Buckyballs - Polyarenes - page 11

N

N

RO 2 C Ar

RO 2 C Ar

Ph 2 P
S

Fe

(S)-DTBM SegPhos
Cu(OTf) 2 /ET 3 N

FeSulphos

C 60

Cu(AcO) 2

+

(2S,5R)-trans

(2S,5S)-cis

OR

Ar N

O

Ph

N

Ph N

P

Ar
Ar

CO 2 R O

O
O
O

CO 2 R

Ar Ar

P

P

Ph

Ph

Ar

P

BPE

Ar

AgAcO

(R)-DTBM SegPhos

(2R,5R)-cis

(2 R ,5 S)-tra ns

Cu(OTf ) 2 /ET 3 N

Scheme 6 Stereodivergent 1,3-dipolar cycloaddition of chiral N-metalated azomethine ylides
onto C 60

obtained by chiral induction from chiral starting reagents or after tedious and
expensive HPLC separation from the racemic mixture [ 40 ].
The non-coordinating nature of fullerenes has hampered the use of asymmetric
metal catalysis and, therefore, the employ of enantiopure fullerene derivatives has
been limited to a few examples [ 42 - 44 ].
In this respect, a major breakthrough has been the chiral activation of a 1,3-
dipole in the cycloaddition of N-metalated azomethine ylides onto C 60 (Scheme 6 )
[ 45 ]. By using catalytic amounts of transition metals and the suitable ligand, the
cycloaddition of a series of -iminoesters occurs under very mild conditions and

good yields, affording pyrrolidinofullerenes (probably the most important class of
fullerene derivatives) with complete control of diastereoselectivity.
More important, chiral complex formed by copper(II) acetate and ( R )-Fesulphos
led the cycloaddition toward the formation of (2 S ,5 S )- cis pyrrolidinofullerenes,
whereas the use of silver acetate and chiral (

α

)-BPE ligand switches the enantios-
electivity toward the opposite (2 R ,5 R )- cis pyrrolidinofullerene enantiomers.
Shortly after, complete control on the stereochemical outcome and a fully
stereodivergent synthesis of all the possible stereoisomers of disubstituted
fulleropyrrolidines was achieved [ 46 ]. The use of the complex Cu(II) triflate/( R )
or ( S )-DTBM segphos switches the diastereoselectivity towards both enantiomers
of the unusual trans pyrrolidine with high enantiomeric excess. For this latter
process, the authors invoke the presence of a stepwise mechanism that justifies a
supra - antara formal [4+2] cycloaddition (Scheme 6 ).