Ruthenium catalysts bearing a benzimidazolylidene ligand for the metathetical ring-closure of tetrasubstituted cycloolefins

Page 1 of 14

Dalton Transactions

View Article Online

DOI: 10.1039/C5DT00433K

Dalton Transactions

RSCPublishing

ARTICLE

Ruthenium catalysts bearing a benzimidazol-

ylidene ligand for the metathetical ring-closure

of tetrasubstituted cycloolefins

Cite this: DOI: 10.1039/x0xx00000x

a

b

a

*a

Yannick Borguet, Guillermo Zaragoza, Albert Demonceau and Lionel Delaude

Received 00th January 2012,

Accepted 00th January 2012

Deprotonation of 1,3ꢀdi(2ꢀtolyl)benzimidazolium tetrafluoroborate with a strong base afforded

,3ꢀdi(2ꢀtolyl)benzimidazolꢀ2ꢀylidene (BTol), which dimerized progressively into the

corresponding dibenzotetraazafulvalene. The complexes [RhCl(COD)(BTol)] (COD is

,5ꢀcyclooctadiene) and cisꢀ[RhCl(CO) (BTol)] were synthesized to probe the steric and

DOI: 10.1039/x0xx00000x

1

www.rsc.org/

1

2

electronic parameters of BTol. Comparison of the percentage of buried volume (%V_B_u_r) and of

the Tolman electronic parameter (TEP) of BTol with those determined previously for

1

,3ꢀdimesitylbenzimidazolꢀ2ꢀylidene (BMes) revealed that the two Nꢀheterocyclic carbenes

displayed similar electron donicities, yet the 2ꢀtolyl substituents took a slightly greater share of

the rhodium coordination sphere than the mesityl groups, due to a more pronounced tilt. The

anti,anti conformation adopted by BTol in the molecular structure of [RhCl(COD)(BTol)]

ensured nonetheless a remarkably unhindered access to the metal center, as evidenced by steric

maps. Secondꢀgeneration ruthenium–benzylidene and isopropoxybenzylidene complexes

featuring the BTol ligand were obtained via phosphine exchange from the first generation

Grubbs and Hoveyda–Grubbs catalysts, respectively. The atropisomerism of the 2ꢀtolyl

substituents within [RuCl (=CHPh)(PCy )(BTol)] was investigated by using variable

2

3

temperature NMR spectroscopy, and the molecular structures of all four possible rotamers of

i

[

RuCl (=CHꢀoꢀO PrC H )(BTol)] were determined by Xꢀray crystallography. Both complexes

2 6 4

were highly active at promoting the ringꢀclosing metathesis (RCM) of model

α,ωꢀdienes. The

replacement of BMes with BTol was particularly beneficial to achieve the ringꢀclosure of

tetrasubstituted cycloalkenes. More specifically, the stable isopropoxybenzylidene chelate

enabled an almost quantitative RCM of two challenging substrates, viz., diethyl 2,2ꢀbis(2ꢀ

methylallyl)malonate and N,Nꢀbis(2ꢀmethylallyl)tosylamide, within a few hours at 60 °C.

Introduction

Over the past two decades, olefin metathesis has become one of

the most powerful tools for the formation of C=C double bonds

N

Cl

1,2

Ru

O

in polymer chemistry and in organic synthesis. The rational

design of wellꢀdefined molybdenum and ruthenium alkylidene

Ph

Cl ^P^C^y3

Cl

i-Pr

3

4

complexes initiated by Schrock and Grubbs in the early 1990s

5

was a crucial milestone in this organometallic success story.

1

2

Another major leap forward was achieved at the turn of the

Chart 1 Second-generation Grubbs and Hoveyda–Grubbs catalysts.

millennium with the introduction of

Nꢀheterocyclic carbene

6

(

NHC) ligands on ruthenium complexes. As a matter of fact,

Very recently, tireless catalytic engineering has led to

7

the soꢀcalled secondꢀgeneration Grubbs (

1

)

and Hoveyda–

significant advances to further improve the rate of olefin

8

Grubbs (

2

) catalysts stand nowadays as references owing to

10

metathesis at low catalyst loading or to achieve high cis or

selectivities.

Z

their high catalytic activity and increased stability compared to

11,12

Despite these spectacular breakthroughs, there

9

their phosphineꢀbased, firstꢀgeneration analogues (Chart 1).

are still many hurdles to overcome in order to make olefin

metathesis a truly universal catalytic process. In particular, the

formation of tetrasubstituted cycloolefins via ringꢀclosing

metathesis (RCM) remains a challenging task for most secondꢀ

This journal is © The Royal Society of Chemistry 2013

J. Name., 2013, 00, 1-3 | 1

Dalton Transactions

Page 2 of 14

View Article Online

ARTICLE

DOI: 10.1039/C5DT00433K

Journal Name

1

3

Chart 2 Second-generation ruthenium–alkylidene catalysts for the RCM of

sterically hindered substrates.

generation catalysts such as

1

or

2

.

This is mainly due to the

difficulty of coordinating a sterically hindered substrate to a

ruthenium active species bearing a bulky NHC ligand such as

,3ꢀdimesitylimidazolinꢀ2ꢀylidene (known as SIMes).

A key observation toward the development of efficient catalyst

precursors for the RCM of tetrasubstituted cycloolefins was

made by Grubbs and coꢀworkers in 2006. During the

desymmetrization of a triene in the presence of ruthenium

As part of our ongoing studies of benzimidazoleꢀbased NHCs,

we recently assessed the ligand properties of 1,3ꢀdimesitylꢀ

benzimidazolꢀ2ꢀylidene (known as BMes) in rutheniumꢀ

1

2

0

1

4

catalyzed olefin metathesis reactions. In the benchmark

cyclization of diethyl 2,2ꢀdiallylmalonate, the [RuCl (PCy )ꢀ

2

3

(

BMes)(=CHPh)] complex

Grubbs secondꢀgeneration catalyst

catalytic efficiency when model diꢀ and trisubstituted

6

performed slightly better than the

complex

unexpected formation of

byproduct took place. It was explained by the presence of only

one orthoꢀsubstituent on each ꢀaryl group of the carbene

3

bearing a chiral NHC ligand (Chart 2), the

1

but gradually lost its

a

tetrasubstituted cycloalkene

α,ωꢀ

dienes were subjected to RCM. These results prompted us to

launch further investigations on the 1,3ꢀdi(2ꢀtolyl)benzimidꢀ

azolꢀ2ꢀylidene ligand (nicknamed BTol). We reasoned that the

synergy between small aryl groups on the nitrogen atoms and a

bulky fused aromatic ring on the backbone carbon atoms of the

central imidazole core should provide an ideal framework for

achieving high efficiencies in the RCM of sterically hindered

substrates (Chart 3). In this contribution, we first discuss the

preparation of suitable precursors for the new BTol ligand and

we assess its steric and electronic properties using rhodium

complexes. Then, we report on the synthesis of two secondꢀ

generation ruthenium–alkylidene complexes derived thereof

and we probe their catalytic activity in the ringꢀclosing

metathesis of various benchmark substrates.

N

ligand, which reduced the steric pressure around the metal

center. Building on this hypothesis, Grubbs et al. designed

several catalysts with low steric demand, which proved very

active for the RCM of challenging tetrasubstituted olefins, but

were also quite unstable. This lack of stability was attributed

to the free rotation of monoꢀortho or unsubstituted ꢀaryl

moieties around the C–N exocyclic bonds of the NHC ligand,

which brings C–H aryl bonds close to the ruthenium center and

promotes their activation, ultimately leading to decomposition

1

5

N

1

6

processes. This assumption was later confirmed by experiꢀ

1

7

18

mental results and theoretical calculations. Subsequent

research efforts aimed at optimizing the balance between

activity and stability by modulating the various C and N

substituents of the NHC ancillary ligand. Catalysts developed

along these lines include the chiral ruthenium–benzylidene

Maintain

conformational

freedom restrictions

1

9

complex

4

reported by Grisi et al. and the chelated

investigated by Grubbs and

isopropoxybenzylidene complex

5

N

1

5b

coꢀworkers (Chart 2). Both compounds were highly efficient

at promoting the RCM of tetrasubstituted olefins under mild

reaction conditions. However, their synthesis required multiple,

lowꢀyielding steps and, in some instances, the use of not readily

available optically active starting materials.

Cl

Ru

[Ru]

Reduce

steric bulk

Ph

Cl PCy₃

6

Chart 3 Tuning of benzimidazole-based ruthenium catalyst

sterically hindered substrates.

6

for the RCM of

Small aryl groups

allow the

coordination of

large substrates

Bulky backbone

substituents

prevent aryl group

rotation and C–H

activation

Results and discussion

N

Synthesis of BTol ligand precursors

H

[

Ru]

H

Because NHCs are most commonly obtained by deprotonation

of the corresponding azolium salts with a strong base, we

began our investigations with the preparation of 1,3ꢀdi(2ꢀtolyl)ꢀ

2

1

Ph

i-Pr

benzimidazolium tetrafluoroborate

7. Unlike 1,3ꢀdimesitylꢀ

N

benzimidazolium tetrafluoroborate, whose synthesis involved

the unexpected formation of a dihydrophenazine intermediate,

Cl

i-Pr

22

Ru

Ph

3

compound

7 was isolated in high yield following the

Cl PCy

straightforward amination/cyclization path pioneered by

3

23,24

Diver.

This twoꢀstep procedure implied the Buchwald–

ꢀtoluidine,

N

Hartwig amination of dibromobenzene with

o

Cl

followed by a classical formylative cyclization with triethyl

orthoformate (Scheme 1). For the sake of convenience, the

resulting hygroscopic benzimidazolium chloride was then

converted into the corresponding tetrafluoroborate by anion

Ru

O

Ph

Cl PCy₃

Cl

i-Pr

4

5

25

exchange with aqueous tetrafluoroboric acid.

2

| J. Name., 2012, 00, 1-3

This journal is © The Royal Society of Chemistry 2012

Page 3 of 14

Journal Name

Dalton Transactions

View Article Online

ART33ICLE

DOI: 10.1039/C5DT004 K

around a symmetry element. Within the compound of interest,

the C1–C1B distance of 1.347(4) Å between the two monoꢀ

meric units was similar to those observed in other dibenzotetraꢀ

Tol

NH₂⁺Cl^–

Tol BF₄^–

Br

N

H

N

a, b

c, d

2

6b,26d,30

azafulvalenes (1.33–1.35 Å).

A strong Nꢀpyramidaliꢀ

NH

Tol

Tol = 2-methylphenyl (2-tolyl)

Scheme 1 Synthesis of 1,3-di(2-tolyl)benzimidazolium tetrafluoroborate (

Reaction conditions: (a) o-toluidine, Pd(OAc) , P(t-Bu) , NaO-t-Bu, PhCH , 110 °C,

overnight; (b) HCl, H O; (c) HC(OEt) , HCl, reflux, overnight; (d) HBF , H O.

⁺Cl^–

2

zation (average C–N–C angle: 107.5°) and a significant twist

Tol

around the central N C=CN₂double bond (average torsion

2

7 (78%)

angle: 14.4°) were noticed. These two distortions presumably

minimize the steric repulsions between the 2ꢀtolyl substituents.

7).

2

3

2

3

4

2

The deprotonation of salt

bis(trimethylsilyl)amide in toluene at room temperature. Within

h, the initially pale yellow solution became progressively

bright orange. This change of color was attributed to the

formation of the highly conjugated dibenzotetraazafulvalene

7 was carried out with potassium

2

8

in solution (Scheme 2). This observation did not come as a

surprise. The dimerization of benzimidazolꢀ2ꢀylidenes bearing

small substituents on their nitrogen atoms is far from

2

6

unprecedented in the literature. Indeed, these benzannulated

carbenes lose less of their aromatic stabilization than imidazolꢀ

2

ꢀylidenes when they dimerize. Hence, they exist as dimers at

ambient temperature, unless bulky nitrogen substituents shift

2

7

the equilibrium toward the monomeric NHCs.

Tol BF₄^–

N

Tol

N

Tol Tol

N

0.5

Fig.

1 ORTEP representation of dibenzotetraazafulvalene 8 with thermal

N

ellipsoids drawn at the 50% probability level. Hydrogen atoms and solvent

molecules were removed for clarity. Selected bond length (Å) and angles (°): C1–

C1B 1.347(4), N1–C1 1.430(4), N2–C1 1.428(4), N1B–C1B 1.430(4), N2B–C1B

Tol

Tol Tol

7

Tol = 2-methylphenyl (2-tolyl)

Scheme 2 Synthesis of the BTol carbene dimer (

KN(SiMe , PhCH , room temp., 2 h.

8 (90%)

1.435(4); N1–C1–N2 107.4(2), N1B–C1B–N2B 107.6(2), C1–N1–C2 107.0(2), C1B–

N1B–C2B 107.1(2), C1–N2–C7 107.4(2), C1B–N2B–C7B 106.9(2).

8

). Reaction conditions:

3

)

2

3

Evaluation of the steric and electronic properties of BTol

1

At room temperature, the H NMR spectrum of the (BTol)

dimer

2

Among the various complexes that were used to determine the

8 featured only poorly resolved signals, a likely

3

1

steric and electronic properties of NHC ligands, rhodium

derivatives with the generic formulas [RhCl(COD)(NHC)]

indication that the 2ꢀtolyl substituents rotated slowly on the

NMR timescale. Warming the sample up to 60 °C led to sharper

peaks with discernable coupling patterns between the aromatic

protons of the fused benzene ring. H NMR spectroscopy also

revealed the complete disappearance of the singlet originally

present at 9.33 ppm in compound

proton of the azolium salt starting material. It is noteworthy that

the C NMR spectrum recorded at room temperature displayed

no resonance within the expected range of chemical shifts for

the carbenic carbon of a benzimidazolꢀ2ꢀylidene (ca. 220–230

(

COD is 1,5ꢀcyclooctadiene) and cisꢀ[RhCl(CO) (NHC)] are

2

probably the most convenient probes, owing to their

1

3

2

straightforward preparation, high stability, and low toxicity.

Hence, we first synthesized [RhCl(COD)(BTol)] ( ) by

deprotonating benzimidazolium salt with potassium tert

butoxide in the presence of the [RhCl(COD)] dimer (Scheme

9

7, assigned to the acidic

7

ꢀ

1

3

2

3

). The desired product was isolated as a microcrystalline

yellow powder in 82% yield after purification by column

chromatography.

2

8

ppm). Thus, there was no evidence for the intervention of a

“

Wanzlick equilibrium” between the monomeric free carbene

26b,29

13

and its dimer,

although the poor sensitivity of C NMR for

a carbenic carbon devoid of any hydrogen could also account

for the lack of signal.

Bright orange crystals of the (BTol) dimer

8 suitable for Xꢀray

2

diffraction analysis were grown by slow evaporation of a

saturated solution in toluene under argon. Determination of

their molecular structure revealed that the asymmetric unit

contained 2.5 molecules of solvent, 0.5 of them distorted

This journal is © The Royal Society of Chemistry 2012

J. Name., 2012, 00, 1-3 | 3

Dalton Transactions

Page 4 of 14

View Article Online

ARTICLE

DOI: 10.1039/C T0043

Jou5Drnal Na3Kme

Tol BF₄^–

Tol

N

H

N

a

b

N

OC

N

Tol

Rh

Tol

Rh

Cl

Rh

Tol

OC

Cl

7

9 (82%)

10 (71%)

) and cis-[RhCl(CO) (BTol)] (10).

Scheme 3 Synthesis of [RhCl(COD)(BTol)] (

Reaction conditions: (a) KO-t-Bu, [RhCl(COD)]

9

2

9

a

9b

2

, THF, room temp., overnight; (b)

(

R

,S

syn,syn)

)

(S

a a

,R )

a

2 2

CO, CH Cl , room temp., 15 min.

(

(anti,anti)

1

H NMR analysis of [RhCl(COD)(BTol)] (

C showed the presence of three different sets of signals for the

ꢀmethyl groups of the tolyl substituents (Fig. 2). This is a

likely consequence of a restricted rotation around the Rh–NHC

9) in CD Cl at 25

2 2

°

o

N

3

and N–Ar bonds. Indeed, there are four possible rotamers for

complex , two of them being enantiomers (Chart 4). Based on

Rh

9

Cl

the Xꢀray crystal structure discussed below and the relative

integrals, we assigned the strongest resonance at 2.02 ppm to

the least sterically hindered anti,antiꢀrotamer 9b and the

weakest resonance at 2.29 ppm to the related symmetrical

9

c

9d

(S ,S )

a a

(anti,syn)

(R ,R )

a

(

syn,anti)

syn,synꢀrotamer 9a. The two lines of similar intensities at 2.11 Chart 4 Possible rotamers of the [RhCl(COD)(BTol)] complex (

a a

9) (R and S

absolute configurations refer to the axial chirality of the two exocyclic C–N

bonds, syn and anti descriptors refer to the relative orientations of each methyl

group with respect to the chlorido ligand).

and 2.25 ppm arose from the unsymmetrical syn,anti and

anti,syn pair of enantiomers 9c,d and the ratios 9a 9c,d 9b were

:

1

3

1

of the order of 5:39:56. In the C{ H} NMR spectrum of

1

complex 11, only two doublets at 197.9 ( ^JRh–C = 51.1 Hz) and

Bright yellow crystals of [RhCl(COD)(BTol)] (9) suitable for

1

98.4 ppm ( ^JRh–C = 50.6 Hz) featured a chemical shift and a

Xꢀray diffraction analysis were grown by slow evaporation of a

concentrated solution in dichloromethane. Only the least

sterically hindered anti,antiꢀrotamer 9b was observed in the

solidꢀstate structure, together with one molecule of coꢀ

crystallized solvent (Fig. 3). As expected, the ligands adopted a

squareꢀplanar disposition around the metal center, with the

NCN plane of the carbene almost perpendicular to the

coordination plane of rhodium. Altogether, the various bond

lengths and angles were similar to those reported previously for

multiplicity compatible with a carbenic carbon coordinated to a

rhodium center. They were assigned to the major rotamers

1

observed on H NMR spectroscopy. The signal due to 9a was

not detected, probably because of its low intensity or an

accidental overlap.

20,34

other complexes of the same family.

Yet, the tilt angle of

the two ꢀtolyl rings in compound 9b (average value 20°) was

o

significantly more pronounced than the one recorded for

mesityl groups in the analogous [RhCl(COD)(BMes)] complex

2

0

(

ca. 5°) (Fig. 4). Translated to ruthenium, this should provide

an easier access to the metal center, which would be beneficial

3

5

to the RCM of tetrasubstituted cycloolefins.

1

Fig. 2 H NMR resonances observed for the o-methyl groups of [RhCl(COD)(BTol)]

(

rotamers).

9

) in CD

2 2

Cl at 298 K (see Chart 4 for the structures of the four possible

4

| J. Name., 2012, 00, 1-3

This journal is © The Royal Society of Chemistry 2012

Page 5 of 14

Journal Name

Dalton Transactions

View Article Online

ART33ICLE

DOI: 10.1039/C5DT004 K

groups are almost perpendicular to the benzimidazole ring and

therefore more distant from the metal center (cf. Fig. 4).

Steric maps generated using a modified version of the Samb

program,

V

ca

courtesy of Prof. L. Cavallo, provided a more

accurate topology of the BMes and BTol ligands in

RhCl(COD)(NHC)] complexes than the %V_B_u_rdescriptor. To

3

5,38

[

build the contour plots depicted in Figure 5, the rhodium atom

was placed at the origin of a Cartesian coordinate system with

the

z

axis corresponding to the Rh–NHC bond and the

axis. Atom

wingspan of the NHC aligned along the

x

coordinates were extracted from the molecular structures

represented in Figures 3 and 4. Positive values of the isocontour

lines refer to the top half coordination sphere of the metal

center, while the bulk of the NHC ligand resides in the bottom

half. Using these conventions, the northern and southern poles

of the map computed for the BMes ligand are both strongly

shielded by the mesityl groups. Contrastingly, access to the

metal via the southern pole is remarkably unhindered with the

Fig. 3 ORTEP representation of [RhCl(COD)(BTol)] (9b) with thermal ellipsoids anti,anti conformation adopted by BTol in the solid state

drawn at the 50% probability level. Hydrogen atoms and a solvent molecule

structure of [RhCl(COD)(BTol)] (9b). Of course, this analysis

were removed for clarity. Selected bond length (Å) and angles (°): Rh1–C1

does not take into account the chlorido and cyclooctadiene

2.003(2), Rh1–Cl1 2.3989(8), Rh1–C22 2.101(3), Rh1–C23 2.112(2), Rh1–C26

2

.185(2), Rh1–C27 2.220(2), C1–Rh1–Cl1 90.32(7), N1–C1–N2 105.6(2), C1–N1– ligands, which were removed for the calculations.

C8–C9 111.9(3), C1–N2–C15–C16 −107.5(3).

Fig. 5 %VBur maps of [RhCl(COD)(BMes)] (left) and [RhCl(COD)(BTol)] (9b) (right)

with the color scale used to display the isocontour levels (in Å).

Bubbling carbon monoxide into a dichloromethane solution of

[

RhCl(COD)(BTol)] (

9) for 15 min at room temperature

4

induced the displacement of the

η

ꢀdiene ligand and afforded

the cisꢀdicarbonyl complex 10 in 71% yield (Scheme 3). This

Fig. 4 Superposition of the molecular structures of [RhCl(COD)(BMes)] (blue) and

[

^p^r^o^d^u^c^t^w^a^s^dⁱ^s^s^o^l^v^e^d^a^g^aⁱⁿⁱⁿ^C^H^C^l2 to record its IR

RhCl(COD)(BTol)] (9b) (yellow).

2

spectrum between NaCl plates. The Tolman electronic

parameter (TEP) of BTol was then computed from the average

In order to quantify the steric demand of the BTol ligand, we

have extracted its percentage of buried volume (%V_B_u_r) from

the XRD structure of complex 9b. This parameter defined by

Cavallo and Nolan gives a measure of the space occupied by a

stretching vibration wavenumber of the carbonyl ligands (^νCO

=

–

1

2

039.5 cm ) using the linear regression proposed by Dröge and

Glorius to correlate data obtained from rhodium complexes

3

9

3

6

with the standard nickelꢀbased TEP scale. This led to a

ligand in the first coordination sphere of a metal center. It was

–

1

3

7

corrected value of 2051.8 cm for BTol, whereas it was 2052.2

computed using the webꢀbased application SambVca. The

–

1

20

cm for BMes. Thus, the electronꢀdonating properties of both

default processing parameters were kept unchanged (sphere

radius: 3.5 Å, distance from the center of the sphere: 2.10 Å,

mesh spacing: 0.05 Å, Bondi radii scaled by 1.17, hydrogen

atoms omitted). Under these conditions, the BTol ligand

exhibited a slightly greater demand than its BMes predecessor,

N,N'ꢀdiarylbenzimidazolylidene species were identical within

–

1

the experimental error range (1 cm ). This result is not

surprising, considering that an almost perpendicular orientation

of the aryl substituents with respect to the central heterocycle

should restrain the transmission of electronic effects from the

sideꢀrings to the carbene center. Electrochemical measurements

could help better discriminate the two NHCs in terms of

electronꢀdonor ability, as they are often more sensitive than IRꢀ

2

0

with a %V_B_u_rvalue of 32.1 vs. 30.0. This counterintuitive

result can be ascribed to the rotation of the 2ꢀtolyl substituent

unhindered side toward the rhodium atom, whereas the mesityl

This journal is © The Royal Society of Chemistry 2012

J. Name., 2012, 00, 1-3 | 5

Dalton Transactions

Page 6 of 14

View Article Online

ARTICLE

DOI: 10.1039/C5DT00433K

Journal Name

3

1f,40

1

based analyses,

option.

but we did not further investigate this in CD Cl₂at 25 °C. Thus, on H NMR spectroscopy, the

2

benzylidene protons of complex 11 resonated as two singlets

centered at 19.26 and 19.22 ppm in a 6:4 ratio (Fig. 6, left). In

Synthesis of ruthenium complexes

line with this observation, two signals in a 4:6 ratio were also

3

1

Wellꢀdefined metathesis initiators analogous to the secondꢀ present at 29.23 and 26.22 ppm in the P{ H} NMR spectrum

7

8

generation Grubbs (

1) and Hoveyda–Grubbs (2) complexes acquired at the same temperature (Fig. 7, left), while the

1

3

1

(

cf. Chart 1) featuring the BTol ligand were obtained from the

C{ H} spectrum featured two highly deshielded absorptions

corresponding firstꢀgeneration catalyst precursors via ligand for the benzylidene carbenic carbons at 297.3 and 294.5 ppm,

exchange of one tricyclohexylphosphine with a slight excess of respectively. All these assignments were confirmed by using

NHC generated in situ by deprotonation of 1,3ꢀdi(2ꢀtolyl)benzꢀ standard COSY, DEPT, HMBC and HSQC sequences, but we

imidazolium tetrafluoroborate (

silyl)amide in toluene (Scheme 4). The synthesis of [RuCl₂_ꢀthe various stereogenic units of complex 11 via 2DꢀNOESY or

=CHPh)(PCy )(BTol)] 11 was carried out at room other advanced NMR techniques.

7) with potassium bis(trimethylꢀ did not further investigate throughꢀspace interactions between

(

)

3

temperature in order to minimize unwanted thermal

degradations in solution. Indeed, a rapid decomposition of this

ruthenium–benzylidene complex occurred upon heating to 50

°

C in toluene. At 20–25 °C, sideꢀreactions were limited and

N

completion was reached within 16 h (overnight). Contrastingly,

Cl

the synthesis of the more stable isopropoxyꢀtethered

Ru

i

[

RuCl (=CHꢀ

o

ꢀO PrC H )(BTol)] complex

(

12

)

could be

2

6

4

Ph

Cl PCy₃

Cy P Cl

3

achieved within 3 h at 60 °C without noticeable thermal

decomposition. In this case, copper(I) chloride was added to the

1

1a

,S

syn,syn)

11a'

(

R

a

)

(R

a a

,S )

reaction mixture after

transformation.

1

h

to further speed up the

(

(syn,syn)

4

1

N

Cl

N

Tol

Ph

Tol

Ru

P Cl

Cl

a

N

b

Tol

Ru

O

Cl PCy

3

Cy

3

–

Cl PCy₃

H

BF

4

Cl

i-Pr

11b

11b'

(S ,R

(anti,anti)

(

S

a

,R

a

)

a

)

(

anti,anti)

7

1

1 (68%)

Scheme 4 Synthesis of second-generation ruthenium–alkylidene complexes 11

and 12. Reaction conditions: (a) KN(SiMe , [RuCl (=CHPh)(PCy ], PhCH , room

(=CH-o-O PrC )(PCy )], PhCH , 60 °C, 1 h

12 (65%)

3

)

2

3

)

2

3

i

temp., overnight; (b) KN(SiMe

then CuCl, 60 °C, 2 h.

3

)

2

, [RuCl

2

6

H

4

3

N

Cl

The enhanced stability of complex 12 compared to 11 was also

evidenced during the workꢀup of the reactions. Purification of

the yellowꢀgreen chelate 12 by column chromatography on

silica gel could be performed with no particular precautions

under a normal atmosphere. On the other hand, the redꢀbrown

compound 11 could only be isolated with a satisfactory yield

when elution was carried out under an inert atmosphere with

dry and degassed solvents.

Ru

Cl PCy₃

3

Cy P Cl

11c

11c'

(R ,R )

a

(S ,S )

a

(

syn,anti)

(anti,syn)

The identity and the purity of [RuCl (=CHPh)(PCy )(BTol)]

N

2

3

(

11) were established by various analytical techniques. It

Cl

Ru

P Cl

should be pointed out that there are eight possible rotamers for

this complex, divided into two groups of four diastereoisomers,

which are non superimposable mirror images of each other

Cl PCy

3

Cy

3

1

1d

,S

anti,syn)

11d'

(R ,R

(syn,anti)

(

Chart 5). They arise from the restricted rotation of the 2ꢀtolyl

substituents within the BTol ligand in conjunction with the

asymmetric nature of the Ru=CHPh fragment. Only two sets of Chart 5 Possible rotamers of the [RuCl

(S

a

)

a

)

(

2

3 a

(=CHPh)(PCy )(BTol)] complex (11) (R and

S

a

absolute configurations refer to the axial chirality of the two exocyclic C–N

This journal is © The Royal Society of Chemistry 2012

signals were visible in the multinuclear NMR spectra recorded

6

| J. Name., 2012, 00, 1-3

Page 7 of 14

Journal Name

Dalton Transactions

View Article Online

ART33ICLE

DOI: 10.1039/C5DT004 K

bonds, syn and anti descriptors refer to the relative orientations of each methyl

group with respect to the benzylidene unit).

When the temperature was lowered to –50 °C, all the rotation

modes could be frozen and four sets of signals became clearly

1

31

1

visible in the H and P{ H} NMR spectra of complex 11

Indeed, the two resonances originally detected at 19.26 and

9.22 ppm for the benzylidene protons were further split into

four singlets and slightly shifted to lower field upon cooling

Fig. 6, right). Hence, their chemical shifts were 19.11, 19.03,

8.96 and 18.81 ppm and the integral ratios were 5:1:3:1.

.

1

(

1

3

1

Likewise, the P{ H} NMR spectrum of [RuCl (=CHPh)ꢀ

2

(

PCy )(BTol)] (11) recorded at –50 °C featured four separate

3

singlets for the phosphine ligand at 30.97, 29.62, 27.17, and

4.20 ppm with the proportions 3:1:5:1 (Fig. 7, right). These

2

data provide unambiguous experimental evidence for the

formation of racemic mixtures containing two major and two

minor rotamers, but a more specific assignment of each

resonance to structures 11a–d/11a'–d' was not carried out.

Despite numerous attempts, we were not able to isolate crystals

of the rutheniumꢀbenzylidene complex 11 suitable for Xꢀray

diffraction analysis. This is most likely due to the limited

stability of this compound in solution. In the case of chelated

species 12, on the other hand, we were very pleased to observe

the formation of two distinct types of crystals during a slow

recrystallization process at room temperature. Crystals of type

1

Fig. 6 H NMR resonances of the benzylidene proton in [RuCl

2

(=CHPh)(PCy

3

)-

(

2 2

BTol)] (11) dissolved in CD Cl at 298 K (left) and 223 K (right).

A

were found to contain the ( _a

and 12d coꢀcrystallized in the same unit cell in a 6:4 ratio.

Conversely, crystals of type mostly consisted of the (R_aR_a)ꢀ

rotamer 12c with a 20% disorder due to the joint presence of

the (R_aS_a)ꢀconformer 12a (Fig. 8). Thus, we were able to

determine the molecular structure of all four possible

S ,R_a) and (S_a,S_a)ꢀrotamers 12b

B

,

i

stereoisomers of [RuCl (=CHꢀoꢀO PrC H )(BTol)] (12). It

6 4

2

should be pointed out that crystals

A and B were not true

enantiomers, because the two diastereoisomers in each of them

were not in the same proportions. Yet, there might exist other

crystals in the sample with different diastereoisomeric ratios.

As a matter of fact, both crystals

belonged to similar space groups that are interchangeable by a

slight metric adaptation ( 2 / and 2 / , respectively).

A and B were monoclinic and

P

c

P

n

1

3

1

Fig. 7 P{ H} NMR resonances of the PCy

3 2 3

ligand in [RuCl (=CHPh)(PCy )(BTol)]

(

11) dissolved in CD Cl at 298 K (left) and 223 K (right).

2

We tentatively assume that the rotation of the 2ꢀtolyl

substituent located above the benzylidene fragment should be

hindered, while the other

Nꢀaryl substituent could rotate freely

at 25 °C, thereby leading to the two observed conformations.

Alternatively, a rotation of the BTol ligand around the Ru–

NHC axis could also justify the observation of two sets of

NMR signals, as it would equilibrate the (syn,syn) and

(

anti,anti) atropisomers on one hand, and their (syn,anti) and

anti,syn) counterparts on the other hand. All the variable

temperature NMR investigations carried out so far to determine

rotational barriers within secondꢀgeneration complexes of type

1

concluded, however, to an easier interconversion of two

rotational isomers around a N–aryl bond than around the Ru–

4

2

NHC axis.

This journal is © The Royal Society of Chemistry 2012

J. Name., 2012, 00, 1-3 | 7

Dalton Transactions

Page 8 of 14

View Article Online

ARTICLE

DOI: 10.1039/C T0043

Jou5Drnal Na3Kme

is reminiscent of a similar case reported by Grubbs et al. when

they determined the molecular structure of [RuCl (=CHꢀ

o

ꢀ

2

i

O PrC H )(SITol)] (

5

) (SITol is 1,3ꢀdi(2ꢀtolyl)imidazolinꢀ2ꢀ

The single crystal of this compound featured two

6

4

1

5b,43d

ylidene).

conformers analogous to 12b and 12d in a 91:9 ratio. In the

major syn rotamer, the 2ꢀtolyl substituent below the basal plane

was rotated 35° away from being orthogonal to the NHC plane,

while the other one lay within 5° of perpendicularity. In the

minor anti rotamer, these angles became 35° and 16°,

respectively. Of note, the tilt of the mesityl groups in the

original secondꢀgeneration Hoveyda–Grubbs catalyst (

2

) were

8

6

° and 9°. Both complexes 5 and 12 should therefore provide

additional space near the ruthenium center to accommodate

large incoming substrates.

In order to investigate more thoroughly the steric requirements

of BTol, we have extracted its %V_B_u_rparameter from the

molecular structures of 12b and 12c. In these two major

conformers, the relative orientation of the orthoꢀmethyl groups

(

syn or anti, cf. Fig. 8) did not seem to have any influence on

V_B_u_rand led to values of 30.3 and 30.2, respectively, down

from 32.1 in rhodium complex 9b These variations

demonstrate once again the structural flexibility of NHCs and

%

.

3

5,36d

their ability to fit with the crowding around a metal center.

A more informative comparison was made with the original

Hoveyda–Grubbs catalyst , in which the SIMes ligand

2

44

8

a

occupied 31.7 or 31.8% of the ruthenium coordination

sphere, depending on the crystal structure used to perform the

calculations. Of course, all these tilt angles and buried volumes

may overestimate actual angle compressions in a solution,

which is free of crystal packing forces.

i

Fig. 8 Molecular structures of [RuCl

2

(=CH-o-O PrC

and S absolute configurations refer to the

axial chirality of the two exocyclic C–N bonds).

6 4

H )(BTol)] (12) derived from

the two types of crystals obtained (R

a

In both crystal forms, the 2ꢀtolyl substituent located above the

isopropoxybenzylidene unit could adopt two distinct

orientations, whereas the other Nꢀaryl group was wellꢀrefined in

Catalytic tests

Complexes 11 and 12 were tested as catalyst precursors for the

RCM of four model

conditions defined by Grubbs and coꢀworkers were applied to

ease the comparison with various other secondꢀgeneration

α,ωꢀdienes. Standard benchmark

a single position. Strong intramolecular hydrogen bonds with

the chlorine atoms might explain this lock. Conversely, when

the aromatic ring of the 2ꢀtolyl substituent is located above the

benzylidene proton, there is a competition between the

4

5

catalysts featuring mesitylꢀsubstituted NHC ligands. We first

investigated the RCM of diethyl diallylmalonate (DEDAM, 13

in CD Cl at 30 °C (Scheme 5). Reactions were carried out

…

)

formation of H Cl bonds and C–H π interactions (see ESI for

details). The ruthenium atom was pentacoordinated and

displayed a distorted squareꢀpyramidal geometry. As expected,

the two chlorine atoms were located trans to each other in the

basal plane, while the two other mutual trans positions were

occupied by the chelating oxygen of the isopropoxy group and

the carbenic carbon of the BTol ligand. The benzylidene unit

took up the apical position and its aromatic part was almost

coplanar with the benzimidazole fused rings. Bond lengths and

angles were in line with those reported previously for various

2

1

using 1 mol% of ruthenium initiator and monitored by H NMR

spectroscopy. Under these conditions, an almost quantitative

conversion of the substrate into cyclopentene diester 14

occurred within 40 min with [RuCl (PCy )(BMes)(=CHPh)] (6)

2

3

(

Fig. 9). Previous work had already established that this BMesꢀ

based initiator and the original Grubbs secondꢀgeneration

catalyst displayed similar reactivities in the RCM of the

model disubstituted cycloolefin under examination.

1

2

0

1

6,19a,43

Replacement of BMes or SIMes with BTol on the ruthenium–

benzylidene scaffold led to a slight rate enhancement.

Conversely, 2 h were needed to reach completion with the

isopropoxybenzylidene complex 12, whereas the Hoveyda–

Grubbs catalyst

activity compared to

other secondꢀgeneration Hoveyda–Grubbs catalysts.

A key structural feature is that the 2ꢀtolyl group located below

the basal plane was significantly more tilted than the one above

the apical benzylidene unit for obvious steric reasons. Thus, the

average deviations from perpendicularity to the benzimidazole

ring were, respectively, 24° and 3° in 12a and 12c, 31° and 9°

in 12d, and 31° and 23° in 12b. The discrepancy between the

2 was reported to keep an almost unchanged

4

5,46

1

.

values measured for the last two diastereoisomers in crystal

A

8

| J. Name., 2012, 00, 1-3

This journal is © The Royal Society of Chemistry 2012

Page 9 of 14

Journal Name

Dalton Transactions

View Article Online

ART33ICLE

DOI: 10.1039/C5DT004 K

EtO

2

C

CO Et

2

EtO C

CO Et

2

Ru cat. (1 mol%)

CD Cl , 30 °C

+

C H

2 4

2

R

1

3: R = H

14: R = H

16: R = Me

15: R = Me

Scheme 5 Ruthenium-catalyzed RCM of diethyl 2,2-diallylmalonate (13) and

diethyl 2-allyl-2-(2-methylallyl)malonate (15).

Fig. 10 Time course of the RCM of diethyl 2-allyl-2-(2-methylallyl)malonate (15

catalyzed by [RuCl

)

2

(PCy

3

)(BMes)(=CHPh)] (

6), [RuCl

2

(PCy

3

)(BTol)(=CHPh)] (11),

i

(=CH-o-O PrC

and [RuCl

2

6

4

H )(BTol)] (12) (1 mol% in CD

2

Cl at 30 °C).

2

Next, we examined the RCM of the two sterically demanding

ꢀdienes 17 and 19 derived from diethyl malonate and

α,ω

tosylamide, respectively (Scheme 6). Standard benchmark

conditions for these challenging substrates implied the recourse

4

5

to 5 mol% of catalyst. Previous assessment of [RuCl (PCy )ꢀ

2

3

(

BMes)(=CHPh)] (6) in tolueneꢀd₈at 80 °C had shown that the

Fig. 9 Time course of the RCM of diethyl 2,2-diallylmalonate (13) catalyzed by

(PCy

BMesꢀbenzylidene complex was largely inefficient at

promoting the RCM of tetrasubstituted cycloalkenes. Despite

[

o-O PrC

RuCl

i

2

(PCy

H

3

)(BMes)(=CHPh)] (

6), [RuCl

)(BTol)] (12) (1 mol% in CD

2

3

)(BTol)(=CHPh)] (11), and [RuCl

2

(=CH-

20

6

4

2

Cl

2

at 30 °C).

a strong thermal activation, it did not afford satisfactory yields

of cycloproducts 18 and 20 and was completely deactivated in

less than 10 min (Figures 11 and 12). Very gratifyingly, its

BTol analogue 11 was much more effective for inducing the

same transformations in CD Cl at 30 °C. With this catalyst

The reduced initiation efficiency of chelate 12 compared to the

mixed phosphine/NHC complexes and 11 became even more

obvious when the RCM of diethyl 2ꢀallylꢀ2ꢀ(methylallyl)ꢀ

6

2

malonate (15) was carried out in CD Cl at 30 °C (Scheme 5

2

precursor, conversion reached 84% after 2 h with the

dimethallylmalonate 17 and 90% with the slightly more

reactive tosylamide 19. Extending the reaction time did not

further increase the yield of 18 and brought the conversion to a

final value of 93% within 3 h in the case of 20. To further

improve these results, we decided to test the chelated

and Fig. 10). When 1 mol% of benzylidene catalyst 11 was

added to the reaction mixture, conversion climbed to 90%

within an hour, but then started to level off and ultimately

stopped at 95% after 2 h. Complex

6, on the other hand,

remained active for a longer period of time and afforded a

quantitative yield of trisubstituted cycloolefin 16 within 3 h. A

different pattern was observed with chelate 12, which suggested

a short induction period at the onset of the reaction, followed by

a slow, albeit steady, progress that led to a 74% conversion

after 2 h. The 90% threshold was reached after 3.5 h and no

sign of deactivation was detected at that point. Yet, the

experiment was not prolonged to reach full conversion.

isopropoxybenzylidene complex 12 in benzeneꢀd₆at 60 °C. We

reasoned that a thermal activation would compensate for the

slow initiation tendency displayed by this chelate in the RCM

of diesters 13 and 14. Indeed, the temperature increase

combined with the reduced steric bulk of the BTol ligand

compared to BMes or SIMes allowed to fully convert substrate

1

9 into cyclic product 20 in less than 2 h, thereby

demonstrating the validity of our approach. In the case of

diester 17, a short induction period of about 15 min was

observed before the reaction took off and a 84% conversion

was recorded after 2 h. it kept slowly increasing and product 18

was eventually obtained in 96% yield after 4 h.

This journal is © The Royal Society of Chemistry 2012

J. Name., 2012, 00, 1-3 | 9

Dalton Transactions

Page 10 of 14

View Article Online

ARTICLE

DOI: 10.1039/C T0043

Jou5Drnal Na3Kme

TEP values computed for BTol in [RhCl(COD)(BTol)] (

9) and

X

Ru cat. (5 mol%)

0–80 °C

cisꢀ[RhCl(CO) (BTol)] 10), respectively, with those

(

2

+

C H

2 4

determined previously from the analogous rhodium complexes

of BMes revealed that the two NHCs displayed similar electron

donicities. Yet, the 2ꢀtolyl substituents took a slightly greater

share of the rhodium coordination sphere than the mesityl

groups, due to a more pronounced tilt. The anti,anti

conformation adopted by BTol in the molecular structure of

complex 9b ensured nonetheless a remarkably unhindered

access to the metal center, as evidenced by the examination of

steric maps.

3

1

7: X = C(CO

9: X = NTs

2

Et)

2

18: R = C(CO

20: X = NTs

2 2

Et)

Scheme 6 Ruthenium-catalyzed RCM of diethyl 2,2-bis(2-methylallyl)malonate

17) and N,N-bis(2-methylallyl)tosylamide (19).

(

Secondꢀgeneration ruthenium–benzylidene and ruthenium–

isopropoxybenzylidene complexes 11 and 12 featuring the new

BTol ligand were synthesized via phosphine exchange from the

first generation Grubbs and Hoveyda–Grubbs catalysts,

respectively. The atropisomerism of the 2ꢀtolyl substituents

within the [RuCl (=CHPh)(PCy )(BTol)] complex (11) was

2

3

1

31

investigated by using variable temperature H and P NMR

spectroscopies, and the molecular structures of all four possible

i

rotamers of [RuCl (=CHꢀ

o

ꢀO PrC H )(BTol)]

(

12

)

were

2

6

4

determined by Xꢀray crystallography. Both complexes were

highly active at promoting the RCM of model ꢀdienes. In

α,ω

line with our expectations, the replacement of BMes with BTol

was particularly beneficial to achieve the ringꢀclosure of

tetrasubstituted cycloalkenes. More specifically, the stable

chelate 12 enabled an almost quantitative RCM of two

Fig. 11 Time course of the RCM of diethyl 2,2-bis(2-methylallyl)malonate (17

catalyzed by [RuCl (PCy )(BMes)(=CHPh)] ( ) (5 mol% in C CD at 80 °C),

RuCl (PCy )(BTol)(=CHPh)] (11) (5 mol% in CD Cl at 30 °C), and [RuCl (=CH-o-

)(BTol)] (12) (5 mol% in C at 60 °C).

)

2

3

6

D

5

3

[

2

3

2

i

O PrC

6

H

4

6 6

D

challenging substrates, viz., diethyl dimethallylmalonate (17

and N,Nꢀdimethallyltosylamide (19), within a few hours at 60

C.

)

°

To sum up, we have demonstrated that 1,3ꢀdi(2ꢀtolyl)benzꢀ

imidazolꢀ2ꢀylidene (BTol) was a very suitable NHC ligand for

achieving the RCM of tetrasubstituted cycloolefins using

secondꢀgeneration ruthenium–alkylidene catalysts. Furtherꢀ

more, the synthesis of this new ancillary ligand proceeded with

remarkable ease, as it required only three steps, among which

one was catalytic, from widely available starting materials.

Further investigations are in progress to evaluate more

thoroughly the efficiency of complexes 11 and 12 in the RCM

of a wide range of

α,ωꢀdienes, and to compare their activities

and stabilities with those of commercially available catalysts

such as

course.

5. Details of these experiments will be reported in due

Fig. 12 Time course of the RCM of N,N-bis(2-methylallyl)tosylamide (19

catalyzed by [RuCl (PCy )(BMes)(=CHPh)] ( ) (5 mol% in C CD at 80 °C),

RuCl (PCy )(BTol)(=CHPh)] (11) (5 mol% in CD Cl at 30 °C), and [RuCl (=CH-o-

)(BTol)] (12) (5 mol% in C at 60 °C).

)

2

3

6

D

5

3

[

2

3

2

Acknowledgements

i

O PrC

6

H

4

6 6

D

The financial support of the “Fonds de la Recherche

Scientifique–FNRS”, Brussels, through grant J.0058.13 is

gratefully acknowledged. The authors would like to thank Prof.

Luigi Cavallo and Dr. Laura Falivene, University of Salerno,

Italy, for generating the steric maps depicted in Figures 5 and

S4.

Conclusion and perspectives

1

,3ꢀDi(2ꢀtolyl)benzimidazolium tetrafluoroborate 7 was easily

obtained via the amination/cyclization of 1,2ꢀdibromobenzene.

Deprotonation of this benzimidazolium salt with a strong base

afforded the new

found to dimerize progressively into the corresponding Notes and references

dibenzotetraazafulvalene . Coordination of BTol to rhodium or

Nꢀheterocyclic carbene BTol, which was

8

ruthenium further led to a small, albeit representative, set of

new organometallic products. Comparison of the %V_B_u_rand

1

0 | J. Name., 2012, 00, 1-3

Page 11 of 14

Journal Name

Dalton Transactions

View Article Online

ARTICLE

DOI: 10.1039/C5DT00433K

a

Laboratory of Organometallic Chemistry and Homogeneous Catalysis,

9

(

a

) P. H. Deshmukh and S. Blechert, Dalton Trans., 2007, 2479–

2491; ( ) C. Samojłowicz, M. Bieniek and K. Grela, Chem. Rev.

2009, 109, 3708–3742; ( ) G. C. Vougioukalakis and R. H. Grubbs,

Chem. Rev., 2010, 110, 1746–1787; ( ) F. B. Hamad, T. Sun, S. Xiao

and F. Verpoort, Coord. Chem. Rev., 2013, 257, 2274–2292.

10 ( ) X. Bantreil, T. E. Schmid, R. A. M. Randall, A. M. Z. Slawin and

C. S. J. Cazin, Chem. Commun., 2010, 46, 7115–7117; ( ) T. E.

Schmid, X. Bantreil, C. A. Citadelle, A. M. Z. Slawin and C. S. J.

Cazin, Chem. Commun., 2011, 47, 7060–7062; ( ) L. H. Peeck, R. D.

Savka and H. Plenio, Chem. Eur. J., 2012, 18, 12845–12853; ( ) P.

Kos, R. Savka and H. Plenio, Adv. Synth. Catal., 2013, 355, 439–447;

) R. Kadyrov, Chem. Eur. J., 2013, 19, 1002–1012; ( ) J. Czaban,

B. M. Schertzer and K. Grela, Adv. Synth. Catal., 2013, 355, 1997–

Institut de Chimie (B6a), Université de Liège, SartꢀTilman par 4000

Liège, Belgium. Eꢀmail: l.delaude@ulg.ac.be; http://www.cata.ulg.ac.be

b

,

c

b

Unidade de Difracción de Raios X, Edificio CACTUS, Universidade de

d

Santiago de Compostela, Campus Vida, 15782 Santiago de Compostela,

Spain

a

†

Electronic Supplementary Information (ESI) available: Experimental

procedures, detailed crystallographic analysis of compounds , and 12

H, C, and P NMR spectra of all the new compounds. CCDC

045783–1045786. For ESI and crystallographic data in CIF or other

electronic format see DOI: 10.1039/b000000x/

b

8,

9

,

1

13

31

c

1

d

(e

f

1

For monographs, see: (

WileyꢀVCH, Weinheim, 2003; (

NATO Science for Peace and Security SeriesꢀA: Chemistry and

Biology, ed. V. Dragutan, A. Demonceau, I. Dragutan and E. S. 11 For molybdenum and tungsten catalysts, see: (

Finkelshtein, Springer, Dordrecht, 2010; ( Metathesis in Natural

Product Synthesis: Strategies, Substrates and Catalysts, ed. J. Cossy,

S. Arseniyadis and C. Meyer, WileyꢀVCH, Weinheim, 2010; (

a

)

Handbook of Metathesis, ed. R. H. Grubbs,

b

)

Green Metathesis Chemistry

,

2006; (

g

) M. Abbas, A. Leitgeb and C. Slugovc, Synlett, 2013, 24

,

1193–1196.

a

) A. Córdova and R.

c

)

Rios, Angew. Chem. Int. Ed., 2009, 48, 8827–8831; (

Peryshkov, R. R. Schrock, M. K. Takase, P. Müller and A. H.

Hoveyda, J. Am. Chem. Soc., 2011, 133, 20754–20757; ( ) M. Yu, I.

Ibrahem, M. Hasegawa, R. R. Schrock and A. H. Hoveyda, J. Am.

Chem. Soc., 2012, 134, 2788–2799; ( ) E. M. Townsend, R. R.

Schrock and A. H. Hoveyda, J. Am. Chem. Soc., 2012, 134, 11334–

11337; ( ) E. T. Kiesewetter, R. V. O'Brien, E. C. Yu, S. J. Meek, R.

b) D. V.

d

)

c

Olefin Metathesis: Theory and Practice, ed. K. Grela, John Wiley &

Sons, Hoboken, NJ, 2014.

d

2

For selected reviews, see: (

a

) A. Fürstner, Angew. Chem. Int. Ed.

) M. R. Buchmeiser, Chem. Rev., 2000, 100

) S. J. Connon and S. Blechert, Angew. Chem. Int. Ed.

003, 42, 1900–1923; ( ) A. Deiters and S. F. Martin, Chem. Rev.

004, 104, 2199–2238; ( ) C. Slugovc, Macromol. Rapid Commun.

004, 25, 1283ꢀ1297; (

,

2

1

2

000, 39, 3012–3043; (

b

,

e

565–1604; (

c

R. Schrock and A. H. Hoveyda, J. Am. Chem. Soc., 2013, 135, 6026–

d

6029.

e

12 For ruthenium catalysts, see: (

Chem. Soc., 2011, 133, 8525–8527; (

Patel, M. B. Herbert and R. H. Grubbs, J. Am. Chem. Soc., 2012, 134

693–699; ( ) R. K. M. Khan, R. V. O'Brien, S. Torker, B. Li and A.

H. Hoveyda, J. Am. Chem. Soc., 2012, 134, 12774–12779; ( ) M. B.

Herbert, V. M. Marx, R. L. Pederson and R. H. Grubbs, Angew.

Chem. Int. Ed., 2013, 52, 310–314; ( ) H. Miyazaki, M. B. Herbert,

a

) K. Endo and R. H. Grubbs, J. Am.

f

) K. C. Nicolaou, P. G. Bulger and D. Sarlah,

) N. Holub and S.

, 1064–1082; ( ) A. H. Hoveyda

and A. R. Zhugralin, Nature, 2007, 450, 243–251; ( ) W. A. L. van

b

) B. K. Keitz, K. Endo, P. R.

Angew. Chem. Int. Ed., 2005, 44, 4490–4527; (

g

,

Blechert, Chem. Asian J., 2007,

2

h

c

i

d

Otterlo and C. B. de Koning, Chem. Rev., 2009, 109, 3743–3782; (

S. Monfette and D. E. Fogg, Chem. Rev., 2009, 109, 3783–3816.

j)

e

3

(a

) R. R. Schrock, Acc. Chem. Res., 1990, 23, 158–165; (

b

) R. R.

P. Liu, X. Dong, X. Xu, B. K. Keitz, T. Ung, G. Mkrtumyan, K. N.

Schrock and A. H. Hoveyda, Angew. Chem. Int. Ed., 2003, 42, 4592–

Houk and R. H. Grubbs, J. Am. Chem. Soc., 2013, 135, 5848–5858;

4

633.

(f

) S. Torker, R. K. M. Khan and A. H. Hoveyda, J. Am. Chem. Soc.

,

4

5

(

a

) T. M. Trnka and R. H. Grubbs, Acc. Chem. Res., 2001, 34, 18–29;

2014, 136, 3439–3455; ( ) J. S. Cannon, L. Zou, P. Liu, Y. Lan, D. J.

g

b

) R. H. Grubbs, Tetrahedron, 2004, 60, 7117–7140.

) D. Astruc, New J. Chem., 2005, 29, 42–56; ( ) T. J. Katz, New J.

) D. Astruc, New J. Chem., 2006, 30

) C. Deraedt, M. d'Halluin and D. Astruc, Eur. J.

Inorg. Chem., 2013, 4881–4908.

) M. Scholl, T. M. Trnka, J. P. Morgan and R. H. Grubbs,

Tetrahedron Lett., 1999, 40, 2247–2250; ( ) T. Weskamp, F. J. Kohl, 15 (

W. Hieringer, D. Gleich and W. A. Herrmann, Angew. Chem. Int. R. H. Grubbs, Org. Lett., 2007,

Ed., 1999, 38, 2416–2419; ( ) L. Ackermann, A. Fürstner, T. Ung, A. A. Pletnev, J. M. Berlin, R. H. Grubbs and Y. Schrodi, Org.

Weskamp, F. J. Kohl and W. A. Herrmann, Tetrahedron Lett., 1999, Lett., 2007, , 1589–1592; ( ) I. C. Stewart, C. J. Douglas and R. H.

, 4787–4790; ( ) J. Huang, H.ꢀJ. Schanz, E. D. Stevens and S. P.

Nolan, Organometallics, 1999, 18, 5375–5380.

O'Leary, K. N. Houk and R. H. Grubbs, J. Am. Chem. Soc., 2014,

a

b

136, 6733–6743.

Chem., 2006, 30, 1844–1847; (

c

,

13 (

a

) Y. Schrodi and R. L. Pederson, Aldrichimica Acta, 2007, 40, 45–

) S.ꢀM. Paek, Molecules, 2012, 17, 3348–3358.

1

848–1852; (

d

52; (b

14 T. W. Funk, J. M. Berlin and R. H. Grubbs, J. Am. Chem. Soc., 2006,

128, 1840–1846.

6

(a

b

a

) J. M. Berlin, K. Campbell, T. Ritter, T. W. Funk, A. Chlenov and

, 1339–1342; ( ) I. C. Stewart, T.

9

b

c

9

c

4

0

d

Grubbs, Org. Lett., 2008, 10, 441–444.

16 C. K. Chung and R. H. Grubbs, Org. Lett., 2008, 10, 2693–2696.

7

8

(

a

) M. Scholl, S. Ding, C. W. Lee and R. H. Grubbs, Org. Lett., 1999, 17 (

, 953–956; ( ) T. M. Trnka, J. P. Morgan, M. S. Sanford, T. E. Chem. Int. Ed., 2007, 46, 5148–5151; (

S. Blechert, Angew. Chem. Int. Ed., 2007, 46, 8082–8085.

18 ( ) J. Mathew, N. Koga and C. H. Suresh, Organometallics, 2008, 27

4666–4670; ( ) A. Poater and L. Cavallo, J. Mol. Catal. A: Chem.

2010, 324, 75–79; (

Chem. Commun., 2011, 47, 6674–6676.

a

) S. H. Hong, A. Chlenov, M. W. Day and R. H. Grubbs, Angew.

1

b

) K. Vehlow, S. Gessler and

Wilhelm, M. Scholl, T.ꢀL. Choi, S. Ding, M. W. Day and R. H.

Grubbs, J. Am. Chem. Soc., 2003, 125, 2546–2558.

a

,

(

a

) S. B. Garber, J. S. Kingsbury, B. L. Gray and A. H. Hoveyda, J.

Am. Chem. Soc., 2000, 122, 8168–8179; ( ) S. Gessler, S. Randl and

S. Blechert, Tetrahedron Lett., 2000, 41, 9973–9976.

b

c) A. Poater, N. BahriꢀLaleh and L. Cavallo,

J. Name., 2012, 00, 1-3 | 11

Dalton Transactions

Page 12 of 14

View Article Online

ARTICLE

DOI: 10.1039/C5DT00433K

Journal Name

1

9 (

a

) F. Grisi, A. Mariconda, C. Costabile, V. Bertolasi and P. Longo,

2009, 28 4988–4995; C. Costabile, A.

S. P. Nolan, Organometallics, 2008, 27, 202–210; (

Demonceau and J. Wouters, Eur. J. Inorg. Chem., 2009, 1882–1891;

) S. Wolf and H. Plenio, J. Organomet. Chem., 2009, 694, 1487–

1492.

e) L. Delaude, A.

Organometallics

,

(b)

Mariconda, L. Cavallo, P. Longo, V. Bertolasi, F. Ragone and F.

(f

Grisi, Chem. Eur. J., 2011, 17, 8618–8629.

2

0 Y. Borguet, G. Zaragoza, A. Demonceau and L. Delaude, Dalton 32 R. Savka and H. Plenio, Dalton Trans., 2015, 44, 891–893.

Trans., 2013, 42, 7287–7296.

33 M. Iglesias, D. J. Beetstra, B. Kariuki, K. J. Cavell, A. Dervisi and I.

1 L. Benhamou, E. Chardon, G. Lavigne, S. BelleminꢀLaponnaz and V.

César, Chem. Rev., 2011, 111, 2705–2733.

A. Fallis, Eur. J. Inorg. Chem., 2009, 1913–1919.

34 (

Commun., 2005, 63–65; (

and E. Herdtweck, Organometallics, 2006, 25, 2437–2448; (

Burling, M. F. Mahon, S. P. Reade and M. K. Whittlesey,

Organometallics, 2006, 25, 3761–3767; ( ) D. M. Khramov, V. M.

Lynch and C. W. Bialewski, Organometallics, 2007, 26, 6042–6049;

) A. P. Blum, T. Ritter and R. H. Grubbs, Organometallics, 2007,

26, 2122–2124; ( ) A. Bittermann, P. Härter, E. Herdtweck, S. D.

a

) P. A. Evans, E. W. Baum, A. N. Fazal and M. Pink, Chem.

) W. A. Herrmann, J. Schütz, G. D. Frey

) S.

2 Y. Borguet, G. Zaragoza, A. Demonceau and L. Delaude, Adv. Synth.

Catal., 2012, 354, 1356–1362.

b

c

3 (

a

) F. M. Rivas, U. Riaz, A. Giessert, J. A. Smulik and S. T. Diver,

, 2673–2676; ( ) F. M. Rivas, A. J. Giessert and S.

Org. Lett., 2001,

3

b

d

T. Diver, J. Org. Chem., 2002, 67, 1708–1711.

2

4 For selected applications in the synthesis of

Nꢀalkyl and

N

ꢀaryl

(e

benzimidazolium derivatives, see: (

C. J. O'Brien and M. G. Organ, Org. Lett., 2005,

D. M. Khramov, A. J. Boydston and C. W. Bielawski, Org. Lett.

006, , 1831–1834; (c) A. R. Chianese, A. Mo and D. Datta, 35 F. Ragone, A. Poater and L. Cavallo, J. Am. Chem. Soc., 2010, 132

a

) N. Hadei, E. A. B. Kantchev,

f

7

, 1991–1994; (

b

)

Hoffmann and W. A. Herrmann, J. Organomet. Chem., 2008, 693

,

2079–2090.

2

8

,

Organometallics, 2009, 28, 465–472; A. R. Chianese, P. T. Bremer,

4249–4258.

) A. C. Hillier, W. J. Sommer, B. S. Yong, J. L. Petersen, L.

Cavallo and S. P. Nolan, Organometallics, 2003, 22, 4322–4326; (

L. Cavallo, A. Correa, C. Costabille and H. Jacobsen, J. Organomet.

Chem., 2005, 690, 5407–5413; ( ) S. DíezꢀGonzález and S. P. Nolan,

C. Wong and R. J. Reynes, Organometallics, 2009, 28, 5244–5252; 36 (

a

(d

) J. A. V. Er, A. G. Tennyson, J. W. Kamplain, V. M. Lynch and C.

b)

W. Bielawski, Eur. J. Inorg. Chem., 2009, 1729–1738.

2

5 As a rule of the thumb, azolium tetrafluoroborates are often more

easily handled and purified than the corresponding chlorides. For

c

Coord. Chem. Rev., 2007, 251, 874–883; (d) H. Clavier and S. P.

other illustrations of this principle, see: (a) A. GómezꢀSuárez, R. S.

Nolan, Chem. Commun., 2010, 46, 841–861.

Ramón, O. Songis, A. M. Z. Slawin, C. S. J. Cazin and S. P. Nolan, 37 A. Poater, B. Cosenza, A. Correa, S. Giudice, F. Ragone, V. Scarano

Organometallics, 2011, 30, 5463–5470; ( ) S. Meiries, K. Speck, D. and L. Cavallo, Eur. J. Inorg. Chem., 2009, 1759–1766.

B. Cordes, A. M. Z. Slawin and S. P. Nolan, Organometallics, 2012, 38 A. Poater, F. Ragone, R. Mariz, R. Dorta and L. Cavallo, Chem. Eur.

, 330–339. J., 2010, 16, 14348–14353.

) F. E. Hahn, L. Wittenbecher, R. Boese and D. Bläser, Chem. Eur. 39 TEP (cm ) = 0.8001

J., 1999, , 1931–1935; ( ) F. E. Hahn, L. Wittenbecher, D. Le Van Angew. Chem. Int. Ed., 2010, 49, 6940–6952.

and R. Fröhlich, Angew. Chem. Int. Ed., 2000, 39, 541–544; ( ) J. W. 40 ( ) M. Süßner and H. Plenio, Chem. Commun., 2005, 5417–5419; (

Kamplain and C. W. Bielawski, Chem. Commun., 2006, 1727–1729; J. P. Moerdyk and C. W. Bielawski, Organometallics, 2011, 30

) D. M. Khramov, A. J. Boydston and C. W. Bielawski, Angew. 2278–2284; ( ) G. A. Blake, J. P. Moerdyk and C. W. Bielawski,

Chem. Int. Ed., 2006, 45, 6186–6189. Organometallics, 2012, 31, 3373–3378.

7 R. W. Alder, M. E. Blake, L. Chaker, J. N. Harvey, F. Paolini and J. 41 For other uses of this phosphine scavenger in the synthesis of

b

3

2

–

1

Rh,average

2

6 (a



CO

+ 420, see: T. Dröge and F. Glorius,

5

b

c

a

b)

,

(d

c

2

Schütz, Angew. Chem. Int. Ed., 2004, 43, 5896–5911.

rutheniumꢀisopropoxyalkylidene complexes, see: (

and S. Blechert, Angew. Chem. Int. Ed., 2002, 41, 794–796; (

Connon, M. Rivard, M. Zaja and S. Blechert, Adv. Synth. Catal.

2003, 345, 572–575; ( ) N. Buschmann, H. Wakamatsu and S.

Blechert, Synlett, 2004, 667–670; ( ) K. Vehlow, S. Maechling and

S. Blechert, Organometallics, 2006, 25, 25–28; ( ) K. Grela, S.

Harutyunyan and A. Michrowska, Angew. Chem. Int. Ed., 2002, 41

4038–4040; ( ) A. Michrowska, S. Harutyunyan, V. Sashuk, G.

Dolgonos and K. Grela, J. Am. Chem. Soc., 2004, 126, 9318–9325;

) M. Bieniek, A. Michrowska, Ł. Gułajski and K. Grela,

Organometallics, 2007, 26, 1096–1099; ( ) D. Rix, F. Caijo, I.

a

) H. Wakamatsu

8 D. Tapu, D. A. Dixon and C. Roe, Chem. Rev., 2009, 109, 3385–

b

) S. J.

3

407.

,

9 (

a

) H.ꢀW. Wanzlick and E. Schikora, Angew. Chem., 1960, 72, 494;

) H.ꢀW. Wanzlick and E. Schikora, Ber. Dtsch. Chem. Ges., 1961,

c

(b

d

9

4

, 2389–2393; (

Chem. Int. Ed., 2000, 39, 4036–4038; (

C.ꢀH. Hu, Mol. Phys., 2004, 102, 2617–2621; (

Hezarkhani and F.ꢀL. Zheng, Eur. J. Inorg. Chem., 2007, 3527–3534.

0 ( ) E. Çetinkaya, P. B. Hitchcock, H. Kücükbay, M. F. Lappert and

S. AlꢀJuaid, J. Organomet. Chem., 1994, 481, 89–95; ( ) J. W.

Kamplain, V. M. Lynch and C. W. Bielawski, Org. Lett., 2007,

401–5404.

) R. Dorta, E. D. Stevens, N. M. Scott, C. Costabille, L. Cavallo, C. 42 (

D. Hoff and S. P. Nolan, J. Am. Chem. Soc., 2005, 127, 2485–2495; Lehmann, R. Mynott, F. Stelzer and O. R. Thiel, Chem. Eur. J., 2001,

) A. Fürstner, M. Alcarazo, H. Krause and C. W. Lehmann, J. Am. , 3236–3253; ( ) G. C. Vougioukalakis and R. H. Grubbs,

Chem. Soc., 2007, 129, 12676–12677; ( ) S. Fantasia, J. L. Petersen, Organometallics, 2007, 26, 2469–2472; ( ) M. M. Gallagher, A. D.

H. Jacobsen, L. Cavallo and S. P. Nolan, Organometallics, 2007, 26 Rooney and J. J. Rooney, J. Organomet. Chem., 2008, 693, 1252–

880–5889; ( ) R. A. Kelly, III, H. Clavier, S. Giudice, N. M. Scott,

c

) V. P. W. Böhm and W. A. Herrmann, Angew.

) M.ꢀJ. Cheng, C.ꢀL. Lai and

) M. K. Denk, A.

e

d

,

e

f

3

a

(g

b

h

9,

Laurent, F. Boeda, H. Clavier, S. P. Nolan and M. Mauduit, J. Org.

Chem., 2008, 73, 4225–4228.

5

1 (a

a) A. Fürstner, L. Ackermann, B. Gabor, R. Goddard, C. W.

(b

7

b

c

,

5

d

1260; (

d

) S. Leuthäusser, V. Schmidts, C. M. Thiele and H. Plenio,

E. D. Stevens, J. Bordner, I. Samardjiev, C. D. Hoff, L. Cavallo and

1

2 | J. Name., 2012, 00, 1-3

Page 13 of 14

Journal Name

Dalton Transactions

View Article Online

ARTICLE

DOI: 10.1039/C5DT00433K

Chem. Eur. J., 2008, 14, 5465–5481; (

Longo and F. Grisi, Organometallics, 2014, 33, 2747–2759.

3 ( ) K. Weigl, K. Köhler, S. Dechert and F. Meyer, Organometallics

005, 24, 4049–4056; (

Makal, K. Jarzembska, K. Woźniak and K. Grela, Chem. Eur. J.

008, 14, 9330–9337; (

C. Virgil and R. H. Grubbs, J. Am. Chem. Soc., 2009, 131, 5313–

320; ( ) I. C. Stewart, D. Benitez, D. J. O'Leary, E. Tkatchouk, M.

W. Day, W. A. Goddard, III and R. H. Grubbs, J. Am. Chem. Soc.

009, 131, 1931–1938; ( ) S. Tiede, A. Berger, D. Schlesiger, D.

Rost, A. Lühl and S. Blechert, Angew. Chem. Int. Ed., 2010, 49

972–3975; ( ) A. Perfetto, C. Costabile, P. Longo, V. Bertolasi and

F. Grisi, Chem. Eur. J., 2013, 19, 10492–10496.

4 M. Barbasiewicz, M. Bieniek, A. Michrowska, A. Szadkowska, A.

Makal, K. Wozniak and K. Grela, Adv. Synth. Catal., 2007, 349

93–203.

e) A. Perfetto, C. Costabile, P.

4

a

,

2

b

) M. Barbasiewicz, A. Szadkowska, A.

2

c) K. M. Kuhn, J.ꢀB. Bourg, C. K. Chung, S.

5

d

,

2

e

,

3

f

4

,

1

4

5 T. Ritter, A. Hejl, A. G. Wenzel, T. W. Funk and R. H. Grubbs,

Organometallics, 2006, 25, 5740–5745.

6 M. Bieniek, A. Michrowska, D. L. Usanov and K. Grela, Chem. Eur.

J., 2008, 14, 806–818.

J. Name., 2012, 00, 1-3 | 13

Dalton Transactions

Page 14 of 14

View Article Online

DOI: 10.1039/C5DT00433K

1

00

80

60

40

20

0

N

Tol

Ts

N

Ts

N

Cl

Ru cat.

0 °C

Ru

O

Cl

i-Pr

120 Time (min)

6

0

20

40

60

80

100

Second-generation ruthenium–alkylidene complexes featuring the 1,3-di(2-tolyl)imidazol-2-

ylidene ligand (BTol) are highly efficient catalysts for the synthesis of tetrasubstituted

cycloolefins via ring-closing metathesis (RCM).

Article Doi

Ruthenium catalysts bearing a benzimidazolylidene ligand for the metathetical ring-closure of tetrasubstituted cycloolefins

DOI: 10.1039/c5dt00433k

Source and publish data:

Authors:

Article abstract of DOI:10.1039/c5dt00433k

Full text of DOI:10.1039/c5dt00433k

Products guided by the article

R&D Labs maybe for 172791-08-1

Relevant to this article

Hot Product