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3 Concepts for Regio- and Stereoselective Multiple Functionalization of C(60)

3 Concepts for Regio- and Stereoselective Multiple Functionalization of C(60)

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10.3 Concepts for Regio- and Stereoselective Multiple Functionalization of C60



(DMA) binds reversibly to C60. A similar effect was observed for 2,6-dimethoxyanthracene [60]. Use of, for example, a ten-fold excess of DMA results in an

equilibrium between the various C60DMAn adducts, with e,e,e-C60DMA3 as the main

component. Hence, synergetic combination of kinetic and thermodynamic control

results in the generation of templates such as e,e,e-C60DMA3, with incomplete

octahedral addition patterns. Since (a) attack of irreversibly binding addends onto

such templates occurs with highly pronounced regioselectivity at free octahedral

sites, (b) facile rearrangement of DMA addends is possible in wrong intermediates,

resulting in the formation of an octahedral isomer, and (c) the reversibly bound

DMA molecules can easily be replaced by the desired addends, the yields of

hexakisadducts such as 17 can be as high as 50%. Another important improvement

was the in situ formation of the bromomalonates, by DBU-initiated reaction between

the corresponding parent malonate and CBr4 [59]. Consequently, a broad variety of

easily available malonates can be used directly to synthesize large quantities of

Th-C66(COOR)12 [26]. A few examples of such adducts with remarkable properties

deserve to be mentioned.



Scheme 10.7 Template mediated synthesis of hexakisadducts of C60

involving an octahedral addition pattern using DMA as equilibrating addend.

(i) Diethyl bromomalonate in the presence of DBU; (ii) in situ formation of

bromomalonate using diethyl malonate and CBr4–DBU.



Lipofullerenes such as 35–37 self-assemble within lipid bilayers into rod-like

structures of nanoscopic dimensions [61, 62]. These anisotropic superstructures

may be important for future membrane technology. Significantly, lipofullerenes

35 and 37 have very low melting points, 22 and 67 °C (DSC, heating scan),

respectively, with 35 being the first liquid fullerene derivative at room temperature.



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10 Regiochemistry of Multiple Additions



10.3 Concepts for Regio- and Stereoselective Multiple Functionalization of C60



The Th-symmetrical hexaaddition pattern also represents an attractive core tecton

for dendrimer chemistry [26, 31, 63–67]. Examples for such dendrimers, involving

a core branching multiplicity of 12, are 38 and 39 [63, 64]. Addition of six mesotropic

cyanobiphenyl malonate addends produced the spherical thermotropic liquid crystal

40 [65]. DSC and POM investigations revealed a smectic A phase between 80 and

133 °C. Interestingly, this spherical and highly symmetrical compound gives rise

to liquid crystallinity despite the absence of molecular anisotropy.



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10 Regiochemistry of Multiple Additions



Fullerene malonates C66(COOR)12 can serve as valuable starting materials for

further side-chain modifications, as demonstrated, for example, with the synthesis

of the highly water-soluble hexamalonic acid derivative C66(COOH)12 (41) by

hydrolysis of 17 [25]. A more efficient synthetic route to water-soluble fullerenoderivatives is cyclopropanation of C60 with bis(3-tert-butoxycarbonyl)propyl malonate

42 to afford the tert-butyl ester 43 (Scheme 10.8) [26]. Subsequent cleavage of the

tert-butyl protecting group leads to the spherical dodecacarboxylic acid 44, which

can then be transformed into dodecaglycine adduct 45.

10.3.1.2 Mixed Hexakisadducts



Using the template mediation technique, mixed hexakisadducts with different types

of addends are also easily available [26]. Here the precursor is not C60 but a C60

adduct with an incomplete octahedral addition pattern. The possible structures of

mixed hexakisadducts with two different addends in octahedral positions are depicted

in Figure 10.12.



10.3 Concepts for Regio- and Stereoselective Multiple Functionalization of C60



Scheme 10.8 Synthesis of highly water-soluble dodecacarboxylic acid 44

and protected glycine derivative 45.



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10 Regiochemistry of Multiple Additions



Figure 10.12 Complete series of octahedral addition patterns in hexakisadducts of C60 with one or two different types of addends and their precursor

adducts; type I adducts are derived from precursors obtained from successive

e-additions, type II adducts from precursors synthesized by other means.



The most important aspect is the synthesis of precursor adducts possessing an

incomplete octahedral addition pattern with one type of addend. Whereas monoadducts or e-bisadducts are easily available, the production of trans-1-bisadducts or

higher adducts with incomplete addition requires more effort. However, production

of the mixed hexakisadducts from all the precursors is generally straightforward,

since advantage can be taken of either the effective template mediation technique

or the highly pronounced e-regioselectivity characteristic of higher adducts to

complete the octahedral addition pattern [1, 26].



10.3 Concepts for Regio- and Stereoselective Multiple Functionalization of C60



Mixed [5:1]-hexakisadducts



This addition pattern is accessible by starting either from a pentakisadduct with

one unchanged octahedral site or from a monoadduct with five unoccupied sites.

However, the stepwise synthesis of pentakisadducts such as 16 (Figure 10.7), with

a C2v-symmetrical addition pattern, is very time-consuming and the overall yield is

unsatisfactory. For a convenient synthesis of this adduct type, an effective protection–

deprotection strategy was developed (Scheme 10.9) [58]. The reaction sequence starts

with the synthesis of the triazoline 46 by [3+2] cycloaddition of methyl azidoacetate

to a [6,6]-double bond of C60. After exhaustive template mediated cyclopropanation

to 47 and thermally induced [3+2] cycloreversion, the pentakisadduct 16 was

obtained in good overall yield. This pentakisadduct is a very valuable starting

material, because attack at the remaining octahedral [6,6]-double bonds proceeds

with quantitative regioselectivity. An example of a [5:1]-hexakisadduct originating

from 16 is the dendrimer 48.



Scheme 10.9 Protection–deprotection technique for the synthesis of

e-pentakisadduct 16. (i) Methyl azidoacetate, 1-chloronaphthalene, 60 °C;

(ii) 10 equiv. DMA, bromomalonate, DBU, toluene, room temp.;

(iii) toluene, reflux.



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10 Regiochemistry of Multiple Additions



The inverse reaction sequence starting from easily available [6,6]-monoadducts,

which are subsequently transformed into [1:5]-hexakisadducts using the template

mediation technique, is even more efficient. This has been demonstrated in the

synthesis of triazoline 47 and several related compounds [26, 58, 63, 64, 66–70].

Examples are functional dendrimers such as 49 [64] and 50 [66]. The globular

amphiphile 50 dissolves in water, forming unilamellar vesicles with diameters

typically between 100 and 400 nm. Stable monolayers of 50 on the air–water interface

have been produced by the Langmuir–Blodgett technique [71]. An example of a

biofunctional fullerene derivative that can intercalate into a DPPC bilayer is the

biotinatyled lipofullerene 51 [68]. This molecule can be used as a transmembrane

anchor for proteins located outside the membrane.



10.3 Concepts for Regio- and Stereoselective Multiple Functionalization of C60



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10 Regiochemistry of Multiple Additions



Diederich and co-workers transformed 52 into the supramolecular cyclophane

53 in quantitative yield by mixing equimolar amounts of 52 with cis-[Pt-(PEt3)2(OTf)2] [70].



10.3 Concepts for Regio- and Stereoselective Multiple Functionalization of C60



Mixed [4:2]-hexakisadducts



As a suitable starting material for the synthesis of [4:2] mixed hexakisadducts the

tetrakisadduct 15 (Figure 10.7) was used [16, 26, 67]. Double cyclopropanation of

this precursor core with the second generation (G2) Fréchet-dendron bromomalonate in the presence of DBU afforded CS-symmetrical C66(COOEt)8(COOG2)4

54 in 75% yield as a yellow powder (Scheme 10.10) [67]. The inverse [2:4] addition

pattern can be obtained by successive fourfold cyclopropanation of the e-bisadduct

4 with second generation (G2) dendron bromomalonates to give C66(COOEt)4(COOG2)8 55 in 73% yield, also as a bright yellow powder (Scheme 10.11) [67].



Scheme 10.10 Double nucleophilic cyclopropanation of all-e tetrakisadduct 15 with second generation (G2) bromomalonates yields

dendrimeric [4:2] hexakisadduct 54. (i) CHBr(COOG2)2, DBU,

toluene–CH2Cl2, 3 d, room temp.



In a similar fashion, 58 was synthesized starting from the e-bisadduct 56 with

four protected terminal carboxylic functions (Scheme 10.12) [26]. Regioselective

approaches to [4:2] hexakisadducts of type II are presented below (Sections 10.3.2

and 10.3.3).



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