Ruthenium nitronate complexes as tunable catalysts for olefin metathesis and other transformations

Article abstract of DOI:10.1039/c2cc37385h

Novel ruthenium(ii) complexes were obtained as a result of a stoichiometric reaction of Grubbs' benzylidene second generation catalysts with 3-nitropropene. These stable complexes, formally ruthenaisoxazole N-oxide derivatives, display activity in both me

Full text of DOI:10.1039/c2cc37385h

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Ruthenium Nitronate Complexes as Tunable Catalysts for Olefin
Metathesis and Other Transformations
a
a
a
b
b
Tomasz Wdowik, Cezary Samojłowicz, Magdalena Jawiczuk, Maura Malińska, Krzysztof Woźniak,
a
and Karol Grela*
Received (in XXX, XXX) Xth XXXXXXXXX 200X, Accepted Xth XXXXXXXXX 200X
First published on the web Xth XXXXXXXXX 200X
DOI: 10.1039/b00000000x
Novel ruthenium(II) complexes were obtained as a result of a
stoichiometric reaction of Grubbs' benzylidene second generation
catalysts with 3-nitropropene. These stable complexes, formally
ruthenaisoxazole N-oxide derivatives, display activity in both
Under the same conditions analogue 8 was obtained from
complex 1c in slightly higher yield.
Cat
+B(OPh)3 high yields of 66
none of was
+
Additive
Results
4
b
metathesis
and
non-metathetic
processes
such
as
O
N
R
R
6
cycloisomerisation, isomerization and hydrogenation.
O
formed, instead
new Ru-alkylidene
complex was
cat
additive
1b/1c+none
6
Since the discovery of stable molybdenum and ruthenium
catalysts, olefin metathesis has became powerful tool in
O
N
observed in NMR
O
1
N
N
organic synthesis. During the last twenty years the large
5
Cl
number of reports concerning new improved catalysts was
published, and the scope of the metathesis transformation was
1
b
Ru
DCM
20 h, RT
O
N
2
P
greatly extended. Moreover, application of these catalysts in
O
7, 55% yield
3
non-metathetic transformations has been also reported. In the
3
ruthenium-branch of metathesis, a great deal of work was
invested to fine-tuning the properties of the “parent”
ruthenium benzylidene (1), indenylidene (2) and 2-alkoxy-
N
N
1
c
Cl
4
,5
benzylidene (3) complexes (Fig. 1).
Ru
DCM
0
.25 h, RT
O
N
L
Ru
P
L
Ru
P
L
P
O
8, 66% yield
Ph
Cl
Cl
Cl
Ru
3
Cl
Ph
Cl
Cl
Scheme 1 Preliminary results of CM of 3-nitropropene 5 with
complexes 4b and 1b and the optimised synthesis of 7 and 8
O
Z
3
3
1
a-c
2a-c
3a-c (Z = H)
a-c (Z = NO2)
These complexes were stable enough for isolation using
standard silica gel column chromatography on air. Structures of
new complexes were determined using NMR spectroscopy and
mass spectrometry. Complexes 7 and 8, bearing a cyclic nitronate
4
a: L =
b: L =
P
3
N
N
N
N
c: L =
(
azinate) skeleton, can be formally classified as ruthenaisoxazole
7
N-oxides. To confirm the connectivity of atoms in the complex 8
(see Fig. 2), the single-crystal X-ray structure analysis has been
determined. The collection and refinement parameters for the
crystallographic analysis are presented in the Supporting
Information. The distance between the ruthenium atom and
carbon atom from NHC ligand is equal to 2.07(1) Å (Ru(1)-C(1))
and the bond length between Ru(1)-P(1) is 2.406(3) Å. The five-
membered azinate ring is flat with the Ru(1)O(1)N(3)C(30)
torsion angle equal to −3(1)º. The formation of the bond between
the ruthenium and oxygen atoms (Ru(1)-O(1) = 2.059(8) Å)
causes the elongation of the N(3)-O(1) bond (1.33(1) Å
comparing to the N(3)-O(2) bond 1.27(1) Å). The bond distance
between the metal centre and carbon atom C(31) is equal to
1.88(1) Å. A repulsion between the azinate, mesyl and trialkylo-
phosphane groups results in the decrease of the P(1)Ru(1)C(1)
angle to 165.0(3)º.
Fig. 1 Selected catalyst for olefin metathesis
Recently we have reported on the application of 3-
nitropropene 5 as a partner for cross-metathesis (CM) of terminal
olefins. This transformation was possible only when catalyst 4b
was applied in the presence of a Lewis acid, while no CM
6
product was observed when Grubbs' second generation catalysts
6
1
b was used. Interestingly, NMR inspection of the reaction
mixture suggested that the reaction of 1b with 3-nitropropene
probably leads to a new alkylidene complex.
As a result of this serendipitous observation, we describe
herewith a study on preparation and chemical properties of
new ruthenium alkylidene complexes formed in the reaction
between Grubbs’ benzylidene complexes 1b,c and 3-
nitropropene 5. Complex 7 was easily obtained in the reaction
of 1b and 3-nitropropene (1.25 equiv.) at room temperature in
5
5% yield as a green micro-crystalline solid (Scheme 1).
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9
additions made possible to discover that CCl
activates strongly catalysts 7 and 8 towards RCM. For example,
reaction of 9 promoted by 8 (5 mol%) in CCl at 60 °C gave
product 10 in 97% yield after 2 h (Table 1). Other haloalkanes,
such as CBr and C Cl , used as additives, activate the nitroniate
4
, used as a solvent,
4
4
2
6
catalyst to a similar extend (Fig. 3 and Table 1).
It may be reasonable to expect that acids (CSA or TMSCl) and
chlorinating agents can open the ruthenaisoxazole ring to
form
catalytically
more
active
or TMS) and
Cl(X’)Ru=CHCH(X)NO . Unfortunately, we were not able
species
L
L
2
Cl(X’)Ru=CHCH=N(O)OR (R
=
H
1
0
2
2
to isolate nor characterise in solution the catalytically active
species formed from 7 or 8 upon action of these additives.
Fig. 2 ORTEP representation of 8 (CCDC 905000; probability ellipsoids
at 50% level, hydrogen atoms omitted for clarity)
Table 1 Influence of additives on RCM of 9
Entry Cat
(mol%)
Solvent
Temp
[°C]
Additive
(mol%)
Yield 10
[a]
[%]
100
1
2
3
7 (5)
8 (5)
7 (5)
CHCl
CCl
Toluene
3
60
60
80
none
none
CBr (20)
4
45
97
74
8
0
0
0
0
0
4
6
4
2
[a] Yield of 10 after 2 h determined by GC using internal standard
Using the optimised conditions, i.e. 1 mol% of 7, 4 mol% of
hexachloroethane in toluene at 80 °C leads to formation of 10 in
high yield (Figure 3, ●). Application of this system to other
11
dienes and alkenes was also successful (Table 2).
0
1
2
3
4
5
6
7
8
9
10
Time / h
Table 2 Catalytic RCM and CM reactions promoted by 7 and 8. TBDMS
tert-butyldimethylsilyl, Ac = acetyl
=
Fig. 3 RCM of 9: promoted by 7 (1 mol%). GC yield of 10 determined by
GC using internal standard. ■ - in toluene (80 °C); ▲ - in CCl (60 °C); □
in toluene (80 °C) with TMSCl (4 mol%); ○ - in toluene (80 °C) with
CSA (4 mol%); ● - in toluene (80 °C) with C Cl (4 mol%)
[
a
4
Entry
Cat Substrate
Product
Yield [%]
-
OTBDMS
OTBDMS
2
6
[b]
1
7
N
O
96
According to Cambridge Structural Database (CSD, version
.33, November 2012) this is the first example of the
ruthenaisoxazole N-oxide catalyst structure reported.
N
O
5
1
2
13
OH
Ph
OH
Ph
The unusual structure of these complexes has raised a
question concerning their chemical reactivity, therefore we tested
[
b]
2
3
7
88
65
1
5
7
and 8 in the ring closing metathesis (RCM) transformation of a
14
16
8
standard model diene – diethyl diallylmalonate 9 (Scheme 2).
OAc
Ph
Ph
[
c]
7
7
7
18
E/Z = 4:1
E
E
E
E
E
E
(
1-5 mol%)
(5 mol%)
AcO
OAc
+
17 (2 equiv.)
see Figure 3
and Table 1
MeOH
65 °C, 50 h
[
a] Isolated yield of analytically pure product; [b] 7 (1 mol%) + C
mol%) in toluene (80 °C), 3 h; [c] 7 (2 mol%) + C Cl (8 mol%) in
2
Cl
6
(4
1
0
up to 100 %
9
82%
11
2 6
Scheme 2 Reactivity profiles of complex 7 with diene 9 (E =
CO Et)
toluene (80°C), 24 h
2
Formation of cycloisomer 11 as a product of reaction of 9
conducted in a protic solvent (Scheme 2) encouraged us to study
a possibility of using the Ru-nitronate complexes in selective
isomerisation and cycloisomerisation reactions (Table 3). It
should be noted that the sulphonamide derivative 19 (Table 3,
entry 2) leads upon action of 7 in methanol to product 20
RCM of diene 9 in the presence of 1 mol% of 7 in DCM at
room temperature was not successful. The same reaction
conducted at higher temperature in toluene gave RCM product 10
but in rather low yield (Fig. 3, curve ■). Interestingly, after
prolonged heating in methanol, a product of cycloizomerization
1
2
(
11) was observed instead of the RCM product 10 (Scheme 2).
containing a tetrasubstituted double bond. Finally, the double
Therefore, we decided to investigate this reaction in more
bond in cyclopentene 10 can be isomerised to yield 21, by using
detail. We found that both activity of 7 and its selectivity towards
the nitronate complex in more acidic CF
3
CH OH (entry 3). Also
2
RCM can be increased by addition of camphorosulfonic acid
other conditions can be used to isomerise the C-C double bond in
10, such as heating in toluene with addition of
vinyloxytrimethylsilane and 8.
(
CSA) or TMSCl (Fig. 3, curves □ and ○). On the other hand,
our unsuccessful trials of application of 7 in Kharasch-type
2
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b
Table
3
Reactivity of complexes
7
in isomerisation and
Department of Chemistry, Warsaw University, Pasteura 1, 02-093
Warsaw (Poland).
cycloisomerisation reactions. Ts = tosyl.
†
Electronic Supplementary Information (ESI) available.
See DOI: 10.1039/b000000x/
Yield [%]
Entry
Substrate
Product
[b]
‡ TW is grateful to the Foundation for Polish Science, for the ‘Ventures’ grant. The
project ‘Ventures/2009-4/9’ was realized within the ‘Ventures’ program of the
Foundation for Polish Science, co-financed from the European Regional Development
Fund within Innovative Economy Operational Programme. CS acknowledges
personal fellowship from the “START-2012” grant for young researchers, which was
given by the Foundation for Polish Science. Special thanks addressed to Michał
Dutkiewicz for the technical assistance during X-Ray sample preparation.
EtO
2
2
C
C
EtO
EtO
2
2
C
C
[
a]
1
2
3
82
88
75
EtO
11
9
1
a) Handbook of Metathesis, ed. R. H. Grubbs, WILEY-VCH,
Weinheim, 2003, vols. 1-3; b) M. Michalak, Ł. Gułajski and K. Grela,
in Science of Synthesis: Houben–Weyl Methods of Molecular
Transformations, ed. A. de Meijere; Georg Thieme Verlag KG, 2010
vol. 47a (Alkenes), pp. 327-438.
[
a]
Ts
N
Ts
N
20
1
9
EtO
2
2
C
EtO
2
2
C
[
c]
2
3
4
5
a) Y. Vidavsky, A. Anaby and N. G. Lemcoff, Dalton Trans., 2012,
EtO
C
EtO
C
4
1
1, 32-43; b) N. Holub and S. Blechert, Chem. Asian J., 2007, 2,
064-1082; c) A. Fürstner, Chem. Commun., 2011, 47, 6505-6511.
1
0
21
[
a] Conditions: 5 mol% of 7, methanol, 65 °C; [b] Yield after 50 h
determined by GC using internal standard. Products identifed by
comparison with independently prepared samples; [c] Conditions: 5 mol
a) B. Alcaide and P. Almendros, Chem. Eur. J., 2003, 9, 1258-1262;
b) B. Schmidt, Eur. J. Org. Chem., 2004, 1865-1880; c) B. Alcaide,
P. Almendros and A. Luna Chem. Rev., 2009, 109, 3817-3858.
a) G. C. Vougioukalakis and R. H. Grubbs, Chem. Rev., 2010, 110,
%
of 8, trifluoroethanol 70 °C
1
2
746-1787 b) C. Samojłowicz, M. Bieniek and K. Grela Chem. Rev.,
009, 109, 3708-3742.
O
O
O
7
, i)
7
, ii)
For selected ruthenium complexes with mixed anionic ligands, see: a)
J. J. Van Veldhuizen, S. B. Garber, J. S. Kingsbury and A. H.
Hoveyda, J. Am. Chem. Soc., 2002, 124, 4954-4955; b) J. O. Krause,
S. Lubbad, O. Nuyken and M. R. Buchmeiser, Adv. Synth. Catal.,
94%
50%
Ph
Ph
Ph
2
2
23
24
7
, iii)
O
2
003, 345, 996-1004; c) P. Teo and R. H. Grubbs, Organometallics,
53%
O
OH
7
, iv)
2010, 29ꢀ ꢁ ꢂꢃꢄꢅꢆꢂꢃꢅꢃꢇ ꢁ ꢈꢉ ꢁ ꢊꢋ ꢁ ꢌꢍꢎꢏꢐꢀ ꢁ ꢑꢋ ꢁ ꢒꢍꢓꢔꢕꢓ ꢁ ꢖꢗꢈ ꢁ ꢘꢋ ꢁ ꢙꢚꢕꢗ
Organometallics, 2011, 30, 3971-3980; e) For a review on olefin
metathesis catalysts with modified anionic ligands, see: E. B.
Anderson and M. R. Buchmeiser, Synlett, 2012, 23, 185-207.
T. Wdowik, C. Samojłowicz, M. Jawiczuk, A. Zarecki and K. Grela,
Synlett, 2010, 2931-2935.
Ph
Ph
78%
Ph
2
5
2
6
27
Scheme 3 Diverse reactivity of 7 in RCM and non-metathetic
processes. i) 7 (1 mol%), C Cl (4 mol%), toluene, 80 °C, 3 h; ii) 7
5 mol%), methanol 65 °C, 50 h; iii) 7 (5 mol%), KHMDS (10 mol
), toluene 80 °C, 65 h; iv) 7 (2 mol%), NaH (7 mol%), i-PrOH
50 equiv.), THF, 70 °C, 5 h.
6
7
2
6
(
%
(
According to our knowledge, this structural motif is quite rare within
organometallic complexes of ruthenium. The most related complex
(
non-alkylidene one) was disclosed: J. N. Coalter, W. E. Streib and K.
G. Caulton Inorg. Chem. 2000, 39, 3749-3756.
T. Ritter, A. Hejl, A. G. Wenzel, T. W. Funk and R. H. Grubbs
Organometallics, 2006, 25, 5740-5745.
For Kharash reaction promoted by Ru olefin metathesis catalysts, see:
B. Schmidt, M. Pohler and B. Costisella J. Org. Chem., 2004, 69,
Next, we have found that depending on conditions, Ru-
8
9
nitronate catalyst can induce a C-C double bond shift over one or
3
,13
over two positions in the RCM product 23 (Scheme 3).
Separate “blind” experiments, conducted under identical
conditions but without the metal complex, have shown that these
transformations do not undergo in the absence of 7. We have also
checked preliminary that reduction of a carbonyl group is also
possible with 7, however, under different conditions. Using
isopropyl alcohol and NaH, we successfully carried out of the
1
421-1424.
10 For an alternative reaction pathway between CCl4 and some Ru(II)
complexes, see: T. Ando, M. Kamigaito and M. Sawamoto,
Tetrahedron, 1997, 53, 15445-15457.
11
For selected chemo-activated Ru-catalysts, see: a) B. De Clercq and
F. Verpoort, Adv. Synth. Catal., 2002, 344, 639-648 b) R. Gawin, A.
Makal, K. Woźniak, M. Mauduit and K. Grela Angew. Chem. Int. Ed.
1
4
reduction of acetophenone 26 (Scheme 3).
In conclusion, our studies shown that the reaction of Grubbs'
second generation catalysts with 3-nitropropene leads to new
chelating Ru-alkylidene complexes bearing a nitronate ligand.
These complexes show catalytic activity both in olefin metathesis
and in non-metathetic processes. Applications of these complexes
in metathesis requires activation by hexachloroethane or
2
007, 46, 7206-7209; c) C. Pietraszuk, S. Rogalski, B. Powała, M.
Miętkiewski, M. Kubicki, G. Spólnik, W. Danikiewicz, K. Woźniak,
A. Pazio, A. Szadkowska, A. Kozłowska and K. Grela Chem. Eur. J.
2
Grela Curr. Org. Chem., 2008, 12, 1631-1647; e) S. Monsaert, A.
Lozano Vila, R. Drozdzak, P. Van Der Voort and F. Verpoort, Chem.
Soc. Rev., 2009, 38, 3360-3372.
012, 18, 6465-6469; For a review, see: d) A. Szadkowska and K.
trimethylsilyl
chloride.
Other
processes
such
as
12 a) B. M. Trost and M. J. Krische, Synlett 1998, 1-16; b) G. C. Lloyd-
Jones, Org. Biomol. Chem., 2003, 1, 215-236.
cycloisomerisation, isomerization of a C-C double bond and
hydrogenations are possible under precisely selected conditions.
While there are known other olefin metathesis catalysts of higher
activity, the diverse reactivity exhibited by 7 and 8 makes these
complexes of interest in organic synthesis.
1
3 For a review, see: a) B. Schmidt, Eur. J. Org. Chem., 2004, 1865-
880. For selected examples, see: a) T. J. Donohoe, T. J. C. O’Riordan
1
and C. P. Rosa, Angew. Chem., Int. Ed., 2009, 48, 1014-1017; c) S.
Fustero, M. Sanchez-Rosello, D. Jimenez, J. F. Sanz-Cervera, C. del
Pozo and J. L. Acena, J. Org. Chem., 2006, 71, 2706-2714; d) S.
Hanessian, S. Giroux and A. Larsson, Org. Lett., 2006, 8, 5481-5484;
e) B. Schmidt, J. Org. Chem., 2004, 69, 7672-7687; f) M. K. Gurjar
and P. Yakambram, Tetrahedron Lett., 2001, 42, 3633-3636.
Notes and references
a
Institute of Organic Chemistry, Polish Academy of Sciences,
14 S. Gladiali, G. Mestroni, in Transition Metals for Organic Synthesis,
eds. M. Beller, C. Bolm, WILEY-VCH, Weinheim, 1998, vol. 2, pp.
97-119.
Kasprzaka 44/52, 01-224, Warsaw (Poland). Fax: (+) 48-22-632-66-81;
E-mail: klgrela@gmail.com; Homepage: http://www.karolgrela.eu/
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