Multiple Cyclopropanations of C₇₀. Synthesis and characterization of bis‐, tris‐, and tetrakis‐adducts and chiroptical properties of bis‐adducts with chiral addends, including a recommendation for the configurational description of fullerene derivatives with a chiral addition pattern

Abstract

The regioselectivity of multiple cyclopropanations of C," with 2-bromopropanedioates in the presence of 1,8-diazahicyclo[5.4.0]undec-7-ene (DBU) as base (Bingel reaction) was investigated in a systematic study. Bisadduct formation occurred preferentially at the 6-6 bonds formed by the most pyramidalized sp2-C-atoms at the two opposite poles of the fullerene and, in the reaction with achiral bis[(ethoxycarhonyl)methyl] 2-bromopropanedioate (13a), yielded three constitutionally isomeric his(methano)fullerenes (Scheme 2). Two of them, C,-symmetrical ( i ) -l and (i)-Z, are chiral; a fact which had not been considered in previous investigations. Formation of the third, C2,-symmetrical isomer 3 was observed for the first time. Configurational descriptions for fullerene derivatives which possess a chiral chromophore as a result of specific functionalization patterns are proposed. Cyclopropanations of C,, with optically active bis[(S)-1-phenylbutyl] 2-bromopropanedioate (13b) yielded five optically active, C2-symmetrical his-adducts 7-1 1 which could be separated by preparative HPLC and fully characterized (Scheme 3, Fig. 4 ) . Compounds 7/8 and 9/10 represent two constitutionally isomeric pairs of diastereoisomers, and their circular dichroism (CD) spectra show pronounced Cotton effects mainly due to strong chiroptical contrihutions from the chirally functionalired fullerene chromophores (Fig. 7). Since the addition patterns on the fullerene surface in each pair of diastereoisomers have an enantiomeric relationship, their CD spectra closely resemble those expected for two enantiomers. In the third constitutional isomer 11, the addition pattern on the fullerene surface is C2,-symmetrical, and optical activity only results from the chiral addends. Its CD spectrum shows weak Cotton effects mainly from induced circular dichroism originating from the perturbation of the achiral fullerene chromophore by the attached chiral addends. Addition of diethyl2-brornopropanedioate (2 equiv.) to the C2-symmetrical racemic bis-adduct (+)-2 yielded a mixture of tris-adducts and one major, C,-symmetrical tetrakis-adduct (&)-4 which was isolated in pure form (Scheme 4 ) . Starting from the achiral C,,.-symmetrical bis-adduct 3, one single C,-symmetrical tris-(5) and one C,,-symmetrical tetrakis-adduct (6) were obtained as major products which were isolated and fully characterized (Scheme 5). The regioselectivity for introduction of a second addend in the same hemisphere of C,o is high and resembles the preferred pattern of bis-addition seen in the functionalization of C60.

Macrocyclization on the fullerene core: Direct regio‐ and diastereoselective multi‐functionalization of [60]fullerene, and synthesis of fullerene‐dendrimer derivatives

“…up to 30 cm2 mmol-I) between 250 and 350 nm result mainly from the chiroptical contributions of the chiral spirobifluorene tether which displays strong optical absorption bands in this range. It is well documented that optically active fullerene derivatives with chiral, weakly absorbing addends but achiral addition patterns do not display strong CD effects in this region [25] [26] (see also the discussion for 25 and ent-25 below). A comparison between the CD spectra of the two enantiomers of 24 to those of the starting bis-malonates ( -) -( S ) -2 3 and ( + ) -( R ) -2 3 (Fig.…”

“…Overall, the chiroptical properties of 24/ent-24 are less pronounced than those measured for fullerenes with optically active addition patterns (see below) [7] [lld] [26] or for optically active derivatives of inherently chiral fullerenes [27], which display Cotton effects with A & values well exceeding 100 cm2 mmolBy the same methodology, the two enantiomeric cis-2 bis-adducts 25 and ent-25, together with the two enantiomeric cis-3 bis-adducts 26 and ent-26, were obtained from the tethered bis-malonates ( -) -2 9 and ( + ) -2 9 , which, in return, were prepared from the commercially available optically pure diols (+)-27 and ( -) -2 7 , respectively (Scheme 5).…”

“…In each reaction, two diastereoisomeric out-out cis-3 bis-adducts are possible due to the chiral addition pattern; however, the very high asymmetric induction in the second intramolecular Bingel addition leads to the formation of 26 or ent-26 only. Transesterification (K,CO,, EtOH/THF) of 26 and ent-26 yielded the cis-3 tetraethyl esters 28 and ent-28, respectivey, as enantiomerically pure compounds; the two enantiomers with the configurational assignments ( ' 0 -2 8 and (fA)-28 (f = fullerene, C = clockwise, A = anticlockwise) [9b] [26] are depicted in Fig. 6.…”

“…The soln. was allowed to slowly warm to r. 13.99;26.81;41.26;61.67;64.53;75.54;110.43;166.24;MH'), 375 ( j'ullerened1,61,62.62-tetracarhoxylute (25) and Diethyl endo,endo- ((4S,diyl)dimethyl] 1 ,2: ~6,17-Bis(niethano) (60]fullerene-61,6~,62,62-tetracurboxylatr (26). DBU (0.25 ml,1.662 mmol) was added at r.t. to C,, (200 mg, 0.277 mmol), I, (155 mg, 0.609 mmol), and (-)- 29 (1 19 mg, 0.305 mmol) in PhMe (400 ml), and the mixture was stirred for 8 h. The crude material was filtered through a short plug (SO,), eluting first with PhMe to remove unreacted C,, and then with CH,CI,/MeOH 97: 3.…”

Regioselectivity in Tether‐Directed Remote Functionalization – The Addition of a Cyclotriveratrylene‐Based Trimalonate to C60 Revisited

“…Such stereogenic elements can be combined with a chiral functionalization pattern of an achiral or a chiral parent fullerene. [1,2,[6][7][8][9] An undirected threefold addition to 6,6-bonds of C 60 can, in principle, yield 46 regioisomeric products, many of which are chiral. Although some of the possible tris-adducts are generally favored in a given reaction, [10][11][12] product mixtures often contain a considerable number of regioisomers that are difficult to separate.…”