stereochemistry

CARLO THILGEN, ISABELLE GOSSE, AND FRANÇOIS DIEDERICH 19
X X
1 2
C 2v
X
cis-1
trans-2
trans-1
1
cis-2
cis-3
trans-4
trans-3
6
eedge eface Regioisomer
cis-1
cis-2
cis-3
e
trans-4
trans-3
trans-2
trans-1
Y Z
Cs Y = Z: Cs C2
Y ≠ Z: C1 (a)
(b)
1 4
Symmetry
C s
C s
C 2
C s
C s
C 2
C 2
D 2h
Y
1
16
Locants
1,2:3,4
1,2:7,21
1,2:16,17
1,2:18,36
1,2:34,35
1,2:33,50
1,2:51,52
1,2:55,60
Figure 1.12. (a) The most characteristic patterns of mono-addition to C60. The symmetry indications
are made under the assumption that the bridging addend X is C2v-symmetric 102 and that
Y and Z are achiral. (b) The possible bis-adduct regioisomers resulting from twofold addition of
identical, C2v-symmetric 102 addends to C60.
located either between two six-membered rings (6–6 bonds) or between
a six- and a five-membered ring (6–5 bonds). The most common monofunctionalization
pattern of C60 14–16 results from 1,2-addition across the
6–6 bond C(1)–C(2) (Figure 1.12a) which has the highest double-bond
character. A number of primary adducts, notably fullerene-fused pyrazolines
and triazolines, can rearrange to so-called homofullerenes, in which the homo
atom is inserted into a 6–5 junction (“6–5 open” methano bridging between
C(1) and C(6)) (Figure 1.12a; cf. also Schemes 1.2, 1.7, and 1.8; for according
derivatives of C70, cf. Section IV.A.2.b. and Scheme 1.9). In combination with
the high symmetry of C60, both addition patterns lead to achiral molecules with
C2v- orCs-symmetrical addends (Figure 1.12a). 102 Many radical reactions,
such as halogenations and a number of nucleophilic additions followed by
the quenching with an electrophile, occur as intrahexagonal 1,4-additions,
especially if sterically demanding groups are introduced (Figure 1.12a). 14–16
If the added groups in such a mono-adduct are not identical, the resulting
functionalization pattern is noninherently chiral; examples of this type will be
Y