Synthesis of electronically modified ru-based neutral 16 VE allenylidene olefin metathesis precatalysts

Article abstract of DOI:10.3390/molecules17055177

Electronic modifications within Ru-based olefin metathesis precatalysts have provided a number of new complexes with significant differences in reactivity profiles. So far, this aspect has not been studied for neutral 16 VE allenylidenes. The first synthe

Full text of DOI:10.3390/molecules17055177

Molecules 2012, 17, 5177-5186; doi:10.3390/molecules17055177
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molecules
ISSN 1420-3049
www.mdpi.com/journal/molecules
Communication
Synthesis of Electronically Modified Ru-Based Neutral 16 VE
Allenylidene Olefin Metathesis Precatalysts
Martin Lichtenheldt, Steffen Kress and Siegfried Blechert *
Technische Universität Berlin, Institut für Chemie, Sekr. C3, Straße des 17. Juni 135,
Berlin 10623, Germany
* Author to whom correspondence should be addressed; E-Mail: blechert@chem.tu-berlin.de;
Tel.: +49-30-314-22255; Fax: +49-30-314-29745.
Received: 1 April 2012; in revised form: 25 April 2012 / Accepted: 26 April 2012 /
Published: 4 May 2012
Abstract: Electronic modifications within Ru-based olefin metathesis precatalysts have
provided a number of new complexes with significant differences in reactivity profiles. So
far, this aspect has not been studied for neutral 16 VE allenylidenes. The first synthesis of
electronically altered complexes of this type is reported. Following the classical
dehydration approach (vide infra) modified propargyl alcohols were transformed to the
targeted allenylidene systems in the presence of PCy3. The catalytic performance was
investigated in RCM reaction (ring closing metathesis) of benchmark substrates such as
diallyltosylamide (6) and diethyl diallylmalonate (7).
Keywords: olefin metathesis precatalysts; neutral allenylidene; electronic modifications;
ring closing metathesis
1. Introduction
Metathesis represents one of the ground-breaking achievements in modern synthetic chemistry.
This powerful tool for carbon-carbon bond formation in hands of synthetic chemists renders the
possibility to cut an (strategically introduced) olefinic double bond in retrosynthetic studies of target
molecules [1,2]. Significant representatives of Ru-based metathesis precatalysts are shown in Figue 1,
where the benzylidene species I and II are most widely used [3–6]. Milestones in catalyst development
have been the introduction of NHC ligands (N-heterocyclic carbenes) and the usage of chelating
carbene moieties to provide highly active catalysts of improved stability (e.g., II, L = NHC). Beyond

Molecules 2012, 17
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that, recent work has shown the dramatic influence of electronic variations within the benzylidene
moiety in precatalysts of type II [7,8]. For instance, electron withdrawing para-substituents successfully
improve the initiation rate by means of decreasing both the electron density on the benzylidene
(increased electrophilicity) and the chelating isopropoxy moiety (decreased Lewis basicity).
Figure 1. Selected examples of Ru-based olefin metathesis precatalysts; L: phosphine,
NHC; X: halide; Y: counter ion.
The systematic investigation of these effects did not only reveal catalysts of enhanced activity, but
moreover a fine tuning of the catalyst´s performance can generally be achieved. Furthermore, the
nature of the carbene moiety is one key factor for a catalyst’s stability during metathesis, thus
structural modifications additionally influence the characteristics of the dormant species and therefore
the lifetime of the metathesis catalyst. Beside I and II Ru-indenylidenes and Ru-allenylidenes also
became of interest. The straightforward and short synthetic approach starting from inexpensive
commercially available materials combined with a remarkable thermal stability makes these
precatalysts a cost-efficient and valuable complement to the well-established benzylidene systems.
Electronic modifications on cationic complexes of type IV have been studied thoroughly [9–12],
whereas in the area of indenylidenes III, only a few examples have been published to date [13,14].
Especially within the class of neutral 16 VE allenylidenes, two complexes of type V are the only
examples known so far [15]. Herein, we present the first synthesis of neutral 16 VE allenylidene
Ru-precatalysts bearing an electronically modified carbene moiety (compounds 5a–c, Scheme 3). The
reactivity profiles of these complexes were investigated during RCM of diallyltosylamide (6) and
diethyl diallylmalonate (7).
2. Results and Discussion
2.1. Concept for the Synthesis of Electronically Modified Neutral Ru-Based 16 VE
Allenylidene Complexes
The discovery of Selegue and coworkers that Ru(II)-complexes promote the dehydration
of propargyl alcohols of type 8 (Scheme 1, dehydration approach) marks the inception of a
straightforward access to a new metathetically active Ru-based catalyst class, the cationic allenylidenes
(IV, Figure 1) [16]. This remarkably simple route could later be extended to the synthesis of
coordinatively unsaturated 16 VE species III and V by Hill [17], Nolan [15] and Fürstner [18,19].
During the first attempts of Hill to obtain neutral allenylidenes a new metathetically active complex
was discovered, which would subsequently be characterized by Nolan as the corresponding
Ru-indenylidene complex (type III, Figure 1). Investigations concerning this unexpected reaction

Molecules 2012, 17
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pathway revealed, that in the presence of protons indenylidene complexes are formed by an acid
catalyzed rearrangement subsequent to the formation of the respective allenylidene complexes
containing PPh3 ligands (Scheme 1; for a detailed discussion see [20–23]). By adding PCy3, replacing
PPh3 during allenylidene formation, the reaction channel to the respective indenylidene is blocked,
most likely due to the increased electron density on the metal center [15]. These results offer a flexible
and reliable access to both precatalyst families.
Scheme 1. Concept for the synthesis of electronically modified neutral allenylidene and
indenylidene precatalysts.
dehydration approach
H+
Ph
Ph
OH
+ PCy3
RuCl2(PPh3)3-4
+
- H2O
- H2O
9
8
indenylidene
route
allenylidene
route
PPh3
Ru
PCy3
Ru
Cl
Cl
Cl
Cl
PPh3
PCy3
4
5
indenylidene complex
allenylidene complex
Within these studies, propargyl alcohols with para-substituted aromatic moieties were chosen to
access allenylidenes in the presence of PCy3 (allenylidene route, Scheme 1); substituents at this
position have shown to exert the most significant electronic effect (vide supra).
2.2. Synthesis of Electronically Modified Propargyl Alcohols 8a–c
The propargyl alcohols needed for these investigations were obtained by addition of
ethynylmagnesium bromide to commercially available disubstituted benzophenone derivatives 10a–c
(Scheme 2). Only compound 10c had to be synthesized by a known three step-procedure involving
nitration and subsequent oxidative cleavage of triphenylmethanol [24–26]. The Grignard reactions
afforded compounds 8a–c in good isolated yields (Scheme 2); as expected by means of polarizing
effects the best result was obtained for electron withdrawing NO2-substituted derivative (8b;
Scheme 2). Due to the prolonged reaction time and slightly forcing conditions (8c, 40 °C, Scheme 2),
the yield is somewhat decreased for the product containing an electron-donating OMe-substitutent.

Molecules 2012, 17
Scheme 2. Synthesis of electronically modified propargyl alcohols 8a–c.
5180
2.3. Synthesis of Ru-Precatalysts 5a–c
The electronically modified allenylidene complexes 5a–c (Scheme 3) were obtained in good yields
via dehydration of propargyl alcohols 8a–c. In the presence of RuCl2(PPh3)3 (9) and 2.3 equiv. PCy3,
while strictly excluding oxygen, indenylidene formation was successfully inhibited. For full
conversions extended reaction times of 16 h in THF (reflux) were necessary.
Scheme 3. Allenylidene (compound 5a–c) formation employing modified propargyl
alcohols 8a–c.
R
yield (%)
R
RuCl2(PPh3)3
HO
5a:
5b:
5c:
Me
NO2
OMe
96
54
76
PCy3
Ru
PCy3 (2.3 equiv.)
Cl
Cl
THF, reflux
16 h
PCy3
R
R
yields obtained
after precipitation
5a-c
8a-c
R
To separate the respective products from remaining phosphines, the crude mixtures were dissolved
in a minimum amount of DCM, followed by addition of an excess of n-hexane to precipitate the
precatalysts. The suspensions were filtrated and the solid matters were washed with cold n-hexane
providing analytically pure compounds; the diminished yields in some cases are partly attributed to the
washing procedure during the purification process. Nevertheless, this route provided the first
electronically modified neutral 16 VE allenylidene complexes as intended. With this new set of
precatalysts in hands we were able to investigate the influence of the modified aromatic systems on the
catalytic performance during RCM of 6 and 7 (Schemes 4 and 5).
2.4. Reactivities of the New Precatalysts
Concerning the effects of electronic modification, disubstituted systems 5a–c were evaluated during
RCM of the benchmark substrates 6 and diethyl diallylmalonate (7); precatalyst 5 (Scheme 1) served
as reference [15,27]. The reactions were performed in DCM-d2 (0.12 M) with a catalyst loading of
5 mol–% at 40 °C under inert atmosphere, conversions were determined by integrating the respective
olefin signals in 1H-NMR spectra.

Molecules 2012, 17
5181
Scheme 4. RCM of 6 employing precatalysts 5a–c relative to 5.
Tos
N
6
[Ru]
(5 mol-%)
CH2Cl2,
40 °C
Tos
N
11
Scheme 5. RCM of 7 employing precatalysts 5a–c relative to 5.
The modified precatalysts show significant differences concerning their reactivity profiles during
the transformation of 6 (Scheme 4) and 7 (Scheme 5). Complex 5a (Me) initiates slightly faster than
the reference system 5, albeit a shorter lifetime is observed, presumably due to a higher concentration
of active species in the early stage of the reaction. For complexes 5b (NO2) and 5c (OMe) the
conversions are significantly slower, but a high stability is observed in both cases, which is reflected in
the reaction progress even after prolonged reaction times. In the course of the transformation, a
conversion of 57% is observed for 5 after 100 min, whereas 5a (42%), 5c (20%) and 5b (9%) catalyze

Molecules 2012, 17
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the reaction less efficiently. In comparison to this, the results for the reaction of 7 show similar trends.
For both complexes 5 and 5a a conversion of 20% is determined after the same time, here again the
Me-derivative 5a initiates considerably faster. Precatalysts 5b and 5c catalyze the RCM inefficiently,
whereas in contrast to the former reaction nitro-derivative 5b shows a higher activity than 5c. In
contrast to the studies on electronic modifications within precatalyst of type II (vide supra), no
correlation with regard to the electron density at the aromatic system can be found at the first glance.
Nevertheless, the enhanced initiation rate for the Me-derivative in comparison to the acceptor and
donor substituted complexes 5b and 5c, respectively, is remarkable; further investigations are required
to get a deeper insight into the important coherencies influencing the stability and reactivity of 5a
relative to 5b and 5c. These first results clearly show, that modifications within the aromatic backbone
of the allenylidene open access to new metathetically active neutral 16 VE complexes with
considerably different characteristics.
3. Experimental
3.1. General
13
1H- (500 MHz; 400 MHz) and C-NMR (125 MHz; 100 MHz) spectra were obtained on a Bruker
DRX-500 and DRX-400 Advanc instrument. IR (ATR) spectra were measured on a Perkin-Elmer
Spektrometer 800. Melting points were obtained on a Leica Galen melting point apparatus with
Wagner-Munz control unit and are uncorrected. Mass spectra were recorded on a Finnigan MAT 95
SQ by FAB-ionisation (fast atom bombardment). TLC analysis was facilitated by the use of
KMnO4/H2O in addition to UV light (254 nm, 366 nm) with fluorescent-indicating plates (silica gel,
Merck, 60 F 254, layer thickness 0.2 mm). The solvents were purchased in absolute quality or dried as
follows: THF was dried over sodium/benzophenone, DCM and n-hexane over CaH2. RCM reactions
were conducted in Schlenk-type glassware or in a Carousel Reaction Station (Radleys Discovery
Technologies Company). If needed, substrates have been distilled prior to use.
3.2. General Procedure for the Synthesis of Propargyl Alcohols 8a–c
A dry and nitrogen-flushed 25 mL Schlenk-flask equipped with a magnetic stirring bar and septum
was charged with the benzophenone-derivative 10 (1 equiv.), which was dissolved in dry THF
(c = 2.2 M). The solution was cooled to 0 °C followed by dropwise addition of ethynylmagnesium
bromide (1 equiv., solution in THF 0.5 M) within 15 min. After 1 h the solution was allowed to reach
RT and was stirred until full conversion was detected by TLC control. The solution was quenched by
adding aqueous saturated NH4Cl and subsequently diluted with Et2O. After separation of the two
layers the aqueous phase was extracted with Et2O (3×). The combined organic phases were washed
with brine and dried over MgSO4. Filtration and subsequent evaporation afforded a crude mixture,
which was purified by flash chromatography to yield the target compound. The spectroscopic data
obtained for 8c[28] were consistent with those reported in the literature.
1,1-bis-(4-Methylphenyl)prop-2-yn-1-ol (8a): Compound 10a (0.5 g, 2.4 mmol) and ethynyl-magnesium
bromide (4.8 mL, solution in THF 0.5 M, 2.4 mmol) were reacted following the general procedure.

Molecules 2012, 17
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Chromatographic purification [n-hexane/EE 7:1 (v:v)]; Rf = 0.26) provided 0.4 g of 8a (1.7 mmol;
1
70%) as yellow solid. H-NMR (CDCl3, 400 MHz): δ 2.35 (s, 6H), 2.72 (s, 1H), 2.87 (s, 1H), 7.80
13
(d, J = 8.5 Hz, 4H), 7.50 (d, J = 8.5 Hz, 4H); C-NMR (CDCl3, 100 MHz): δ 21.0, 74.2, 75.3, 86.7,
126.0, 129.2, 137.8, 142.0; IR (ATR): 3539, 3450, 3286, 2923, 1717, 1510, 821 cm−1; EI-MS (60 °C):
m/z (%) 236 (100) [M+], 221 (84), 145 (57), 91 (57), 53 (60); HR-MS: (M+ = C17H16O) found:
236.1208, calc.: 236.1201.
1,1-bis-(4-Nitrophenyl)prop-2-yn-1-ol (8b): Compound 10b (0.8 g, 2.9 mmol) and ethynylmagnesium
bromide (5.9 mL, solution in THF 0.5 M, 2.9 mmol) were reacted following the general procedure.
Chromatographic purification [n-hexane/EE 3:1 (v:v)]; Rf = 0.29) provided 0.7 g of 8b (2.3 mmol;
78%) as yellow solid. 1H-NMR (CD2Cl2, 400 MHz): δ 3.05 (s, 1H), 3.07 (s, 1H), 7.80 (d, J = 9.2 Hz,
13
4H), 8.21 (d, J = 8.8 Hz, 4H); C-NMR (CD2Cl2, 100 MHz): δ 73.3, 77.9, 83.9, 123.9, 126.9, 147.7,
149.8; IR (ATR): 3486, 3288, 2927, 2588, 2117, 1700, 1608, 1519, 1345, 702 cm−1; EI-MS (200 °C):
m/z (%) 298 (14) [M]+, 281 (43), 251 (52), 176 (100); HR-MS (m/e) found for (M+) 298.0588, calc.
for C15H10N2O5 298.0590.
3.3. General Procedure for the Synthesis of the Precatalysts 5a–c
A dry and nitrogen-flushed Schlenk-tube was successively charged with RuCl2 (PPh3)3
(1.00 equiv.), X (1.15 equiv.), PCy3 (2.30 equiv.) and dry THF (c = 0.025 M). The solution was heated
to 70 °C for 16 h, whereupon the solvent was removed to afford the crude mixture. Dissolved in a
minimum amount of DCM the residue was treated with an excess of n-hexane to obtain a precipitate,
which was filtered and washed with cold n-hexane to yield the desired precatalyst.
(PCy3)2Cl2Ru[bis-(4-methylphenyl)-allenylidene] (5a): The reaction of RuCl2(PPh3)3 (100 mg,
0.10 mmol), 8a (28.3 mg, 0.12 mmol), PCy3 (67.3 mg, 0.24 mmol) afforded 5a (94.9 mg, 0.10 mmol,
1
96%) as an orange brown powder after precipitation, following the general procedure. H-NMR
(CD2Cl2, 500 MHz): δ 1.13–1.26 (m, 24H), 1.42–1.49 (m, 12H), 1.65–1.67 (m, 12H), 1.98 (d, J = 12.2 Hz,
13
12H), 2.18 (s, 6H), 2.60–2.65 (m, 6H), 7.10 (d, J = 8.0 Hz, 4H), 7.70 (d, J = 8.2 Hz, 4H); C-NMR
(CD2Cl2, 125 MHz): δ 21.5, 26.6, 28.0, 29.7, 32.1, 129.5, 129.8, 139.0, 143.9, 144.3, 186.3, 214.2;
31P-NMR (CD2Cl2, 125 MHz): δ 40.6; IR (ATR): cm−1 3411, 2924, 2849, 1913, 1715, 1436; FAB-MS:
m/z (%) 951 (40) [M]+, 878 (100) [M−2Cl]+.
(PCy3)2Cl2Ru[bis-(4-nitrophenyl)-allenylidene] (5b): The reaction of RuCl2(PPh3)3 (100 mg, 0.10 mmol),
8b (35.8 mg, 0.12 mmol), PCy3 (67.3 mg, 0.24 mmol) afforded 5b (57.3 mg, 0.06 mmol, 57%) as a
dark brown powder after precipitation, following the general procedure. 1H-NMR (CD2Cl2, 500 MHz):
δ 1.12–1.25 (m, 24H), 1.41–1.48 (m, 12H), 1.66 (d, J = 10.7 Hz, 12H), 1.92 (d, J = 11.9 Hz, 12H),
2.55–2.65 (m, 6H), 7.95 (d, J = 8.7 Hz, 4H), 8.18 (d, J = 9.1 Hz, 4H); 13C-NMR (CD2Cl2, 125 MHz): δ
26.4, 27.1, 29.5, 32.4, 114.9, 131.7, 140.5, 146.2, 160.4, 190.1, 216.8; 31P-NMR (CD2Cl2, 125 MHz): δ
44.4; IR (ATR): cm−1 3348, 3076, 2929, 2854, 1926, 1525, 1347, 696; FAB-MS: m/z (%) 940 (39)
[M−2Cl]+, 843 (100), 819 (60).

Molecules 2012, 17
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(PCy3)2Cl2Ru[bis-(4-methoxyphenyl)-allenylidene] (5c): The reaction of RuCl2(PPh3)3 (200 mg,
0.21 mmol), 8c (64.4 mg, 0.24 mmol), PCy3 (134 mg, 0.48 mmol) afforded 5c (155 mg, 0.16 mmol,
1
76%) as a reddish brown powder after precipitation, following the general procedure. H-NMR
(CD2Cl2, 500 MHz): δ 1.13–1.24 (m, 24H), 1.42–1.50 (m, 12H), 1.65–1.67 (m, 12H), 1.99 (d, J = 12.4 Hz,
12H), 2.37 (s, 3H), 2.57–2.69 (m, 6H), 6.76 (d, J = 8.8 Hz, 2H), 7.23 (t, J = 8.1 Hz, 2H), 7.45
(t, J = 7.1 Hz, 1H), 7.67 (d, J = 7.5 Hz, 2H), 7.75 (d, J = 9.6 Hz, 2H); 13C-NMR (CD2Cl2, 125 MHz): δ
26.6, 28.0, 29.7, 32.1, 55.5, 114.1, 131.3, 139.9, 160.3, 188.6, 215.5; 31P-NMR (CD2Cl2, 125 MHz): δ
40.1; IR (ATR): cm−1 3368, 3056, 2928, 2846, 1932, 1595, 1252, 1169, 695; FAB-MS: m/z (%) 983
(28) [M]+, 910 (100) [M−2Cl]+, 797 (30), 613 (60), 531 (46).
4. Conclusions
In this communication we reported on the first synthesis of electronically modified neutral 16 VE
Ru-based allenylidene precatalysts. With the aim of modifying the carbene moiety in 5 (Figure 1) at
the para-positions within the aromatic systems, the classical dehydration approach rendered a flexible
access for the intended alteration. With regard to this, substituted propargyl alcohols 8a–c were
derived in good isolated yields (63–87%) by the addition of ethynylmagnesium bromide to the
respective benzophenone derivatives 10a–c. The subsequent dehydration in the presence of 9 and PCy3
provided electronically modified allenylidenes 5a–c in moderate to excellent isolated yields (54–96%)
after precipitation. Compounds 8a–c were evaluated in RCM of benchmark substrates 6 and 7; these
investigations demonstrated the dramatic differences concerning the initiation rate and stability during
metathesis reactions comparing the Me-substituted allenylidene 8a to the NO2-substituted 8b and
OMe-substituted complexes 8c.
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