Stereogenic addends

Higher Fullerene Derivatives with Stereogenic Elements Located Exclusively in the Addends... 

A Ci-symmetric cis-1 bis-adduct with an additional bridge between the addends was obtained in the reaction between two molecules of ethyl propio-late and C6o in the presence of triphenylphosphane.318 Its structure (( )-162, Figure 1.37) was established with the help of the HMBC- (heteronuclear multiple bond correlation) NMR technique and includes stereogenic centers in the addend moiety as well as a noninherently chiral addition pattern. The latter can be related to the head-to-tail connectivity of the two propiolate units. 

A complex, multiply bridged 1,2,3,4-adduct of C60 (168) (Scheme 1.11) including a noninherently chiral addition pattern as well as a multitude of stereogenic centers in the addend moiety was obtained in a tandem reaction between the alkaloid scandine and Ceo-324 The sequence included a photoin-duced addition of the tertiary amine subunit of the alkaloid and a [2 + 2] cycloaddition of its vinyl group to the adjacent intrahexagonal formal double bond of the fullerene. The structural elucidation included 1H-1 H COSY-, HMQC- (heteronuclear multiple quantum coherence), HMBC-, and ROESY-(rotating frame Overhauser enhancement spectroscopy) NMR experiments and... 

Figure 1.44. The different stereoisomers obtained by [2 + 2] photocycloaddition of 3-methyl-cyclohex-2-enone to Cgo (left, the newly formed stereogenic centers are marked by an asterisk), and illustration of a sector mle devised for the correlation of the absolute configuration of a 6-6 closed 1,2-adduct of Cgo to the CD band around 430 nm (right). According to the sign ((+) or (-)) of the sector in which an addend moiety (R1, R2,R3, or R4) is located, it is postulated to add a positive or negative contribution to the Cotton effect associated with the ca. 430 nm UV/Vis absorption of the fullerene chromophore.

Various other [3 + 2] cycloadditions, affording chiral, anellated C6o derivatives with stereogenic centers in the addends are reported in literature. The products were generally obtained as racemates and resulted from reaction of buckminsterfullerene with species like 2,3-disubstituted 2//-azirincs (via nitrile ylides [under direct irradiation] or via 2-azaallenyl radical cations [sensitization by photoinduced electron transfer]),365 1-substituted 5-diazopentane-1,4-diones (via cyclic carbonyl ylides),366 7-alkylidene-2,3-diazabicyclo[2.2.1] hept-2-ene (via a diradicaloid trimethylenemethane derivative),367 1-benzylpy-razolidine-3-ones in the presence of aldehydes (via pyrazolidinium ylides),368 2-trifluoromethyl-2,5-dihydro-l,3-oxazol-5-ones (via nitrile ylides),369 nitro-alkanes in the presence of triethylamine and trimethylsilyl chloride (via N-silyloxynitrones),370 or dv-HOCH2 CH=C H C H 2 OCO 2 H( in the presence of... 

The photochemically induced addition of bulky disilanes to C6o has already been discussed in Section IV.A.l.h, as it affords the unusual inherently chiral 1,16-addition pattern (( )-72, Figure 1.20). In this reaction some phenyl-substituted disilanes give rearranged adducts of the type ( )-73 (Figure 1.20).197,404 Further [60]fullerene derivatives with stereogenic centers in the addends were obtained in photoinduced addition-rearrangement reactions with 3,4-bis(alkylidene)-l,2-disilacyclobutanes 405 and with octaaryl-substi-tuted cyclotetrasilanes and -germanes406... 

Besides stereoselectivity requirements, transition metal catalyzed addition reactions pose several other selectivity problems. Thus, regioselectivity must be achieved with unsymmetrical addends and isomerization of substrates and products, such as double-bond isomerization or epimerization of stereogenic centers, must be suppressed. Manipulation of catalytic systems might not only influence catalytic activities but also disturb a rather delicate reaction control and thus lead to different product selectivities. 

In many cases, the chirality of fullerene derivatives is neither inherent to the core of the unfunctionalized carbon cage, nor is it due to stereogenic units in the addends, but it can be attributed to an addition pattern lacking reflection symmetry [33,34]. 

Addition patterns that are chiral due only to structural differences in the addends (nature, sequence, and steric arrangement of atoms) are termed non-inherently chiral. The analogy to the case of the stereogenic center with a tetrahedral arrangement of ligands is to be noted ... 

---