Studies on synthesis of quinonylidene Hoveyda-type complexes

Article abstract of DOI:10.1002/aoc.3294

In a quest of redox-switchable metathesis catalysts we attempted synthesis of ruthenium quinonylidene complexes using two synthetic pathways. First, Hoveyda-type complexes bearing chelating benzylidene and naphthylidene ligands substituted with two alkoxy/hydroxy groups were synthesized and characterized. The catalysts were tested in model ring-closing metathesis reactions, and displayed interesting correlations between structure and catalytic activity. Unfortunately, numerous attempts at oxidation of the complexes to derivatives of benzo- and naphthoquinone were unsuccessful. However, the second approach, using exchange reaction of ruthenium precursor with vinylquinone ligand, gave a transient unstable product observed with ¹H NMR. The experimental data suggest that conjugation of electron-deficient quinones to the ruthenium centre results in intrinsically unstable species, which undergo secondary reactions under ambient conditions.

Full text of DOI:10.1002/aoc.3294

Full Paper
Received: 20 October 2014
Revised: 21 January 2015
Accepted: 21 January 2015
Published online in Wiley Online Library
(wileyonlinelibrary.com) DOI 10.1002/aoc.3294
Studies on synthesis of quinonylidene
Hoveyda-type complexes
Krzysztof Grudzień and Michał Barbasiewicz*
In a quest of redox-switchable metathesis catalysts we attempted synthesis of ruthenium quinonylidene complexes using two syn-
thetic pathways. First, Hoveyda-type complexes bearing chelating benzylidene and naphthylidene ligands substituted with two
alkoxy/hydroxy groups were synthesized and characterized. The catalysts were tested in model ring-closing metathesis reactions,
and displayed interesting correlations between structure and catalytic activity. Unfortunately, numerous attempts at oxidation of
the complexes to derivatives of benzo- and naphthoquinone were unsuccessful. However, the second approach, using exchange
1
reaction of ruthenium precursor with vinylquinone ligand, gave a transient unstable product observed with H NMR. The exper-
imental data suggest that conjugation of electron-deficient quinones to the ruthenium centre results in intrinsically unstable spe-
cies, which undergo secondary reactions under ambient conditions. Copyright © 2015 John Wiley & Sons, Ltd.
Additional supporting information may be found in the online version of this article at the publisher’s web-site.
Keywords: ruthenium; quinones; redox-switchable complexes; metathesis; catalytic activity
Introduction
Results and Discussion
[
1]
Switchable catalysts offer useful advantages over persistent cata-
lytic systems, enabling control of activity and selectivity in the
course of chemical transformations. In the area of olefin
Synthesis of Ligands
Our studies began from the synthesis of several organic precursors
derived from 1,2- and 1,4-dialkoxybenzenes, and 1,2- and
[2]
[3,4]
metathesis, stimuli such as protonation,
addition of Lewis
and changes of redox
have been demonstrated to be effective factors that
2,3-dialkoxynaphthalenes (Scheme 2, top). Compounds 3a, 3b
[4]
[5]
[6,7]
acids, temperature, UV irradiation
and 5a–c were synthesized from commercially available 2,5-
dimethoxybenzaldehyde, o-vanilin and 2,3-dihydroxybenzaldehyde.
Sterically demanding ligand 4 was constructed using the AlCl
catalysed Friedel–Crafts alkylation of 1,4-dimethoxybenzene
with 2,5-dichloro-2,5-dimethylhexane,
[8–11]
potential
may be applied to switch catalysts between the on and off states. In
the latter class of complexes, for which oxidation controls the cata-
lytic activity, structures modified with redox-active ferrocenyl group
present in the N-heterocyclic carbene (NHC) 1a (Fig. 1) or in the
chelating nitrogen ligand 1b were described. The studies were
3
-
[18]
followed by Vilsmayer
[8]
formylation. Naphthalene derivatives 6 and 7 were obtained
by methylation of isomeric 1,2- and 2,3-dihydroxynaphthalenes,
[
9]
performed on the assumption that the redox process runs on the
followed by ortho-directed metalation and reaction with
[12]
ancillary ligands, and the catalytic centre of the metathesis reac-
tion remains stable under the reaction conditions. Interestingly, ox-
idant additives also displayed useful effects on metathesis reactions
catalysed with unmodified Grubbs and Hoveyda (2) complexes.
It was demonstrated that the presence of quinones in the reaction
mixtures preserves undesired isomerizations of the olefinic
and facilitates the removal of residual ruthenium
species.
Following our research programme
[15b]
DMF.
So-formed aldehydes were subjected to Wittig re-
[19]
agents (Me or Et) under usual conditions.
Interestingly, the synthesized ligands 3–7 seem to be also valu-
able substrates for the preparation of VQ ligands by direct oxidation
to quinones. Unfortunately, a preliminary literature search revealed
that the transformation can be a difficult task. In fact, preliminary
screening of oxidation of the precursors revealed only formation of
complicated mixtures or dark polar degradation products. Fortu-
nately, in a further synthetic attempt we succeeded in preparation
of a VQ ligand 11 (Scheme 2, bottom). Starting from a commercially
[
13]
[20]
[
14]
products,
[
10]
[21]
[
5a,b,15]
focused on the
[16]
Hoveyda-type metathesis catalysts, we considered chelating li-
gand of the complexes as a potential redox-active fragment. The
benzylidene ring, incorporating two alkoxy or hydroxy substituents,
can be recognized as a reduced hydroquinone form, which upon
oxidation may transform into an unprecedented quinonylidene li-
available naphthoquinone (8), reduction with a SnCl /MeOH/HCl
2
[22]
mixture
followed by alkylation with CH I afforded 1,4-
3
dimethoxynaphthalene (9). The product was formylated with
[
17]
gand (Scheme 1).
In our report we present attempts at the synthesis of
quinonylidene ruthenium complexes by oxidation of selected
Hoveyda-type metathesis catalysts (Scheme 1, left), and li-
gand exchange reaction with vinylquinone (VQ; Scheme 1,
right).
*
Correspondence to: Michał Barbasiewicz, Faculty of Chemistry, University of
Warsaw, Pasteura 1, 02-093 Warsaw, Poland. E-mail: barbasiewicz@chem.uw.
edu.pl
Faculty of Chemistry, University of Warsaw, Pasteura 1, 02-093, Warsaw, Poland
Appl. Organometal. Chem. (2015)
Copyright © 2015 John Wiley & Sons, Ltd.

K. Grudzień and M. Barbasiewicz
ruthenium complexes with different NHC ligands: 1,3-bis(2,4,6-
Fe
MesN
Cl
NMes
Ru
N
NMes
Ru
MesN
Cl
NMes
Ru
trimethylphenyl)imidazoline-2-ylidene (SiMes, 12a; Fig. 2, left) and
[24]
Cl
Cl
1,3-bis(2,6-diisopropylphenyl)imidazoline-2-ylidene (SiPr, 12b).
6
4
Cl
Cl
Recently, it was demonstrated that the more sterically demanding
SiPr ligand offers better stabilization of ruthenium complexes,
and results in more persistent catalytic species in metathesis
Fe
O 2
5
O
N
Mes
3
2
[25]
1
a
1b
Hoveyda complex
reactions.
Reaction of precursor 12b with
a
derivative of 1,4-
Figure 1. Selected redox-active metathesis catalysts (Mes = 2,4,6-
trimethylphenyl).
dimethoxybenzene 3b gave complex 13 in very good yield (91%;
Fig. 2, right). A similar reaction with modified tetraline ligand 4 re-
sulted in the formation of a green-coloured mixture, which
[8–10]
1
displayed a broadened resonance in the H NMR spectrum (ca
[
21]
1
6.6 ppm). However, in a preparative experiment after chroma-
tography we collected only undefined products of secondary trans-
formations, lacking signals in the benzylidene region (15–20 ppm).
In turn naphthalene ligands 6, 7 and 10b displayed interesting dif-
ferences in reactivity depending on the arrangement of coordinat-
[5a,15a]
ing sites on the naphthalene core.
Reaction of 6 with
precursors 12a and 12b gave complexes 14 (66%) and 15 (79%), re-
spectively. Similar attempts with ligand 7 failed to give the ex-
pected products, most probably due to steric hindrance around
Scheme 1. Alternative syntheses of ruthenium quinonylidene complex:
ligand oxidation of a substituted Hoveyda-type complex (left), and ligand
exchange reaction of ruthenium precursor with vinylquinone (VQ; right). In
our studies we considered derivatives of 1,2- and 1,4-dialkoxybenzene,
and 1,2-, 1,4- and 2,3-dialkoxynaphthalene.
[
15b]
the C = C bond in the multiply substituted aromatic system.
In
turn exchange reactions with 10b were only partially successful:
[26]
precursor 12b (SiPr) led to an unstable complex 16 in moderate
1
yield (57%), while reaction with 12a (SiMes) monitored with H
NMR revealed formation of only traces of benzylidene product
(
4
2 2
16.56 ppm), which decomposed within hours in CD Cl at
0°C.
[
21]
[19]
Scheme 2. Structures of alkoxyaromatic ligands 3–7 (top), and synthesis
of vinylquinone ligand 11 (bottom). Reagents and conditions: a) SnCl O,
Á2H
, DMF, 45 °C, 70 h, 9, 64%
, 80 °C, 48 h, 68%; d) MePPh Br or
Br, t-PeOK, THF, 0 °C to room temperature, 30 min or 1 h, 10a, 96%,
0b, 95%; e) CAN, H O, MeCN, 0 °C to room temperature, 10 min, for R = H,
product not isolated, for R = CH , 11, 54%. DMF = dimethylformamide;
CAN = cerium ammonium nitrate.
2
2
[
22]
MeOH, conc. HClaq, reflux, 3 h ; b) CH
over 2 steps; c) POCl , DMF, C Cl
EtPPh
3 2 3
I, K CO
3
2
H
4
2
3
Figure 2. Indenylidene ruthenium precursors (12a, 12b, left) and Hoveyda-
type complexes synthesized in ligand exchange reactions (13–16, right).
Complex 16 was isolated with limited purity.
a)
3
1
2
3
Vilsmayer reagent, and olefinated to two derivatives bearing vinyl
10a) and propenyl (10b) substituents. Oxidation of the latter prod-
(
[
22]
uct with a CAN/H O/CH CN mixture ran selectively and gave li-
2
3
gand 11, which was isolated by chromatography in moderate
yield (54%).
Scheme 3. Syntheses of ruthenium complexes 20a and 20b bearing
phenolic substituents. Reagents and conditions: a) CH
temperature, 23 h, then chromatography, 18a, 33%, or iPrBr, K
room temperature, 20 h, then 50 °C, 16 h, then chromatography, 18b, 22%;
b) EtPPh Br, t-PeOK, THF, 0 °C to room temperature, 30 min, 19a, 83%,
9b, 93%; c) 12b (1.0 equiv.), 19a or 19b (1.2 equiv.), CuCl (1.5 equiv.),
CH Cl , 40 °C, 30 min, 20a, 88%, 20b, 83%.
3
I, K
2
CO
3
, DMF, room
Synthesis of Ruthenium Complexes
2
CO , DMF,
3
The synthesized ligands were used for exchange reactions with ru-
thenium precursors under usual conditions (CH
As precursors we applied commercially available indenylidene
3
[
23]
2 2
Cl , CuCl, 40 °C).
1
2
2
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Appl. Organometal. Chem. (2015)

Quinonylidene Hoveyda-type complexes
Activity Studies
In the next step we tested the synthesized catalysts in model ring-
closing metathesis (RCM) reactions of diethyldiallylmalonate (DEDAM)
using standard conditions (0.2 M of substrate with 0.2 mol% of cata-
lyst in CD
2
Cl
H NMR spectroscopy.
The obtained data revealed interesting correlations:
2
at 25 °C), and monitored their conversions with
1
[15b]
The results are presented in Figure 4.
(1) Complex 20a, bearing an OMe coordination site and free hy-
droxyl group attached to position 5 of the benzylidene ring
(cf. Fig. 1), was less active than 13, an analogue with two
methoxy substituents. In general the difference agrees with
the electronic control of activity of the Hoveyda-type
[29]
complexes, and the effect of substituents expressed with
[30]
p
the Hammett parameters (σ ), where the OH group is a more
[
31]
powerful electron donor than OMe (À0.37 versus À0.27).
(
2) Faster product accumulation for catalyst 20b with OiPr coor-
dinating site as compared with 20a (OMe) seems to be
puzzling in the light of experimental results of Plenio and
co-workers, where the opposite trend was observed for
[28]
Figure 3. ORTEP
representation of complex 20b, shown as
displacement ellipsoids drawn at the 50% probability level. Selected bond
distances (Å) and angles (°): Ru1–C1 = 1.9782(18), Ru1–C28 = 1.8273(18),
Ru1–O1 = 2.2885(13), Ru1–Cl1 = 2.3411(5), Ru1–Cl2 = 2.3141(5), C1–Ru1–
C28 = 99.43(8), C1–Ru1–O1 = 172.68(6), Cl1–Ru1–Cl2 = 152.37(2).
[
31a]
SiMes complexes.
One can speculate that more bulky
SiPr ligand causes a severe barrier to substrate approach in
[33]
the initiation step and thus a dissociative mechanism of
initiation predominates. As a consequence complex 20b
with bulkier OiPr group initiates faster, due to destabilization
of the chelate ring.
Finally, for studies of oxidation of the ruthenium complexes we
prepared two more metathesis catalysts bearing one alkoxy and
one hydroxy substituent in the benzylidene ring. The correspond-
ing ligands were prepared in parallel syntheses starting from
monoalkylation of 2,5-dihydroxybenzaldehyde with MeI and iPrBr,
followed by the Wittig reaction. Then exchange reactions of ligands
(
3) In turn, the behaviour of naphthalene complexes 14–16 can
be attributed to the spectrum of steric activation, caused by
the repulsion of etheral coordinating site with adjacent sub-
stituents of the benzylidene ring. The effect, first demon-
[
34]
strated by Blechert and co-workers
and quantified by
19a and 19b with SiPr precursor 12b gave complexes 20a (OMe)
[
25]
Percy and co-workers, arises from out-of-plane distortion
of the alkoxy substituent that weakens the O → Ru bond
and accelerates the initiation process. As steric hindrance
increases for a series of ligands, the destabilization reaches
the limit required for complex isolation, as displayed by 4
and 20b (iPrO) isolated in 88 and 83% of yield, respectively
(Scheme 3).
The synthesized complexes 13–16 and 20a,b were characterized
1
13
[27]
with H NMR and C NMR, IR and HRMS. In addition the detailed
structure of SiPr complex 20b was obtained from X-ray studies. Key
parameters of the structure were consistent with data of similar
complexes described in the literature (Fig. 3).
(Scheme 4). At the same time the presence of sterically
demanding NHC ligand (SiPr) acts in the opposite direction,
stabilizing the complexes, and interplay of both factors
[25,35]
controls activity profiles of the metathesis catalysts.
Attempts at Synthesis of Quinonylidene Complexes
In the following we focused on the synthesis of ruthenium
quinonylidenes, starting from attempts at oxidation of the
synthesized Hoveyda-type complexes 13–16 and 20a,b. In the
Scheme 4. Properties of Hoveyda-type complexes as a function of
structure of benzylidene and NHC ligands. a) Numbers in parentheses
1
[25]
Figure 4. Reaction profiles of RCM of DEDAM determined using H NMR
spectroscopy (0.2 M concentration of substrate, 25 °C, CD
2 2
Cl ) with 0.2 mol
describe maximum conversions of DEDAM substrate (Fig. 4), when the
catalysts are deactivated.
[32]
%
of the catalyst.
Appl. Organometal. Chem. (2015)
Copyright © 2015 John Wiley & Sons, Ltd.
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K. Grudzień and M. Barbasiewicz
reactions we tested the following oxidants: CAN, Fremy’s salt
potassium nitrosodisulfonate), DDQ (2,3-dichloro-5,6-
(
dicyanoquinone), chloranil (1,4-tetrachloroquinone), and for
hydroxy-substituted complexes 20a,b also PCC (pyridinium
[
21,36]
chlorochromate), and Dess–Martin periodinane.
Unfortu-
nately, all of the reactions ran non-selectively, and partial or com-
plete decomposition of substrates occurred, detected as a
1
disappearance of resonance in the benzylidene region of H NMR
spectrum (15–20ppm). Although in some cases we observed trace
amounts of unidentified species, their transitory character discour-
aged us from attempts of isolation on a preparative scale.
Finally, we attempted alternative syntheses of quinonylidene
complexes in exchange reactions of SiPr precursor 12b with VQ li-
gand 11. The experiments were performed under protective atmo-
sphere, and a variety of conditions were tested. In most cases the
mixtures turned dark red violet within minutes after combining
the reagents, and TLC analyses revealed the presence of less polar
orange substances (mainly substrates, c-hexane:ethyl acetate 6:1,
Rf ≥ 0.5), accompanied by a very polar red violet spot (c-hexane:
ethyl acetate 6:1, R = 0; ethyl acetate, R ≈ 0.3). The latter product
f
f
was isolated by column chromatography giving a dark red violet
coloration of the collected fractions, and a dark glassy solid after
evaporation. To our surprise H NMR analyses of the substance
Scheme 5. Results of reactions between ruthenium precursor 12b (SiPr)
and VQ ligand 11 (top) and 1,4-naphthoquinone (8; bottom).
1
revealed no signals in the benzylidene region, and only few broad-
ened resonances corresponding to aromatic and aliphatic
[
21]
13
fragments. In the same way a long-accumulated C NMR spec-
trum was poor, suggesting a structure of oligomers, or paramag-
netic species. To shed more light on the process we repeated the
ligand exchange reaction in an NMR tube and monitored it using
substrates), simplified pattern in the aliphatic region of the spec-
trum, consistent with symmetric trans-Cl geometry of the product
2
(considering a cis-Cl2 geometry of the starting 12b), and the
chemical shift of resonance attributed to the Ru = CH group
(deshielded ca +0.4ppm as compared with other SiPr Hoveyda-
type complexes) support the hypothesis. However, at the same
time the structure seemed to be intrinsically unstable, and vanished
within hours giving only minor amounts of isomerized or
1
H NMR spectroscopy (Fig. 5; Scheme 5, top).
Interestingly, in the spectrum we observed a large signal at
6.71 ppm (s, 1H), which slowly decayed with the formation of a mi-
nor resonance at 16.55 ppm. At the same time in the aliphatic re-
gion (corresponding to structure of the NHC ligand) other
resonances observed at 4.20 (s, 4H), 3.54 (sept, J = 6.8Hz, 4H) and
.27 ppm (d, J = 6.8 Hz, 24H) decreased in a parallel manner, with
the formation of only minor amounts of secondary by-products.
Although the exact structure of the major kinetic product was not
determined, it is reasonable to assign it as the quinonylidene com-
plex 21. Fast accumulation of the intermediate under the reaction
conditions (observed at first acquisition after combining of the
1
1
rearranged products observed with H NMR.
Additional support for the idea was obtained from studies of re-
action between ruthenium precursor 12b and unsubstituted
naphthoquinone 8 (Scheme 5, bottom). In contrast to the reactivity
of VQ ligand 11, naphthoquinone 8 failed to react with 12b in a tol-
1
[
21]
[
37]
uene solution (40 °C, 1 h). From the result we suppose that reac-
tion with 11 most likely begins from metathesis of the olefinic
fragment, following a common mechanism of synthesis of the
Hoveyda-type complexes. Unfortunately, direct conjugation of
quinone to the ruthenium centre manifests in destabilization of
[
38]
the resulting structure
interaction,
plausibly by weakening of the RuÁÁÁO
[
29,39]
or an intramolecular redox process giving para-
magnetic Ru(III) species.
[
40]
Conclusions
We synthesized a set of chelating Hoveyda-type ligands derived
from 1,2- and 1,4-dialkoxybenzene, and 1,2-, 1,4- and 2,3-
dialkoxynaphthalene (3–7 and 10). First, the organic precursors
were subjected to different oxidants to transform into VQs. The at-
tempts succeeded in CAN oxidation of 10b to afford 11. In the next
step dialkoxyligands 3–7 and 10 were used for exchange reactions
with indenylidene ruthenium precursors 12a,b (SiMes and SiPr, re-
spectively). We observed that ligands that display pronounced ste-
ric hindrance around the olefinic group (4) or etheral coordinating
site (7) fail to react, or destabilize the resulting chelates. However,
less sterically demanding substrates resulted in the formation of ru-
thenium Hoveyda-type complexes 13–16. Analogously we
1
Figure 5. H NMR studies of an exchange reaction between quinone ligand
1
1 and ruthenium precursor 12b (SiPr). The array experiment was
conducted in CD Cl with CuCl catalyst at 40 °C, and the acquisitions 1–15
were repeated in 10 min intervals. Horizontal scale on both insets is given
in ppm.
2
2
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Copyright © 2015 John Wiley & Sons, Ltd.
Appl. Organometal. Chem. (2015)

Quinonylidene Hoveyda-type complexes
[
[
10] E. L. Rosen, C. D. Varnado Jr., K. Arumugam, C. W. Bielawski,
J. Organometal. Chem. 2013, 745–746, 201–205.
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Dalton Trans. 2013, 42, 13251–13264.
synthesized two other ruthenium benzylidenes bearing phenolic
substituent in position 5 of the benzylidene ring (20a,b). All of the
catalysts were tested in model RCM reactions, and revealed correla-
tions of activities with electronic and steric effects of the substitu-
ents. Attempts at oxidative transformations of the complexes to
ruthenium quinonylidenes failed, leading mainly to decomposi-
[12] Redox activity of the coordination spheres (so-called non-innocent be-
haviour of ligands) is well-documented for catalytic systems:
V. Lyaskovskyy, B. de Bruin, ACS Catal. 2012, 2, 270–279.
1
[13] It was demonstrated, that the Grubbs second-generation catalyst
tions. However, H NMR studies of exchange reaction between
(
2
SiMes) converts into carbyne complex, when treated with I :
VQ ligand 11 and SiPr complex 12b revealed formation of transient
complex assigned to the ruthenium quinonylidene 21. Under ambi-
ent conditions the product underwent secondary reactions, giving
polar red violet destructs. We suspect that the intrinsic instability
of ruthenium quinonylidene derives from destabilization of the
chelate ring, or intramolecular redox processes, which leads to
paramagnetic Ru(III) species.
M. Shao, L. Zheng, W. Qiao, J. Wang, J. Wang, Adv. Synth. Catal. 2012,
354, 2743–2750.
14] S. H. Hong, D. P. Sanders, C. W. Lee, R. H. Grubbs, J. Am. Chem. Soc. 2005,
[
1
27, 17160–17161.
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171–3177; b) K. Grudzień, M. Malińska, M. Barbasiewicz,
Organometallics 2012, 31, 3636–3646.
[
3
[16] a) Y. Vidavsky, A. Anaby, N. G. Lemcoff, Dalton Trans. 2012, 41, 32–43; b)
Y. Ginzburg, G. N. Lemcoff, Hoveyda-type olefin metathesis complexes,
in Olefin Metathesis: Theory and Practice (Ed.: K. Grela), John Wiley,
Hoboken, NJ, 2014; c) M. Barbasiewicz, Novel concepts in catalyst
design—a case study of development of Hoveyda-type complexes, in
Olefin Metathesis: Theory and Practice (Ed.: K. Grela), John Wiley,
Hoboken, NJ, 2014.
Experimental Section
Text and figures giving NMR spectra and experimental procedures
for the syntheses of 3–7, 9–11, 13–16 and 18–20 are available in
the supporting information. CCDC-1026979 (complex 20b) con-
tains the supplementary crystallographic data for this paper. These
data can be obtained free of charge from the Cambridge Crystallo-
graphic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
[
17] The term ‘quinonylidene’ means the ‘methylenequinone’ structure of li-
gands. In the literature are two similar terms, ‘quinolinylidene’ and
‘quinolidene’, both referring rather to methylenequinoline and related
species. Interestingly, the latter also form ruthenium chelates, see:
M. Barbasiewicz, A. Szadkowska, R. Bujok, K. Grela, Organometallics
2
006, 25, 3599–3604.
[
18] J. Maignan, G. Malle, A. Deflandre, G. Lang (L’Oreal, France), New
derivatives of 5,6,7,8-tetrahydro-1-naphthalenol, a process for their
preparation and cosmetic and pharmaceutical compositions
containing them. US Patent 5043482, 1991.
19] The Wittig reactions of o-vanilin and 2,3-dihydroxybenzaldehyde were
performed with an excess of potassium t-amylate base to deprotonate
the phenolic OH groups. Despite deactivating effect of the anionic sub-
stituents, isolated yields of the products were good (94 and 81%, re-
spectively). Alkoxysubstituted ligands bearing propenyl substituents
Acknowledgements
This work was financed by the Iuventus Plus programme of the
Polish Ministry of Science and Higher Education (grant no. IP2012
[
001972, 2013-2015). NMR studies presented in the report were car-
ried out at the Biological and Chemical Research Centre, University
of Warsaw, established within the project co-financed by the Euro-
pean Union from the European Regional Development Fund under
the Operational Programme Innovative Economy, 2007–2013. The
authors thank: Mr Robert Pawłowski for preliminary syntheses of li-
gands 3–5 and 11, Umicore AG & Co. KG for donation of the M2 and
M23 complexes, and Prof. Karol Grela for support and the opportu-
nity to perform independent research programme in the field.
(3b, 5a–c, 6, 7, 10b, and 19a,b) were isolated as mixtures of E/Z iso-
mers, while sterically hindered tetraline derivative 4 was a Z isomer,
predominantly.
[
20] Simple unsubstituted vinyl- and propenylquinones remain virtually
unknown, and only a limited number of reports characterized
related structures as prone toward Diels–Alder reactions, and
electrocyclization processes: a) H. Irngartinger, B. Stadler, Eur. J. Org.
Chem. 1998, 605–626; b) K. A. Parker, T. L. Mindt, Org. Lett. 2001, 3,
3
9
875–3878; c) K. A. Parker, T. L. Mindt, Tetrahedron 2011, 67,
779–9786.
[
21] See the supporting information for details.
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[
[
[
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Appl. Organometal. Chem. (2015)
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m
, rather than σ
p
values,
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[
35] Usually less active catalysts produce more RCM product at the
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2
013, 19, 16403–16414.
[
[
36] Silver-based oxidants were omitted from the study, due to the possible
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Supporting Information
37] Under ligand exchange conditions with the VQ ligand 11 complexes 2
1
H
2 2
and 13 remained virtually unchanged (CD Cl , 40 °C, overnight,
Additional supporting information may be found in the online ver-
sion of this article at the publisher’s web-site.
NMR). For an example of intermolecular reaction of ruthenium com-
plexes with quinones, see: M. B. Herbert, Y. Lan, B. K. Keitz, P. Liu,
wileyonlinelibrary.com/journal/aoc
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