A study towards the regioselective synthesis of the e , e , e trisadduct of C 60 via the [4+2] Diels-Alder reaction with tethers bearing ortho - quinodimethane precursors

The regioselective synthesis of an e , e , e trisadduct of C 60 via the Diels-Alder reaction with ortho - quinodimethanes has been attempted employing the tether-directed remote functionalization approach. Opened-structure tether 10 and macrocyclic tethers 16 and 21 were synthesized for this purpose. The functionalization of C 60 afforded inseparable mixtures of regiomeric trisadducts and the regioselective formation of the e , e , e trisadduct was not feasible even when the more preorganized tethers 16 and 21 were employed. The in situ thermal generation of ortho - quinodimethanes from the 1,2-bis(bromomethyl)benzene precursors requires high temperatures and is followed by fast, irreversible cycloaddition with C 60 to afford thermally stable products, which prevents the achievement of high regioselectivities.


Introduction
The fascinating properties of [60]fullerene have inspired chemists to design and synthesize derivatives with specific function and unique architectures targeting future applications in advanced nanoscale materials and devices. 1,2For synthetic chemists, the limited solubility of C 60 in common organic solvents has been an obstacle but at the same time triggered an enormous development of its covalent chemistry. 32][3] While single additions on C 60 deliver one regioisomer (all double bonds are equivalent), the double and triple covalent attachment of addends leads to the formation of different regioisomers with the number increasing to 46 for trisadducts.The multifunctionalization of C 60 became one of the first targets in fullerene chemistry and following the tether-directed remote functionalization concept introduced by Diederich, 4 there has been a significant increase of research activity in this area.
A challenging topic in the areas of fullerene chemistry, material and biological sciences is the regioselective synthesis of [60]fullerene trisadducts with C 3 -symmetrical addition patterns (Scheme 1).The well-defined three-dimensional molecular architecture combined with the unique physicochemical properties of the fullerene chromophore render this family of molecules candidates for future applications.Apart from the aesthetically pleasing architecture, this family of trisadducts showed pronounced biological activity and were investigated as potent drugs for numerous diseases. 2The strong antioxidant character of these compounds 2 together with their potential to act as platforms for the construction of functional supramolecules (e.g., lipofullerenes, dendrofullerenes) 5 have set the development of synthetic methods for the regioselective triple additions on C 60 a highly desirable target.7][18] Opened-and closed-structure tethers equipped with three malonate groups were utilized for this purpose and all possible C 3 -symmetrical addition patterns have been accessed except the cis-1,cis-1,cis-1 (all-cis-1) which is not energetically favored owing to the steric hindrance of the addends.In all these cases, the covalent functionalization was performed by the Bingel cyclopropanation of the reactive [6,6]-double bonds of C 60 , a widely used derivatization method in fullerene chemistry.
A disadvantage of the Bingel reaction is the reversibility under reductive (chemical and electrochemical) conditions [19][20][21] [Scheme 2, (a)], a limiting factor for possible applications of C 60 trisadducts in electron transfer processes mimicking the photosynthetic system.To overcome this problem, Diels-Alder cycloadditions can be employed that lead to the formation of stable fullerene cycloadducts, at least under reductive conditions.A cycloadduct of C 60 [Scheme 2, (b)] derived from the Diels-Alder addition of a diene to C 60 is, however, sensitive to oxidation which proceeds via the "ene" reaction of singlet oxygen ( 1 O 2 ) 22 with the double bond of the cyclohexene ring to afford the corresponding allylic hydroperoxide 3. [23][24][25] The fullerene core acts as a photosensitizer and thus, adducts of this kind should be handled in the dark.This problem can be tackled if ortho-quinodimethanes are used as dienes [Scheme 2, (c)] and in such a case the corresponding cycloadducts 4 are thermally and photochemically stable.While a plethora of C 60 monoadducts have been reported in the literature using ortho-quinodimethanes as reactive dienes, 26,27 there are only a few reports on the synthesis of bisadducts. 28,29ith respect to the trisadducts, to the best of our knowledge, there are no reports on either stepwise or tether-directed regioselective synthesis of a Diels-Alder trisadduct of C 60 employing ortho-quinodimethanes as reactive dienes.The synthesis of the equatorial,equatorial,equatorial (e,e,e) trisadduct derived from the stepwise Diels-Alder reaction of 9,10-dimethylanthracene with C 60 is the only example of [4+2] trisadduct reported by Kräutler in 2008. 30

Results and Discussion
Our interest was focused on the regioselective synthesis of a redox-stable and singlet oxygen insensitive trisadduct of C 60 , functionalized by the [4+2] Diels-Alder reaction with orthoquinodimethanes. Specifically, we designed e,e,e trisadduct 5 (Figure 1) which is expected to be thermally stable and inert to 1 O 2 photooxidation owing to the presence of the aromatic benzenes formed during the cycloaddition reactions.The choice of the e addition pattern was not accidental as it is well documented that it is favored over the others (cis and trans). 31,32For the synthesis of trisadduct 5 we employed the tether-directed remote functionalization method utilizing C 3 -symmetrical tethers equipped with ortho-quinodimethane precursor moieties for the three-fold Diels-Alder reaction on the fullerene sphere.The reports of Hirsch [9][10][11][12] and Nierengarten 13,14 on the synthesis of e,e,e trisadducts of C 60 with tripodal tethers equipped with malonate moieties, prompted us to design opened-structure tether 10 bearing ortho-quinodimethane precursors covalently connected to the phloroglucinol focal point via two-carbon alkyl chains (Scheme 3).Bromoethanol was firstly protected as a THP ether 33 followed by a three-fold Williamson etherification with phloroglucinol to afford tripodal protected alcohol 7. The highest yield of 7 (70%) was obtained when the reaction was carried out in acetone heated at reflux, and by using K 2 CO 3 as a base in the presence of 18crown-6.Subsequent cleavage of the protecting groups using p-TSA furnished triol 8 which has been synthesized before in one step by the reaction of phloroglucinol with ethylene carbonate. 34his stepwise, protection-deprotection strategy offers the possibility of synthesizing similar tethers differing in the length of the alkyl spacers connecting the reactive groups with phloroglucinol.This can be accomplished by employing the appropriate bromoalcohols with variable number of carbon atoms, which in the case of ethylene carbonate is limited to two.In the last step, triol 8 was subjected to a three-fold esterification with acid 9 35 using DCC and DMAP, in THF solvent.Tether 10 was isolated by column chromatography in 28% yield and characterized by NMR spectroscopy and MALDI-TOF mass spectrometry.Scheme 3. Synthesis of tripodal tether 10.
We next investigated the remote three-fold Diels-Alder functionalization of C 60 with tether 10 under the experimental conditions used for the generation of ortho-quinodimethanes from the corresponding 1,2-bis(bromomethyl)benzene precursors (Scheme 4).The reaction was carried out under high dilution conditions (C 60 concentration 10 -5 mol/L), in toluene solvent and by using KI/18-crown-6 as the 1,4-debrominating reagent.As the 1,4-elimination step requires high temperature, the reaction mixture was refluxed at 110 o C and the progress of the reaction was monitored by TLC, HPLC and MALDI-TOF.According to the HPLC elugram of the crude mixture (see Supplementary Material), the regioselective formation of a specific trisadduct was not observed but instead, the reaction led to the formation of an unseparable mixture of fullerene adducts.In the MALDI-TOF spectrum (see Supplementary Material), the dominant peak at 1368 m/z corresponding to the [M-H] -ion confirmed that the three-fold Diels-Alder reaction was successful and the reaction products were trisadducts of C 60 .The lack of regioselectivity in the remote functionalization of C 60 with tether 10 can be attributed to the open structure of the tripodal tether and thus, to an insufficient preorganization of the reactive groups.Furthermore, for adducts formed from opened-structure tethers, in-out stereoisomerism 36,37 has been observed which is attributed to the relative orientation of the ester moieties connecting the reactive groups with phloroglucinol.As a consequence, a specific addition pattern is represented by a number of stereoisomers and in the case of e,e,e trisadduct 5, there are four possible (Figure 2).According to PM3 calculations, 38 the in-in, out-in, out-out stereoisomer was the most thermodynamically stable.The remote functionalization of C 60 with cyclo-[n]-malonate tethers 7,8,15-18 has been proved advantageous in the regioselective synthesis of cyclopropanated fullerene multi-adducts, compared to the acyclic tether analogues.In addition, when macrocyclic tethers are used, the inout stereoisomerism cannot operate due to the restricted flexibility of the reactive groups.With this in mind, we designed macrocyclic tether 16 (Scheme 5) where the ortho-quinodimethane precursors are well-preorganized as they are incorporated in a macrocyclic system.For the synthesis of 16, pyrocatechol was firstly mono-protected as a benzyl ether 39,40 and subjected to a Williamson etherification with 1,5-dichloropentane to afford 12 in very good yield.Deprotection of the benzyl ethers by hydrogenation furnished diol 13 which was allowed to react with dichloride 14 derived from the two-fold Williamson etherification of pyrocatechol with 1,5dichloropentane.The cesium-templated intermolecular cyclization of 13 and 14 led to the exclusive formation of macrocycle 15 isolated in 50% yield after column chromatography.

Scheme 5. Synthesis of macrocyclic tether 16.
Finally, the benzylic bromide groups on the aromatic rings were installed in a one-pot, twostep process.A six-fold electrophilic aromatic substitution with formaldehyde followed by the in situ substitution of hydroxyl groups with bromine atoms led to the successful synthesis of tether 16 (isolated in 50%).Its structural assignment was accomplished by 1 H, 13 C NMR, IR spectroscopies and by MALDI-TOF mass spectrometry.
The remote functionalization of C 60 with tether 16 was then investigated under high dilution conditions (C 60 concentration 10 -5 mol/L), in toluene and by using KI/18-crown-6 for the generation of the reactive ortho-quinodimethane dienes.In the MALDI-TOF spectrum (see Supplementary Material) of the crude reaction mixture the intense peak at 1334 m/z corresponding to the [M+H] + ion confirmed the successful formation of trisadducts of C 60 .HPLC analysis (see Supplementary Material) revealed the non-regioselective behavior of tether 16 in the remote Diels-Alder cycloaddition reaction which led to an inseparable mixture of trisadducts.Attempts to separate at least some major adducts by column chromatography failed.
In tether 16, the ortho-quinodimethane precursors are connected with C5 alkyl chains which prefer an anti confirmation.This might render the adoption of a concave geometry difficult, leading to an inappropriate orientation of the reactive groups and consequently to the poor regioselectivity of the Diels-Alder remote functionalization of C 60 .With this assumption in mind, we modified the tether's structure by replacing the alkyl chains with glycol moieties.Thus, we designed tether 21 which was synthesized following the same synthetic strategy (Scheme 6) with that for tether 16.To introduce the glycol groups, we employed in the Williamson etherification steps bis(2-chloroethyl)ether instead of 1,5-dichloropentane.Compounds 18 41 and 19 42 were synthesized according to literature reports while, 17 41 and 20 43 are known compounds in the literature but they were synthesized following our approach.Worthy of note was that the higher yields of these synthetic steps may be attributed to the presence of the glycol groups which helped improve the solubility of the intermediates.Thus, tether 21 was synthesized in very good overall yield and its structure was assigned by NMR spectroscopy and MALDI-TOF mass spectrometry.Scheme 6. Synthesis of macrocyclic tether 21.
The three-fold Diels-Alder functionalization of C 60 with tether 21 was successful as confirmed from the MALDI-TOF spectrum (see Supplementary Material) of the crude reaction mixture which showed a clear peak at 1339 m/z corresponding to the [M + ] ion of C 60 trisadducts.Unfortunately, HPLC analysis (see Supplementary Material) revealed that the remote functionalization of C 60 was not regioselective, and column chromatographic separation of any of the formed trisadducts was not feasible.
As the Diels-Alder remote functionalization of C 60 with macrocyclic tethers 16 and 21 was non-regioselective, we focused our attention on the 1,4-debromination of the 1,2-bis(bromomethyl)benzene precursors of the tethers leading to the formation of the reactive orthoquinodimethanes.In all cases, KI/18-crown-6 was used as the source of I -which acts as the 1,4elimination reagent preceding the cycloaddition reaction with C 60 .To exclude the possibility that K + also binds into the macrocyclic cavity of 16 and especially of 21 which bears glycol chains rigidifying the structure of the macrocyclic rings, we tested different iodine salts.As such, we repeated the reactions of C 60 with tethers 16 and 21 in toluene heated at 110 o C, by using Et 4 N + I - and CsI as debrominating reagents.The reactions were monitored by HPLC and MALDI-TOF but in all cases, non-separable mixtures of C 60 trisadducts were formed.At temperatures lower than 110 o C the formation of ortho-quinodimethanes from the corresponding 1,2-bis(bromomethyl)benzene precursors was notably suppressed.
In the three-fold Diels-Alder reaction of C 60 with tethers 10, 16 and 21, thirty double bonds of the fullerene skeleton are available for functionalization and the possible trisadducts that can be formed are 46.The final approach of our study focused on finding a way to reduce the number of possible regioisomers and obtain a better understanding regarding the regioselective behavior of the synthesized tethers.As such, we chose as a starting material a synthetically valuable derivative of C 60 namely, e,e,e trisadduct 22 (Scheme 7).In the trisadduct 22, one hemisphere of the fullerene core is protected by the cyclo- [3]-octyl malonate moiety and thus, the number of double bonds available for functionalization is reduced to half in comparison with the parent C 60 .In case that the synthesized tethers will react in a regioselective manner with trisadduct 22 and succeed in accessing the e,e,e addition pattern, hexakis adduct 23 (Scheme 7) was expected to form.Such hexaadducts of C 60 are called type I [3:3] 44 and the addends are located at the octahedral sites of the fullerene sphere.6][47][48][49][50] Trisadduct 22 was synthesized according to the literature 7 and the remote functionalization with tethers 10, 16 and 21 was investigated in toluene heated at 110 o C and by using KI/18-crown-6 as the 1,4debrominating reagent.The crude reaction mixtures were analyzed with the aid of TLC, HPLC and MALDI-TOF mass spectrometry.The successful formation of hexakis adducts was confirmed by the measured MALDI-TOF spectra but according to the HPLC elugrams, the regioselective formation of a specific hexaadduct did not occur.The Diels-Alder functionalization reactions of the trisadduct 22 with tethers 10, 16 and 21 furnished inseparable mixtures of fullerene hexakis adducts and, as such, the targeted hexakis adduct 23 could not be isolated.

Conclusions
Summarizing, the results derived from the present study on the regioselective tether-directed synthesis of an e,e,e trisadduct of C 60 via the [4+2] Diels-Alder reaction with orthoquinodimethanes, we designed and synthesized three novel C 3 -symmetrical tethers equipped with 1,2-bis(bromo-methyl)benzene moieties.Under the appropriate experimental conditions, these groups lead to the in situ formation of the corresponding ortho-quinodimethanes via an 1,4elimination transformation.Tether 10 has an open structure, while 16 and 21 are macrocyclic molecules where the ortho-quinodimethane precursors are better preorganized as they are linked with alkyl and glycol chains, respectively.The three-fold [4+2] cycloaddition reactions of tethers 10, 16 and 21 with C 60 were carried out under high dilution conditions, in toluene heated at 110 o C and by using different 1,4-debrominating reagents.In all cases, inseparable mixtures of regiomeric trisadducts were formed and the regioselective formation of the targeted e,e,e trisadduct was not feasible even when the more preorganized tethers 16 and 21 were employed.The results of the present study lead to the conclusion that the [4+2] cycloaddition of orthoquinodimethanes with the C 60 is a kinetically controlled reaction.The in situ thermal generation of ortho-quinodimethanes from the corresponding 1,2-bis(bromomethyl)benzene precursors requires high temperatures (110 o C) and is followed by the fast, irreversible [4+2] cycloaddition reaction with C 60 to afford thermally stable products.To reduce the number of possible regioisomeric trisadducts, the tether-directed remote [4+2] functionalization was also investigated with e,e,e trisadduct 22 towards the synthesis of hexakis adduct 23 functionalized at the octahedral sites of the fullerene.Also in this case, tethers 10, 16 and 21 reacted in a nonregioselective manner strengthening the conclusion that the in situ thermal formation of orthoquinodimethanes from 1,2-bis(bromomethyl)benzene precursors followed by the fast and irreversible [4+2] cycloaddition prevents the achievement of high regioselectivities even if the reactive groups are incorporated in well-preorganized molecular systems.

Scheme 1 .
Scheme 1.The four possible C 3 -symmetrical addition patterns of C 60 .

Figure 1 .
Figure 1.The designed e,e,e trisadduct of C 60 functionalized with tethers bearing orthoquinodimethane precursors.

Figure 2 .
Figure 2. The four possible stereoisomers of e,e,e trisadduct 5 and their PM3 calculated ∆H f energies.