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Synthesis of enol esters catalysed by ‘early–late’ Ti–Ru complexes
Inorganica Chimica Acta 350 (2003) 289 /292 www.elsevier.com/locate/ica
Synthesis of enol esters catalysed by ‘early late’ Ti Ru complexes /
/
Pierre Le Gendre, Virginie Comte, Ame´lie Michelot, Claude Moı¨se * Laboratoire de Synthe`se et d’Electrosynthe`se Organome´talliques, LSEO-UMR 5632, Universite´ de Bourgogne, Faculte´ des Sciences Gabriel, 6 bd Gabriel, 21000 Dijon, France Received 28 June 2002; accepted 2 September 2002 Dedicated to Dr Pierre Braunstein, a friend and a distinguished scientist
Abstract The titane /ruthenium heterobimetallic compounds (p -cymene)[(h5-C5H5)(m-h5:h1-C5H4(CH2)m PR2)TiCl2]RuCl2 4 /6 have been revealed to be quite good catalysts for the addition of formic acid to 1-hexyne and phenylacetylene. These preliminary results led us to synthesize new tetrametallic complexes 10 /12 via the reaction of the titanocene phosphanes 1 /3 with the polymer [Ru(CO)2(mO2CH)]n . Their catalytic ability for the enol esters formation has been studied. # 2003 Elsevier Science B.V. All rights reserved. Keywords: Titanocene; Ruthenium; Bimetallic complexes; Enol esters
1. Introduction The preparation and the study of early /late heterobimetallic complexes constitute a particularly active research area in organometallic chemistry owing to their potential application as catalysts [1], materials [2], or as biomimetic models [3]. As a part of our ongoing research in this field [4] we focused our attention on the study of bimetallic complexes built from an arene ruthenium and a titanocene dichloride fragments [5]. Both moieties are able to catalyse a wide variety of reactions [6] and therefore their combination constitute a new material with powerful catalytic ability. Recently we reported the synthesis of the bimetallic complexes 4 /6 through the complexation of the titanocene monophosphanes 1/3 with the binuclear complex [(p -cymene)RuCl2]2 (Scheme 1) [5a]. Preliminary assessment of the performance of these complexes in ringclosing metathesis revealed an excellent Ti /Ru precatalyst [5c]. Herein we describe a new application of these bimetallic systems as catalysts for the synthesis of enol formate. Indeed the monometallic (arene)RuCl2(PR3) * Corresponding author. Tel.: /33-3-8039 6081; fax: /33-3-8039 6076. E-mail address: [email protected] (C. Moı¨se).
complexes are known to catalyse the Markovnikov addition of carboxylic acids to terminal alkynes resulting in the formation of enol esters [7]. Good insight of the influence of the titanocene fragment on its late metal neighbour were thus obtained by comparing the performance of the bimetallic complexes 4/6 in the addition of formic acid to 1-hexyne with the activity of their monometallic analog. These preliminary results led us to synthesize and characterize a serie of new tetrametallic complexes Ru2(m-O2CH)2(CO)4[(h5-C5H5)(mh5:h1-C5H4(CH2)m PR2)TiCl2]2 10 /12. Their catalytic potential has also been evaluated.
2. Experimental 2.1. General procedures All reactions were carried out under an atmosphere of purified argon. The solvents and eluents were dried by the appropriate procedure and distilled under argon immediately before use. Standard Schlenk techniques and conventional glass vessels were employed. Elemental analyses were carried out with a EA 1108 CHNS-O FISONS Instruments. 1H (300 MHz) and 31P{1H} (75 MHz) spectra were collected on a Bruker Avance 300 MHz spectrometer. Chemical shifts are relative to
0020-1693/03/$ - see front matter # 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0020-1693(02)01563-3
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2.3. Ru2(m-O2CH)2(CO)4[(h5-C5H5)(m-h5:h1-C5H4PPh2)TiCl2]2 (10)
Scheme 1.
internal TMS (1H) or external H3PO4 (31P). IR spectra were recorded with a IR-FT Bruker Vector 22. The conversion of hexyne for the catalytic enol formate synthesis was monitored on a GC-17A SHIMADZU with a 007 Methyl 5% Phenyl Silicone Capillary column (30 m). The heterobimetallic complexes 4 /6 were synthesized as previously reported 5a,5c. (p-cymene)RuCl2(PPh3) (7), (p -cymene)RuCl2(PCy3) (8) [8] and the polymer [Ru(m-O2CH)(CO)2]n [9] were prepared according to the literature methods.
2.2. Catalytic enol formate synthesis A 5-ml Schlenk flask was charged under argon with 0.025 mmol of catalyst 4/8 (0.012 mmol in case of 9/ 12). A toluene solution (4 ml) of 1-hexyne (205 mg, 2.5 mmol) and n -decane (205 mg, 1.4 mmol) as internal standard were then added followed by 95 ml (2.5 mmol) of formic acid. The solution was stirred at 90 8C and the reaction monitored by GC (Tables 1 and 2). After cooling and evaporation of the solvent, the crude product mixture was analysed using 1H NMR spectroscopy. The collected data compared with those in the literature confirm the formation of enol formate.
A suspension of [Ru(m-O2CH)(CO)2]n (30 mg, 0.15 mmol) and (h5-C5H5)(h5-C5H4-PPh2)TiCl2 (1) (64 mg, 0.15 mmol) in methylcyclohexane (5 ml) was heated at 50 8C for 5 h. The resulting orange precipitate was collected by filtration and dried under vacuum. The residue was redissolved in 5 ml of dichloromethane and an orange/brown powder was isolated after filtration and evaporation of the solvent. 10 was recrystallized from toluene as dark crystals (35 mg, 37% yield). IR (KBr): 2029 (s, nCO), 1993 (m, nCO), 1957 (s, nCO), 1580 (s, n O2CH) cm1. 1H NMR (CDCl3): d/6.46 (s, 10H, Cp), 6.76 (m, 4H, Cp?), 7.60 /7.19 (m, 24H, Ph/ Cp?), 8.18 (s, 2H, O2CH). 31P{1H} NMR (CDCl3): d/ 1.09 (s). Anal. Calc. for C50H40O8P2Cl4Ru2Ti2: C, 47.27; H, 3.17. Found: C, 47.24; H, 3.03.
2.4. Ru2(m-O2CH)2(CO)4[(h5-C5H5)(m-h5:h1C5H4(CH2)2PPh2)TiCl2]2 (11) A suspension of [Ru(m-O2CH)(CO)2]n (30 mg, 0.15 mmol) and (h5-C5H5)[h5-C5H4(CH2)2-PPh2]TiCl2 (2) (68 mg, 0.15 mmol) in methylcyclohexane (5 ml) was heated under reflux for 1.5 h. The resulting orange precipitate was treated by the same procedure as above and isolated as red/orange crystalline powder after recrystallization from a dichloromethane/hexane mixture (50 mg, 51% yield). IR (KBr): 2020 (s, n CO), 1977 (m, nCO), 1945 (s, n CO), 1589 (s, n O2CH) cm 1. 1H NMR (CDCl3): d /2.84 (m, 8H, CH2), 6.33 (m, 4H, Cp?), 6.38 (m, 4H, Cp?), 6.50 (s, 10H, Cp), 7.44 /7.29 (m, 20H, Ph), 8.25 (s, 2H, O2CH). 31P{1H} NMR (CDCl3): d /11.9 (s). Anal. Calc. for C54H48O8P2Cl4Ru2Ti2: C, 48.89; H, 3.65. Found: C, 49.52; H, 4.02.
2.5. Ru2(m-O2CH)2(CO)4[(m-h5-C5H5)(h5:h1C5H4(CH2)2PCy2)TiCl2]2 (12) Table 1 Addition of formic acid to 1-hexyne catalysed by the heterobimetallic complexes 4 /6 or by the monometallic complexes 7 /8 Catalyst
Conversion of hexyne (%) a
Gem/(Z/E ) ratio
Ti /PPh2/Ru (4) Ti /(CH2)2PPh2/Ru (5) Ti /(CH2)2PCy2/Ru (6) PPh3/Ru (7) Pcy3/Ru (8)
86 70 66 92 81
82/18 60/40 66/34 80/20 82/18
Conditions: catalyst (0.025 mmol), 1-hexyne (2.5 mmol), formic acid (2.5 mmol), toluene (4 ml), 90 8C, 21 h. a Determinated by GC using n -decane as internal standard.
The same procedure as above with [Ru(mO2CH)(CO)2]n (30 mg, 0.15 mmol) and (h5-C5H5)[h5C5H4(CH2)2-PCy2]TiCl2 (3) (70 mg, 0.15 mmol) led to 12 which was isolated as orange crystals after recrystallization in a dichloromethane/hexane mixture (60 mg, 60% yield). IR (KBr): 2019 (s, nCO), 1972 (m, nCO), 1943 (s, nCO), 1599 (s, n O2CH) cm 1. 1H NMR (CDCl3): d /3.00 /1.25 (m, 52H, Cy/CH2), 6.37 (m, 4H, Cp?), 6.40 (m, 4H, Cp?), 6.52 (s, 10H, Cp), 8.15 (s, 2H, O2CH). 31P{1H} NMR (CDCl3): d/20.9 (s). Anal . Calc. for C54H72O8P2Cl4Ru2Ti2: C, 48.01; H, 5.37. Found: C, 49.08; H, 6.40.
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Table 2 Addition of formic acid to 1-hexyne catalysed by the tetrametallic complexes 10 /12 or by the homobimetallic one 9 Catalyst
Conversion of hexyne (%) a
Gem/(Z/E ) ratio
t (h)
[PPh3/Ru(m-O2CH)]2 (9) [Ti /PPh2/Ru(m-O2CH)]2 (10) [Ti /(CH2)2PPh2/Ru(m-O2CH)]2 (11) [Ti /(CH2)2PCy2/Ru(m-O2CH)]2 (12)
95 94 63 60
90/10 85/15 70/30 62/38
6 21 15 21
Conditions: catalyst (0.012 mmol), 1-hexyne (2.5 mmol), formic acid (2.5 mmol), toluene (4 ml), 90 8C. a Determinated by GC using n -decane as internal standard.
3. Results and discussion We carried out the addition of formic acid to 1hexyne in toluene at 90 8C by using the bimetallic complexes 4/6. For comparative purposes the monometallic complexes (p -cymene)RuCl2(PPh3) (7) and (p cymene)RuCl2(PCy3) (8) were also tested in similar conditions. A similar reactivity was first observed in a sense that all catalyse the reaction and give exclusively the coupling product (Table 1). No by-product can be observed in the 1 H NMR spectrum of the crude reaction mixture. Looking at these results more in details, it appears that only the bimetallic complex 4 is able to compete with 7 and 8 in terms of reactivity and selectivity. Both others led to lower conversions and selectivities. In line with previous observations, the nature of the phosphane
Fig. 1. Consumption of hexyne vs. time. Catalysts used: (^), Ti / PPh2/Ru (4); (I), Ti /(CH2)2PPh2/Ru (5); (k),Ti/(CH2)2PCy2/Ru (6); (j), PPh3/Ru (7); (m), PCy3/Ru (8). (Reaction conditions as in Table 1.)
does not seem to be crucial for this reaction [7c]. Hence, although the phosphanes 2 and 3 are very different in term of cone angle and basicity, their catalytic performances are quite similar. Thus the difference of reactivity between 4 and 5 /6 can be attributed to the titanocene fragment. A kinetic study which shows the conversion of hexyne vs. time highlights this phenomenon (Fig. 1). The shape of the curve with the catalyst 4 differs greatly from the others. Indeed, after an induction period, a turnover frequency of 19 h1, which is almost four times higher than 5 /6, is reached. The catalytic addition of formic acid to phenylacetylene in toluene at 90 8C with the aid of the bimetallic complexes 4 /6 has also been attempted. The better catalyst is again the complex 4, it permitted to obtain the geminal enol formate with 90% of conversion and 90% of regioselectivity. It is noteworthy that in the above reactions, the color of the reaction mixture changes from brick red to yellow. In the case of (p -cymene)RuCl2(PPh3) (7) this phenomenon is explained by the in situ formation of a canary yellow binuclear derivative Ru2(mO2CH)2(CO)4(PPh3)2 (9). This homobimetallic complex has appeared to be one of the most efficient catalyst for the generation of enol esters [7c]. Considering this result, we focused our attention on the synthesis of the formate tetrametallic complexes 10/12 which are the homolog of 9 with two pendent titanocene dichloride groups. We achieved our goal by using a method employed for a polymeric phosphane [10]. The reaction of the phosphanes 1/3 with the polymer [Ru(CO)2(m-O2CH)]n in methylcyclohexane proceeds smoothly and give the targeted complexes 10 /12 in a very clean manner (Scheme 2).
Scheme 2.
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contribute to demonstrate the versatility of these systems toward various catalytic reactions [5c]. Moreover, this study led us to develop the synthesis of new tetrametallic complexes which are also good candidates for the alkyne electrophilic activation. Further applications of these complexes in cyclopropanation and polymerisation are under study.
References
Fig. 2. Consumption of hexyne vs. time. Catalysts used: (j), [PPh3/ Ru(m-O2CH)]2 (9); (^), [Ti /PPh2/Ru(m-O2CH)]2 (10); (I), [Ti/ (CH2)2PPh2/Ru(m-O2CH)]2 (11); (k), [Ti/(CH2)2PCy2/Ru(m-O2CH)]2 (12). (Reaction conditions as in Table 2.)
Their infrared spectra show four characteristic stretching bands, three n (CO) bands slightly shifted to lower wave number than the polymer (2038, 1998, 1943 cm 1) and a m-formate stretching band in the 1600 cm 1 region. The 31P{1H} NMR spectra of 11 and 12 exhibit a singlet at d/12 and 20 ppm respectively, which are consistent with coordinated phosphane ligands. It is worth mentioning that 10 gives an unusual 31 P chemical shift at d/1 ppm which might be explained by a constraint geometry around the phosphorus atom. We therefore tested the catalytic behaviour of the tetrametallic complexes 10/12 in the enol formate reaction as previously described. For comparative purpose, a reaction with the homobimetallic complex 9 has been carried out. Although the tetrametallic compounds 10/12 appeared to be less active than 9 (Table 2), they were revealed generally more efficient than their bimetallic counterparts (Fig. 2). The tetrametallic complex 10 built from the phosphane 1 emerged as a very efficient catalyst for the regioselective addition of formic acid to hexyne. Indeed it led to 94% of conversion and 85% of the geminal isomer. Both others gave lower conversions and selectivities which confirms the dramatic influence of the spacer between the titanium and the ruthenium atoms on the catalyst.
4. Conclusion These results constitute a new example of application of ‘early/late’ complexes in the catalysis field and so
[1] (a) N. Wheatley, P. Kalck, Chem. Rev. (1999) 3379; (b) L. Gelmini, D.W. Stephan, Organometallics 5 (1986) 67; (c) R. Choukroun, D. Gervais, P. Kalck, F. Senocq, J. Organomet. Chem. 335 (1987) C9; (d) M.J. Hostetler, M.D. Butts, R.G. Bergman, J. Am. Chem. Soc. 115 (1993) 2743; (e) A.M. Trzeciak, J.J. Ziolkowski, R. Choukroun, J. Mol. Catal. 110 (1996) 135; (f) Y. Yamaguchi, N. Suzuki, T. Mise, Y. Wakatsuki, Organometallics 18 (1999) 996; (g) B.E. Bosch, I. Bru¨mmer, K. Kunz, G. Erker, R. Fro¨hlich, S. Kotila, Organometallics 19 (2000) 1255. [2] S.J. Tauster, S.C. Fung, L.R. Garten, J. Am. Chem. Soc 100 (1978) 170. [3] (a) D. Coucouvanis, Acc. Chem. Res. 14 (1981) 201; (b) R. Kra¨mer, Coord. Chem. Rev. 182 (1999) 243. [4] G. Boni, M.M. Kubicki, C. Moı¨se, in: P. Braunstein, L.A. Oro, P.R. Raithby (Eds.), Metal Clusters in Chemistry, vol. 1, WileyVCH, Weinheim, 1999, p. 110. [5] (a) P. Le Gendre, P. Richard, C. Moı¨se, J. Organomet. Chem. 605 (2000) 151; (b) P. Le Gendre, E. Maubrou, O. Blacque, G. Boni, C. Moı¨se, Eur. J. Inorg. Chem. (2001) 1437; (c) P. Le Gendre, M. Picquet, P. Richard, C. Moı¨se, J. Organomet. Chem. 643 /644 (2002) 227. [6] (a) W. Kaminsky, J. Chem. Soc., Dalton Trans. (1998) 1413; (b) P.C. Mohring, N.J. Coville, J. Organomet. Chem. 479 (1994) 1; (c) P. Jutzi, T. Redeker, B. Neumann, H.G. Stammler, Organometallics 15 (1996) 4153; (d) F. Simal, D. Jan, A. Demonceau, A.F. Noels, Tetrahedron Lett. 40 (1999) 1653; (e) M. Kitamura, M. Tokunaga, R. Noyori, J. Org. Chem. 57 (1992) 4053; (f) A. Fu¨rstner, M. Liebl, C.W. Lehmann, M. Picquet, R. Kunz, C. Bruneau, D. Touchard, P.H. Dixneuf, Chem. Eur. J. 6 (2000) 1847. [7] (a) C. Ruppin, P.H. Dixneuf, Tetrahedron Lett. 29 (1986) 6323; (b) C. Bruneau, M. Neveux, Z. Kabouche, P.H. Dixneuf, Synlett (1991) 755; (c) C. Bruneau, P.H. Dixneuf, J. Chem. Soc., Chem. Commun. (1997) 507. [8] S. Serron, S.P. Nohan, Organometallics 14 (1995) 4611. [9] G.R. Crooks, B.F.G. Johnson, J. Lewis, I.G. Williams, J. Chem. Soc. A (1969) 2761. [10] O. Lavastre, P. Bebin, O. Marchaland, P.H. Dixneuf, J. Mol. Catal. A 108 (1996) 29.
Synthesis of enol esters catalysed by ‘early late’ Ti Ru complexes /
/
Pierre Le Gendre, Virginie Comte, Ame´lie Michelot, Claude Moı¨se * Laboratoire de Synthe`se et d’Electrosynthe`se Organome´talliques, LSEO-UMR 5632, Universite´ de Bourgogne, Faculte´ des Sciences Gabriel, 6 bd Gabriel, 21000 Dijon, France Received 28 June 2002; accepted 2 September 2002 Dedicated to Dr Pierre Braunstein, a friend and a distinguished scientist
Abstract The titane /ruthenium heterobimetallic compounds (p -cymene)[(h5-C5H5)(m-h5:h1-C5H4(CH2)m PR2)TiCl2]RuCl2 4 /6 have been revealed to be quite good catalysts for the addition of formic acid to 1-hexyne and phenylacetylene. These preliminary results led us to synthesize new tetrametallic complexes 10 /12 via the reaction of the titanocene phosphanes 1 /3 with the polymer [Ru(CO)2(mO2CH)]n . Their catalytic ability for the enol esters formation has been studied. # 2003 Elsevier Science B.V. All rights reserved. Keywords: Titanocene; Ruthenium; Bimetallic complexes; Enol esters
1. Introduction The preparation and the study of early /late heterobimetallic complexes constitute a particularly active research area in organometallic chemistry owing to their potential application as catalysts [1], materials [2], or as biomimetic models [3]. As a part of our ongoing research in this field [4] we focused our attention on the study of bimetallic complexes built from an arene ruthenium and a titanocene dichloride fragments [5]. Both moieties are able to catalyse a wide variety of reactions [6] and therefore their combination constitute a new material with powerful catalytic ability. Recently we reported the synthesis of the bimetallic complexes 4 /6 through the complexation of the titanocene monophosphanes 1/3 with the binuclear complex [(p -cymene)RuCl2]2 (Scheme 1) [5a]. Preliminary assessment of the performance of these complexes in ringclosing metathesis revealed an excellent Ti /Ru precatalyst [5c]. Herein we describe a new application of these bimetallic systems as catalysts for the synthesis of enol formate. Indeed the monometallic (arene)RuCl2(PR3) * Corresponding author. Tel.: /33-3-8039 6081; fax: /33-3-8039 6076. E-mail address: [email protected] (C. Moı¨se).
complexes are known to catalyse the Markovnikov addition of carboxylic acids to terminal alkynes resulting in the formation of enol esters [7]. Good insight of the influence of the titanocene fragment on its late metal neighbour were thus obtained by comparing the performance of the bimetallic complexes 4/6 in the addition of formic acid to 1-hexyne with the activity of their monometallic analog. These preliminary results led us to synthesize and characterize a serie of new tetrametallic complexes Ru2(m-O2CH)2(CO)4[(h5-C5H5)(mh5:h1-C5H4(CH2)m PR2)TiCl2]2 10 /12. Their catalytic potential has also been evaluated.
2. Experimental 2.1. General procedures All reactions were carried out under an atmosphere of purified argon. The solvents and eluents were dried by the appropriate procedure and distilled under argon immediately before use. Standard Schlenk techniques and conventional glass vessels were employed. Elemental analyses were carried out with a EA 1108 CHNS-O FISONS Instruments. 1H (300 MHz) and 31P{1H} (75 MHz) spectra were collected on a Bruker Avance 300 MHz spectrometer. Chemical shifts are relative to
0020-1693/03/$ - see front matter # 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0020-1693(02)01563-3
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2.3. Ru2(m-O2CH)2(CO)4[(h5-C5H5)(m-h5:h1-C5H4PPh2)TiCl2]2 (10)
Scheme 1.
internal TMS (1H) or external H3PO4 (31P). IR spectra were recorded with a IR-FT Bruker Vector 22. The conversion of hexyne for the catalytic enol formate synthesis was monitored on a GC-17A SHIMADZU with a 007 Methyl 5% Phenyl Silicone Capillary column (30 m). The heterobimetallic complexes 4 /6 were synthesized as previously reported 5a,5c. (p-cymene)RuCl2(PPh3) (7), (p -cymene)RuCl2(PCy3) (8) [8] and the polymer [Ru(m-O2CH)(CO)2]n [9] were prepared according to the literature methods.
2.2. Catalytic enol formate synthesis A 5-ml Schlenk flask was charged under argon with 0.025 mmol of catalyst 4/8 (0.012 mmol in case of 9/ 12). A toluene solution (4 ml) of 1-hexyne (205 mg, 2.5 mmol) and n -decane (205 mg, 1.4 mmol) as internal standard were then added followed by 95 ml (2.5 mmol) of formic acid. The solution was stirred at 90 8C and the reaction monitored by GC (Tables 1 and 2). After cooling and evaporation of the solvent, the crude product mixture was analysed using 1H NMR spectroscopy. The collected data compared with those in the literature confirm the formation of enol formate.
A suspension of [Ru(m-O2CH)(CO)2]n (30 mg, 0.15 mmol) and (h5-C5H5)(h5-C5H4-PPh2)TiCl2 (1) (64 mg, 0.15 mmol) in methylcyclohexane (5 ml) was heated at 50 8C for 5 h. The resulting orange precipitate was collected by filtration and dried under vacuum. The residue was redissolved in 5 ml of dichloromethane and an orange/brown powder was isolated after filtration and evaporation of the solvent. 10 was recrystallized from toluene as dark crystals (35 mg, 37% yield). IR (KBr): 2029 (s, nCO), 1993 (m, nCO), 1957 (s, nCO), 1580 (s, n O2CH) cm1. 1H NMR (CDCl3): d/6.46 (s, 10H, Cp), 6.76 (m, 4H, Cp?), 7.60 /7.19 (m, 24H, Ph/ Cp?), 8.18 (s, 2H, O2CH). 31P{1H} NMR (CDCl3): d/ 1.09 (s). Anal. Calc. for C50H40O8P2Cl4Ru2Ti2: C, 47.27; H, 3.17. Found: C, 47.24; H, 3.03.
2.4. Ru2(m-O2CH)2(CO)4[(h5-C5H5)(m-h5:h1C5H4(CH2)2PPh2)TiCl2]2 (11) A suspension of [Ru(m-O2CH)(CO)2]n (30 mg, 0.15 mmol) and (h5-C5H5)[h5-C5H4(CH2)2-PPh2]TiCl2 (2) (68 mg, 0.15 mmol) in methylcyclohexane (5 ml) was heated under reflux for 1.5 h. The resulting orange precipitate was treated by the same procedure as above and isolated as red/orange crystalline powder after recrystallization from a dichloromethane/hexane mixture (50 mg, 51% yield). IR (KBr): 2020 (s, n CO), 1977 (m, nCO), 1945 (s, n CO), 1589 (s, n O2CH) cm 1. 1H NMR (CDCl3): d /2.84 (m, 8H, CH2), 6.33 (m, 4H, Cp?), 6.38 (m, 4H, Cp?), 6.50 (s, 10H, Cp), 7.44 /7.29 (m, 20H, Ph), 8.25 (s, 2H, O2CH). 31P{1H} NMR (CDCl3): d /11.9 (s). Anal. Calc. for C54H48O8P2Cl4Ru2Ti2: C, 48.89; H, 3.65. Found: C, 49.52; H, 4.02.
2.5. Ru2(m-O2CH)2(CO)4[(m-h5-C5H5)(h5:h1C5H4(CH2)2PCy2)TiCl2]2 (12) Table 1 Addition of formic acid to 1-hexyne catalysed by the heterobimetallic complexes 4 /6 or by the monometallic complexes 7 /8 Catalyst
Conversion of hexyne (%) a
Gem/(Z/E ) ratio
Ti /PPh2/Ru (4) Ti /(CH2)2PPh2/Ru (5) Ti /(CH2)2PCy2/Ru (6) PPh3/Ru (7) Pcy3/Ru (8)
86 70 66 92 81
82/18 60/40 66/34 80/20 82/18
Conditions: catalyst (0.025 mmol), 1-hexyne (2.5 mmol), formic acid (2.5 mmol), toluene (4 ml), 90 8C, 21 h. a Determinated by GC using n -decane as internal standard.
The same procedure as above with [Ru(mO2CH)(CO)2]n (30 mg, 0.15 mmol) and (h5-C5H5)[h5C5H4(CH2)2-PCy2]TiCl2 (3) (70 mg, 0.15 mmol) led to 12 which was isolated as orange crystals after recrystallization in a dichloromethane/hexane mixture (60 mg, 60% yield). IR (KBr): 2019 (s, nCO), 1972 (m, nCO), 1943 (s, nCO), 1599 (s, n O2CH) cm 1. 1H NMR (CDCl3): d /3.00 /1.25 (m, 52H, Cy/CH2), 6.37 (m, 4H, Cp?), 6.40 (m, 4H, Cp?), 6.52 (s, 10H, Cp), 8.15 (s, 2H, O2CH). 31P{1H} NMR (CDCl3): d/20.9 (s). Anal . Calc. for C54H72O8P2Cl4Ru2Ti2: C, 48.01; H, 5.37. Found: C, 49.08; H, 6.40.
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Table 2 Addition of formic acid to 1-hexyne catalysed by the tetrametallic complexes 10 /12 or by the homobimetallic one 9 Catalyst
Conversion of hexyne (%) a
Gem/(Z/E ) ratio
t (h)
[PPh3/Ru(m-O2CH)]2 (9) [Ti /PPh2/Ru(m-O2CH)]2 (10) [Ti /(CH2)2PPh2/Ru(m-O2CH)]2 (11) [Ti /(CH2)2PCy2/Ru(m-O2CH)]2 (12)
95 94 63 60
90/10 85/15 70/30 62/38
6 21 15 21
Conditions: catalyst (0.012 mmol), 1-hexyne (2.5 mmol), formic acid (2.5 mmol), toluene (4 ml), 90 8C. a Determinated by GC using n -decane as internal standard.
3. Results and discussion We carried out the addition of formic acid to 1hexyne in toluene at 90 8C by using the bimetallic complexes 4/6. For comparative purposes the monometallic complexes (p -cymene)RuCl2(PPh3) (7) and (p cymene)RuCl2(PCy3) (8) were also tested in similar conditions. A similar reactivity was first observed in a sense that all catalyse the reaction and give exclusively the coupling product (Table 1). No by-product can be observed in the 1 H NMR spectrum of the crude reaction mixture. Looking at these results more in details, it appears that only the bimetallic complex 4 is able to compete with 7 and 8 in terms of reactivity and selectivity. Both others led to lower conversions and selectivities. In line with previous observations, the nature of the phosphane
Fig. 1. Consumption of hexyne vs. time. Catalysts used: (^), Ti / PPh2/Ru (4); (I), Ti /(CH2)2PPh2/Ru (5); (k),Ti/(CH2)2PCy2/Ru (6); (j), PPh3/Ru (7); (m), PCy3/Ru (8). (Reaction conditions as in Table 1.)
does not seem to be crucial for this reaction [7c]. Hence, although the phosphanes 2 and 3 are very different in term of cone angle and basicity, their catalytic performances are quite similar. Thus the difference of reactivity between 4 and 5 /6 can be attributed to the titanocene fragment. A kinetic study which shows the conversion of hexyne vs. time highlights this phenomenon (Fig. 1). The shape of the curve with the catalyst 4 differs greatly from the others. Indeed, after an induction period, a turnover frequency of 19 h1, which is almost four times higher than 5 /6, is reached. The catalytic addition of formic acid to phenylacetylene in toluene at 90 8C with the aid of the bimetallic complexes 4 /6 has also been attempted. The better catalyst is again the complex 4, it permitted to obtain the geminal enol formate with 90% of conversion and 90% of regioselectivity. It is noteworthy that in the above reactions, the color of the reaction mixture changes from brick red to yellow. In the case of (p -cymene)RuCl2(PPh3) (7) this phenomenon is explained by the in situ formation of a canary yellow binuclear derivative Ru2(mO2CH)2(CO)4(PPh3)2 (9). This homobimetallic complex has appeared to be one of the most efficient catalyst for the generation of enol esters [7c]. Considering this result, we focused our attention on the synthesis of the formate tetrametallic complexes 10/12 which are the homolog of 9 with two pendent titanocene dichloride groups. We achieved our goal by using a method employed for a polymeric phosphane [10]. The reaction of the phosphanes 1/3 with the polymer [Ru(CO)2(m-O2CH)]n in methylcyclohexane proceeds smoothly and give the targeted complexes 10 /12 in a very clean manner (Scheme 2).
Scheme 2.
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contribute to demonstrate the versatility of these systems toward various catalytic reactions [5c]. Moreover, this study led us to develop the synthesis of new tetrametallic complexes which are also good candidates for the alkyne electrophilic activation. Further applications of these complexes in cyclopropanation and polymerisation are under study.
References
Fig. 2. Consumption of hexyne vs. time. Catalysts used: (j), [PPh3/ Ru(m-O2CH)]2 (9); (^), [Ti /PPh2/Ru(m-O2CH)]2 (10); (I), [Ti/ (CH2)2PPh2/Ru(m-O2CH)]2 (11); (k), [Ti/(CH2)2PCy2/Ru(m-O2CH)]2 (12). (Reaction conditions as in Table 2.)
Their infrared spectra show four characteristic stretching bands, three n (CO) bands slightly shifted to lower wave number than the polymer (2038, 1998, 1943 cm 1) and a m-formate stretching band in the 1600 cm 1 region. The 31P{1H} NMR spectra of 11 and 12 exhibit a singlet at d/12 and 20 ppm respectively, which are consistent with coordinated phosphane ligands. It is worth mentioning that 10 gives an unusual 31 P chemical shift at d/1 ppm which might be explained by a constraint geometry around the phosphorus atom. We therefore tested the catalytic behaviour of the tetrametallic complexes 10/12 in the enol formate reaction as previously described. For comparative purpose, a reaction with the homobimetallic complex 9 has been carried out. Although the tetrametallic compounds 10/12 appeared to be less active than 9 (Table 2), they were revealed generally more efficient than their bimetallic counterparts (Fig. 2). The tetrametallic complex 10 built from the phosphane 1 emerged as a very efficient catalyst for the regioselective addition of formic acid to hexyne. Indeed it led to 94% of conversion and 85% of the geminal isomer. Both others gave lower conversions and selectivities which confirms the dramatic influence of the spacer between the titanium and the ruthenium atoms on the catalyst.
4. Conclusion These results constitute a new example of application of ‘early/late’ complexes in the catalysis field and so
[1] (a) N. Wheatley, P. Kalck, Chem. Rev. (1999) 3379; (b) L. Gelmini, D.W. Stephan, Organometallics 5 (1986) 67; (c) R. Choukroun, D. Gervais, P. Kalck, F. Senocq, J. Organomet. Chem. 335 (1987) C9; (d) M.J. Hostetler, M.D. Butts, R.G. Bergman, J. Am. Chem. Soc. 115 (1993) 2743; (e) A.M. Trzeciak, J.J. Ziolkowski, R. Choukroun, J. Mol. Catal. 110 (1996) 135; (f) Y. Yamaguchi, N. Suzuki, T. Mise, Y. Wakatsuki, Organometallics 18 (1999) 996; (g) B.E. Bosch, I. Bru¨mmer, K. Kunz, G. Erker, R. Fro¨hlich, S. Kotila, Organometallics 19 (2000) 1255. [2] S.J. Tauster, S.C. Fung, L.R. Garten, J. Am. Chem. Soc 100 (1978) 170. [3] (a) D. Coucouvanis, Acc. Chem. Res. 14 (1981) 201; (b) R. Kra¨mer, Coord. Chem. Rev. 182 (1999) 243. [4] G. Boni, M.M. Kubicki, C. Moı¨se, in: P. Braunstein, L.A. Oro, P.R. Raithby (Eds.), Metal Clusters in Chemistry, vol. 1, WileyVCH, Weinheim, 1999, p. 110. [5] (a) P. Le Gendre, P. Richard, C. Moı¨se, J. Organomet. Chem. 605 (2000) 151; (b) P. Le Gendre, E. Maubrou, O. Blacque, G. Boni, C. Moı¨se, Eur. J. Inorg. Chem. (2001) 1437; (c) P. Le Gendre, M. Picquet, P. Richard, C. Moı¨se, J. Organomet. Chem. 643 /644 (2002) 227. [6] (a) W. Kaminsky, J. Chem. Soc., Dalton Trans. (1998) 1413; (b) P.C. Mohring, N.J. Coville, J. Organomet. Chem. 479 (1994) 1; (c) P. Jutzi, T. Redeker, B. Neumann, H.G. Stammler, Organometallics 15 (1996) 4153; (d) F. Simal, D. Jan, A. Demonceau, A.F. Noels, Tetrahedron Lett. 40 (1999) 1653; (e) M. Kitamura, M. Tokunaga, R. Noyori, J. Org. Chem. 57 (1992) 4053; (f) A. Fu¨rstner, M. Liebl, C.W. Lehmann, M. Picquet, R. Kunz, C. Bruneau, D. Touchard, P.H. Dixneuf, Chem. Eur. J. 6 (2000) 1847. [7] (a) C. Ruppin, P.H. Dixneuf, Tetrahedron Lett. 29 (1986) 6323; (b) C. Bruneau, M. Neveux, Z. Kabouche, P.H. Dixneuf, Synlett (1991) 755; (c) C. Bruneau, P.H. Dixneuf, J. Chem. Soc., Chem. Commun. (1997) 507. [8] S. Serron, S.P. Nohan, Organometallics 14 (1995) 4611. [9] G.R. Crooks, B.F.G. Johnson, J. Lewis, I.G. Williams, J. Chem. Soc. A (1969) 2761. [10] O. Lavastre, P. Bebin, O. Marchaland, P.H. Dixneuf, J. Mol. Catal. A 108 (1996) 29.