Z-Selective Monothiolate Ruthenium Indenylidene Olefin Metathesis Catalysts

Wietse Smit, Jonas B. Ekeli, Giovanni Occhipinti,* Bartosz Wozniak, Karl W. Tornroos,

Article

Z‑Selective Monothiolate Ruthenium Indenylidene Oleﬁn

Metathesis Catalysts

and Vidar R. Jensen *

Cite This: https://dx.doi.org/10.1021/acs.organomet.9b00641

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sı Supporting Information

ABSTRACT: Ru-alkylidenes bearing sterically demanding arylthiolate ligands (SAr)

constitute one of only two classes of catalyst that are Z-selective in metathesis of 1-

alkenes. Of particular interest are complexes bearing pyridine as a stabilizing donor ligand,

[

RuCl(SAr)(CHR)(NHC)(py)] (R = phenyl or 2-thienyl, NHC = N-heterocyclic

carbene, py = pyridine), which initiate catalysis rapidly and give appreciable yields

combined with moderate to high Z-selectivity within minutes at room temperature. Here,

we extend this chemistry by synthesizing and testing the ﬁrst two such complexes (5a and

b) bearing 3-phenylindenylidene, a ligand known to promote stability in other

ruthenium-based oleﬁn metathesis catalysts. The steric pressure resulting from the three

bulky ligands (the NHC, the arylthiolate, and the indenylidene) forces the thiolate ligand

to position itself trans to the NHC ligand, a conﬁguration diﬀerent from that of the

corresponding alkylidenes. Surprisingly, although this conﬁguration is incompatible with

Z-selectivity and slows down pyridine dissociation, the two new complexes initiate readily at room temperature. Although their

thermal stability is lower than that of typical indenylidene-bearing catalysts, 5a and 5b are fairly stable in catalysis (TONs up to

200) and oﬀer up to ca. 80% of the Z-isomer in prototypical metathesis homocoupling reactions. Density functional theory (DFT)

calculations conﬁrm the energetic cost of dissociating pyridine from 5a (= M1-Py) to generate 14-electron complex M1. Whereas

the latter isomer does not give a metathesis-potent allylbenzene π-complex, it may isomerize to M1-trans and M2, which both form

π-complexes in which the oleﬁn is correctly oriented for cycloaddition. The oleﬁn orientation in these complexes is also indicative of

Z-selectivity.

INTRODUCTION

Chart 1. Ru-Based Oleﬁn Metathesis Catalysts

■

Oleﬁn metathesis (OM) is the most versatile catalytic method

for carbon−carbon double-bond formation. Several well-

deﬁned oleﬁn metathesis catalysts have been developed, based

−8

on molybdenum, tungsten, or ruthenium.

In general,

ruthenium-based catalysts are the most robust against oxygen,

moisture, and substrates with heteroatom-containing functional

groups. Grubbs ﬁrst-generation catalyst (1a, Chart 1) as well as

the more stable and active second-generation catalyst, 1c, are

both frequently used in a variety of OM reactions.

Nevertheless, challenges in Ru-catalyzed oleﬁn metathesis still

remain. For example, these catalysts do not mediate ring-closing

metathesis (RCM) reactions that form tetrasubstituted cyclic

alkenes, and already trisubstituted alkenes are diﬃcult to obtain

via RCM. Furthermore, the temperature-stability of 1a, and to

some extent also 1c, is limited, reducing their applicability in

reactions that require high temperatures. In addition, synthesis

of 1a−c requires costly and dangerous chemicals such as

diazoalkane derivatives.

Received: September 24, 2019

The above challenges have motivated the investigation of

alternatives to the traditional alkylidene ligands. Reaction of

RuCl (PPh ) ] with the cheap and easily available 1,1-

[

3 3

diphenylpropargyl alcohol was believed to have given the

XXXX American Chemical Society

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Article

diphenylallenylidene complex [RuCl (CCCPh )-

selective catalysis. However, both the indenylidene and

selectivity-inducing thiolate are sterically demanding ligands.

Replacing a chloride ligand in 2b−c with a bulky, selectivity-

inducing arylthiolate would probably not be possible. Instead,

complexes 3b and 4, the latter being commercially available, are

more promising starting points for this substitution. Presumably,

at least one pyridine ligand would readily dissociate to make

room for the incoming thiolate, and a pyridine ligand might ﬁt

next to the thiolate aryl group in the catalyst precursor.

To test this hypothesis, we have pursued the synthesis of

monothiolate analogues of NHC-coordinated ruthenium

indenylidene complexes, with the goal to obtain Z-selective

metathesis catalysts with attractive properties inherited in

particular from the indenylidene ligand.

(

PPh ) ]. However, X-ray crystallographic analysis and a

3 2

review of previously recorded spectroscopic data later showed

that a rearrangement had taken place, to a cyclized vinyl carbene

11,13

or indenylidene complex.

Subsequently, substitution of

PPh with PCy and various N-heterocyclic carbenes (NHC) led

to a family of indenylidene-bearing ruthenium complexes,

corresponding to both ﬁrst- (2a) and second-generation (2b,c)

11,13−16

Grubbs catalysts.

Consistent with the intention behind

the alkylidene replacement, 2a and 2c are thermally much more

stable than their benzylidene analogues, with no signs of

decomposition even after 10 days of heating at 80 °C in toluene-

14,17,18

d . Catalysts 2a−c performed well in RCM

and in the

In addition, 2c

3,15,19−24

synthesis of complex natural products.

mediates RCM leading to tetrasubstituted cyclic alkenes.

0,17

RESULTS AND DISCUSSION

However, the catalytic activities of 2a−c are, in general, not

■

higher than those of 1a−c.

Treatment of [RuCl (IMes)(py) (3-phenylindenylidene)]

Major improvements in catalytic activity, in particular in the

initiation and early phase of the metathesis experiments, have

been obtained by substituting the remaining phosphine ligand in

(3b), which was ﬁrst prepared according to known proce-

dures, with 1 equiv of potassium 2,4,6-triphenyl-benzenethio-

late in tetrahydrofuran (THF) readily led to compound 5a

(Scheme 1). Diﬀusion of n-pentane into a concentrated solution

b with two pyridine ligands. Starting from 2a−c, the same

26,27 28

strategy has oﬀered complexes 3a,b

and 4, respectively.

Although these mono- (4) and bis-pyridine (3a,b) complexes

initiate catalysis faster, they also deactivate faster, with catalytic

activity typically ceasing well before full conversion has been

Scheme 1. Synthesis of Compound 5a

reached.

The advantages of fast initiation are striking in Z-selective

oleﬁn metathesis. The ruthenium monothiolate congeners of

Grubbs third-generation catalysts, a class of catalyst originally

29−32

stabilized exclusively via benzylidene ether ligands,

not

only initiate faster, but these pyridine-coordinated ruthenium

monothiolate catalysts are also less inclined to isomerize the

substrate and thus are more selective for metathesis than their

3,34

counterparts bearing benzylidene ether ligands.

Thus, even if the monothiolate catalysts are less Z-selective

than Grubbs’s cyclometalated catalysts

of 5a in dichloromethane (DCM) at low temperature (−32 °C)

gave dark red crystals suitable for X-ray crystallographic analysis.

The molecular structure, relevant bond lengths, and angles are

shown in Figure 1.

35−38

and higher Z-

selectivities can also be reached when using Z-conﬁgured

substrates or cosubstrates in conjunction with Hoveyda’s

stereoretentive catechothiolate-coordinated catalysts,

Compound 5a adopts a slightly distorted square pyramidal

geometry, with the indenylidene occupying the apical position, a

conﬁguration typical of pentacoordinate Grubbs catalyst

precursors. Surprisingly, however, in the basal plane, the two

ligands with the strongest trans inﬂuence, the thiolate ligand

(S1) and IMes (C1) are positioned trans to each other (C1−

Ru1−S1 = 146°). The total steric pressure, larger than usual due

to the indenylidene ligand, prevents the cis-coordination of the

thiolate and the NHC observed in corresponding alkylidene

9−41

they do have attractive catalytic properties: The pyridine-

coordinated monothiolate catalysts can reach appreciable yields

combined with moderate to high Z-selectivity within minutes in

prototypical metathesis reactions of terminal oleﬁns at room

temperature. These catalysts can also be used in Z-selective

metathesis of diﬃcult substrates, such as dienes to form

macrocyclic rings and ω-alkenoic acids to form α,ω-dicarboxylic

acid alkenes.

29−31,33

monothiolate complexes.

Instead, in 5a, the two anionic

Summarizing, the ruthenium monothiolate catalyst design is

ﬂexible, and even allows for immobilization of the catalyst by

ligands are positioned cis to each other. While this cis-

conﬁguration of the anionic ligands has not been observed in

other monothiolate-coordinated ruthenium-based oleﬁn meta-

thesis catalysts, this isomer is not uncommon among other

tethering the catalyst to a silica support via the thiolate. Most

importantly, Z-selective monothiolate analogues of ﬁrst-,

second-, and third-generation Grubbs-type catalysts exist. The

main reason for this ﬂexibility is that these Z-selective catalysts

share key structural and mechanistic features with the Grubbs

catalysts: they bind the reacting oleﬁns and form the

ruthenacyclobutane intermediate trans to the donor ligand (L

ruthenium-based oleﬁn metathesis catalysts. Presumably,

during the synthesis, pyridine dissociates to make room for

the incoming bulky thiolate, followed by recoordination of

pyridine cis to IMes. This unusual conﬁguration, if maintained

during catalysis, is not expected to be suitable for fast-initiating

and Z-selective metathesis. First, with its position trans to

chloride (Cl1), a ligand of little trans inﬂuence, pyridine is not

guaranteed to dissociate rapidly. When it eventually dissociates,

the incoming oleﬁn and the ruthenacyclobutane (RCB)

intermediate are expected to experience a roughly equally

distributed steric pressure from the trans-positioned IMes and

the thiolate, as opposed to selectivity-inducing steric pressure

1,42

in Chart 1).

These similarities suggest that another structural facet found

among the ruthenium dichloride metathesis catalysts, the Ru-

indenylidene architecture, might also be transferred to the

ruthenium monothiolate framework. If so, then the attractive

properties of the indenylidene catalysts, such as improved

temperature stability (see above), might be exploited in Z-

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Figure 1. X-ray structure of 5a with displacement ellipsoids drawn at

the 50% probability level. Except for the NHC backbone (C2 and C3),

hydrogen atoms and solvent molecules (DCM) are omitted for clarity.

Selected geometric parameters: Ru1−C1 = 2.0911(16) Å, Ru1−Cl1 =

Figure 2. X-ray structure of 5b with displacement ellipsoids drawn at

the 50% probability level. Except for the NHC backbone (C2 and C3),

hydrogen atoms and solvent molecules (DCM) are omitted for clarity.

Selected geometric parameters: Ru1−C1 = 2.0795(15) Å, Ru1−Cl1 =

.3903(4) Å, Ru1−N3 = 2.1113(14) Å, Ru1−C51 = 1.8731(16) Å,

.3879(4) Å, Ru1−N3 = 2.1134(14) Å, Ru1−C51 = 1.8745(15) Å,

Ru1−S1 = 2.3066(4) Å, C2−C3 = 1.346(3) Å; N1−C1−N2 =

Ru1−S1 = 2.3181(4) Å, C2−C3 = 1.520(2) Å; N1−C1−N2 =

03.24(14)°, C1−Ru1−Cl1 = 88.52(5)°, C1−Ru1−N3 = 89.71(6)°,

06.56(14)°, C1−Ru1−Cl1 = 87.21(4)°, C1−Ru1−N3 = 91.37(6)°,

C1−Ru1−C51 = 106.20(7)°, C1−Ru1−S1 = 145.78(5)°, C51−Ru1−

C1−Ru1−C51 = 107.23(6)°, C1−Ru1−S1 = 145.07(4)°, C51−Ru1−

Cl1 = 91.76(5)°, C51−Ru1−N3 = 93.03(6)°, C51−Ru1−S1 =

Cl1 = 91.25(5)°, C51−Ru1−N3 = 93.03(6)°, C51−Ru1−S1 =

08.01(5)°, N3−Ru1−S1 = 88.53(4)°, Cl1−Ru1−S1 = 90.408(14)°,

07.66(5)°, N3−Ru1−S1 = 88.48(4)°, Cl1−Ru1−S1 = 90.385(13)°,

Ru1−S1−C27 = 114.26(5)°.

Ru1−S1−C27 = 113.73(5)°.

toward a single face of the RCB in known Z-selective

monothiolate catalysts.

parameters (see Table S1). The packing eﬀects should thus be

very similar for the two compounds and allow for meaningful

comparison of structural parameters. This is a welcome

opportunity to gauge the eﬀect of the two diﬀerent NHC

ligands, IMes and SIMes, in 5a and 5b. The electronic and steric

The corresponding SIMes-bearing compound, 5b (Scheme

), was obtained by reacting [RuCl (SIMes)(py)(3-phenyl-

Scheme 2. Synthesis of Compound 5b

44−48

eﬀects of these two ligands are still a matter of debate.

As in 5a, compound 5b adopts a slightly distorted square

pyramidal geometry, with the indenylidene occupying the apical

position and with trans-positioning of the SIMes and the thiolate

in the basal plane. The Ru−C(NHC) bond is slightly shorter in

b than in 5a (Ru1−C1 = 2.079 vs Ru1−C1 = 2.091 Å),

indicating that SIMes is more strongly bound to ruthenium than

IMes. This is consistent with the slightly stronger L → Ru σ-

donation and d → L back-donation calculated using density

indenylidene)] (4), commercially available as Umicore M31,

with 1 equiv of potassium 2,4,6-triphenyl-benzenethiolate in

THF. Unfortunately, we could not reach conversions above

functional theory (DFT), for SIMes compared to IMes.

Furthermore, the N1−C1−N2 angle is wider in 5b (107°) than

in 5a (103°). The wider N1−C1−N2 angle and shorter Ru−

C(NHC) bond in 5b imply higher steric pressure from the

mesityl groups on neighboring ligands. This, together with the

stronger L → Ru σ-donation should translate into longer bond

distances to ruthenium for the other ligands. Indeed, this holds

true for the covalent and dative metal−ligand bonds. The Ru

C(ind) and Ru−N(py) bonds are slightly longer in 5b (1.875

and 2.113 Å, respectively) than in 5a (1.873 and 2.111 Å),

whereas the eﬀect is more pronounced for the Ru−S bond trans

to the NHC, which is more than 1 pm longer in 5b than in 5a. In

contrast, the ionic Ru−Cl bond appears to adapt to the overall

arrangement of the other ligands and is 0.2 pm shorter in 5b than

in 5a. Similar results are obtained when the DFT-optimized

geometries of 5a and 5b are compared, again conﬁrming that in

this particular case packing eﬀects are not overriding the

electronic and steric di ﬀ erences between IMes and SIMes; see

the Supporting Information for details.

0%. Extension of the reaction time beyond 30 min and addition

of more thiolate led to massive product decomposition. Puriﬁed

b was obtained by ﬁltering the reaction mixture, dissolving the

residue in DCM, and precipitating or crystallizing using mixtures

of DCM and n-pentane at low temperature (−32 °C). Attempts

at synthesizing analogues of 5a,b by using an even more

sterically demanding thiolate, potassium 2,4,6-tri(di-m-

methylphenyl)benzenethiolate, resulted in either decomposi-

tion or yields that were too low to allow for product isolation.

Diﬀusion of n-pentane into a concentrated solution of 5b in

DCM at low temperature (−32 °C) yielded dark red crystals

suitable for X-ray crystallographic analysis. The molecular

structure, relevant bond lengths, and angles are shown in Figure

The crystals of 5a and 5b are isomorphous, that is, they belong

to the same space group P1 and have almost identical cell

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As shown by the complexity of the H and C NMR spectra

Table 1. Homocoupling of Neat Substrates at RT

(

see the Supporting Information), the pyridine ligand in both 5a

and 5b dissociates to give mixtures of 5a and 5b and the

corresponding pyridine-free ruthenium complexes. These

complexes have trans-conﬁgured anionic ligands, as discussed

in the “Molecular-Level Calculations” subsection below.

The ease with which pyridine dissociates from 5a and 5b is

surprising given the trans-position of this ligand relative to

chlorine. To help understand the unexpected lability of pyridine,

cat. load

(mol %)

time

(min)

conv.

(%)

run

cat.

sub.

(%)

0.5

0.1

0.5

0.1

0.01

0.5

OCT

variable-temperature H NMR experiments of complex 5a in

CDCl were conducted; see the “Variable-Temperature H

240

NMR Experiments on 5a” section in the Supporting

Information for details. The free pyridine was identiﬁed by the

two multiplets at 8.62 (CH (2,6)) and 7.68 ppm (CH (4)),

while the third multiplet at 7.29 ppm (CH (3,5)) was in part

masked by other aromatic peaks. These experiments reveal that

pyridine dissociation from 5a is spontaneous, albeit relatively

slow, at room temperature. Whereas the rate of dissociation is

much slower than for the closely related alkylidene-based

OCT

catalysts, the equilibrium is shifted more to the right: The

degree of pyridine dissociation is estimated to reach ca. 80% (see

the Supporting Information), compared to up to 32% for the

AB = allylbenzene, PB = 4-phenyl-1-butene, OCT = 1-octene.

Determined by H NMR using hexamethylbenzene as internal

standard. Freshly crystallized 5a (batch B; see Table 2 and associated

alkylidene-based catalysts. Still, equilibrium is not reached

instantaneously, and dissociation is associated with a non-

negligible barrier (see the estimate in the computational section

below). The reverse reaction is even slower since the equilibrium

is ﬁrmly shifted to the right.

discussion) + 0.1 equiv of pyridine. Additive: 0.1 equiv of pyridine.

However, with 4-phenyl-1-butene, and especially at high

conversion (entry 8), double-bond migration becomes signiﬁ-

cant. To gauge the eﬀects of the thiolate ligand, tests of the

dichloride precursors (3b and 4, respectively; see Schemes 1 and

Surprisingly, the variable-temperature NMR experiments also

reveal that 5a and 5b are much less thermally stable than other

indenylidene-bearing catalysts such as 2a−c and 3a−c.

Decomposition products were detected within minutes when

the sample was heated to 313 K or above. These are much lower

temperatures than those tolerated by alkylidene-bearing

analogues, suggesting that the poor thermal stability of 5a

and 5b is caused by the steric congestion resulting from the

combination of the indenylidene with two other sterically

demanding ligands, the thiolate and the NHC. Very little of the

additional steric congestion in these thiolate-coordinated

complexes compared to the corresponding dichlorides is due

to sulfur itself; see the “Molecular-Level Calculations” section

below.

) for the synthesis of 5a and 5b in metathesis of allylbenzene

have also been included (entries 11 and 12). A comparison

between entries 1 and 11 (IMes-based catalysts) and 5 and 12

(

SIMes-based), respectively, illustrates the dramatic eﬀect on

the Z-selectivity, which increases from 16% to above 75%, on

exchanging a chloride for a thiolate. However, the increased Z-

selectivity comes at the price of lower productivity, especially for

5a, which also promotes more oleﬁn isomerization than parent

dichloride precursor 3b.

Whereas 5b loses much of its catalytic activity and selectivity

for oleﬁn metathesis on addition of pyridine (entry 6), the

opposite is true for 5a. Loss of pyridine from microcrystalline 5a

reduces the selectivity for both metathesis and the Z-isomer

product. The reason is that, in contrast to 5b, freshly crystallized

5a contains slightly less than one equivalent of pyridine and even

more pyridine is lost on recrystallization. Whereas the eﬀect of

pyridine on catalysis using 5a is investigated explicitly below, in

Table 2 and the associated discussion, the catalytic tests of 5a

reported in Table 1 were performed in the presence of 0.1 equiv

of pyridine to ensure reproducibility between diﬀerent batches

of 5a. As indicated above, 5a contains less than one equivalent of

pyridine. For example, a recrystallized batch of 5a (batch A), was

Catalytic Results. As already noted, 5a and 5b are prone to

lose the pyridine ligand. This dissociation creates a vacant

coordination site. As expected from this unsaturation, both

complexes readily initiate metathesis at room temperature (see

Table 1), at a rate much faster than that of the 2-

9,30

isopropoxybenzylidene congeners.

Still, in accordance

with the already noted barrier to pyridine dissociation, the

initiation rate of the indenylidene catalysts is clearly lower than

that of the closely related pyridine-coordinated alkylidene

counterparts. Interestingly, the indenylidene catalysts oﬀer Z-

selectivities up to and above 80% in standard metathesis

homocoupling of 1-alkenes, even though the two potentially

selectivity-inducing bulky ligands, the NHC and the arylthiolate,

are positioned trans to each other in the pyridine-stabilized

precursors 5a and 5b. This result strongly suggests that a cis−

determined (by H NMR; see the Supporting Information) to

contain 0.92 equiv of pyridine, while a freshly crystallized batch

(batch B) contained 0.96 equiv. This suggests that the loss of

pyridine originates from cocrystallization of 5a with the trans-

conﬁgured pyridine-free species (see the “Molecular-Level

Calculations” section below) resulting from dissociation of

pyridine from 5a in solution. The latter dissociation is

irreversible; see the Supporting Information for experimental

results and the computational analysis below. In contrast, no

pyridine was lost when the ﬁrst crystallized material (batch B)

was dried under vacuum.

trans isomerization of the anionic ligands occurs during the

initiation of these catalysts. Similar to the 2-isopropoxybenzy-

lidene congeners, SIMes-coordinated catalyst 5b is more

active and Z-selective, albeit with lowering of the selectivity at

higher conversions (entry 5). Relatively little double-bond

isomerization of the substrate is observed with 5b, in particular

with allylbenzene (entries 5 and 6) and 1-octene (entry 9).

Surprisingly, despite the minor loss of pyridine (0.04 equiv)

on recrystallization, batch A is much less Z-selective than batch

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Table 2. Catalysis Using 0.5 mol % 5a: Inﬂuence of Pyridine

NMR experiments in CDCl , were included using a continuum

solvent model as described in the Supporting Information.

Whereas relative free energies are reported for the catalyst

concentration used in the variable-temperature NMR experi-

run batch pyridine (equiv) time (min) conv. (%)

O/I

Z (%)

0.2

71|

ments (1.86 mM in CDCl ), the free energies discussed in the

0.1

0.2

following are also, unless otherwise stated, the ones obtained for

a standard state corresponding to a 1 M solution at room

temperature.

The calculations ﬁrst conﬁrmed that pyridine dissociation is

energetically costly. From M1-Py, the DFT model of 5a,

dissociation leads to a kinetic product, M1, that is 14−16 kcal/

mol less stable depending on the solvent (Scheme 3). In

120

Scheme 3. Isomerization of Compound 5a

0.1

120

The content of pyridine in batches A and B was determined by H

NMR to be 0.08 and 0.04 equiv lower than the stoichiometric

amount. Determined by H NMR using hexamethylbenzene as

internal standard.

B (Table 2: cf. entries 1−2 and 14−15). Remarkably, the Z-

selectivity and the selectivity for oleﬁn metathesis of batch A are

improved on addition of 0.1 equiv of pyridine to the reaction

mixture. The pyridine addition thus makes the catalytic

performance of the two batches become similar (entries 3−6

and 16−19), albeit at the expense of reducing the catalytic

activity of batch A. Addition of more than 0.1 equiv of pyridine

reduces the initiation rate signiﬁcantly and does not improve the

Z-selectivity any further (entries 7−13).

The most likely cause for the drop in Z-selectivity following

loss of pyridine is catalyst decomposition to isomerization-active

ruthenium complexes. Indeed, the beneﬁt of excess pyridine is

more pronounced at higher conversion and at longer reaction

time. The complexes resulting from loss of pyridine are much

less stable than the 16-electron catalyst precursors, and

ruthenium metathesis catalysts are known to decompose to

species catalyzing double-bond migration in the substrate. This

double-bond migration is most likely accompanied by Z−E

isomerization of the product, either catalyzed by the same

species or, more likely (see below), by related metathesis catalyst

decomposition products. The isomerization-active species, such

as the 12-electron RuCl (SIMes), tend to be coordinatively

less saturated than metathesis catalysts. Thus, although addition

of pyridine reduces the catalytic activity for both metathesis and

isomerization, the low-coordinate isomerization catalysts are

deactivated more easily than the metathesis catalysts. Moreover,

the Z−E alkene isomerization catalysts seem to be more easily

deactivated than the double-bond migration catalysts.

Molecular-Level Calculations. As detailed above, the

ligand arrangement in 5a and 5b is neither expected to lead to

rapid initiation, nor to Z-selective metathesis. The initial

pyridine dissociation from 5a and the subsequent rearrangement

into isomers expected to be Z-selective were thus investigated

using DFT. Solvent eﬀects of toluene, mimicking the catalytic

experiments in the neat, largely apolar substrates, and chloro-

form, mimicking the above-described variable-temperature

addition, the dissociation requires geometric strain and is

calculated to involve a barrier of 25−27 kcal/mol. Although this

barrier is higher than barriers obtained for the initiation of

2,53

phosphine-stabilized Ru indenylidene catalysts,

it is

qualitatively consistent with pyridine dissociation taking place

at a time scale of hours instead of minutes; see the “Variable-

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Temperature H NMR Experiments on 5a” section in the

Supporting Information for details. This is much slower than for

the closely related alkylidene-based catalysts.

The 14-electron complex, M1, generated by pyridine

dissociation, may coordinate the oleﬁn to form a π-complex

(M1-AB) only about 1 kcal/mol less stable than M1. However,

this π-complex does not have a conﬁguration suitable for

cycloaddition and metathesis. Instead of the substrate

RHCCH plane being positioned parallel to that of

RuInd, which is required for formation of the new CC

bond, the oleﬁn approaches the indenylidene sideways.

However, metathesis-potent π-complexes may be formed by

ﬁrst isomerizing M1. By moving the thiolate from a cis to a trans

position relative to the chloride ligand, M1-trans is obtained.

The latter complex has, just like the closely related Z-selective

ruthenium alkylidene catalysts, an empty coordination site

trans to the NHC ligand and is only 4−5 kcal/mol less stable

than M1-Py.

Moreover, a transition state for the rearrangement from M1 to

M1-trans could not be obtained, but this isomerization is

associated with a very soft mode and a negligible barrier.

However, even if the rearrangement is fast and the calculated

free energy of M1-trans is not much higher than that of M1-Py,

the calculations do not, at ﬁrst glance, seem to be consistent with

the observation (above and in the Supporting Information) that

pyridine dissociation from 5a (= M1-Py) is spontaneous. The

puzzle is resolved by considering the conditions under which the

dissociation experiments were performed: The catalyst was

Figure 3. Optimized geometry of M1-trans indicating secondary

interactions between a thiolate phenyl substituent and ruthenium.

Scheme 4. Isomerization of Compound 5b

dissolved in CDCl at a much lower concentration (1.86 mM)

than the standard thermodynamic state (1 M). Correcting for

the low concentration of 5a (as explained in the Supporting

Information) leads to the prediction that pyridine dissociation is

spontaneous at room temperature (with a relative free energy of

M1-trans essentially at zero), as observed in the variable-

temperature NMR experiments. Under these conditions, i.e.,

mimicking the low-concentration, variable-temperature

NMR experiments as well as the catalytic experiments (which

are also conducted at catalyst concentrations closer to 1.86 mM

than to 1 M), recoordination of the dissociated pyridine in trans-

position to the NHC ligand (to give M1-trans-Py, Scheme 3) is

predicted to be endergonic by 2 kcal/mol. The surprising

stability of M1-trans is in part due to intramolecular donor−

acceptor interactions between one of the ortho-phenyl

substituents of the thiolate ligand and the ruthenium center,

as shown in Figure 3. Thus, diﬀerent from M1 which lacks such

stabilizing interactions, M1-trans has an eﬀective electron count

higher than 14.

The same stabilizing interactions are found for the

corresponding active complex (SM1-trans, Scheme 4) of

SIMes-coordinated catalyst 5b. DFT calculations (Scheme 4)

predict that this active complex is generated from 5b at

approximately the same rate as M1-trans from 5a. The

calculated pyridine dissociation barriers (via SM1-Py-TS and

M1-Py-TS, respectively) for the two catalysts are similar.

However, the free energy of the active species generated from 5b

Allylbenzene may bind to the empty coordination site in M1-

trans to form M1-trans-AB, a π-complex that is 4 kcal/mol less

stable than M1-AB, but which may subsequently evolve via

initiation metathesis to replace the indenylidene by a phenyl-

ethylidene ready for productive self-metathesis of allylbenzene.

That the latter will be Z-selective is suggested already by the

preferred allylbenzene coordination mode in M1-trans-AB,

which has the allylbenzene benzyl group oriented away from the

bulky thiolate. This allylbenzene orientation is also preferred in

the 3 kcal/mol less stable M2-AB, which may be formed by a

∼

180° rotation of indenylidene in M1-trans followed by

(SM1-trans) is ca. 1 kcal/mol lower than for 5a. The higher

substrate coordination.

catalytic activity of 5b (c.f., entries 1 and 5, Table 1) may thus, at

least in part, be due to a stabilizing eﬀect of the SIMes ligand

We have investigated alternatives to these pyridine dissoci-

ation and Ru complex isomerization pathways. First, since the

unusual isomer of 5a and 5b is due to the steric requirement of

the indenylidene, the relatively fast initiation and the Z-

selectivity of these complexes could, in principle, be due to

indenylidene isomerization to the less sterically demanding

(

both a stronger σ-donor and π acceptor than IMes, see above)

on the low-coordinate active species. This stabilization shifts the

equilibrium between the precursor and the active species more

in favor of the latter.

Organometallics XXXX, XXX, XXX−XXX

Organometallics

Article

4−56

allenylidene.

However, the allenylidene isomers lie more

%V_B_u_rof 67.7% (ﬁrst coordination sphere) and 94.9% (full

than 20 kcal/mol higher in energy than the indenylidene

counterparts (see the Supporting Information), making them

unlikely catalytic intermediates. More importantly, reaching

these intermediates is all but impossible, as the indenylidene−

allenylidene isomerization barriers have, in general, been found

to be prohibitively high. Another possibility is formation of a

new catalytic species via secondary bond formation between a

thiolate phenyl and ruthenium. Such a bond is formed upon

dissociation of chloride from a 14-electron complex. However,

even if chloride dissociation from M2 is compensated for by

Ru −phenyl interactions (in M5, see the Supporting Informa-

tion ), the energetic cost of reaching M5 is higher than 30 kcal/

mol, eﬀectively excluding chloride dissociation and Ru−phenyl

bond formation as part of the initiation mechanism.

In conclusion, while chloride dissociation and indenylidene

isomerization to allenylidene are unlikely to take part in the

catalyst initiation mechanism, pyridine may dissociate from M1-

Py (= 5a) to generate 14-electron complex M1. Whereas the

latter isomer does not give a metathesis-potent π-complex, it

may isomerize to M1-trans and M2, which both form π-

complexes in which the oleﬁn is correctly oriented for

cycloaddition. The oleﬁn orientation in these complexes is

also indicative of Z-selectivity.

model), respectively.

CONCLUSION

■

The ﬁrst Ru-monothiolate complexes (5a and 5b) bearing

indenylidene ligands have been prepared. These complexes

contain three sterically demanding ligands: the NHC, the

arylthiolate, and the indenylidene. The resulting steric pressure

forces the thiolate ligand to position itself trans to the NHC

ligand. This ligand conﬁguration is not found in any other

monothiolate-based OM catalyst. In fact, this conﬁguration is

both incompatible with Z-selectivity and expected to initiate

metathesis slowly as pyridine might not readily dissociate when

trans to a chloride. Surprisingly, even though this dissociation is

indeed associated with a barrier, 5a and 5b not only initiate more

rapidly than their 2-isopropoxybenzylidene congeners but also

are fairly long-lived (TONs up to 2200) and Z-selective, oﬀering

up to ca. 80% of the Z-isomer in prototypical metathesis

homocoupling reactions.

To uncover the causes behind the unexpected selectivity and

fairly rapid initiation, the initial pyridine dissociation and the

subsequent rearrangement into isomers expected to be Z-

selective were investigated using DFT. The calculations

conﬁrmed a barrier (25−27 kcal/mol) to pyridine dissociation,

reﬂecting the cost of generating an empty coordination site trans

to chloride in 14-electron complex M1. Next, the empty

coordination site in M1 may be taken up by the thiolate to give

much more stable M1-trans, which may bind allylbenzene to

form a π-complex (M1-trans-AB). The latter may subsequently

evolve via initiation metathesis to mediate productive self-

metathesis of allylbenzene. That this metathesis is Z-selective is

indicated already by the preferred orientation, away from the

bulky thiolate, of the allylbenzene benzyl group in M1-trans-AB.

The stereodirecting properties of the thiolate ligand have been

analyzed using the SambVca web application. The thus-

generated steric map of DFT-optimized oleﬁn complex M1-

trans-AB, with the substrate allylbenzene removed for clarity, is

shown in Figure 4 . The steric map illustrates, as expected, that

EXPERIMENTAL SECTION

Unless otherwise stated, reactions were performed under a dry argon

atmosphere by using Schlenk techniques or a glovebox. [RuCl (IMes)-

■

(

Py) (3-phenylindenylidene)] (3b) and potassium 2,4,6-triphenylben-

zene-thiolate were prepared according to their respective literature

7,30

procedures.

[RuCl (SIMes)(Py)(3-phenylindenylidene)] (4),

commercially available as Umicore M31, was obtained from Sigma-

Aldrich and used as received. Inhibitor-free THF, obtained from Sigma-

Aldrich, was dried over potassium hydride and ﬁltered over Celite prior

to use. Anhydrous n-pentane was purchased from Sigma-Aldrich and

used as received. DCM was dried using an MBraun solvent puriﬁcation

system (“Grubbs column”) before use. Allylbenzene, 1-octene, 4-

phenyl-1-butene, and pyridine were obtained from Sigma-Aldrich.

Allylbenzene and 4-phenyl-1-butene were degassed, dried overnight

Figure 4. Steric map of M1-trans-AB (cf. Scheme 3), obtained for a

model in which the atoms of the allylbenzene substrate have been

removed.

over CaH , ﬁltered over a pad of activated basic alumina (Brockmann

I), and stored overnight over activated Selexsorb CD. 1-Octene was

degassed and ﬁltered over a pad of activated basic alumina (Brockmann

I).

the thiolate ligand induces Z-selectivity by acting on one side of

the substrate cavity and forcing substituents to predominantly

occupy the other side of the cavity.

NMR spectra were recorded on Bruker Biospin AV500, AV600, and

To further analyze the steric eﬀects of the thiolate, the

50 Ascend spectrometers. Chemical shifts were reported relative to the

percentage of buried volume (%V_B_u_r) of the active species of

b (SM1-trans, Scheme 4) was compared to that of the

residual solvent peaks. HRMS ESI mass spectra were recorded by

means of an orthogonal electron spray ionization ion source (ESI)

interfaced to a JMS-T100LC AccuTOF mass spectrometer from JEOL

USA, Inc. (Peabody, MA). The ions were transported into the

orthogonal accelerating time-of-ﬂight (TOF) single-stage reﬂectron

mass analyzer by a high-frequency and high-voltage quadrupole ion

guide. Detection was achieved with a dual microchannel plate detector.

Elemental analyses were performed using an Elementar Vario EL III

analyzer.

corresponding dichloride, i.e., 4 without pyridine, again using

DFT-optimized geometries. With 95.0% for 5b and 86.4% for 4,

respectively, the thiolate is seen to add considerably to the steric

congestion. Moreover, only a small part of this diﬀerence

originates from the sulfur atom itself, i.e., from replacing chlorine

with sulfur. When only ruthenium and the atoms directly

^b^oⁿ^d^e^d^t^o^r^u^t^h^eⁿⁱ^u^m^w^e^r^e^c^oⁿ^sⁱ^d^e^r^e^d^,^%^VBur was only

marginally larger for 5b (67.7%) than for 4 (67.0%). 5a

experiences a steric congestion very similar to that of 5b, with a

Suitable crystals for diﬀraction experiments were immersed in

Paratone-N (Hampton Research) in a nylon loop. Data collection was

done on a Bruker AXS TXS rotating anode system with an APEXII Pt

Organometallics XXXX, XXX, XXX−XXX

Organometallics

The Supporting Information is available free of charge at

Article

CCD detector using graphite-monochromated Mo Kα radiation (λ =

.71073 Å). Data collection and data processing were done using

(s, 1.11H), 2.29 (s, 1.80H), 2.26 (s, 1.88H), 2.15 (s, 1,90H), 2.10 (s,

1.82H), 1.73 (s, 1.11H), 1.67 (s, 1.84H), 1.57 (s, 1.11H), 1.16 (s,

APEX2, SAINT, and SADABS version 2008/1 or TWINABS,

whereas structure solution and ﬁnal model reﬁnement were done using

1.11H). C{ H} NMR (213.77 MHz, CDCl ): δ = 271.16, 215.07,

211.21, 159.17, 156.85, 154.61, 151.77, 149.93, 147.98, 147.72, 146.60,

145.50, 145.14, 143.00, 142.91, 142.60, 142.52, 141.71, 141.57, 141.39,

141.19, 140.36, 140.07, 139.91, 139.75, 139.55, 139.07, 138.79, 128.67,

138.19, 138.07, 137.97, 137.87, 137.63, 137.59, 137.56, 137.36, 137.14,

137.02, 136.86, 136.76, 136.53, 136.30, 136.02, 135.78, 135.01, 134.83,

134.61, 134.44, 133.85, 131.97, 130.99, 130.65, 130.13, 130.08, 130.04,

129.94, 129.79, 129.68, 129.54, 129.44, 129.26, 129.21, 129.09, 129.02,

128.98, 128.88, 128.86, 128.81, 128.77, 128.47, 128.37, 128.23, 128.14,

128.11, 128.07, 128.04, 127.98, 127.95, 127.84, 127.74, 127.66, 127.60,

127.53 127.50, 127.30, 127.18, 127.03, 127.01, 126.96, 126.91, 127.71,

126.67, 126.57, 126.47, 126.40, 126.36, 125.99, 125.72, 125.31, 124.61,

124.31, 124.05, 123.19, 122.47, 121.56, 121.29, 117.44, 116.62, 100.37,

99.95, 94.67, 91.81, 89.88, 87.41, 79.40, 62.50, 54.35, 53.68, 53.56,

53.11, 52.67, 51.31, 50.88, 34.27, 31.74, 22.49, 21.21, 21.12, 21.05,

20.98, 20.96, 20.90, 20.80, 20.23, 20.04, 19.82, 19.47, 19.15, 18.83,

18.45, 18.25, 17.82, 14.21. Anal. Calcd (%) for C H ClN RuS: C

SHELXS₆version 2013/1 or SHELXT version 2014/4 and

SHELXL version 2014/7.

[

RuCl(IMes)(-S-2,4,6-Ph-C H )(Py)(3-phenylindenylidene)]

6 2

(

5a). Under an inert atmosphere, [RuCl (IMes)(Py) (3-phenyl-

2 2

indenylidene)] (3b, 100.0 mg, 0.121 mmol) and potassium 2,4,6-

triphenylbenzenethiolate (47.6 mg, 0.126 mmol) were suspended in 5

mL of THF. After 2 h of stirring at room temperature, the mixture was

ﬁltered through a short pad of Celite. Subsequently, the ﬁltrate was

concentrated to about 2 mL in vacuo. Slow addition of 3 mL of n-

pentane yielded after 1 day at −32 °C dark-red microcrystals (92 mg,

yield 72%) of 5a. Crystals suitable for X-ray diﬀraction analysis were

grown by slow diﬀusion of pentane into a concentrate solution of 5a in

dichloromethane at −32 °C. H NMR (850.13 MHz, CDCl ): δ = 8.62

(

br m, 0.401H), 8.49 (d, J = 5.5 Hz, 0.77H), 8.13 (br d, J = 7.1

HH HH

Hz, 1.57H), 7.90 (d, J = 2.0 Hz 0.19H), 7.72−7.27 (m, 14.38H),

65 58

.23−7.18 (m, 2,13H), 7.11 (d, J = 2.0 Hz, 0.19H), 7.08−6.99 (m,

74.37, H 5.57, N 4.00. Found: C 74.35, H 5.29, N 3.96. HRMS (ESI )

102

.53H), 6.95 (td, J = 7.1 and J = 1.0 Hz, 0.79H), 6.87−6.77 (m,

found (calcd): m/z 1072.3026 (1072.2981) [C H ClN₃RuS

65 58

.55H), 6.58−651 (m, 0.51H), 6.44−6.39 (m, 1.60H), 6.29 (nr m,

+Na] .

.70H), 6.17 (s, 1.58H), 6.00−5.93 (m, 2.70H), 5.86 (t, J = 5.9 Hz,

Catalytic Tests, General Procedure. In a glovebox, a 4 mL vial

equipped with a magnetic stirring bar and a screw cap was charged with

the substrate, 2 mg of hexamethylbenzene (internal standard), and the

catalyst. The vial was closed, and the reaction mixture was stirred at

room temperature. The reaction was quenched with an excess of ethyl

vinyl ether (EVE). Determination of conversions, O/I ratio, and Z-

.20H), 5.77 (d, J = 7.8 Hz, 0.20H), 5.25 (br d, J = 5.7 Hz,

.20H), 5.22 (br d, J = 7.3 Hz, 0.79H), 5.19 (br t, J = 5.7 Hz,

.20H), 5.11 (s, 0.19H), 4.17 (t, J = 6.2 Hz, 0.20H), 2.38 (s, 2.33H),

.34 (s, 0.62H), 2.31 (s, 2.33H), 2.26 (s, 0.62H), 2.21 (s, 2.32H), 2.16

(

s, 0.62H), 2.10 (s, 0.61H), 2.07 (s, 0.62H), 1.65 (s, 2.31H), 1.64 (s,

.63H), 1.58 (s, 2.31H), 1.02 (s, 2.33H). ¹³C{ H} NMR (150.91 MHz,

29,36

selectivities were done according to literature procedures.

CDCl ): δ = 270.76, 183.30, 180.29, 159.11, 154.02, 150.02, 149.75,

For catalyst loading of 0.5 mol %, 0.50 mmol of substrate and 0.0025

mmol of catalyst were used.

For catalyst loading of 0.1 mol %, 0.76 mmol of substrate and

0.00076 mmol of catalyst were used.

For catalyst loading of 0.01 mol %, 7.6 mmol of substrate and

0.00076 mmol of catalyst were used.

In the catalytic tests with pyridine, the small amount of pyridine

(0.02 mg) was added by preparing a stock solution of pyridine in the

substrate. The stock solution was obtained by dilution of another stock

solution prepared by dissolving 1 mg of pyridine into 2.5 mmol of

substrate [e.g., 10-fold dilution (0.1 equiv), 5-fold dilution (0.2 equiv),

no dilution (1.0 equiv), and 50-fold dilution (Table 1, entry 4)].

47.97, 147.61, 146.70, 145.50, 145.14, 142.85, 142.81, 142.69, 141.53,

41.13, 140.49, 140.09, 139.64, 139.58, 139.14, 139.04, 138.76, 138.58,

38.03, 137.33, 137.25, 137.09, 137.04, 136.79, 136.67, 135.99, 135.95,

35.90, 135.46, 134.61, 134.54, 134.09, 133.76, 133.69, 132.97, 131.96,

30.74, 130.34, 130.00, 129.87, 129.73, 129.50, 129.43, 129.26, 129.21,

29.14, 129.09, 128.67, 128.43, 128.36, 128.22, 128.06, 127.86, 127.72,

27.64, 127.64, 127.45, 127.27, 127.14, 126.91, 126.82, 126.68, 126.55,

26.46, 126.35, 126.27, 125.88, 125.77, 125.58, 125.35, 125.31, 125.11,

24.38, 124.33, 123.98, 123.84, 123.40, 122.48, 121.56, 117.38, 99.73,

3.89, 90.89, 89.46, 87.79, 78.93, 63.75, 21.20, 21.13, 20.96, 20.90,

0.79, 20.15, 20.01, 19.84, 19.57, 18.59, 18.42, 18.23. Anal. Calcd (%)

for C H ClN RuS: C 74.51, H 5.39, N 4.01. Found: C 74.42, H 5.34,

N 4.00. HRMS (ESI ) found (calcd): m/z 933.28172 (933.28164)

C H N RuS] .

60 51 2

ASSOCIATED CONTENT

sı Supporting Information

102

■

[

RuCl(SIMes)(-S-2,4,6-Ph-C H )(Py)(3-phenylindenylidene)]

6 2

(

5b). Under an inert atmosphere, [RuCl (SIMes)(Py)(3-phenyl-

https://pubs.acs.org/doi/10.1021/acs.organomet.9b00641.

indenylidene)] (4, 99.7 mg, 0.133 mmol) and potassium 2,4,6-

triphenylbenzenethiolate (52.3 mg, 0.139 mmol) where suspended in 5

mL of THF. After 30 min of stirring at room temperature, the mixture

was ﬁltered over a glass ﬁber ﬁlter. The residue on the ﬁlter was

dissolved in DCM. Subsequent addition of n-pentane yielded a

precipitate. The precipitate was ﬁrst washed with a small amount of n-

pentane, then dried and ﬁnally redissolved in DCM. Slow addition of 3

mL of n-pentane yielded after some days at −32 °C dark-red

microcrystals (28.4 mg, yield 20%) of 5b. H NMR (850.13 MHz,

CDCl ): δ = 8.61 (br, 1.34H), 8.28 (br, 0.37H), 8.12 (br d, J = 6.7

Hz, 0.73H), 7.86 (d, J = 1.8 Hz, 0.62H), 7.75 (br d, J = 7.7 Hz,

.64H), 7.71−7.66 (m, 1.88 H), 7.64−7.58 (m, 1.65H), 7.56−7.26 (m,

0.23H), 7.23 (br t, J = 7.7 Hz, 0.78H), 7.19 (br t, J = 7.3 Hz,

.34H), 7.06 (br s, 0.57H), 7.04 (br s, 0.31H), 7.02−6.99 (m, 0.96H),

.98 (br s, 0.68H), 6.92 (t d, J = 7.2, 0.7 Hz, 0.33H), 6.87 (br t, J

Spectra ( H, ¹³C NMR, and HRMS) and X-ray reﬁne-

ment data for new compounds, computational details,

calculated electronic and free energies, and additional

computational results (PDF)

Computed molecular Cartesian coordinates in a format

convenient for visualization (XYZ)

Accession Codes

CCDC 1942925 and 1942926 contain the supplementary

crystallographic data for this paper. These data can be obtained

free of charge via www.ccdc.cam.ac.uk/data_request/cif , or by

emailing data_request@ccdc.cam.ac.uk , or by contacting The

Cambridge Crystallographic Data Centre, 12 Union Road,

Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

3 3

5.6 Hz, 0.32H), 6.82−6.75 (m, 1.48H), 6.54 (br, 1.11H), 6.50 (br t,

= 6.1 Hz, 0.37H), 6.41 (t d, J = 7.1, 0.7 Hz, 0.37H), 6.36 (d,

J_H_H= 6.0 Hz, 0.64H), 6.27 (br, 0.77H), 6.24 (br s, 0.66H), 6.19 (br s,

.38H), 6.06 (s, 0.62H), 6.00 (d, J = 7.0 Hz, 0.74H), 5.94 (br t, J

6.9 Hz, 0.37H), 5.91 (t, J = 6.0 Hz, 0.64H), 5.85 (s, 0.37H), 5.74

d, J = 8.1 Hz, 0.73H), 5.24 (d, J = 7.4 Hz, 0.37H), 5.21 (d, J

5.4 Hz, 0.64H), 5.10 (t, J = 5.8 Hz, 0.64H), 4.11 (t, J = 6.1 Hz,

HH HH

.63H), 4.04−3.90 (m, 1.82H), 3.89−3.79 (m, 0.62H), 3.79−3.70 (m,

.74H), 3.65−3.53 (m, 0.96H), 2.65 (s, 1.09H), 2.52 (m, 3.76H), 2.44

(

HH HH

AUTHOR INFORMATION

■

Corresponding Authors

HH HH HH

Giovanni Occhipinti − Department of Chemistry, University of

Bergen N-5007 Bergen, Norway; orcid.org/0000-0002-

7279-6322 ; Email: Giovanni.Occhipinti@uib.no

Organometallics XXXX, XXX, XXX−XXX

Organometallics

N-5007 Bergen, Norway; orcid.org/0000-0003-2444-3220 ;

Article

Vidar R. Jensen − Department of Chemistry, University of Bergen

Email: Vidar.Jensen@uib.no

(8) Schwab, P.; Grubbs, R. H.; Ziller, J. W. Synthesis and Applications

of RuCl (CHR ′)(PR ) : The Influence of the Alkylidene Moiety on

3 2

Metathesis Activity. J. Am. Chem. Soc. 1996, 118, 100−110.

9) Scholl, M.; Ding, S.; Lee, C. W.; Grubbs, R. H. Synthesis and

Activity of a New Generation of Ruthenium-Based Olefin Metathesis

Authors

Wietse Smit − Department of Chemistry, University of Bergen N-

007 Bergen, Norway; orcid.org/0000-0002-0142-6025

Jonas B. Ekeli − Department of Chemistry, University of Bergen

N-5007 Bergen, Norway; orcid.org/0000-0002-6137-6322

Catalysts Coordinated with 1,3-Dimesityl-4,5-dihydroimidazol-2-yli-

dene Ligands. Org. Lett. 1999, 1, 953−956.

(11) Jafarpour, L.; Schanz, H.-J.; Stevens, E. D.; Nolan, S. P.

10) Dragutan, V.; Dragutan, I.; Verpoort, F. Ruthenium Indenylidene

Complexes. Platinum Met. Rev. 2005, 49, 33−40.

Bartosz Wozn

Bergen N-5007 Bergen, Norway; orcid.org/0000-0002-

460-3344

Karl W. Tornroos − Department of Chemistry, University of

Bergen N-5007 Bergen, Norway; orcid.org/0000-0001-

140-5915

iak − Department of Chemistry, University of

Indenylidene-Imidazolylidene Complexes of Ruthenium as Ring-

Closing Metathesis Catalysts. Organometallics 1999, 18, 5416 −5419.

(12) Harlow, K. J.; Hill, A. F.; Wilton-Ely, J. D. E. T. The First Co-

Ordinatively Unsaturated Group 8 Allenylidene Complexes: Insights

into Grubbs ’ vs . Dixneuf-Furstner Olefin Metathesis Catalysts. J. Chem.

Soc., Dalton Trans. 1999, 285−292.

13) Furstner, A.; Guth, O.; Duffels, A.; Seidel, G.; Liebl, M.; Gabor,

(

Complete contact information is available at:

https://pubs.acs.org/10.1021/acs.organomet.9b00641

B.; Mynott, R. Indenylidene Complexes of Ruthenium: Optimized

Synthesis, Structure Elucidation, and Performance as Catalysts for

Olefin Metathesis Application to the Synthesis of the ADE-Ring

System of Nakadomarin A. Chem. - Eur. J. 2001, 7, 4811−4820.

Notes

The authors declare no competing ﬁnancial interest.

(14) Furstner, A.; Liebl, M.; Hill, A. F.; Wilton-Ely, J. D. E. T.

Coordinatively Unsaturated Ruthenium Allenylidene Complexes:

Highly Effective, Well Defined Catalysts for the Ring-Closure

Metathesis of α ,ω -Dienes and Dienynes. Chem. Commun. 1999,

ACKNOWLEDGMENTS

■

We gratefully acknowledge ﬁnancial support from the Research

Council of Norway (RCN) via the GASSMAKS (grant no.

08335), FORNY2020 (239288), and FRIPRO (262370)

programs, as well as via the Norwegian NMR Platform, NNP

226244). The RCN is also thanked for CPU (NN2506K) and

storage resources (NS2506K). Bjarte Holmelid is thanked for

assistance with the HRMS ESI ) analyses and Inger Johanne

Fjellanger for assistance with the elemental analysis. W.S. is

grateful to the University of Bergen for a Ph.D. scholarship.

15) Furstner, A.; Grabowski, J.; Lehmann, C. W. Total Synthesis and

01−602.

Structural Refinement of the Cyclic Tripyrrole Pigment Non-

ylprodigiosin. J. Org. Chem. 1999, 64, 8275−8280.

(

(16) Boeda, F.; Clavier, H.; Nolan, S. P. Ruthenium-Indenylidene

Complexes: Powerful Tools for Metathesis Transformations. Chem.

Commun. 2008, 2726−2740.

(17) Furstner, A.; Thiel, O. R.; Ackermann, L.; Schanz, H.-J.; Nolan, S.

P. Ruthenium Carbene Complexes with N, N ′-Bis(Mesityl)Imidazol-2-

Ylidene Ligands: RCM Catalysts of Extended Scope. J. Org. Chem.

18) Schmidt, B.; Wildemann, H. Single and Double Ring Closing

000, 65, 2204−2207.

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