5022
V. Theodorou, K. Skobridis / Tetrahedron Letters 46 (2005) 5021–5023
Earlier, Stieglitz had proposed that sodium methoxide
promoted aryl rearrangement of N-chlorotritylamines6
with initial loss of a proton from the amine, followed
by loss of a chloride anion. The mechanism seems to
be a concerted aryl migration from carbon to nitrogen,
involving the loss of a chloride anion (Eq. 6).
on similar reactions of lithium tritylamide involving a
phenyl migration from carbon to nitrogen and a hydride
loss.
An intramolecular view of the reaction may lead to a
mechanism where the migrating group moves either
without its electron pair (electrophilic or cationotropic
rearrangement) or with its electron pair (nucleophilic
or aniotropic rearrangement). Such a view suggests a
nucleophilic attack of the nitrogen on the adjacent C-
bonded phenyl group and a bridging anionic intermedi-
ate (I), which may rearrange to a carbanion (Ia),
(Scheme 1, A), or an attack of the phenyl on the nitro-
gen atom, with the loss of a hydride, and a bridging cat-
ionic intermediate (II), (Scheme 1, B). In order to attain
the intermediate (I) or (II), in which the migrating phe-
nyl group is partially bonded to both the N and C cen-
ters, it is clear that the migrating phenyl group must
present its [p] face to the N atom, so that its antibonding
[p] orbitals can accept or release, respectively, electron
density.
Ar C–NHCl CH ONa Ar C@N–Ar þ Clꢁ
ð6Þ
3
!
3
2
An anionic rearrangement of tertiary benzylic amines,
on treatment with n-BuLi was demonstrated by Eisch
et al.,7 where the base-promoted1,2 intramolecular shift
of a phenyl (or aryl) group from nitrogen to a benzylic
carbon occurred by way of a bridging anionic aryl inter-
mediate, leading to the formation of benzhydrylamine
(Eq. 7).
PhCH2–NPh2 1. n-BuLi Ph CH–NHPh
ð7Þ
!
2
2. H2O
On the other hand, Grovenstein and Williams8 discov-
ered that attempted preparation of triphenylethyllithium
from the corresponding chloride leads, through a carb-
anion, to an analogous rearrangement and to the forma-
tion of 1,1,2-triphenylethyllithium. Confirmation of
these results was provided by a direct carbonation of
the lithium compound, with the production of the corre-
sponding propanoic acid (Eq. 8).
Further, phenyl rearrangement followed by the expul-
sion of a hydride anion with the aid of the lithium cat-
ion (path a) or loss of a hydride and subsequent phenyl
rearrangement (path b), would lead to the formation of
the imine (Scheme 1). This abstraction may seem to be
difficult, however, there is reason to think that trityl-
amine contains considerable strain relative to the
imine. The release of this strain by phenyl rearrange-
ment and hydride abstraction may act as the driving
force.
Li
CO2
Li/THF
-60 ˚C
Ph2C CH2Ph
COOH
Ph2C CH2Ph
Ph3C CH2Cl
Ph3C CH2Li
ð8Þ
In order to elucidate further the mechanism of this base
promoted tritylamine rearrangement, we are exploring
its sensitivity to the electronic nature of several aryl sub-
stituents and their relative migratory aptitudes, taking
into account that the charge development on the migrat-
ing aryl group is influenced by the leaving group and the
mechanism. We are also attempting readdition
reactions.
While the formation of imines in the Stieglitz reactions
of triarylmethylamines through a cationic intermediate
and the base-induced rearrangement of tertiary benzylic
amines, as well as the rearrangement of triphenylethyl-
lithium, through an anionic intermediate, as mentioned
above, are well known, we are not aware of any reports
Li
Li
Ph
Ph
Ph
H
path a
C
N
Ph
C
N
Li
Ph
H
(Ia)
(I)
(A)
Ph2C
N
Ph
+
LiH
Li
H
Ph
Ph
Ph
Ph
C
N
C
N
H
Li
path b
(I)
Ph
C
N
Ph
H
(I)
Li
Ph2C
N
Ph
(B)
+
LiH
Ph
Ph
Ph
Ph
C
N
C
N
H
(II)
Scheme 1. Proposed mechanisms for the imine formation: (A) electrophilic phenyl rearrangement; (B) nucleophilic phenyl rearrangement.