Catalysis of linear alkene metathesis by Grubbs-type ruthenium alkylidene complexes containing hemilabile α,α-diphenyl-(monosubstituted-pyridin-2-yl)methanolato ligands

Article abstract of DOI:10.3762/bjoc.15.19

Four new Grubbs-type precatalysts [RuCl(H₂IMes)(O^N)(=CHPh)], where [O^N = α,α-diphenyl-(3-methylpyridin-2-yl)methanolato, α,α-diphenyl-(4-methylpyridin-2-yl)methanolato, α,α-diphenyl-(5-methylpyridin-2-yl)methanolato and α,α-diphenyl-(3-methoxypyridin-2-yl)methanolato] were synthesized and tested for their activity, stability and selectivity in the 1-octene metathesis reaction. Overall the precatalysts showed good activity and high stability for the metathesis of 1-octene at temperatures above 80 °C and up to 110 °C. Selectivities towards the primary metathesis products, i.e., 7-tetradecene and ethene, above 85% were obtained with all the precatalysts at 80 and 90 °C. High selectivities were also observed at 100 °C for the 4-Me- and 3-OMe-substituted precatalysts. With an increase in temperature an increase in isomerisation products and secondary metathesis products were observed with the latter reaching values >20% for the 3-OMe- and 3-Me-substituted precatalysts at 110 and 100 °C, respectively. All the precatalysts exhibits first-order kinetics at 80 °C with the 3-substituted precatalysts the slowest. The behaviour of the 3-substituted precatalysts can be attributed to electronic and steric effects associated with the adjacent bulky phenyl groups.

Full text of DOI:10.3762/bjoc.15.19

Catalysis of linear alkene metathesis by Grubbs-type
ruthenium alkylidene complexes containing hemilabile α,α-
diphenyl-(monosubstituted-pyridin-2-yl)methanolato ligands
Tegene T. Tole1,2, Johan H. L. Jordaan1 and Hermanus C. M. Vosloo*1
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2019, 15, 194–209.
1Research Focus Area for Chemical Resource Beneficiation,
Catalysis and Synthesis Research Group, North-West University,
Hoffmann Street, 2531 Potchefstroom, South Africa and 2Department
of Chemistry, College of Natural and Computational Sciences,
Hawassa University, Hawassa, Ethiopia
doi:10.3762/bjoc.15.19
Received: 02 September 2018
Accepted: 21 December 2018
Published: 22 January 2019
Email:
This article is part of the thematic issue "Progress in metathesis
chemistry III".
Hermanus C. M. Vosloo* - manie.vosloo@nwu.ac.za
* Corresponding author
Guest Editors: K. Grela and A. Kajetanowicz
Keywords:
© 2019 Tole et al.; licensee Beilstein-Institut.
License and terms: see end of document.
Grubbs-type precatalyst; hemilabile; 1-octene metathesis;
pyridinyl-alcoholato ligand
Abstract
Four new Grubbs-type precatalysts [RuCl(H2IMes)(O^N)(=CHPh)], where [O^N = α,α-diphenyl-(3-methylpyridin-2-
yl)methanolato, α,α-diphenyl-(4-methylpyridin-2-yl)methanolato, α,α-diphenyl-(5-methylpyridin-2-yl)methanolato and α,α-
diphenyl-(3-methoxypyridin-2-yl)methanolato] were synthesized and tested for their activity, stability and selectivity in the
1-octene metathesis reaction. Overall the precatalysts showed good activity and high stability for the metathesis of 1-octene at tem-
peratures above 80 °C and up to 110 °C. Selectivities towards the primary metathesis products, i.e., 7-tetradecene and ethene, above
85% were obtained with all the precatalysts at 80 and 90 °C. High selectivities were also observed at 100 °C for the 4-Me- and
3-OMe-substituted precatalysts. With an increase in temperature an increase in isomerisation products and secondary metathesis
products were observed with the latter reaching values >20% for the 3-OMe- and 3-Me-substituted precatalysts at 110 and 100 °C,
respectively. All the precatalysts exhibits first-order kinetics at 80 °C with the 3-substituted precatalysts the slowest. The behaviour
of the 3-substituted precatalysts can be attributed to electronic and steric effects associated with the adjacent bulky phenyl groups.
Introduction
The alkene metathesis reaction is now well established as a the reaction including the application of these in industrial pro-
powerful synthetic tool in organic and polymer chemistry [1,2]. cesses (Figure 1). Of course, the role of the so-called Schrock
The development of metal alkylidene precatalysts based on ru- metal carbenes based on tungsten and molybdenum should not
thenium, starting with the so-called Grubbs 1 (1) and 2 (2) be ignored in the success story of the alkene metathesis reac-
metal carbenes, played a major role to extend the versatility of tion but it is not the focus of this article.
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Beilstein J. Org. Chem. 2019, 15, 194–209.
diethyl diallylmalonate with 100% conversion after 30 min,
results comparable to the unimolecular catalyst.
Denk et al. [9] synthesised N-heterocyclic (NHC) ruthenium
alkylidene complexes containing pyridinyl-alcoholato ligands
(4). These complexes were tested as precatalysts at different
temperatures in the ROMP of norbornene and cyclooctene.
Oligomers were obtained at room temperature in the presence
of 4a and 4d, while 4b and 4c yielded polymers. At 60 °C,
Figure 1: Structures of Grubbs 1 (1) and 2 (2) precatalysts.
The large number of ruthenium alkylidene precatalysts that has ROMP was observed with norbornene (98–100%) and cyclo-
been developed is based on the design concepts illustrated in octene (72–80%) in the presence of 4.
Scheme 1 [3]. The design concept C is of interest because of the
potential hemilabile nature and latent metathesis activity of We investigated a number of Grubbs 1- and Grubbs 2-type (5)
these complexes [4]. Of particular interest to us are the rutheni- metal carbenes with pyridinyl alcoholato ligands for the
um alkylidene complexes containing the pyridinyl alcoholato 1-octene metathesis reaction (Figure 3) [10-14]. The incorpora-
bidentate ligands investigated by a number of research groups tion of pyridinyl-alcoholato ligands in the Grubbs-type precata-
[5].
lysts has shown an increase in the thermal stability, activity and
lifetime of the precatalysts when compared to 1 and 2 [10]. The
pyridinyl-alcoholato Grubbs 2-types exhibited higher activities
and selectivities than the Grubbs 1-types and were investigated
in more detail. It is clear from the results that the chelating
ability of the pyridinyl alcoholato ligands combined with the
NHC ligand is responsible for the activity and improved
stability of the precatalyst at high temperatures. In general 5d
performed the best in the 1-octene metathesis reactions when
compared to complexes 5a–c and 5e–h. The catalytic perfor-
Scheme 1: Design concepts for ruthenium alkylidene precatalysts [3].
The pyridinyl alcohol found its way to the Grubbs-type com-
plexes from research by Van Der Schaaf and co-workers on the
Schrock-type analogues [6,7]. Grubbs 1-type complexes 3a–f
(Figure 2) were used to catalyse the ring-closing metathesis
(RCM) of dialkenes, ring-opening metathesis polymerisation
(ROMP), isomerisation of alkenes and cross-metathesis (CM)
of alkenes [7]. The complexes were synthesized by reacting the
lithium salts of the corresponding pyridinyl alcohols with
[RuCl2(=CHC6H5)(P(iPr3))2]. These complexes catalysed inter
alia the cyclisation of hex-5-enyl undec-10-enoate to oxacyclo-
hexadec-11-en-2-one (50% at 60 °C in toluene) and the ROMP
of dicyclopentadiene. They were also able to immobilise these
Grubbs 1-type precatalysts using dendritic pyridinyl alcohols
[8]. These complexes catalysed the RCM reaction (at 80 °C) of
Figure 3: Structures of Grubbs 2-type (5) pyridinyl-alcoholato precata-
lysts.
Figure 2: Structures of Grubbs 1-type (3) and 2-type (4) pyridinyl-alcoholato precatalysts.
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Beilstein J. Org. Chem. 2019, 15, 194–209.
mance could be further tuned by the incorporation of an elec- positions of the pyridine ring of the pyridinyl-alcoholato ligands
tron-donating (e.g., OMe, 5k) or electron-withdrawing (e.g. Cl, at high temperatures (80–110 °C). The increase in stability is at-
5i and 5j) group at the 2- or 4-position of one of the α-phenyl tributed to the electronic and steric influence of the Me and
groups of 5d [14]. At 80–110 °C these complexes showed im- OMe groups on Ru–N chelation. The activity of the precata-
proved catalytic performance in the metathesis of oct-1-ene. At lysts also showed a significant improvement upon increasing
110 °C complex 5k, with 96% conversion and 95% selectivity the reaction temperature from 40 to 110 °C. The increase in the
towards the primary metathesis products tetradec-7-ene and activity of the precatalysts is relatively low in the 40–60 °C
ethene, outperformed the other complexes. In a computational range, but a high activity difference is observed upon increas-
study the improved catalytic performance was attributed to ing the temperature in 10 °C intervals between 70 and 110 °C.
strengthening of the Ru–N bond due to steric repulsion be-
tween the substituted phenyl group and the NHC ligand [14].
An 8-quinolinolate Grubbs 2-type derivative, patented by
Slugovc and Wappel [15] for use in ROMP reactions, was
found to be inactive (<1% conversion) for 1-octene metathesis
at 60 °C [12].
Schachner et al. [16] evaluated the catalytic activity of 5b, 5d
and related complexes for the ROMP of cyclooctene, CM of
hex-5-enyl acetate with dec-5-ene and the RCM of hex-5-en-1-
yl undec-10-enoate. Superior (CM, RCM) to moderate (ROMP)
activities were observed for most of these precatalysts. An inter-
esting result was the very high affinity (“stickiness”) to
untreated, unmodified and commercially available chromatogra-
phy-grade silica. This was exploited further by Cabrera et al.
[17,18] when 5b and related complexes were investigated as
heterogeneous precatalysts in biphasic RO-RCM and CM reac-
tions. The substrate and catalyst were adsorbed on a thin layer
Figure 4: Structures of pyridinyl-substituted Grubbs 2-type pyridinyl-
alcoholato precatalysts.
silica plate and developed in EtOAc/hexane (1:7 v/v) for the
CM of methyl 9-dodecene and in hexane for the RO-RCM of
cis-cyclooctene.
Results and Discussion
The above-mentioned studies clearly illustrate the versatility A mixture of products, summarised in Table 1, is obtained
and use of ruthenium alkylidene complexes with pyridinyl-alco- during the metathesis of 1-octene, i.e., primary metathesis prod-
holato ligands. In principle these studies had one approach in ucts (PMPs), isomerisation products (IPs) and secondary me-
common concerning the pyridinyl-alcoholato ligand, and that
was to focus on substituents on the α-carbon of the ligand. To
Table 1: Mixture of products formed during 1-octene metathesis in the
our knowledge, there are no reports on investigations of elec-
tronic and/or steric effect(s) of pyridinyl substituents on the
chelation efficiency of pyridinyl alcoholato ligands, and subse-
quently its metathesis activity. Therefore, in this paper, we in-
vestigated the influence of a monosubstituent on the pyridinyl
moiety on the 1-octene metathesis activity of a Grubbs 2-type
precatalyst with an α,α-diphenyl methanolato ligand. For the
synthesis of the pyridinyl methanol compounds, commercially
available substituted bromopyridines were reacted with benzo-
phenone followed by a reaction of the lithiated alcohol with 2.
Four new ruthenium alkylidene complexes, i.e., 6–9 (Figure 4),
were successfully obtained and investigated as precatalyst in
1-octene metathesis in the temperature range 40–110 °C. The
stability, selectivity and turnover frequency (TOF) of 2 in-
creased upon substituting Me and OMe groups on the various
presence of ruthenium alkylidene precatalysts.
Reaction
Substratea
Productsa
Abbrev.
PMPs
primary metathesis
self-metathesis
isomerisation
C=C7
C=C7
C=C + C7=C7
C2=C6 +
C3=C5 +
C4=C4
IPs
secondary
metathesis
SMPs
cross metathesis
C=C7 +
C2=C6
C2=C7 +
C=C6 + C=C2
+ C6=C7
self-metathesis
C2=C6
C2=C2 +
C6=C6
aGeometrical isomers and hydrogens are not shown for simplicity.
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Beilstein J. Org. Chem. 2019, 15, 194–209.
tathesis products (SMPs). The PMPs, 7-tetradecene (cis and high PMPs formation are observed at 100 °C (>85% after
trans) and ethene, forms as a result of the self-metathesis (SM) ca. 200 min).
of 1-octene. Simultaneously 1-octene is isomerised to 2-, 3- and
4-octene (IPs). The subsequent SM and CM reactions of the The formation of PMPs did not equilibrate within 540 min for
internal alkenes yield alkenes (cis and trans) in the C3–C13 the temperature range 50 to 80 °C; however, its formation
range (SMPs).
equilibrated at ca. 400 min at 90 °C and ca. 200 min at 100 °C.
This shows that 7 is stable at high temperatures, with
All the reactions were followed by GC at regular sampling moderate to very good PMPs (ca. 25–80%) formation. On the
intervals until 540 min. Because the observed formation of IPs other hand, the formation of SMPs (ca. 0.2–2.0%) and IPs
is mostly below 2% and never above 4% it is also not shown in (ca. 0.2–0.5%) is negligible in the range 40–80 °C, while a
the figures.
more significant amount is formed at temperatures greater than
90 °C (ca. 10–14% SMPs and 1.7–2.1% IPs) after 540 min.
Effect of the reaction temperature
The results of the metathesis of 1-octene at temperatures Table 2 summarises the overall catalytic performance of 7 at
40–100 °C are presented in Figure 5 and Table 2 for precatalyst 420 min. In this period it can be seen that the PMPs and SMPs
7. The rate of self-metathesis of 1-octene showed an increase formation, TON, and TOF of the precatalyst show a direct rela-
upon raising the reaction temperature from 40 to 100 °C. The tionship with temperature. The highest PMPs formation is ob-
metathesis reaction is insignificant at lower temperatures (40 to served for the temperature changes from 80 to 90 °C (29.3%)
60 °C). Upon raising the temperature beyond 70 °C, an in- and the least for 40 to 50 °C (0.7%) at 420 min.
crease in the reaction rate was observed, resulting in a signifi-
cant increase in 1-octene conversion greater than 80% at 90 °C The selectivity towards PMPs showed a dramatic increase upon
after 540 min. Increasing the temperature further to 100 °C increasing the temperature from 40 to 80 °C (23–97%); howev-
showed a dramatic increase in the metathesis reaction rate and a er, it showed a decrease going from 80 (97%) to 90°C (89%),
Figure 5: The influence of the reaction temperature on the (a) conversion of 1-octene, (b) formation of PMPs and (c) formation of SMPs using precat-
alyst 7 (Ru/1-octene = 1:9000). [ 40 °C, ■ 50 °C, ▲ 60 °C, ● 70 °C, ◊ 80 °C, □ 90 °C, ∆ 100 °C].
Table 2: Summary of the catalytic performance of precatalyst 7 at different temperatures (Ru/1-octene molar ratio 1:9 000, 420 min).
Entry
Temp. [°C]
Conv.a
PMPsa
SMPsa
IPsa
Sb
TONc
TOFd
1
2
3
4
5
6
7
40
50
60
70
80
90
100
0.8
0.2
0.2
0.4
1.1
0.9
1.1
8.0
11.0
0.4
0.8
0.3
0.2
0.2
1.0
1.1
23
44
80
95
97
89
87
18
0.07 × 10−2
0.32 × 10−2
2.07 × 10−2
7.57 × 10−2
15.39 × 10−2
25.86 × 10−2
29.68 × 10−2
2.1
0.9
81
7.2
5.8
522
1908
3879
6516
7479
22.3
44.4
81.4
95.2
21.2
43.1
72.4
83.1
aConversion or yield in mol %; bS (selectivity) in percent toward PMPs; cTON (turnover number) = [%PMPs × (Oct/Ru)]/100; dTOF (turnover frequen-
cy) = TON/time in s.
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and then to 100 °C (87%). An overall assessment of the results period the formation of IPs remained below 3% even at the high
show that at 80 °C the catalyst showed a high selectivity for temperatures. At 100 °C and 540 min, a larger amount of SMPs
PMPs with a negligible amount of SMPs and IPs. Although the is formed for precatalyst 8 than that of 7.
activity of the precatalyst increased a great deal at 90 and
100 °C, the selectivity for PMPs decreased as a result of the Table 3 summarises the overall catalytic performance of precat-
high amount of SMPs and IPs formation. The TOF increased alyst 8 at 420 min. The PMPs and SMPs formation, TON and
significantly as a result of increasing the temperature. The TOF all show a direct relationship with temperature. Precata-
highest TOF increase was observed upon increasing the temper- lysts 7 and 8 share similarities in having the same temperature
ature from 80 to 90 °C. Generally, precatalyst 7 showed very range for the highest PMPs formation, i.e., 70 to 80 °C at
good activity, selectivity and stability at high temperatures.
420 min. The biggest difference, however, is observed for 8
(37%). Relatively higher SMPs are formed for 8 (11%, 19%)
The results of the metathesis of 1-octene at temperatures than that of 7 (8%, 11%) at 90 and 100 °C, respectively. The
40–100 °C are presented in Figure 6 and Table 3 for precatalyst relatively low PMPs formation of 8 compared to that of 7 is due
8. A similar overall trend for 8 is observed, i.e., very low reac- to the relatively high SMPs and IPs formations with precatalyst
tion rates at temperatures below 60 °C with a rapid increase in 8. The IPs formation in 8 follows a similar pattern to that of 7,
reaction rates above 70 °C resulting in 1-octene conversions i.e., it showed an increase upon increasing the temperature from
above ca. 70% after 540 min. Although the formation of PMPs 40 to 50 °C, followed by a decrease from 50 to 60 °C and then
equilibrated quickly at ca. 70% after ca. 150 min at 100 °C and an increase from 60 to 100 °C. The selectivity in 8 increased
at ca. 65% only after ca. 400 min for 90 °C it did not equili- upon increasing the temperature from 40 to 70 °C, and then
brate at 80 °C even after 540 min.
showed a decrease from 70 to 100 °C.
The formation of SMPs is very low (below 4%) for 8 in the A maximum selectivity for 8 is observed at 70 °C (94%) (see
temperature range 40–80 °C after 540 min, while it is relatively Table 3, entry 4) and for that of 7 at 80 °C (97%) (see Table 2,
high at 14 and 21% at 90 and 100 °C, respectively. In the same entry 5). Generally, precatalyst 7 showed a better selectivity
Figure 6: The influence of the reaction temperature on the (a) conversion of 1-octene, (b) formation of PMPs and (c) formation of SMPs using precat-
alyst 8 (Ru/1-octene = 1:9000). 40 °C, ■ 50 °C, ▲ 60 °C, ● 70 °C, ◊ 80 °C, □ 90 °C, ∆ 100 °C].
Table 3: Summary of catalytic performance of precatalyst 8 at different temperatures (Ru/1-octene molar ratio 1:9 000, 420 min).
Entry
Temp. [°C]
Conv.a
PMPsa
SMPsa
IPsa
Sb
TONc
TOFd
1
2
3
4
5
6
7
40
50
60
70
80
90
100
1.5
0.8
0.3
0.4
0.9
0.1
0.2
0.8
1.3
1.8
56
49
85
94
92
84
78
72
0.28 × 10−2
0.50 × 10−2
1.61 × 10−2
8.11 × 10−2
21.61 × 10−2
23.21 × 10−2
26.07 × 10−2
2.9
1.4
0.6
126
5.3
4.5
0.7
405
24.0
65.4
77.3
93.8
22.7
60.5
65.0
73.0
1.1
2043
5445
5850
6570
4.1
11.0
19.0
aConversion or yield in mol %; bS (selectivity) in percent toward PMPs; cTON (turnover number) = [%PMPs × (Oct/Ru)]/100; dTOF (turnover frequen-
cy) = TON/time in s.
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Beilstein J. Org. Chem. 2019, 15, 194–209.
compared to that of 8 at 420 min. Although the TOF of 8 is in precatalyst 6 showed an increase in the activity of the precata-
direct relation with temperature, it follows the following order lyst upon increasing the temperature from 60 to 110 °C. A large
upon comparing with 7; 8 > 7 at 40 and 50 °C (see Table 2 and increase in the rate of the metathesis reaction is observed upon
Table 3, entries 1–7), at 60 °C 7 > 8 (see Table 2 and Table 3, increasing the temperature from 80 to 90 °C. Although the ac-
entry 3), at 70 and 80 °C 8 > 7 (see Table 2 and Table 3, entries tivity of the precatalyst has shown an increase upon increasing
4 and 5) and 7 > 8 at 90 and 100 °C (see Table 2 and Table 3, the temperature from 60 to 110 °C, a very high (ca. 45%) in-
entries 6 and 7).
crease in the PMPs formation is observed upon increasing the
temperature from 80 to 90 °C at 540 min. The PMPs formation
Summarising the comparisons of precatalysts 7 and 8, it is did not equilibrate at 90 °C and this shows the stability of
noted that precatalyst 7 showed better activity, selectivity and precatalyst 6 at high temperatures. The PMPs formation, how-
stability in the 60–100 °C temperature range, except for 80 °C, ever, equilibrated from ca. 270 min at 100 °C and ca. 140 min at
at 420 min. It also showed higher TOF at 60, 90 and 100 °C at 110 °C. At 80 °C, the activity of precatalyst 6 (ca. 25%) is very
420 min. According to a DFT study by Getty et al. [19] the low compared to precatalysts 7 (ca. 50%) and 8 (ca. 70%)
more positively charged the Ru, the slower the initiation rate of during the course of PMPs formation, at 540 min. Generally, in
the catalyst. The calculated Mulliken atomic charge of Ru in 7 the first 100 to 300 minutes, the rate of formation of PMPs in-
(0.934) is less positive than in 8 (0.976).
creases dramatically and slows down afterwards in the tempera-
ture range of 90 to 110 °C. A similar trend is observed during
The results of the metathesis of 1-octene at temperatures of 60 the course of SMPs formation. Significant amounts (16–36%)
to 110 °C are presented in Figure 7 and Figure 8 for precata- of SMPs are formed by 6 for temperatures 90 to 110 °C, while
lysts 6 and 9, respectively. Because of their low activity and negligible amounts (1.3–3.6%) of SMPs are formed from 60 to
high stability, the metathesis reactions were done between 60 80 °C. Large amounts of SMPs (ca. 36%) and IPs (5%) were
and 110 °C. Metathesis of 1-octene by the 3-Me-substituted formed by 6 at 540 min at 100 °C. A comparison of precata-
Figure 7: The influence of the reaction temperature on the (a) conversion of 1-octene, (b) formation of PMPs and (c) formation of SMPs using precat-
alyst 6 (Ru/1-octene = 1:9000). [▲ 60 °C, ● 70 °C, ◊ 80 °C, □ 90 °C, ∆ 100 °C, ○ 110 °C].
Figure 8: The influence of the reaction temperature on the (a) conversion of 1-octene, (b) formation of PMPs and (c) formation of SMPs using precat-
alyst 9 (Ru/1-octene = 1:9000). [▲ 60 °C, ● 70 °C, ◊ 80 °C, □ 90 °C, ∆ 100 °C, ○ 110 °C].
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Beilstein J. Org. Chem. 2019, 15, 194–209.
lysts 6, 7 and 8 with regard to SMPs and IPs formation at both 7 (0.934) and 8 (0.976). Its high activity at 110 °C with
100 °C and 540 min shows a decreasing order of 6 > 8 > 7. 69% selectivity is, however, remarkable for linear alkene me-
Generally, a relatively large amount of IPs is formed by precat- tathesis catalysed by ruthenium alkylidene precatalysts.
alyst 6. Although the rate of PMPs formation is slow at 80 °C,
high selectivity and stability are attained at this temperature for The 3-OMe-substituted precatalyst 9 showed a negligible activi-
precatalyst 6, similar to precatalysts 7 and 8. The highest differ- ty for the metathesis of 1-octene at 60 °C (Figure 8), similar to
ence in the PMPs formation (ca. 45%) is observed between 80 the 3-Me-substituted precatalyst 6. The overall activity of the
and 90 °C. Although the SMPs formation has shown a direct precatalyst, however, showed a significant increase upon in-
relationship with temperature, the formation of high SMPs creasing the temperature from 60 to 110 °C. In a similar way to
(16–36%) from 90 to 110 °C limited the PMPs formation to that of 6, the largest increase in the activity of the precatalyst is
only a maximum of ca. 68% at 540 min. The contribution of the observed upon increasing the temperature from 80 to 90 °C
IPs formation in limiting the formation of PMPs is not negli- (ca. 38%) at 420 min. The activity of the catalyst showed a
gible as a result of the relatively high (ca. 3.5%) IPs formation. small difference between 100 and 110 °C, on the overall me-
The SMPs formation increased approximately six-fold upon in- tathesis reaction.
creasing the temperature from 80 to 90 °C, which was then fol-
lowed by approximately a two-fold increase upon increasing the During the course of PMPs formation, high catalytic activity for
temperature from 90 to 100 °C.
9 is observed within 200 min at temperatures above 90 °C
(ca. 60%), while the activity of the precatalyst showed a
Table 4 presents the overall catalytic performance of precata- dramatic increase from 70 to 90 °C after ca. 500 min, similar to
lyst 6 at 420 min. At 420 min, 1-octene conversion is 98.0% that of 6 (see Figure 7b). For both 6 and 9 the highest PMPs
(110 °C), 93.0% (100 °C), 70.5% (90 °C), 19.4% (80 °C), 6.8% (>60%) is observed from 90 to 110 °C after 420 min. The rate
(70 °C) and 2.5% (60 °C). This shows the dramatic increase of of formation of SMPs is very high for 9 at 110 °C within
the catalytic activity upon increasing the temperature. An inves- 420 min. This is the reason for the decrease in the formation of
tigation of the PMPs formation reveals a huge 66% increase for PMPs from 71% at 100 °C to 64.2% at 110 °C.
the PMPs formation upon increasing the reaction temperature
from 60 to 110 °C. The IPs formation, on the other hand, in- Table 5 presents the overall catalytic performance of precata-
creased two-fold upon increasing the temperature from 60 to lyst 9 at 420 min. Firstly, PMPs formation increased from 0.5 to
70 °C, 80 to 90 °C and 90 to 100 °C.
71% when increasing the temperature from 60 to 100 °C; how-
ever, from 100 to 110 °C the PMPs yield decreased from 71 to
The selectivity toward PMPs (67.2%) and the SMPs (28.8%) 64.2%. The reason for this is the formation of a very large
are relatively high at 110 °C. The TOF is also directly related to amount of SMPs (23.2%) and IPs (2.6%), which is, more than
the reaction temperature. The TOF of precatalyst 6 are general- twice the amount at 100 °C. Similarly, the selectivity and TOF
ly lower than for precatalysts 7 and 8. This, therefore, shows the showed a decrease when going from 100 to 110 °C.
relatively high stability of precatalyst 6 compared to those of 7
and 8. As a result of having a more positive Ru charge, precata- The selectivity showed a dramatic (50%) increase upon increas-
lyst 6 showed a low initiation rate. This is also in agreement ing the temperature from 60 (43%) to 80 °C (93%) and then
with the DFT study of Getty et al. [19], i.e., precatalyst 6 begins to decrease to 91% (at 90 °C), 85% (at 100 °C) and
(0.988) has more positive Mulliken’s atomic charge on Ru than finally to 71% (at 110 °C). Although the catalyst showed signif-
Table 4: Summary of catalytic performance of precatalyst 6 at different temperatures (Ru/1-octene molar ratio 1:9000, 420 min).
Entry
Temp. [°C]
Conv.a
PMPsa
SMPsa
IPsa
Sb
TONc
TOFd
1
2
3
4
5
6
60
2.5
1.0
0.9
0.6
1.3
0.7
1.2
3.9
2.1
41
66
85
83
66
69
93
0.36 × 10−2
1.61 × 10−2
5.86 × 10−2
21.07 × 10−2
21.88 × 10−2
24.01 × 10−2
70
6.8
4.5
1.0
406
80
19.4
70.5
93.0
98.0
16.4
59.0
61.3
67.2
2.3
1479
5310
5514
6051
90
10.3
28.5
28.8
100
110
aConversion or yield in mol %; bS (selectivity) in percent toward PMPs; cTON (turnover number) = [%PMPs × (Oct/Ru)]/100; dTOF (turnover frequen-
cy) = TON/time in s.
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Table 5: Summary of catalytic performance of precatalyst 9 at different temperatures (Ru/1-octene molar ratio 1:9000, 420 min).
Entry
Temp. [°C]
Conv.a
PMPsa
SMPsa
IPsa
Sb
TONc
TOFd
1
2
3
4
5
6
60
1.1
0.5
0.3
0.3
0.9
0.3
0.6
1.2
2.6
43
73
93
91
85
71
45
0.18 × 10−2
1.89 × 10−2
8.57 × 10−2
20.82 × 10−2
25.36 × 10−2
22.93 × 10−2
70
7.3
5.3
1.1
477
80
25.7
64.0
83.3
90.0
24.0
58.3
71.0
64.2
1.4
2160
5247
6390
5778
90
5.1
100
110
11.1
23.2
aConversion or yield in mol %; bS (selectivity) in percent toward PMPs; cTON (turnover number) = [%PMPs × (Oct/Ru)]/100; dTOF (turnover frequen-
cy) = TON/time in s.
icant stability and very good (71%) selectivity at 110 °C, it is at all temperatures (except at 80 °C). In an attempt to under-
advisable not to go beyond 100 °C, as the formation of SMPs stand the significance of these results, DFT calculations were
and IPs doubled that will affect the overall PMPs yield. It is performed on the precatalysts.
also worthwhile to note the decrease in the turnover frequency
at 110 °C.
Precatalyst 6 showed the lowest activity of all precatalysts in
the specified temperature ranges. It is also worthwhile to note
Similar to the Me-substituted precatalysts 6 (85%) (see Table 4, that increasing the reaction temperature showed a significant in-
entry 3), 7 (97%) (see Table 2, entry 5) and 8 (92%) (see crease in the activity of 6. Precatalyst 9, on the other hand,
Table 3, entry 5), precatalyst 9 showed high selectivity (93%) showed better performance at high temperatures (≥70 °C) com-
towards PMPs and good stability at 80 °C after 420 min. It is pared to 6. This may be explained by the longer Ru–N bond
also observed from the results that precatalyst 9 showed rela- (2.181 Å) in the geometry-optimised structure (Figure 9) of
tively high selectivity (91%) for PMPs and good activity, higher precatalyst 9 compared to that of the Ru–N bond (2.166 Å) of 6.
than its methyl counterparts 6 (83%) (see Table 4, entry 4), 7 A longer bond suggests a weaker Ru–N chelation thus a more
(84%) (see Table 2, entry 6) and 8 (89%) (see Table 3, entry 6), active hemilabile complex. The difference in the Ru–N bond
at 90 °C after 420 min.
length may be attributed to the electron-withdrawing inductive
effect of the OMe group making the Ru–N chelation weaker.
A general comparison of the overall performance of the precata- Furthermore, a type of orbital interaction between the oxygen of
lysts, in terms of PMPs, SMPs, IPs, selectivity, TON and TOF, the 3-OMe group and the two α-phenyl rings, i.e., an oxygen
exhibits the decreasing order of 7 > 9 > 8 > 6 at 60, 90 and lone pair-aromatic π interaction illustrated in Figure 10, may
100 °C. The order, however, changes at 80 °C to 8 > 7 > 9 > 6 add to the inductive effect. The longer Ru–O bond (2.031 Å),
and at 70 °C to 7 8 > 9 > 6. In all cases, the small amounts of shorter Cα–O bond (1.420 Å) and Cα–C2 bond (1.541 Å) ob-
SMPs and IPs are positive for the application of these systems served in precatalyst 9 when compared to the corresponding
at higher temperatures. Overall precatalyst 7 performed the best bonds in 6, i.e., 2.028, 1.425 and 1.544 Å, respectively, supports
Figure 9: Geometry-optimised structures of precatalyst 9, 6 and 8.
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Beilstein J. Org. Chem. 2019, 15, 194–209.
such a premise. It may also be a plausible explanation for the In order to overcome the combined effect of the resistance that
envelope geometry of the five-membered ruthenacycle. In addi- resulted from the steric crowdedness and the inductive electron-
tion, the relatively low ruthenium metal positive charge on 9 donation by the 3-Me group and open the strong Ru–N chela-
would cause it to have a high initiation rate constant [19].
tion, it needs relatively high energy. In 8 the methyl group is in
exactly the opposite orientation to the two α-phenyl groups.
Therefore, the steric crowdedness that is observed in 6 that will
lead to steric resistance to open the Ru–N chelation does not
exist. Thus 8 is more susceptible to hemilability than 6 and ex-
hibits higher activity. Therefore, for 4-Me- and 5-Me-subtsti-
tuted precatalysts, only the inductive electron-donation effect of
the methyl group is the reason for the increased stability. In the
3-Me- and 3-OMe-substituted precatalysts, however, the steric
effect and orbital interactions work towards the stability of the
precatalyst in addition to inductive effects.
Stability of precatalysts
Figure 10: An illustration of the envisaged methoxy oxygen lone pair-
aromatic π-electron interaction.
In previous studies [11,12,14] we investigated the stability of
pyridinyl-alcoholate Grubbs-type precatalysts as seen in the im-
proved catalytic lifetimes of these complexes. Plots of
On the other hand, the 3-Me group in 6 will strengthen the ln([starting material]) versus time, proposed by Grubbs and
Ru–N chelation via inductive electron-donation and steric co-workers [20], were used as a measure of the stability of the
repulsion between the methyl group and the two phenyl rings. precatalyst, i.e., a linear plot indicates a reaction with pseudo-
As a result of the steric interaction 6 has a planar five-mem- first order rate kinetics, while a curved plot points towards cata-
bered ruthenacycle geometry (Figure 9). In the absence of sub- lyst decomposition. We used the conversion of 1-octene at a
stituents on the pyridinyl moiety it is expected that the resulting Ru/1-octene molar ratio of 1:9000 and a reaction temperature of
precatalyst will be more active at lower temperatures. This is 80 °C to compare the stability of 5d with that of 5i, 5j and 5k
indeed the case when 5d is used as catalyst.
[14]. In Figure 11 the literature data (% 1-octene conversion
and ln(% 1-octene)) of 5d is compared with that of precatalysts
As we have discussed earlier, the 4-Me-substituted precatalyst 7 6 – 9 at a Ru/1-octene molar ratio of 1:9000 and a reaction tem-
has shown better catalytic performance in all temperatures perature of 80 °C over 540 min.
under investigation except at 80 °C. The reason for this is that
the Me group is, relatively speaking, further removed from the The overall activity order of the catalysts follows the order
pyridine nitrogen so that the inductive electron-donation by the 5d > 8 > 7 > 9 > 6 up to ca. 540 min. The order 8 > 5d 7 >
methyl group cannot significantly influence the electron densi- 9 > 6 is observed for both the overall metathesis and the PMPs
ty on the pyridine nitrogen. There is also no steric effect that formation. All the precatalysts exhibits first-order kinetics over
would interfere with the Ru–N bond strength. The strength- the first ca 540 min when the ln(% 1-octene) plots (Figure 11b)
ening effect on the Ru–N chelation would, therefore, possibly are considered. The substituted precatalysts show better stability
be low compared to the other precatalysts.
than 5d, thus longer lifetimes, with 6 and 9 the slowest and 8
close to but slower than 5d.
If this is a plausible explanation for the relatively better perfor-
mance of the 4-Me-substituted precatalyst 7, one might ask It is interesting to note that the stability of 5j and 5k correlates
what about the difference between the 3-Me-, 6, and 5-Me- very well with that of 7, while 5i is more stable than 7 but less
substituted, 8, precatalysts that are at the same distance from the than 9 (comparison of current results with results in [14]). This
pyridine nitrogen? In the optimised structure of 6, the Me group clearly indicates that a substituent on one of the α-phenyl
is in a crowded environment due to its proximity to the two groups or the pyridinyl moiety has a stabilising effect on the
α-phenyl groups, which upon opening the Ru–N chelation, corresponding precatalyst with a substituent on the 3-position (6
would even become more sterically crowded. This results in a and 9) of the pyridinyl rendering the precatalyst the most stable.
planar geometry of the five-membered ruthenacycle while 8 ex- The latter two is also active at higher temperatures.
hibits an envelope geometry. The Ru–N (2.179 Å) bond length
in 8 is longer and the Cα–O (1.417 Å) and Cα–C2 (1.532 Å) Table 6 presents the overall catalytic performance of precata-
bonds are shorter than the corresponding bonds in 6.
lysts 5d, and 6–9 at a Ru/1-octene molar ratio of 1:9000, 80 °C
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Beilstein J. Org. Chem. 2019, 15, 194–209.
Figure 11: Influence of precatalysts 6–9 and 5d on the (a) conversion of 1-octene and (b) ln([n%1-octene]) versus time plots (Ru/1-octene = 1:9000;
80 °C). [■ 6, 7, ▲ 8, ● 9, □ 5d]
Table 6: Summary of catalytic performances of different precatalyst (Ru/1-octene molar ratio 1:9000, 80 °C, 420 min).
Entry
Precat.
Conv.a
PMPsa
SMPsa
IPsa
Sb
TONc
TOFd
1
2
3
4
5
5de
8
71.2
65.4
44.4
25.7
19.4
68.0
60.5
43.1
24.0
16.4
3.0
4.1
1.1
1.4
2.3
0.3
0.8
0.2
0.3
0.7
96
92
97
93
85
6120
5445
3879
2160
1476
24.29 × 10−2
21.61 × 10−2
15.39 × 10−2
8.57 × 10−2
5.86 × 10−2
7
9
6
aConversion or yield in mol %; bS (selectivity) in percent toward PMPs; cTON (turnover number) = [%PMPs × (Oct/Ru)]/100; dTOF (turnover frequen-
cy) = TON/time in s; eSee reference [14].
and 420 min. According to these results precatalyst 5d shows Effect of catalyst concentration
the highest PMPs, TON and TOF. Although it has relatively Earlier studies indicated that 80 °C is the optimum temperature
high SMPs compared to most of the precatalysts, its overall for 5d [10,11]. It was therefor decided to investigate the effect
performace prevails over the other precatalysts. The second best of the concentration of the precatalyst on the metathesis of
performance was observed for 8, as it resulted in relatively high 1-octene at 80 °C. Precatalyst 8 was chosen for this investiga-
PMPs, TON and TOF compared to the rest of the precatalysts, tion at Ru/1-octene molar ratios of 1:6000, 1:9000, 1:10000 and
although its SMPs ranks as first. The rest of the precatalysts can 1:15000.
be ranked in a decreasing order of activity of 7 > 9 > 6. It is
clear from the data in Table 6 that the unsubstituted precatalyst
5d is more active compared to the substituted precatalysts at
420 min. This will only be due to the substituent effect on the
activity of the precatalyst.
Table 7 presents the overall catalytic performance of precata-
lyst 8 at different Ru/1-octene molar ratios, 80 °C and 420 min.
With a decrease in precatalyst concentration a direct relation-
ship was observed with the conversion of 1-octene and PMPs,
Table 7: Summary of the catalytic performance of precatalyst 8 present in different concentrations (80 °C, 420 min).
Entry
C8:Ru
Conv.a
PMPsa
SMPsa
IPsa
Sb
TONc
TOFd
1
2
3
4
6000
76.9
65.4
60.2
52.8
70.7
60.5
55.4
48.4
5.9
4.1
4.5
3.8
0.4
0.8
0.4
0.7
92
92
92
92
4240
5445
5539
7254
16.83 × 10−2
21.61 × 10−2
21.98 × 10−2
28.78 × 10−2
9000
10000
15000
aConversion or yield in mol %; bS (selectivity) in percent toward PMPs; cTON (turnover number) = [%PMPs × (Oct/Ru)]/100; dTOF (turnover frequen-
cy) = TON/time in s.
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Beilstein J. Org. Chem. 2019, 15, 194–209.
they all decreased, while the TON and TOF increased. The of the pyridine ring were also observed. The latter three signals
SMPs and IPs did not follow a specific trend while the overlapped too much to be useful.
selectivity remained the same, i.e., 92%, at all the concentra-
tions.
We performed a 1H NMR investigation of the metathesis of
1-octene by precatalyst 7 in the temperature region 60–90 °C in
1H NMR investigation of precatalyst 7 and 5d order to gain some insight into the reaction mechanism. The
Proton nuclear magnetic resonance spectrometry (1H NMR) is a carbene-Hα 1H NMR signals at 90 °C over a period of 345 min
powerful tool to study ruthenium alkylidene complexes and was are presented in Figure 12. Three signals attributed to the
used to study 1-octene metathesis in the presence of 1 and 2 benzylidene (δ 17.33 ppm, singlet), heptylidene (δ 16.85 ppm,
[10,21,22]. The conversion of the benzylidene, [Ru]=CHPh, to triplet) and methylidene (δ 15.68 ppm) were observed. A small
the heptylidene, [Ru]=CHC6H13, and methylidene, [Ru]=CH2, signal at δ 16.66 ppm appeared at 270 min and was not assigned
(where [Ru] = RuL2Cl2) could be clearly distinguished using (inter alia multiplicity not discernable). A different develop-
the carbene-Hα signals; they appeared as a singlet, triplet and ment of carbene signals over time is observed than what was re-
singlet in the δ 18.5–20.2 ppm region, respectively. We also in- ported before for 5a [10], i.e., the methylidene signal starts to
vestigated 5a and observed five carbene-Hα signals attributing appear at 12 min while the heptylidene signal only starts to
three to the alkylidene species when the pyridinyl-alcoholato appear at 165 min. The benzylidene signal rapidly declines after
ligand was in the “closed” (coordinated) position; δCHPh 194 min and is not observed at 345 min. No clear indication of
18.05 ppm, δCHHx 16.71 ppm and δCHH 16.08 ppm [10]. The an “open” or “closed” complex was observed, so it assumed
other two was attributed to the benzylidene (δCHPh 19.48 ppm) that the signals represent the “closed” species. It can be con-
and methylidene (δCHH 19.76 ppm) species in the “open” cluded that the alkylidene species of the pyridinyl-alcoholato
(uncoordinated) position with the uncoordinated heptylidene Grubbs 2-type precatalysts are quite stable at high temperatures
signal not appearing probably due to the fast reaction of this explaining the activity of these precatalyst at high temperatures
species. Four signals at δ 9.48 ppm (for the coordinated ligand), and the slow rate of disappearance/formation of these signals
9.05 ppm, 9.22 ppm, and 9.71 ppm attributed to the Hα signals confirms the longer lifetimes observed in the catalytic reactions.
Figure 12: 1H NMR spectra of the carbene-Hα region at different time intervals of the 1-octene/7 reaction mixture in toluene-d8 at 90 °C.
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Beilstein J. Org. Chem. 2019, 15, 194–209.
The Hα pyridine ring 1H NMR signals at 90 °C at 345 min are crease at 420 min. This affected the other performance indica-
presented in Figure 13. Five Hα signals of the pyridine ring that tors, i.e., PMPs, S, TON and TOF; lower values than the neat
are not observed at the beginning of the reaction were observed reactions were obtained. The results suggest that no significant
at δ 9.57 ppm (doublet), δ 9.22 ppm (doublet), δ 9.08 ppm solvent effect appears to exist .However, the increase in SMPs
(unknown multiplicity), δ 8.91 ppm (doublet) and δ 8.85 ppm (associated with an increase in IPs) indicates decomposition of
(doublet). The signal at δ 9.70 ppm (singlet) was the only signal the precatalyst to active isomerisation species, probably metal
observed at the beginning of the reaction. These signals is prob- hydride species. In our NMR study no indication of the exis-
ably due to the “open” and “closed” pyridinyl-alcoholato tence of metal hydride species was found.
ligands of alkylidene species and a possible assignment is
shown in Figure 13. Further research is required to gain a Conclusion
comprehensive understanding of the operation of the active The aim of our research is to control the Ru–N bond strength of
species of the pyridinyl-alcoholato ruthenium alkylidene precat- the bidentate hemilabile pyridinyl-alcoholato ligands in precata-
alysts.
lyst 5d in an attempt to synthesise a precatalyst with high per-
formance for linear alkene metathesis at high temperatures. To
reach this aim, we synthesised ruthenium alkylidene precata-
lysts by substituting one of the hydrogens of the pyridine ring of
Effect of solvent on 1-octene metathesis
using precatalyst 7
Because toluene-d8 was used in the 1H NMR study it was the bidentate pyridinyl-alcoholato ligand by Me and OMe
decided to investigate if toluene as solvent has any effect on the groups. We synthesised the 3-, 4-, and 5-methyl and 3-methoxy-
1-octene metathesis reaction using precatalyst 7. Results of this substituted 5d precatalysts. The catalytic activity, selectivity
investigation are presented in Table 8.
and stability results of the Me- and OMe-substituted 5d precata-
lysts, in 1-octene metathesis, showed promising results at high
An increase in SMPs formation is the only difference that was temperatures. The high stability, very good activity, selectivity,
observed when toluene was used as solvent with an 8.9% in- TON and TOF of the four precatalysts, at high temperatures,
Figure 13: 1H NMR spectra of the Hα region of the pyridine ring of the 1-octene/7 reaction mixture in toluene-d8 at 90 °C at 345 min.
Table 8: Summary of the catalytic performance of precatalyst 7 present in the presence of toluene as solvent (Ru/1-octene = 1:9000, 90 °C, 420 min).
Entry
Solvent
Conv.a
PMPsa
SMPsa
IPsa
Sb
TONc
TOFd
1
2
neat
81.4
79.8
72.4
61.8
8.0
1.0
1.2
89
77
6516
5564
25.86 × 10−2
22.08 × 10−2
toluenee
16.9
aConversion or yield in mol %; bS (selectivity) in percent toward PMPs; cTON (turnover number) = [%PMPs × (Oct/Ru)]/100; dTOF (turnover frequen-
cy) = TON/time in s; eVtoluene = 4 mL.
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Beilstein J. Org. Chem. 2019, 15, 194–209.
proved that the hemilability of the bidentate hemilabile 99%) and toluene (99%) were purchased from Sigma-Aldrich.
pyridinyl alcoholato ligand can be influenced by monosubstitu- Toluene-d8 (99.5%) was purchased from MERCK and tert-
tion on the pyridinyl moiety. A Ru/1-octene precatalyst concen- butyl hydrogen peroxide (80%) from Riedel-de Haen. Diethyl
tration of 1:9000 and 80 °C were found to be the best reaction ether and THF were dried over Na in the presence of benzo-
conditions for the precatalysts. Although 8 performed better phenone. Pentane was distilled over CaH2 in an inert atmo-
than the rest of the precatalysts at 80 °C, 7 showed the best per- sphere before using as solvent. 2-N,N-Dimethylaminoethanol
formance in the other temperatures under investigation. and n-hexane were dried over molecular sieves (4 Å) and kept
1H NMR spectrometry was used to investigate precatalyst 7 and under nitrogen before use. Gas-tight Hamilton syringes were
the active alkylidene species, i.e., benzylidene, heptylidene and used to add reagents and dried solvents to the reactor. Acrodisc
methylidene, were observed. NMR evidence of the hemilabile Premium 25 mm syringe filter with GxF/0.45 µm GHP mem-
nature of these precatalysts was found in the Hα region of the brane (PALL) was used to filter the lithium salt from the precat-
pyridine ring of the pyridinyl alcoholato ligand.
alyst.
Experimental
Experimental procedures
Instruments and reagents
Precatalyst synthesis: The well-established methods of
1H NMR (600 MHz) spectra were obtained using a Bruker Herrmann et al. [23] and Van Der Schaaf et al. [7] were used to
Ultrashield Plus 600 Avance III spectrometer. synthesize precatalysts 6–9. This is illustrated in Scheme 2.
GC/FID: The progress of the metathesis reactions was fol- Benzylidene chloro(1,3-bis(2,4,6-trimethylphenyl)-2-
lowed on an Agilent 6890 gas chromatograph equipped with an imidazolidinylidene)[1-(3′-methyl-2′-pyridinyl)-1,1-
Agilent 7683 auto sampler, HP-5 5% phenyl methyl siloxane diphenylmethanolato]ruthenium (6): Yield 0.422 g, 89%,
capillary column and a flame ionisation detector (FID). The green powder, decomp.: 190 °C, 1H NMR (600 MHz, CDCl3) δ
following general GC settings were used: Column: HP-5, 16.89 (s, 1H, H of Ru=CHPh), 9.70 (t, J = 3.8 Hz, 1H, H-6 of
30.0 m × 320 μm × 0.25 μm, nominal; detector: FID at 250 °C; C5H3N), 7.24 (m, 1H, para H of Ru=CHPh), 7.24 (m, 2H, meta
H2 flow rate: 40 mL/min at 20 °C; air flow rate: 450 mL/min at H of Ru=CHPh), 7.15 (m, 2H, ortho H of Ph), 7.05/7.28 (m,
20 °C; inlet temperature: 200 °C, 60.6 kPa; N2 carrier gas flow 4H, meta H of mesityl), 7.00 (m, 1H, H-4 of C5H3N), 6.90 (s,
rate: 45 mL/min at 20 °C; injection volume: 2 µL (auto injec- 2H, para H of Ph), 6.97 (d, J = 3.7 Hz, 2H, ortho H of
tion); syringe size 10.0 μL; split ratio: 50.4:1; split flow Ru=CHPh), 6.49 (s, 1H, H-5 of C5H3N), 6.75 (s, 2H, meta H of
94.3 mL/min; oven programming: 60 °C for 5 min; 60 to Ph), 4.01–3.87 (m, 4H, H of NHC), 2.56/2.18/2.11 (3 × s,
110 °C at 25 °C/min; 110 °C hold for 10 min; 110 to 290 °C at 3 × 6H, H of 6-CH3 on mesityl), 1.18 (s, 3H, H of CH3 on
25 °C/min; 290 to 300 °C at 25 °C for 5 min.
C 5 H 3 N ) ; M A L D I – M S ( m / z ) : [ M ] + 8 0 7 . 2 6 4 6
(C47H48ClN3ORu); IR (in cm−1): v(OH, moisture) = 3386,
GC/MSD analyses was performed on an Agilent 6890 gas chro- v(=C-H) = 3054, 3018, 776, v(CH3) = 2921, 2852, 1396,
matograph equipped with an Agilent 7683B autosampler, HP-5 v(C=N) = 1604, v(C=C) = 1584–1443, v(C-N) = 1254, v(C-O) =
capillary column and an Agilent 5973 mass selective detector 1157; 13C NMR (150 MHz) δ 290.4, 214.5, 169.9, 151.2, 148.4,
(MSD). The oven programme was used with either a 144.6, 144.4, 139.2, 137.8, 132.4, 128.8, 128.7, 126.8, 126.7,
two-minute solvent delay or no solvent delay. Helium was used 121.3, 93.7, 51.2, 21.0, 19.1, 18.8.
as carrier gas with a 1.5 mL/min flow rate at 20 °C. The
following general GC settings were used: Column: HP-5, Benzylidene chloro(1,3-bis(2,4,6-trimethylphenyl)-2-
30.0 m × 320 μm × 0.25 μm; Split ratio: 0.1:1; Split flow: imidazolidinylidene)[1-(4′-methyl-2′-pyridinyl)-1,1-
0.1 mL/min; Inlet: 250 °C, 16.6 kPa; Injection volume: 0.2 μL; diphenylmethanolato]ruthenium (7): Yield 0.456 g, 96%,
Detector: 50–550 Dalton mass range; scan speed of green powder, decomp.: 190 °C, 1H NMR (600 MHz, CDCl3) δ
2.94 seconds per decade; oven programming: 60 °C (hold time 17.09 (s, 1H, H of Ru=CHPh), 9.42 (d, J = 5.9 Hz, 1H, H-6 of
2 min); 60 to 110 °C at 25 °C/min; 110 °C (hold time 10 min); C5H3N), 7.27 (m, 1H, H-3 of C5H3N), 7.27 (m, 2H, para H of
110 to 290 °C at 25 °C/min (hold time 16 min).
Ru=CHPh), 7.24 (s, 2H, meta H of Ru=CHPh), 7.24/7.04 (m,
4H, meta H of mesityl), 7.10 (m, 4H, ortho H of Ph), 6.96 (t,
Reagents: 2-Bromo-3-methylpyridine (95%), 2-bromo-4- J = 7.6 Hz, 2H, para H of Ph), 6.92 (s, 2H, ortho H of
methylpyridine (97%), 2-bromo-5-methylpyridine (98%), Ru=CHPh), 6.76 (s, 1H, H-5 of C5H3N), 6.63 (s, 4H, meta H of
2-bromo-3-methoxypyridine (97%), n-BuLi (2.5 M in hexane), Ph), 4.04–3.90 (m, 4H, H of NHC), 2.60/2.25/2.19 (3 × s,
benzophenone (99%), 2-N,N-dimethylaminoethanol (≥ 99.5%), 3 × 6H, H of 6-CH3 on mesityl), 1.99 (s, 3H, H of CH3 on
Grubbs 2 (2) (97%), 1-octene (99% GC), nonane (reagent plus C5H3N); IR (in cm−1): v(OH, moisture) = 3393, v(=C-H) =
206

Beilstein J. Org. Chem. 2019, 15, 194–209.
Scheme 2: Synthesis of pyridinyl-alcohol ligands and Grubbs 2-type pyridinyl-alcoholato complexes.
3052, 3018, 755, v(CH3) = 2919, 2850, 1290, v(C=N) = 1611, C5H3N), 7.30 (m, 1H, para H of Ru=CHPh), 7.29 (m, 2H, meta
v(C=C) = 1448–1443, v(C-N) = 1258, v(C-O) = 1248; H of Ru=CHPh), 7.25 (m, 4H, ortho H of 2Ph), 7.16 (m, 1H,
13C NMR (150 MHz, CDCl3) δ 291.7, 214.7, 170.6, 151.5, H-4 of C5H3N), 6.91/6.54 (s, 4H, meta H of mesityl), 7.04 (m,
149.8, 146.2, 143.8, 139.2, 137.8, 136.7, 129.0, 128.5, 126.8, 1H, H-4 of C5H3N), 6.99 (m, 2H, para H of 2Ph), 6.99 (m, 1H,
126.6, 122.3, 93.0, 51.3, 21.0, 19.1, 18.8.
H-5 of C5H3N), 6.73 (m, 4H, meta H of 2Ph), 6.73 (m, 2H,
para H of 2Ph), 6.43 (d, J = 7.7 Hz, ortho H of Ru=CHPh),
Benzylidene chloro(1,3-bis(2,4,6-trimethylphenyl)-2- 4.01–3.89 (m, 4H, H of NHC), 2.57/2.19/2.16 (3 × s, 3 × 6H,
imidazolidinylidene)-[1-(5′-methyl-2′-pyridinyl)-1,1- 6-CH3 of mesityl), 2.90 (s, 3H, H of OCH3); MALDI–MS
diphenylmethanolato]ruthenium (8): Yield 0.304 g, 64%, (m/z): [M]+ 823.2417 (C47H48ClN3O2Ru); IR (in cm−1): v(OH,
dark-green crystalline powder, decomp.: 125 °C, 1H NMR moisture) = 3388 v(=C-H) = 3054, 3015, 755, v(CH3) = 2920,
(600 MHz, CDCl3) δ 17.07 (s, 1H, H of Ru=CHPh), 9.37 (s, 2848, 1316, v(C=N) = 1601, v(C=C) = 1586–1457, v(C-N) =
1H, H-6 of C5H3N), 7.26 (m, 1H, para H of Ru-CHPh), 7.24 (s, 1258, v(C-O) = 1230; 13C NMR (150 MHz, CDCl3) δ 290.5,
4H, meta H of mesityl), 7.14 (m, 1H, H-3 of C5H3N), 7.10–7.02 214.5, 162.9, 151.5, 151.4, 149.1, 142.3, 139.2, 137.3, 137.2,
(m, 4H, ortho H of Ph), 6.97 ( t, J = 7.5 Hz, 2H, meta H of 129.0, 128.6, 126.8, 122.1, 93.6, 55.4, 51.3, 20.9, 19.1, 18.8.
Ru=CHPh), 6.92 (s, 2H, ortho H of Ru=CHPh), 6.92 (s, 1H,
H-4 of C5H3N), 6.75 (t, J = 7.6 Hz, 2H, para H of 2Ph), 6.68/ Metathesis reactions: The metathesis reactions were carried
6.58 (2d, J = 7.3 Hz, 4H, meta H of 2Ph), 4.00–3.95 (m, 4H, H out in 5 mL small-scale glass reactors. The reactor containing a
of NHC), 2.60/2.26/2.19 (3 × s, 3 × 6H, 6-CH3 of mesityl), 2.17 small magnetic bar was flushed with nitrogen and an appro-
(s, 3H, H of CH3 on C5H3N); MALDI–MS (m/z): [M]+ priate amount of precatalyst added by weighing. Once again,
807.2660 (C47H48ClN3ORu); IR (in cm−1): v(OH, moisture) = the contents of the reactor were carefully flushed with nitrogen
3391, v(=C-H) = 3055, 3020, 755, v(CH3) = 2920, 2849, 1377, and the reactor was sealed. The sealed reactor was placed in an
v(C=N) = 1605, v(C=C) = 1481–1410, v(C-N) = 1261, v(C-O) = aluminium block on a magnetic stirrer. The temperature was set
1163; 13C NMR (150 MHz, CDCl3) δ 291.2, 214.5, 168.2, to the desired temperature and allowed to stabilise prior to the
151.6, 149.9, 146.3, 143.9, 139.2, 137.3, 134.9, 129.0, 128.5, reactor being placed in the block. The temperature was regu-
126.8, 126.5, 121.7, 92.9, 51.3, 20.9, 19.1, 18.8.
lated throughout the reaction using a temperature controller
fitted with a thermocouple. After one minute of heating nonane
Benzylidene chloro(1,3-bis(2,4,6-trimethylphenyl)-2- (0.25 mL) was added via gastight syringe (1 mL) as an internal
imidazolidinylidene)-[1-(3′-methoxy-2′-pyridinyl)-1,1- standard, followed by the addition of 1-octene (5 mL) via
diphenylmethanolato]ruthenium (9): Yield 0.465 g, 96%, gastight syringe (5 mL). Samples (0.1 mL) were withdrawn at
green powder, decomp.: 197 °C, 1H NMR (600 MHz, CDCl3) δ time intervals for ca. 520 min with a gastight syringe (1 mL),
16.97 (s, 1H, H of Ru=CH), 9.38 (d, J = 5.5 Hz, 1H, H-6 of transferred to a GC vial (1 mL), quenched with toluene
207

Beilstein J. Org. Chem. 2019, 15, 194–209.
(0.3 mL) and tert-butyl hydrogen peroxide (2 drops), and then BL460C G6 Storage Server, Operating system on compute
injected into a GC/FID by auto sampler. The metathesis reac- nodes: Scientific Linux SL release 5.3, Cluster operating
tion was terminated after 1440 minutes and analysed by system: Rocks 5.2 - Scientific Linux SL release 5.3.
GC/FID. Some samples were also analysed by GC/MSD. Each
experiment was repeated at least three times.
Acknowledgements
The financial support of the North-West University (NWU) and
1H NMR investigation of metathesis reaction: An NMR tube the South African Department of Science and Technology
was placed in a Schlenk tube, evacuated with a vacuum pump (DST)-National Research Foundation (NRF) Centre of Excel-
and then flushed with a stream of argon. The same procedure lence in Catalysis (c*change) towards this research is hereby
was repeated and then 12 mg (0.015 mmol) of precatalyst 7 was gratefully acknowledged. Opinions expressed and conclusions
added to the NMR tube. Once again, the contents of the NMR arrived at, are those of the authors and are not necessarily to be
tube were flushed with argon and toluene-d8 (0.65 mL) added. attributed to the NWU or c*change.
The catalyst was dissolved by shaking the contents and 1-octene
ORCID® iDs
Tegene T. Tole - https://orcid.org/0000-0002-5858-9239
(0.1 mL, 0.64 mmol) added immediately before putting the tube
in the spectrometer for temperature ranges 30–50 °C. 1H NMR
Johan H. L. Jordaan - https://orcid.org/0000-0002-8134-6753
spectra were recorded at 5–6 minute intervals for 5–8.5 h. For
Hermanus C. M. Vosloo - https://orcid.org/0000-0002-5879-323X
temperature ranges 60–90 °C, 1H NMR of the precatalyst was
done alone before adding the 1-octene. The precatalyst
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