Functionalization of Fullerene C₆₀ with Organic Carbonates in the Presence of a Grignard Reagent and Ti(Oi-Pr)₄  ^†

Abstract

Fullerene C₆₀ is by far the most studied of all allotropic modifications of carbon. Chemical modification of the double bond over the years has led to the emergence of a variety of fullerene derivatives. These derivatives have now found numerous applications in medicine, materials and supramolecular chemistry, and as efficient electron acceptors in organic photovoltaic devices. The main method for the functionalization of C₆₀ fullerenes, which makes it possible to obtain its derivatives in a preparative volume, is the Bingel–Hirsch reaction. But this method makes it possible to obtain fullerocyclopropanes containing only carboxyl substituents at the bridging carbon atom. Therefore, in order to obtain new materials, we began to study the interaction with organic carbonates in combination with Grignard reagents in the presence of Ti-containing complex catalysts. We hope that replacing the olefin in the Kulinkovich reaction with a C₆₀ fullerene molecule will lead to new and hard-to-find functionalization products of the latter. Organic carbonates were chosen as the object of study due to the fact that they are used in the industry as solvents for natural and synthetic resins, cellulose ethers, dispersants, blowing agents, emulsifiers, absorbents of hydrogen sulfide and carbon dioxide, starting materials for the industrial synthesis of fibers and plastics, as well as plasticizers, pharmaceuticals and plant protection products.

1. Introduction

Organic carbonates play an important role in modern organic synthesis [1,2]. They are used in the chemical industry as plasticizers, emulsifiers and solvents for natural and synthetic resins. In addition, carbonates are part of biologically active compounds in pharmaceuticals and agrochemicals [3,4]. Based on the above, in order to obtain new and promising materials for the chemical, pharmaceutical and agricultural industries and for photoelectronics, we decided to study the interaction of fullerene C₆₀ with organic carbonates in combination with Grignard reagents in the presence of Ti-containing complex catalysts.

2. Results and Discussion

In the classical version of the Kulinovich reaction, hydroxy cyclopropanation of olefins with carboxylic acid esters is carried out using EtMgBr and Ti(Oi-Pr)₄ [5]. We assumed that replacing the olefin with a fullerene and the carboxylic acid ester with an organic carbonate will lead us to completely new fullerene derivatives (Scheme 1).

In order to test our idea and select the most optimal conditions, it was necessary to obtain the simplest organic carbonates. The latter were obtained by the interaction of alcohols with ethyl chloroformate in the presence of triethylamine in a ratio of 1:1:1 (r.t., 3 h). Unfortunately, this method did not lead to a pure target product. According to [6], replacing triethylamine with N,N-Dimethylaniline (1.5 mol) leads to the required carbonates, which have a pleasant floral–fruity aroma (Scheme 2).

Preliminary experiments have established that the interaction of C₆₀ fullerene can be realized only with an aryl-containing carbonate. Thus, ethyl phenyl carbonate interacts with fullerene C₆₀ in the presence of Ti(Oi-Pr)4 and EtMgBr at 80 °C with a reagent ratio of 5:1:15:15 for 1 h.

The reaction progress was monitored by analytical HPLC (Figure 1). The peak at 5.542 min belongs to the fullerene derivative, and the peak at 6.712 to unreacted free fullerene. Increasing the temperature and reaction time and changing the reagent ratio led to an increase in the intensity of the peaks at the fourth minute, which corresponds to polyaddition products.

Using semi-preparative HPLC, an individual adduct was isolated from the resulting reaction mass. One-dimensional (¹H and ¹³C) experiments showed that under the conditions we selected, a product different from our idea was formed. At the moment, this derivative is being developed for two-dimensional (HHCOSY, HSQC, HMBC) NMR spectroscopy and mass spectrometry in order to reliably establish the structure of the fullerene derivative (Figure 2).

In order to develop our idea, we plan to study the influence of the nature of the alkyl and halogen substituents of the Grignard reagent RMgX on the yield of target adducts, the influence of hydrolysis conditions (RCOH, D₂O) on the quality of the target product and the use of a wider range of carbonates with different substituents.

3. Conclusions

Thus, preliminary experiments have established that the interaction of fullerene C₆₀ under the conditions of the Kulinkovich reaction is possible only with aryl-containing organic carbonates.