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Organometallics 2010, 29, 5722–5724 DOI: 10.1021/om100168x
Activation of Mesitylene Involving Multiple Carbon-Carbon Bond Formation under Ambient Conditions with Regeneration of the Organoiron Precursor† Anett C. Sander, Jaime Ruiz, and Didier Astruc* Institut des Sciences Mol eculaires, UMR CNRS No. 5255, Universit e Bordeaux 1, 351 Cours de la Lib eration, 33405 Talence Cedex, France Received March 2, 2010 þ
Summary: The CpFe -induced nona-allylation of mesitylene was largely improved by scaling up and carrying out the reaction of [FeCp(η6-1,3,5-C6H3Me3][PF6] with KOH/DME under ambient conditions (95% yield), followed by recycling this organoiron precursor by visible-light photolysis in CH2Cl2 using a 100 W desk lamp in the presence of mesitylene (95% recovery yield).
Introduction Whereas catalysis is the ideal process in organic transformations,1 many reactions remain stoichiometric and produce waste chemicals. In arene chemistry, nucleophilic substitution of halide substituents has been the subject of considerable interest during the past decade,1b,c whereas benzylic activation followed by functionalization with electrophiles remains stoichiometric. The most useful organometallic families in term of stoichiometric arene activation and functionalization are the robust 18-electron complexes [Cr(η6-arene(CO)3],2 [Mn(η6-arene)(CO)3]þ,3 [Fe(η6-arene)(η5-C5H5)]þ,4,5 and [Ru(η6-arene)(η5-C5H5)]þ.6 The relative strengths of activation of the 12-electron activating groups were found to follow their electron-withdrawing properties:7
CrðCOÞ3 < MCpþ < ½MnðCOÞ3 þ with Cp ¼ η5 -C5 H5 ; M ¼ Fe or Ru
†
Part of the Dietmar Seyferth Festschrift. Dedicated to our esteemed colleague Professor Dietmar Seyferth, on the occasion of his retirement from the Editorship of Organometallics. *Corresponding author. E-mail:
[email protected]. (1) (a) Applied Homogeneous Catalysis with Organometallic Compounds; Cornils, B.; Herrmann, W. A., Eds.; Wiley-VCH: Weinheim, 1996. (b) Astruc, D. Organometallic Chemistry and Catalysis; Springer: Heidelberg, 2007; Chapters 14-21. (c) Modern Arene Chemistry; Astruc, D., Ed.; Wiley-VCH: Weinheim, 2003. (2) (a) Semmelhack, M. F. N. Y. Acad. Sci. 1977, 95, 36–58. (b) Jaouen, G. N. Y. Acad. Sci. 1977, 295, 59–77. (c) K€undig, E. P. Pure Appl. Chem. 1985, 57, 1855–1864. (3) (a) Pike, R. D.; Sweigart, D. A. Synlett 1990, 10, 565–571. (b) Jacques, B.; Chanaewa, A.; Chavarot-Kerlidou, M.; Rose-M€unch, F.; Rose, E.; Gerard, M Organometallics 2008, 27, 626–636. (c) Eloi, A.; RoseM€ unch, F.; Rose, E. J. Am. Chem. Soc. 2009, 131, 14178–14179. (4) (a) Green, M. L. H.; Pratt, L.; Wilkinson, G. J. Chem. Soc. 1960, 989–992. (b) Nesmeyanov, A. N.; Volkenau, N. A.; Bolesova, I. N. Tetrahedron Lett. 1963, 1725–1729. (c) Nesmeyanov, A. N. Adv. Organomet. Chem. 1972, 10, 1. (d) Astruc, D. Tetrahedron 1983, 39, 4027–4095. (5) Abd-El-Aziz, A. S.; Bernardin, S. Coord. Chem. Rev. 2000, 203, 219–267. pubs.acs.org/Organometallics
Published on Web 05/06/2010
The readily available family of complexes [FeCp(η6arene)][PF6]1b,c has attracted our attention because of useful electron8 and proton9 transfer reactions. Activation of polymethylbenzenes by CpFeþ9 is indeed particularly productive, because it allows the multiple formation of carbon-carbon bonds upon deprotonation-allylation sequences in one-pot reactions.10 In particular, the reactions of the hexamethylbenzene and mesitylene complexes are very practical and efficient. It is known that, in the presence of t-BuOK and allyl bromide, [FeCp(η6-C6Me6][PF6], 1, leads either to the hexa-allylated product [FeCp(η6-C6(CH2)2CHdCH2)6][PF6], 2, resulting from six single deprotonation-allylation sequences at each benzylic carbon, or to the dodeca-allylated product [FeCp(η6-C6{CH(CH2CHdCH2)2}6][PF6], 3, resulting from double branching on each benzylic carbon following 12 deprotonation-allylation sequences depending on the reaction time (1 day vs 3 weeks).10a,b Likewise, the reaction of [FeCp(η6-1,3,5-C6H3Me3][PF6], 4, with the same reactants leads to the nona-allylated complex [FeCp(η61,3,5-C6H3C(CH2CHdCH2)3][PF6], 5, resulting from nine deprotonation-allylation sequences in one pot.10c The hexabutenyl complex 2 and the nona-allylated complex 5 are routinely synthesized in our laboratory, because they lead to a star and dendritic core, respectively.10 Decomplexation of 2 and 5 is usually carried out using visible light in acetonitrile in the presence of triphenylphosphine to trap the CpFeþ moiety in a stable 18-electron cationic complex [FeCp(PPh3)(MeCN)2]][PF6], 6, which is not used further (eq 1).12 (6) (a) Pearson, A. J. Acc. Chem. Res. 1980, 13, 463–469. (b) Pearson, A. J. Synlett 1990, 1, 10–19. (7) Kane-Maguire, L. A. P.; Honig, E. D.; Sweigart, D. A. Chem. Rev. 1984, 84, 525. (8) (a) Hamon, J.-R.; Astruc, D.; Michaud, P. J. Am. Chem. Soc. 1981, 103, 758–766. (b) Green, J. C.; Kelly, M. R.; Payne, M. P.; Seddon, E. A.; Astruc, D.; Hamon, J.-R.; Michaud, P. Organometallics 1983, 2, 211– 218. (c) Lacoste, M.; Varret, F.; Toupet, L.; Astruc, D. J. Am. Chem. Soc. 1987, 109, 6504–6506. (d) Desbois, M.-H.; Astruc, D.; Guillin, J.; Varret, F.; Trautwein, A. X.; Villeneuve, G. J. Am. Chem. Soc. 1989, 111, 5800–5809. (9) (a) Hamon, J.-R.; Saillard, J.-Y.; Le Beuze, A.; McGlinchey, M.; Astruc, D. J. Am. Chem. Soc. 1982, 104, 7549–7555. (b) Astruc, D. Acc. Chem. Res. 1986, 19, 377–383. (10) (a) Moulines, F.; Astruc, D. Angew. Chem., Int. Ed. Engl. 1988, 27, 1347–1349. (b) Moulines, F.; Gloaguen, B.; Astruc, D. Angew. Chem., Int. Ed. Engl. 1992, 28, 458–460. (c) Moulines, F.; Djakovitch, L.; Boese, R.; Gloaguen, B.; Thiel, W.; Fillaut, J.-L.; Delville, M.-H.; Astruc, D. Angew. Chem., Int. Ed. Engl. 1993, 32, 1075–1077. (11) (a) Sartor, V.; Djakovitch, L.; Fillaut, J.-L.; Moulines, F.; Neveu, F.; Marvaud, V; Guittard, J.; Blais, J.-C.; Astruc, D. J. Am. Chem. Soc. 1999, 121, 2929–2930. (b) Ruiz, J.; Lafuente, G.; Marcen, S.; Ornelas, C.; Lazare, S.; Cloutet, E.; Blais, J.-C.; Astruc, D. J. Am. Chem. Soc. 2003, 125, 7250–7257. (c) Ornelas, C.; Ruiz, J.; Belin, C.; Astruc, D. J. Am. Chem. Soc. 2009, 131, 590–601. r 2010 American Chemical Society
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Scheme 1
It requires the follow-up chromatographic purification of the organic product (in particular because of the presence of residual phosphine) that is time, silica gel, and energy consuming. We now report (i) the optimization of the perallylation reaction of 4 using KOH/1,2-dimethoxyethane (DME) instead of t-BuOK/tetrahydrofuran (THF) and (ii) the recycling of 4 by carrying out the visible-light photolysis reactions in the presence of the starting arene ligand mesitylene in order to avoid the production of waste organometallics and to reuse the CpFeþ group. The reaction 1 with KOH for the hexa-alkylation reaction in the presence alkyl iodides has been reported, whereas t-BuOK provoked the dehydrohalogenation of the alkyl iodide instead of benzylic alkylation.13
Results and Discussion The known multiple deprotonation-allylation reactions of 4 in THF were efficient but required multiple daily additions of t-BuOK and allyl bromide into the reaction flask. We now find that the use of KOH/DME instead of t-BuOK/THF routinely results in completion (95% yield) of the one-pot nona-allylation of 4 on a 10 g scale to give 5 under ambient conditions (the temperature may vary at least between 18 and 25 °C) in three days without the need of supplementary additions of the organic reactants (see Experimental Section). This good reactivity of the KOH/DME system under these very convenient conditions results from the chelation of Kþ by DME, which makes the hydroxide anion more reactive upon chelation-induced ion-pair separation.14 The complex 5 is stable to moist air at room temperature (only), but light sensitive. It can be handled in the laboratory in solution for some time in the absence of light but also be photolyzed upon irradiation with visible light. Instead of photolysis in the presence of triphenylphosphine in acetonitrile, as routinely experienced earlier, we find that visiblelight photolysis of 5 in the presence of a large excess of mesitylene in dichloromethane leads to 4 and the nona-allyl arene derivative 7 in 95% yield. This visible-light photolysis reaction is also very convenient, because it involves the use of an external 100 W desk lamp (Scheme 1). Visible-light photolysis of [FeCp(η6-C6H5Me][PF6], 8, in the presence of functional cyclopentadienyl ligands in this solvent was recently shown to be a useful route to functional (12) (a) Gill, T. P.; Mann, K. R. Inorg. Chem. 1980, 19, 3008–3011. (b) Gill, T. P.; Mann, K. R. Inorg. Chem. 1983, 22, 1986–1989. (c) Catheline, D.; Astruc, D. J. Organomet. Chem. 1983, 248, C9–C12. (d) Catheline, D.; Astruc, D. J. Organomet. Chem. 1984, 272, 417–426. (e) Ruiz, J.; Astruc, D. Inorg. Chim. Acta 2008, 361, 1–4. (13) Moulines, F.; Astruc, D. J. Chem. Soc., Chem. Commun. 1989, 614–615. Fillaut, J.-L.; Linares, J.; Astruc, D. Angew. Chem., Int. Ed. Engl. 1994, 33, 2460–2462. (14) (a) Reichardt, C. Solvent Effects in Organic Chemistry, 2nd ed.; VCH: Weinheim, 1988. (b) Loupy, A.; Tchoubar, B. Salt Effects in Organic and Organometallic Chemistry; VCH: Weinheim, 1992. (c) Loupy, A.; Tchoubar, B.; Astruc, D. Chem. Rev. 1992, 92, 1142–1165. (15) (a) Diallo, A.; Ruiz, J.; Astruc, D. Org. Lett. 2009, 11, 2635– 2637. (b) Diallo, A.; Ruiz, J.; Astruc, D. Inorg. Chem. 2010, 49, 1913–1920.
ferrocene15 and biferrocene15b derivatives, and the visiblelight photolyses of 4, 5, and 8 are closely related, because the electronic density in the iron-arene bond is about the same. The bulk of the arene ligand of 4 also favors visible-lightinduced decomplexation and disfavors back exchange of 5 to re-form 4. The principle of the visible-light-induced recovery of the complex [FeCp(η6-polymethylbenzene][PF6], namely 1, was further examined with the complex [CpFe(η6-hexabutenylbenzene][PF6], 2, obtained by hexa-allylation of 1. Thus, visible-light photolysis of 2 was carried out in dichloromethane in the presence of excess C6Me6. This reaction did not result in decomplexation of 2, which is relatively stable in the presence of C6Me6 and visible light, contrary to 4, 5, and 8, although the same reaction in MeCN in the presence of PPh3 instead of C6Me6 was efficient (eq 1). Although 1 is rapidly decomposed in the presence of UV light, it is very stable to visible light,8a and so is 2. The reason for this difference of behavior among the complexes [FeCp(η6-polymethylbenzene][PF6] is that the UV-vis absorption band of these complexes is progressively shifted from the visible to the UV region upon increasing the number of methyl groups on the arene ligand as a result of enhanced electron density in the arene-ligand bond. The visible-light photolysis of 2 in the presence of PPh3 in MeCN instead of excess mesitylene in CH2Cl2 leads to [CpFe(PPh3)(MeCN)2][PF6], 6, and hexabutenylbenzene, 9, however11 (Scheme 2). This difference is due to the fact that PPh3 is a much stronger nucleophile than mesitylene. The distorted a3E1 ligand-field photoexcited state features a “hole” in low-lying d orbital that increases its susceptibility toward nucleophilic attack relative to the ground state and population of a σ* Fe-arene orbital that lengthens and weakens the Fe-arene bond.12 Light-induced arene decoordination may proceed according to the stepwise hexahapto f tetrahapto arene coordination change, thereby leaving the possibility for a two-electron phosphine ligand to further coordinate to a 16-electron intermediate iron species much more efficiently than an incoming arene. The arene-exchange strategy, which works well for the recycling of the organometallic compound in the CpFeþinduced nona-allylation of mesitylene, does not apply to the CpFeþ-induced hexa-allylation of C6Me6, because 2 is not sensitive to visible light in the presence of an arene ligand, contrary to 4. The complex 1 can be obtained in 76% yield, however, upon visible photolysis of 5 in CH2Cl2 in the presence of C6Me6 (Scheme 2, top).
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Sander et al. Scheme 2
In conclusion, the improvement and scaling up of the CpFeþ-induced nona-allylation of mesitylene and the visible-light photolytic recovery of its complex 4 represent a significant improvement in the synthesis of the dendrimer core 7, which is of valuable help for dendrimer chemistry with 1f3 connectivity16 and its applications.17
Experimental Section Nona-allylation of 4. A mixture of [CpFe(η6-mesitylene)](PF6), 44a,b,18 (1 equiv, 10 g), and KOH (45 equiv, 66 g) was dried under vacuum for 1 h in the dark; then DME (100 mL) and allyl bromide (45 equiv) were added. After 72 h at RT, the solvent was evaporated under reduced pressure at RT (heating would cause decomposition), and the crude product was dissolved in dichloromethane, filtered on Celite, and washed with dichloromethane. The organic solution was acidified with an aqueous HPF6 solution (1 equiv) at -78 °C (lower yields were obtained when HPF6 was added at RT instead of -78 °C) and dried over Na2SO4 at RT. After evaporation of the solvent, pentane was added to the brown oil and the product 5 was obtained as a dark yellow powder in 95% (18.37 g) (16) (a) Newkome, G. R.; Yao, Z.; Baker, G. R.; Gupta, G. K. J. Org. Chem. 1985, 50, 2003–2006. (b) Nlate, S.; Ruiz, J.; Blais, J.-C.; Astruc, D. Chem. Commun 2000, 417–418. (c) Astruc, D.; Ruiz, J. Tetrahedron Report No. 903. Tetrahedron 2010, 66, 1769–1785. (d) Newkome, G. R.; Shreiner, C. Chem. Rev. 2010, 110, 000. (17) (a) Astruc, D.; Ornelas, C.; Ruiz Acc. Chem. Res. 2008, 41, 841–856. (b) Astruc, D.; Boisselier, E.; Ornelas, C. Chem. Rev. 2010, 110, 1857–1959. (18) Khand, I. U.; Pauson, P. L.; Watts, W. E. J. Chem. Soc. 1968, 2257–2260.
yield following trituration, filtration of pentane, and drying in vacuum. Visible-Light Photolysis of 5 in the Presence of Mesitylene. Complex 5 (100 mg, 0.134 mmol, 1 equiv) was dried under vacuum for 1 h before adding 5 mL of dichloromethane and mesitylene (2 mL, 13.4 mmol, 100 equiv). Then, the reaction mixture was irradiated for 2 h using a 100 W desk lamp located near the Schlenk flask. The solvent was removed under reduced pressure, and Et2O was added to the brown oil. The solid that formed was filtered off and dissolved in acetone. Evaporation of the solvent under reduced pressure provided the complex [CpFe(η6-mesitylene)](PF6) (4) as a yellow powder (49 mg, 0.13 mmol, 95%). The solvent from the filtrate was removed under reduced pressure, and the excess of mesitylene was recovered upon evaporation under high vacuum. The solid obtained is the nona-allylated arene derivative 7 (95 mg, 0.127 mmol, 95%). On a 1 g scale, the yield of 4 using the same procedure is 80%, and the yield of 7 is 95%. Visible-Light Photolysis of 5 in the Presence of C6Me6. A mixture of 5 (100 mg, 0.134 mmol, 1 equiv) and C6Me6 (0.434 g, 2.68 mmol, 20 equiv) was dried under vacuum for 1 h before adding 10 mL of dichloromethane. The reaction mixture was irradiated for 1 h 10 min using a 100 W desk lamp located near the Schlenk flask. The precipitate was filtered off and dissolved in acetone. Evaporation of the solvent provided [CpFe(η6C6Me6)](PF6), 1,8a,18 as a yellow powder (44 mg, 0.102 mmol, 76%). The solvent from the filtrate was removed under reduced pressure, and the two remaining products, C6Me6 and 7 (61 mg, 95%), were separated by three successive precipitations from pentane at -5 °C.
