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Stereochemical Studies of Fluorescent Tröger’s Bases
David E. Lewis, Department of Chemistry, University of Wisconsin-Eau Claire, Eau Claire, WI 54702
The Tröger’s base skeleton is a rigid framework containing two chiral nitrogen atoms
at bridgehead positions. Under acid catalysis, the ring system undergoes inversion,
but two mechanisms for the inversion have been proposed Our primary goal is to
use symmetrically substituted chiral Tröger’s bases to probe the mechanism of this
ring inversion. The synthesis of the required chiral compounds will require the
synthesis of the optically pure 4-amino-3-(1-hydroxyethyl)-N-alkyl-1,8-naphthal-imides,
and then condensing them. The synthesis of the required alcohols is proving to be
problematic.
Two Unexpected Results
It is conventional wisdom based on over 6 decades of
synthesis that the heterocyclic ring of naphthalimides,
once formed, is resistant to nucleophilic attack. We find,
however, that when 4-substituted-N-aryl-1,8-naphthalimides are treated with primary amines, replacement of
the aryl group occurs, regarrdless of the substituent on
the naphthalimide ring or the n-aryl ring.
X
X
N
Ar
O
RNH2/r.t.
X = Cl, NMe2
Ar = Ph, p-BrPh, p-MePh,
p-H2NSO2Ph
O
N
R
O
X = Cl, NMe2
R = n-C4H9, n-C6H13
NH2 OH
N
O
N
R
O
N
R
O
R = n-C4H9, n-C6H13
N
R
O
O
NHR
O
N
O
N
O
aprotic
solvent
Br2
O
N
O
NHR

O
O
We have found an unanticipated participation of the
naphthalimide ring system during the addition of bromine to
N-allyl-4-alkylamino-1,8-naphthaliimides.
In
aprotic
solvents, the product of ring bromination is isolated; in protic
solvents, the “normal” addition products are isolated.
The results are rationalized in terms of an
intermediate iminium ion which exists as an
NHR
intimate ion pair in aprotic solvents, and as a
Br
solvent-separated ion pair in protic solvents. We
have now isolated the iminium ion intermediate.
NHR
RNH2/r.t.
R
N
O
O
N
NHR
protic
solvent
O
Br
O
N
O
Br
Br