Chiral addition patterns

Herrmann A, Ruettimann M, Thilgen C, Diederich F (1995) 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. Helv. Chim. Acta 78 1673-1704. 

Adducts with an Inherently Chiral Addition Pattern... 

Figure 10.9 Schematic representation of some examples of inherently chiral bis- and tris-addition patterns of Qq adducts.

Their isolation by flash chromatography on silica gel was comparatively easy. The CD spectra of related pairs of diastereomers whose addition pattern represent pairs of enantiomers, reveal pronounced Cotton effects and mirror image behavior. It is the chiral arrangement of the conjugated Jt-electron system within the fullerene core that predominantly determines the chiroptical properties. Adducts with a C2-... 

Systematic investigations of twofold additions of malonates to C70 revealed that the second addition takes place at one of the five a-bonds of the unfunctionalized pole [17, 26], With achiral, C2v-symmerical malonate addends, three constitutionally isomeric bisadducts are formed An achiral one (C2v-symmetrical 1), and two chiral ones (C2-symmetrical 2 and 3), which are obtained as pairs of enantiomers with an inherently chiral addition pattern (Figure 13.5). Twofold addition of chiral malonates leads to the formation of five optically active isomers, two constitutionally isomeric pairs of C2-symmetrical diastereomers and a third constitutional C2-symmetrical isomer (Figure 13.5). Twofold additions of azides to C70 lead to diazabis[70]homo-fullerenes, which served as starting material for the synthesis of bis-(aza[70]-fullerenyl) (Cg9N)2 (Chapter 12) [27]. As further bisadditions, addition reaction to C70 [2+2]cycloaddition of electron-rich bis(diethylamino)ethyne and 1-alkylthio-2-(diethylamino)ethynes [28] and the addition of transition metal fragments have been reported [29-32],... 

Figure 13.5 Bisadducts of Cy obtained from twofold cyclopropanation reactions. Both addends with chiral and achiral substituents have been used. Adducts 2 and 3 exhibit an inherently chiral addition pattern and are formed together with the corresponding enantiomer or diastereomer involving the mirror image addition pattern.

Fig. 1 Frequency distribution of the %ee in repeated experiments of the Soai reaction performed with different starting aldehydes but without initial addition of chiral substances. Cray bars 2-t-Bu-C=C-5-pyrimidinyl aldehyde ([35]) black bars 2-t-Bu-C=C-5-pyrimidinyl aldehyde with achiral silica gel ([40]) patterned bars (CH3)2-Si-C=C-5-pyrimidinyl aldehyde ([41])...

Derivatives of achiral parent fullerenes in which the functionalization creates a chiral addition pattern on the spheroid, regardless of the addends being identical or different, have an inherently chiral functionalization pattern. The derivatives of chiral parent fullerenes automatically have an inherently chiral functionalization pattern. 

Figure 1.6. Schlegel diagrams of two stereoisomers of tetrakis[( )-l-phenylbutyl] l,2 56,57-bis (methano)[70]fullerene-71,71J2,72-tetracarboxylate.35 54 The two types of stereogenic elements (inherently chiral addition pattern and stereogenic centers in the ester groups) are specified independently of each other. As can be seen from the descriptors (S,S,S,S, C) and (S fA), the depicted molecules are diastereoisomers.

It should be mentioned that the addition of chiral residues to an achiral fullerene does not necessarily lead to a chiral addition pattern (cf. Section IV.C and Figure 1.1). In such a case the configuration of the chiral addend(s) only... 

Figure 1.13. Bis-adducts of C6o with an inherently chiral addition pattern. Each regioisomer is represented by a single, fC-configured structure. The configuration of actually obtained products is indicated in each case by a descriptor preceding the compound number.

R,R,R,R, A)-20 (Figure 1.14), each as a diastereoisomeric pair in which the (R,R)-configured side groups are combined with inherently chiral addition patterns of opposite configuration (fC or fA) ,( After the separation of all isomers, they found that the formation of the bis-adducts 13-16 was only weakly diastereoselective (d.e. <10%). Furthermore the CD spectra of the diastereoisomeric pairs 13/14,15/16, and 17/18 are nearly mirror-image shaped which clearly proves the opposite configuration of the addition patterns in each pair and also the predominant chiroptical contribution of the distorted residual fullerene n-chromophorcs in comparison to that of the addends. 

This corroborates observations by Diederich and co-workers who had found analogous results for diastereoisomeric pairs of C70 derivatives combining a uniformly configured chiral side chain with fullerene addition patterns of opposite configuration (cf. Section IV.A.2.e).35,54... 

An equally high diastereoselectivity (d.e. >96%) was observed in the generation of the inherently chiral cis-3 addition pattern when (R,R,fA)-32 and (S,S, C)-32 (Scheme 1.4) were prepared (next to the corresponding cis-2... 

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