- Email: [email protected]
On understanding the unique role of electrophilicity on C60 reactivity and strain on exothermicity in Diels–Alder and epoxidation reactions
6 November 1998
Chemical Physics Letters 296 Ž1998. 429–434
On understanding the unique role of electrophilicity on C 60 reactivity and strain on exothermicity in Diels–Alder and epoxidation reactions M. Manoharan
)
School of Chemistry, UniÕersity of Hyderabad, Hyderabad-500 046, India Received 21 May 1998; in final form 18 September 1998
Abstract Semiempirical AM1 calculations have revealed that the electrophilic character of C 60 causes the barrier for the Diels–Alder addition of butadiene to be lowered Žrelative to ethylene. whereas for the epoxidation reaction Žwith dimethyldioxirane. the barrier is raised Žrelative to ethylene.. This is because the electronic factor appears to be relatively greater than the steric influence of fullerene during the reaction. The loss of aromaticity in the six-membered faces of C 60 as well as the strain relief in the cluster during adduct formation leads the reaction to a less exothermic path compared to the ethylene reaction. These estimations have led us to provide a common criterion for any kind of cycloaddition and oxygen-transfer reaction involving the 6-6 ring junction of fullerene. q 1998 Elsevier Science B.V. All rights reserved.
1. Introduction The most prominent representative of the fullerene family is C 60 and it has been used extensively for numerous methods of functionalization w1,2x in recent years. Of the functionalizations, the Diels–Alder w3–6x and oxygen transfer reactions w7–12x of C 60 have attracted much attention. These reactions readily occur through the 6-6 ring junction of such clusters with reasonable product yields. The significant effect of this strained, bulky electrophile on the mechanism of these reactions is unclear. Although a few experimental w13,14x and theoretical w15–17x reports on the mechanism of the Diels–Alder reac-
)
Corresponding author. E-mail: [email protected]
tion involving fullerenes are available, there have been no studies on the mechanism of oxygen transfer reactions. Therefore, this Letter presents a theoretical study of the kinetic and thermodynamic control of C 60 –butadiene ŽBD. w1x and C 60 –dimethyldioxirane ŽDMD. w11x reactions in three parts: Ž1. the relative reactivity of an olefin embedded in a C 60 cage compared to a free olefin; Ž2. the role of cluster strain and the electrophilicity of C 60 on the barrier; and Ž3. the effect of strain relief and aromatic destabilization of C 60 on the reaction exothermicity. The main goal of this investigation is to understand the flexibility of the fullerene environment in the reaction path and to throw light on the specific factors that decide the kinetic and thermodynamic properties when fullerene reacts with either a nucleophile or an electrophile.
0009-2614r98r$ - see front matter q 1998 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 Ž 9 8 . 0 1 0 6 3 - X
M. Manoharanr Chemical Physics Letters 296 (1998) 429–434
430
2. Computational details
From these values, the average percentage of bond make–break at the TS can be obtained:
Semiempirical AM1 w18x calculations have been performed throughout by using the MOPAC 6.0 program w19x. The size of the system studied prohibits the use of more expensive ab initio and density functional ŽDF. methods. Baker’s eigenvector following ŽEF. procedure w20x has been used for all geometry optimizations. It has been used extensively w21–24x for locating the stationary points, viz. equilibrium and transition state ŽTS. geometries, and this method has been found to be far better, both in terms of the quality of refined geometry and converged energy. FORCE calculations have been carried out to confirm the single imaginary frequency for all TSs and zero for reactants and products. The progress of a reaction can be followed through the changes in the bond order of certain bonds during the reaction. The extent of bond formingr cleaving of a specific bond i at the TS can be calculated as:
BFi or BC i s
BOiTS y BOiR BOiP y BOiR
= 100 ,
where BOiTS , BOiR and BOiP are the bond order values of the bond i that is forming or cleaving during the reaction at the TS, reactant and product.
BFC ave s
1 2
ž
1 ni
1 ÝBFi q
nj
/
ÝBC j .
This is proposed with a view to bookkeeping the bond make–break during the course of the reaction. This will further indicate the ‘earliness’ and ‘lateness’ of the TS in the reaction.
3. Results and discussions The molecular size, the effect of pyramidalization and the presence of embedded aromatic sextets on C 60 , all of which help to stabilize its lowest unoccupied molecular orbital ŽLUMO. considerably w25x, lead to a molecule that behaves as an electron-deficient cluster. Therefore, the interaction between the LUMO of C 60 and the highest occupied molecular orbital ŽHOMO. of BD Ž2. in the Diels–Alder reaction is more dominant than the HOMOŽC 60 . – LUMOŽBD. interaction as has been examined in the prototype reactionŽ1. drawn in Fig. 1. In the epoxidation reaction, the HOMO of the simple olefin readily interacts with the peroxide antibonding orbital ŽLUMO. of DMDŽ3., which causes the maximum stabilization and is the normal expectation w26,27x. On the other hand, the C 60 –DMD reaction involves the interaction between the oxygen lone-pair HOMO
Fig. 1. Schematic diagram of the FMO interaction in the Diels–Alder and epoxidation reactions.
M. Manoharanr Chemical Physics Letters 296 (1998) 429–434
of DMD and the C 60 LUMO Ž4b. rather than the one anticipated in the normal case Ž4a., and the electron-accepting ability of C 60 allows one to stabilize this reverse interaction. Moreover, the strength of the two frontier orbital interaction in 2 and 4 appears to be greater than that in 1 and 3, since the distorted antibonding orbital of the 6-6 junction matches the interacting orbitals of the approaching BD and DMD. Here, in the FMO analysis, the calculated frontier orbital energy gaps ŽTable 1. clearly reflect the electron-affinity character of C 60 on reactivity due to the strain on the spherical curvature of fullerene and further predict the Diels–Alder reaction of C 60 to be a normal-electron demand type whereas the C 60 epoxidation is predicted to be an inÕerse-electron demand type. The present investigation thus seeks to establish the mechanism of these reactions more quantitatively than through this qualitative argument. The earlier AM1 results of Diels–Alder w15,22– 24x and epoxidation w21x reactions reproduced the experimental observations reasonably and have also been in excellent agreement with the high-level ab initio and DF calculations w26–28x, which thus provides credibility for the use of AM1 for such reactions. Our calculations reveal that the C 60 –BD reaction passes through a concerted synchronous transition structure ŽTS. A shown in Fig. 2, which leads to the formation of a cyclohexene fused fullerene adduct, whereas the epoxidation of C 60 by DMD concertedly attains the synchronous ‘butterfly’ spiro TS B ŽFig. 2. and generates acetone and fullerene epoxide as observed in typical reactions w21,26,27x. Owing to the electrophilic 6-6 junction of fullerene, the two new C PPP C bonds at the TS are formed to a
431
greater extent in the former reaction and the two C PPP O bonds are formed at a slower rate in the latter compared to their respective typical reactions. This is clearly reflected in the bond lengths as noted in the TSs A and B. In addition, the extent of all bond forming and cleaving processes have been monitored in the TSs through bond order analysis and are accumulated in a BFC ave index. This further confirms that the TS of 2 occurs earlier whereas 4 involves the ‘late’ TS compared with the TSs of ethylene reactions, as can be seen from the BFC ave values ŽTable 1.. The quantum of charge transfer Ž qCT . presented in Table 1 clearly shows a flexible role of the electrophilicity of C 60 in both Diels–Alder and epoxidation reactions. An ‘early’ TS and the higher qCT values would have a lower activation barrier w21–24x. A notable feature of the TSs A and B is in the pyramidalized angles, the extent of pyramidalization at the reacting centres in the former TS is found to be higher than that in the latter, since the geometry A is pyramidalized 108 from C 60 whereas B is 3.38 from the fullerene structure. This happens because the involvement of four reacting centres in a Diels–Alder reaction leads to a less strained sixmembered TS whereas the three reacting centres of the epoxidation lead to a more strained three-membered spiro TS. The calculated activation energies presented in Table 1 predict that the C 60 –BD reaction is expected to be faster than the typical reaction Ž1. whereas the epoxidation of C 60 by DMD is found to be slower than that of ethylene Ž3., despite the fact that C 60 reacts with a nucleophile in the former reaction and an electrophile is involved in the latter. This is in good accordance with the experimental observations
Table 1 Calculated AM1 frontier orbital energy gaps ŽeV., charge transfer, activation and reaction energies Žkcal moly1 ., deformation energies Žkcal moly1 . of reactants for the title reactions, and average percentage of bond forming and cleaving at the TS Reaction
D E1 a
D E2 b
qCT c
D E/
DE1d
DE2 d
D Er
BFC ave
1 2 3 4
10.8 6.4 12.4 11.6
11.0 10.1 12.9 8.6
y0.001 0.243 0.197 0.160
23.2 16.3 15.6 28.1
15.9 12.0 17.8 21.5
9.9 5.8 0.4 1.1
y58.0 y50.8 y85.3 y72.3
42.7 37.4 18.4 27.8
a
D E1 s E HOMO Ždiene. y ELUMO dienophile.rEHOMO Žolefin. y ELUMO ŽDMD.. D E2 s E LUMO Ždiene. y EHOMO Ždienophile.rELUMO Žolefin. y EHOMO ŽDMD.. c Quantum of charge transfer from diene to dienophilerolefin to DMD at the TS. d DE1 and DE2: deformation energy of BDrDMD and deformation energy of ethylenerC 60 , respectively, at the TS. b
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M. Manoharanr Chemical Physics Letters 296 (1998) 429–434
Fig. 2. AM1 optimized transition structures ŽA and B. of the Diels–Alder and epoxidation reactions involving C 60 with selected geometric parameters. Parentheses denote the TS of the ethylene reaction.
w11,13,14x. A substantial difference in the barrier between C 60 Ž2 and 4. and ethylene Ž1 and 3. reactions is most likely due to the electron withdraw-
ing ability of such a cluster. Deformation energies 1 of reactants ŽTable 1. show that both BD and C 60 deform to a lesser extent in the Diels–Alder reaction and the deformation of DMD as well as C 60 significantly increases in the epoxidation compared to those found in the ethylene reactions. In epoxidation, the weak peroxide bond of DMD w29,30x cleaves quickly during the formation of the TS and hence it deforms heavily and to a greater degree compared to a typical olefin as seen in the deformation energies. From DE1 and DE2, one can observe that the trend in deformation energies parallels the trend in the reaction barriers. A contrasting reactivity of C 60 observed in the Diels–Alder and epoxidation reactions obviously shows the real electron-deficient character of C 60 , although the strain factor has brought to light information on the electrophilic nature of the isolated fullerenes w25,31x. Moreover, when the reactivity of fullerene is compared with ethylene, the dominant role of the electrophile over strain on the barrier can be seen. It is to be noted that the reacting junction of fullerene is a strained nonplanar alkene whereas ethylene is a simple planar alkene. When two electronically different systems ŽBD and DMD. react with the strained alkene, the pyramidalized angle is found to increase monotonically on going from reactant to product via the TS in both Diels–Alder and epoxidation reactions and hence the strain is released accordingly due to the marginal changes in the orbital distortion. Nevertheless, it does not help to show the above reactivity pattern on the barrier. This quantitative prediction thus indicates the fact that the electronic factor offsets the steric influence of C 60 and dominates in the barriers. In both of these reactions, there is a relief of strain in the cluster during the formation of the TS as well as in the product, since the reacting carbon atoms are rehybridized from sp 2 to sp 3. At the same time, as the reacting bond of C 60 is abutted between the two aromatic hexagonal rings, it destabilizes the resonance rings during the TS and product formation. The concept of aromaticity on fullerene is evident from the fact that 1 Deformation energy of a reactant is defined as the energy acquired by the reactant to distort it from the ground state geometry to the geometry of it in the TS. Deformation energy analysis gives a clue for why the barrier increasedrdecreased during the course of the reaction.
M. Manoharanr Chemical Physics Letters 296 (1998) 429–434
the six-membered faces of fullerene have been predicted to be aromatic hexagons with strong diamagnetic exaltation w31–33x, although they have alternative single and double bonds. Owing to the strain relief as well as the loss of aromaticity at the reacting centre, the reaction energy ŽTable 1. of the fullerene reaction is significantly reduced compared to the ethylene reaction. The strain relief should normally increase the reaction energy to some extent instead of decreasing the energy, however, it controls the reaction exothermicity. In view of the above discussion, it is clear that the lower exothermicity of C 60 reactions compared to ethylene reactions seems to be due mainly to the loss of aromatic stabilization during the product formation. A recent theoretical prediction w24x further supports this criterion, because the gain or loss of aromaticity of the benzenoid ring in the product plays a major role in the thermodynamic control of the reaction. Interestingly, both these effects – the loss of aromaticity and the strain relief – thus uniquely support the thermodynamic aspect of these reactions, but not the kinetic part. Overall, AM1 calculations conclude that the Diels–Alder reaction of C 60 is kinetically but not thermodynamically more favoured compared to typical reactions. The epoxidation of C 60 is predicted to be both kinetically and thermodynamically less favoured than that of ethylene. The highly exothermic reaction should have an ‘early TS or reactant-like TS’ on the reaction energy surface, as one would normally expect based on the Hammond’s postulate w34x. In this context, the former reaction appears to be contradictory to the postulate. This investigation finally establishes that the electrophilicity of fullerene, rather than the strain, is the determining factor of reactivity, whereas the resonance destabilization as well as the strain release in the cluster during the product formation determines the reaction exothermicity, and through the specific factor, both the kinetic and thermodynamic path of the reactions involving the 6-6 ring bond of any fullerene can be identified. Acknowledgements The author is very grateful to Professors E.D. Jemmis ŽSchool of Chemistry, University of Hyder-
433
abad, India. and P. Venuvanalingam ŽDepartment of Chemistry, Bharathidasan University, India. for helpful discussions and for providing the computational facilities. The author also thanks the Department of Science and Technology, India, for financial support in the form of Research Associate.
References w1x C. Thilgen, A. Herrmann, F. Diederich, Angew. Chem., Int. Ed. Engl. 36 Ž1997. 2268. w2x F. Diederich, C. Thilgen, Science 271 Ž1996. 317, and references cited therein. w3x Y. Rubin, S.I. Khan, D.I. Freedberg, C. Yeretzian, J. Am. Chem. Soc. 115 Ž1993. 344. w4x B. Krautler, J. Maynollo, Angew. Chem., Int. Ed. Engl. 107 ¨ Ž1995. 69. w5x P. Belik, A. Gugel, J. Spickermann, K. Mullen, Angew. ¨ ¨ Chem., Int. Ed. Engl. 105 Ž1993. 95. w6x M. Ohno, S. Kojima, S. Eguchi, J. Chem. Soc., Chem. Commun. Ž1995. 565. w7x R. Malhotra, S. Kumar, A. Satyam, J. Chem. Soc., Chem. Commun. Ž1994. 1339. w8x J.P. Deng, C.Y. Mou, C.C. Han, J. Phys. Chem. 99 Ž1995. 14907. w9x A.L. Balch, D.A. Costa, B.C. Noll, M.M. Olmstead, J. Am. Chem. Soc. 117 Ž1995. 8926. w10x R.W. Murray, K. Iyanar, Tetrahedron Lett. 38 Ž1997. 335. w11x Y. Elemes, K. Silverman, C. Sheu, M. Kao, C.S. Foote, M.M. Alvarez, R.L. Whetten, Angew. Chem., Int. Ed. Engl. 31 Ž1992. 351. w12x K.M. Creegan, J.L. Robbins, W.K. Robbins, J.M. Millar, R.D. Sherwood, P.J. Tindall, D.M. Cox, A.B. Smith III, J.P. McCauley, D.R. Jones, R.T. Gallagher, J. Am. Chem. Soc. 114 Ž1992. 1103. w13x L.M. Giovane, J.W. Barco, T. Yadav, A.L. Lafleuer, J.A. Marr, J.B. Howard, V.M. Rotello, J. Phys. Chem. 97 Ž1993. 856. w14x L.S.K. Pang, M.A. Wilson, J. Phys. Chem. 97 Ž1993. 6762. w15x J. Mestres, M. Duran, M. Sola, ` J. Phys. Chem. 100 Ž1996. 7449. w16x A. Chikama, H. Fueno, H. Fujimoto, J. Phys. Chem. 99 Ž1995. 8541. w17x M. Sola, ` M. Duran, J. Mestres, J. Am. Chem. Soc. 118 Ž1996. 8920. w18x M.J.S. Dewar, E.G. Zoebisch, E.F. Healy, J.J.P. Stewart, J. Am. Chem. Soc. 107 Ž1985. 3902. w19x J.J.P. Stewart, MOPAC 6.0, QCPE No. 455, Dep. Chem., Indiana Univ., Bloomington, IN, 1990. w20x J. Baker, J. Comput. Chem. 7 Ž1986. 385. w21x M. Manoharan, P. Venuvanalingam, J. Mol. Struct. ŽTheochem. 394 Ž1997. 41. w22x M. Manoharan, P. Venuvanalingam, J. Chem. Soc., Perkin Trans. 2 Ž1997. 1799.
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w23x M. Manoharan, P. Venuvanalingam, Int. J. Quantum Chem. 66 Ž1998. 309. w24x M. Manoharan, P. Venuvanalingam, J. Phys. Org. Chem. 11 Ž1998. 133. w25x M. Sola, ` J. Mestres, M. Duran, J. Phys. Chem. 99 Ž1995. 10752. w26x K.N. Houk, J. Liu, N.C. DecMello, K.R. Condroski, J. Am. Chem. Soc. 119 Ž1997. 10147. w27x R.D. Bach, M.N. Glukhovtsev, C. Gonzalez, M. Marquez, C.M. Estevez, A.G. Baboul, H.B. Schlegel, J. Phys. Chem. ´ 101 Ž1997. 6092.
w28x O. Wiest, K.N. Houk, Top. Curr. Chem. 183 Ž1996. 1. w29x R.W. Murray, Chem. Rev. 89 Ž1989. 1187. w30x W. Adam, R. Curci, J.O. Edward, Acc. Chem. Res. 22 Ž1989. 205. w31x R.C. Haddon, Science 261 Ž1993. 1545. w32x R. Zanasi, P. Lazzeretti, P.W. Fowler, Chem. Phys. Lett. 278 Ž1997. 251. w33x K. Ruud, H. Agren, T. Helgaker, P. Dahle, H. Kock, P.R. Taylor, Chem. Phys. Lett. 285 Ž1998. 205. w34x G.S. Hammond, J. Am. Chem. Soc. 77 Ž1955. 334.
Chemical Physics Letters 296 Ž1998. 429–434
On understanding the unique role of electrophilicity on C 60 reactivity and strain on exothermicity in Diels–Alder and epoxidation reactions M. Manoharan
)
School of Chemistry, UniÕersity of Hyderabad, Hyderabad-500 046, India Received 21 May 1998; in final form 18 September 1998
Abstract Semiempirical AM1 calculations have revealed that the electrophilic character of C 60 causes the barrier for the Diels–Alder addition of butadiene to be lowered Žrelative to ethylene. whereas for the epoxidation reaction Žwith dimethyldioxirane. the barrier is raised Žrelative to ethylene.. This is because the electronic factor appears to be relatively greater than the steric influence of fullerene during the reaction. The loss of aromaticity in the six-membered faces of C 60 as well as the strain relief in the cluster during adduct formation leads the reaction to a less exothermic path compared to the ethylene reaction. These estimations have led us to provide a common criterion for any kind of cycloaddition and oxygen-transfer reaction involving the 6-6 ring junction of fullerene. q 1998 Elsevier Science B.V. All rights reserved.
1. Introduction The most prominent representative of the fullerene family is C 60 and it has been used extensively for numerous methods of functionalization w1,2x in recent years. Of the functionalizations, the Diels–Alder w3–6x and oxygen transfer reactions w7–12x of C 60 have attracted much attention. These reactions readily occur through the 6-6 ring junction of such clusters with reasonable product yields. The significant effect of this strained, bulky electrophile on the mechanism of these reactions is unclear. Although a few experimental w13,14x and theoretical w15–17x reports on the mechanism of the Diels–Alder reac-
)
Corresponding author. E-mail: [email protected]
tion involving fullerenes are available, there have been no studies on the mechanism of oxygen transfer reactions. Therefore, this Letter presents a theoretical study of the kinetic and thermodynamic control of C 60 –butadiene ŽBD. w1x and C 60 –dimethyldioxirane ŽDMD. w11x reactions in three parts: Ž1. the relative reactivity of an olefin embedded in a C 60 cage compared to a free olefin; Ž2. the role of cluster strain and the electrophilicity of C 60 on the barrier; and Ž3. the effect of strain relief and aromatic destabilization of C 60 on the reaction exothermicity. The main goal of this investigation is to understand the flexibility of the fullerene environment in the reaction path and to throw light on the specific factors that decide the kinetic and thermodynamic properties when fullerene reacts with either a nucleophile or an electrophile.
0009-2614r98r$ - see front matter q 1998 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 Ž 9 8 . 0 1 0 6 3 - X
M. Manoharanr Chemical Physics Letters 296 (1998) 429–434
430
2. Computational details
From these values, the average percentage of bond make–break at the TS can be obtained:
Semiempirical AM1 w18x calculations have been performed throughout by using the MOPAC 6.0 program w19x. The size of the system studied prohibits the use of more expensive ab initio and density functional ŽDF. methods. Baker’s eigenvector following ŽEF. procedure w20x has been used for all geometry optimizations. It has been used extensively w21–24x for locating the stationary points, viz. equilibrium and transition state ŽTS. geometries, and this method has been found to be far better, both in terms of the quality of refined geometry and converged energy. FORCE calculations have been carried out to confirm the single imaginary frequency for all TSs and zero for reactants and products. The progress of a reaction can be followed through the changes in the bond order of certain bonds during the reaction. The extent of bond formingr cleaving of a specific bond i at the TS can be calculated as:
BFi or BC i s
BOiTS y BOiR BOiP y BOiR
= 100 ,
where BOiTS , BOiR and BOiP are the bond order values of the bond i that is forming or cleaving during the reaction at the TS, reactant and product.
BFC ave s
1 2
ž
1 ni
1 ÝBFi q
nj
/
ÝBC j .
This is proposed with a view to bookkeeping the bond make–break during the course of the reaction. This will further indicate the ‘earliness’ and ‘lateness’ of the TS in the reaction.
3. Results and discussions The molecular size, the effect of pyramidalization and the presence of embedded aromatic sextets on C 60 , all of which help to stabilize its lowest unoccupied molecular orbital ŽLUMO. considerably w25x, lead to a molecule that behaves as an electron-deficient cluster. Therefore, the interaction between the LUMO of C 60 and the highest occupied molecular orbital ŽHOMO. of BD Ž2. in the Diels–Alder reaction is more dominant than the HOMOŽC 60 . – LUMOŽBD. interaction as has been examined in the prototype reactionŽ1. drawn in Fig. 1. In the epoxidation reaction, the HOMO of the simple olefin readily interacts with the peroxide antibonding orbital ŽLUMO. of DMDŽ3., which causes the maximum stabilization and is the normal expectation w26,27x. On the other hand, the C 60 –DMD reaction involves the interaction between the oxygen lone-pair HOMO
Fig. 1. Schematic diagram of the FMO interaction in the Diels–Alder and epoxidation reactions.
M. Manoharanr Chemical Physics Letters 296 (1998) 429–434
of DMD and the C 60 LUMO Ž4b. rather than the one anticipated in the normal case Ž4a., and the electron-accepting ability of C 60 allows one to stabilize this reverse interaction. Moreover, the strength of the two frontier orbital interaction in 2 and 4 appears to be greater than that in 1 and 3, since the distorted antibonding orbital of the 6-6 junction matches the interacting orbitals of the approaching BD and DMD. Here, in the FMO analysis, the calculated frontier orbital energy gaps ŽTable 1. clearly reflect the electron-affinity character of C 60 on reactivity due to the strain on the spherical curvature of fullerene and further predict the Diels–Alder reaction of C 60 to be a normal-electron demand type whereas the C 60 epoxidation is predicted to be an inÕerse-electron demand type. The present investigation thus seeks to establish the mechanism of these reactions more quantitatively than through this qualitative argument. The earlier AM1 results of Diels–Alder w15,22– 24x and epoxidation w21x reactions reproduced the experimental observations reasonably and have also been in excellent agreement with the high-level ab initio and DF calculations w26–28x, which thus provides credibility for the use of AM1 for such reactions. Our calculations reveal that the C 60 –BD reaction passes through a concerted synchronous transition structure ŽTS. A shown in Fig. 2, which leads to the formation of a cyclohexene fused fullerene adduct, whereas the epoxidation of C 60 by DMD concertedly attains the synchronous ‘butterfly’ spiro TS B ŽFig. 2. and generates acetone and fullerene epoxide as observed in typical reactions w21,26,27x. Owing to the electrophilic 6-6 junction of fullerene, the two new C PPP C bonds at the TS are formed to a
431
greater extent in the former reaction and the two C PPP O bonds are formed at a slower rate in the latter compared to their respective typical reactions. This is clearly reflected in the bond lengths as noted in the TSs A and B. In addition, the extent of all bond forming and cleaving processes have been monitored in the TSs through bond order analysis and are accumulated in a BFC ave index. This further confirms that the TS of 2 occurs earlier whereas 4 involves the ‘late’ TS compared with the TSs of ethylene reactions, as can be seen from the BFC ave values ŽTable 1.. The quantum of charge transfer Ž qCT . presented in Table 1 clearly shows a flexible role of the electrophilicity of C 60 in both Diels–Alder and epoxidation reactions. An ‘early’ TS and the higher qCT values would have a lower activation barrier w21–24x. A notable feature of the TSs A and B is in the pyramidalized angles, the extent of pyramidalization at the reacting centres in the former TS is found to be higher than that in the latter, since the geometry A is pyramidalized 108 from C 60 whereas B is 3.38 from the fullerene structure. This happens because the involvement of four reacting centres in a Diels–Alder reaction leads to a less strained sixmembered TS whereas the three reacting centres of the epoxidation lead to a more strained three-membered spiro TS. The calculated activation energies presented in Table 1 predict that the C 60 –BD reaction is expected to be faster than the typical reaction Ž1. whereas the epoxidation of C 60 by DMD is found to be slower than that of ethylene Ž3., despite the fact that C 60 reacts with a nucleophile in the former reaction and an electrophile is involved in the latter. This is in good accordance with the experimental observations
Table 1 Calculated AM1 frontier orbital energy gaps ŽeV., charge transfer, activation and reaction energies Žkcal moly1 ., deformation energies Žkcal moly1 . of reactants for the title reactions, and average percentage of bond forming and cleaving at the TS Reaction
D E1 a
D E2 b
qCT c
D E/
DE1d
DE2 d
D Er
BFC ave
1 2 3 4
10.8 6.4 12.4 11.6
11.0 10.1 12.9 8.6
y0.001 0.243 0.197 0.160
23.2 16.3 15.6 28.1
15.9 12.0 17.8 21.5
9.9 5.8 0.4 1.1
y58.0 y50.8 y85.3 y72.3
42.7 37.4 18.4 27.8
a
D E1 s E HOMO Ždiene. y ELUMO dienophile.rEHOMO Žolefin. y ELUMO ŽDMD.. D E2 s E LUMO Ždiene. y EHOMO Ždienophile.rELUMO Žolefin. y EHOMO ŽDMD.. c Quantum of charge transfer from diene to dienophilerolefin to DMD at the TS. d DE1 and DE2: deformation energy of BDrDMD and deformation energy of ethylenerC 60 , respectively, at the TS. b
432
M. Manoharanr Chemical Physics Letters 296 (1998) 429–434
Fig. 2. AM1 optimized transition structures ŽA and B. of the Diels–Alder and epoxidation reactions involving C 60 with selected geometric parameters. Parentheses denote the TS of the ethylene reaction.
w11,13,14x. A substantial difference in the barrier between C 60 Ž2 and 4. and ethylene Ž1 and 3. reactions is most likely due to the electron withdraw-
ing ability of such a cluster. Deformation energies 1 of reactants ŽTable 1. show that both BD and C 60 deform to a lesser extent in the Diels–Alder reaction and the deformation of DMD as well as C 60 significantly increases in the epoxidation compared to those found in the ethylene reactions. In epoxidation, the weak peroxide bond of DMD w29,30x cleaves quickly during the formation of the TS and hence it deforms heavily and to a greater degree compared to a typical olefin as seen in the deformation energies. From DE1 and DE2, one can observe that the trend in deformation energies parallels the trend in the reaction barriers. A contrasting reactivity of C 60 observed in the Diels–Alder and epoxidation reactions obviously shows the real electron-deficient character of C 60 , although the strain factor has brought to light information on the electrophilic nature of the isolated fullerenes w25,31x. Moreover, when the reactivity of fullerene is compared with ethylene, the dominant role of the electrophile over strain on the barrier can be seen. It is to be noted that the reacting junction of fullerene is a strained nonplanar alkene whereas ethylene is a simple planar alkene. When two electronically different systems ŽBD and DMD. react with the strained alkene, the pyramidalized angle is found to increase monotonically on going from reactant to product via the TS in both Diels–Alder and epoxidation reactions and hence the strain is released accordingly due to the marginal changes in the orbital distortion. Nevertheless, it does not help to show the above reactivity pattern on the barrier. This quantitative prediction thus indicates the fact that the electronic factor offsets the steric influence of C 60 and dominates in the barriers. In both of these reactions, there is a relief of strain in the cluster during the formation of the TS as well as in the product, since the reacting carbon atoms are rehybridized from sp 2 to sp 3. At the same time, as the reacting bond of C 60 is abutted between the two aromatic hexagonal rings, it destabilizes the resonance rings during the TS and product formation. The concept of aromaticity on fullerene is evident from the fact that 1 Deformation energy of a reactant is defined as the energy acquired by the reactant to distort it from the ground state geometry to the geometry of it in the TS. Deformation energy analysis gives a clue for why the barrier increasedrdecreased during the course of the reaction.
M. Manoharanr Chemical Physics Letters 296 (1998) 429–434
the six-membered faces of fullerene have been predicted to be aromatic hexagons with strong diamagnetic exaltation w31–33x, although they have alternative single and double bonds. Owing to the strain relief as well as the loss of aromaticity at the reacting centre, the reaction energy ŽTable 1. of the fullerene reaction is significantly reduced compared to the ethylene reaction. The strain relief should normally increase the reaction energy to some extent instead of decreasing the energy, however, it controls the reaction exothermicity. In view of the above discussion, it is clear that the lower exothermicity of C 60 reactions compared to ethylene reactions seems to be due mainly to the loss of aromatic stabilization during the product formation. A recent theoretical prediction w24x further supports this criterion, because the gain or loss of aromaticity of the benzenoid ring in the product plays a major role in the thermodynamic control of the reaction. Interestingly, both these effects – the loss of aromaticity and the strain relief – thus uniquely support the thermodynamic aspect of these reactions, but not the kinetic part. Overall, AM1 calculations conclude that the Diels–Alder reaction of C 60 is kinetically but not thermodynamically more favoured compared to typical reactions. The epoxidation of C 60 is predicted to be both kinetically and thermodynamically less favoured than that of ethylene. The highly exothermic reaction should have an ‘early TS or reactant-like TS’ on the reaction energy surface, as one would normally expect based on the Hammond’s postulate w34x. In this context, the former reaction appears to be contradictory to the postulate. This investigation finally establishes that the electrophilicity of fullerene, rather than the strain, is the determining factor of reactivity, whereas the resonance destabilization as well as the strain release in the cluster during the product formation determines the reaction exothermicity, and through the specific factor, both the kinetic and thermodynamic path of the reactions involving the 6-6 ring bond of any fullerene can be identified. Acknowledgements The author is very grateful to Professors E.D. Jemmis ŽSchool of Chemistry, University of Hyder-
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abad, India. and P. Venuvanalingam ŽDepartment of Chemistry, Bharathidasan University, India. for helpful discussions and for providing the computational facilities. The author also thanks the Department of Science and Technology, India, for financial support in the form of Research Associate.
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