Organometallics 1995, 14, 2565-2569
2565
The R~(q~-naphthalene)(~~-cycloocta-l,5-diene)/ Acetonitrile System in the Selective Dimerization of Methyl Acrylate to trans-Dimethyl-2-hexenedioate Paolo Pertici," Valter Ballantini, and Piero Salvadori Centro di Studio del CNR per le Macromolecole Stereordinate ed Otticamente Attive, Dipartimento di Chimica e Chimica Industriale, University of Pisa, via Risorgimento 35, 56100 Pisa, Italy
Martin A. Bennett Research School of Chemistry, Australian National University, Canberra, ACT 0200, Australia Received September 29, 1994@ The complex Ru(~6-naphthalene)(~4-cycloocta-1,5-diene), I, in the presence of acetonitrile, selectively catalyzes the tail-to-tail dimerization of methyl acrylate to trans-dimethyl-2hexenedioate a t 140 "C. An examination of the stoichiometric reaction between I and methyl acrylate shows that acetonitrile promotes removal of the dimer from the coordination sphere as solvents, a long-lived of ruthenium. Using tetrahydrofuran or N-methyl-2-pyrrolidinone catalyst is obtained which can be re-used without appreciable loss of activity.
Introduction The selective tail-to-tail dimerization of substituted alkyl acrylates is a useful method for synthesizing important intermediates in fine and industrial chemistry. Tail-to-tail dimers of methyl acrylate la, l b and 2a, 2b (Scheme 1) are potential precursors to adipic acid,l and one of them, trans-dimethyl-2-hexenedioate, lb, is a starting material for the preparation of biologically active compounds.2 A variety of transition metal-based systems of rhodium, palladium, nickel, and ruthenium catalyze acrylate So far, the most active of these catalyst dimeri~ation.~ is the agostic cation [Cp*(C2H4)RhCH2CH2-p-H]+obtained by protonation of cp*Rh(C~H4)2.~,~ Unlike most systems, this operates between room temperature and 60 "C with high selectivity for tail-to-tail coupling, although it requires the presence of H2 (1 bar) to maintain catalyst lifetime. Ruthenium trichloride in methanol is active only at fairly high temperatures (210 "Q5 but good conversion can be obtained at 125 "C by adding reducing agents, especially zinc.6 The zerovalent metal complexes Ru(?,%6H6)(v4-1,3-cyclohexadiene) and Ru(q6-C6Hs)(v2-CH2=CHCO2Me)2 have also been used as catalysts for acrylate dimerization, and high conversions to l d l b have been obtained by addition of sodium naphthalide, suggesting that the active catalysts may be anionic ruthenium s p e ~ i e s Other .~ ruthenium-based catalysts for methyl acrylate dimerization are Ru~(C0)1p @Abstractpublished in Advance ACS Abstracts, April 1, 1995. (1) McKinney, R. J. U.S. Patent 4 504 674, 1985. (2)(a) Nugent, W. A.; Hobbs, F. W. J. Org. Chem. 1983,48,5364. (b) A. Mitra In The Synthesis of Prostaglandins; Wiley: New York, 1979;pp 337-352. (3)Brookhart, M.; Sabo-Etienne, S. J. Am. Chem. SOC.1991,113, 2777 and references cited therein. (4)(a) Brookhart, M.;Hauptman, E. J. Am. Chem. SOC.1992,114, 4437.(b) Hauptman, E.; Sabo-Etienne, S.;White, P. S.; Brookhart, M.; Garner, J. M.; Fagan, P. J.; Calabrese, J. C. J.Am. Chem. SOC.1994, 116,8038. (5)(a) Alderson, T.US.Patent 3 013 066, 1961.(b) Alderson, T.; Jenner, E. L.; Lindsay, R. V. J.Am. Chem. SOC.1966,87,5638. ( 6 )McKinney, R. J.; Colton, M. C. Organometallics 1986,5, 1080.
Scheme 1. Tail-to-Tail Dimerization of Methyl Acrylate
\&\ CLl b (trans) Ir(cf.9
\
0
\@LI+qf\ 2.w 2b (tans)
0
in the presence of tertiary phosphines8and RuHCl(C0)(PPr'3)2 to which equiv of CF3SOAg has been added.g We recently reported that the complex Ru($-naphthalene)(v4-COD),I (COD = cycloocta-1,5-diene),in the presence of acetonitrile, shows a high catalytic activity in the isomerization and hydrogenation of unsaturated compounds.lOJ1We observed that the monocyclic arene complexes Ru(v6-arene)(y4-COD)(arene = benzene, pcymene) are less reactive than I in these reactions, and we attributed this behavior to the greater lability of the naphthalene-ruthenium bond: polynuclear arenes are generally less firmly bonded than mononuclear arenes to transition metals.12 In reactions catalyzed by I, naphthalene can be easily removed from the metal, thus making available coordination sites that are required for catalytic processes. "he interesting catalytic properties of I deriving from the lability of the naphthaleneRu bond prompted an investigation of its catalytic activity in other reactions. We describe here the results obtained in the dimerization of methyl acrylate. (7) McKinney, R. J. Organometallics 1986,5, 1752.
(8)Ren, C. Y.; Cheng, W. C.; Chan, W. C.; Yeung, C. H.; Lau, C. P. J . Mol. Catal. 1990. 59. L1. (9)Sustmann, R.'; Hornung, H. J.; Schupp, T.; Patzke, B. J. Mol. Catal. 1993. , 85. - - , 149. ~~(lO)Pe&i, P.; Uccello Barretta, G.; Burzagli, F.; Salvadori, P.; Bennett, M. A. J. Organomet. Chem. 1991,413,303. (11)Bennett, M. A,; Neumann, H.; Thomas, M.; Wang, X-q; Pertici, P.; Salvadori, P.;Vitulli, G. Organometallics, 1991,10,3237. (12)See for example: Muetterties, E. L.; Bleeke, J. R.; Wucherer, E. J.;Albright, T. A. Chem. Rev. 1982,82,499.
Q276-7333l95/2314-2565$Q9.QQlO 0 1995 American Chemical Society
Pertici et al.
2566 Organometallics, Vol. 14, No. 5, 1995
Table 1. Catalytic Dimerization of Methyl Acrylate with Ru(@-naphthalene)(v4-COD),"I r e a m productsb run
solvent
1
THF
2* 3e 4f 5
THF THF THF
NMP reaction mixture of run 1 reaction mixture of run 5
time (h)
conv
4 8 24 8 8 8 4 8 24
70 80 100 50 30 5 75 90 100
(%)
0.61
linear dimers(1)
lb/l
T@
70 70 75 65 70
80 80 75 70 70
181 207 278 120 78
70 70 75
70 70 80
194 233 278
.1
-
-1,s -
0
-2,s
ab
lb
Is
*
bo
b W
Figure 1. First-order plot for the reaction of Ru($CloHs)(v4-COD),I, with methyl acrylate in toluene-dg at -30 "C. 7g 24 100 75 80 278 -COOMe (see below), appeared, while the intensity of the resonances due t o I decreased. 108 8 65 45 80 Bh NMP 8 80 70 80 207 Some information about the kinetics of replacement 9' THF of naphthalene in I by methyl acrylate at -30 "C was Reaction conditions: I L0.02 g (0.06 mmol)], acetonitrile L0.063 obtained by recording the extent of decrease of I at mL (1.2 mmol)], solvent (5 mL), methyl acrylate E2 mL (22.2 mmol)], heated a t 140 "C in a thick-walled Carius tube. Decane different times, determined by integrating the CloHs was used as internal standard. The other reaction products are protons (multiplet at 6 4.1 ppm of AA' system).ll The branched dimers (ca. 5%) and trimers (20-30%). Turnover plot, displayed in Figure 1, shows the reaction is firstnumber = mol of linear dimers produced per mol of Ru. Heated order, with a rate constant of 2.74 x lo-, sec-'. a t 100 "C. e Carried out without acetonitrile. f Catalytic precursor: When there was no further change in the intensity Ru($-p-cymene)(y4-COD), 0.021 g (0.06 mmol). g Fresh methyl acrylate (2 mL) has been added. Sodium naphthalide (0.12 mmol) of the signals in two successive 5-min-intervals, a 2Din THF has been added. The reaction products are linear dimers COSY NMR experiment was carried out in an effort to (45%), branched dimers (5%),and trimers (50%). Under hydrogen learn about the species present in solution (Figure 2). atmosphere. Cross-peak analysis shows that two systems of three protons are present, in which each proton is correlated Results to only two others. They correspond t o the signals at 6 The results obtained in the dimerization of methyl 4.95,3.85,2.15, ppm and 3.95,3.65, and 2.85 ppm. The acrylate are reported in Table 1. In all the runs trunsfact that the spectrum also shows two singlets a t 6 3.37 dimethyl-2-hexenedioate, lb, was the predominant and 3.34 ppm assignable to COOMe resonances suggests product. After 4 h at 140 "C in THF the conversion was that a t least two molecules of methyl acrylate are 70% and after 24 h was quantitative (run 1). The attached to ruthenium. The other signals, to which the off diagonal peaks correspond, do not belong t o a simple reaction also proceeded a t 100 "C (run 2) with a similar chemoselectivity but a lower rate. The presence of three proton system. The signal at 6 4.5 ppm is related acetonitrile is very important to achieve high catalytic to the signals a t 6 3.7 and 2.85 ppm. The latter is related to the signals at 6 2.15 and 1.7 ppm, and they activity: after 8 h in the absence of acetonitrile a t 140 form cross-peaks with other signals. We think that "C only 30% of methyl acrylate was converted into as these resonances are due t o COD, or to a Cg fragment products (run 3). Using R~(11~-p-c~ene)(11~-COD) catalytic precursor, only 5% conversion was obtained derived from COD, which is not symmetrically bonded to ruthenium. after 8 h (run4). When the solution was kept at 25 "C for 24 h, two A slight increase in conversion, without change in new resonances appeared at 6 5.4 and 0.9 ppm as double selectivity, was obtained in N-methyl-2-pyrrolidinone doublets, which were coupled, as shown by spin decou(NMP) as solvent (run 5). At the end of the reaction the THF or NMP solutions were clear red-brown and pling experiments. Comparison with literature data on contained no suspended solids. These solutions mainthe isolated complex Ru(MUC)dP(OMe)33(MUC = dimtained their activity unchanged, additional methyl ethyl muconateY allows these signals be assigned to the acrylate being dimerized at the same rate as that olefinic protons of MUC bound to ruthenium. Other observed during the previous reaction (compare runs 6 coupled signals are a quartet at 6 2.05 ppm and a triplet and 7 with runs 1 and 5, respectively). In order to at 6 1.05 ppm that can be assigned to methyl propionate, obtain information on the function of acetonitrile and which could be formed as shown in Scheme 2. on the reaction mechanism, the reaction between I and Scheme 2 methyl acrylate (molar ratio 1:2) has been examined in cat detail, both without and with acetonitrile. 2CH,=CHCOOMe Without Acetonitrile. A 2 equiv amount of methyl MeOOCCH=CHCH=CHCOOMe H, acrylate and 1 equiv of I were dissolved in toluene-dg a t -70 "C, and the reaction was followed by 'H-NMR CH,=CHCOOMe H, 2% CH,CH,COOMe spectroscopy a t various temperatures. Reaction began Interestingly, neither methyl acrylate nor free dimers only at -30 "C. At this temperature two multiplets a t 6 7.35 ppm and 7.75 ppm, due t o free naphthalene, and were present in the solution at this time and the NMR two singlets a t 6 3.34 ppm and 3.37 ppm, assignable to spectrum did not change after 3 days a t room temper@
24
100
75
75
278
-
+
+
Organometallics, Vol. 14,No. 5, 1995 2567
Selective Dimerization of Methyl Acrylate
l ' " ' " " ' ~ " ' ' ~ ' ~ ' ' ~ ' ~ 1
S A
4.s
1.5
4.4
c.5
3.0
' . ' I
e.4
l ' " ' l " ' ' I ' " ~
1.5
1.0
4.5
P P
Figure 2. 2D-COSY spectrum for the reaction between Ru(176-C10H6)(174-COD), I, and methyl acrylate at -30 "C. Below 5 ppm, resonances due to known species are present. Scheme 3
55 1
443
ature. The mass spectrum of the residue, after removal of the solvent, showed a peak at mle = 551.7 and other peaks a t 443.7, 357.8 and 271.8. These data are consistent with a species of empirical formula Ru(MUC)(CH~=CHCO~M~)~(CEHI~) (m le = 551.7) that fragments as shown in Scheme 3. These results show that, even in the absence of acetonitrile, methyl acrylate removes naphthalene from ruthenium in I, the final product being a compound containing methyl acrylate, MUC, and C8H12. Attempts to isolate the compound in a pure state have not been successful so far. Dimethyl-2hexenedioate or other dimers do not form even after a long period at room temperature, as shown by IH-NMR spectroscopy.
357
271
With Acetonitrile. The behavior of the reaction of I and methyl acrylate in the presence of acetonitrile-& (2 moVmol of I) is completely different. The 'H-NMR spectrum of this reaction mixture, recorded after 5 h a t room temperature, clearly shows multiplets at 6 6.86 and 5.76 ppm due t o the olefinic protons of free dimethyl trans-2-hexenedioate, together with peaks due to a trace of methyl propionate. It is also worth noting that resonances due to ruthenium-hydride species were never observed, either in the presence or absence of acetonitrile. Comparison of the results of these two reactions shows that lb is formed only in the presence of acetonitrile.
2568 Organometallics, Vol. 14,No. 5, 1995
Pertici et al.
Scheme 4. Proposed Catalytic Cycles for the Dimerization of Methyl Acrylate (MA) by R U ( ~ ~ ~ - C ~ ~ H * ) ( ~I,~ in ~ - the C O Presence D), of CH&Nu
Discussion
The results do not suffice t o give a detailed mechanistic scheme for the dimerization of methyl acrylate W$-C&d(q4-COJ.V catalyzed by I/CH&N, but some general conclusions can be drawn. L LCH3CN,MA,Sok (1)The system based on RU(T,%IOHS)(T,I~-COD)/M~CN is far more active than that based on Ru($-p-cymene)(r4-COD)/MeCN,as is true also for catalyzed hydrogenation and isomerization of olefins. This is because of the greater stability of the mononuclear arene-metal COD bond.12 (2) In contrast to the latter processes, the function of the acetonitrile is not to promote the q6 to v4 transformation of naphthalene, since methyl acrylate readily displaces naphthalene from I, even in the absence of acetonitrile. Instead, as a consequence of its high affinity for ruthenium,1° it may help to eliminate l b from the coordination sphere after hydrogen transfer has taken place. (3) In contrast to the behavior of RU(y6-C6H6)(?$ of CH2=CHC02Me)2 as catalyst p r e c u r ~ o r ,addition ~ sodium naphthalide does not increase the catalytic activity of I/CH&N (run 8). Hence intervention of an anionic ruthenium complex seems unlikely, although such a species might not be stable in the presence of CH3CN. However, studies of the chemistry of I in the presence of THFll have given no indication of disproportionation into ionic fragments containing Ru(I1) and Ru(-I) or Ru(-11). Thus, the simplest assumption is that the catalyst precursor is a neutral ruthenium(0) species of the type RU(CH~=CHCO~M~)~(NCM~), formed by complete displacement of COD and naphthalene from I. Several mechanisms for dimerization based on such h-3 a catalyst can be considered.', One possible cycle starts with vinylic C-H oxidative addition of methyl acrylate C02Me to Ru(0) t o generate a hydrido(vinyl)ruthenium(II) speLnk-1 cies (Scheme 4a). Insertion of a second molecule of methyl acrylate into the Ru-H bond and acetonitrilepromoted elimination of the two Ru-C o-bonds gives a The upper part of this scheme represents the catalyst lb, thus completing the cycle. Models for these steps activation, and the lower parts (a) and (b) represent the cycles are provided by the reaction of ethyl methacrylate with themselves. Ru(C2H4)(PPh& (eq 1)14and by the reaction of methyl assisted by acetonitrile, completes the cycle (Scheme 4b). acrylate with [RuCl(CO)H(PPh3)31(eq 2).15 This type of mechanism has been proposed for the linear dimerization of methyl acrylate catalyzed by [Pd(acac)Ru(C,H,)(PPh,), CH,=C(Me)CO,Et (solv)21BF4.16 A model for the first step is provided by
1
""7
-
+
I
1
RuH{ CH=C(Me)COOEt}(PPh,), RuCl(CO)H(PPh,), I
(1)
+ CH2=CHC02Me+
Ru{ CH2CH2CbOMe}C1(CO)(PPh3)2PPh, (2) A second possibility is the initial coupling of two MA units at ruthenium(0) to form a ruthenacyclopentane, which then undergoes P-hydride elimination to form a hydrido 7,- allyl complex. Transfer of hydride to the carbon atom bearing the COzMe group, which may be (13)Parshall, G. W.; Ittel, S. D. In Homogeneous Catalysis: The Applications and Chemistry of Soluble Transition Metal Complexes; Wilev: New York. 1992: DD 83-84. (12)Komiya, S:;Ito, y.iCowie, M.; Yamamoto, A,; Ibers, J. A. J . Am. Chem. SOC.1976,98, 3874. (15)Hiraki, K.; Ochi, N.; Sasada, Y.; Hayashida, H.; Fuchita, Y.; Yamanaka, S.J. Chem. SOC.,Dalton Trans. 1985, 873.
the formation of the ferracycle (OC)4FeCH(C02Me)C1
H2CH(C02Me)in the photochemical reaction of methyl acrylate with Fe(C0)5.17 The corresponding reaction with RuQ(CO)IPgives only the bis(o1efin) complex Ru( C O ) ~ ( T , I ~ - C H ~ = C H C Oalthough ~ M ~ ) ~ ,this does form a ruthenacyclopentane on treatment with dimethyl 3-cyclobutene-cis-1,2-dicarboxylate.la In conclusion, the Ru($-naphthalene)(y4-COD)/acetonitrile system represents a new catalyst for the selective tail-to-tail dimerization of methyl acrylate. The turnover numbers are better than or comparable with those achieved with other ruthenium-based catalysts such Ru(v6-c6H6)(M.A)2. The system is also longer-lived than (16) Guibert, I.; Neibecker, D.; Tkatchenko, I. J. Chem. SOC.,Chem. Commun. 1989, 1850. (17)Grevels, F-W.; Schulz, D.; Koerner von Gustorf, E. Angew. Chem., Int. Ed. Engl. 1974, 13, 534. (18)Grevels, F-W.; Reuvers, J. G. A,; Takats, J. Angew. Chem., Int. Ed. Engl. 1981, 20, 452.
Organometallics, Vol.14,No. 5, 1995 2569
Selective Dimerization of Methyl Acrylate
that derived from the somewhat more active catalyst RuHCl(CO)(FPr3)~AgO~ and does not show the complications arising from tertiary phosphine-catalyzed dimerization of MA. It is, however, less active and somewhat less selective toward linear dimers than recently described systems based on r h o d i ~ m ( I I 1and )~~~ palladium(II),lg but, in contrast to the former, it does not require the presence of hydrogen to mantain catalyst lifetime. Further investigations of the mechanism of dimerization are planned.
Experimental Section The complex R~(77~-p-cymene)(tl~-COD) was made from [RuC12(y6-p-cymene)12,COD, and Na2C03 in propan-2-01~~ and identified by comparison of its lH NMR spectrum with authentic material.21 The complex Ru(q6-naphthalene)(q4COD), I, was prepared as already described." All catalytic experiments were carried out in thick-walled Carius tubes under nitrogen atmosphere. THF was dried by refluxing over N d K alloy before distillation. N-methyl-2~
~
pyrrolidinone (Aldrich product), stored under nitrogen, was used directly. Methyl acrylate was degassed and kept under nitrogen before use. Acetonitrile was purified by distillation from P4010under nitrogen. In a typical experiment I (20 mg, 0.06 mmol) was placed in a tube and dissolved in the appropriate solvent (5 mL). Acetonitrile (0.063 mL, 1.2 mmol) was introduced, and the solution was stirred at room temperature for a few minutes. Methyl acrylate (2 mL, 22.2 mmol) was added. The solution was heated at 140 "C in a constanttemperature bath (f0.2 "C). The reaction mixture was analysed by gas chromatography using a Perkin-Elmer 8500 apparatus equipped with a 12 m x 0.22 BP1 capillary column, using helium as carrier gas. The dimers were characterized by GCMS, lH-NMR, and 13C-NMRand by comparison with authentic samples. Some trimers that are also formed were not characterized. Proton NMR spectroscopic studies were carried out on solutions of I in toluene-de or benzene-& in a 5 mm 0.d. NMR tube t o which methyl acrylate or a mixture of methyl acrylate and acetonitrile was added. The growth of the signals of the products was monitored on a Varian VXR-300 spectrometer at various probe temperatures in the range -70 to 25 "C.
~~
(19) Grenouillet, P.;Neibecker, D.; Tkachenko, I. Fr.Patent 2 596 390, 1987. (20) Bennett, M. A.; McMahon, I. J.; Pelling, S.; Brookhart, M.; Lincoln, D.M.Organometallics 1992,11, 127. (21) Pertici, P.; Bertozzi, S.; Lazzaroni, R.; Vitulli, G.; Bennett, M. A.J. Organomet. Chem. 1988,354,117.
Acknowledgment. This work was supported by the CNR research program Progetto Finalizzato per la Chimica Fine 11. OM940755C
