and 5 illustrate conversion/time graphs for reduction of
15 and 16, respectively, using catalysts 8 and 10 (see ESI†).
For acetophenone, N¢-methylation resulted in significant loss of
catalytic activity. The non-methylated catalyst (R,R)-10 gave an
alcohol of 96.5% ee (R)11 in 100% conversion after 3 h, whilst
(S,S)-8 gave the same product in 73% ee (R) in just 6% conversion
Fig. 5 Time course of imine 16 reduction using tethered catalysts 10
(N–H) and 8 (N–Me). MeCN cosolvent, FA/TEA = 5 : 2, [imine] = 0.45 M,
S/C = 100, 25 ◦C, S/C = 100. Followed by 1H-NMR (400 MHz).
after 18 h (example for [ketone] = 2 M). Complex (R,R)-9 gave a
product of 36% ee (R) in 17% yield after 4 days ([ketone] = 2 M).
In the case of imine reduction, N¢-alkylated tethered complexes
were less active than the parent catalysts, however reactions did
generally proceed to >95% within 20 h, even in the presence of
a cosolvent. In the example shown in Fig. 5, the amine product
was of 22% ee (S) at the end of the reaction using catalyst (R,R)-8
and 34.5% ee (S) with catalyst (R,R)-10. Although not illustrated,
complex (R,R)-9 gave an amine product of 7% ee (S) in 95%
conversion after 24 h under the same conditions. The relative rate
of reduction of an imine using 8 was not as sharply different to
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Scheme 2 Reduction reactions used to test catalyst activity.
Fig. 6 Time course of RuH peak of catalyst 8 (red line) during reduction
of imine 16 (conversion shown by green line), FA/TEA = 5 : 2, [imine] =
0.50 M, 30 ◦C. Followed by 1H-NMR (700 MHz).
that observed using catalyst 10 containing the ‘N–H’ bond, as it
was for a ketone.
The low reactivity of the N¢-alkylated catalysts relative to the
non-alkylated ones could be due to a number of reasons12,14
including: (i) catalyst decomposition, (ii) slow formation of the
Ru–H species or (iii) slow transfer of the hydride from the
Ru–H species to the substrate. We followed reduction reac-
tions of 15 and 16 and attempted to observe a Ru–H peak
in the reaction.12 Ikariya and Koike have obtained evidence
that the assistance of the N–H bond in 1 is required for
the formation of [(p-cymene)Ru(TsNCH2CH2NH2)OCHO] from
the hydride [(p-cymene)Ru(TsNCH2CH2NH2)H] by insertion
into carbon dioxide; no formation of the Ru–formate complex
was observed in an attempt to add carbon dioxide to [(p-
cymene)Ru(TsNCH2CH2NMe2)H].10 It was not possible to es-
tablish whether the same N–H bond interaction with Ru-bound
formate was required for the decarboxylation to form a hydride. A
similar formate precursor to a rhodium hydride complex has also
been reported.14
alkyl group on the basic nitrogen atom in 8, this and the slower
hydride formation may be responsible for the lower rate of imine
reduction observed with 8 compared to 10. If this is the case,
then our observations suggest that the mechanism of reduction of
imines3f may not rely on the directing effect of the N–H bond in
the catalyst to the same extent.3,7a,14 The extra alkyl groups in 7–9
also reduce the enantioselectivities of reduction reactions which
are catalysed by Ru(II)/TsDPEN complexes.
Experimental section
General experimental details, and procedures for synthesis of
complex precursors and of complex 9, tables, graphical data,
X-ray crystallographic data and NMR spectra may be found in
the ESI.†
In the case of catalyst 7, derived from N¢N¢-dimethylTsDPEN
6, no strong Ru–H peak could be observed during the attempted
reduction of either ketone or imine. This suggests that either 7, or
its hydride derivative, may be undergoing decomposition,10 which
may explain, in part, its lower reactivity. In one case (see ESI†)
where a RuH peak was observed (reduction of 16 using 3 mol%
7) and measured by NMR, this decreased over the course of the
reaction and was <0.2 mol% by the time full imine reduction was
achieved (ca. 180 h). The results obtained with catalyst 8 were more
encouraging. Clear evidence of a Ru–H peak was observed at ca. d
-5.3 which persisted throughout the reduction of both ketone and
imine. At 3 mol% catalyst it was possible to establish that the level
of the Ru–H species increased gradually and eventually remained
constant at a maximum value (Fig. 6); ca. 5 h was required for
90% imine reduction. This would suggest that, although hydride
formation is slow, the rate-limiting step is likely to be the transfer
of the hydride to the imine substrate.
Preparation of N-[(1R,2R)-2-(dimethylamino)-1,2-diphenylethyl]-
4-methylbenzenesulfonamide benzene ruthenium chloride 7
A mixture of N-[(1R,2R)-2-(dimethylamino)-1,2-diphenylethyl]-
4-methylbenzenesulfonamide 6 (0.250 g, 0.635 mmol), ben-
zeneruthenium(II) chloride dimer (0.318 g, 0.635 mmol, 1.0 eq)
and triethylamine (0.353 mL, 2.54 mmol, 4.0 eq) in IPA (25 mL)
was heated at 80 ◦C for 1 h under an inert atmosphere. The reaction
mixture was cooled to room temperature and concentrated to give
a residue. The residue was filtered and washed with water to leave
a solid. The solid was purified by flash column chromatography
on Florisil. The complex was eluted in hexane : EtOAc : MeOH
(5 : 4 : 1) to give compound 7 as a light brown solid (0.145 g,
0.242 mmol, 38%). m.p. 146–148 ◦C with decomposition; [a]D
=
20
+1687 (c = 0.0048 in CHCl3); nmax = 2921, 1452, 1437, 1252, 1129,
1086, 942, 809, 699, 663 cm-1; 1H NMR (300 MHz, CDCl3, TMS):
d 7.37 (2H, d, J = 8.0 Hz, o-CH of -SO2C6H4CH3), 7.15–7.11 (2H,
m, ArH), 6.98–6.92 (4H, m, ArH), 6.80 (2H, d, 3J = 8.0 Hz, m-CH
of -SO2C6H4CH3), 6.60–6.54 (4H, m, ArH), 5.79 (6H, s, C6H6),
4.90 (1H, d, J = 11.7 Hz, CHN(CH3)2), 4.58 (1H, d, J = 11.7 Hz,
CHNHTs), 3.20 (3H, s, N(CH3)2), 2.89 (3H, s, N(CH3)2), 2.20
(3H, s, CH3); 13C NMR (75 MHz, CDCl3, TMS): d 142.27, 139.65,
139.19, 130.11, 129.65, 128.56, 128.10, 126.80, 126.33, 125,46,
84.46, 76.88, 66.38, 52.24, 50.15, 21.17; m/z ESI-MS [M - Cl]+
573.0; HRMS found 573.1147 (C29H31ClN2O2RuS - Cl requires
573.1147, error = 0.7 ppm).
Conclusions
Taken together, these results suggest that, in [(arene)Ru(TsDPEN-
H)H] catalysts, the presence of the N–H bond is (i) beneficial, but
not essential, for formation of the ruthenium hydride species,10 (ii)
essential for the transfer of hydrogen to ketones in the reduction
step and (iii) beneficial but not essential for the transfer of
hydrogen to imines. Whilst it is difficult to factor in the clearly
important effect of the extra steric hindrance created by a second
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Reduction of acetophenone 15 and imine 16 or its salt
Preparation of {N-[(1R,2R)-2-((3-cyclohexa-1,4-dienyl)propyl)-
(methyl)ammonium chloride)-1,2-diphenylethyl]-4-methylbenzene-
sulfonamide} ruthenium chloride dimer 14
In neat FA/TEA. A mixture of imine/ketone (50 mg), catalyst
(1 mol%) in FA : TEA (5 : 2) (0.2 mL) was stirred at 30 ◦C for
18–22 h under an inert atmosphere. For reaction monitoring, an
aliquot of the reaction mixture was filtered through a plug of silica
and analyzed by chiral GC for % conversion and ee.
To a solution of N-[(1R,2R)-2-((3-cyclohexa-1,4-dienyl)propyl)-
(methyl)amino)-1,2-diphenylethyl]-4-methylbenzenesulfonamide
13 (0.355 g, 0.710 mmol) in DCM (10 mL) was added a 2 M
solution of HCl in diethyl ether (0.89 mL, 1.77 mmol) and the
mixture was stirred at 22 ◦C for 30 min under an inert atmosphere.
The solvents were removed under reduced pressure to give a
residue. This was dissolved in ethanol (20 mL) and ruthenium
trichloride trihydrate (0.139 g, 0.532 mmol) was added. The
resulting mixture was heated at 78 ◦C for 16 h. The reaction
mixture was cooled, solid separated out, filtered and washed with
ethanol to give compound 14 as a green solid (0.250 g, 0.177 mmol,
50%) which was used directly in the next step, m.p. > 300 ◦C; m/z
ESI-MS [M - Cl]+ 599.1 (monomer formed by dimer cleavage and
In solvent. A mixture of imine/ketone (50 mg), catalyst
(1 mol%) and FA : TEA (5 : 2) (0.2 mL) in solvent (0.4 mL) was
stirred at 30 ◦C for 18–22 h under an inert atmosphere. For reaction
monitoring, an aliquot of the reaction mixture was filtered through
a plug of silica and analyzed by chiral GC, with comparison to an
authentic sample of the required material, for % conversion and
ee (retention times given in ESI†).
400 MHz NMR kinetic study of the reduction of acetophenone.
To a 5 mm NMR tube were added catalyst (0.01 mmol), and
formic acid/triethylamine 5 : 2 complex (1 mL). After 30 min,
acetophenone was added (120 mg, 1 mmol) followed by 0.05 mL
of C6D6 hence providing a substrate solution of initially ca.
0.86 M. The reaction was followed by 1H-NMR until the specified
conversion was achieved. The conversion was calculated by the
integration of the methyl peak from the starting material at ca.
2.44 ppm and the CH from the product at ca. 4.87 ppm. Note that
the exact positions of these peaks vary slightly depending on the
exact nature of each sample (solvent, concentration etc.). At the
end of the reaction the reaction mixture was flushed through a
short pad of silica using EtOAc to elute. The alcohol was isolated
by flash chromatography on silica gel and its ee was determined by
chiral GC with comparison to an authentic sample of the required
material, for % conversion and ee (retention times are given in
ESI†).
1
loss of HCl in situ); H NMR (300 MHz, d6-DMSO, TMS): d
9.30–8.50 (2H, 4 ¥ brs, NH), 7.60–6.80 (30H, m, ArH), 6.08–6.00
6
6
(4H, m, h C6H5), 5.95–5.80 (6H, m, H h C6H5), 5.15–5.05 (2H,
m, CH), 4.95–4.80 (2H, m, CH), 2.90–2.00 (12H, m, CH2), 2.40
(12H, brs, CH3).
Preparation of {N-[(1R,2R)-2-((3-cyclohexa-1,4-dienyl)propyl)-
(methyl)amino)-1,2-diphenylethyl]-4-methylbenzenesulfonamide}
ruthenium chloride monomer 8
A
mixture
of
{N-[(1R,2R)-2-((3-cyclohexa-1,4-
dienyl)propyl)(methyl)ammonium chloride)-1,2-diphenylethyl]-
4-methylbenzenesulfonamide} ruthenium chloride dimer 14
(0.275 g, 0.195 mmol) and triethylamine (0.162 mL, 1.168 mmol,
6.0 eq) in IPA (15 mL) was heated at 80 ◦C for 1 h under an
inert atmosphere. The reaction mixture was cooled to room
temperature and concentrated to give a residue. This was
filtered and washed with water. The solid was purified by flash
column chromatography on Florisil. The complex was eluted in
hexane : EtOAc : MeOH (5 : 4 : 1) to give compound 8 as a light
brown solid (0.175 g, 0.275 mmol, 70%). m.p. 184–186 ◦C with
400 MHz NMR kinetic study for reduction of imine. To a
5 mm NMR tube were added catalyst (0.005 mmol), and formic
acid/triethylamine 5 : 2 complex (0.25 mL). After 30 min a solution
of imine 16 (0.5 mmol) in acetonitrile (0.8 mL) was added followed
by 0.05 mL of C6D6 hence providing a substrate solution of initially
ca. 0.45 M. The reaction was followed by 1H-NMR until complete
reduction was observed. The conversion was calculated by the
integration of the aromatic proton peak from the starting material
(two singlets at ca. 6.99, 6.69 ppm) and the product (two singlets
at ca. 6.50, 6.40 ppm). Note that the exact positions of these
peaks vary slightly depending on the exact nature of each sample
(solvent, concentration etc.). At the end of the reaction the reaction
mixture was flushed through a short pad of silica using EtOAc to
elute. The amine product was isolated by flash chromatography on
silica gel and its ee was determined by chiral GC with comparison
to an authentic sample of the required material, for % conversion
and ee (retention times are given in ESI†).
24
decomposition; [a]D = +1394 (c = 0.0052 in CHCl3); nmax 3435,
2973, 2924, 1600, 1454, 1267, 1129, 1085, 1045, 940, 841, 699,
1
664 cm-1; H NMR (300 MHz, CDCl3, TMS): d 7.44 (1H , br s,
ArH), 7.28 (2H ,d, J = 7.6 Hz, ArH), 7.22 (1H, br s, ArH), 7.11
(1H, t, J = 6.9 Hz, ArH), 6.97 (1H, br s, ArH), 6.89 (2H, br d, J =
5.2 Hz, ArH), 6.73 (2H, d, J = 7.6 Hz, m-CH of -SO2C6H4CH3),
6.62–6.58 (2H, m, ArH), 6.53–6.49 (2H, m, ArH), 6.44 (1H, t,
6
J = 4.9 Hz, p-CH of h C6H5), 6.29 (1H, d, J = 4.4 Hz, o-CH of
6
6
h C6H5), 5.74 (1H, t, J = 5.3 Hz, m-CH of h C6H5), 5.46 (1H, t,
6
J = 5.2 Hz, m-CH of h C6H5), 5.31 (1H, d, J = 5.6 Hz, o-CH of
6
h C6H5), 4.87 (1H, d, J = 11.8 Hz, CHN(CH2)3-), 4.70 (1H, d, J =
11.8 Hz, CHNTs), 3.34–3.28 (1H, m, NCH2), 2.93 (1H, br d, J =
13.2 Hz, NHCH2), 2.83 (3H, s, NCH3), 2.77 (1H, br d, J = 9.2 Hz,
-CH2CH2CH2), 2.43–2.33 (1H, m, CH2CH2CH2), 2.33–2.24 (2H,
m, CH2CH2CH2), 2.17 (3H, s, CH3); 13C NMR (75 MHz, CDCl3,
TMS): d 141.90, 139.52, 139.32, 134.40, 130.41, 130.12, 128.52,
127.94, 126.74, 126.24, 125.20, 88.05, 87.12, 85.97, 85.05, 84.79,
84.59, 79.14, 66.42, 53.53, 48.40, 28.60, 23.77, 21.14; m/z ESI-MS
[M - Cl]+ 599.1; HRMS found 599.1314 (C31H33ClN2O2RuS - Cl
requires 599.1308, error = -1.0 ppm).
700 NMR reactions for reduction of imine. To a 5 mm NMR
tube were added the imine 16 (0.731 mmol), catalyst (1 or 3 mol%),
and formic acid/triethylamine 5 : 2 complex (0.6 mL), followed
1
by 0.05 mL of C6D6. The reaction was followed by H-NMR
with hydride detection until the maximum level of reduction was
observed. The conversion was calculated, and the product isolated,
by following the procedure in the paragraph above.
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5 (a) M. Yamakawa, I. Yamada and R. Noyori, Angew. Chem., Int. Ed.,
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Acknowledgements
We thank Warwick University for support of RS through a
scholarship, EPSRC for financial support of JEDM (EP/F019424)
and EPSRC/AstraZeneca for support of FKC via a collaborative
studentship. The X-ray diffractometer was funded through the
AWM Science City Advanced Materials project and ERDF
support.
7 (a) J. E. D. Martins, G. J. Clarkson and M. Wills, Org. Lett., 2009, 11,
847–850; (b) M. P. Magee and J. R. Norton, J. Am. Chem. Soc., 2001,
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triclinic, space group P1, a = 10.3603(2), b = 11.0519(3), c = 14.0747(3)
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◦
◦
◦
˚
A, a = 90.6376(19) , b = 103.5283(17) , g = 107.614(2) , U = 1487.56(6)
3
˚
A
(by least squares refinement on 22 067 reflection positions), T =
100(2) K, l = 0.71073 A, Z = 2, D(calc) = 1.461 Mg m-3, F(000) =
676. m(MoK-a) = 0.721 mm-1. Crystal character: orange block. Crystal
dimensions 0.40 ¥ 0.18 ¥ 0.14 mm. Further details are given in the ESI.
10 T. Koike and T. Ikariya, Adv. Synth. Catal., 2004, 346, 37–41.
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˚
12 (a) F. K. Cheung, C. Lin, F. Minissi, A. Lorente Criville´, M. A. Graham,
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13 X-ray data for 8; CCDC no. 793888, C31H33ClN2O2RuS, M = 634.17, or-
thorhombic, space group P2(1)2(1)2(1), a = 8.39707(8), b = 9.14896(9),
´
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◦
◦
◦
3
˚
˚
c = 35.0858(3) A, a = 90 , b = 90 , g = 90 , U = 2695.44(4) A (by
least squares refinement on 7176 reflection positions), T = 100(2) K,
˚
Soc., 2005, 127, 1883–1894; (f) J. B. Aberg, J. S. M. Samec and J.-E.
-3
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l = 1.54184 A, Z = 4, D(calc) = 1.563 Mg m , F(000) = 1304. m(MoK-
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a) = 6.436 mm-1. Crystal character: orange block. Crystal dimensions
0.2108 ¥ 0.1391 ¥ 0.0319 mm. Further details are given in the ESI.
14 A. J. Blacker, E. Clot, S. B. Duckett, O. Eisenstein, J. Grace, A. Nova,
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4 K. J. Haack, S. Hashiguchi, A. Fujii, T. Ikariya and R. Noyori, Angew.
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3294 | Org. Biomol. Chem., 2011, 9, 3290–3294
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