π-face donor properties of N-heterocyclic carbenes in grubbs II complexes

FULL PAPER

DOI: 10.1002/chem.200800139

p-Face Donor Properties of N-Heterocyclic Carbenes in Grubbs II

Complexes

Steffen Leuthäußer,^[a]Volker Schmidts,^[b]Christina M. Thiele,*^[b]and Herbert Plenio*^[a]

Abstract: The electron-donating prop-

erties of eighteen N-heterocyclic car-

benes (N,N’-bis(2,6-dimethylphenyl)i-

midazol)-2-ylidene and the respective

dihydro ligands) with 4,4’-R substituted

aryl rings (4,4’-R = NEt₂, OMe, Me,

H, SMe, F, Cl, Br, I) in the respective

Grubbs II complexes were studied

using electrochemical techniques. The

nature of the 4-R substituent has a

strong influence on the Ru^II/IIIredox

potentials ranging between DE_1/2=+

0.196 and +0.532V. Three unsymmet-

rical Grubbs II complexes with 4-R ¼

4-R’ were also synthesized. Dynamic

NMR spectroscopy revealed the re-

tuted mesityl ring with respect to the

R=CHPh unit, gives rise to different

redox potentials (4-R = NEt₂, 4-R’ =

Br: DE_1/2=+0.232 and +0.451 V). In

the oxidized Grubbs II complex (4-R

= NEt₂, H) and in the cathodic isomer

the electron rich aryl ring is located

above the Ru=CHPh unit. This orien-

tational effect provides clear evidence

for strong p–p through-space interac-

tions in the Ru^IIIcomplexes, assuming

that the alternative through-bond

transfer of electron density is equally

efficient in both isomers.

À

stricted rotation around the (NHC)C

Ru bond (DG = 89 kJmol^À1at 333 K)

resulting in two atropisomers, respec-

tively, with an isomer ratio close to

unity. Each of the isomers, that is the

two orientations of the 4-R/4-R’ substi-

Keywords: dynamic NMR spectros-

copy · metathesis · N-heterocyclic

carbenes

·

redox chemistry

·

ruthenium

Introduction

carried out.^[16–28]Notable, is work from Nolan who studied

various (NHC)Ni(CO)₃complexes^[29]and of Herrmann et al.

who recently investigated the donating properties of a large

number of different NHC ligands.^[30]From these and related

studies the overall donation of NHC ligands can be deter-

mined experimentally, but the decomposition into different

factors and their individual contribution is more difficult.

The traditional view of NHC ligands being predominantly

s-donors^[25,30]was refined and extended to NHC ligands

acting as p-donors in electron-deficient metal complexes.^[31]

Recently, evidence is accumulating that NHC ligands can be

regarded as p-acceptors;^[32–37]the extent to which this hap-

pens is under debate.^[34]

The use of N-heterocyclic carbenes in catalytically active

transition-metal complexes is rapidly growing.^[1]In olefin

metathesis this class of ligands has convincingly demonstrat-

ed its superiority over the classic phosphorusACHTREU(NG III)-based li-

gands.^[2–7]The performance of NHC-based metal complexes

in other fields of catalysis, such as for example Pd-catalyzed

cross-coupling reactions, can also be outstanding.^[8–13]

A detailed understanding of the steric and electronic

properties of NHC ligands in metal complexes is essential to

gain full control over the catalytic properties of transition

metals^[14,15]and a number of experimental studies have been

However, in certain metal complexes the donating/accept-

ing properties of NHC ligands may not to be limited to ef-

fects via the carbene carbon. A structural peculiarity of

Grubbs II complexes is the near coplanarity of the Ru=

CHAr unit and the N-aryl ring belonging to the N-heterocy-

clic carbene ligand. Fürstner et al. were the first to relate

the short carbon–carbon distance reported in the crystal

structure (around 300 pm for the respective a-carbon

atoms) to possible p-stacking interactions between the ben-

zylidene unit and the N-aryl rings of the NHC ligand.^[38]

Short distances between the aryl rings were observed in sev-

eral solid-state structures of Grubbs II complexes.^[39–42]

[a] Dr. S. Leuthäußer, Prof. Dr. H. Plenio

Anorganische Chemie im Zintl-Institut, TU Darmstadt

Petersenstr. 18, 64287 Darmstadt (Germany)

E-mail: plenio@tu-darmstadt.de

[b] V. Schmidts, Dr. C. M. Thiele

Clemens-Schçpf-Institut für Organische Chemie und Biochemie

TU Darmstadt, Petersenstr. 22, 64287 Darmstadt (Germany)

E-mail: cmt@punk.oc.chemie.tu-darmstadt.de

Supporting information for this article is available on the WWW

1

under http://www.chemeurj.org/ or from the author: H, ¹³C NMR as-

signment of 6a, 6m-1, 6m-2 and signal intensities of 1D PFGSE

NOE spectra of 6a and color versions of Figures 1–3 and 6–8.

Chem. Eur. J. 2008, 14, 5465 – 5481

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5465

In a preliminary study we have determined the redox po-

tentials of several Grubbs-type complexes with various sub-

stituents at the 4-position of the mesityl flaps.^[43]Demonceau

et al. demonstrated with different Ru^IIcomplexes for

ROMP and Kharasch reactions, the use of electrochemical

studies to rationalize the catalytic behavior.^[44,45]In our ex-

periments we observed a strong influence of the electronic

nature of the 4-substituent on the Ru^II/IIIredox potentials.

This was unexpected, since seven bonds separate the 4-R

substituent from the redox-active Ru center and since the

through bond electronic communication is hampered by the

orthogonality of the five- and six-membered ring systems

(as concluded from solid-state structures). This was seen as

evidence for p–p interactions^[46]between an aromatic ring at

the NHC ligand and the Ru–benzylidene unit. In the mean-

time, it was shown that even in Grubbs II-type complexes

with mixed aryl, alkyl–NHC^[40,47–50]or related ligands^[49]the

aryl groups are located above the benzylidene unit and the

alkyl group above the empty coordination site. For bulky

alkyl groups steric arguments account for the observed ori-

entation of the N-substituents.^[42]However, Ledoux et al.

have shown that even in methyl, aryl-substituted NHC li-

gands, the cofacial orientation of N-aryl and the Ru–benzyli-

dene group is the preferred one.^[47]Grubbs et al. recently re-

ported a Grubbs II-type complex with an aryl, aryl’–NHC

ligand displaying a preferential orientation of the more elec-

tron-rich aryl above the Ru=CHR group.^[51]

Scheme 1. Synthesis of the imidazolium and imidazolinium salts (3a, 4a:

4-R=NEt₂, 3b, 4b: 4-R=OMe, 3b, 4c: 4-R=Me, 3d, 4d: 4-R=H, 3e,

4e: 4-R=SMe, 3 f, 4 f: 4-R=F, 3g, 4g: 4-R=Cl, 3h, 4h: 4-R=Br, 3i, 4i:

4-R=I). a) Ethanol, glyoxal, HCOOH, RT; b) THF, LiAlH₄, RT, HCl/

H₂O; c) THF, (CH₂O)_n, HCl/dioxane, RT; d) HCACHTER(UNG OEt)₃, HCOOH,

1208C.

synthesis of such compounds have been reported in the liter-

ature.^[51–57]Typically, in a stepwise manner an unsymmetrical

oxalyl diamide is converted into the respective diamine

under strongly reducing conditions using BH₃or LiAlH₄,

followed by ring closure. This approach is not compatible

with substituents sensitive towards reduction. In order to

avoid the use of reductants, we have developed a different

route (Scheme 2). Starting from the N-b-hydroxyethyl sub-

stituted 2,6-dimethylanilines,^[58]treatment with HI or Ph₃P/I₂

generated the respective N-b-iodoethyl anilinium salt, fol-

lowed by nucleophilic substitution of the iodide with 2,6-di-

methylanilines (4-R = Br or NEt₂). This synthesis can be

easily upscaled yielding dekagrams of the corresponding un-

symmetrical diamines in good yields (50–70%). The dia-

mines then undergo cyclisation to the respective imidazolini-

Based on detailed electrochemical studies of symmetrical

and unsymmetrical Grubbs II-type complexes and extensive

NMR studies, we now want to describe in more detail ex-

periments related to the question of whether p–p interac-

tions are of significance in Grubbs II complexes and wheth-

er such interactions influence the electron density at the Ru

center and the catalytic properties.

Results and Discussion

Synthesis of imidazolium and imidazolinium salts and of the

respective saturated and unsaturated Grubbs II complexes:

In order to systematically cover a large range of electronic

effects in Grubbs II complexes, several symmetrical imida-

zolium and imidazolinium salts with variable 4-R substitu-

um salts with HCACHTRE(UNG OEt)₃. The respective symmetrical and

unsymmetrical Grubbs II-type complexes were obtained

from Grubbs I complex and the respective carbene follow-

ing standard procedures (Scheme 3).^[59]This synthesis works

for all NHC ligands with the exception of those with strong-

ly electron-withdrawing substituents. Upon exposure of the

ents were synthesized (4-R

(Scheme 1); others with (4-R =

NEt₂, Br, H, Me, S(O)₂tolyl)

were available from our previ-

ous work.^[16]

= OMe, SMe, F, Cl, I)

Since this study was under-

taken to elucidate potential p–

p interactions between the aryl

rings and the Ru=CHPh unit,

we were also interested in un-

symmetrical

imidazolinium

salts, which originate from two

different anilines. Several useful

Scheme 2. Synthesis of unsymmetrical imidazolium and imidazolinium salts. a) Neat, 65% aq. HI or I₂/PPh₃;

b) DMF, NaHCO₃, 4-R-aniline (4-R, 4-R’=Br, NEt₂), 508C, 50–70% yield; c) HC(OEt)₃, HCOOH, 1208C,

AHCTREUNG

synthetic approaches for the NH₄Cl.

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Chem. Eur. J. 2008, 14, 5465 – 5481

N-Heterocyclic Carbenes

FULL PAPER

Grubbs I complex to the 4-S(O₂)tolyl substituted carbene

(or the respective Ag⁺–NHC complex) no reaction took

place. We attribute this failure to the relatively poor elec-

tron donation of the respective carbene with 4-R

=

S(O)₂tolyl; which we have recently shown to be comparable

to that of PCy₃.^[16]

Figure 1. Ring-closing olefin metathesis of diethyl diallylmalonate in

CH₂Cl₂(0.1m, 1 mol% Grubbs II catalyst, T=308C) (color version avail-

able in the Supporting Information).

Scheme 3. Synthesis of the 4-R substituted Grubbs II complexes with un-

saturated ligands (R=R’=NEt₂5a, OCH₃5b, Me 5c, H 5d, SCH₃5e, F

5 f, Cl 5g, Br 5h, I 5i), with saturated ligands (analogous lettering 6a, 6b,

6c, 6d, 6e, 6 f, 6g, 6h, 6i) and R ¼ R’=NEt₂/H 6m, Br/H 6n, NEt₂/Br

6o. a) Toluene, KOtBu, RT; Grubbs I complex.

Olefin metathesis activity of the 4-R substituted saturated

and unsaturated Grubbs II complexes: In order to assess the

catalytic activity of olefin-metathesis catalysts in a compara-

ble manner, Grubbs et al. recently suggested a number of di-

agnostic test reactions.^[60]We have applied a few of the

newly synthesized complexes in some of these reactions to

learn more about the influence of electron density on the

olefin-metathesis reactivity.

In the ring-closing metathesis of diethyl diallylmalonate

(Figure 1) the activity of the respective Grubbs II catalysts

within the two series of complexes with saturated and unsa-

turated N-heterocyclic carbene ligands is ranked according

to the electron richness of the respective Ru centres

(Grubbs rule):^[61,62]4-NEt₂> 4-H @ 4-Cl. This order of re-

activity was also found in the ring-closing metathesis of dia-

llyltosylamine (Figure 2). In both metathesis reactions the

reactivity differences between 4-NEt₂and 4-H substituted

complexes are small compared with what could have been

expected from the large differences in the redox potentials.

This was unexpected considering the strong donation of the

4-R = NEt₂group. However, p-stacking interactions can

only be operative while the Ru=CHR substructure exists,

which is not always the case during the catalytic cycle.

On comparing the catalytic activity of Ru complexes with

saturated and unsaturated NHC ligands with identical 4-R

group (Figures 1 and 3) the former group is significantly

more active, as was previously reported by Grubbs^[63]and

Fürstner.^[38]The activity of the saturated Grubbs II complex

with 4-Cl is almost the same as that of the unsaturated

Grubbs II complex with 4-NEt₂, while the electron releasing

capacity of the unsaturated NHC 4a (4-R = NEt₂) is signifi-

cantly higher than that of the saturated NHC 3g (4-R =

Cl)^[16]which also translates into very different redox poten-

Figure 2. Ring-closing olefin metathesis of diallyltosylamine in CH₂Cl₂

(0.1m, 1 mol% Grubbs II catalyst 6a, d, g, T=08C) (color version avail-

able in the Supporting Information).

tials for 5a and 6g. Based on the electrochemical experi-

ments (see below) we thus conclude, that the small differ-

ence in the electron-releasing capacity of the saturated und

unsaturated NHC ligands can hardly account for the much

larger differences in the reactivity of the respective saturat-

ed and unsaturated Grubbs II complexes.

UV/Vis spectroscopy of Grubbs II complexes: In order to

study the influence of the 4-R substituents on the d–d chro-

mophore, UV/Vis spectra of the series of complexes 5 and 6

were recorded (Table 1). All complexes display a typical d–

d transition around 500 nm. The use of progressively more

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H. Plenio, C. M. Thiele et al.

Table 2. Redox potentials of various Grubbs II complexes (CH₂Cl₂/

NBu₄PF₆(0.1m); internal reference octamethylferrocene (FcMe₈); 293 K,

scan rate 100 mVs^À1).

Saturated DE_1/2[V]

(E_aÀE_c[mV])

Unsaturated DE_1/2[V]

(E_aÀE_c[mV])

4-R=4-R’

6a, 4-NEt₂

6b, 4-OMe

6c, 4-Me

6d, 4-H

6e, 4-SMe

6 f, 4-F

6g, 4-Cl

0.196 (83)

0.454 (74)

0.455 (76)

0.469 (81)

0.479 (76)

0.516 (78)

0.529 (78)

0.538 (82)

0.532(84)

5a 0.271 (80)

5b 0.453 (76)

5c 0.450 (77)

5d 0.470 (85)

5e 0.479 (71)

5 f 0.514 (82)

5g 0.535 (84)

5h 0.532(84)

5i 0.528 (82)

6h, 4-Br

6i, 4-I

4-R ¼ 4-R’

6m, 4-NEt₂/H

0.219 (102)^[a]

0.410 (72)^[a]

0.503 (85)^[a]

0.226 (104)^[a]

0.461 (82)^[a]

0.224^[b]

0.420^[b]

0.498^[b]

0.232^[b]

0.451^[b]

6n, 4-Br/H

6o, 4-Br/NEt₂

Figure 3. Cross metathesis of hexenylacetate/methacrylate in CH₂Cl₂

(0.4m, 2.5 mol% Grubbs II catalyst, T=358C) (color version available in

the Supporting Information).

[a] Cyclic voltammetry. [b] Square wave voltammetry.

et al., who claims that PCy₃ligands transfer more electron

density on the metal center than an NHC ligand.^[66]Howev-

er, a redox potential denotes the energy difference between

two redox states, which is not necessarily correlated with the

electron densities of a metal complex in only one oxidation

state.

Table 1. UV/Vis data of various Grubbs II complexes (CH₂Cl₂; c=

0.0028m.

4-R=4-R’

Saturated l_max[nm]

(e [Lmol^À1cm^À1])

Unsaturated l_max[nm]

(e[Lmol^À1cm^À1])

4-NEt₂

4-OMe

4-H

4-Cl

4-I

493 (193)

501 (253)

502(169)

501 (221)

497 (240)

499 (280)

502 (357)

504 (146)

503 (273)

504 (212)

The range of the redox potentials in the related

(NHC)IrClACHTER(UNG cod) complexes with the same 4-substituents re-

cently studied by us is significantly smaller (saturated NHC:

DE=247 mV, unsaturated NHC: DE=214 mV).^[16]Care has

to taken when comparing redox potentials of Ir and Ru

complexes, nonetheless, the notably stronger influence of

the 4-substituents on the redox potential of Grubbs II com-

plexes prompted us to consider additional interactions—

other than the normal through-bond component.^[67–70]In this

respect, interesting transannular interactions between cofa-

cial aromatic ring systems were reported by Gleiter et al.

(cofacial cyclobutadienyl–cobalt complexes),^[71–73]Boekel-

heide, Jordan et al. and Speiser et al. (ruthenium-

electron-donating substituents in the 4-position has only a

small effect on the d–d transitions.

Electrochemical studies

Symmetrical Grubbs II complexes: Ru^II/IIIredox potentials

of a large number of Grubbs II complexes 5 and 6 were de-

termined (Table 2) to study how substituents at the aromatic

rings influence the electron density at ruthenium. In general,

such complexes are characterized by a highly reversible

electrochemistry.^[43,64]Pronounced differences in the redox

potentials between 4-NEt₂and 4-Br substituted NHC li-

gands in the saturated (DE_1/2=0.196 ! 0.538 V, DE=

344 mV) and in the unsaturated series (DE_1/2=0.271 !

0.532V, DE=261 mV) of NHC ligands were observed. The

much larger effect of the NEt₂group on the redox potentials

of Ru^II/IIIcompared to that of the OMe group is based on

the much more negative Hammett parameter of the NEt₂

group.^[65]The Ru^II/IIIredox potentials of the various Grubbs

II complexes are significantly more cathodic than that of the

Grubbs I complex (DE_1/2=0.585 V).^[43]This is indicative of a

higher electron density at the metal center in the NHC/

PCy₃-substituted Grubbs II complexes, than in the PCy₃/

PCy₃-substituted Grubbs I species. There appears to be a

conflict with results of recent XAS studies by Kennepohl

AHCTREUNG

[2.2]paracyclophane)^[74,75]to significantly influence cobalt

redox potentials.^[76]

The redox potentials of Grubbs II complexes with saturat-

ed and unsaturated NHC (but identical 4-R groups) are

very similar (with the exception of 4-R = NEt₂); indicative

of similar donor properties in the two classes of ligands.^[77]

However, the catalytic properties of the two series of com-

plexes are quite different (see section on Catalysis). We con-

clude that differences in the catalytic properties of saturated

and unsaturated Grubbs II complexes do not originate from

dissimilar electron densities at the metal centres. Instead the

slightly different steric requirements of saturated and unsa-

turated NHC ligands (non-planar versus planar five-mem-

bered ring) may be responsible.^[4,78]

Unsymmetrical Grubbs II complexes: Grubbs II complexes

6m, 6n and 6o with different 4-R, R’ substituents were

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N-Heterocyclic Carbenes

FULL PAPER

shown to exist as pairs of atropisomers by NMR spectrosco-

py. In one the 4-R substituted aryl flap is located above the

Ru–benzylidene unit and the 4-R’ group above the vacant

site; in the other one the 4-R’ group is located above the

Ru–benzylidene unit and 4-R above the vacant site

(Scheme 4). It is an important question whether the unsym-

a peak separation of 191 mV. The redox potential of the

cathodic isomer is close to that of the symmetrically substi-

tuted 6a (DE_1/2=0.196 V, 4-R=NEt₂, NEt₂), while the po-

tential of the anodic isomer is close to that of 6d (DE_1/2

=

0.469 V, 4-R=H, H). We thus conclude that the orientation

of the 4-R substituted aryl rings relative to the Ru=CHPh

group is very important for the redox behavior of the re-

spective Grubbs II complexes.

Scheme 4. Atropisomerism in unsymmetrically 4-substituted Grubbs II

complexes.

metrical substitution and the different orientations of the 4-

R and 4-R’ group have a significant influence on the redox

potentials of the respective Grubbs II complexes. Should the

two atropisomers persist on the timescale of the electro-

chemical experiment, two extreme situations are conceiva-

ble. In the first scenario, the electron density of the aryl

flap, which is modulated by the nature of 4-R and 4-R’

groups, is exclusively transferred to the Ru–benzylidene unit

via transannular interactions; then the two different atro-

pisomers must be characterized by significantly different

redox potentials. In the second scenario, the electron density

of the 4-R group is transferred solely via bonds to the Ru

atom. It follows then, that the redox potentials of the two

atropisomers have to be identical; as the relative orientation

of the 4-R or 4-R’ substituted aryl flaps with respect to the

Ru–benzylidene unit should not influence the through-bond

transfer. In reality, a mixed situation is likely and it remains

to be shown experimentally, whether the two isomers are

characterized by sufficiently different redox potentials.

In order to observe two reversible redox waves, the rota-

À

Figure 4. Cyclic voltammogram of 6m in CH₂Cl₂Ru^II/III0.219 V (DE=

102mV) and Ru ^II/III0.410 V (DE=72mV) referenced vs. FcMe ₈

À0.010 V (DE=84 mV).

Electrochemical experiments with 6o (4-R=Br, NEt₂)

again revealed a two wave situation (DE_1/2=0.226 and

0.461 V) (Figure 5). As expected, the splitting of the redox

potentials is even larger (215 mV). The redox potential of

the cathodic isomer of 6o (DE_1/2=0.226 V) is close to that

of the cathodic isomer of 6m (DE_1/2=0.219 V). This again

demonstrates that the relative orientation of the 4-R groups

of the NHC ligand relative to the Ru–benzylidene unit pri-

marily governs the Ru redox potential! In line with this, the

redox potentials of the anodic atropisomers of 6m and 6o

tion of the NHC ligand around the Ru CACHTRE(UGN NHC) should be

slow on the timescale of the electrochemical experiment.

differ significantly (6m DE_1/2=0.410 V vs. 6o DE_1/2=

Based on variable temperature NMR experiments (see sec-

0.461 V).

À

tion on NMR) the rates of the Ru C

A

With 6n (4-R=Br, H) only a single redox wave was ob-

served in the cyclic voltammogram and in the square-wave

experiment, even though NMR spectroscopy confirms the

Grubbs II complexes are known, while the relevant rates of

the corresponding process in the paramagnetic Ru^IIIspecies

could not be studied.

Based on an estimate of Ru^IIdynamic process our initial

electrochemical experiments (cyclic voltammetry and

square-wave voltammetry) were carried out at temperatures

of À208C applying scan rates of between 50–1000 mVs^À1. In

all of these initial experiments using 6m (4-R = H, NEt₂)

two distinct and reversible redox waves were observed. This

two wave situation persists even when carrying out the ex-

periments at ambient temperature. It is very important to

note here, that we never observed two distinct redox events

in any of the symmetrically substituted Grubbs II com-

plexes.

The two redox potentials for the atropisomers of 6m

(Table 2, Figure 4) were determined as DE_1/2=0.219 V

(cathodic isomer) and DE_1/2=0.410 V (anodic isomer), with

Figure 5. Square-wave voltammogram of 6o (FcMe₈DE_1/2À0.010 V).

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5469

H. Plenio, C. M. Thiele et al.

existence of two atropisomers. The absence of two redox

events for 6n is not surprising, as the estimated difference

of the redox potentials (based on DE_1/2of 6d and 6h) of the

two rotamers should be significantly below 50 mV, which

can hardly be resolved using cyclic or square wave voltam-

metry. The observed DE_1/2=0.503 V for 6n is half way be-

tween the redox potentials of 6d and 6h.

NMR Spectroscopy

Symmetrical Grubbs II complexes: We analyzed the various

chemical shifts in the H, ¹³C and ³¹P NMR resonances of

1

the Grubbs II complexes reported here, hoping to observe

characteristic changes depending on the electronic nature of

the NHC ligands with variable 4-R substituents. However,

no such correlations could be found. With the same basic

It could be argued that other events, such as the adventi-

tious protonation of the NEt₂groups or the coordination of

the aniline nitrogen to Ru, lead to two redox events. In

order to exclude this, we deliberately added stoichiometric

amounts (0.5–2equivalents) of acid (HBF ₄) to 6m in the

electrochemical cell. The presence of acid immediately led

to irreversible voltammograms. Nonetheless, we tried to syn-

thesize the respective protonated complex 6m·H⁺. This

again turned out to be unsuccessful due to very significant

decomposition (³¹P NMR) of the Grubbs II complexes upon

attempted protonation. In order to exclude the potential co-

ordination of the aniline nitrogen we deliberately added

N,N’-diethylaniline to a Grubbs II complex in an electro-

chemical cell. No change in the CV trace was observed. We

thus conclude that the two wave situation is not caused by

adventitious protonation of an amino group or its coordina-

tion to Ru, but is an intrinsic property of the unsymmetrical-

ly substituted Grubbs II complexes.

idea, we have determined one bond H–¹³C coupling con-

1

stants in the carbene unit Ru=CHPh of several complexes.

In the complexes studied, the J_C,Hcoupling constants are in-

sensitive towards electronic and structural changes: J_C,H

1

(6a; 4-R=NEt₂)=148.2Hz ( ꢀ0.2Hz)

=

¹J_C,H(6b; 4-R=

OMe)=148.2Hz ( ꢀ0.2Hz); ¹J_C,H(5b; 4-R=OMe)=

1

147.5 Hz (ꢀ0.2Hz) = J_C,H(5i; 4-R = I) not even the re-

placement of a NHC ligand by a PCy₃resulting in a Grubbs

I complex gave significant changes in the one bond hetero-

nuclear coupling constant (¹J_C,H=147.1 Hz ꢀ0.2Hz).

Furthermore, we also determined the two bond ¹³C-³¹P

coupling constants across ruthenium in the (NHC)C-Ru-

PCy₃unit of the Grubbs complexes reported here. In the

2

series of saturated Grubbs II complexes the J_{P, C}varies be-

tween 77–78 Hz, in the unsaturated series complexes be-

tween 82-84 Hz. Obviously, there is a significant difference

between saturated and unsaturated complexes. The nature

of the 4-R substituents does not influence this coupling con-

stant, indicative of a fairly invariant bonding situation in this

segment of the complex.

X-ray crystalstructure anaylsis of 5a (4-R =NEt₂): Within

the series of Grubbs II complexes studied here 5a stands

out as the most electron-rich complex and we were interest-

ed to learn from the respective crystal structure, whether

the rather different electron-donating property of the NHC

ligands influences structural parameters of the complex.

However, in the solid state the geometric parameters of 5a

(Figure 6) are comparable to those observed in related com-

Unsymmetrical Grubbs II complexes: In the NMR spectra

of the unsymmetrical complexes 6m, 6n and 6o two differ-

ent atropisomers are observed, which are characterized by

different orientations of 4-R and 4-R’ with respect to the

Ru–benzylidene unit (Scheme 4).

plexes, as compiled by Fürstner.^[38]The Ru–C

A

Consequently, the two sets of signals for the two isomers

can be distinguished. Information about the orientation of

the 4-R groups is derived from a complete assignment of

both atropisomers at 238 K (see Supporting Information)

and the corresponding nuclear Overhauser enhancement

(NOE/ROE) between the benzylidene unit and the mesityl

flap above it. At that temperature the signals of both mesi-

tyl flaps as well as those of the benzylidene unit are com-

pletely sharp, whereas at RT virtually all signals are broad-

ened such that a complete assignment is impossible. The as-

signment of the isomers is aided by the fact that the reso-

nances of this mesityl flap are strongly shielded when locat-

ed in the anisotropy cone of the benzylidene group. The

tance of 205.8(4) pm is within the typical range. The

À

carbon–carbon distance between the Ru=C and the N

C(Ph) is fairly short (296.4 pm). The two nitrogen atoms in

the 4-position are in a trigonal-planar environment (average

CNC angle 119.98), indicative of efficient nitrogen lone pair

donation into the aromatic ring.

1

isomer populations were determined using the respective H

and ³¹P integrals. Based on this the following assignment

were made: 6m: 43/57 (major isomer with -NEt₂group

above Ru=CHPh), 6n: 64/36 (major isomer with -Br above

Ru=CHPh), 6o: 58/42(major isomer with -Br above Ru =

CHPh). The isomer populations are close to unity. Obvious-

ly, the energy differences between the isomers are very

small (in the range of 1–2kJmol ^À1) not providing evidence

in favor of p-stacking interactions.

Figure 6. Crystal structure of Grubbs II complex 6a (4-R=NEt₂) (color

version available in the Supporting Information).

5470

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ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Chem. Eur. J. 2008, 14, 5465 – 5481

N-Heterocyclic Carbenes

FULL PAPER

Dynamic NMR spectroscopy: We have extracted rotational

barriers to obtain information on the influence of the 4-R

substituents in complexes 6a and 6m. The investigation of

the symmetrical Grubbs II complex 6a revealed five impor-

tant dynamic processes (Scheme 5), denoted with the corre-

sponding rate constants k₀, k₁, k₂, k₃and k₄. It was noted ear-

save way to assign magnetisation transfer pathways. Because

of its superior performance we used JS-ROESYs at all tem-

peratures below 268 K.^[79]The different exchange pathways

are characterized by different symmetry operations.

When determining k₁and k₂(rotations of the mesityl

flaps) great care has to be taken, as also the rotation of the

Ru=CHPh unit (k₃) would lead to an interconversion of the

corresponding signals (Me1+Me2and H1 +H2for k₁and

Me3+Me4 and H3+H4 for k₂). k₃, however, can be moni-

tored by the interconversion of the diastereotopic NHC

backbone protons. As long as k₃cannot be detected (up to

248 K for 6a and 6m-1, see Figures 7 and 8) it is safe to de-

termine k₁and k₂from the interconversion of the corre-

sponding methyl groups or aromatic protons. One difficulty

poses problems for the determination of k₃and the monitor-

ing of the onset of the corresponding rotational motion: as

these protons are geminal diastereotopic protons, they ex-

hibit significant NOE/ROE and their mutual scalar coupling

could also lead to TOCSY transfer in the ROE spectrum.

The latter is avoided by adjusting the spinlock angle to 45

degrees.^[79]The former problem cannot be avoided and leads

to the following consequences.

In ROESY experiments ROE and exchange have differ-

ent signs, such that a signal could still look like a pure ROE,

even if some exchange is already contributing (leading to a

decrease in signal intensity). So, an ROE-type (different

sign of cross peak as compared to diagonal) signal does not

automatically mean that there is no exchange (just that

ROE is larger as compared to exchange).^[95]In NOE experi-

ments, however, at temperatures below the zero cross-over

of the NOE (ca. 250 K for the mesityl flaps) NOE and ex-

change have the same sign.

lier,^[38,51]that the rotation about the Ru NHC bond (k₄) is

À

restricted in complexes of this kind.

Scheme 5. Dynamic processes in Grubbs II complexes.

All exchange pathways could be nicely followed via 2D

exchange spectroscopy (EXSY, Figures 7 and 8). Down to

268 K NOESY spectra, in which signals originating from

(the positive) NOE (in the extreme narrowing limit) have

different phase with respect to the diagonal, whereas signals

due to chemical exchange have the same phase as the diago-

nal, could be used. When reducing the temperature, howev-

er, one easily reaches the intermediate exchange region

(zero cross-over of the NOE), where parts of the molecule

can exhibit positive NOE signals with other parts showing

negative NOE signals. Thus signals due to the negative

NOE and due to chemical exchange cannot be distinguished

safely. In this motional regime ROESY spectra provide a

Thus exchange would lead to a

positive change in peak intensi-

ty. We did not observe any

change in signal intensity in 1D

PFGSE NOE experiments^[80–82]

(see below) except that due to

temperature dependence of the

NOE up to 248 K (see Figure

S4, Supporting Information).

Above 248 K k₃starts being ob-

servable and k₁and k₂cannot

be determined reliably any-

more. Consequently, we restrict

our interpretation of these data

to temperatures up to 238 K for

k₁and k₂.

To monitor/determine k₄we

used the signals of the diaste-

reotopic NEt₂group as the only

symmetry operation intercon-

Figure 7. EXSY/JS-ROESY of 6a at 238 K (200 ms mixing time) and expansion of the region 1.8–3 ppm. Solid,

black signals are either diagonal signals or cross peaks indicating chemical exchange, dotted signals show

ROE. The different rotational barriers observable at that temperature are indicated by boxes connecting the

two diagonal signals with the two cross peaks: Solid black line (see also expansion) for process with corre-

sponding rate constant k₂, dashed line for k₁and dashed dotted line for k₀(color version available in the Sup-

porting Information).

verting the signals is the rota-

À

tion around the Ru NHC bond

(see Figure 8, no exchange ob-

served at 248 K, crosspeaks

appear at 333 K).

Chem. Eur. J. 2008, 14, 5465 – 5481

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5471

H. Plenio, C. M. Thiele et al.

Figure 8. EXSY/JS-ROESY of 6m at 248 K (200 ms mixing time) and expansion of the region 3.7–4.2 ppm. Solid, black signals are either diagonal signals

or cross peaks indicating chemical exchange, dotted signals show ROE. The only rotational process observable at that temperature, k₃for 6m-2, is indi-

cated by boxes (solid black lines) connecting the two diagonal signals with the two cross peaks. For 6m-1 this rotational process is not observed at 248 K.

Dashed lines show the ROE signals between the respective NHC-backbone signals at 248 K (ROE @ exchange, see comments in the text and Figure

SI4, supporting informations) and indicate where k₃starts being observable at temperatures of 258 K and higher (data not shown). The dashed dotted

line indicates that k₄(corresponding to the interconversion of 6m-1 and 6m-2) is not observed at 258 K and at which position k₄can be observed at

333 K (color version available in the Supporting Information).

Having examined all exchange pathways qualitatively, we

performed quantitative dynamic studies by selective 1D

PFGSE NOE experiments at different mixing times and dif-

ferent temperatures using the PANIC approach^[83,84]for

quantification (see Supporting Information).

(H1 and H2) are isochronous and rather sharp at room tem-

perature. This has been interpreted to be due to two differ-

ent rotational barriers (k₁¼ k₂, k₁fast at RT, k₂slow at

RT) and indicative of p-stacking, for which additional evi-

1

dence was provided by comparing reactivity and H spectra

The fastest rotation occurs about the C Ph bond in the

in chloroform and benzene.^[38]

À

Ru–benzylidene unit with the corresponding rate constant

k₀. We can estimate this barrier, since at the lowest accessi-

ble temperature for NMR experiments (193 K) the inter-

change is almost locked on the NMR timescale, correspond-

ing to a barrier of circa DG=52kJmol ^À1. k₄describes the

slowest process with DG=89.2kJmol ^À1(k₄=0.1 s^À1at 333 K

for 6a, k₄=0.2s ^À1at 333 K for unsymmetrical 6m), which is

different from the corresponding barrier in related fluorine-

substituted Grubbs complexes (75 kJmol^À1), but comparable

that in the Grubbs II complex (91 kJmol^À1).^[51]The unsym-

metrical Grubbs II complexes such as 6m, 6n and 6o there-

fore exist as two atropisomers at room temperature,^[85]

whose interconversion is very slow on the NMR timescale.

It is, however, fast enough to preclude the purification of

the two atropisomeric complexes by conventional tech-

niques.

On closer examination of the spectra of complexes 6a

and 6m, (especially after being able to assign all resonances,

see Experimental Section and Supporting Information), we

realized that H1 and H2are accidentally isochronous (the

corresponding methyl groups Me1 and Me2, however, are

still anisochronous). We were able to extract rate constants

for k₁and k₂at four temperatures (223–238 K). At all tem-

peratures k₁equals k₂(k₁=k₂) within experimental error

(see Table 3) as determined from the signals of the methyl

groups (Me1+Me2and Me3 +Me4, respectively). For the

mesityl flap above the benzylidene unit we were even able

to check for consistency by using H3 and H4. From these

temperature dependent measurements of the rate constants

we had access to DG^ꢀ, DH^ꢀand DS^ꢀ. The latter two, how-

ever, are notoriously error prone, so that comparisons have

to be viewed with caution.

1

As reported earlier H signals of the aromatic proton of

When comparing the complexes 6a and 6m-1 (NEt₂

above benzylidene moiety) only minute differences are ob-

served; for 6m-2 (H above the benzylidene moiety), howev-

er, the rotation of the Ru=CHPh group (k₃) sets in much

earlier. Whether this trend is observable also in other com-

the mesityl ring above the benzylidene unit (denoted H3

and H4) are extensively broadened in spectra recorded at

1

room temperature; whereas H signals of the aromatic pro-

tons of the mesityl ring above the empty coordination site

5472

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Chem. Eur. J. 2008, 14, 5465 – 5481

N-Heterocyclic Carbenes

FULL PAPER

Table 3. Rate constants for the different rotations in complex 6a and 6m (two atropisomers, 4-R=NEt₂, 4-

R’=H, 6m-1, 4-R above benzylidene unit, 6m-2; 4-R’ above benzylidene unit) and corresponding DG^ꢀ, DH^ꢀ

and DS^ꢀ. The latter two determined from measurements at temperatures 223 to 238 K (as at temperatures

above 248 K a small contribution of k₃is observed additionally to k₁/k₂; see main text and Figure SI4). For

6m-1 comparable values are obtained, for 6m-2, however, the rotational process corresponding to k₃sets in

much earlier.

spectrum of the paramagnetic

species were unsuccessful due

to extreme line broadening

and/or decomposition of the

Ru^IIIspecies. Following the oxi-

dation of the Grubbs II com-

plex the reduction was effected

using FcMe₈(octamethylferro-

cene) at temperatures low

enough to preclude the equili-

bration of the isomers. This re-

action proceeds to completion

within 30 min at À788C (TLC

control). For the following

work-up it is mandatory to

keep the temperature always

below À308C to slow down iso-

merization. Simple filtration of

the reaction mixture over silica

sufficed to remove the para-

magnetic impurities. Evapora-

tion of the solvents (CH₂Cl₂,

Et₂O) was done at À788C. Fol-

lowing this simple procedure

the enriched cathodic isomer

(anodic/cathodic 15:85, com-

pared with 43:57 at RT equilib-

rium) was isolated. This experi-

ment also establishes the full

Complex

T [K]

Rate constant

k_x[s^À1

]

DG^ꢀ[kJmol^À1

]

DH^ꢀ[kJmol^À1

]

DS^ꢀ[Jmol^À1

]

223

228

233

238

248

258

268

k₁

k₁+ k₃

0.005

0.009

0.021

0.040

0.27

64.0 (ꢀ0.1)

64.2( ꢀ0.2)

64.1 (ꢀ0.1)

64.2( ꢀ0.1)

63.0 (ꢀ0.1)

ꢀ0.1)

61.6 (ꢀ2)

À11.1 (ꢀ9)

0.9263.0 (

3.09

62.9 (ꢀ0.3)

6a

223

228

233

238

248

258

268

333

k₂

0.005

0.010

0.023

0.044

0.24

1.08

2.51

0.10

63.8 (ꢀ0.2)

64.1 (ꢀ0.2)

63.9 (ꢀ0.1)

64.0 (ꢀ0.1)

63.3 (ꢀ0.1)

62.7 (ꢀ0.1)

63.3 (ꢀ0.3)

89.3 (ꢀ0.2)

61.4 (ꢀ4)

À10.8 (ꢀ15)

k₂

k₂+ k₃

k₄

n.d.

238

333

k₁

k₂

k₄

0.10

0.20

62.4 (ꢀ0.1)

87.1 (ꢀ0.2)

n.d.

6m-1

238

333

k₁+ k₃

k₂+ k₃

k₄

0.22

0.24

0.20

60.9 (ꢀ0.1)

60.7 (ꢀ0.1)

87.1 (ꢀ0.2)

n.d.

6m-2

plexes and can be related to the properties of these Grubbs

II complexes will be investigated in the future.

reversibility of the cyclic voltammograms. Following the full

RT isomerisation, the initial 43:57 ratio of the atropisomers

of 6m was detected. The ¹H and ³¹P NMR spectra of the

85% component allow the assignment of the enriched

isomer as the one with the 4-R = NEt₂located above the

Ru=CHPh group. This provides strong support for the p-p

interaction between the two aryl groups. The electrochemi-

cal data demonstrate that the electronic density at the Ru is

primarily modulated by the nature of the 4-R group located

above the Ru–benzylidene group; the influence of the 4-R

group above the empty coordination site seems to be much

weaker; effectively being limited to the much weaker

through bond component.

Orientation of the 4-R substituted mesitylversus the Ru–

benzylidene unit: The electrochemical experiments show

that the orientation of the 4-R group relative to the Ru–

benzylidene unit exerts a strong influence on the Ru redox

potentials. A very important question is which redox poten-

tial (cathodic and anodic isomer) corresponds to which ori-

entation of the 4-R substituted aryl group. However, the in-

1

tegration of H and ³¹P spectra of 6m, 6n and 6o revealed

that the energy differences between the two orientations are

in the 1–2kJmol ^À1range. With a view to the very small

energy differences between the respective atropisomers, the

integration of the square-wave voltammograms is not suita-

ble to resolve the question of which orientation of the 4-R

group gives rise to which redox potential. The same argu-

ment applies to the use of X-ray crystal structure analysis.

We have therefore taken a different approach. The iso-

meric mixture of 6m was chemically oxidized using Fc⁺

Summary and Conclusions

We studied the catalytic, electrochemical, dynamic and

structural properties of several Grubbs II complexes, in

which the nature of the substituents in the 4-position of the

mesityl flaps of the NHC ligand was varied systematically.

From these experiments we can draw a number of conclu-

sions:

À [86]

PF₆. This process leads to the selective oxidation of the

cathodic isomer. NHC ligands are known to also stabilize

metals in higher oxidation states.^[87]When the reaction is

carried out under conditions which allow the equilibration

of the two Ru^IIatropisomers, the isomer mixture is convert-

ed almost quantitatively into the cathodic isomer.^[88]The

presence of only a single oxidized isomer was verified by re-

ductive cyclic voltammetry. Attempts to resolve the NMR

a) The differences in the reactivity of Grubbs II complexes

with saturated and unsaturated NHC ligands do not orig-

inate from different electron density at the Ru center.

The redox potentials of the various Grubbs II complexes

Chem. Eur. J. 2008, 14, 5465 – 5481

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5473

H. Plenio, C. M. Thiele et al.

For quantitative determination of rate constants from transient 1D NOE

experiments, first T₁times were determined using the inversion-recovery

method. The relaxation delays in the 1D PFGSE NOE experiments^[82]

were set accordingly (10 s). A 20 ms Gaussian pulse was chosen for selec-

tive irradiation in most cases. For each rate constant (at each tempera-

ture) five NOE experiments were performed with mixing times between

100 and 600 ms. The integral ratio of exchange peak to irradiated peak

was used (PANIC approach^[83,84]) for quantification. Only those values

within the initial rate approximation were used for the fit of peak volume

versus mixing time (up to 400 ms in most cases) leading directly to the

rate constant as slope of the corresponding plot.^[89]DG^ꢀ, DH^ꢀand DS^ꢀ

were obtained using the Eyring equation.^[90]

confirm that the electron donation of saturated and un-

saturated NHC ligands is similar. Nonetheless, saturated

complexes Grubbs II complexes are initially more active

in olefin metathesis than the unsaturated ones; subtle

steric effects could play a decisive role.

b) The nature of the 4,4’-substituents on the mesityl flaps of

the NHC ligands has a significant influence on the elec-

tron density at Ru and on the catalytic properties of

Grubbs II complexes; a range of 336 mV is covered be-

tween the most and the least electron donating substitu-

ents attached to the mesityl flaps.

GC analysis were performed on CP-Sil 8 CB column (15 m, d_i=0.25 mm,

Varian) with Perkin Elmer Clarus 500 GC AutoSystem. Electrochemis-

try: The standard electrochemical instrumentation consisted of an

EG&G 273 A-2 potentiostat galvanostat. A three-electrode configuration

was employed. The working electrode was a Pt disk (diameter 1 mm)

sealed in soft glass with a Pt wire as counter electrode. The pseudo refer-

ence electrode was an Ag wire. Potentials were calibrated internally

against the formal potential of octamethylferrocene (À0.010 V vs Ag/

AgCl). All cyclic voltammograms and square wave voltammograms were

recorded in dry CH₂Cl₂under an atmosphere of Ar. As supporting elec-

trolyte NBu₄PF₆(c=0.1 molL^À1) was used. Square wave voltammetry

(pulse height 50 mV; frequency 15 Hz). Thin layer chromatography

(TLC) was performed using silica gel 60 F 254 (0.2 mm) on aluminium

plates. For preparative chromatography E. Merck silica gel 60 (0.063–

0.20 mesh) was used.

c) The use of unsymmetrically substituted NHC ligands

(bearing different 4,4’-substituents on the mesityl rings)

in Grubbs II complexes results in two atropisomers,

which are characterized by different orientations of the 4

and 4’-substituents relative to the Ru–benzylidene unit

and more importantly also by different redox potentials.

More specifically we learnt that the mesityl flap located

above the Ru=CHPh unit is primary responsible for the

redox potentials. This observation is not compatible with

an exclusive through-bond electron transfer of the elec-

tron density from the 4-substituents to Ru, but matches

well with considerable transannular interactions of the

mesityl flap and the Ru=CHR unit. The fact that the

ratio of the different atropisomers is close to unity, indi-

cates that the effect of p-stacking on isomer ratio is

weak in the Ru^IIcomplexes, while it appears to be much

stronger in the oxidized Grubbs II complexes.

The following compounds were prepared according to literature proce-

dures: 2,6-dimethyl-4-iodoaniline,^[91]as described for the 2,6-diisopropyl

derivative, 2,6-dimethyl-4-fluoroaniline,^[92]2,6-dimethyl-4-(methylthio)a-

niline,^[93]N,N’-bis(2,4,6-trimethylphenyl)imidazolium chloride and N,N’-

bis(2,4,6-trimethylphenyl)imidazolinium chloride,^[94]N,N’-bis(2,6-dime-

thylphenyl)imidazolium chloride and N,N’-bis(2,6-dimethyl-4-bromophe-

nyl)imidazolium chloride,^[16]N,N’-bis(2,6-dimethyl-4-bromophenyl)imida-

zolinium chloride,^[16]N,N’-bis(2,6-dimethyl-4-N,N’-diethylaminophenyl)-

imidazolium chloride,^[16]N,N’-bis(2,6-dimethyl-4-N,N’-diethylaminophe-

nyl)imidazolinium chloride.^[16]

The work described here provides firm evidence for N-

aryl-substituted NHC ligands acting as p-face donors in the

oxidized Grubbs II complexes. This quality will be of rele-

vance for other NHC metal complexes.

Synthesis of the anilines

2,6-Dimethyl-4-fluoroaniline:^[92]3,5-Dimethylaniline (34.5 mL, 268 mmol,

1 equiv) was diazotized at 08C using NaNO₂and HCl. The resulting solu-

tion of the corresponding diazonium chloride was treated with aq. HBF₄

(8m, 33.50 mL, 268 mmol, 1 equiv) stirred at 08C for 2h and the diazoni-

um tetrafluoroborate was filtered off. The white solid was washed with

cold water (20 mL), a cold mixture of methanol/Et₂O 1:1 (20 mL) and

Et₂O (20 mL). The product was dried in vacuo. The diazonium tetrafluor-

oborate was heated to 758C at which point the evolution of N₂und BF₃

started. After the evolution had ceased the crude product was distilled.

3,5-Dimethylfluorbenzene (14.51 g, 55%) was obtained as a colorless oil.

¹H NMR (200 MHz, CDCl₃): d=2.29 (s, 6H, CH₃), 6.68 ppm (m, 3H,

arom.).

ExperimentalSection

Generalexperimentalmethods : All chemicals were purchased as reagent

grade from commercial suppliers and used without further purification,

unless otherwise noted. THF was distilled over potassium and benzophe-

none under argon. ¹H, ¹³C and ³¹P NMR spectra were recorded on

Bruker DRX 500 at 500.15, 125.75 and 202.46 MHz, respectively, or on

Bruker DRX 300 at 300 or 75.07 MHz. The chemical shifts are given in

parts per million (ppm) on the delta scale (d) and are referenced to tetra-

methylsilane (¹H, ¹³C NMR=0 ppm), ³¹P NMR (65% aq. H₃PO₄=

0 ppm), Abbreviations for NMR data: s=singlet; d=doublet; t=triplet;

q=quartet; m=multiplet; brs=broad signal; arom.=aromatic protons.

To neat 3,5-dimethylfluorobenzene (14.51 g, 117 mmol, 1 equiv) was

added dropwise fuming HNO₃(96%, 4.77 mL, 7.25 g, 1 equiv) at À158C.

The mixture was stirred for 3 h at room temperature, poured into water

(100 mL) and extracted with Et₂O (3100 mL). The combined organic

layers were washed with saturated Na₂CO₃solution and dried over

MgSO₄. The solvent was evaporated to give a mixture of 2,6-dimethyl-4-

fluoronitrobenzene and 2,4-dimethyl-6-fluoronitrobenzene. The two iso-

mers were separated by column chromatography (silica gel, cyclohexane/

ethyl acetate 8:1, R_f=0.40). 2,6-Dimethyl-4-fluoronitrobenzene (3.63 g,

18%) was obtained as a colorless oil. ¹H NMR (500 MHz, CDCl₃): d =

¹J_C,Hcoupling constants were measured using w₂coupled HSQC spectra

2

(with a digital resolution of 0.2Hz), J_C,Pfrom the ¹³C spectrum. All vari-

able temperature NMR experiments were performed with a TBI probe

with selective ³¹P coil, which was also used for ³¹P decoupling, equipped

with a BTO-2000 (temperature reference stabilizing unit, no temperature

correction necessary). Assignment at low temperature was performed

using ¹H, ¹³C, HSQC (³¹P decoupled, using adiabatic pulses for inversion

and refocusing pulses on ¹³C), HMBC and JS-ROESY spectra, which are

(with exception of JS-ROESY) available in the Bruker pulse sequence li-

brary. JS-ROESY^[79]was implemented and recorded with typical mixing

times of 200 to 600 ms and relaxation delays of typically 1 s for qualita-

tive exchange/NOE mapping.

2.32 (s, 6H, CH₃), 6.82ppm (d, ³J_H,F=9 Hz, 2H, arom.); ¹³C NMR

3

(126 MHz, CDCl₃): d=17.8, 115.6 (d, ²J_C,F=24 Hz), 132.8 (d, J_C,F

=

9 Hz), 148.7, 162.2 ppm (d, ¹J_C,F=252 Hz). 2,6-Dimethyl-4-fluoronitroben-

zene (3.63 g, 21.5 mmol, 1 equiv) was dissolved in glacial acetic acid

(150 mL) and hydrogenated using palladium on charcoal as catalyst

(10 wt% Pd, 2.29 g, 2.2 mmol, p(H₂)=5 bar). After 5 h the reaction mix-

ture was filtrated over Celite, neutralized with NaOH solution (1m) and

extracted with CH₂Cl₂(3100 mL). The combined organic layers were

5474

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Chem. Eur. J. 2008, 14, 5465 – 5481

N-Heterocyclic Carbenes

FULL PAPER

dried over MgSO₄and the solvent was evaporated to yield 2,6-dimethyl-

4-fluoroaniline (1.97 g, 66%). ¹H NMR (300 MHz, CDCl₃): d= 2.15 (s,

6H, CH₃), 3.39 (brs, 2H, NH₂), 6.66 ppm (d, ³J_H,F=9.0 Hz, 2H, arom.);

¹³C NMR (75.5 MHz, CDCl₃): d = 17.8, 114.4 (d, ²J_C,F=22.4 Hz), 123.1

Workup B: The reaction mixture was basified using NaOH and extracted

with Et₂O (3250 mL). The combined organic layers were dried over

MgSO₄and the solvent was evaporated in vacuo.

N,N’-Bis(2,6-dimethyl-4-methoxyphenyl)ethylenediamine dihydrochlor-

ide: N,N’-Bis(2,6-dimethyl-4-methoxyphenyl)ethylenediimine (415 mg,

1.28 mmol, 1 equiv); LiAlH₄(97 mg, 2.56 mmol, 2 equiv). Workup A:

3

1

(d, J_C,F=7.5 Hz), 138.6, 155.6 ppm (d, J_C,F=235 Hz).

2,6-Dimethyl-4-methoxyaniline: 2,4-Dinitroaniline (36.62 g, 0.2 mol,

1 equiv) was diazotized using NaNO₂und HCl. The resulting solution of

the diazonium chloride was transferred via cannula to a solution of 3,5-

dimethylanisole (15.0 g, 110 mmol) in glacial acetic acid (350 mL) at 08C.

The reaction mixture was stirred for 24 h at room temperature and

poured into water (1 L). The resulting red azo dye was filtered off and re-

dissolved in CHCl₃. The solution was washed with saturated NaHCO₃so-

lution and dried over MgSO₄. The solvent was evaporated to yield the

crude azo dye (23.97 g).

1

Yield: 310 mg (60%). H NMR (300 MHz, [D₆]DMSO): d=2.43 (s, 12H,

ArCH₃), 3.54 (s, 4H, NCH₂CH₂N), 3.72(s, 6H, OMe), 3.0–4.2(brs, 4H,

NH₂), 6.72ppm (s, 4H, arom.); ¹³C NMR (75 MHz, [D₆]DMSO): d=18.3,

46.6, 55.2, 114.6, 133.3 ppm.

N,N’-Bis(2,6-dimethyl-4-thiomethylphenyl)ethylenediamine dihydrochlor-

ide

(2e):

N,N’-Bis(2,6-dimethyl-4-thiomethylphenyl)ethylenediimine

(920 mg, 2.58 mmol, 1 equiv); LiAlH₄(196 mg, 5.16 mmol, 2equiv).

Workup A: Yield: 944 mg (84%). ¹H NMR (300 MHz, [D₆]DMSO): d=

2.44 (s, 6H, SCH₃), 2.45 (s, 12H, ArCH₃), 3.63 (s, 4H, CH₂), 7.03 (s, 4H,

arom.), 6.0–8.5 ppm (brs, 4H, NH₂); ¹³C NMR (75 MHz, [D₆]DMSO):

d=14.7, 18.2, 46.2, 126.7, 132.4, 133.3, 137.0 ppm.

A sample (10 g, 30.3 mmol, 1 equiv) was dissolved in THF/ethanol 8:1

(150 mL) and hydrogenated using palladium on charcoal as catalyst

(10 wt% Pd, 3.22 g, 3.0 mmol, p(H₂)=5 bar). After 5 h the reaction mix-

ture was filtrated over Celite and the solvent was evaporated. The crude

product was purified by column chromatography (silica gel, cyclohexane/

ethyl acetate 2:1) to yield 2,6-dimethyl-4-methoxyaniline (0.89 g, 19%).

¹H NMR (300 MHz, CDCl₃): d= 2.17 (s, 6H, CH₃), 3.34 (s, 2H, NH₂),

3.73 (s, 3H, OCH₃), 6.55 ppm (s, 2H, arom.); ¹³C NMR (75.5 MHz,

CDCl₃): d = 18.0, 55.7, 113.9, 123.2, 136.4, 152.0 ppm.

N,N’-Bis(2,6-dimethyl-4-fluorophenyl)ethylenediamine dihydrochloride

(2 f):

N,N’-Bis(2,6-dimethyl-4-fluorophenyl)ethylenediimine

(3.24 g;

10.8 mmol, 1 equiv); LiAlH₄(820 mg, 21.6 mmol, 2 equiv). Workup A:

Yield: 3.33 g (82%). ¹H NMR (300 MHz, [D₆]DMSO): d=2.45 (s, 12H,

3

ArCH₃), 3.53 (s, 4H, NCH₂CH₂N), 7.00 ppm (d, J_H,F=9 Hz, 4H, arom.);

¹³C NMR (75 MHz,[D₆]DMSO): d=18.3, 46.6, 115.7 (d, ²J_C,F=23 Hz),

3

1

133.6, 134.5 (d, J_C,F=17 Hz), 159.8 ppm (d, J_C,F=243 Hz).

Generalprocedure for the synthesis of diimines 1

N,N’-Bis(2,6-dimethyl-4-chlorophenyl)ethylenediamine dihydrochloride

The corresponding aniline (2equiv) was dissolved in ethanol (2mL per

mmol), treated with aqueous glyoxal solution (40% weight; 1 equiv) and

three drops of formic acid. The reaction mixture was stirred over night.

The yellow solid was filtered off, washed with cold MeOH and dried in

vacuo. The volume of the mother liquor was halved and the remaining

solution kept at 48C over night for a second batch of product.

(2g):

N,N’-Bis(2,6-dimethyl-4-chlorophenyl)ethylenediimine

(4.20 g,

12.6 mmol, 1 equiv); LiAlH₄(547 mg, 14.4 mmol, 2equiv). Workup A:

Yield: 3.98 (77%). ¹H NMR (300 MHz, [D₆]DMSO): d=2.19 (s, 12H,

ArCH₃), 3.26 (s, 4H, NCH₂CH₂N), 6.96 (s, 4H, arom.), 6.9–7.5 ppm (brs,

4H, NH₂); ¹³C NMR (75 MHz, [D₆]DMSO): d=18.5, 47.0, 129.1, 130.2,

134.0, 137.6 ppm.

N,N’-Bis(2,6-dimethyl-4-methoxyphenyl)ethylenediimine (1b): 2,6-Di-

methyl-4-methoxyaniline (0.89 g, 5.89 mmol, 2equiv), glyoxal (0.34 mL,

428 mg, 2.95 mmol, 1 equiv). Yield: 708 mg (74%). ¹H NMR (300 MHz,

CDCl₃): d = 2.20 (s, 12H, CH₃), 3.79 (s, 6H, CH₃), 6.57 (s, 4H, arom.),

8.11 ppm (s, 2H, CH); ¹³C NMR (75 MHz, CDCl₃): d=18.5, 55.3, 113.7,

128.6, 143.3, 156.7, 163.5 ppm.

N,N’-Bis(2,6-dimethyl-4-iodophenyl)ethylenediamine

(2i): N,N’-Bis(2,6-dimethyl-4-iodophenyl)ethylenediimine

dihydrochloride

(2.04 g,

3.95 mmol, 1 equiv); LiAlH₄(300 mg, 7.9 mmol; 1 equiv). Workup A:

Yield: 2.34 g (99%). ¹H NMR (300 MHz, [D₆]DMSO): d=2.31 (s, 12H,

ArCH₃), 3.41 (s, 4H, NCH₂CH₂N), 6.4–7.3 (brs, 4H, NH₂), 7.36 ppm (s,

4H, arom.); ¹³C NMR (75 MHz, [D₆]DMSO): d=17.8, 46.4, 91.1, 133.7,

137.2, 138.3 ppm.

N,N’-Bis(2,6-dimethyl-4-thiomethylphenyl)ethylenediimine (1e): 2,6-Di-

methyl-4-thiomethylaniline (3.0 g, 17.9 mmol, 2equiv), glyoxal (1.03 mL,

1.30 g, 8.97 mmol, 1 equiv). Yield: 2.44 g (76%). ¹H NMR (200 MHz,

CDCl₃): d=2.17 (s, 12H, CH₃), 2.49 (s, 6H, CH₃), 7.02(s, 4H, arom.),

8.09 ppm (s, 2H, CH); ¹³C NMR (75 MHz, CDCl₃): d=16.4, 18.3, 127.0,

127.6, 134.1, 147.5, 163.4 ppm.

N-(2-Iodoethyl)-2,6-dimethylaniline hydroiodide: N-(2-Hydroxyethyl)-

2,6-dimethylaniline hydroiodide (45.1 g, 273 mmol, 1 equiv) was placed in

around bottom flask and cooled to 08C. Aqueous HI (57%, 108 mL,

819 mmol, 3 equiv) was added dropwise with vigorous stirring. After the

addition was completed the reaction mixture was heated to reflux for

24 h. After cooling to room temperature the product was filtered off and

washed with Et₂O until the filtrate remained colorless. N-(2-Iodoethyl)-

2,6-dimethylaniline hydroiodide was obtained as a yellow solid (90 g,

82%). ¹H NMR (300 MHz, [D₆]DMSO): d=2.41 (s, 6H, ArCH₃), 3.44 (t,

2H, NCH₂CH₂I), 3.44 (t, 2H, NCH₂CH₂I), 7.21 (s, 3H, arom.), 8.4 ppm

(brs, 2H, NH₂); ¹³C NMR (75 MHz, [D₆]DMSO): d=À0.3, 18.7, 52.5,

128.5, 130.6, 132.3, 135.9 ppm.

N,N’-Bis(2,6-dimethyl-4-fluorphenyl)ethylenediimine (1 f): 2,6-Dimethyl-

4-fluoroaniline (6.00 g, 43.1 mmol, 2 equiv), glyoxal (2.5 mL, 21.6 mmol,

1 equiv). Yield: 4.85 g (75%). ¹H NMR (300 MHz, CDCl₃): d=2.18 (s,

12H, CH₃), 6.81 (d, ³J_H,F=9 Hz, 4H, arom.), 8.09 ppm (s, 2H, CH);

¹³C NMR (75 MHz, CDCl₃): d=17.4, 113.7 (d, ²J_C,F=22 Hz), 127.7 (d,

4

³J_C,F=7.5 Hz), 144.4 (d, J_C,F=2.3 Hz), 158.7 (¹J_C,F=243 Hz), 162.9 ppm.

N,N’-Bis(2,6-dimethyl-4-chlorophenyl)ethylenediimine (1g): 2,6-Dimeth-

yl-4-chloroaniline (7.19 g, 46 mmol, 2 equiv), glyoxal (2.63 mL, 23 mmol,

1 equiv). Yield: 4.27 g (56%). ¹H NMR (500 MHz, CDCl₃): d=2.15 (s,

12H, CH₃), 7.08 (s, 4H, arom), 8.07 ppm (s, 2H, CH); ¹³C NMR

(126 MHz, CDCl₃): d=18.2, 128.3, 128.4, 129.4, 148.2, 163.7 ppm.

N-(2-Hydroxyethyl)-2,6-dimethyl-4-bromoaniline:

2,6-Dimethyl-4-bro-

moaniline (103 g, 515 mmol, 3 equiv) and 2-chloroethanol (11.5 mL,

172mL, 1 equiv) were heated to 100 8C for 48 h. The reaction mixture

was poured into H₂O (300 mL), basified with solid KOH and extracted

with CH₂Cl₂(300 mL). The organic layer was dried over MgSO₄, the sol-

vent was evaporated in vacuo and the residue distilled under reduced

pressure (1278C, 0.2mbar). Yield: 19.7 g (69%), viscous yellow oil.

¹H NMR (300 MHz, CDCl₃): d= 2.0–3.0 (brs, 2H, NH + OH), 2.27 (s,

6H, ArCH₃), 3.10 (t, 2H, NCH₂CH₂OH), 3.75 (t, 2H, NCH₂CH₂I),

7.12ppm (s, 2H, arom.); ¹³C NMR (75 MHz, CDCl₃): d=18.3, 50.1, 62.1,

114.5, 131.3, 132.0, 144.7 ppm.

N,N’-Bis(2,6-dimethyl-4-iodophenyl)ethylenediimine (1i): 2,6-Dimethyl-4-

iodoaniline (2.87 g, 11.6 mmol, 2 equiv), glyoxal (0.67 mL, 5.81 mmol,

1 equiv). Yield: 2.04 g (68%). ¹H NMR (300 MHz, CDCl₃): d=2.12 (s,

12H, CH₃), 7.43 (s, 4H, arom), 8.05 ppm (s, 2H, CH); ¹³C NMR

(75 MHz, CDCl₃): d=18.0, 89.2, 129.0, 137.0, 149.6, 163.6 ppm.

Generalprocedure for the synthesis of diamine dihydrochol rides 2

The corresponding ethylenediimine (1 equiv) was placed in a Schlenk

flask and dissolved in anhydrous THF (10 mL per mmol) under argon.

The solution was cooled to 08C and LiAlH₄pellets (2equiv) were added.

The reaction mixture was stirred over night at room temperature and

poured carefully into an excess of an ice/conc. HCl mixture.

N-(2-Iodoethyl)-2,6-dimethyl-4-bromoaniline: Ph₃P (32.7 g, 125 mmol,

2equiv) was dissolved in CH ₂CH₂(400 mL). Imidazole (8.48 mg,

125 mmol, 2 equiv) and iodine (31.6 g, 125 mmol, 2 equiv) were added

and the mixture was stirred for 30 min at room temperature. N-(2-Hy-

droxyethyl)-2,6-dimethyl-4-bromoaniline (15.2 g, 62.3 mmol, 1 equiv) dis-

solved in CH₂Cl₂(100 mL) was added, the reaction mixture was stirred

Workup A: The white precipitate was collected by filtration, washed with

cold water and dried in vacuo.

Chem. Eur. J. 2008, 14, 5465 – 5481

ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

www.chemeurj.org

5475

H. Plenio, C. M. Thiele et al.

for another 30 min and poured into H₂O (500 mL). The mixture was basi-

fied with solid NaHCO₃and the organic layer was separated and dried

over MgSO₄. The solvent was evaporated in vacuo and the residue was

purified by column chromatography (silica, cyclohexane/ethyl acetate

10:1, R_f=0.36) to yield the title compound as a brown oil (17.5 g, 80%).

¹H NMR (300 MHz, CDCl₃): d=2.27 (s, 6H, ArCH₃), 3.25–3.30 (m, 4H,

NCH₂CH₂I), 3.35 (brs, 1H, NH), 7.12ppm (s, 2H, arom.); ¹³C NMR

(75 MHz, CDCl₃): d=6.9, 18.6, 50.0, 114.8, 131.4, 131.8, 143.5 ppm.

413 mg (79%). ¹H NMR (300 MHz, [D₆]DMSO): d=2.15 (s, 12H,

ArCH₃), 2.50 (s, 6H, SMe), 7.27 (s, 4H, arom.), 8.30 (s, 2H, NCHCHN),

9.78 ppm (s, imidazolium H); ¹³C NMR (75 MHz, [D₆]DMSO): d=14.4,

16.9, 125.2, 126.5, 130.1, 135.1, 138.8, 141.8ppm.

N,N’-Bis(2,6-dimethyl-4-fluorophenyl)imidazolium chloride (3 f·HCl):

N,N’-Bis(2,6-dimethyl-4-fluorophenyl)ethylenediimine

(355 mg,

1.18 mmol, 1 equiv), paraformaldehyde (44 mg, 1.48 mmol, 1.25 equiv);

HCl in dioxane (4m, 0.44 mL, 1.77 mmol, 1.5 equiv). Yield: 366 mg

(89%). ¹H NMR (300 MHz, [D₆]DMSO): d=2.11 (s, 12H, ArCH₃), 7.26

(d, ³J_H,F=9 Hz), 8.28 (s, 2H, NCHCHN), 9.77 ppm (s, 1H, imidazolium

H); ¹³C NMR (75 MHz, [D₆]DMSO): d=17.1, 115.4 (d, ²J_C,F=23 Hz),

124.8, 129.7, 137.6 (d, ³J_C,F=10 Hz), 139.1, 162.3 ppm (d, ¹J_C,F=247 Hz).

Generalprocedure for the synthesis of unsymmetricaldiamines

N-(2-Iodoethyl)-2,6-dimethylaniline hydroiodide or N-(2-iodoethyl)-2,6-

dimethyl-4-bromoaniline (1 equiv), the corresponding aniline (1 equiv)

and NaHCO₃(2–3 equiv) were dissolved in DMF (150 mL). The reaction

mixture was stirred for 48 h at 508C, poured into an excess of water and

extracted with Et₂O (3200 mL). The combined organic layers were

dried over MgSO₄and the solvent was evaporated. The crude product

was purified by column chromatography (silica gel, cyclohexane/ethyl

acetate).

N,N’-Bis(2,6-dimethyl-4-chlorophenyl)imidazolium chloride (3g·HCl):

N,N’-Bis(2,6-dimethyl-4-chlorophenyl)ethylenediimine

(680 mg,

2.04 mmol, 1 equiv), paraformaldehyde (77 mg, 2.55 mmol, 1.25 equiv);

HCl in dioxane (4m, 0.77 mL, 3.08 mmol, 1.5 equiv). Yield: 360 mg

(46%). ¹H NMR (300 MHz, [D₆]DMSO): d=2.18 (s, 12H, ArCH₃), 7.56

(s, 4H, arom), 8.35 (s, 2H, NCHCHN), 9.84 ppm (s, 1H, imidazolium H);

¹³C NMR (75 MHz, [D₆]DMSO): d=15.9, 123.8, 127.5, 131.3, 134.1,

136.3, 137.9 ppm.

N-(2,6-Dimethylphenyl)-N’-(2,6-dimethyl-4-diethylaminophenyl)ethylene-

diamine (2m): N-(2-Iodoethyl)-2,6-dimethylaniline hydroiodide (12.09 g,

30 mmol, 1 equiv); 2,6-dimethyl-4-N,N’-diethylaminoaniline (5.77 g,

30 mmol, 1 equiv); NaHCO₃(7.56 g, 90 mmol, 3 equiv). R_f=0.32(cyclo-

hexane/ethyl acetate 4:1); yield: 5.29 g (52%), brown oil. ¹H NMR

(300 MHz, [D₆]DMSO): d=1.13 (t, 6H, CH₂CH₃), 2.29 (s, 6H, ArCH₃),

2.32 (s, 6H, ArCH₃), 3.08 (t, 2H, NCH₂CH₂N), 3.21 (t, 2H, NCH₂CH₂N),

3.28 (q, CH₂CH₃), 6.42(s, 2H, arom.), 6.81 (t, ³J_H,H=7.5 Hz, 1H, p-H),

6.99 ppm (d, ³J_H,H=7.5 Hz, 2H, m-H); ¹³C NMR (75 MHz, [D₆]DMSO):

d=12.7, 18.8, 49.0, 49.7, 113.3, 121.6, 128.9, 129.1, 132.2, 146.3 ppm.

N,N’-Bis(2,6-dimethyl-4-iodophenyl)imidazolium chloride (3i·HCl): N,N’-

Bis(2,6-dimethyl-4-iodophenyl)ethylenediimine

(1.00 g,

1.94 mmol,

1 equiv), paraformaldehyde (73 mg, 2.42 mmol, 1.25 equiv); HCl in diox-

ane (4m, 0.73 mL, 2.91 mmol, 1.5 equiv). Yield: 912 mg (83%). ¹H NMR

(300 MHz, [D₆]DMSO): d=2.12 (s, 12H, ArCH₃), 7.82(s, 4H, arom),

8.33 (s, 2H, NCHCHN), 9.86 ppm (s, 1H, imidazolium H); ¹³C NMR

(75 MHz, [D₆]DMSO): d=16.5, 97.9, 124.6, 133.2, 137.0, 137.3,

138.5 ppm.

N-(2,6-Dimethylphenyl)-N’-(2,6-dimethyl-4-bromophenyl)ethylenedi-

amine (2n): N-(2-Iodoethyl)-2,6-dimethylaniline hydroiodide (4.00 g,

10 mmol, 1 equiv); 2,6-dimethyl-4-bromoaniline (2.00 g, 10 mmol,

1 equiv); NaHCO₃(2.52 g, 30 mmol, 3 equiv). R_f=0.50 (cyclohexane/

ethyl acetate 4:1); yield: 1.40 g (68%), yellow solid. ¹H NMR (300 MHz,

[D₆]DMSO): d=2.30 (s, 6H, ArCH₃), 2.34 (s, 6H, ArCH₃), 3.23 (m, 4H,

NCH₂CH₂N), 3.38 (s, 2H, NH), 6.88 (t, ³J_H,H=7.5 Hz, 1H, p-H), 7.04 (d,

³J_H,H=7.5 Hz, 2H, m-H), 7.16 ppm (s, 2H, arom.); ¹³C NMR (75 MHz,

[D₆]DMSO): d=18.6, 18.7, 48.8, 49.0, 114.4, 122.2, 122.4, 129.0, 129.6,

131.5, 145.3, 145.9 ppm.

General procedure for the synthesis of imidazolinium chlorides 4·HCl

A) The corresponding diamine dihydrochloride was suspended in HC-

AHCTRE(UNG OEt)₃(5 mL per mmol), three drops of formic acid were added and the

reaction mixture stirred at 1208C over night. The white precipitate was

filtered off, washed several times with Et₂O and dried in vacuo.

B) The corresponding diamine (1 equiv) and NH₄Cl (1 equiv) were sus-

pended in HCACHTER(UNG OEt)₃and three drops of formic acid were added. The re-

action mixture was stirred at 1208C over night and poured into an excess

of water. The aqueous phase was washed with Et₂O (2100 mL) and ex-

tracted with CH₂Cl₂(3100 mL). The combined organic layers were

dried over MgSO₄and the solvent was evaporated in vacuo.

N-(2,6-Dimethyl-4-bromophenyl)-N’-(2,6-dimethyl-4-diethylaminophenyl)-

A

(2o):

N-(2-Iodoethyl)-2,6-dimethyl-4-bromoaniline

(6.00 g, 17.0 mmol, 1 equiv); 2,6-dimethyl-4-diethylaminoaniline (3.26 g,

17.0 mmol, 1 equiv); NaHCO₃(2.85 g, 33.9 mmol, 2 equiv). R_f=0.23 (cy-

clohexane/ethyl acetate 4:1); yield: 4.27 g (60%), brown oil. ¹H NMR

(300 MHz, CDCl₃): d=1.26 (t, 6H, CH₂CH₃), 2.40 (s, 6H, ArCH₃), 2.43

(s, 6H, ArCH₃), 3.17–3.20 (m, 2H, NCH₂CH₂N), 3.27–3.31 (m, 2H,

NCH₂CH₂N), ꢁ3.4 (s, 2H, NH), 3.41 (q, 6H, CH₂CH₃), 6.55 (s, 2H,

arom.), 7.23 ppm (s, 2H, arom.); ¹³C NMR (75 MHz, CDCl₃): d=11.7,

17.5, 17.8, 43.5, 47.9, 48.5, 112.2, 112.7, 130.1, 130.2, 131.2, 134.0. 143.1,

144.4 ppm.

N,N’-Bis(2,6-dimethyl-4-methyloxyphenyl)imidazolinium

chloride

(4b·HCl): Procedure A: N,N’-Bis(2,6-dimethyl-4-methoxyphenyl)ethyl-

enediamine dihydrochloride (310 mg, 0.77 mmol). Yield: 228 mg (79%).

¹H NMR (300 MHz, [D₆]DMSO): d=2.35 (s, 12H, ArCH₃), 3.74 (s, 6H,

OMe), 4.41 (s, NCH₂CH₂N), 6.84 (s, 4H, arom), 9.01 ppm (s, 1H, imida-

zolinium H); ¹³C NMR (75 MHz, [D₆]DMSO): d=17.5, 51.0, 55.4, 113.9,

126.2, 137.2, 159.6, 160.7 ppm.

N,N’-Bis(2,6-dimethyl-4-thiomethylphenyl)imidazolinium

chloride

General procedure for the synthesis of imidazolium chlorides 3·HCl

(4e·HCl): Procedure A: N,N’-Bis(2,6-dimethyl-4-thiomethylphenyl)ethyl-

enediamine dihydrochloride (944 mg, 2.18 mmol). Yield: 607 mg (69%).

¹H NMR (300 MHz, [D₆]DMSO): d=2.41 (s, 12H, ArCH₃), 2.50 (s, 6H,

SMe), 4.45 (s, 4H, NCH₂CH₂N), 7.15 (s, 4H, arom), 9.12ppm (s, 1H, imi-

dazolinium H); ¹³C NMR (75 MHz, [D₆]DMSO): d=14.4, 17.3, 50.9,

125.4, 130.1, 136.3, 140.6, 160.4 ppm.

The corresponding diimine (1 equiv) was dissolved in anhydrous THF

(10 mL per mmol) under an atmosphere of Ar. A solution of paraformal-

dehyde (1.25 equiv) in HCl in dioxane (4m; 1.5 equiv) was prepared and

added to the diimine solution at 08C via syringe. The reaction mixture

was stirred at room temperature for 4 h, the white precipitate was filtered

off, washed with Et₂O and dried in vacuo.

N,N’-Bis(2,6-dimethyl-4-fluorophenyl)imidazolinium chloride (4 f·HCl):

Procedure A: N,N’-Bis(2,6-dimethyl-4-fluorophenyl)ethylenediamine di-

hydrochloride (1.13 g, 2.99 mmol, 1 equiv). Yield: 880 mg (84%).

¹H NMR (500 MHz, [D₆]DMSO): d=2.30 (s, 12H, ArCH₃), 4.40 (s, 4H,

N,N’-Bis(2,6-dimethyl-4-methyloxyphenyl)imidazolium

chloride

(3b·HCl): N,N’-Bis(2,6-dimethyl-4-methoxyphenyl)ethylenediimine

(234 mg, 0.72 mmol, 1 equiv), paraformaldehyde (27 mg, 0.90 mmol,

1.25 equiv); HCl in dioxane (4m, 0.27 mL, 1.08 mmol, 1.5 equiv). Yield:

199 mg (74%). ¹H NMR (300 MHz, [D₆]DMSO): d=2.13 (s, 12H,

ArCH₃), 3.81 (s, 6H, OMe), 6.96 (s, 4H, arom.), 8.26 (s, 2H, NCHCHN),

9.71ppm (s, 1H, imidazolium H); ¹³C NMR (75 MHz, [D₆]DMSO): d=

17.2, 55.6, 113.9, 125.0, 126.3, 136.1, 139.0, 160.2 ppm.

3

NCH₂CH₂N), 7.10 (d, J_H,F=9 Hz), 9.02ppm (s, 1H, imidazolinium H);

¹³C NMR (126 MHz, [D₆]DMSO): d=17.8, 51.3, 115.8 (d, ²J_C,F=22 Hz),

1

130.1 (d, ⁴J_C,F=2Hz), 139.3 ( ³J_C,F=11 Hz), 161.2, 162.2 ppm (d, J_C,F

=

247 Hz).

N,N’-Bis(2,6-dimethyl-4-chlorophenyl)imidazolinium chloride (4g·HCl):

Procedure A: N,N’-Bis(2,6-dimethyl-4-chlorophenyl)ethylenediamine di-

hydrochloride (2.26 mg, 5.51 mmol, 1 equiv). Yield: 1.51 mg (71%).

¹H NMR (500 MHz, [D₆]DMSO): d=2.28 (s, 12H, ArCH₃), 4.37 (s, 4H,

NCH₂CH₂N), 7.29 (s, 4H, arom.), 9.08 ppm (s, 1H, imidazolinium H);

N,N’-Bis(2,6-dimethyl-4-thiomethylphenyl)imidazolium

chloride

(3e·HCl): N,N’-Bis(2,6-dimethyl-4-thiomethylphenyl)ethylenediimine

(460 mg, 1.29 mmol, 1 equiv), paraformaldehyde (48 mg, 1.61 mmol,

1.25 equiv); HCl in dioxane (4m, 0.48 mL, 1.92mmol, 1.5 equiv). Yield:

5476

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Chem. Eur. J. 2008, 14, 5465 – 5481

N-Heterocyclic Carbenes

FULL PAPER

¹³C NMR (126 MHz, [D₆]DMSO): d=17.6, 51.2, 128.9, 132.7, 134.5,

138.7, 160.9 ppm.

benzylidene), 7.33 (t, J=7.5 Hz, 1H, p-H benzylidene), 7.07 (t, J=

7.5 Hz, 2H, m-H benzylidene), 7.0 (brs, 1H, o-H benzylidene), 6.92(s,

2H, NCHCHN), 6.68 (s, 2H, m-H aryl_NHC), 6.4 (brs, 1H, m-H aryl_NHC),

5.5 (brs, 1H, m-H aryl_NHC), 3.76 (s, 3H, OMe), 3.48, (s, 3H, OMe), 2.4

(brs, 6H, ArCH₃), 2.11 (q, 3H, PCH), 1.8 (brs, 6H, ArCH₃), 1.44 (m,

8H, Cy), 1.32(m, 7H, Cy), 0.8–1.0 ppm (m, 15H, Cy); ¹³C NMR (CDCl₃,

126 MHz): d=293.9, 189.2 (d, J_C,P=83 Hz), 158.6, 158.0, 150.8, 138.8,

131.3, 129.8, 126.9, 123.8, 123.5, 112.5, 54.1, 53.6, 30.7 (d, J_C,P=17 Hz),

28.2, 26.8, 26.7, 26.0, 25.9, 25.4, 25.2, 19.0, 17.7 ppm; ³¹P NMR (CDCl₃,

202 MHz): d=31.5 ppm.

N,N’-Bis(2,6-dimethyl-4-iodophenyl)imidazolinium chloride (4i·HCl):

Procedure A: N,N’-Bis(2,6-dimethyl-4-iodophenyl)ethylenediamine dihy-

drochloride (1.30 g, 2.19 mmol, 1 equiv). Yield: 944 mg (76%). ¹H NMR

(300 MHz, [D₆]DMSO): d=2.34 (s, 12H, ArCH₃), 4.45 (s, 4H,

NCH₂CH₂N), 7.69 (s, 4H, arom.), 9.08 ppm (s, 1H, imidazolinium H);

¹³C NMR (75 MHz, [D₆]DMSO): d=16.8, 50.7, 96.9, 133.3, 137.3, 138.3,

160.2ppm.

N-(2,6-Dimethylphenyl)-N’-(2,6-dimethyl-4-diethylaminophenyl)imidazo-

linium chloride (4m): Procedure B: N-(2,6-Dimethylphenyl)-N’-(2,6-di-

methyl-4-N,N’-diethylaminophenyl)ethylenediamine (4.35 g, 12.8 mmol,

Cl₂Ru=CHPh

chloride (313 mg, 1.00 mmol, 1.5 equiv), KOtBu (112mg, 1.00 mmol,

1.5 equiv), dichlorobenzylidenebis(tricyclohexylphosphine)ruthenium

ACHRTUNEG(3d)PCy₃ACHTRE(UNG 5d): N,N’-Bis(2,6-dimethylphenyl)imidazolium

1 equiv); NH₄Cl (685 mg, 12.8 mmol, 1 equiv); HCACHTER(UNG OEt)₃(21.3 mL,

128 mmol, 10 equiv). Yield: 3.52 g (71%). ¹H NMR (300 MHz,

[D₆]DMSO): d=1.07 (t, 6H, CH₂CH₃), 2.30 (s, 6H, ArCH₃), 2.38 (s, 6H,

ArCH₃), 3.34 (q, 4H, CH₂CH₃), 4.43 (m, 4H, NCH₂CH₂N), 6.47 (s, 2H,

arom.), 7.26 (m, 2H, m-H), 7.35 (m, 1H, p-H), 9.02ppm (s, 1H, imidazo-

linium H); ¹³C NMR (75 MHz, [D₆]DMSO): d=12.4, 17.3, 17.8, 43.6,

50.7, 51.4, 110.7, 121.0, 128.9, 129.9, 133.5, 135.8, 136.0, 147.8, 160.4 ppm.

(550 mg, 0.67 mmol, 1 equiv). R_f=0.20; yield: 405 mg (74%); red solid.

¹H NMR (CDCl₃, 500 MHz): d=19.9 (s, 1H, RuCH), 8.3 (brs, 1H, o-H

benzylidene), 7.15 (t, J=7.0 Hz, 1H, p-H benzylidene), 7.11 (s, 2H, ar-

yl_NHC), 7.07 (brs, 3H, NCHCHN + o-H benzylidene), 6.97 (t, J=7.5 Hz,

2H, m-H benzylidene), 6.54 (t, J=7.5 Hz, 1H, p-H aryl_NHC), 6.4 (brs,

1H, aryl_NHC), 6.09 (s, 2H, aryl_NHC), 2.61 (s, 6H, ArCH₃), 2.45 (m, 3H,

PCH), 2.21 (brs, 6H, ArCH₃), 1.4–1.7 (m, 15H, Cy), 1.0–1.2ppm (m,

15H, Cy); ¹³C NMR (CDCl₃, 126 MHz): d=294.5, 188.7 (d, J_C,P=83 Hz),

151.2, 138.2, 137.6, 137.0, 135.5, 131.3, 128.7, 127.9, 127.4, 123.0, 122.6,

30.9 (d, J_C,P=16 Hz), 28.3, 26.9 (d, J_C,P=10 Hz), 26.3, 25.9, 25.3, 18.9,

17.5 ppm; ³¹P NMR (CDCl₃, 202 MHz): d=31.4 ppm.

N-(2,6-Dimethylphenyl)-N’-(2,6-dimethyl-4-bromophenyl)imidazolinium

chloride (4n): Procedure B: N-(2,6-Dimethylphenyl)-N’-(2,6-dimethyl-4-

bromophenyl)ethylenediamine (2.26 g, 6.50 mmol, 1 equiv); NH₄Cl

(348 mg, 6.50 mmol, 1 equiv); HCACHTRE(UNG OEt)₃(10.8 mL, 65 mmol, 10 equiv).

Yield: 2.00 g (79%). ¹H NMR (300 MHz, [D₆]DMSO): d=2.42 (s, 12H,

ArCH₃), 4.51 (s, 4H, NCH₂CH₂N), 7.25–7.40 (m, 3H, arom.), 7.56 (s, 2H,

arom.), 9.18 ppm (s, 1H, imidazolinium H); ¹³C NMR (75 MHz,

[D₆]DMSO): d=17.1, 17.3, 50.7, 50.9, 122.8, 128.9, 130.0, 131.4, 132.8,

133.3, 135.7, 138.6, 160.1 ppm.

Cl₂Ru=CHPhACTHERU(NG 3e)PCy₃AHCTRE(UGN 5e): N,N’-Bis(2,6-dimethyl-4-thiomethylphenyl)-

imidazolium chloride (178 mg, 0.44 mmol, 1.5 equiv), KOtBu (49 mg,

0.44 mmol, 1.5 equiv), dichlorobenzylidenebis(tricyclohexylphosphine)ru-

thenium (241 mg, 0.29 mmol, 1 equiv). R_f=0.24; yield: 210 mg (80%);

1

red solid. H NMR (CDCl₃, 500 MHz): d=19.43 (s, 1H, RuCH), 9.0 (brs,

N-(2,6-Dimethyl-4-diethylaminophenyl)-N’-(2,6-dimethyl-4-bromophenyl)-

imidazolinium chloride (4o): Procedure B: N-(2,6-dimethyl-4-bromo-

phenyl)-N’-(2,6-dimethyl-4-diethylaminophenyl)ethylenediamine (4.29 g,

1H, o-H benzylidene), 7.41 (t, J=7.3 Hz, 1H, p-H benzylidene), 7.2(brs,

1H, o-H benzylidene), 7.16 (t, J=7.3 Hz, 2H, m-H benzylidene), 7.05 (s,

2H, m-H aryl_NHC), 6.96 (s, 1H, NCHCHN), 6.95 (s, 1H, NCHCHN), 6.9

(brs, 1H, m-H aryl_NHC), 5.9 (brs, 1H, m-H aryl_NHC), 2.50 (s, 3H, SCH₃),

2.5 (brs, 6H, ArCH₃), 2.28 (s, 3H, SCH₃), 2.2 (brs, 3H, ArCH₃), 2.23 (m,

3H, PCH), 2.0 (brs, 3H, ArCH₃), 1.53 (brs, 9H, Cy), 1.42(brs, 6H, Cy),

0.9–1.1 ppm (brs, 15H, Cy); ¹³C NMR (CDCl₃, 126 MHz): d=295.5,

190.0 (d, J_C,P=82 Hz), 151.9, 140.3, 139.3, 138.9, 137.1, 136.2, 134.8, 128.2,

125.4, 124.9, 124.7, 124.5, 31.8 (d, J_C,P=16 Hz), 26.4, 27.9 (d, J_C,P=10 Hz),

27.0, 26.5, 19.9, 18.7, 15.0, 14.8 ppm; ³¹P NMR (CDCl₃, 202 MHz): d=

31.7 ppm.

10.3 mmol, 1 equiv); NH₄Cl (548 mg, 10.3 mmol, 1 equiv); HCACHTRE(UNG OEt)₃

(17.1 mL, 103 mmol, 10 equiv). Yield: 1.21 g (25%). ¹H NMR (300 MHz,

[D₆]DMSO): d=1.07 (t, 6H, CH₂CH₃), 2.29 (s, 6H, ArCH₃), 2.38 (s, 6H,

ArCH₃), 3.34 (q, 4H, CH₂CH₃), 4.41 (m, 4H, NCH₂CH₂N), 6.46 (s, 2H,

arom.), 7.53 (s, 2H, arom.), 9.02ppm (s, 1H, imidazolinium H);

¹³C NMR (75 MHz, [D₆]DMSO): d=12.4, 17.1, 17.8, 43.6, 50.6, 51.5,

110.7, 120.9, 122.7, 131.4, 133.0, 136.0, 138.6, 147.9, 160.5 ppm.

GeneralProcedure for the synthesis of Grubbs II compel xes 5 and 6

The corresponding azolium salt (1.5 or 2equiv) and KO tBu (1.5 or

2equiv) were weighed in a Schlenk tube under an atmosphere of argon.

Toluene was added and the mixture was stirred for 30 min. Dichloroben-

zylidenebis(tricyclohexylphosphine)ruthenium (1 equiv) was added as a

solid under a stream of argon. The mixture was stirred for 1 h at 508C.

The solvent was evaporated in vacuo and the crude product was purified

by column chromatography (silica gel, cyclohexane/ethyl acetate 10:1).

Cl₂Ru=CHPhACTHERU(NG 3 f)PCy₃AHCTRE(UGN 5 f): N,N’-Bis(2,6-dimethyl-4-fluorophenyl)imida-

zolium chloride (148 mg, 0.42mmol, 1.5 equiv), KO tBu (48 mg,

0.42mmol, 1.5 equiv), dichlorobenzylidenebis(tricyclohexylphosphine)ru-

thenium (233 mg, 0.28 mmol, 1 equiv). R_f=0.21; yield: 216 mg (90%);

pink solid. ¹H NMR (CDCl₃, 500 MHz): d=19.46 (s, 1H, RuCH), 9.0

(brs, 1H, o-H benzylidene), 7.43 (t, J=6.8 Hz, 1H, p-H benzylidene), 7.2

(brs, 1H, o-H benzylidene), 7.16 (t, J= Hz, 2H, m-H benzylidene), 7.00

Cl₂Ru=CHPhACHTREUNG(3a)PCy₃ACHTREUNG(5a): N,N’-Bis(2,6-dimethyl-4-N,N’-diethylamino-

(s, 1H, NCHCHN), 6.96 (s, 1H, NCHCHN), 6.94 (d, JACHTER(UNG H,F)=8.5 Hz,

phenyl)imidazolium chloride (341 mg, 0.75 mmol, 1.5 equiv), KOtBu

(84 mg, 0.75 mmol, 1.5 equiv), dichlorobenzylidenebis(tricyclohexylphos-

phine)ruthenium (412mg, 0.5 mmol, 1 equiv). R_f=0.22; yield: 384 mg

(82%); brown solid. ¹H NMR (CDCl₃, 500 MHz): d=19.42(s, 1H,

RuCH), 9.0 (brs, 1H, o-H benzylidene), 7.33 (t, J=7.0 Hz, 1H, p-H ben-

2H, m-H aryl_NHC), 6.6 (brs, 1H, m-H aryl_NHC), 5.8 (brs, 1H, m-H ar-

yl_NHC), 2.48 (brs, 6H, ArCH₃), 2.22 (q, 3H, PCH), 1.8–2.4 (brs, 6H,

ArCH₃), 1.53 (brs, 9H, Cy), 1.42(brs, 6H, Cy), 1.03 (brs, 9H, Cy),

0.95 ppm (brs, 6H, Cy); ¹³C NMR (CDCl₃, 126 MHz): d=294.1, 189.5 (d,

J

_C,P=81 Hz), 162.2 (d, J_C,F=67 Hz), 160.3 (d, J_C,F=67 Hz), 150.6, 140.1,

zylidene), 7.08 (brs, 3H, o-

+ m-H benzylidene), 6.96 (s, 1H,

138.0, 133.9, 132.6, 127.5, 127.2, 123.8, 123.5, 114.2 (d, J_C,F=21 Hz), 113.5

(d, J_C,F=23 Hz), 30.8 (d, J_C,P=18 Hz), 28.2, 26.8 (d, J_C,P=10 Hz), 25.2,

19.0, 17.7 ppm; ³¹P NMR (CDCl₃, 202 MHz): d=31.6 ppm.

NCHCHN), 6.93 (s, 1H, NCHCHN), 6.46 (s, 2H, m-H aryl_NHC), 6.2(brs,

1H, m-H aryl_NHC), 5.4 (brs, 1H, m-H aryl_NHC), 3.39 (q, 4H, CH₂CH₃),

3.15 (q, 4H, CH₂CH₃), 2.45 (brs, 6H, ArCH₃), 2.27 (m, 3H, PCH), 2.0

(brs, 6H, ArCH₃), 1.4–1.6 (m, 15H, Cy), 1.23 (t, 6H, CH₂CH₃), 1.13 (t,

6H, CH₂CH₃), 0.9–1.1 ppm (m, 15H, Cy); ¹³C NMR (CDCl₃, 126 MHz):

d=293.6, 189.2 (d, J_C,P=84 Hz), 151.0. 147.0, 146.5, 138.1, 136.2, 127.1,

126.6, 125.4, 124.1, 123.6, 109.2, 108.4, 43.0, 42.9, 30.5 (d, J_P,C=16 Hz),

28.3, 26.9, 26.8, 25.4, 19.3, 11.9, 11.8 ppm; ³¹P NMR (CDCl₃, 202 MHz):

d=31.0 ppm.

Cl₂Ru=CHPhACTHERNGU(3g)PCy₃ACHTRE(UGN 5g): N,N’-Bis(2,6-dimethyl-4-chlorophenyl)imi-

dazolium chloride (586 mg, 1.54 mmol, 1.5 equiv), KOtBu (172mg,

1.54 mmol, 1.5 equiv), dichlorobenzylidenebis(tricyclohexylphosphine)ru-

thenium (632mg, 0.77 mmol, 1 equiv). R_f=0.41; yield: 651 mg (95%);

pink solid. ¹H NMR (CDCl₃, 500 MHz): d=19.45 (s, 1H, RuCH), 8.9

(brs, 1H, o-H benzylidene), 7.47 (t, J=7.0 Hz, 1H, p-H benzylidene),

7.24 (s, 2H, m-H aryl_NHC), 7.20 (m, 3H, m-H benzylidene + o-H benzyli-

dene), 7.00 (s, 1H, NCHCHN), 6.97 (s, 1H, NCHCHN), 7.0 (brs, 1H, m-

H aryl_NHC), 6.1 (brs, 1H, m-H aryl_NHC), 2.47 (s, 6H ArCH₃), 2.23 (q, 3H,

PCH), 2.2 (brs, 3H, ArCH₃), 1.9 (brs, 3H, ArCH₃), 1.56 (brs, 9H, Cy),

1.43 (brs, 6H, Cy), 0.8–1.1 ppm (m, 15H, Cy); ¹³C NMR (CDCl₃,

126 MHz): d=294.3, 189.3 (d, J_C,P=82Hz), 150.5, 139.3, 136.5, 135.5,

Cl₂Ru=CHPhACHTREUNG(3b)PCy₃CAHTRE(UGN 5b): N,N’-Bis(2,6-dimethyl-4-methoxyphenyl)-

imidazolium chloride (150 mg, 0.40 mmol, 1.5 equiv), KOtBu (45 mg,

0.40 mmol, 1.5 equiv), dichlorobenzylidenebis(tricyclohexylphosphine)ru-

thenium (220 mg, 0.27 mmol, 1 equiv). Yield: 210 mg (90%); red solid.

¹H NMR (CDCl₃, 500 MHz): d=19.38 (s, 1H, RuCH), 9.0 (brs, 1H, o-H

Chem. Eur. J. 2008, 14, 5465 – 5481

ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

www.chemeurj.org

5477

H. Plenio, C. M. Thiele et al.

135.2, 134.3, 133.4, 127.7, 127.6, 127.3, 126.9, 123.7, 123.3, 30.8 (d, J_C,P

=

dene), 6.58 (t, J=7.5 Hz, 1H, p-H aryl_NHC), 6.1 (brs, 1H, m-H aryl_NHC),

4.04 (m, 2H, NCH₂CH₂N), 3.90 (brs, 2H, NCH₂CH₂N), 2.7 (brs, 9H,

ArCH₃), 2.2 (brs, 3H, ArCH₃), 2.16 (q, 3H, PCH), 1.49 (m, 9H, Cy),

1.34 (m, 6H, Cy), 1.01, (m, 9H, Cy), 0.7–0.9 ppm (m, 6H, Cy); ¹³C NMR

(CDCl₃, 126 MHz): d=293.9, 219.1 (d, J_C,P=77 Hz), 150.4, 138.3, 136.5,

136.2, 128.1, 127.9, 127.3, 127.1, 126.7, 51.0, 50.3, 30.5 (d, J_C,P=18 Hz),

30.5, 28.9, 28.0, 26.7, 25.4, 25.1, 19.6, 17.8 ppm; ³¹P NMR (CDCl₃,

202 MHz): d=29.0 ppm.

17 Hz), 28.3, 26.8, 26.7, 25.3, 18.7, 17.5 ppm; ³¹P NMR (CDCl₃,

202 MHz): d=32.0 ppm.

Cl₂Ru=CHPhACHTREUNG(3h)PCy₃ACHTREUNG(5h): N,N’-Bis(2,6-dimethyl-4-bromophenyl)imi-

dazolium chloride (214 mg, 0.45 mmol, 1.5 equiv), KOtBu (51 mg,

0.45 mmol, 1.5 equiv), dichlorobenzylidenebis(tricyclohexylphosphine)ru-

thenium (249 mg, 0.30 mmol, 1 equiv). R_f=0.38; yield: 274 mg (92%);

pink solid. ¹H NMR (CDCl₃, 500 MHz): d=19.45 (s, 1H, RuCH), 8.9

(brs, 1H, o-H benzylidene), 7.48 (t, J=7.3 Hz, 1H, p-H, benzylidene),

7.40 (s, 2H, m-H aryl_NHC), 7.22 (m, 3H, m-H benzylidene + o-H benzyli-

dene), 7.2(brs, 1H, m-H aryl_NHC), 6.99 (s, 1H, NCHCHN), 6.96 (s, 1H,

NCHCHN), 6.2(brs, 1H, m-H aryl_NHC), 2.47 (brs, 6H, ArCH₃), 2.23 (q,

3H, PCH), 1.7–2.1 (brs, 6H, ArCH₃), 1.57 (brs, 9H, Cy), 1.43 (brs, 6H,

Cy), 0.8–1.1 ppm (m, 15H, Cy); ¹³C NMR (CDCl₃, 126 MHz): d=294.4,

189.3 (d, J_C,P=82 Hz), 150.5, 139.6, 137.7, 137.0, 135.8, 130.6, 129.8, 127.6,

127.4, 123.7, 123.3, 122.9, 122.0, 30.8 (d, J_C,P=17.6 Hz), 28.3, 26.8, 26.7,

25.3, 18.6, 17.4 ppm; ³¹P NMR (CDCl₃, 202 MHz): d=32.0 ppm.

Cl₂Ru=CHPhACTHERU(NG 4e)PCy₃AHCTRE(UGN 6e): N,N’-Bis(2,6-dimethyl-4-thiomethylphenyl)-

imidazolinium chloride (163 mg, 0.40 mmol, 2equiv), KO tBu (45 mg,

0.40 mmol, 2equiv), dichlorobenzylidenebis(tricyclohexylphosphine)ru-

thenium (165 mg, 0.20 mmol, 1 equiv). Yield: 111 mg (61%); red solid.

¹H NMR (CDCl₃, 500 MHz): d=19.08 (s, 1H, RuCH), 8.94 (brs, 1H, o-

H benzylidene), 7.31 (t, J=7.0 Hz, 1H, p-H benzylidene), 7.09 (t, J=

7.0 Hz, 2H, m-H benzylidene), 7.0 (brs, 1H, o-H benzylidene), 6.96 (s,

2H, m-aryl_NHC), 6.75 (brs, 1H, m-aryl_NHC), 5.75 (s, 1H, m-aryl_NHC), 3.7–

3.9 (m, 4H, NCHCHN), 2.56 (brs, 3H, ArCH₃), 2.51 (brs, 3H, ArCH₃),

2.41 (s, 3H, SMe), 2.18 (s, 3H, SMe), 2.12 (m, 3H, PCH), 2.11 (brs, 3H,

ArCH₃), 1.3–1.5 (m, 15H, Cy), 0.8–0.9 ppm (m, 15H, Cy); ¹³C NMR

(CDCl₃, 126 MHz): d=150.3, 138.2, 137.2, 135.6, 133.5, 127.1, 124.8,

124.0, 54.7, 51.2, 30.5 (d, J_C,P=17.6 Hz), 28.0, 26.8, 26.7, 25.3, 19.1, 17.8,

14.0, 13.7 ppm; RuCH and RuC_NHCwere not observed; ³¹P NMR

(CDCl₃, 202 MHz): d=29.6 ppm.

Cl₂Ru=CHPh

lium chloride (210 mg, 0.37 mmol, 1.5 equiv), KOtBu (42mg, 0.37 mmol,

1.5 equiv), dichlorobenzylidenebis(tricyclohexylphosphine)ruthenium

ACHTREUNG(3i)PCy₃ACHTREUNG(5i): N,N’-Bis(2,6-dimethyl-4-iodophenyl)imidazo-

(204 mg, 0.25 mmol, 1 equiv). Yield: 200 mg (75%); pink solid. ¹H NMR

(CDCl₃, 500 MHz): d=19.43 (s, 1H, RuCH), 8.9 (brs, 1H, o-H benzyli-

dene), 7.60 (s, 2H, m-H aryl_NHC), 7.50 (t, 1H, p-H benzylidene), 7.25

(brs, 4H, o-H benzylidene + m-H benzylidene + m-H aryl_NHC), 6.98 (m,

1H, NCHCHN), 6.95 (m, 1H, NCHCHN), 6.5 (brs, 1H, m-H aryl_NHC),

2.5 (brs, 9H ArCH₃), 2.23 (q, 3H, PCH), 2.0 (brs, 3H, ArCH₃), 1.57 (brs,

9H, Cy), 1.42(brs, 6H, Cy), 0.9–1.1 ppm (m, 15H, Cy); ¹³C NMR

(CDCl₃, 126 MHz): d=294.4, 189.1 (d, J_C,P=82Hz), 150.5, 139.6, 137.8,

136.6, 135.8, 127.6, 127.5, 123.6, 123.2, 95.6, 94.9, 30.7 (d, J_P,C=17.6 Hz),

28.3, 26.8, 26.7, 26.3, 26.1, 26.0, 25.9, 18.3, 17.2 ppm; ³¹P NMR (CDCl₃,

202 MHz): d=31.9 ppm.

Cl₂Ru=CHPhACTHERU(NG 4 f)PCy₃AHCTRE(UGN 6 f): N,N’-Bis(2,6-dimethyl-4-fluorophenyl)imida-

zolinium chloride (256 mg, 0.73 mmol, 2 equiv), KOtBu (82mg,

0.73 mmol, 2equiv), dichlorobenzylidenebis(tricyclohexylphosphine)ru-

thenium (300 mg, 0.365 mmol, 1 equiv). R_f=0.21; yield: 187 mg (60%);

pink solid. ¹H NMR (CDCl₃, 500 MHz): d=19.17 (s, 1H, RuCH), 9.00

(brs, 1H, o-H benzylidene), 7.40 (t, J=7.5 Hz, 1H, p-H benzylidene),

7.14 (t, J=7.5 Hz, 2H, m-H benzylidene), 7.05 (brs, 1H, o-H benzlidene),

6.89 (d, JCATHRE(UNG H,F)=8.5 Hz, 2H, m-H aryl_NHC), 6.63 (brs, 1H, m-H aryl_NHC),

Cl₂Ru=CHPhACHTREUNG(4a)PCy₃ACHTREUNG(6a): N,N’-Bis(2,6-dimethyl-4-N,N’-diethylamino-

5.67 (brs, 1H, m-H aryl_NHC), 3.8–4.1 (m, 4H, NCH₂CH₂N), 2.4–2.8 (m,

9H, ArCH₃), 2.18 (q, 3H, PCH), 2.06 (brs, 3H, ArCH₃), 1.5 (brs, 9H,

Cy), 1.4 (brs, 6H, Cy), 1.0 (brs, 9H, Cy), 0.9 ppm (brs, 6H, Cy);

¹³C NMR (CDCl₃, 126 MHz): d=293.5, 220.6 (d, J_C,P=77 Hz), 161.8 (d,

phenyl)imidazolinium chloride (808 mg, 1.77 mmol, 2equiv), KO tBu

(199 mg, 1.77 mmol, 2equiv), dichlorobenzylidenebis(tricyclohexylphos-

phine)ruthenium (727 mg, 0.88 mmol, 1 equiv). R_f=0.29; yield: 660 mg

(78%); brown solid. ¹H NMR (CDCl₃, 500 MHz): d=19.14 (s, 1H,

RuCH), 9.1 (brs, 1H, o-H benzylidene), 7.30 (t, J=7.5 Hz, 1H, p-H ben-

zylidene), 7.1 (m, 3H, o- + m-H benzylidene), 6.45 (s, 2H, m-H aryl_NHC),

6.2(brs, 1H, m-H aryl_NHC), 5.3 (brs, 1H, m-H aryl_NHC), 3.9 (brs, 4H,

NCH₂CH₂N), 3.37 (q, 4H, CH₂CH₃), 3.12(brs, 4H, CH ₂CH₃), 2.4–2.7

(brs, 9H, ArCH₃), 2.23 (m, 3H, PCH), 2.0 (brs, 3H, ArCH₃), 1.3–1.6 (m,

15H, Cy), 1.22 (t, 6H, CH₂CH₃), 1.11 (t, 6H, CH₂CH₃), 0.9–1.1 ppm (m,

15H, Cy); ¹³C NMR (CDCl₃, 126 MHz): d=292.7, 219.9 (d, J_C,P=78 Hz),

150.7. 146.6, 146.0, 139.1, 137.0, 127.4, 126.6, 215.0, 109.8, 108.8, 51.7,

50.6, 42.9, 42.8, 30.4 (d, J_C,P=16 Hz), 28.1, 26.8, 26.7, 25.9, 25.6, 25.4, 21.3,

19.7, 18.3, 11.9 ppm; ³¹P NMR (CDCl₃, 202 MHz): d=29.3 ppm.

J

_C,F=79 Hz), 159.8 (d, J_C,F=79 Hz), 150.2, 140.9, 138.5, 133.4 (d, J_C,F

250 Hz), 127.6, 127.1, 114.5 (d, J_C,F=22 Hz), 113.7 (d, J_C,F=22 Hz), 51.1,

50.3, 30.6 (d, _C,P=17 Hz), 28.1, 26.7 (d, _C,P=12Hz), 25.1, 19.4,

17.9 ppm; ³¹P NMR (CDCl₃, 202 MHz): d=29.3 ppm.

Cl₂Ru=CHPh(4g)PCy₃(6g): N,N’-Bis(2,6-dimethyl-4-chlorophenyl)imi-

=

J

A

ACHTREUNG

dazolinium chloride (384 mg, 1.00 mmol, 2equiv), KO tBu (112mg,

1.00 mmol, 2equiv), dichlorobenzylidenebis(tricyclohexylphosphine)ru-

thenium (412mg, 0.50 mmol, 1 equiv). R_f=0.25; yield: 307 mg (69%);

pink solid. ¹H NMR (CDCl₃, 500 MHz): d=19.08 (s, 1H, RuCH), 8.90

(brs, 1H, o-H benzylidene), 7.37 (t, J=7.5 Hz, 1H, p-H benzylidene),

7.11 (m, 4H, m-H benzylidene + m-H aryl_NHC), 6.9 (m, 2H, o-H benzyli-

Cl₂Ru=CHPhACHTREUNG(4b)PCy₃CAHTRE(UGN 6b): N,N’-Bis(2,6-dimethyl-4-methoxyphenyl)-

dene

+ m-H aryl_NHC), 5.89 (brs, 1H, m-H aryl_NHC), 3.7–3.9 (m, 4H,

imidazolinium chloride (150 mg, 0.40 mmol, 2equiv), KO tBu (45 mg,

0.40 mmol, 2equiv), dichlorobenzylidenebis(tricyclohexylphosphine)ru-

thenium (165 mg, 0.20 mmol, 1 equiv). Yield: 99 mg (56%); red solid.

¹H NMR (CDCl₃, 500 MHz): d=19.15 (s, 1H, RuCH), 9.0 (brs, 1H, o-H

benzylidene), 7.35 (t, J=8.0 Hz, 1H, p-H benzylidene), 7.1 (brs, 3H, m-

H benzylidene + o-H benzylidene), 6.71 (s, 2H, m-H aryl_NHC), 6.5 (brs,

1H, m-H aryl_NHC), 5.5 (brs, 1H, m-H aryl_NHC), 3.9–4.0 (brs, 4H,

NCH₂CH₂N), 3.80 (s, 3H, OMe), 3.51, (s, 3H, OMe), 2.4–2.8 (brs, 9H,

ArCH₃), 2.19 (q, 3H, PCH), 2.1 (brs, 3H, ArCH₃), 1.3–1.6 (m, 15H, Cy),

0.8–1.0 ppm (m, 15H, Cy); ¹³C NMR (CDCl₃, 126 MHz): d=293.1, 220.0

(d, J_C,P=77 Hz), 158.0. 157.4, 139.7, 137.4, 131.7, 131.2, 129.6, 128.6,

127.0, 125.1, 112.8, 111.2, 54.0, 53.6, 51.3, 50.4, 30.5 (d, J_C,P=16 Hz), 28.1,

26.8, 26.7, 25.2, 19.5, 18.0 ppm; ³¹P NMR (CDCl₃, 202 MHz): d=

29.3 ppm.

NCH₂CH₂N), 2.52 (brs, 3H, ArCH₃), 2.47 (brs, 6H, ArCH₃), 2.12 (q,

3H, PCH), 2.0 (brs, 3H, ArCH₃), 1.2–1.4 (m, 15H, Cy), 0.8–0.9 ppm (m,

15H, Cy); ¹³C NMR (CDCl₃, 126 MHz): d=293.6, 220.4 (d, J_C,P=77 Hz),

150.0, 140.1, 138.1, 136.9, 135.0, 133.5, 132.5, 130.8, 128.7, 128.0, 127.6,

127.1, 51.0, 50.9, 30.5 (d, J_C,P=16.3 Hz), 28.2, 28.0, 26.7, 26.6, 25.9, 25.3,

25.2, 19.1, 17.6 ppm; ³¹P NMR (CDCl₃, 202 MHz): d=29.8 ppm.

Cl₂Ru=CHPhACTHERNGU(4h)PCy₃ACHTRE(UGN 6h): N,N’-Bis(2,6-dimethyl-4-bromophenyl)imi-

dazolinium chloride (149 mg, 0.32mmol, 2equiv), KO tBu (35 mg,

0.32mmol, 2equiv), dichlorobenzylidenebis(tricyclohexylphosphine)ru-

thenium (130 mg, 0.16 mmol, 1 equiv). R_f=0.20; yield: 126 mg (81%);

1

pink solid. H NMR (CDCl₃, 500 MHz): d=19.14 (s, 1H, RuCH), 8.97 (s,

1H, o-H benzylidene), 7.44 (t, J=7.8 Hz, 1H, p-H benzylidene), 7.35 (s,

2H, m-H aryl_NHC), 7.0–7.2(brs, 4H, o-H benzylidene + m-H benzyli-

dene

+ m-H aryl_NHC), 6.13 (brs, 1H, m-H aryl_NHC), 3.7–4.1 (m, 4H,

Cl₂Ru=CHPh

ACHTREUNG(4d)PCy₃ACHTREUNG(6d): N,N’-Bis(2,6-dimethylphenyl)imidazolinium

NCH₂CH₂N), 2.78 (brs, 3H ArCH₃), 2.5–2.6 (m, 6H, ArCH₃), 2.19 (q,

3H, PCH), 2.03 (brs, 3H, ArCH₃), 1.2–1.6 (m, 15H, Cy), 0.8–1.1 ppm (m,

15H, Cy); ¹³C NMR (CDCl₃, 126 MHz): d=293.7, 220.4 (d, J_C,P=77 Hz),

150.1, 137.5, 135.6, 131.0, 130.2, 127.7, 127.3, 122.1, 121.2, 50.9, 50.2, 30.6

chloride (157 mg, 0.5 mmol, 2equiv), KO tBu (56 mg, 0.5 mmol, 2equiv),

dichlorobenzylidenebis(tricyclohexylphosphine)ruthenium

(205 mg,

0.25 mmol, 1 equiv). R_f=0.31; yield: 105 mg (51%); red solid. ¹H NMR

(CDCl₃, 500 MHz): d=19.16 (s, 1H, RuCH), 9.0 (brs, 1H, o-H benzyli-

dene), 7.33 (t, J=7.0 Hz, 1H, p-H benzylidene), 7.22 (m, 3H, m-H ben-

zylidene + aryl_NHC), 7.08 (s, 3H, aryl_NHC), 7.0 (brs, 1H, o-H benzyli-

(d,

J_C,P=17 Hz), 28.3, 26.8 (d, J_C,P=10 Hz), 25.3, 19.0, 17.7 ppm;

³¹P NMR (CDCl₃, 202 MHz): d=30.0 ppm.

5478

www.chemeurj.org

ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Chem. Eur. J. 2008, 14, 5465 – 5481

N-Heterocyclic Carbenes

FULL PAPER

Cl₂Ru=CHPh

linium chloride (296 mg, 0.52 mmol, 2 equiv), KOtBu (59 mg, 0.52mmol,

2equiv), dichlorobenzylidenebis(tricyclohexylphosphine)ruthenium

(215 mg, 0.26 mmol, 1 equiv). R_f=0.23; yield: 137 mg (49%); pink solid.

¹H NMR (CDCl₃, 500 MHz): d=19.14 (s, 1H, RuCH), 8.96 (brs, 1H, o-

H benzylidene), 7.55 (s, 2H, m-H aryl_NHC), 7.46 (t, J=7.3 Hz, 1H, p-H

A

(6i): N,N’-Bis(2,6-dimethyl-4-iodophenyl)imidazo-

11.9 ppm; ³¹P NMR (CDCl₃, 202 MHz): d=29.8 (major isomer),

28.6 ppm (minor isomer).

Oxidation and reduction of 6m: Compound 6m (140 mg, 0.171 mmol,

1 equiv) and ferrocenium tetrafluoroborate (47 mg, 0.171 mmol, 1 equiv)

were placed in a Schlenk tube and dissolved in dry and degassed CH₂Cl₂

(5 mL). The solution was stirred for 1 h at room temperature and the vol-

atiles were removed in vacuo. The residue was suspended in toluene

(5 mL), vigorously stirred for 30 min and filtered off. The orange-brown

solid was washed with pentane until the washings remained colorless and

dried in vacuo. Yield: 124 mg (80%). Some of this material (35 mg,

0.039 mmol, 1 quiv) was placed in a Schlenk tube, dissolved in dry and

degassed CH₂Cl₂(3.5 mL) and cooled to À788C. Octamethylferrocene

(12mg, 0.039 mmol, 1 quiv) was added and the mixture was stirred for

30 min at À788C. During that time, a color change from dark red to a

dark green-blue occurred. The reaction mixture was poured onto a

column (silica gel) and eluted with Et₂O (50 mL precooled to À308C).

The volatiles were removed in vacuo at À788C and the residue was dis-

solved in CD₂Cl₂(0.8 mL, precooled to À788C) for NMR analysis. The

filtrate is free from paramagnetic impurities.

benzylidene), 7.1–7.4 (m, 3H, m-H benzylidene

+ o-H benzylidene),

7.01 (brs, 1H, m-H aryl_NHC), 6.35 (brs, 1H, m-H aryl_NHC), 3.7–4.1 (brs,

4H, NCH₂CH₂N), 2.74 (brs, 3H, ArCH₃), 2.4–2.6 (m, 6H, ArCH₃), 2.19

(m, 3H, PCH), 2.01 (brs, 3H, ArCH₃), 1.2–1.6 (m, 15H, Cy), 0.8–1.1 ppm

(m, 15H, Cy); ¹³C NMR (CDCl₃, 126 MHz): d=294.8, 221.3 (d, J_C,P

=

77 Hz), 151.1, 141.8, 139.6, 139.4, 138.1, 137.5, 137.1, 128.7, 128.5, 96.0,

95.2, 31.7 (d, J_C,P=17 Hz), 29.4, 29.1, 27.8 (d, J_C,P=8 Hz), 26.5, 19.9,

18.6 ppm; ³¹P NMR (CDCl₃, 202 MHz): d=29.9 ppm.

Cl₂Ru=CHPhACHTREUNG(4m)PCy₃HCATRE(UGN 6m): N-(2,6-Dimethylphenyl)-N’-(2,6-dimethyl-

4-diethylaminophenyl)imidazolinium chloride (386 mg, 1.00 mmol,

2equiv), KO tBu (112mg, 1.00 mmol, 2equiv), dichlorobenzylidenebis-

(tricyclohexylphosphine)ruthenium (412mg, 0.50 mmol, 1 equiv). R_f=

1

0.29; yield: 421 mg (94%); brown solid. H NMR (CDCl₃, 500 MHz): d=

CCDC 665252 contains the supplementary crystallographic data for this

paper. These data can be obtained free of charge from The Cambridge

Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.

(mixture of 2isomers) 19.19 (s, RuCH, minor isomer), 19.11 (s, RuCH,

major isomer), 9.00 (brs, o-H benzylidene), 7.27–7.33 (m, p-H benzyli-

dene + p-H aryl_NHC(H)), 7.18–7.23 (m, m-H benzylidene), 7.06 (brs, o-H

benzylidene + m-H aryl_NHC(H)), ca. 6.9 (brs, m-H aryl_NHC), 6.55 (t, J=

7.3 Hz, p-H aryl_NHC), 6.44 (s, m-H aryl_{NHC(NEt )}), ca. 6.2(brs, m-H aryl_NHC),

AHCTREUNG

2

5.33 (brs, m-H aryl_NHC), 3.7–4.1 (m, NCH₂CH₂N), 3.36 (q, CH₂CH₃), 3.11

(q, CH₂CH₃), 2.65 (brs, ArCH₃), ca. 2.5 (brs, ArCH₃), 2.18 (m, PCH), ca

2.1 (brs, ArCH₃), 1.49 (brs, Cy), 1.37 (brs, Cy), 1.20 (t, CH₂CH₃), 1.09 (t,

CH₂CH₃), 0.8–1.1 ppm (m, Cy); ¹³C NMR (CDCl₃, 126 MHz): d=220.2

(d, J_C,P=79 Hz), 218.9 (d, J_C,P=79 Hz), 150.6, 146.7, 146.2, 138.6, 136.8,

128.0, 127.9, 127.2, 127.2, 126.9, 126.9, 126.7, 109.9, 51.8, 50.9, 50.8, 50.2,

42.9, 42.8, 30.6 (d, J_C,P=16 Hz), 30.5 (d, J_C,P=16 Hz), 28.1, 28.8, 28.7,

25.4, 24.2, 19.7, 19.1, 18.3, 13.1, 11.9 ppm; ³¹P NMR (CDCl₃, 202 MHz):

d=28.5 (minor isomer), 27.8 ppm (major isomer).

Acknowledgements

This work was supported by the Deutsche Forschungsgemeinschaft and

the Fonds der Chemischen Industrie. We wish to thank Ms. I. Svoboda

for the crystal structure analysis and Prof. Dr. R. Schäfer for helpful dis-

cussions. C.M.T. thanks Professor M. Reggelin for helpful discussions and

for his support.

Cl₂Ru=CHPhACHTREUNG(4n)PCy₃ACHTREUNG(6n): N-(2,6-Dimethylphenyl)-N’-(2,6-dimethyl-4-

bromophenyl)imidazolinium chloride (310 mg, 0.787 mmol, 2equiv),

KOtBu (88 mg, 0.787 mmol, 2equiv), dichlorobenzylidenebis(tricyclohex-

ylphosphine)ruthenium (324 mg, 0.394 mmol, 1 equiv). R_f=0.29; yield:

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1

255 mg (72%); pink solid. H NMR (CDCl₃, 500 MHz): d=(mixture of 2

isomers) 19.20 (s, RuCH, major isomer), 19.12 (s, RuCH, minor isomer),

9.01 (brs, o-H benzylidene), 7.44 (t, J=7.5 Hz, p-H benzylidene), 7.35 (s,

m-H aryl_NHC(Br), minor isomer), 7.33 (t, J=7.5 Hz, m-H benzylidene),

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PCH), 2.06 (brs, 3H, ArCH₃), 1.2–1.6 (brs, Cy), 0.7–1.1 ppm (brs, Cy);

¹³C NMR (CDCl₃, 126 MHz): d=293.4, 219.7 (d, J_C,P=77 Hz), 150.3,

150.1, 138.2, 137.6, 136.3, 130.9, 130.1, 128.3, 128.0, 127.5, 127.4, 127.3,

127.2, 122.0, 121.1, 51.0, 50.3, 50.1, 30.6 (d, J_C,P=17 Hz), 30.5 (d, J_C,P

=

17 Hz), 28.1, 26.8, 26.7, 19.1, 19.1, 17.8 ppm; ³¹P NMR (CDCl₃,

202 MHz): d=29.6 (major isomer), 29.2 ppm (minor isomer).

Cl₂Ru=CHPh

ACHTREUNG(4o)PCy₃ACHTRENUG

dimethyl-4-diethylaminophenyl)imidazolinium

0.47 mmol, 2equiv), KO tBu (53 mg, 0.47 mmol, 2equiv), dichlorobenzyli-

denebis(tricyclohexylphosphine)ruthenium (193 mg, 0.23 mmol, 1 equiv).

R_f=0.30; yield: 163 mg (72%); brown solid. ¹H NMR (CDCl₃,

500 MHz): d=(mixture of 2isomers) 19.14 (s, RuCH, major isomer),

18.98 (s, RuCH, minor isomer), 9.01 (brs, o-H benzylidene), 7.43 (t, J=

7.5 Hz, p-H benzylidene), 7.34 (s, arom., minor isomer), 7.19 (t, J=

7.5 Hz, m-H benzylidene), 7.07 (brs, orho H benzylidene

+ m-H ar-

yl_NHC), 6.43 (s, m-H aryl_NHC(NEt2), major isomer), 6.58 (t, 8.2Hz, p-H ar-

yl_NHC, major isomer), 6.13 (brs, m-H aryl_NHC), 3.7–4.1 (m, NCH₂CH₂N),

3.35 (q, CH₂CH₃), 3.11 (q, CH₂CH₃), 2.4–2.8 (m, ArCH₃), 2.1–2.3 (m,

PCH), 2.06 (brs, ArCH₃), 1.3–1.6 (brs, Cy), 1.20 (t, CH₂CH₃), 1.09 (t,

CH₂CH₃), 0.88 (t, CH₂CH₃), 0.8–1.1 ppm (brs, Cy); ¹³C NMR (CDCl₃,

126 MHz): d=292.6, 220.6 (d, J_C,P=77 Hz), 219.4 (d, J_C,P=77 Hz),150.5,

150.3, 146.8, 146.2, 137.9, 136.1, 130.9, 130.4, 127.3, 127.2, 127.0, 124.5,

121.8, 120.9, 109.9, 51.9, 50.8, 50.0, 42.9, 42.8, 30.5 (d, J_C,P=15 Hz), 30.2

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5480

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Chem. Eur. J. 2008, 14, 5465 – 5481

N-Heterocyclic Carbenes

FULL PAPER

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Received: January 23, 2008

Published online: May 6, 2008

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Article Doi

π-face donor properties of N-heterocyclic carbenes in grubbs II complexes

DOI: 10.1002/chem.200800139

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