New (carbene)ruthenium-arene complexes: Preparation and uses in catalytic synthesis of furans

Article abstract of DOI:10.1021/om960002l

A variety of neutral ruthenium-carbene complexes RuCl₂(carbene)(arene) 1 (carbene = C(NR)C₆H₄(NR), R = Me (a), Et (b); arene = p-Me-C₆H₄-ⁱPr (1), C₆H₃Me₃ (2), C₆Me₆ (3)) have been prepared by reaction of [RuCl₂(carene)]₂ precursors with the enetetraamines (RN)C₆H₄(RN)C=C(NR)C₆H₄(NR) I (R = Me) and II (R = Et). Ru=C(NCH₂Ph)CH₂-CH₂(NCH₂Ph)Cl ₂(p-cymene) (4) was prepared similarly, whereas the reaction of [RuCl₂-(cycloocta-1,5-diene)]_n with I led to the formation of the 16-electron neutral complex RuCl₂[=C(NMe)C₆H₄(NMe)]₃ (6). One of them (3a) was transformed into the dihydride derivative RuH₂(C(NMe)C₆H₄NMe)(C₆Me ₆) (5). The cyclic voltammograms of derivatives 1-3 show that they are oxidized in the range E_1/2 = 1.03-1.31 V vs SCE and are more electrophilic than the isoelectronic RuCl₂(PR₃)(arene) complexes. Derivatives 1a, 3a, and 3b act as efficient catalyst precursors for the electrophilic activation of the C≡C bond of (Z)-3-methylpent-2-en-4-yn-1-ol to afford 2,3-dimethylfuran in good yield via intramolecular cyclization.

Full text of DOI:10.1021/om960002l

2434
Organometallics 1996, 15, 2434-2439
Articles
New (Ca r ben e)r u th en iu m -Ar en e Com p lexes:
P r ep a r a tion a n d Uses in Ca ta lytic Syn th esis of F u r a n s†
Hasan Ku¨cu¨kbay,‡ Bekir Cetinkaya,*,‡ Salaheddine Guesmi,§,| and
Pierre H. Dixneuf*,§
Ino¨nu¨ University, Fen Edebiyat Faku¨ltesi, Kimya Bolu¨mu¨, 44069 Malatya, Tu¨rkey, and
Laboratoire de Chimie de Coordination Organique, URA CNRS 415, Campus de Beaulieu,
University of Rennes, 35042 Rennes, France
Received J anuary 2, 1996X
A variety of neutral ruthenium-carbene complexes RuCl2(carbene)(arene) 1 (carbene )
C(NR)C6H4(NR), R ) Me (a ), Et (b); arene ) p-Me-C6H4-iPr (1), C6H3Me3 (2), C6Me6 (3))
have been prepared by reaction of [RuCl2(arene)]2 precursors with the enetetraamines
(RN)C6H4(RN)CdC(NR)C6H4(NR) I (R ) Me) and II (R ) Et). RudC(NCH2Ph)CH2-
CH2(NCH2Ph)Cl2(p-cymene) (4) was prepared similarly, whereas the reaction of [RuCl2-
(cycloocta-1,5-diene)]n with I led to the formation of the 16-electron neutral complex
RuCl2[dC(NMe)C6H4(NMe)]3 (6). One of them (3a ) was transformed into the dihydride
derivative RuH2(C(NMe)C6H4NMe)(C6Me6) (5). The cyclic voltammograms of derivatives
1-3 show that they are oxidized in the range E1/2 ) 1.03-1.31 V vs SCE and are more
electrophilic than the isoelectronic RuCl2(PR3)(arene) complexes. Derivatives 1a , 3a , and
3b act as efficient catalyst precursors for the electrophilic activation of the CtC bond of
(Z)-3-methylpent-2-en-4-yn-1-ol to afford 2,3-dimethylfuran in good yield via intramolecular
cyclization.
In tr od u ction
unsaturated carbonyl compounds,5 butatrienes,6 buteno-
lides,7 and unsaturated aldehydes.8
During the last few years, the study of ruthenium
complexes has revealed their power in promoting novel
activation processes and catalytic reactions that have
strengthened organic synthesis methodologies. Thus,
the activation of terminal alkynes into vinylidene
intermediates1 and the promoted alkene-alkyne cou-
pling2,3 have led to the discovery of powerful new
methods for the synthesis of vinylcarbamates,4 R,â-
A promising aspect of ruthenium chemistry deals with
the generation of ruthenium carbene species that are
expected to be efficient catalyst intermediates for olefin
cyclopropanation or metathesis9 and ring-opening me-
tathesis polymerization of cyclic olefins.9a,10 For in-
stance, RuCl2(PCy3)(p-cymene) and even RuCl2(PPh3)3
on addition of diazoacetic esters generate very efficient
catalysts for the polymerization of low-strained cyclic
olefins.10,11 Recently, Grubbs et al. have shown that the
† Dedicated to Professor Michael F. Lappert, University of Sussex,
Brighton, U.K.
coordinatively
unsaturated
carbene
complex
RudCHCHdCPh2(Cl)2(PCy3)2 is a precursor for the
‡ Ino¨nu¨ University.
§University of Rennes.
| Present address: Laboratoire de Chimie de Coordination et
Analytique, Universite´ Chouaib Doukkali, Faculte´ des Sciences, BP
20, El-J adida, Morocco.
(5) (a) Trost, B. M.; Dyker, G.; Kulawiec, R. J . J . Am. Chem. Soc.
1990, 112, 7809-7811. (b) Trost, B. M.; Kulawiec, R. J . J . Am. Chem.
Soc. 1992, 114, 5579-5584.
X Abstract published in Advance ACS Abstracts, April 15, 1996.
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Ouzzine, K.; Dixneuf, P. H. Organometallics 1991, 10, 2768-2772. (c)
Touchard, D.; Haquette, P.; Pirio, N.; Toupet, L.; Dixneuf, P. H.
Organometallics 1993, 12, 3232-3139. (d) Le Lagadec, R.; Roman, E.;
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1992; Vol. 2, pp 73-94.
S0276-7333(96)00002-7 CCC: $12.00 © 1996 American Chemical Society

New (Carbene)ruthenium-Arene Complexes
Organometallics, Vol. 15, No. 10, 1996 2435
displacement from ruthenium-phosphine complexes.20-22
To our knowledge, no neutral (arene)RuX2(carbene)
complexes of type E have been prepared so far. The
syntheses of enetetraamine derivatives (D: R ) Me,23
Et24) have been reported recently, and only rhodium-
carbene complexes have been prepared from D.23 (Car-
bene)metal complexes arising from the cleavage of D
are expected to contain a bulky carbene ligand that,
moreover, is a less electron-donating ligand than the
Sch em e 1
related electron-rich carbene CN(Me)CH2CH2NMe,19
due to electron delocalization onto the aromatic rings.
Among the ruthenium(II) complexes the (arene)RuCl2-
(PR3) derivatives have been shown to be the promoters
of many catalytic reactions related to the electrophilic
activation of alkynes in the synthesis of vinylcarbam-
ates,4 â-oxopropyl esters,25 and enol esters.26 The
isoelectronic complexes (arene)RuCl2(dC(NR)C6H4NR)
of type E, even if they are not expected to promote olefin
metathesis catalysis due to the presence of heteroatoms
linked to the carbene carbon, are of interest in providing
activation of terminal alkynes. Thus, we have studied
the possibility of direct cleavage of the chloro bridges
of the binuclear complexes [RuCl2(arene)]2 (A) with
enetetraamines of type D in an attempt to generate
(carbene)ruthenium species E (Scheme 1).
polymerization of low-strain olefins.12 Moreover, due
to the tolerance of ruthenium catalysts toward a variety
of functional groups, these ruthenium-carbene species
are finding new applications for organic synthesis.13,14
By displacement of the arene ligand of [RuCl2(p-
cymene)]2 in the presence of an optically active triden-
tate ligand and N2CHSiMe3, an 18-electron RudCHSiMe3
carbene complex15 has been isolated and shown to
catalyze asymmetric cyclopropanation of olefins with
N2CHCO2R derivatives. Moreover, a very efficient
catalyst for the polymerization of non-strained olefins
has just been reported by Noels et al.16 on treatment of
RuCl2(PCy3)(p-cymene) with N2CHSiMe3 and is consis-
tent with the generation, after displacement of the
arene, of a RudCHSiMe3 coordinatively unsaturated
species containing a bulky PCy3 ligand. In contrast,
coordinatively saturated cationic Fischer-type (carbene)-
ruthenium complexes C,1b,17 arising from (arene)ruthe-
nium precursors A and B (Scheme 1), have been shown
to be inactive in catalysis.18 The efficiency in catalysis
and organic synthesis of neutral ruthenium-carbene
species RudCHR generated from (arene)ruthenium(II)
precursors10,15,16 has led us to study the possibility of
generating neutral (arene)ruthenium-carbene com-
plexes E from the aromatic enetetraamines derivatives
D (Scheme 1).
We now report (i) the detailed preparation of novel
neutral (arene)RuCl2(dC(NR)C6H4NR) complexes of
type E, (ii) an electrochemical study showing that they
cannot be considered as electron-rich ruthenium(II)
complexes and (iii) the promotion by these complexes
of the efficient catalytic activation of (Z)-enynols and a
convenient synthesis of furans.
Resu lts a n d Discu ssion
P r ep a r a tion of Com p lexes. The precursor [RuCl2-
(p-cymene)]2 (A1) was reacted with 1 equiv of bis(1,3-
dimethylbenzimidazolidine-2-ylidene) (I) in toluene at
100 °C for 4 h. The brown complex 1a was isolated in
93% yield (Scheme 2). Its analytical and spectroscopic
data correspond to the addition of only one carbene
ligand to the 16-electron ruthenium species [RuCl2-
(p-cymene)]: the p-cymene ligand is retained (1H NMR),
and the 13C NMR spectrum revealed the presence of the
carbene moiety, by the low-field singlet at δ 188.89 ppm
for the RudC carbon nucleus. Analogously, the carbene
complex 1b was obtained in 88% yield from the reaction
of precursor A1 with the olefin II (Scheme 2).
Lappert et al.19 have shown that electron-rich olefins
closely related to D are useful precursors to generate a
variety of electron-donating (carbene)metal complexes,
and several ruthenium-carbene complexes have been
prepared by this method, essentially via phosphine
The [(arene)RuCl2]2 precursors containing 1,3,5-tri-
methylbenzene (A2) and hexamethylbenzene (A3) are
known to lead to a stronger arene-ruthenium bond
(12) (a) France, M. B.; Paciello, R. A.; Grubbs, R. H. Macromolecules
1993, 26, 4739. (b) Nguyen, S. T.; Grubbs, R. H.; Ziller, J . W. J . Am.
Chem. Soc. 1993, 115, 9858-9859. (c) Wu, Z.; Nguyen, S. T.; Grubbs,
R. H.; Ziller, J . W. J . Am. Chem. Soc. 1995, 117, 5503-5511. (d)
Schwab, P.; France, M. B.; Ziller, J . W.; Grubbs, R. H. Angew. Chem.,
Int. Ed. Engl. 1995, 34, 2039-2041.
(13) (a) Fu, G. C.; Nguyen, S. T.; Grubbs, R. H. J . Am. Chem. Soc.
1993, 115, 9856-9857. (b) Kim, S.-H.; Bowden, N.; Grubbs, R. H. J .
Am. Chem. Soc. 1994, 116, 10801-10802. (c) Kinoshita, A.; Mori, M.
Synlett 1994, 1020.
(14) Schmalz, H.-G. Angew. Chem., Int. Ed. Engl. 1995, 34, 1833-
1836.
(15) Park, S.-B.; Nishiyama, H.; Itoh, Y.; Itoh, K. J . Chem. Soc.,
Chem. Commun. 1994, 1315-1316.
(16) Stumpf, A. W.; Saive, E.; Demonceau, A.; Noels, A. F. J . Chem.
Soc., Chem. Commun. 1995, 1127-1128.
(20) Lappert, M. F.; Pye, P. L. J . Chem. Soc., Dalton Trans. 1978,
837-844.
(21) Hitchcock, P. B.; Lappert, M. F.; Pye, P. L. J . Chem. Soc., Dalton
Trans. 1978, 826-836.
(22) Hitchcock, P. B.; Lappert, M. F.; Pye, P. L.; Thomas, S. J . Chem.
Soc., Dalton Trans. 1979, 1929-1942.
(23) C¸ etinkaya, E.; Hitchcock, P. B.; Ku¨c¸u¨kbay, H.; Lappert, M. F.;
Al-J uaid, S. J . Organomet. Chem. 1994, 481, 89-95.
(24) C¸ etinkaya, B.; C¸ etinkaya, E.; Ku¨c¸u¨kbay, H.; O¨ zdemir, I.;
Gu¨rbu¨z, N. Abstracts, 35th IUPAC Congress; Tu¨rkish Chem. Soc.:
Istanbul, Tu¨rkey, 1995; Vol. II, p 1380.
(17) Pilette, D.; Ouzzine, K.; Le Bozec, H.; Dixneuf, P. H.; Rickard,
C. E. F.; Roper, W. R. Organometallics 1992, 11, 809-817.
(18) Lavastre, O.; Dixneuf, P. H. J . Organomet. Chem. 1995, 488,
C9-C10.
(25) Devanne, D.; Ruppin, C.; Dixneuf, P. H. J . Org. Chem. 1988,
53, 925-928.
(26) (a) Ruppin, C.; Dixneuf, P. H. Tetrahedron Lett. 1986, 27, 6323-
6324. (b) Bruneau, C.; Neveux, M.; Kabouche, Z.; Ruppin, C.; Dixneuf,
P. H. Synlett 1991, 755-763.
(19) Lappert, M. F. J . Organomet. Chem. 1988, 358, 185-214.

2436 Organometallics, Vol. 15, No. 10, 1996
Ku¨cu¨kbay et al.
Sch em e 2a
a
Reagents and conditions: (i) [RuCl2(p-MeC6H4CH(Me)2)]2 (A1), 4 h, 100 °C; (ii) [RuCl2(C6H3Me3)]2 (A2), 5 h, 100 °C; (iii)
[RuCl2(C6Me6)]2 (A3), 4 h, 100 °C; (iv) NaBH4, 10 h, room temperature; (v) [RuCl2(COD)]n, 4 h, 100 °C.
than does p-cymene in the cationic (carbene)ruthenium
is likely to adopt an orthogonal position to the ruthe-
nium-arene axis, as observed in the X-ray diffraction
structure of the cationic carbene complex [(η6-C6Me6)-
RudC(OMe)CHdCPh2(Cl)(PMe3)]PF6.17
-
complexes [(arene)Ru(PR3)(Cl)(dC(OMe)CH2R)]+PF6
(C)27 originating from the direct activation of terminal
alkynes. Thus, both olefins I and II were reacted in
toluene at 100 °C with 1 equiv of [RuCl2(η6-C6H3Me3)]2
(A2) and [RuCl2(η6-C6Me6)]2 (A3) successively. The
brown, stable (carbene)ruthenium complexes 2a (96%),
2b (95%), 3a (96%), and 3b (79%) were obtained. Each
of these complexes retains the arene ligand and contains
one carbene ligand (R ) Me) (a ) or (R ) Et) (b), as
shown by 1H NMR. The 13C NMR shows the typical
low-field signal (singlet) for such a (carbene)metal
complex as in the two corresponding rhodium deriva-
tives23 (δ (ppm): 196.0 (2a ), 195.0 (2b), 195.5 (3a ), 194.0
(3b)) without a noticeable influence of the nature of the
arene ligand.
The above synthesis of neutral (carbene)ruthenium
complexes 1-3 shows that it is possible to cleave the
chloro bridges of precursors A, not only with phosphorus
derivatives or two-electron-donating ligands, but also
with the olefins I and II that are cleaved, although
under more drastic conditions (100 °C/4 h). It consti-
tutes a new way to generate neutral (arene)ruthe-
nium(II)-carbene derivatives E and contrasts with that
of the cationic carbene complexes C (Scheme 1).
To examine whether this transformation A f E was
restricted to the 1,2-substituted aryl olefins I and II,
the precursor A1 was reacted with 1 equiv of the more
electron-rich bis(1,3-dibenzylimidazolidine-2-ylidene) (III)
in toluene at 90 °C for 4 h. The brown complex 4 was
isolated in 87% yield (eq 1). Its NMR spectra showed
the presence of one arene and one carbene ligand, with
the typical singlet in 13C NMR for the RudC carbon
nucleus at lower field (δ 208.3 ppm) than for the
analogous complexes 1-3 (δ 194-196 ppm).
Although complexes 1-3 have a formal symmetry
plan, the (NCH2CH3) methylene protons of the carbene
ligand of derivatives b are nonequivalent in 1H NMR,
showing that the privileged position of the carbene
ligand deviates from this symmetry plan. The carbene
(27) Devanne, D.; Dixneuf, P. H. J . Organomet. Chem. 1990, 390,
371-378.

New (Carbene)ruthenium-Arene Complexes
Organometallics, Vol. 15, No. 10, 1996 2437
Ta ble 1. Cyclic Volta m m etr ic Da ta for
(Ca r ben e)r u th en iu m Com p lexesa
E1/2(Ru3+/Ru2+),
compd
V vs SCE
∆Ep, mV
1a
1b
2a
2b
3a
3b
4
1.31
1.23
1.18
1.12
1.12
1.03
1.27
1.50
0.77
0.92
1.06
120
150
170
160
165
140
125
200
80
6
RuCl2(PMe3)(C6Me6)1b
RuCl2(PPh3)(C6Me6)1b
RuCl2(CNBut)(C6Me6)30
70
70
(1)
a
E vs SCE; Pt working electrode; 200 mV/s; recorded in CH2Cl2
solution with 0.026 M Bu4NPF6 as supporting electrolyte.
Electr och em ica l Stu d ies of Com p lexes 1-6. The
electrophilicity concept of ruthenium(II) catalyst precur-
sors has been used to explain selective, mild catalytic
transformations of alkynes.4,26,32 The method of choice
to evaluate either the electron deficiency or richness of
a metal complex remains the measure of its reversible
oxidation potential (E1/2) by cyclic voltammetry. In
addition, it offers a direct comparison of (arene)RuCl2-
(carbene) with related derivatives such as (arene)-
RuCl2L (L ) PR3,1b,27 CNR30). Complexes 1-6 were
studied by cyclic voltammetry in CH2Cl2 solution with
Bun4NPF6 as supporting electrolyte. Each of them gives
a reversible oxidation process at 200 mV/s. The corre-
sponding data are gathered in Table 1. It shows that
(arene)RuCl2(carbene) complexes (1-4) are oxidized at
a much higher potential (E1/2 ) 1.03-1.31 V (SCE)) than
the corresponding phosphine complexes (arene)RuCl2-
(PR3). The electron-donating capability of the arene
ligand is reflected in the sequence Me-C6H4-Pri (1a ) <
C6H3Me3 (2a ) < C6Me6 (3a ). The coordinatively unsat-
urated complex 6 possesses the highest potential of
oxidation. The (arene)RuCl2(carbene) derivatives 1-3
a priori offer a good balance of electron deficiency to
provide electrophilic activation of alkynes with respect
to (arene)RuCl2(PR3) complexes.1b
Fischer-type carbene ligands have been shown to
insert into a cis-hydrido-metal bond to generate an η2-
coordinated alkyl ligand,28 and the subsequent coordi-
nation of the ligand heteroatom led to the formation of
a stable 18-electron derivative. To study the behavior
of the carbene ligand in complexes 1-3 toward ruthe-
nium hydrides, complex 3a was reacted with an excess
of NaBH4 in dry ethanol. After 10 h at room temper-
ature the light yellow, unstable complex 5 was isolated
in 72% yield. The infrared spectrum indicated two
Ru-H absorptions at ν 1924 and 1885 cm-1 (KBr), and
in 1H NMR (300 MHz) a signal (2 H) at δ -9.71 ppm
was consistent with the formation of a dihydridoruthe-
nium carbene complex ((C6H6)RuH2(PPh3): δ (Ru-H)
-10.70 ppm, ν(Ru-H) 1963, 1932 cm-1).29 The 1H NMR
shows only one singlet for the NMe groups and rules
out the coordination of one nitrogen atom. This trans-
formation 3a f 5 indicates that the carbene ligand of
3a stabilizes the dihydrido intermediates rather than
leads to (carbene) carbon insertion into the Ru-H bond.
During the formation of complexes 1-3 the fixation
of a second neutral carbene ligand was never observed,
whereas from the same precursors A, phosphorus
derivatives easily undergo coordination of two PR3
ligands in [(arene)Ru(PR3)2Cl]X complexes and mixed
bis(carbene)ruthenium-arene derivatives were already
obtained via an indirect route.30
Ca t a lyt ic Syn t h esis of F u r a n Der iva t ives.
Whereas ruthenium(II) catalysts allow the electrophilic
activation of (E)-3-methylpent-2-en-4-yn-1-ol (7) toward
the addition of carboxylates to provide a direct access
to a variety of dienes,33 we have shown that the
activation of its Z isomer 8 could constitute a way to
generate furan derivatives34 but that catalytic efficiency
could be improved. The catalytic activity of the (arene)-
RuCl2(carbene) derivatives 1-3 in the transformation
8 f 9 was thus investigated (eq 2).
In order to coordinate several carbene ligands to the
ruthenium(II) site, [RuCl2(COD)]n,31 which is known to
easily release the cyclooctadiene ligand, was reacted
with a large excess of the functional alkene I in toluene.
A yellow precipitate, identified as RuCl2(dC(NMe)C6-
H4NMe)3 (6), was isolated in 98% yield (Scheme 2). The
coordination of three carbene ligands to the RuCl2
moiety was shown by the presence of one 13C signal at
δ 195.5 ppm (RudC) and one at δ 34.3 ppm (NMe). This
synthesis of the formal 16-electron ruthenium complex
6, isoelectronic with RuCl2(PPh3)3 which appears to be
a key catalyst precursor, may have also a potential for
catalysis.
(2)
(32) (a) Doucet, H.; Ho¨fer, J .; Bruneau, C.; Dixneuf, P. H. J . Chem.
Soc., Chem. Commun. 1993, 850-851. (b) Darcel, C.; Bruneau, C.;
Dixneuf, P. H.; Neef, G. J . Chem. Soc., Chem. Commun. 1994, 333-
334.
(33) Bruneau, C.; Kabouche, Z.; Neveux, M.; Seiller, B.; Dixneuf, P.
H. Inorg. Chim. Acta 1994, 222, 155-163.
(34) Seiller, B.; Bruneau, C.; Dixneuf, P. H. J . Chem. Soc., Chem.
Commun. 1994, 493-494.
(28) (a) Le Bozec, H.; Fillaut, J .-L.; Dixneuf, P. H. J . Chem. Soc.,
Chem. Commun. 1986, 1182-1185. (b) Osborn, V. A.; Parker, C. A.;
Winter, M. J . J . Chem. Soc., Chem. Commun. 1986, 1185-1186.
(29) Werner, H.; Kletzin, H. J . Organomet. Chem. 1982, 228, 289.
(30) Dussel, R.; Pilette, D.; Dixneuf, P. H.; Fehlhammer, W. P.
Organometallics 1991, 10, 3287-3291.
(31) Mu¨ller, J .; Fischer, E. O. J . Organomet. Chem. 1966, 5, 275.

2438 Organometallics, Vol. 15, No. 10, 1996
Ku¨cu¨kbay et al.
3
Ta ble 2. Ca ta lytic Syn th esis of
2,3-Dim eth ylfu r a n (9)a
7.29-7.25 (m, 4 H, C6H4), 5.45 and 5.17 (4 H, MeC6H4-, J HH
3
) 6 Hz), 4.10 (s, 6 H, NCH3), 2.96 (sept, 1 H, CHMe2, J HH
)
7 Hz), 1.99 (s, 3 H, C6H4CH3), 1.23 (d, 6 H, (CH3)2CHC6H4,
3J HH ) 7 Hz). 13C{1H} NMR (125.8 MHz, CDCl3, 297 K, δ in
ppm): 188.89 (s, RudC), 135.40, 122.93, 109.62, and 99.08 (s,
C6H4), 85.87 (s, MeC6H4-), 82.82 (s, MeC6H4-), 36.21 (s,
NCH3), 30.58 (s, -C6H4CHMe2), 22.28 (s, -C6H4CH3), 18.59
(s, CH(CH3)2).
catalyst
time (h)
yield (%)b
1a
1a
1a
3a
3b
3b
6
2
10
25
25
15
25
25
45
67
79
75
48
72
23
Syn th esis of [Ru Cl2(LEt)(p-MeC6H4CHMe2)] (1b). Simi-
lar to the above procedure, compound 1b was synthesized from
bis(1,3-diethylbenzimidazolidine-2-ylidene) (II; 1.1 mmol) and
[RuCl2(p-MeC6H4CHMe2)]2 (1.1 mmol) (yield 88%). Mp: 215-
216 °C. Anal. Calcd for C21H28N2RuCl2: C, 52.50; H, 5.83;
N, 5.83. Found: C, 52.26; H, 5.92; N, 5.84. Mass (m/ z): 480
(21, M+), 445 (50, [M - 35]+), 174 (6, [M - 306]+). 1H NMR
(250 MHz, CDCl3, 297 K, δ in ppm): 7.40-7.20 (m, 4 H, C6H4),
Experimental conditions: 10-2 mol of (Z)-3-methylpent-2-en-
a
4-yn-1-ol (8) and 10-4 mol of catalyst 1, 3, or 6 without solvent at
b
60 °C. Yield calculated on distilled products.
The reaction of (Z)-3-methylpent-2-en-4-yn-1-ol (8; 10
mmol) in the presence of 1a , 3a , or 3b (0.1 mmol) as
catalyst precursor, without solvent, led to a complete
conversion of the alkyne after 20-25 h at 60 °C and to
the quantitative formation of the 2,3-dimethylfuran 9
which was isolated on distillation in 79, 75 and 72%
yield, respectively (Table 2). Under these conditions,
with 1 mol % of catalyst, 1a appeared to be the best
catalyst. The slightly less electron deficient (carbene)-
ruthenium complexes 3a ,b show a slightly weaker
activity, and the electron-deficient but bulky tris-
(carbene)ruthenium complex 6 did not show a reason-
able activity.
3
5.40 and 5.10 (4 H, MeC6H4-, J HH ) 6 Hz), 4.90 (2 HA) and
4.60 (2 HB) (m, 2 NCH2Me; the 2J H
value could not be
H
A
B
determined), 2.90 (sept, 1 H, CHMe2,3J HH ) 7 Hz), 1.94 (s, 3
3
H, C6H4CH3), 1.56 (t, 6 H, CH3CH2N, J HH ) 7.9 Hz), 1.21 (d,
6 H, (CH3)2CHC6H4), 3J HH ) 7 Hz). 13C{1H} NMR (125.8 MHz,
CDCl3, 297 K, δ in ppm): 187.91 (s, RudC), 134.89, 122.58,
110.54, and 99.11 (s, C6H4), 86.29 (s, MeC6H4-), 82.77 (s,
MeC6H4-), 44.68 (s, CH2Me), 30.48 (s, -C6H4CHMe2), 22.35
(s, CH3C6H4), 18.43 (s, -NCH2CH3), 15.86 (s, CH(CH3)2).
Syn th esis of [Ru Cl2(LMe)(C6H3Me3)]2 (2a ). To a solution
of bis(1,3-dimethylbenzimidazolidine-2-ylidene) (I; 0.5 mmol)
in 10 mL of toluene was added [RuCl2(C6H3Me3)]2 (0.5 mmol)
and the mixture was heated for 5 h at 100 °C. A light brown
precipitate appeared. The product was filtered off from the
hot solution, washed with Et2O and then dried under vacuum
(yield 96%). Mp: >300 °C dec. Anal. Calcd for C18H22N2-
RuCl2: C, 49.31; H, 5.02; N, 6.39. Found: C, 49.68; H, 5.23;
N, 6.49. 1H NMR (300.13 MHz, CD2Cl2, 297 K, δ in ppm):
7.45-7.25 (m, 4 H, C6H4), 4.90 (s, 3 H, C6H3Me3), 4.20 (s, 6 H,
-NCH3), 2.05 (s, 9 H, C6H3(CH3)3). 13C{1H} NMR (50.289
MHz, CDCl3, 297 K, δ in ppm): 196.00 (s, RudC), 137.74,
123.64, and 111.72 (s, C6H4), 107.34 (s, C6H3Me3), 82.04 (s,
C6H3Me3), 38.37 (s, NCH3), 21.06 (s, C6H3(CH3)3).
Exp er im en ta l Section
Gen er a l Da ta . All reactions were performed under an
argon or dinitrogen atmosphere with use of Schlenk tech-
niques. The solvents were deoxygenated and dried by stand-
ard methods. Infrared spectra were recorded on a Nicolet 205
FT-IR spectrometer. 1H (300.13 and 250 MHz) and 13C (50.289
and 125.8 MHz) NMR spectra were recorded on Gemini-Varian
and Bruker AMX-500 and AC300P spectrometers at 297 K and
referenced to TMS. Mass spectra were obtained with Varian
MAT 711 and Kratos MS902 instruments. Elemental analyses
were performed by the Service Central de Microanalyse du
CNRS at Vernaison.
Syn th esis of [Ru Cl2(LEt)(C6H3Me3)]2 (2b). Similar to the
above procedure, compound 2b, [RuCl2(LEt)(C6H3Me3)]2, was
synthesized from bis(1,3-diethylbenzimidazolidine-2-ylidene)
(II) and [RuCl2(C6H3Me3)]2 (yield 95%). Mp: 293-295 °C.
Anal. Calcd for C20H26N2RuCl2: C, 51.50; H, 5.58; N, 6.00.
Cyclic Volta m m etr y. Conventional electrochemical equip-
ment was used: EGG PAR Model 362 scanning potentiostat
with a BD90 X-Y recorder. The working electrode was a
stationary platinum-disk electrode of 1-mm diameter. The
auxiliary electrode was a platinum electrode, and the reference
electrode was an aqueous saturated calomel electrode (SCE).
In a typical experiment, 4 × 10-5 mol of complex was dissolved,
under a dinitrogen atmosphere, in 15 mL of dried dichloro-
methane containing 0.15 g of pure Bu4NPF6 as electrolyte.
The complexes [RuCl2(p-MeC6H4CHMe2)]2,35 [RuCl2(COD)]n,31
[RuCl2(1,3,5-C6H3Me3)]2,36 and [RuCl2(C6Me6)]235 and electron-
rich olefins I,23 II,24 and III37 were prepared by literature
methods. The commercial (J anssen) (Z)-3-methyl-2-penten-
4-yn-1-ol was used as received.
Found: C, 51.65; H, 5.58; N, 6.35. Mass (m/ z): calcd for
102
C
20H26N2Cl2 Ru 466.056, found 466.052. 1H NMR (300.13
MHz, CD2Cl2, 297 K, δ in ppm): 7.45-7.25 (m, 4 H, C6H4),
4.90 and 4.35 (dq, 4 H, (2 HA, 2 HB), 2 NCH2Me, 2J H B ) 13.4
H
A
3
Hz, J HH ) 7 Hz), 4.75 (s, 3 H, C6H3Me3), 1.90 (s, 9 H, C6H3-
(CH3)3), 1.50 (s, 6 H, -NCH2CH3, 3J HH ) 7 Hz). 13C{1H} NMR
(50.289 MHz, CDCl3, 297 K, δ in ppm): 195.00 (s, RudC),
137.34, 124.66, and 112.64 (s, C6H4), 107.60 (s, C6H3Me3), 81.86
(s, C6H3Me3), 46.84 (s, CH2Me), 20.93 (s, -C6H3(CH3)3), 18.21
(s, -NCH2CH3).
Syn th esis of [Ru Cl2(LMe)(C6Me6)] (3a ). A solution of bis-
(1,3-dimethylbenzimidazolidine-2-ylidene) (I; 1.0 mmol) in 10
mL of toluene was added to [RuCl2(C6Me6)]2 (1.0 mmol), and
the mixture was heated for 4 h at 100 °C. A brown precipitate
appeared. The product was filtered off from the hot solution,
washed with Et2O and then dried under vacuum (yield 96%).
Mp: 323-325 °C. Anal. Calcd for C21H28N2RuCl2: C, 52.50;
H, 5.83; N, 5.83. Found: C, 52.45; H, 5.77; N, 5.476. Mass
(m/ z): calcd for C21H28N2102RuCl2 480.067, found 480.071. 1H
NMR (300.13 MHz, CD2Cl2, 297 K, δ in ppm): 7.35-7.18 (m,
4 H, C6H4), 4.00 (s, 6 H, NCH3), 1.98 (s, 18 H, C6(CH3)6).
13C{1H} NMR (50.289 MHz, CDCl3, 297 K, δ in ppm): 195.50
(s, RudC), 137.98, 124.84, and 111.49 (s, C6H4), 96.42 (s,
C6Me6), 37.99 (s, NCH3), 17.57 (s, C6(CH3)6).
Syn th esis of [Ru Cl2(LMe)(p-MeC6H4CHMe2)] (1a ).
A
solution of bis(1,3-dimethylbenzimidazolidine-2-ylidene) (I; 1.4
mmol) in 10 mL of toluene was added to [RuCl2(p-MeC6H4-
CHMe2)]2 (1.4 mmol), and the mixture was heated for 4 h at
100 °C. A brown precipitate appeared. The product formed
was filtered off from the hot solution, washed with Et2O, and
then dried under vacuum (yield 93%). Mp: 195-196 °C. Anal.
Calcd for C19H24N2RuCl2: C, 50.44; H, 5.31; N, 6.19. Found:
C, 50.65; H, 5.38; N, 6.85. Mass (m/ z): 452 (20, M+), 417 (40,
[M - 35]+). 1H NMR (250 MHz, CDCl3, 297 K, δ in ppm):
(35) Bennett, M. A.; Huang, T.-N.; Matheson, T. W.; Smith, A. K.
Inorg. Synth. 1982, 21, 74-78.
Syn th esis of [Ru Cl2(LEt)(C6Me6)] (3b). Similar to the
above procedure, compound 3b was synthesized from bis(1,3-
diethylbenzimidazolidine-2-ylidene) (II; 1.2 mmol) and
[RuCl2(C6Me6)]2 (1.2 mmol) and obtained in 79% yield. Mp:
(36) (a) Bennett, M. A.; Smith, A. K. J . Chem. Soc., Dalton Trans.
1974, 233-241.
(37) Winberg, H. E.; Carnahan, J . E.; Coffman, D. D.; Brown, M. J .
Am. Chem. Soc. 1965, 87, 2055.

New (Carbene)ruthenium-Arene Complexes
Organometallics, Vol. 15, No. 10, 1996 2439
275-277 °C. Anal. Calcd for C23H32N2RuCl2: C, 54.33; H,
6.02; N, 5.51. Found: C, 54.39; H, 6.20; N, 5.45. Mass (m/ z)
calcd for C23H32N2RuCl2 508.099; found 508.102. 1H NMR
stable even under an inert atmosphere, and its spectroscopic
data had to be immediately obtained. Its elemental analysis
could not be determined. Mp: >300 °C. IR (cm-1, KBr): 1924,
1885 (s, Ru-H). 1H NMR (300 MHz, C6D6, 297 K, δ in ppm):
6.93-6.68 (m, 4 H, C6H4), 3.70 (s, 6 H, NCH3), 2.15 (s, 18 H,
C6(CH3)6), -9.71 (s, 2 H, Ru-H).
(300.13 MHz, CD2Cl2, 297 K, δ in ppm): 7.45-7.18 (m, 4 H,
2
C6H4), 4.95 and 4.10 (dq, 4 H, (2HA, 2HB), NCH2Me, J H
)
H
A
B
3
12.9 Hz, J HH ) 6.9 Hz), 1.95 (s, 18 H, C6(CH3)6), 1.50 (t, 6 H,
CH3CH2N, 3J HH ) 6.9 Hz). 13C{1H} NMR (50.289 MHz, CDCl3,
297 K, δ in ppm): 194.00 (s, RudC), 137.31, 124.40, and 112.25
(s, C6H4), 96.46 (s, C6Me6), 46.30 (s, CH2Me), 18.09 (s,
-NCH2CH3), 17.44 (s, C6(CH3)6).
Syn th esis of [Ru Cl2(LMe)3] (6). To a solution of bis(1,3-
dimethylbenzimidazolidine-2-ylidene) (I; 1.3 mmol) in 20 mL
of toluene was added [RuCl2(COD)]n (0.43 mmol) and the
mixture was heated for 4 h at 100 °C. A yellow precipitate
appeared. The product was filtered off from the hot solution,
washed with Et2O and then dried under vacuum (yield: 98%).
Mp: 303-305 °C. Anal. Calcd for C27H30N6RuCl2: C, 53.11;
H, 4.92; N, 13.77. Found: C, 54.08; H, 5.01; N, 13.93. Mass
(m/ z): 610 (3, M+), 575 (65, [M - 35]+), 464(10, [M - 146]+),
140 (35, [M - 464]+). 1H NMR (250 MHz, CDCl3, 297 K, δ in
ppm): 7.26-7.22 (m, 12 H, C6H4), 3.63 (s, 18 H, -NCH3).
13C{1H} NMR (125.8 MHz, CDCl3, 297 K, δ in ppm): 195.50
(s, RudC), 136.19, 121.63, and 108.53 (s, C6H4), 34.33 (s,
NCH3).
Syn th esis of [Ru Cl2(LBz)(p-MeC6H4CHMe2)] (4). A solu-
tion of bis(1,3-dibenzylimidazolidine-2-ylidene) (III; 0.6 mmol)
in 10 mL of toluene was added to [RuCl2(p-MeC6H4CHMe2)]2
(0.6 mmol), and the mixture was heated for 4 h at 90 °C. A
brown precipitate appeared. The product was filtered off from
the hot solution, washed with Et2O, and then dried under
vacuum (yield 87%). Mp: 230-232 °C. Anal. Calcd for
C
27H32N2RuCl2: C, 58.27; H, 5.76; N, 5.03. Found: C, 58.48;
H, 5.91; N, 5.33. Mass (m/ z): 556 (5, M+), 350 (94, [M -
206]+), 249 (18, [M - 307]+. 1H NMR (250 MHz, CDCl3, 297
K, δ in ppm): 7.55-7.26 (m, 10 H, C6H5), 5.41 and 5.14 (m, 4
Ca ta lytic Syn th esis of 2,3-Dim eth ylfu r a n (9). In a
typical experiment, to 10 mmol of (Z)-3-methylpent-2-en-4-yn-
1-ol (8) was added 0.1 mol of the ruthenium catalyst 1, 3, or 6
without additional solvent. The mixture was stirred in an oil
bath at 60 °C for 25 h. The mixture was distilled at
atmospheric pressure; bp 90-91 °C. Yields are indicated in
Table 2. IR (pure, ν): 1605 cm-1 (CdC). Mass (m/ z): calcd
for C6H8O 96.0572, found 96.0575. 1H NMR (300.13 MHz,
3
H, MeC6H4-, J HH ) 6 Hz), 5.39 and 4.90 (AB system, 4 H, 2
NCH2Ph, J H
) 14.7 Hz), 3.51 (m, 4 H, NCH2CH2N), 2.89
H
A
B
3
(sept, 1 H, CHMe2, J HH ) 7 Hz), 2.14 (s, 3 H, -C6H4CH3),
1.26 (d, 6 H, (CH3)2CHC6H4-), J HH ) 7 Hz). 13C{1H} NMR
3
(125.8 MHz, CDCl3, 297 K, δ ppm): 208.36 (s, RudC), 136.96,
128.62, 127.65, and 127.55 (s, C6H4), 108.20 (s, MeC6H4-),
97.69 (s, MeC6H4-), 85.68 (s, MeC6H4-), 83.50 (s, MeC6H4-),
55.75 (s, NCH2Ph), 48.81 (s, -NCH2CH2N-), 30.63 (s, CHMe2),
22.49 (s, -C6H4CH3), 18.67 (s, (CH3)2CHC6H4-).
3
CDCl3, δ in ppm): 7.19 (d, H5, J HH ) 1.8 Hz), 6.14 (d, H4),
2.19 (s, Me(3)), 1.94 (s, Me(2)).
Syn t h esis of [R u H2(LMe)(C6Me6)] (5). A suspension of
[RuCl2(LMe)(C6Me6)] (0.6 mmol) in 10 mL of dried EtOH was
added to NaBH4 (2.5 mmol), and the mixture was stirred at
room temperature for 10 h. The color changed progressively
from brown to yellow, and the suspension became nearly a
solution in this period. The mixture was filtered, and the clear
filtrate was then concentrated to ca. 5 mL; Et2O (3 mL) was
added, and the solution was cooled to -25 °C to yield the
cream-colored compound (yield 72%). The complex 5 is not
Ack n ow led gm en t. We are grateful to TU¨ BITAK
and CNRS for support (cooperative program 2504), to
the University Chouaib Doukkali of El J adida, Morocco,
for a leave of absence (S.G.), and to Prof. C. Moinet,
University of Rennes, for helpful assistance in electro-
chemical studies.
OM960002L