Synthesis and metathesis reactions of a phosphine-free dihydroimidazole carbene ruthenium complex

Article abstract of DOI:10.1016/S0040-4039(00)01808-6

Synthesis and activity in ring closure metathesis (RCM) and cross metathesis (CM) of the phosphine-free 1,3-dimesityl-4,5-dihydroimidazole-2-ylidene (IHMes) ruthenium alkoxybenzylidene complex 6 are reported. (C) 2000 Elsevier Science Ltd.

Full text of DOI:10.1016/S0040-4039(00)01808-6

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 41 (2000) 9973–9976
Synthesis and metathesis reactions of a phosphine-free
dihydroimidazole carbene ruthenium complex
Simon Gessler, Stefan Randl and Siegfried Blechert*
Institut fu¨r Organische Chemie, Technische Universita¨t Berlin, Straße des 17. Juni 135, D-10623 Berlin, Germany
Received 8 September 2000; accepted 9 October 2000
Abstract
Synthesis and activity in ring closure metathesis (RCM) and cross metathesis (CM) of the phosphine-
free 1,3-dimesityl-4,5-dihydroimidazole-2-ylidene (IHMes) ruthenium alkoxybenzylidene complex 6 are
reported. © 2000 Elsevier Science Ltd. All rights reserved.
Keywords: carbene complex; olefin metathesis; ruthenium; homogenous catalysis.
The introduction of N-heterocyclic carbenes as Lewis basic ligands into ruthenium–alkylidene
complexes of the Grubbs type 1 has strongly encouraged the development of new highly active
metathesis catalysts (Fig. 1).1 It was shown that complexes 2, in which both phosphines are replaced
by more Lewis basic diaminocarbene ligands, exhibit a higher stability but are less reactive in RCM
and CM reactions.1a,f Probably, the catalytically active 14e− species is formed more slowly in
comparison with the bisphosphine complex due to the stronger carbene metal bond.1c
Figure 1. Ruthenium carbene complexes for olefin metathesis (Cy=cyclohexyl, Mes=C6H2-2,4,6-Me3)
This drawback has been overcome by the use of the sterically demanding IMes- and
IHMes-ligands because they enable the selective replacement of only one phosphine moiety by
* Corresponding author. Fax: +49 30 3142 3619; e-mail: blechert@chem.tu-berlin.de
0040-4039/00/$ - see front matter © 2000 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(00)01808-6

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the N-heterocyclic carbene. These mixed complexes such as 3 bearing one phosphine as a leaving
ligand show increased stability and activity in RCM and CM reactions.1b,f
We were interested in IHMes–Ru complexes with a non-phosphine leaving ligand with regard
to different selectivities and reactivities. Recently, Hoveyda et al. have shown that O-chelating
benzylidene moieties can be used for such purposes, providing complexes such as 4 exhibiting
extraordinary stability against water and oxygen. Another advantage is the more facile purification
by flash chromatography (FC).2
Herein, we report the synthesis of IHMes-o-isopropoxybenzylidene-ruthenium dichloride 6 and
its behaviour in some metathesis reactions.3
As shown above, 6 can be obtained in 75% yield in two steps starting from 4. Treatment of
4 with 1.2 equiv. of IHMesCl and 1.2 equiv. KOtBu in THF/toluene at 80°C leads to the formation
of a pink intermediate 5 still bearing the PCy3 moiety. This suggests that the IHMes ligand replaces
the isopropoxy group. Compound 5 can be isolated and was fully characterised. Formation of
the desired complex is achieved by stirring of 5 at room temperature in CHCl3 for two hours
(Scheme 1). The green crystalline product 6 can be separated from the liberated phosphine by
flash chromatography using CH2Cl2 as the eluent. The structure of this complex was confirmed
by 1H, 13C NMR, HR-MS and X-ray analysis.4 Like 4, complex 6 shows a great stability against
water and oxygen and can be stored under ambient atmosphere at room temperature.
Scheme 1.
Catalyst 6 was tested in several metathesis reactions and compared with 3.5 We have found
that the exchange of the PCy3 ligand with the isopropyl ether leads to different reactivities.
In contrast to 3, which proved to be an excellent catalyst for yne-ene CM,6 analogous
reactions with catalyst 6 yielded only traces of the desired products. Polymerisation of the
alkyne component was not observed.
In CM reactions both 3 and 6 show similar behaviour. CM between olefin 7 and electron-
deficient olefins is easily accomplished using 6 within 2 h (Table 1).7
Ring closure of dienes such as 10 is complete in less than 15 min at room temperature using
6, whereas 3 requires higher temperatures.
Substantial differences were found in RCM reactions with dienes of types 12 accessible via the
Baylis–Hillman reaction.8,9 These substrates contain a sterically hindered electron deficient and
a monosubstituted double bond (Table 2).

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Table 1
CM between 7 and various electron-deficient olefinsa
EWG=
CO2CH3
COCH3
CHO
CON(CH3)2
\98%b (\20:1)
Yield of 9 (E/Z)
87% (\20:1)
85% (\20:1)
93% (1:1)
a Conditions: CH2Cl2, 40°C, [7]=0.05 M, 2 equiv. of 8.
b Yield determined by NMR.
Using 6 dimerisation via the monosubstituted double bond is much more favoured than
cyclisation when compared to 3. However, the dimerisation is reversible. Thus, longer reaction
times lead to higher amounts of products of type 13. For example, in the case of 12a the ratio
of the products switches from 1:5 to 2:1 when the reaction mixture is stirred for 48 h instead of
2 h with no significant change in yield.
Strikingly, catalyst 3 shows a much lower reactivity towards acrylates 12e and 12f with
unprotected hydroxyl groups than 6. Diene 12e is not converted at all and with 12f a longer
reaction time (48 versus 12 h) is required, whereas in the other cases, reaction times with both
catalysts are similar.
Another difference in reactivity was found in the reaction of diene 15.
Table 2
RCM reactionsa
EWG=CO2CH3 (entries a–d)
EWG=CN (entries e–h)
5 mol% 3
ratio 13:14 (yield)
5 mol% 6
ratio 13:14 (yield)
(a) n=0, R=H
(b) n=1, R=H
(c) n=0, R=TBS
(d) n=1, R=TBS
(e) n=0, R=H
(f) n=1, R=H
(g) n=0, R=TBS
(h) n=1, R=TBS
2:1 (60%)
3:2 (42%)
1:5 (50%)
1:2 (47%)
3:1 (86%)
1:2 (90%)
1:5 (40%)
1:1 (42%)
2:1 (92%)
10:1 (90%)
\20:1 (87%)
\20:1 (89%)
No reaction
\20:1 (46%)
6:1 (90%)
\20:1 (86%)
a Product ratio determined by NMR spectroscopy after complete conversion of the substrate. Conditions: CH2Cl2,
40°C, [12]=0.05 M.

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Both 3 and 6 efficiently catalyse the RCM of 15. In the presence of acrylonitrile, the use of
6 results in the chemoselective formation of cross metathesis product 16 only. On the other
hand, neither RCM nor CM was observed using 3 under identical conditions.
Our results indicate that complex 6 is different from 3 in its catalytic properties with regard
to both selectivity and reactivity. As a first conclusion, 6 seems to be a promising catalyst,
especially for CM.
Further studies, particularly in the field of cross metathesis, are in progress in our laborato-
ries.
Acknowledgements
S.G. and S.R. thank the Graduiertenkolleg ‘Synthetische, mechanistische und reaktions-tech-
nische Aspekte von Metallkatalysatoren’ of the TU Berlin for their Ph.D. grants.
References
1. For examples, see: (a) Weskamp, T.; Schattenmann, W. C.; Spiegler, M.; Herrmann, W. A. Angew. Chem., Int. Ed.
Engl. 1998, 37, 2490–2493; (b) Weskamp, T.; Kohl, F. J.; Hieringer, W.; Gleich, D.; Herrmann, W. A. Angew.
Chem., Int. Ed. Engl. 1999, 38, 2416–2419; (c) Huang, J.; Stevens, E. D.; Nolan, S. P.; Petersen, J. L. J. Am. Chem.
Soc. 1999, 121, 2674–2678; (d) Jafarpour, L.; Nolan, S. P. Organometallics 2000, 19, 2055–2057; (e) Scholl, M.;
Trnka, T. M.; Morgan, J. P.; Grubbs, R. H. Tetrahedron Lett. 1999, 40, 2247–2250; (f) Scholl, M.; Ding, S.; Lee,
C. W.; Grubbs, R. H. Org. Lett. 1999, 6, 953–956.
2. Kingsbury, J. S.; Harrity, J. P. A.; Bonitatebus, P. J.; Hoveyda, A. H. J. Am. Chem. Soc. 1999, 121, 791–799.
3. During preparation of this manuscript, Hoveyda et al. described a different procedure for the synthesis of 6:
Garber, S. B.; Kingsbury, J. S.; Gray, B. L.; Hoveyda, A. H. J. Am. Chem. Soc. 2000, 122, 8168–8179.
4. Selected analytical data for complexes 5 and 6:
1
Complex 5: 31P NMR (80 MHz, CDCl3), l=29.36; H NMR (200 MHz, CDCl3), l=19.88 (1H, br), 7.41–6.00
(8H), 4.41 (1H, sept), 4.18–3.48 (4H, m), 2.76, 2.59, 2.47, 2.35, 2.13, 1.89 (18H, s), 1.27 (6H, d), 1.83–0.84 (33H);
HR-MS (FAB): calcd: 906.372, found: 906.374.
Complex 6: 13C NMR (125 MHz, CDCl3), l=296.5 (br), 211.1, 152.2, 145.3, 139.1 (br), 138.8, 129.5, 129.3, 122.7,
1
122.2, 112.9, 74.9, 51.5, 21.0, 19.4 (br); H NMR (500 MHz, CDCl3), l=16.56 (1H, s), 7.52–6.77 (8H), 4.90 (1H,
sept), 4.18 (4H, s), 2.48 (12H, s), 2.40 (6H, s), 1.27 (6H, d); HR-MS (FAB): calcd: 626.140, found: 626.139.
5. All new compounds have been fully characterised. Given yields are isolated yields after FC on silica gel.
6. Stragies, R.; Voigtmann, U.; Blechert, S. Tetrahedron Lett. 2000, 41, 5465–5468.
7. Similar yields and E/Z ratios were obtained using a slightly modified catalyst of type 3: Chatterjee, A. K.; Morgan,
J. P.; Scholl, M.; Grubbs, R. H. J. Am. Chem. Soc. 2000, 122, 3783–3784.
8. Baylis, A. B.; Hillman, M. E. D. DE-B 2155113, 1972 [Chem. Abstr. 1972, 77, 34174q].
9. Reviews: (a) Ciganek, E. Org. React. 1977, 51, 201–350; (b) Drewes, S. E.; Roos, G. H. P. Tetrahedron 1988, 44,
4653–4670; (c) Basaviah, D.; Rao, P. D.; Hyma, R. S. Tetrahedron 1996, 52, 8001–8062.
.
.