Subject Number : S-18-MS-0033
Support Type : Collaborative research
Proposal Title (English) : [2+2] cycloaddition reaction on the basis of fullerenes actuated via nitrene
Username (English) : Xiang Zhao
Affiliation (English) : Xi’an Jiaotong University, China

1. Summary
The perfect symmetry molecule C60, has attracted great interest from larger number of scientists due to its unique π-electron structure leading to their diversiform chemical and physical features. Thus chemical reactions occurring to normal π-electron systems, like ethylene, benzene and cyclohexene, would behave peculiarly when happening to fullerenes and their derivatives.[1] It is revealed in 2015 that (2+2) cycloadditions of benzyne to endohedral metallofullerenes M3N@C80 (M=Sc and Y)follow a diradical mechanism, accompanying with a rotating-intermediate state, rather than normal caebene mechanism.[2]
[2+2] cycloaddition reaction, one of the most important reactions to π bond, nevertheless, between fullerenes is hindered because of spatial and electrostatic interaction among themselves. Hereof, our focus is on investigating how to realize the [2+2] cycloaddition reactions to double fullerenes or more easily and the mechanism of [2+2] cycloaddition reactions to fullerenes. The electrostatic interaction between two independent fullerenes would be weakened with endohedral and exohedral functionalization, but spatial obstruction is still not ignored.
Then intramolecular cycloaddition would be a better choice to fullerenes. Bis-carbene and bis-nitrene possess double sites to react with π bonds, furthermore, 9,10-bis(azidomethyl)anthracene has been successfully isolated and it will be a better precursor to bis-nitrene following with eliminating of N2.[3,4] Nucleophilic addition would like to happen between 9,10-bis(azenylmethyl)anthracene and two electron-deficient fullerenes C60, and in this way, double C60 are close enough to each other to react with surmounting spatial and electrostatic resistance. The mechanism of [2+2] cycloaddition reactions would be further explored on the basis of the reaction model shown in Scheme 1. The fancy mechanism of [2+2] cycloaddition reactions to fullerenes is expected. This project will extend the functionalization of fullerenes and carbene is in another expectation of functionalized molecule to drive the [2+2] cycloaddition to fullerenes. The reason why intermediate 2 could not form another intermediate 3 is due to the easily hydrogen migration in intermediate 2.
2. Computational method
The reactants and possible products have been optimized based on M06-2X/6-31G(d) level showing in Figure 1. 2 and 3 molecules are triplet and all of the other molecules are singlet. Furthermore, 4c isomer possesses lowest energies among 4a, 4b and 4c isomers, and 5d also possesses the lowest relative energies among 5a, 5b, 5c and 5d. Especially, 5d is derived from the intramolecular [2 + 2] cycloaddition reaction, and the lower relative energy of 5d than 5c means that the intramolecular [2 + 2] cycloaddition reaction to 5c is thermodynamically supported which will be further discussed in this work.
3. Results ad Discussion
The In order to discuss the kinetic reaction about nitrene and fullerene or the intramolecular [2 + 2] cycloaddition reaction, the reaction conditions have been optimized for 1 to 2 based on M06-2X/6-31G(d) in PCM model with different solvents and the results have been shown in Table 1. It is clearly that the dG’ and dG determine that the reaction would be thermodynamically and kinetically preferred in solvent than in gas. The dG’ is almost same at the same temperature and different solvents with the ignorable ΔΔG’ (0.2 or 0.1 kcal/mol). As is same with the dG at the same temperature in different solvents with the largest ΔΔG(1.1 kcal/mol) at 500 K. These results mean that the polarity of solvent affects less on the reaction. Furthermore, ΔΔG’ are 0.9 or 0.8 at different temperature meaning that the temperature also plays negligible role in kinetically chemical reaction. Although the ΔΔGmax are larger arriving at 22.4 or 22.5 kcal/mol at different temperature at distinguished solvents respectively, the difference change of Gibbs free energies is no more than 8.0 kcal/mol between 300 K and 500 K. It is not economical to decreasing 8.0 kcal/molchange of Gibbs free energies with increasing 200 K. Thus following kinetic and thermodynamic study for the reaction in this work will be carried out on the level of M06-2X/6-31G(d)-PCM (Tetrahydrofuran), with lowest active Gibbs free energies and change of Gibbs free energies seeing in Table 1, at 300 K. The decompositions of alkyl azides are first order, with activation Gibbs energies in range of 38 – 47 kcal/mol,[5]and the activation Gibbs energies from 1 two 2 intermediate in tetrahydrofuran is 40.4 kcal/mol at 300 K which is well comparable with reference values.[5]
In the future, chemical reaction kinetic would be carried out based on Scheme 1 to elucidate the possible reaction mechanism between nitrine and C60on M06-2X/6-31G(d)-PCM(Tetrahydrofuran) level at 300 K. Furthermore, the orbital interaction between the two reactants would be studied via natural bond orbital analysis. The key factor determining the transition states would also be explored with the energy decomposition analysis of the transition states.

Figure 1. Figure 1The structures of main reactants and products, and the relative energies, in kcal/mol based on M06-2X/6-31G(d) level, of 4 isomers (marked in red) and 5 isomers (marked in brown) respectively.
4. References
[1] Lu, X.; Feng, L.; Akasaka, T.; Nagase, S. Chem. Soc. Rev.2012, 41, 7723-7760.
[2] Yang, T.; Nagase, S.; Akasaka, T.; Poblet, J. M.; Houk, K. N.; Ehara, M.; Zhao, X. J. Am. Chem. Soc. 2015, 137, 6820-6828.
[3] Reuter, R.; Wegner, H. A. Chem. Comm. 2013, 49, 146-148.
[4] Kannan, A.; Rajakumar, P. RSC Adv. 2015, 5, 51834-51840.
[5] Abramovitch, R. A.; Kyba, E. P.; J. Am. Chem. Soc.1974, 96, 480-488.
5. Publication/Presentation
(1) M. Y. Li, Y. X. Zhao. M. Ehara, X. Zhao, to be submitted.
6. Patent N/A