Studies on the asymmetric cycloaddition of 2-azaallyl anions with alkenes

Article abstract of DOI:10.1016/S0040-4039(01)01539-8

A preliminary survey of strategies for the asymmetric cycloaddition of nonstabilized 2-azaallyllithiums with alkenes is reported. The chiral diamine (-)-sparteine exerted little, if any, absolute stereocontrol. The attachment of a chiral auxiliary to the anion was more successful, leading to the first example of an asymmetric cycloaddition of a 2-azaallyllithium (24 to 25, 98% e.e.).

Full text of DOI:10.1016/S0040-4039(01)01539-8

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 42 (2001) 7361–7365
Studies on the asymmetric cycloaddition of 2-azaallyl anions
with alkenes
William H. Pearson,* Erland P. Stevens† and Aaron Aponick‡
Department of Chemistry, University of Michigan, Ann Arbor, MI 48109-1055, USA
Received 13 June 2001; accepted 22 August 2001
Abstract—A preliminary survey of strategies for the asymmetric cycloaddition of nonstabilized 2-azaallyllithiums with alkenes is
reported. The chiral diamine (−)-sparteine exerted little, if any, absolute stereocontrol. The attachment of a chiral auxiliary to the
anion was more successful, leading to the first example of an asymmetric cycloaddition of a 2-azaallyllithium (24 to 25, 98% e.e.).
© 2001 Elsevier Science Ltd. All rights reserved.
The anionic cycloaddition of 2-azaallyl anions with
alkenes has been known since the early 1970s, begin-
ning with the initial work by Kauffmann on 1,3-
diphenyl-2-azaallyllithium1 and continuing to our own
work on less-stabilized alkyl-substituted 2-azaallyllithi-
ums derived from tin–lithium exchange2 (Scheme 1). To
the best of our knowledge, no examples of asymmetric
versions of these cycloadditions have been reported.3
Herein we report our initial survey of strategies for
asymmetric 2-azaallyl anion cycloadditions, culminat-
ing in the first example of a highly enantioselective
example of this process.
The most convenient strategy for the asymmetric
cycloaddition of 2-azaallyllithiums with alkenes would
be to use a chiral additive that would ligate the lithium
ion. In the Kauffmann process, wherein a base is used
to deprotonate an imine (e.g. 5), a chiral solvating
agent could be used, or perhaps a chiral lithium dialkyl-
amide would serve as both a base and a chiral lithium-
solvating agent. The disadvantages of this approach are
several-fold. First, it is difficult to rationally design a
ligand/azaallyllithium system due to the uncertainties
surrounding the structure of such complexes. Second,
the stereochemical information embodied in the
lithium-ligating agent may be too remote from the site
of cycloaddition for high enantioselectivity. Third, even
if a particular system could be optimized, it may not be
relevant to a broader group of anions. Finally, the
Kauffmann-type anions are limited in scope, producing
polyarylpyrrolidines such as 6. Nonetheless, we exam-
ined this approach briefly. Our initial attempts at using
a chiral lithium-solvating agent in the Kauffman strat-
egy were unsuccessful, e.g. Eq. (1). While the cycloaddi-
tion was very efficient in the presence of the chiral
diamine (−)-sparteine,4 the cycloadduct 6 was produced
in low e.e.5
R2
H
R2
H
H
R3
SnBu3
R3
R1
N
R1
N
2
1
R1-R3: at least
two aryl groups
R1-R3: aryl stabilization
not needed
R2NLi
n-BuLi
R2
H
R2
Li
R3
N
Li
4
R1
Ph
Ph
6 (92%, 0.6% e.e.)
Ph
R1
N
R3
1) LDA, THF
3
Ph
N
Ph
2) (-)-sparteine
3) E-stilbene
Ph
N
(1)
H
5
Scheme 1.
* Corresponding author. E-mail: wpearson@umich.edu
† Present address: Department of Chemistry, Davidson College, PO
Box 1719, Davidson, NC 28036, USA.
Similarly, the presence of (−)-sparteine during cycload-
ditions of less-stabilized 2-azaallyllithiums was also
ineffective (Scheme 2), producing the racemic pyrroline
‡ Kodak Fellow.
0040-4039/01/$ - see front matter © 2001 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(01)01539-8

7362
W. H. Pearson et al. / Tetrahedron Letters 42 (2001) 7361–7365
NH
Table 1.
Me
Ph
NHMe
OH
ICH2SnBu3
Na2CO3
1) BrCN
Me
1) n-BuLi
(-)-sparteine
solvent
N
O
CHO
N
2) NaOCH3
(84%)
Ph
Ph
Ph
DMF
+
L-ephedrine
13
(50%)
2) H2O
3) HCO2Ac
NEt3, CH2Cl2
N
SnBu3
Ph
Ph
SnBu3
7
8
N
Me
Ph
Me
n-BuLi
N
Me
N
O
Solvent
Additive
Yield (%)
% e.e.
Li
N
O
Ph
THF
THF
Toluene
None
(−)-Sparteine
(−)-Sparteine
49
17
39
0
0
6
14
15
Ph
Ph
Me
R
N
or
or
SiEt3
Ph
R'
N
Ph
8 from 7 in lower yield than in the absence of the
ligand. Running the cycloaddition in toluene with
sparteine restored the efficiency of the cycloaddition
and produced non-racemic material, although in only
6% e.e. (Table 1).6
OH
Ph
16
Scheme 3.
The failure of the cycloaddition of 15 may be due to
unfavorable electron-pair repulsion between the 4p-
electron system of the 2-azaallyllithium and the lone
pairs present on the nitrogen and oxygen of the oxazo-
lidine ring, as shown by structure 15 of Fig. 1. We thus
redesigned our chiral auxiliary based on several points.
First, from the allyl anion literature, it is known that a
heteroatom at the terminus of an anion prefers to
orient its lone pair perpendicular to the allyl anion
p-system, thus minimizing repulsive interactions
between filled orbitals.10 Second, structural studies on a
number of allyl anions show the metal to be p-com-
plexed.11 Particularly relevant are solution12 and solid-
Due to the limitations of the chiral ligand approach
outlined above, we sought to examine the placement of
a chiral auxiliary on the 2-azaallyllithium. We chose to
use a heteroatom-linked auxiliary, since we had previ-
ously shown that heteroatom-substituted 2-azaallyllithi-
ums could be generated from tin-bearing imidates (e.g.
9), amidines (e.g. 10), and thioimidates, producing 1-
pyrrolines such as 11 and 12 after ejection of the
heteroatom group after the cycloaddition (Scheme 2).7,8
Ph
Ph
SnBu3
SnBu3
n-BuLi
state
structural13
studies
on
1,3-diphenyl-2-
+
MeO
N
azaallylsodium, which clearly show the metal to be
p-complexed. The first design element we considered
was to excise the ring in 15 that enforces unfavorable
N
N
9
Ph
67%
(8:1)
11
12
n-BuLi
88%
(10:1)
N
N
11
12
Me
Me
10
H
Ph
N
Z
:
:
Ph
N
N
N
vs.
O
H
H
H
H
Li+
Li+
Scheme 2.
15
17
Our first attempt at using a heteroatom-linked chiral
auxiliary focused on the 2-iminooxazolidine 14, which
by virtue of its two points of attachment should pro-
duce a relatively rigid and facially biased 2-azaallyl-
lithium intermediate 15. Attaching the auxiliary via two
heteroatoms would have an additional benefit in that it
remains attached after the cycloaddition, allowing the
separation of any diastereomers that may result, as well
as producing a material at the oxidation state of a
OMe
R1
..
E
1
N
NZ
?
R2
N
..
3
H
Li
2 N
R1
n-BuLi
1
H
O
Me
R2
3
SnBu3
18
19
R1
E
OMe
..
Z
N
N
N
R2
LDA
Enders, et al.
N
..
lactam, i.e. the amidine 16. Transformation of
L-
H
R1
R2
Figure 1.
ephedrine into the known9 2-iminooxazolidine 13, fol-
lowed by N-alkylation afforded 14. Unfortunately,
transmetalation of 14 in the presence of trans-stilbene,
triethylvinylsilane or diphenylacetylene did not afford
any identifiable products (Scheme 3).
Li
H
O
Me
20
21

W. H. Pearson et al. / Tetrahedron Letters 42 (2001) 7361–7365
7363
OMe
SnBu3
electronic interactions, but retain a heteroatom (or
O
23
perhaps two) at one terminus of the 2-azaallyllithium
system. For example, a more flexible anion such as
1-pyrrolidino-2-azaallyllithium 17 allows bond rotation
such that the nitrogen lone pair can be orthogonal to
the 2-azaallyllithium p-system, as shown in Fig. 1.
Simple MNDO calculations support this structure.
Note that the Z geometry of 17 is shown, in accord
with the literature of 1-amino allyl anions10 and 1-
amino-1-azaallyl anions (i.e. a-lithiated hydrazones),14
where a strong preference for such a geometry is found.
The success of the cyclizations of 9 and 10 (Scheme 2)
further supports the idea that such conformationally
flexible heteroatom-substituted anions are viable. The
second design element that was applied was to intro-
duce a chiral center on the pyrrolidine ring of 17 that
bears a lithium-chelating substituent in order to provide
internal chelation of the p-complexed lithium ion,
directing it to one of the two diastereotopic faces of the
anion. Based on these ideas, an O-methyl prolinol-
derived auxiliary was deemed attractive, generalized by
19, available from transmetalation of the stannyl
amidine 18.15 This strategy is similar to that employed
by Enders and co-workers in their work on chiral
lithiated hydrazones. For example, deprotonation of
the SAMP-hydrazone 20 is thought to produce the
1-azaallyl anion 21, which differs from 19 by a reversal
of the CN linkage.14a,d,i Two significant issues will
remain unresolved until experimental results are
obtained. First, does the geometry of 18 correlate with
the geometry of 19? While the E isomer of the amidine
18 is preferred, the 1Z,2E isomer of 19 should be
preferred. Will the barrier to bond rotation be low
enough to allow 19 to form, or will the geometry of 18
be conserved, producing the 1E,2E isomer of 19? A
similar issue arises in Enders’ chemistry; the geometry
of 21 is independent of the geometry of 20, indicating
that bond geometrical interconversion is possible. Sec-
ond, which face of the anion 19 will combine with the
anionophile? The top face (as drawn) appears least
sterically hindered, and would thus result in inversion
of stereochemistry with respect to the lithium atom. On
the other hand, organolithiums often prefer to react
with electrophiles with retention of configuration. Fur-
ther confusing the issue is the mechanism of the
cycloaddition itself. A stepwise cycloaddition (via car-
bolithiation of the anionophile followed by a 5-endo-
trig cyclization) is a viable alternative to a concerted
cycloaddition.16–18 Thus, for example, a concerted
cycloaddition might proceed from the least hindered
face of the anion, while a stepwise cycloaddition involv-
ing carbolithiation as the first step might involve a
retentive pathway proceeding from the same face as the
lithium ion. For the analogous 1-azaallyl anions 21,
Collum favors approach of the electrophile from the
side of the anion opposite to the lithium,14f while
Enders and Boche favor a ‘metallo-retentive SE2%-front’
mechanism.14h
H2N
NEt3
MeOTf
N
CH2Cl2
reflux
CH2Cl2
22
OMe
Ph
SnBu3
n-BuLi
N
N
THF, -78 ˚C
N
24
51% (1:1)
25 (46%)
98.2% e.e.
Ph
MeO
(+ 13% of 2
other isomers)
NLi
Ph
Ph
Me
Me
N
N
N
Me
26
N
H
Li
Li
O
H
27
O
Me
Me
Scheme 4.
in moderate yield as a 1:1 mixture of diastereomers.
Transmetalation in the presence of a-methylstyrene
gave the pyrroline 25, presumably via 26 and 27. A
small amount of a mixture of two other isomers was
also isolated, but their structures were not determined.
The major product 25 was found to have an e.e. of
greater than 98%, as determined by HPLC using an
amylose carbamate stationary phase.22 The absolute
configuration of 25 has not yet been determined, and is
proposed based on our rationale for the structure of the
anion 19 combined with a concerted cycloaddition via
an invertive pathway (see above).
In conclusion, a survey of various strategies for accom-
plishing an asymmetric cycloaddition of a 2-azaallyl
anion with an alkene has resulted in the first successful
example of such a process. Details of the scope23 and
absolute stereoselectivity of this process will follow in a
full report.
Experimental
(2S,3%RS)-1-(2-Aza-1-methyl-3-(tri-n-butylstannyl)-1-
butenyl)-2-(methoxymethyl)pyrrolidine (24): Methyl tri-
flate (90 mL, 0.80 mmol) was added to a solution of the
known24 amide 22 (120 mg, 0.78 mmol) in CH2Cl2 (10
mL). After heating at reflux for 24 h, the mixture was
allowed to cool to room temperature. In a separate
flask, N-[1-(tri-n-butylstannyl)ethyl]phthalimide (360
mg, 0.78 mmol)8 and hydrazine monohydrate (3.0 mL,
62 mmol) were dissolved in EtOH (25 mL) and heated
at reflux. After 6 h, the mixture was concentrated to
provide a white mass, which was diluted with ether and
washed five times with water. The organic phase was
dried (Na2SO4), filtered, and concentrated to provide
the crude amine 23 as an oil.8 This material was dis-
To study this approach, the amidine 24 was synthesized
as shown in Scheme 4. O-Methylation of the acetamide
22 (derived from
L-proline) followed by combination of
the resultant iminium salt19,20 with racemic 2321 gave 24

7364
W. H. Pearson et al. / Tetrahedron Letters 42 (2001) 7361–7365
solved in dichloromethane (3 mL), mixed with triethyl-
amine (0.20 mL, 1.4 mmol), and added to the triflate
salt prepared above. After 12 h, the mixture was diluted
with ether and washed with 10% aqueous NaOH, and
the organic phase was dried (Na2SO4), filtered, and
concentrated. Chromatography (hexane to EtOAc to
15% MeOH/EtOAc gradient on alumina) afforded 190
mg (51%) of the title compound as an inseparable 1:1
mixture of diastereomers, Rf=0.20 (15% MeOH/EtOAc
on alumina). Partial data for one diastereomer: 1H
NMR (CDCl3, 360 MHz) l 3.34 (s, 3H). Partial data
for the other diastereomer: 1H NMR (CDCl3, 360
MHz) l 3.32 (s, 3H). Data for mixture: IR (neat) 1605
the American Chemical Society, and the Eastman
Kodak Company for financial support of this research.
References
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2.3, CDCl3).
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1
(s) cm−1; H NMR (CDCl3, 360 MHz) l 4.10–4.00 (br
s, 1H), 3.64–3.55 (m, 1H), 3.46–3.40 (m, 1H), 3.40–3.28
(m, 4H), 3.28–3.05 (m, 2H), 1.88–1.72 (m, 7H), 1.50–
1.40 (m, 6H), 1.40–1.23 (m, 6H), 0.95–0.69 (m, 15H);
13C NMR (CDCl3, 90 MHz) l 151.4, 150.4, 74.1, 74.0,
58.9, 58.9, 56.1, 56.0, 47.9, 45.8, 29.3, 28.3, 28.1, 27.8,
27.6, 27.3, 27.1, 23.5, 23.4, 23.0, 22.8, 15.6, 13.8, 13.7,
8.9; MS (CI, NH3) m/z (rel. int.) 475 (9, M+H), 474 (7,
M+), 185 (38), 184 (97), 183 (100); HRMS (CI, NH3)
calcd for C22H47N2O120Sn (M+H)+ 475.2710, found
475.2709.
(2R,3S or 2S,3R)-2,3,5-Trimethyl-3-phenyl-3,4-dihydro-
2H-pyrrole (25): A solution of the amidine 24 (435 mg,
0.92 mmol) and a-methylstyrene (240 mg, 2.0 mmol) in
THF (2 mL) was added in a dropwise fashion to a
solution of n-butyllithium (1.7 mL, 4.6 mmol, 2.7 M in
hexanes) in THF (2 mL) at −78°C. After 2 h, excess
methanol was added and the mixture was diluted with
ether and washed with water. The organic phase was
dried (Na2SO4) and concentrated. Chromatography (0–
100% EtOAc/hexane gradient) first afforded 21 mg
(13%) of a mixture of two pyrrolines of undetermined
structure, Rf=0.15 (EtOAc), presumed to be
diastereomeric and/or regioisomeric with the pyrroline
1
22. Partial data for the first minor isomer: H NMR
(CDCl3, 360 MHz) l 1.92 (s, 3H), 1.53 (s, 3H), 1.37 (d,
3H, J=6.9 Hz). Partial data for the second minor
1
isomer: H NMR (CDCl3, 360 MHz) l 1.80 (s, 3H),
1.62 (s, 3H), 1.42 (d, 3H, J=6.9 Hz). Further elution
gave 80 mg (46%) of the title compound, the relative
configuration of which was determined by difference
NOE spectroscopy. Separation of the enantiomers of 22
by chiral HPLC revealed an enantiomeric excess of
98.2%. Data for 22: Rf=0.10 (EtOAc); IR (neat) 1646
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(m) cm−1; H NMR (CDCl3, 360 MHz) l 7.34–7.21 (m,
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using this separation technique. Examination of 25
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