Fullerene chemistry under microwave irradiation

PERGAMON

Carbon 38 (2000) 1641–1646

Fullerene chemistry under microwave irradiation a ´ ´ a , M.C. Perez ´ a , A. de la Hoz b F. Langa a , *, P. de la Cruz a , E. Espıldora , J.J. Garcıa a

´ ´ Departamento de Quımica Organica , Facultad de Ciencias del Medio Ambiente, Universidad de Castilla-La Mancha, 45071 Toledo, Spain b ´ ´ ´ , Facultad de Quımica , Universidad de Castilla-La Mancha, 13071 Ciudad Real, Spain Departamento de Quımica Organica Dedicated to Professor Jose Elguero on the occasion of his 65th birthday

Abstract Microwave irradiation can be efficiently used as a source of energy in fullerene chemistry. A wide range of reactions have been performed and a drastic reduction in reaction times and a general improvement of yields in isolated products is obtained.  2000 Elsevier Science Ltd. All rights reserved. Keywords: A. Fullerene

1. Introduction Microwave radiation is a clean methodology for introducing energy into reactions [1]. A solid or liquid can be heated by converting electromagnetic energy into thermal energy. With microwaves the energy transfer is produced by dielectric loss, so the propensity of a sample to undergo microwave heating depends on the dielectric loss factor (´ ») and the dielectric constant (´ 9). A high dissipation factor (tan d 5 ´ » /´ 9) is responsible for a high susceptibility to microwave energy. As a guide, compounds with high dielectric constant tend to heat under microwave irradiation, while less polar substances and highly ordered crystalline materials are poorly absorbing. Microwave irradiation produces spectacular increases in temperature, many times greater than those achievable by conventional heating. However, the most attractive characteristics relevant to chemistry are: (i) The magnitude of the energy absorbed depends on the dielectrical properties of the molecule and, as a consequence, it can be selectively absorbed. In this way compounds thermally insulated absorb microwaves and can be heated efficiently. (ii) The energy absorption is volumetric. Microwave energy is deposited in the interior of the heated material *Corresponding author.

so the interior can be heated without the mediation of conductive heating. The volume of the material can be heated more homogeneously than using conventional heating even without stirring. This effect modifies also the shape of the reaction flask to be used. Materials of spherical appearance, with low volume / surface ratio, will concentrate an electromagnetic field at their center. Consequently, cylindrical flasks, with high volume / surface ratio, are more suitable for microwave applications. (iii) The possibility of performing reactions in solventfree conditions or on solid inorganic supports has considerable attractions to the industry and converts microwave chemistry into an environmentally benign method [2]. Some recyclable solids such as montmorillonite, clays, alumina, silica, bentonites, . . . , absorb microwaves and can be used efficiently as supports and catalysts in microwave assisted chemistry [3]. Microwave irradiation has been successfully applied in chemistry. Several examples have been described in analytical chemistry [4], environmental chemistry [4], chemistry of materials [5,6], organometallic chemistry [7] and organic chemistry [8–11]. Microwave Assisted Organic Synthesis is known to achieve spectacular accelerations in many reactions as a consequence of a heating rate that can not be reproduced by classical heating. However, many reports points to even

0008-6223 / 00 / $ – see front matter  2000 Elsevier Science Ltd. All rights reserved. PII: S0008-6223( 99 )00284-5

1642

F. Langa et al. / Carbon 38 (2000) 1641 – 1646

more important applications of microwave radiation. Typically, some reactions that do not occur by classical heating or that occur in very low yields can be performed in good yields under microwaves. This includes: (i) reactions that require harsh conditions, long reaction times and / or high temperatures, because reaction times can be drastically reduced; (ii) reactions that use or produce temperaturesensitive reagents or products, because the heating time is reduced and, as a consequence, decomposition is avoided; (iii) reactions that occur with the loss of a volatile polar molecule, for example water, because the equilibria can be easily displaced; (iv) reaction on solid supports, oxidants, acid catalysts, etc. Some authors postulate the existence of a specific effect derived from the microwave field, known as the ‘microwave effect’, and not from the rapid heating [12]. This effect has been postulated to occur by: (i) the presence of hot spots, implying that in certain zones within the sample the temperature is much greater than the macroscopic temperature [13]; (ii) an increase in the diffusion rate within solids using microwaves [14]; (iii) changes in the mechanism in relation to conventional heating produced by the heating rate induced by microwaves [15]; or (iv) changes in competitive reactions involving polar transition states possessing different polarities [16]. Considering these possibilities, numerous processes can not only be improved upon, but can be controlled in the case of competitive reactions, or, with respect to the chemo-, regio- and stereoselectivity, may be modified or inverted with respect to classical heating. We have focused our research on the application of microwave irradiation in the preparation of derivatives of [60]fullerene. Among the suitable procedures for realising C 60 , cycloadditions provide a powerful tool because C 60 behaves as an electron-deficient olefin with a relatively low lying LUMO. In this context Diels–Alder and 1,3-dipolar cycloadditions have been performed [17–20] and the conditions for the cycloadduct formation strongly depend on the energy gap between the controlling orbitals, thus requiring frequent (several hours, or days) reflux in high boiling point solvents. Diels–Alder [21] and 1,3-dipolar [22] cycloadditions are easily performed under microwave irradiation, problems of reversibility or decomposition of reagents and / or products have been nicely solved using this methodology. Consequently, we considered it of interest to investigate the potential of microwave irradiation in the preparation of fullerene derivatives when this type of reaction is involved. In the reactions reported herein, we have used two kind of apparatus modified by our group: a domestic microwave oven perforated on the top to accommodate a reflux condenser and with a 10-cm pipe to avoid microwave leakage; the turnable dish was also replaced by a magnetic stirrer as shown in Fig. 1 and a focused microwave reactor (Maxidigest MX350) equipped with an infrared temperature detector and with full control of temperature and incident power through a computer.

Fig. 1. Modified microwave oven.

In order to test the ability of microwave irradiation to activate the formation of fullerene derivatives, with respect to conventional heating, we have chosen several well known Diels–Alder reactions of C 60 : with anthracene [23– 25], o-quinodimethane [26] (generated in situ from sultine) and o-quinone methide [27] (formed in situ from o-hydroxybenzyl alcohol); we have also tried the hetero-Diels– Alder reaction with chalcone and 4-benzyliden-1,2-dimethyl-2-imidazolin-5-one.

2. [412] Cycloadditions The facile reaction of C 60 with anthracene, extensively studied [23–25], has been reported to form, under thermal conditions, multiply addended cycloadducts which undergo cycloreversion to the starting materials. The reaction of C 60 with 10 equiv. of anthracene in toluene as solvent using the modified microwave oven (Method A) produced cycloadduct 1 in 35% yield (Scheme 1); this result improves the yields obtained by conventional heating, 13% [23–25] (reflux, toluene, 3 days); 25% [23–25] (reflux, benzene, 12 h), reduces the reaction time to 15 min and shows how this methodology not only reduces, as expected, the reaction times to minutes, but yields can also be improved. Reaction with o-quinodimethane, formed from sultine, for 20 min at 800 W, produces cycloadduct 2 (Scheme 1) in higher yield (39%) in comparison with

F. Langa et al. / Carbon 38 (2000) 1641 – 1646

1643

Scheme 1. Cycloadditions to C 60 studied under microwave.

thermal conditions (30% after 18 h, refluxing) [26]. Longer reaction times result in lower yields due to the formation of polyadducts. [412] cycloaddition of heterocyclic o-quinodimethanes have been also performed in order to determine the redox properties of the novel C 60 -pyrazine-3a–c and C 60 thiophene-containing adducts 4a–b [27]. The new adducts 3a–c were prepared in moderate yields by reaction of C 60 with pyrazino-2,3-quinodimethane, generated in situ by treatment of the corresponding 2,3-bis(bromomethyl)pyrazine derivatives with o-dichlorobenzene (ODCB) (Scheme 1), thus providing an expeditious access to [6,6]closed fullerene derivatives covalently attached to different heterocycles. Microwave assisted reactions were performed in a focused microwave reactor (Method B) at 105 W and higher yields were obtained when 2,3quinodimethanes were used as dienes, similar to classical heating. Nevertheless, lower yields were obtained under

microwave irradiation when cycloadducts 4a–b were prepared (Scheme 1). The reaction of C 60 with o-quinone methide, prepared from o-hydroxybenzyl alcohol, was performed using Method A, at 800 W, yielding 5 in only 4 min within 27% yield (Scheme 1). Although Eguchi et al. [28] described the reaction with a slightly better yield (31%) by thermolysis in a sealed vessel, the microwave approach to this adduct offers the simplicity of the procedure, thus avoiding the risk of high-pressure conditions. We tried the reaction with chalcone, where the resulting cycloadduct (6) can not be stabilised by aromatization. Frontier Orbital calculations indicate that the energy gap between the controlling orbitals is not higher than that observed for the reaction with o-quinone methide. Nevertheless, adduct 6 was not formed under any thermal or microwave conditions. In the case of the reaction with 4-benzyliden-1,2-dimethyl-2-imidazolin-5-one [29], the

F. Langa et al. / Carbon 38 (2000) 1641 – 1646

1644

cycloaddition adduct (7), should be stabilised by aromatization of the imidazol ring, thus avoiding cycloreversion of this allowed reaction (Scheme 1), according to the energy gap between the HOMO of diene and the LUMO of C 60 . However, the reaction does not work, probably due to the steric hindrance between the fullerene surface and the phenyl group of the heterodiene system. Another contributing factor to the lack of reactivity observed for this heterodiene could be the larger distance between the ˚ than in extremes in the heterodiene moiety (R 1,4 53.08 A) ˚ this disthe case of o-quinone methide (R 1,4 52.84 A); tance, R 1,4 , has been related to the reactivity of some outer-ring dienes [30].

3. [312] Cycloadditions A general method for the functionalization of C 60 is the 1,3-dipolar cycloaddition of azomethine ylides, firstly described by Prato [31–34], leading to fulleropyrrolidines. We have prepared several fulleropyrrolidines, 8a–c, under microwave irradiation and observed that microwave irradiation competes again favourably, and, thus, derivative 8a was prepared in 37% using a focused microwave

reactor (Method B); fulleropyrrolidines 8b and 8c, not previously reported, were also prepared in 30% and 15% yield, respectively. N-Alkyl and N-arylpyrrolidino[60]fullerenes are not easily accessible by cycloaddition reactions. Alkylation and arylation of N-H-pyrrolidino[60]fullerenes is the best alternative. However, pyrrolidino[60]fullerenes (or fulleropyrrolidines) are several orders of magnitude less basic than the correspondent pyrrolidines [35], probably due to an interaction between the nitrogen lone pair and the C 60 cage. Consequently, the alkylation of the pyrrolidine nitrogen is a difficult process. Microwave irradiation coupled with Phase Transfer Catalysis in the absence of a solvent has proved to be a very useful technique for anionic activation [36–41]. Application of this methodology permitted the alkylation of N-H-pyrrolidino[60]fullerenes and the synthesis of compounds 9a–e listed in Scheme 2 with good yields [42]. In order to evaluate the synergy between dry media and microwave irradiation [43] in this reaction, several experiments were tried: If the preparation of 9e is carried out in refluxing toluene as solvent, the yield is only 16% after 24 h (17% of C 60 is obtained and 9% of the starting material is recovered); moreover, if the reaction is done in dry

Scheme 2. Pyrrolidino[60]fullerene systems prepared under microwave irradiation.

F. Langa et al. / Carbon 38 (2000) 1641 – 1646

1645

Scheme 3. Pyrazolo[60]fullerene systems prepared under microwave.

media but under classical heating at a higher temperature (1008C) instead of microwave irradiation, after 10 min no reaction is detected by TLC and after 3 h (18 times more than in the microwave situation) the yield is 31%, but only 18% of the starting material is recovered and 7% of C 60 and other non-identified decomposition products are obtained. Nevertheless, the S N Ar reaction of a series of N-Hpyrrolidino[60]fullerenes as nucleophiles and 2,4-dinitrochlorobenzene as substrate to prepare new C 60 -acceptor dyads 10a–c, which incorporate a 2,4-dinitrophenyl moiety as acceptor, were unsuccessful under microwave irradiation and after a few seconds of irradiation an electrical arc occurred in the microwave cavity, and then no starting material was recovered. Cycloadducts 10a–c were obtained, however, under classical heating by phase transfer catalysis in the absence of a solvent [44] (Scheme 2). The formation of several 1-aryl-3-(1-phenyl-pyrazol-4yl)-pyrazolo-[49,59:1,2]-[60]fullerene adducts 11a–c from nitrile imines, generated in situ from the corresponding hydrazone and NBS in the presence of Et 3 N and reacted

with C 60 under microwave irradiation (Method B) (Scheme 3) has been described by us [45]. This approach to the 2-pyrazoline ring fused to C 60 is simpler than that previously described by cycloaddition to nitrile imines generated in situ from the corresponding N-chlorobenzylidene derivatives. In a similar way, 39-(N-phenylpyrazolyl)isoxazolino[49,59:1,2][60]fullerene dyads 12–13 have been prepared from the corresponding nitrile oxides [46]. Although the most common way to generate nitrile oxides uses hydroximoyl chlorides as starting materials, in our case the oxime was treated with NBS in the presence of Et 3 N and reacted in situ with C 60 under microwave irradiation. Under these conditions (Scheme 4), adducts 12–13 were obtained in 22% in both cases after isolation by flash chromatography (silica gel, toluene). Longer reaction times afforded greater amounts of bisadducts. The same reactions were carried out under thermal conditions and lower yields were obtained (12: 14%; 13: 17%). A significant accelerating effect (10 min vs. 24 h) occurs under microwave irradiation. These new isoxazoline-fused

Scheme 4. Isoxazolo[60]fullerene systems prepared under microwave.

1646

F. Langa et al. / Carbon 38 (2000) 1641 – 1646

organofullerenes present a better acceptor ability than unaddended C 60 as the reduction potential values show an anodic shift (70 mV for 12 and 50 mV for 13) in comparison to the parent C 60 .

4. Conclusion In summary, we have shown that microwave irradiation can be efficiently used as a source of energy in fullerene chemistry. In the wide range of reactions studied, important improvements have been achieved: drastic reductions in reaction times; general improvement of yields in isolated products; reduction in decomposition of products; and an efficient control of the reaction when polyadducts are possible.

Acknowledgements Financial support from the Spanish DGICYT (Grant No. PB97-0429) is gratefully acknowledged. One of us (E.E.) thanks the Junta de Comunidades de Castilla-La Mancha for a fellowship.

References [1] Mingos DMP, Whittaker AG. Microwave dielectric heating effects. In: van Eldik R, Hubbard CD, editors, Chemical synthesis in chemistry under extreme or non-classical conditions, New York: John Wiley, 1997. [2] Varma RS. Green Chem 1999;1:43. [3] Loupy A, Petit A, Hamelin J, Texier-Boullet F, Jacquqult P, Mathe´ D. Synthesis 1998:1213. [4] Zlotorzynski A. Critical Rev Anal Chem 1995;25:43. [5] Clark DE, Sutton WH. Annu Rev Mater Sci 1996;26:299. [6] Jacob J, Chia LHL, Boey FYC. J Mat Sci 1995;30:532. [7] Van Eldik R, Hubbard CD. New J Chem 1997;21:825. [8] Caddick S. Tetrahedron 1995;52:10403. [9] Majetich G, Hicks R. Radiat Phys Chem 1995;45:567. [10] Bose AK, Manhas MJ, Banik BK, Robb EW. Res Chem Intermed 1994;20:1. [11] Strauss CR, Trainor RW. Aust J Chem 1995;48:1665. [12] Langa F, de la Cruz P, de la Hoz A, Diaz-Ortiz A, DiezBarra B. Contemporary Org Synth 1997:373. [13] Zhang X, Hayward DO, Mingos DMP. Chem Commun 1999:975. [14] Wroe R, Rowley AT. J Mater Sci 1996;31:2019. [15] Arrieta A, Lecea B, Cossio FP. J Org Chem 1998;63:5869. ´ ´ FP, [16] Langa F, de La Cruz P, de la Hoz A, Espıldora B, Cossıo Lecea B. J Org Chem, in press. [17] Wudl F. Acc Chem Res 1992;25:157. [18] Hirsch A. The chemistry of the fullerenes, Stuttgart: Thieme, 1994.

[19] Hirsch A. Synthesis 1995:895. [20] Diederich F, Thilgen C. Science 1996;271:317. [21] Diaz-Ortiz A, Carrillo JR, Diez-Barra B, de la Hoz A, Gomez-Escalonilla MJ, Moreno A, Langa F. Tetrahedron 1996;52:9237, and references therein. [22] Diaz-Ortiz A, Diez-Barra E, de la Hoz A, Prieto P, Moreno A, Langa F, Prange´ T, Neuman A. J Org Chem 1995;60:4160, and references therein. [23] Schlueter JA, Seaman JM, Taha S, Cohen H, Lykke KR, Wang HH, Williams JM. J Chem Soc, Chem Commun 1993:972. [24] Tsuda M, Ishida T, Nogami T, Kurono S, Ohashi M. J Chem Soc, Chem Commun 1993:1296. [25] Komatsu K, Murata Y, Sugita N, Takeuchi K, Wan TSM. Tetrahedron Lett 1993;34:8473. ´ N, Seoane C, de la Cruz P, Langa F, Wudl [26] Illescas B, Martın F. Tetrahedron Lett 1995;36:8307. ´ ´ N, Seoane C, [27] Fernandez-Paniagua UM, Illescas B, Martın De la Cruz P, de la Hoz A, Langa F. J Org Chem 1997;62:3705. [28] Ohno M, Azuma T, Eguchi S. Chem Lett 1993:1833. [29] Klein B, Texier-Boullet F, Hamelin J. Tetrahedron Lett 1993;34:4639. [30] Torres-Garcia G, Mattay J. Tetrahedron 1996;52:5421, and references therein. [31] Maggini M, Scorrano G, Prato M. J Am Chem Soc 1993;115:9798. [32] Prato M, Maggini M, Giacometti C, Scorrano G, Sandona` G, Famia G. Tetrahedron 1996;52:5221. [33] Prato M, Maggini M, Scorrano G. Synthetic Metals 1996;77:89. [34] Wilson SR, Wang Y, Cao J, Tan X. Tetrahedron Lett 1996;37:775. [35] Prato M, Maggini M. Acc Chem Res 1998;31:519. [36] Brain G, Loupy A, Villemin D. Microwave activation of reactions on inorganic solid supports. In: Smith K, editor, Solid supports and catalysts in organic synthesis, Ellis Harwood, 1992. [37] Loupy A, Bram G, Sansoulet J. New J Chem 1992;16:233. [38] Tavener SJ, Clark JH. Chem Ind 1997;1:22. ´ ´ [39] Langa F, de la Cruz P, de la Hoz A, Dıaz-Ortiz A, DıezBarra E. Contemporary Org Synth 1997;4:373. [40] Bogdal D, Pielichowski J, Boron A. Synlett 1996:873. ´ [41] Almena I, Dıaz-Ortiz A, Diez-Barra E, de la Hoz A, Loupy A. Chem Lett 1996:333. ´ [42] de la Cruz P, de la Hoz A, Font LM, Langa F, Perez MC. Tetrahedron Lett 1998;39:6053. [43] de la Cruz P, Diez-Barra E, Loupy A, Langa F. Tetrahedron Lett 1996;37:1113. ´ [44] de la Cruz P, de la Hoz A, Langa A, Martin N, Perez MC, ´ Sanchez L. Eur J Org Chem 1999:3433. ´ JJ, Gomez-Escalonilla ´ [45] de la Cruz P, Diaz-Ortiz A, Garcıa MJ, de la Hoz A, Langa F. Tetrahedron Lett 1999;40:1587. ´ ´ JJ, de la Hoz A, Langa F, [46] de la Cruz P, Espıldora E, Garcıa ´ N, Sanchez ´ Martın L. Tetrahedron Lett 1999;40:4889.