Salicylaldimine ruthenium alkylidene complexes: Metathesis catalysts tuned for protic solvents

Article abstract of DOI:10.1002/adsc.200600264

Tuning the electronic and steric environment of olefin metathesis catalysts with specialized ligands can adapt them to broader applications, including metathesis in aqueous solvents. Bidentate salicylaldimine ligands are known to stabilize ruthenium alkylidene complexes, as well as allow ringclosing metathesis in protic media. Here, we report the synthesis and characterization of exceptionally robust olefin metathesis catalysts bearing both bidentate salicylaldimine and N-heterocyclic carbene ligands, including a trimethylammonium-functionalized complex adapted for polar solvents. NMR spectroscopy and X-ray crystallographic analysis confirm the structures of the complexes. Although the N-heterocyclic carbene-salicylaldimine ligand combination limits the activity of these catalysts in non-polar solvents, this pairing enables efficient ring-closing metathesis of both dienes and enynes in methanol and methanol/water mixtures under air.

Full text of DOI:10.1002/adsc.200600264

FULL PAPERS
DOI: 10.1002/adsc.200600264
Salicylaldimine Ruthenium Alkylidene Complexes: Metathesis
Catalysts Tuned for Protic Solvents
Joseph B. Binder,a Ilia A. Guzei,a and Ronald T. Rainesa,b,*
a
Department of Chemistry, University of Wisconsin–Madison, 1101 University Avenue, Madison, WI 53706-1322, USA
Department of Biochemistry, University of Wisconsin–Madison, 433 Babcock Drive, Madison, WI 53706-1544, USA
b
Fax : (+1)-608-262-3453; e-mail: raines@biochem.wisc.edu
Received: June 3, 2006; Revised: November 3, 2006
Supporting information for this article is available on the WWW under http://asc.wiley-vch.de/home/.
Abstract: Tuning the electronic and steric environ- ized complex adapted for polar solvents. NMR spec-
ment of olefin metathesis catalysts with specialized troscopy and X-ray crystallographic analysis confirm
ligands can adapt them to broader applications, in- the structures of the complexes. Although the N-het-
cluding metathesis in aqueous solvents. Bidentate erocyclic carbene–salicylaldimine ligand combination
salicylaldimine ligands are known to stabilize ruthe- limits the activity of these catalysts in non-polar sol-
nium alkylidene complexes, as well as allow ring- vents, this pairing enables efficient ring-closing meta-
closing metathesis in protic media. Here, we report thesis of both dienes and enynes in methanol and
the synthesis and characterization of exceptionally methanol/water mixtures under air.
robust olefin metathesis catalysts bearing both biden-
tate salicylaldimine and N-heterocyclic carbene li- Keywords: metathesis; N-heterocyclic carbenes;
gands, including a trimethylammonium-functional- ruthenium; Schiff bases; water; X-ray diffraction
Introduction
plexes display impressive stability and activity, and
their readily accessible imine ligands lend themselves
As an efficient, selective means for forming carbon- to catalyst tuning.[14] In addition, we were encouraged
carbon bonds, olefin metathesis has revolutionized or- by the capacity of phosphine-bearing catalysts similar
ganic synthesis and polymer chemistry.[1] The high cat- to 6a to perform ring-closing metathesis (RCM) in
alytic activity and functional group tolerance of well- methanol.[15] Aware of the benefits that N-heterocyclic
defined ruthenium-based complexes 1–4a allow exten- carbene (NHC) ligands provide other ruthenium
sive use of metathesis chemistry (Figure 1).[2–5] Still metathesis catalysts, we chose to investigate salicylal-
more diverse utilization of metathesis necessitates cat- dimine complexes such as 7a, b reported by Verpoort
alysts tailored to specific applications.[6] For instance, and co-workers.[16–18] According to their work, this
aqueous olefin metathesis is attractive for carbon- family of NHC-imine complexes efficiently catalyzes
carbon bond formation in biological applications[7] ring-opening metathesis polymerization (ROMP),
and green chemistry,[8,9] yet complexes 1–4a are in- RCM, and Kharasch addition. Imine-bearing rutheni-
soluble in water and soluble versions such as 5 have um complexes are proposed to enter the olefin meta-
proven to be unstable to air and incompatible with in- thesis catalytic cycle with decoordination of the imine
ternal olefins.[10–12] Grubbs and co-workers have nitrogen rather than the phosphine dissociation ob-
begun to address this challenge by modifying the served for initiation of 1 and 2.[14,17] As a result, we ex-
local environment of the catalyst with a polyethylene pected that an NHC ligand such as 1,3-dimesityl-4,5-
glycol-bearing ligand, as in complex 4b.[9] Here, our dihydroimidazol-2-ylidene (H2IMes) could beneficial-
intent is to complement this approach by tuning the ly replace the spectator tricyclohexylphosphine of 6a–
primary coordination sphere of the catalyst to the de- c, leading to improved thermal stability and enhanced
mands of aqueous media.
olefin metathesis activity.[19,20] This report details our
With the goal of creating designer catalysts for synthesis of NHC-imine metathesis catalysts and the
aqueous metathesis, we were attracted to reported crystal structure of a member of this class, complex
ruthenium complexes containing bidentate salicylald- 7a, as well as the activity of water-adapted complex
imine ligands, such as 6a, b and 7a, b.[13] These com- 7c.
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Joseph B. Binder et al.
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Figure 1. Olefin metathesis catalysts.
Results and Discussion
the green hexacoordinate starting material quickly
formed wine-red complexes 7a–c as pyridine and
chloride were displaced by the bidentate ligand. After
the removal of thallium chloride and excess ligand,
Synthesis of NHC-Salicylaldimine Complexes
As shown in Scheme 1, our initial efforts concentrated complexes 7a–c were isolated in good (76–87%)
on using the standard means for introducing the yield. Notably, this robust route even allowed access
H2IMes ligand, here, in situ formation of the free car- to cationic complex 7c with polar ligand 9c in DMF.
bene from the imidazolinium salt with potassium tert-
Manipulation of the NHC-imine complexes 7a–c re-
butoxide and subsequent reaction with the phosphine- vealed their dramatic stability to air, heat, and water.
bearing ruthenium complex 6a.[17] Attempts to pre- These complexes tolerate storage for months as solids
pare reported complex 7a by this published method under vacuum without degradation as judged by
resulted, however, in decomposition of the ruthenium 1H NMR spectroscopy. Although 2 decomposes com-
benzylidene starting material. Faced with this road- pletely in less than 1 day in C6D6 at 558C under air,
block, we chose to approach 7a–c by introducing sali- ꢀ 95% of complex 7b is intact after 8 days according
1
cylaldimine ligands to a complex already bearing the to H NMR spectroscopy. Complex 7c tolerates water,
H2IMes ligand. Alkoxide and aryloxide ligands, as and remains ~40% intact after 2 days in CD3OD/
well as neutral donors, react readily with bis(pyridine) D2O (3:1) under air at 208C (according to integration
G
1
precursor 8.[21] The chloride ligands in 8 are far more of ArCH3 and alkylidene H NMR signals). In con-
labile than those in 2 and are displaced easily by in- trast, previous catalysts bearing ligands with cationic
coming ligands, especially with the assistance of thalli- functional groups (such as 5[12]) decompose in minutes
um ion. Accordingly, the thallium salts of salicylald- when exposed to traces of oxygen in solution.
imine ligands 9a–c were prepared in situ with thallium
Interestingly, no exchange of the alkylidene proton
ethoxide and reacted with 8 (Scheme 1). Gratifyingly, of complex 7c with solvent deuterons is observed in
396
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Salicylaldimine Ruthenium Alkylidene Complexes
FULL PAPERS
Scheme 1. Synthesis of NHC-imine complexes 7a–c by the reported route from 6a[17] and our route from 8.
CD3OD/D2O (3:1). Such H/D exchange is observed resonance appears at d=18.56 for 7a while the litera-
for ruthenium alkylidene and vinylidene complexes ture reports d=19.42, which is nearly the same as
without NHC ligands, including 5 and 6a, b,[23] but not that reported for the parent phosphine-bearing com-
1
for complex 4b.[9] Ligand electronics are known to plex 6a.[14] Evidenced by H and 13C NMR spectra of
affect the rate of alkylidene H/D exchange,[23] and the 1–4a, replacement of the tricyclohexylphosphine
presence of the NHC ligand could slow this process.
ligand with H2IMes typically results in a shielding
effect on the alkylidene proton that shifts its reso-
nance approximately 0.9 ppm upfield.[2–5] In addition,
the observed H NMR spectrum of 7a includes eight
distinct ArCH3 singlets, consistent with the asymme-
try of the putative structure, while the literature spec-
1
Spectral and Structural Characterization of NHC-
Salicylaldimine Complexes
1H and 13C NMR spectral data for complexes 7a, b trum reveals only six ArCH3 resonances, including an
synthesized by the route in Scheme 1 were consistent aliphatic three-proton doublet. No coalescence of the
with their putative structures, but surprisingly did not methyl resonances of 7a was observed at elevated
match those previously reported for these com- temperature (558C in CDCl3 or 708C in C6D6). Spec-
plexes.[17] As listed in Table 1, the benzylidene proton tra for 7b, c agree with these findings, with both com-
Table 1. 1H NMR (CDCl3) data for 7a and related complexes.
Compound
d (ppm), Ru=CHAr
d (ppm), ArCH3
7a
18.56 (s, 1H)
2.60 (s, 3H), 2.41 (s, 3H), 2.28 (s, 3H), 2.25 (s, 3H), 2.19 (s, 3H), 2.00 (s, 3H),
1.46 (s, 3H), 1.04 (s, 3H)
2.49 (s, 6H), 2.31 (s, 3H), 2.20 (s, 6H), 2.09 (s, 3H), 1.76 (s, 3H), 1.72 (d, 3H)
2.45 (q, 3H), 2.33 (s, 3H), 1.78 (d, 3H), 1.71–1.14 (m, 30H)
2.62–1.15 (m, 66H)
2.56–0.15 (m, 51H)
2.37–1.20 (m, 33H), 1.80 (d, 6H)
2.48 (s, 12H), 2.40 (s, 6H), 1.27 (d, 6H)
7a[17]
19.42 (s, 1H)
19.45 (d, 1H)
20.02 (s, 1H)
19.16 (s, 1H)
17.44 (d, 1H)
16.56 (s, 1H)
6a[14]
1[a]
2[a]
3[3]
,
,
[2]
[4]
4a[5]
[a]
In CD2Cl2.
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397

Joseph B. Binder et al.
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reaction solvent to hexanes and the base to the more
soluble potassium tert-amylate enhanced the synthesis,
enabling the clean conversion of 6a to 7a as con-
firmed by 1H NMR. Once again, spectral characteriza-
tion failed to reveal the signals of the proposed imine
complex reported by De Clercq and Verpoort.[17] Al-
though these results confirm that 7a is the authentic
product of both our synthesis from H2IMes complex 8
and the preparation from 6a, the identity of the prod-
uct reported previously[17] still remains unclear.
After we submitted this manuscript, Verpoort and
co-workers reported the synthesis of complex 7b and
three similar NHC-imine complexes by a route similar
to ours.[28] These workers found the RCM activity of
their NHC-imine complexes in non-polar solvents to
be similar to that of the NHC-imine complexes re-
ported herein (vide infra). Although they confirmed
the structure of these complexes by X-ray crystallo-
graphic analysis, they did not comment on the dis-
crepancies with their earlier paper[17] nor did they ex-
amine RCM catalysis in an aqueous environment.
Figure 2. Solid-state molecular structure of complex 7a. Hy-
drogen atoms are omitted for clarity. Thermal ellipsoids are
shown at 50% probability. Selected bond lengths (Š) and
À
À
À
angles (8): Ru C37 1.838(2), Ru C16 2.032(2), Ru Cl
À
À
À
2.0530(15), Ru O 2.1080(18), Ru N1 2.3976(6), C16 N2
À
À
À
1.347(3), C16 N3 1.346(3), C7 N1 1.301(3), C37 H37
À
À
À
À
À
0.9500, C37 C38 1.476(3); C37 Ru C16 98.28(9), C37 Ru
À
À
À
À
O 98.70(9), C16 Ru N1 158.34(8), O Ru N1 89.40(6),
À
À
À
À
À
À
C37 Ru Cl 88.79(8), C16 Ru O 83.79(7), C37 Ru N1
À
À
À
À
À
103.07(8), C16 Ru Cl 94.73(6), O Ru Cl 172.50(4), N1
À
Ru Cl 89.35(5).
Ring-Closing Metathesis Activity of 7a–c
plexes displaying benzylidene proton resonances Having confirmed the structure of the NHC-imine
about 0.9 ppm upfield from those of the parent com- complexes, we investigated their activity for diene
plexes[15,24] and eight distinct methyl resonances.
RCM. In contrast to previous reports,[16] complexes
To resolve this conflict, we sought crystallographic 7a–c are dramatically less active in non-polar solvents
evidence. Suitable crystals of complex 7a were grown (e.g., benzene and CH2Cl2) than their phosphine-bear-
by slow diffusion of pentane into a concentrated ben- ing counterpart, 6a (Table 2). Elevated temperatures
zene solution over a few days. The crystalline material are necessary for metathesis. Even at 558C, com-
1
both displayed a H NMR spectrum that was indistin- plexes 7a and 7b require 40–72 h to achieve a high
guishable from that of complex 7a prior to crystalliza- conversion of most substrates in toluene or benzene,
tion and retained RCM activity (vide infra). The X- but standard complexes 2, 4a, and 4c and phosphine-
ray data confirmed the structure of 7a, as shown in bearing complexes such as 6a achieve such transfor-
Figure 2 (see the Supporting Information for addition- mations in a few hours or less.[14,15] Cationic complex
al details). Both 7a and phosphine-bearing complexes 7c shows similarly low activity in benzene, requiring
similar to 6a, b adopt a distorted trigonal-bipyramidal 40 h for high conversions.
geometry; the common bond lengths and angles in
Yet, under similar conditions in methanol, com-
the two classes of complex are virtually identical.[15] plexes 7b and 7c catalyze the complete ring-closing of
As in the phosphine-bearing complexes, the anionic both polar and non-polar substrates to form five-, six-,
À
À
moieties are trans with an O Ru Cl bond angle of and seven-membered cyclic products in only 6–12 h.
172.58 in NHC-imine complex 7a. Just as in oxygen-li- Although complex 7b is less soluble in methanol than
gated complex 4a,[5] the Ru C16 bond is shorter than complex 7c, these two catalysts display similar activi-
À
that in complex 2 (2.085 Š),[25] possibly reflecting the ty, demonstrating that the trimethylammonium sub-
lesser trans influence of the imine ligand relative that stituent influences solubility more than reactivity.
of a phosphine.[26]
Unlike other metathesis catalysts adapted to protic
solvents, complex 7c does not require inert atmos-
phere conditions for catalysis of RCM. In addition,
Synthesis of Authentic 7a by Phosphine Displacement complex 7c is effective for RCM of N,N-dimethyl-
with H2IMes
N,N-diallylammonium chloride (10e), which has
proven to be recalcitrant for other catalysts.[29,30] This
Returning to the original synthesis of complex 7a, we specialized catalyst is also active in methanol/water
sought to reproduce the published results by using the mixtures, with 10 mol% of 7c achieving the highest
improved method for introducing the H2IMes ligand conversion of N,N-diallylamine hydrochloride (10d)
reported by Nolan and co-workers.[27] Switching the to RCM product 11d yet reported in an aqueous envi-
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Salicylaldimine Ruthenium Alkylidene Complexes
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Table 2. RCM of representative dienes and enynes catalyzed by ruthenium complexes under air.[a]
Substrate
Product
Solvent (Substrate Conc. [M])
Complex (mol%)
Time [h]
Conversion [%][b]
CH2Cl2 (0.01)
CH2Cl2 (0.01)
C6D5Cl (0.1)
C6D6 (0.1)
C6D6 (0.1)
C7D8 (0.05)
CD3OD (0.025)
C6D6 (0.05)
CD3OD (0.05)
2:1 CD3OD/D2O (0.025)
CD3OD (0.025)
CD3OD (0.05)
2:1 CD3OD/D2O (0.025)
2 (1)
1.5
0.16
4
99[c]
99[c]
100[d]
100[e]
90
76
94
>95
>95
60
4c (1)
6a (5)
7a (5)
7a (5)
7b (5)
7b (5)
7c (5)
7c (5)
7c (10)
7b (5)
7c (5)
7c (5)
4
72
70
23
40
12
6
9
9
6
36
34
29
CH2Cl2 (0.01)
C6D6 (0.05)
C6D6 (0.05)
C6D6 (0.05)
C7D8 (0.05)
C7D8 (0.05)
CD3OD (0.025)
C6D6 (0.05)
CD3OD (0.05)
2:1 CD3OD/D2O (0.025)
2:1 CD3OD/D2O (0.05)
2:1 CD3OD/D2O (0.025)
D2O (0.2)
4a (1)
7a (5)
7a (5)
7b (5)
7b (5)
7b (5)
7b (5)
7c (5)
7c (5)
7c (5)
7c (5)
7c (10)
4b (5)
4.5
6
26
24
6
70
9
6
6
6
6
9
91[f]
8
68
10
43 (+ 8)[g]
92
>95
7
>95
93
59
75
36
67 (+28)[h]
CD3OD (0.05)
2:1 CD3OD/D2O (0.025)
7c (5)
7c (10)
12
6
79
40
C7D8 (0.05)
CD3OD (0.025)
C6D6 (0.05)
CD3OD (0.05)
2:1 CD3OD/D2O (0.025)
C7D8 (0.05)
C7D8 (0.05)
CD3OD (0.025)
C6D6 (0.05)
CD3OD (0.05)
C7D8 (0.05)
C7D8 (0.05)
CD3OD (0.025)
C6D6 (0.05)
CD3OD (0.05)
2:1 CD3OD/D2O (0.025)
C7D8 (0.05)
C7D8 (0.05)
CD3OD (0.025)
C6D6 (0.05)
7b (5)
7b (5)
7c (5)
7c (5)
7c (5)
7a (5)
7b (5)
7b (5)
7c (5)
7c (5)
7a (5)
7b (5)
7b (5)
7c (5)
7c (5)
7c (5)
7a (5)
7b (5)
7b (5)
7c (5)
7c (5)
7c (10)
40
9
40
9
45
94
65
>95
57
6
70
40
9
40
9
40
40
5
40
7
41
10
<5
77
<5
>95
>95
95
>95
>95
85
90
90
93
89
6
40
40
9
40
7
CD3OD (0.05)
2:1 CD3OD/D2O (0.025)
>95
>95
6
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Joseph B. Binder et al.
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Table 2. (Continued)
Substrate
Product
Solvent (Substrate Conc. [M])
Complex (mol%)
Time [h]
Conversion [%][b]
C7D8 (0.05)
C7D8 (0.05)
CD3OD (0.025)
C6D6 (0.05)
CD3OD (0.05)
7a (5)
7b (5)
7b (5)
7c (5)
7c (5)
36
36
36
36
36
7
7
25
42
32
C7D8 (0.05)
C7D8 (0.05)
7a (5)
7b (5)
7b (5)
7c (5)
7c (5)
7c (10)
4d (5)
36
18
2
5
2
93
>95
90
>95
>95
>95
92[i]
CD3OD (0.025)
C6D6 (0.05)
CD3OD (0.05)
5:2 CD3OD/D2O (0.02)
5:2 CH3OH/H2O (0.02)
6
0.5
[a]
Conditions for RCM catalyzed by complexes 7a–c: 558C, air.
[b]
[c]
[d]
[e]
[f]
1
Conversions determined by H NMR spectroscopy.
208C, ref.[44]
558C, ref.[14]
558C, ref.[16]
Room temperature, ref.[45]
808C.
[g]
[h]
[i]
Room temperature, ref.[9]
258C, ref.[33]
ronment.[9,11,30–32] Moreover, 7c accomplishes this Table 3. Solvent dependence of RCM of N-tosyldiallylamine
(10c) catalyzed by complex 7b.[a]
transformation cleanly. For comparison, Grubbs and
co-workers reported 67% ring-closing of 10d along
Solvent
Dielectric constant (e)
Conversion (%)
with 28% conversion to cycloisomer 12d using 5
mol% of complex 4b in water at room temperature
over 36 h.[9] With a variety of non-polar dienes 7c also
achieves good to excellent conversions to five-, six-,
and seven-membered rings in methanol/water mix-
tures. In contrast to RCM with complexes 7a–c in
non-polar solvents or methanol, RCM in the aqueous
environment is limited by decomposition of the cata-
C6D6
CD2Cl2
CD3OD
2.28
8.9
32.6
14
36
69
[a]
Reaction conditions: 5 mol% 7b, 0.025M substrate 10c,
408C, 24 h, under air.
lyst or intermediates – in the former solvents the dependence is consistent with the initiation rate trend
intact complex is observable by NMR spectroscopy for catalysis by 2, wherein switching from toluene to
throughout the timecourse of the reaction, but in CH2Cl2 increased the initiation rate by 30%.[20] To-
methanol/water the resonances of the complex disap- gether, these data suggest that the activity of NHC-
pear.
imine complexes 7a–c is limited by their rate of initia-
Enyne metathesis by salicylaldimine alkylidene tion. Direct observations of metathesis reaction mix-
1
ruthenium complexes has not been reported previous- tures by H NMR also support this conclusion. For
ly. We found that complexes 7a–c are efficient cata- both NHC-imine catalysts and several other chelated
lysts of enyne RCM. Substrate 10k is smoothly trans- catalysts,[35] propagating alkylidene species are not de-
formed in non-polar solvents (5–36 h) and in protic tected during metathesis, and, in non-polar solvents,
1
media (2–6 h). For comparison, Grela and coworkers the H NMR signals for the complexes are typically
recently reported metathesis of 10k with complex 4d unchanged upon completion. These results imply that
in 92% conversion in an aqueous solvent.[33] With 7a– the ligand combination in 7a–c allows only a small
c, the more difficult substrate 10j also undergoes ring fraction of the complex species to participate in meta-
closure to a moderate extent.
thesis. Although NHC ligands improve metathesis
The solvent dependence of ring-closing metathesis propagation activities for ruthenium complexes, they
with the NHC-imine complexes offers an explanation tend to decrease their initiation rates drastically.[20]
for their low activity. Catalysts bearing the H2IMes Switching from a tricyclohexylphosphine spectator
ligand are typically more active in aromatic solvents ligand to an NHC could likewise decrease the initia-
than in CH2Cl2.[34] In contrast, complex 7b is more tion rate for imine complexes, such that they remain
active in more polar solvents (Table 3). This solvent largely dormant as 16-electron species.[20] Increasing
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Salicylaldimine Ruthenium Alkylidene Complexes
FULL PAPERS
ware was either oven- or flame-dried. Manipulation of or-
ganometallic compounds was performed using standard
Schlenk techniques under an atmosphere of dry Ar(g),
except as noted otherwise.
the temperature and solvent polarity apparently ena-
bles the complexes to surmount this hurdle, resulting
in the high RCM activity of 7b, c in a polar solvent.
Thus, ruthenium NHC-imine complexes are rare ex-
amples of complexes that are more active as metathe-
sis catalysts in methanol than in non-polar solvents.[31]
These characteristics of high stability and slow ini-
tiation suit salicylaldimine-based catalysts to specific
applications. For example, complexes 7b, c could
enable ROMP in protic media, although their slow in-
itiation rates are likely to result in large polydisper-
sities that could be inappropriate for some applica-
tions.[36] Similarly, the low concentrations of propagat-
ing alkylidene species produced by salicylaldimine
catalysts prevent them from supporting cross-meta-
thesis. Nonetheless, these same qualities make NHC-
salicylaldimine catalysts promising for RCM in aque-
ous environments. Slow initiation appears to protect
the complex from water while providing a steady
supply of active species with which to accomplish
RCM. These advantages could be combined with the
protective local environment provided by polyethy-
lene glycol-functionalized ligands[9] to provide even
more efficient catalysts for aqueous RCM.
The term “concentrated under reduced pressure” refers to
the removal of solvents and other volatile materials using a
rotary evaporator at water aspirator pressure (<20 torr)
while maintaining the water-bath temperature below 408C.
In other cases, solvent was removed from samples at high
vacuum (<0.1 torr). The term “high vacuum” refers to
vacuum achieved by a mechanical belt-drive oil pump.
NMR spectra were acquired with a Bruker DMX-400
Avance spectrometer (1H, 400 MHz; 13C, 100.6 MHz),
Bruker Avance DMX-500 spectrometer (1H, 500 MHz; 13C,
125.7 MHz), or Bruker Avance DMX-600 spectrometer (1H,
600 MHz) at the National Magnetic Resonance Facility at
Madison (NMRFAM). NMR spectra were obtained at ambi-
ent temperature unless indicated otherwise. Coupling con-
stants J are given in Hertz.
Mass spectrometry was performed with a Micromass LCT
(electrospray ionization, ESI) in the Mass Spectrometry Fa-
cility in the Department of Chemistry. Elemental analyses
were performed at Midwest Microlabs (Indianapolis, IN).
N-(4-Bromo-2,6-dimethylphenyl)-salicylaldimine (9a), N-
(4-bromo-2,6-dimethylphenyl)-5-nitrosalicylaldimine
(9b),
and ruthenium imine complex 6a were prepared according
to the methods of De Clercq and Verpoort.[14] (3-Formyl-4-
hydroxyphenyl)-trimethylammonium iodide was prepared
according to the method of Ando and Emoto[37] and con-
verted to the chloride salt by anion exchange. (H2IMes)-
Conclusions
(C5H5N)2(Cl)2Ru=CHPh (8) was synthesized by the method
of Grubbs and co-workers.[21]
Specialized ligands can tune the characteristics of
olefin metathesis catalysts, adapting them to demand-
ing applications, including metathesis in aqueous envi-
ronments. We prepared ruthenium complexes bearing
both NHC and salicylaldimine ligands, and confirmed
their structures by NMR spectroscopy and X-ray crys-
tallographic analysis. These new complexes initiate
slowly, but are highly effective catalysts for diene and
enyne RCM in protic media. The enhanced stability
engendered by the salicylaldimine ligands allows tri-
methylammonium-functionalized complex 7c to ach-
ieve clean RCM of N,N-diallylamine hydrochloride
(10d) with the highest conversion yet reported in an
aqueous environment. Further investigations will ad-
dress the potential of these catalysts for RCM of
more complex substrates as well as the possible en-
hancement of salicylaldimine-based catalysts with
polyethylene glycol-bearing ligands.
The following RCM substrates were obtained from com-
mercial sources and used without further purification: dieth-
yl diallylmalonate (10a), N,N-diallyl-2,2,2-trifluoroacetamide
(10b), diallyl ether (10f), and 1,7-octadiene (10h) from Al-
drich (Milwaukee, WI); N,N-diallyl-N,N-dimethylammoni-
um chloride (10e) from Fluka (Buchs, Switzerland); and di-
allyldiphenylsilane (10g) from Acros Organics (Geel, Bel-
gium).
N,N-Diallyl-4-methylbenzenesulfonamide (10c) was pre-
pared by the method of Lamaty and co-workers.[38] Diallyl-
amine hydrochloride (10d) was prepared from the corre-
sponding amine (Aldrich) by treatment with ethereal HCl.
N-(2-Propenyl)-4-methylbenzenesulfonamide was prepared
by the method of Pagenkopf and co-workers.[39]
Preparation of N-(4-Bromo-2,6-dimethylphenyl)-5-
trimethylammoniumsalicylaldimine Chloride (9c)
4-Bromo-2,6-dimethylaniline (1.003 g, 5.02 mmol) and (3-
formyl-4-hydroxyphenyl)-trimethylammonium
chloride
(1.082 g, 5.02 mmol) were dissolved in ethanol (25 mL), and
the resulting solution was stirred at reflux for 16 h. The reac-
tion mixture was allowed to cool and concentrated under re-
duced pressure. The residue was treated with hexanes
(15 mL), removed by filtration, and dried under high
vacuum to afford 9c as a yellow powder; yield: 1.544 g
(3.88 mmol, 77%). 1H NMR (CD3OD): d=8.61 (s, 1H),
8.20 (s, 1H), 8.00 (d, J=8.5 Hz, 1H), 7.31 (s, 2H), 7.22 (d,
J=8.5 Hz, 1H), 3.70 (s, 9H), 2.18 (s, 6H); 13C NMR
(CD3OD): d=168.5, 163.0, 148.4, 140.1, 132.2, 131.9, 126.3,
Experimental Section
General Considerations
Commercial chemicals were of reagent grade or better, and
were used without further purification. Anhydrous THF,
DMF, and CH2Cl2 were obtained from CYCLE-TAINERꢁ
solvent delivery systems (J.T. Baker, Phillipsburg, NJ).
Other anhydrous solvents were obtained in septum-sealed
bottles. In all reactions involving anhydrous solvents, glass-
Adv. Synth. Catal. 2007, 349, 395 – 404
ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.asc.wiley-vch.de
401

Joseph B. Binder et al.
FULL PAPERS
125.6, 120.4, 120.0, 119.3, 58.2, 18.5; HR-MS-ESI (m/z):
[MÀCl]+ calcd. for C18H22BrN2O: 361.0916; found: 361.0906.
red. After 1 h the solvent was removed under high vacuum.
The residue was dissolved in CH2Cl2 (20 mL), and the re-
sulting solution was filtered to remove the thallium chloride
by-product. Subsequent manipulations were performed
under air with reagent-grade solvents. The filtrate was
washed twice with deionized water (20 mL) and once with
brine (20 mL), and the organic layer was concentrated
under reduced pressure. The residue was dissolved in
CH2Cl2 (1 mL), and the resulting solution was transferred
into pentane (20 mL) to precipitate a red-brown solid. The
precipitate was removed by filtration, washed with pentane
(10 mL), and dried under high vacuum to afford 7c as a red-
brown powder; yield: 132 mg (142 mmol, 87%). 1H NMR
(CD2Cl2): d=18.37 (s, 1H), 7.90 (d, J=8.7 Hz, 1H), 7.64 (s,
1H), 7.46–7.59 (m, 3H), 7.38 (t, J=6.8 Hz, 1H), 7.14 (d, J=
8.7 Hz, 1H), 7.02 (m, 4H), 6.94 (s, 1H), 6.78 (s, 1H), 6.44 (s,
1H), 6.32 (s, 1H), 3.94–4.16 (m, 4H), 3.79 (s, 9H), 2.52 (s,
3H), 2.44 (s, 3H), 2.39 (s, 3H), 2.26 (s, 3H), 2.03 (s, 3H),
1.97 (s, 3H), 1.42 (s, 3H), 1.09 (s, 3H); 13C NMR (CD2Cl2):
d=300.7, 219.4, 170.3, 166.9, 152.1, 151.0, 140.3, 139.3, 139.2,
138.5, 137.7, 137.1, 136.7, 134.6, 134.1, 133.7, 132.2, 130.6,
130.5, 129.8, 129.7, 129.4, 129.3, 128.4, 127.6, 126.1, 124.9,
118.3, 118.0, 58.4, 51.8, 51.3, 21.1, 21.0, 20.0, 18.8, 18.3, 18.1,
18.0, 17.9; HR-MS-ESI (m/z): [MÀCl]+ calcd. for
C46H53BrClN4ORu: 887.2167; found: 887.2125.
Preparation of NHC-Imine Complex7a
N-(4-Bromo-2,6-dimethylphenyl)-salicylaldimine 9a (60 mg,
197 mmol) was dissolved in anhydrous THF (5 mL). Thalliu-
m(I) ethoxide (14 mL, 197 mmol) was added to this solution,
and the resulting yellow mixture was stirred for 1.5 h. The
green complex 8 (114 mg, 158 mmol) was added as a solid,
resulting in a rapid color change of the solution from yellow
to red. After 1.5 h, the solvent was removed by high
vacuum. The residue was dissolved in benzene (5 mL), and
the resulting solution was filtered through a glass wool plug
to remove the thallium chloride by-product. The solvent was
removed under high vacuum, and pentane (10 mL) was
added to the residue to make a slurry. The red solid was re-
moved by filtration, washed with pentane (3”5 mL), and
dried under high vacuum to afford 7a as a red powder;
yield: 101 mg (121 mmol, 76%). Crystals suitable for X-ray
diffraction analysis were obtained by layering pentane over
1
a solution of 7a in benzene. H NMR (CD2Cl2): d=18.47 (s,
1H), 7.50 (s, 1H), 7.48 (d, J=8.0 Hz, 2H), 7.34 (t, J=
7.5 Hz, 1H), 7.26 (t, J=7.7 Hz, 1H), 7.01–6.95 (m, 6H), 6.91
(s, 1H), 6.77 (s, 1H), 6.48 (t, J=7.3 Hz, 1H), 6.40 (s, 1H),
6.36 (s, 1H), 4.14–3.94 (m, 4H), 2.56 (s, 3H), 2.44 (s, 3H),
2.36 (s, 3H), 2.25 (s, 3H), 2.11 (s, 3H), 1.97 (s, 3H), 1.42 (s,
3H), 1.06 (s, 3H); 13C NMR (CD2Cl2): d=298.7, 220.7,
170.3, 167.5, 152.2, 151.7, 140.2, 139.5, 138.2, 137.6, 137.4,
137.0, 136.9, 135.1, 134.3, 132.7, 129.6, 129.4, 128.6, 128.2,
123.8, 119.1, 117.7, 114.0, 51.8, 51.2, 21.1, 21.0, 20.1, 18.8,
18.3, 18.2, 17.9, 17.8; HR-MS-ESI (m/z): [M]+ calcd. for
C43H45BrClN3ORu: 829.1511; found: 829.1517; anal. calcd.
for C43H45BrClN3ORu: C 61.76, H 5.42, N 5.02; found: C
61.70, H 5.43, N, 4.90.
Preparation of N,N-Di-3-butenyl-2-
nitrobenzenesulfonamide (10i)
A procedure from the literature[40] was modified as follows.
2-Nitrobenzenesulfonamide (1.01 g, 5.00 mmol) and 4-
bromo-1-butene (4.05 g, 30.0 mmol) were dissolved in ace-
tone (25 mL), and potassium carbonate (1.73 g, 12.5 mmol)
was added to this solution. The resulting mixture was stirred
for 5 d. After filtration, the reaction mixture was acidified
with formic acid until no additional evolution of CO2(g) was
observed, and then concentrated under reduced pressure.
The residue was dissolved in EtOAc, washed once with 1M
HCl (50 mL), twice with saturated aqueous NaHCO3
(50 mL), and once with brine (50 mL). The organic layer
was dried with MgSO4(s) and concentrated under reduced
pressure. The crude product was purified by flash chroma-
tography (20% EtOAc v/v in hexane) to afford 10i as a
yellow oil; yield: 180 mg (0.580 mmol, 11.6%). 1H NMR
(CDCl3): d=8.07–8.01 (m, 1H), 7.72–7.61 (m, 3H), 5.77–
5.64 (m, 2H), 5.13–4.98 (m, 4H), 3.39 (t, J=7.6 Hz, 4H),
2.31 (q, J=7.6 Hz, 4H); 13C NMR (CDCl3): d=148.2, 134.3,
133.9, 133.6, 131.8, 131.0, 124.3, 117.6, 46.9, 32.8. HR-MS-
ESI (m/z): [M+Na]+ calcd. for C14H18N2O4SNa: 333.0885;
found: 333.0891.
Preparation of NHC-Imine Complex7b
Complex 7b was prepared in 79% yield from 9b and 8 by
using a procedure similar to that for the preparation of 7a.
1H NMR (CD2Cl2): d=18.43 (s, 1H), 8.06 (dd, J=9.4 Hz,
2.5 Hz, 1H), 8.02 (d, J=2.5 Hz, 1H), 7.58 (s, 1H), 7.49 (b,
2H), 7.40 (t, J=7.4 Hz, 1H), 7.07–7.00 (m, 4H), 6.96 (d, J=
9.4 Hz, 1H), 6.93 (s, 1H), 6.76 (s, 1H), 6.42 (s, 1H), 6.37 (s,
1H), 4.17–3.96 (m, 4H), 2.53 (s, 3H), 2.41 (s, 3H), 2.34 (s,
3H), 2.25 (s, 3H), 2.07 (s, 3H), 1.99 (s, 3H), 1.45 (s, 3H),
1.04 (s, 3H); 13C NMR (CD2Cl2): d=297.2, 214.4, 170.2,
163.0, 147.4, 146.0, 135.6, 134.9, 134.4, 134.0, 133.1, 132.4,
131.9, 131.1, 130.6, 129.9, 129.1, 127.5, 125.9, 125.2, 125.1,
125.0, 124.9, 124.8, 123.8, 123.5, 119.6, 113.8, 113.6, 47.2,
46.6, 16.4, 16.3, 15.4, 14.3, 13.7, 13.5, 13.3, 13.1; HR-MS-ESI
(m/z): [M]+ calcd. for C43H44BrClN4O3Ru: 874.1361; found:
874.1324.
Preparation of N-(2-Propenyl)-N-(2-butynyl)-4-
methylbenzenesulfonamide (10j)
Following the procedure of Pagenkopf and co-workers,[39] N-
Preparation of NHC-Imine Complex7c
N-(4-Bromo-2,6-dimethylphenyl)-5-trimethylammoniumsali-
cylaldimine chloride 9c (71 mg, 180 mmol) was dissolved in
anhydrous DMF (3 mL). Thallium(I) ethoxide (12.7 mL, 180
mmol) was added to the solution, and the resulting yellow
mixture was stirred for 1 h. The green complex 8 (119 mg,
164 mmol) was added to this mixture as a solution in THF
(0.5 mL), resulting in a rapid color change from yellow to
(2-propenyl)-4-methylbenzenesulfonamide
(2.00 g,
9.51 mmol), 1-bromo-2-butyne (3.79 g, 28.5 mmol), and po-
tassium carbonate (1.58 g, 11.41 mmol) afforded 10j as a
yellow oil following silica gel flash chromatography (10%
EtOAc v/v in hexane); yield: 1.15 g (4.36 mmol, 46%).
1H NMR data were in agreement with those reported by
Buchwald and co-workers.[41]
402
www.asc.wiley-vch.de
ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2007, 349, 395 – 404

Salicylaldimine Ruthenium Alkylidene Complexes
FULL PAPERS
correction was based on fitting a function to the empirical
transmission surface as sampled by multiple equivalent
measurements.[43]
Preparation of Allyl 1,1-Diphenylpropargyl Ether
(10k)
Allyl 1,1-diphenylpropargyl ether was prepared by the
The systematic absences in the diffraction data were con-
method of Dixneuf and co-workers.[42] Rf =0.52 (silica gel,
¯
sistent for the space groups P1and P1. The E-statistics
1
5% EtOAc v/v in hexane). H NMR (CDCl3): d=7.58 (d,
¯
strongly suggested the centrosymmetric space group P1that
J=7.6 Hz, 4H), 7.30 (t, J=7.4 Hz, 4H), 7.23 (t, J=7.2 Hz,
2H), 6.05–5.93 (m, 1H), 5.36 (d, J=17.5, 1H), 5.16 (d, J=
10.4 Hz, 1H), 4.04 (d, J=5.1 Hz, 2H), 2.86 (s, 1H);
13C NMR (CDCl3): d=143.3, 134.9, 128.4, 127.9, 126.7,
116.3, 83.4, 80.2, 77.8, 66.1; HR-MS-ESI (m/z): [M+Na]+
calcd. for C18H16ONa: 271.1099; found: 271.1106.
yielded chemically reasonable and computationally stable
results of refinement.[43]
A successful solution by the direct methods provided
most non-hydrogen atoms from the E-map. The remaining
non-hydrogen atoms were located in an alternating series of
least-squares cycles and difference Fourier maps. All non-
hydrogen atoms were refined with anisotropic displacement
coefficients. All hydrogen atoms were included in the struc-
ture factor calculation at idealized positions and were al-
lowed to ride on the neighboring atoms with relative iso-
tropic displacement coefficients.
The final least-squares refinement of 459 parameters
against 7597 data resulted in residuals R (based on F2 for I
ꢀ 2s) and wR (based on F2 for all data) of 0.0262 and
0.0739, respectively. The final difference Fourier map was
featureless.
Crystallographic data (excluding structure factors) for
compound 7a have been deposited with the Cambridge
Crystallographic Data Centre as supplementary publication
no. CDC 628401. Copies of the data can be obtained free of
charge on application to CCDC, 12 Union Road, Cambridge
CB2 1EZ, UK [fax: (internat.) +44 1223/336–033; e-mail:
deposit@ccdc.cam.ac.uk].
Representative Procedure for RCM Reactions
On the benchtop under air, the ruthenium complex 7a
(1.6 mg, 1.9 mmol) was dissolved in non-distilled, non-de-
gassed C6D6 (0.75 mL) in an NMR tube, and substrate 10c
(8.1 mL, 38 mmol) was added to this solution. The tube was
capped, wrapped with parafilm, shaken, and placed in a
temperature-controlled water bath at 558C. Product forma-
tion was monitored by 1H NMR integration of the allylic
methylene signals.
Alternative Preparation of NHC-Imine Complex7a
A flame-dried Schlenk flask was charged with 1,3-bis(2,4,6-
trimethylphenyl)-4,5-dihydroimidazolium tetrafluoroborate
(12.2 mg, 30.9 mmol) and anhydrous hexanes (5 mL). After
the addition of 1.7M potassium tert-amylate in toluene
(18.2 mL, 30.9 mmol), the suspension was stirred for 5 min.
Complex 6a (5.0 mg, 6.2 mmol) was added as a solid, and the
resulting reddish mixture was heated to 758C for 1 h, after
which the solvent was removed under high vacuum. The
Acknowledgements
1
crude product was analyzed by H NMR in CD2Cl2.
We are grateful to H. E. Blackwell, L. L. Kiessling, D. M.
Lynn, and M. Kim for contributive discussions. J.B.B. was
supported by an NDSEG Fellowship sponsored by the Air
Force Office of Scientific Research. This work was supported
by grant GM44783 (NIH). NMRFAM was supported by
grant P41RR02301 (NIH).
Structure Determination of NHC-Imine Complex7a
A red crystal of complex 7a with approximate dimensions
0.36”0.31”0.30 mm3 was selected under oil under ambient
conditions and attached to the tip of a nylon loop. The crys-
tal was mounted in a stream of cold N2(g) at 100(2) K and
centered in the X-ray beam by using a video camera.
The crystal evaluation and data collection were performed References
on a Bruker CCD-1000 diffractometer with Mo Ka (l=
0.71073 Š) radiation and a diffractometer to crystal distance
of 4.9 cm.
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