Genenation of 1-azapentadienyl anion from N-(tert-butyldimethylsilyl)-3-buten-1-amine

Article abstract of DOI:10.1021/jo0162336

N-(tert-Butyldimethylsilyl)-3-buten-1-amine undergoes allylic deprotonation at the 2-position when exposed to 2 equiv of nBuLi in THF. This allylic anion undergoes lithium hydride elimination to generate a 1-azapentadienyl anion. The anion is generated cleanly and completely.

Full text of DOI:10.1021/jo0162336

J . Org. Chem. 2002, 67, 3915-3918
3915
Sch em e 1
Gen er a tion of 1-Aza p en ta d ien yl An ion fr om
N-(ter t-Bu tyld im eth ylsilyl)-3-bu ten -1-a m in e
Madeleine A. J acobson and Paul G. Williard*
Department of Chemistry, Brown University,
Providence, Rhode Island 02912
Paul_Williard@brown.edu
Received October 24, 2001
pentadienyl derivatives from silicon,5 germanium and
tin,2c,6 and related compounds7 have been made and
studied to investigate the related chemistry of these
species, and to explore their use in transmetalation
reactions. Wu¨rthwein and co-workers have carried out
extensive studies on the experimental and theoretical
chemistry of lithium azapentadienyl species,8 and Pear-
son et. al. have, among other things, synthesized pyrro-
lidines, 1-pyrrolines, and pyrroles by the [4πs + 2πs]
cycloaddition of nonstabilized 2-azaallyl anions with
alkenes.9
1-Azapentadienyl anions, upon which this report fo-
cuses, are useful ambident nucleophiles in organic
synthesis.5a,7a,10 Wu¨rthwein has generated 1-azapenta-
dienylanions from N-(tert-butyl)crotanaldimine, using
LDA in THF at -15 °C.8c These anions are synthetically
useful and add to R,â-unsaturated carbonyls in a Michael
fashion to generate, after hydrolysis, the corresponding
aldehydes and ketones.
Abstr a ct: N-(tert-Butyldimethylsilyl)-3-buten-1-amine un-
dergoes allylic deprotonation at the 2-position when exposed
to 2 equiv of nBuLi in THF. This allylic anion undergoes
lithium hydride elimination to generate a 1-azapentadienyl
anion. The anion is generated cleanly and completely.
Much work has been done on pentadienyl anions.1 They
have been shown to be useful reagents in the preparation
of interesting acyclic complexes.2 Pentadienyl anions are
also valuable precursors for pentadienylsilanes and stan-
nanes, through which aldehydes or ketones can be
converted to dienyl alcohols.3 Silylated and tin penta-
dienyl complexes are useful intermediates in synthetic
organic synthesis, either as reagents for selective trans-
formations or as intermediates for the creation of carbon-
carbon bonds. The high selectivity of the tin-carbon bond
cleavage in transmetalations, transition-metal catalyzed
couplings, and direct reactions is well established.4 As
an extension of the wealth of pentadienyl anion chemistry
that abounds, azapentadienyl anions have attracted
attention in both organic and inorganic chemistry. Aza-
In this work, we have discovered that N-(tert-but-
yldimethylsilyl)-3-buten-1-amine (1) unexpectedly gener-
ates the 1-azapentadienyl anion (3) when exposed to 2
equiv of nBuLi in THF, as depicted in Scheme 1. The
anion is generated cleanly and completely.
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10.1021/jo0162336 CCC: $22.00 © 2002 American Chemical Society
Published on Web 04/26/2002

3916 J . Org. Chem., Vol. 67, No. 11, 2002
Notes
F igu r e 1. 1H NMR (stacked plots) showing the conversion of 1 into 3, in THF-d8 at room temperature. Unlabeled peaks correspond
to starting material, 1. Labeled peaks correspond to 3.
Our interest in the dilithiation of silyl-protected allyl-
amines and related compounds11 led us to investigate the
reactivity of N-(tert-butyldimethylsilyl)-3-buten-1-amine
1. This compound has previously been reported to form
the bis-silyl-3-buten-1-amine upon reaction with 2 equiv
of nBuLi in diethyl ether, followed by quench with
TMSCl.12 The investigators did not report any products
resulting from allylic or vinylic deprotonation of the
starting material.
Contrary to these results, we note the formation of
1-azapentadienyl anions when 1 is exposed to 2 equiv of
nBuLi in THF, as depicted in Scheme 1. The initial anion,
(2), is formed by allylic deprotonation at the 2-position
of the amine by nBuLi. This allylic anion then im-
mediately undergoes lithium hydride elimination8h,13 at
room temperature, to form the azapentadienyl anion 3,
and in fact, 2 cannot be detected. The overall reaction,
however, can be followed by 1H NMR, as shown in Figure
1. The conversion is clean and the 1-azapentadienyl anion
is fully generated within 12 h (Figure 2). The anion has
F igu r e 2. COSY of 3, in THF-d8 at room temperature.
been fully characterized by 1- and 2D NMR,14 and the
structure was further confirmed by comparison with
which was generated from the tert-butyl protected cro-
literature data for N-(tert-butyl)-1-azapentadienyllithium,
tonaldimine, using LDA as the base.8d Analogy to the
same literature data also suggested that the carbon
backbone of 3 adopts a “W” conformation in solution. The
aggregation state of 3 was not determined.
(11) (a) Williard, P. G.; J acobson, M. A. Org. Lett. 2000, 2, 2753. (b)
J acobson, M. A.; Williard, P. G. J . Org. Chem. 2002, 67, 32.
(12) Burns, S. A.; Corriu, R. J . P.; Huynh, V.; Moreau, J . J . E. J .
Organomet. Chem. 1987, 333, 281.
However, when the reaction is carried out at lower
temperatures, LiH elimination is slower than dianion
formation and thus samples containing high concentra-
tions of 2 could be prepared.15 Careful NMR analysis of
a sample containing an approximately 1:1 mixture of 2
and 3 at -90 °C in THF-d8 confirmed the structure of 2
(13) For lithium hydride elimination in lithium amides, see: (a)
Ziegler, Von K.; Gellert, H.-G. Liebigs Annal. Chem. 1950, 567, 179.
(b) Wittig, G.; Schmidt, H.-J .; Renner, H. Chem. Ber. 1962, 95, 2377.
(c) Bach, R. D.; Bair, K. W.; Willis, C. L. J . Organomet. Chem. 1974,
77, 31. (d) Kowalski, C.; Creary, X.; Rollin, A. J .; Burke, M. C. J . Org.
Chem. 1978, 43, 2601. (e) Melamed, U.; Feit, B.-A.; J . Chem. Soc.,
Perkin Trans. 1 1980, 1267. (f) Richey, H. G., J r.; Erickson, W. F. J .
Org. Chem. 1983, 48, 4349. (g) Li, M.-Y.; San Fillipo, J ., J r. Organo-
metallics 1983, 2, 554. (h) Newcomb, M.; Burchill, M. T. J . Am. Chem.
Soc. 1984, 106, 8276. (i) Majewski, M. Tetrahedron Lett. 1988, 29, 4057
and references therein. (j) Barluenga, J .; Canteli, R.-M.; Florez, J . J .
Org. Chem. 1996, 61, 3753.
1
as an allylic dianion. The H and 13C chemical shifts of 2
(15) A sample containing nBuLi (2 equiv) and 1 (1 equiv) was
prepared at -78 °C. No reaction could be observed at this temperature
over several hours. The temperature was gradually raised to 0 °C,
where after about 6 h a very small amount of deprotonation could be
observed. The sample was stored at -20 °C for 48 h, and at this point
most of the starting material had been converted to the allylic anion
2 which was then observed at -90 °C.
(14) 1H NMR (THF-d8) data for compound 3: δ 7.27 (dm, 1H, J )
12.0 Hz), 6.23 (dddd, 1H, J ) 16.4 Hz, 10.6 Hz, 9.9 Hz, 0.6 Hz), 4.85
(ddm, 1H, J ) 12.0 Hz, 10.6 Hz), 3.99 (ddm, 1H, J ) 16.4 Hz, 2.9 Hz),
3.70 (ddm, 1H, J ) 9.9 Hz, 3.0 Hz), 0.86 (s, 9H), -0.06 (s, 6H).
Multiplicities and coupling constants were extracted using resolution
enhancement (unshifted sine bell multiplication).

Notes
J . Org. Chem., Vol. 67, No. 11, 2002 3917
Ta ble 1. 1H- a n d 13C NMR Da ta for 2 in THF -d 8 a t -90 °C
Sch em e 2
atom
C-1
13C NMRa
45.3
atom
1H NMR
H-1a
H-1b
H-2
H-3
H-4c
H-4t
3.37
3.72
3.98
6.24
1.17
0.88b
C-2
C-3
C-4
91.4
145.3
28.9
a
b
Determined from 2D HMQC. Determined from 2D COSY.
products from decomposition of the solvent under the
influence of the base.
It is also interesting to note the different reactivity of
N-allyl-2,2-dimethylpropionamide (7), where 1 and 7
differ only in the nature of the protecting group. When 7
is exposed to similar conditions (2 equiv of nBuLi, THF,
20 °C), no identifiable products are recovered. However,
if less than 2 equiv of nBuLi is used, a different reactivity
is observed. In this reaction lithium hydride is not
eliminated following the allylic deprotonation. Instead,
one molecule of 9 presumably deprotonates one molecule
of the mono-lithium amide of 8, to give 10 and regenerate
9. This is then repeated until the reaction reaches its
thermodynamic sink, 2,2-dimethyl-N-(lithiopent-1-enyl)-
propionamide (12) (Scheme 3). The overall reaction is not
clean, but 12 can be identified as one of the components
of the mixture at the end of the experiment, by 1D and
2D NMR.
It is apparent that the nature of the protecting group
plays a great role in the reactivity of 3-buten-1-amines.
When the amine is protected as an amide, and exposed
to alkyllithium base, several products are formed. One
of the major products can be identified as the rearranged
product 12. On the other hand, if a trialkylsilyl-protected
amine is used, allylic deprotonation is followed by lithium
hydride elimination, generating the 1-azapentadienyl
anion 3. In this communication we have demonstrated a
new and simple method of generating these synthetically
useful 1-azapentadienyl anions. Future work will focus
on applying these dianions to significant synthetic tar-
gets. The reactivity of related amines and their deriva-
tives will also be explored.
F igu r e 3. Proposed structural unit of dianion 2.
could be fully assigned based on 2D COSY and HMQC
experiments. The aggregation state of 2 was not deter-
mined. We found that the methylene protons of 2 are
chemically nonequivalent suggesting that the dianion
exists in a rigid conformation. 13C chemical shifts are
consistent with a localized allylic anion with the lithium
atom situated at the terminal position16 (Table 1). Based
on these results, and drawing analogy to our previous
work on the solid-state structure of related dianions,11a
we propose that the fundamental structural unit of
dianion 2 is cyclic with the two lithium atoms bridging
between the nitrogen and the terminal carbon atom
(Figure 3). This structure is fully consistent with the
NMR spectra and was found to be a minimum on the
potential energy surface by PM3 semiempirical calcula-
tions.17
Silylation has been shown to occur in different posi-
tions of azapentadienyl anions.5a,18 Silylation of 3 with
TMSCl (2 equiv) resulted in the exclusive formation of
N-(tri-methylsilyl)(tert-butyldimethylsilyl))-1,3-dibuten-
1-amine, as confirmed by GC/MS and NOESY experi-
ments.
N-(tert-Butyldimethylsilyl)phenethylamine (4) gener-
ates a 3-phenyl-1-azaallyl anion, (6), when exposed to 2
equiv of nBuLi in THF at room temperature.19 As with
1, the azaallyl anion is formed from lithium hydride
elimination of a stabilized dianion, in this case the
benzylic anion (5) (Scheme 2). However, the reaction of
4, in contrast to that of 1, does not go to completion. The
base (nBuLi) appears to deprotonate the solvent faster
than it deprotonates 4, and only 25% of the starting
material 4 goes on to form product, N-(tert-butyldimeth-
ylsilyl)styrylamine (6), with 2 equiv of nBuLi. With a
large excess of base the reaction could be pushed to
completion, but this would generate large amounts of side
Exp er im en ta l Section
Gen er a l Meth od s. Diethyl ether and THF were distilled
from sodium/benzophenone under a nitrogen atmosphere. n-
Buthyllithium was obtained from Aldrich Chemical Co. (2.5 M
in hexanes), and the exact concentration of the solution was
determined by direct titration with 2,5-dimethoxybenzyl alcohol
in THF. All chemicals were purchased from Aldrich Chemical
Co. and distilled prior to use when appropriate. Deuterated NMR
solvents were dried over 3 Å Linde sieves prior to use. All
reactions and NMR studies were carried out under argon
atmosphere, using flame-dried glassware. GC analyses were
performed using a mass selective detector. IR spectra were
recorded on a FTIR instrument. NMR spectra were collected on
spectrometers operating at 400 and 300 MHz for 1H and 100
and 75 MHz for 13C observation. The chemical shifts were
referenced to CDCl3 (1H 7.27 ppm and 13C 77.23 ppm), CD3OD
(1H 3.31 ppm and 13C 49.15 ppm) or THF-d8 (1H 1.73 ppm and
13C 25.37 ppm) as internal standards. HRMS were performed
at the Brown University Mass Spectrometry laboratory.
(16) (a) Fraenkel, G.; Duncan, J . H.; Wang, J . J . Am. Chem. Soc.
1999, 121, 432. (b) Fraenkel, G.; Cabral, J .; Lanter, C.; Wang, J . J .
Org. Chem. 1999, 64, 1302.
(17) PM3: Stewart, J . J P. J . Comput. Chem. 1989, 10, 209. Li
parameters: Anders, E.; Koch, R.; Freunscht, P. J . Comput. Chem.
1993, 14, 1301. The calulations were performed with PC Spartan Pro,
Wave function Inc. 18401 Von Karman Ave., Suite 370, Irvine, CA
92715.
N-(ter t-Bu tyld im eth ylsilyl)-3-bu ten -1-a m in e (1). LiAlH4
(1.7 g, 45 mmol) was weighed into a flask, and 70 mL of dry
diethyl ether was added. The flask was placed in a 0 °C icebath,
(18) Addition of acylating agents as electrophiles, will yield ther-
modynamically stable dienamides: See footnote 10b.
(19) As observed by 1H and COSY NMR experiments.

3918 J . Org. Chem., Vol. 67, No. 11, 2002
Notes
Sch em e 3
and AlCl3 (6.0 g, 45 mmol) was added in small portions over 10
min. The resulting slurry was stirred for 5 min. Allyl cyanide
(3.6 mL, 45 mmol) was placed in a flask with 20 mL of dry
diethyl ether. The mixture was added to the LiAlH4/AlCl3 slurry
over 10 min. Effervescence was observed upon addition. The
reaction mixture was stirred at 0 °C for 2 h, after which the
reaction was quenched by careful addition of 10% NaOH, also
at 0 °C. NaOH was added until the reaction mixture tested basic
on pH paper and effervescence was no longer observed. The
aqueous layer was extracted with diethyl ether (4 × 40 mL) and
dried over NaOH pellets.20 1H NMR confirmed the presence of
3-butene-1-amine, which was silylated directly, without further
workup.
Triethylamine (6.0 mL, 43 mmol), followed by tert-butyldi-
methylsilyl chloride (6.4 g, 42.8 mmol) dissolved in 20 mL of
diethyl ether, was added to the ether layer containing the amine,
and the reaction mixture was stirred at room temperature for
72 h. The presence of 1 was confirmed by GC/MS. The reaction
mixture was filtered through florisil and the filtrate stirred with
CaH2 for 20 h. Filtration, removal of the ether and low-boiling
impurities by distillation, followed by Kugelrohr distillation
(bp125 °C, 0.25 mmHg) of the remaining liquid, resulted in 0.39
g (22% yield) of 1. Despite repeated efforts, 1 was isolated with
12% 1,3-di-tert-butyl-1,1,3,3-tetramethyldisiloxane and 10% tert-
butyldimethylsilyl hydroxide. FTIR (neat): ν (cm-1) 3401, 3078,
2953, 2855, 1640. 1H NMR (CDCl3): δ 5.77 (m, 1H). 5.05 (m,
2H), 2.82 (q, 2H), 2.16 (m, 2H), 0.88 (s, 9H), 0.05 (s, 6H). 13C
NMR (CDCl3): δ -4.6, 26.1, 26.8, 39.6, 42.1, 116.5, 137.2. HRMS
calculated for C10H23NSi: 185.1599. Found: 185.1597.
florisil, and the filtrate was stirred with CaH2 for 3 h. Filtration,
followed by removal of the solvent in vacuo, resulted in 15.3 g
(82% yield) of 4. FTIR (neat): ν (cm-1) 3401, 3027, 2951, 2853,
1
1603. H NMR (CD3OD): δ 7.36 (m, 2H). 7.27 (m, 3H), 2.95 (m,
2H), 2.82 (m, 2H), 1.02 (s, 9H), 0.02 (s, 6H). 13C NMR (CD3OD):
δ -4.04, 18.3, 25.6, 39.0, 43.2 126.4, 128.7, 128.9, 139.8.
N-Allyl-2,2,-d im eth ylp r op ion a m id e (7). 3-Butene-1-amine
was prepared in the same fashion as described for compound 1,
except all amounts were doubled. 1H NMR confirmed the
presence of 3-butene-1-amine, which was pivaloyl protected
directly, without further workup. Triethylamine (11.3 mL, 81
mmol) was added to the ether layer containing the 3-buten-1-
amine. Trimethylacetyl chloride (10 mL, 81 mmol) was placed
in an addition funnel and added dropwise to the reaction
mixture, at 0 °C. Immediately upon addition of the trimethyl-
acetyl chloride, fumes developed and a white solid precipitated.
The reaction mixture was stirred at room temperature for 12 h,
and the presence of 7 was confirmed by GC/MS. The reaction
mixture was filtered through florisil, and the ether was removed
in vacuo, resulting in 1.7 g (12% yield) of 7. FTIR (neat): ν (cm-1
)
1
3351, 3078, 2966, 1634. H NMR (CDCl3): δ 5.83 (bs, 1H), 5.66
(m, 1H), 4.97 (m, 2H), 3.18 (m, 2H), 2.15 (m, 2H), 1.07 (s, 9H).
13C NMR (CDCl3): δ 27.4, 33.6, 38.0, 38.4, 116.9, 135.3, 178.1.
HRMS calculated for C9H17NONa: 178.1208. Found: 178.1213.
Gen er al P r ocedu r e for th e P r epar ation of NMR Sam ples
To F ollow th e Rea ction of 1, 4, a n d 7 w ith n Bu Li. A 0.13
mmol amount of n-butyllithium in hexanes was placed in a
flame-dried NMR tube under argon atmosphere. The hexanes
were removed in vacuo, and 0.6 mL of THF-d8 was added. To
this clear and colorless solution 1 or 4 (6.6 × 10-5 mol), or 7
(9 mg, 6.0 × 10-5 mol), was added.
N-(ter t-Bu tyld im eth ylsilyl)p h en eth yla m in e (4). Triethyl-
amine (11.1 mL, 0.080 mol) and phenethylamine (10 mL, 0.080
mol) was added to 240 mL of diethyl ether. tert-Butyldimethyl-
silyl chloride (12.0 g, 0.080 mol), in 100 mL of diethyl ether, was
added to the amine/triethylamine mixture. Within a few minutes
white solid precipitated out. The reaction mixture was stirred
at room temperature for 6 days, and the presence of 4 was
confirmed by GC/MS. The reaction mixture was filtered through
Ack n ow led gm en t. This work was supported by
PHS grant GM35980.
Su p p or tin g In for m a tion Ava ila ble: 1H and 13C NMR for
1
compounds 1, 4, and 7. H NMR for compound 2 and 3. This
material is available free of charge via the Internet at
http://pubs.acs.org.
(20) Procedure adapted from Sato, T.; Nakamura, N.; Ikeda, K.;
Okada, M.; Ishibashi, H.; Ikeda, M. J . Chem. Soc., Perkin Trans. 1
1992, 2399.
J O0162336