Intramolecular "hydroiminiumation" of alkenes: Application to the synthesis of conjugate acids of Cyclic Alkyl Amino Carbenes (CAACs)

Article abstract of DOI:10.1002/anie.200605083

(Chemical Equation Presented) A rival for the hydroamination reaction: The intramolecular "hydroiminiumation" reaction features some distinct advantages over intramolecular hydroamination since the resulting prochiral iminium ions potentially allow the su

Full text of DOI:10.1002/anie.200605083

Angewandte
Chemie
DOI: 10.1002/anie.200605083
Carbene Precursors
Intramolecular “Hydroiminiumation” of Alkenes: Application to the
Synthesis of Conjugate Acids of Cyclic Alkyl Amino Carbenes
(CAACs)**
Rodolphe Jazzar, Rian D. Dewhurst, Jean-Baptiste Bourg, Bruno Donnadieu, Yves Canac, and
Guy Bertrand*
Nitrogen-containing heterocyclic systems have attracted
considerable interest over the years because they form the
core structures, and are key intermediates, of natural prod-
ucts.[1] One of the most appealing synthetic approaches for
their preparation is the intramolecular hydroamination of
alkenes, in which the nitrogen–carbon bond is formed by
addition of an amine to an olefin.[2] Various catalysts have
been used to effect this transformation, which include alkali
metals,[3] early[4] and late transition metals,[5] and f-block
elements.[6] Interestingly, despite the buffering effect of
amines, intramolecular[7] and even intermolecular[8] acid-
catalyzed hydroaminations have recently been developed.
Schlummer and Hartwig[7a] reported the cyclization of amino
alkenes bearing an electron-withdrawing group on the nitro-
gen atom by catalysis with triflic or sulfuric acid (20 mol%;
Scheme 1). A mechanistic study of this process suggested that
in contrast to similar transformations using various electro-
philes as promoters (for example, iodine-,[9] bromine-,[10] and
selenium-based electrophiles[11]), the first step was protona-
tion of the amine, followed by intramolecular transfer of the
proton to the double bond in the rate-determining step, and
lastly trapping of the generated cation by the amino group.
Accordingly, in the absence of electron-withdrawing groups
on the nitrogen atom, the cyclization does not occur because
of the excessive basicity of the amino group, which prevents
the transfer of the proton to the olefin.
Imines are certainly less basic than amines, and therefore
it was decided to investigate the feasibility of “hydroiminium-
ation” reactions, which would be an atom-economical route to
cyclic iminum salts A (Scheme 1). Providing there is a bulky
aryl substituent on the nitrogen atom and that there is a
quaternary carbon atom in the position a to the aldiminium
carbon atom, salts A’ are the direct precursors of stable cyclic
alkyl amino carbenes (CAACs) B.[12] We have shown that
CAACs can compete with N-heterocyclic carbenes (NHCs)[13]
as ligands for transition-metal-based catalysts,[12a] and also
allow the preparation of very low coordinate transition-metal
centers.[12b] We report herein our preliminary results on the
scope of the thermally induced hydroiminiumation reaction
and its application to the synthesis of a variety of CAAC
precursors.
To establish the viability of this hydroiminiumation
methodology, the synthesis of the previously reported
CAAC/H+ compound 4a[12a] was chosen as an initial test.
Deprotonation of aldimine 1a, derived from 2,6-diisopropyl-
aniline (DippNH2) and cyclohexane carboxaldehyde, with
lithium diisopropylamide (LDA) leads to the corresponding
1-aza-allyl anion, which readily reacts at room temperature
with 3-bromo-2-methylpropene (or 3-chloro-2-methylpro-
pene) to afford the alkenyl aldimine 2a in 94% yield
(Scheme 2). Addition of a stoichiometric amount of a 2m
solution of HCl/Et2O to a toluene solution of 2a at ꢀ788C
resulted in the immediate formation of a white precipitate.
After 15 minutes at ꢀ788C, the mixture was allowed to warm
to room temperature and stirring was continued for an
additional 15 minutes. After filtration and recrystallization
from chloroform, a new compound 3a was isolated as white
crystals in 92% yield. The ionic character of 3a was apparent
from its low solubility in toluene, while its acyclic nature was
revealed by the presence of a 13C NMR signal at d =
117.0 ppm from an ethylenic CH2 fragment. The protonation
of the nitrogen atom was indicated by a 1H NMR signal at d =
Scheme 1. a) Schematic representation of acid-catalyzed intramolecular
hydroamination; EWG=electron-withdrawing group, n=1,2. b) The
hydroiminiumation reaction as a potential synthetic route to cyclic
iminium salts A. c) CAAC/H+ salts A’, the precursors of CAACs B;
R1,R2 ¼H, Ar=bulkyaryl group.
[*] Dr. R. Jazzar, Dr. R. D. Dewhurst, J.-B. Bourg, B. Donnadieu,
Dr. Y. Canac, Prof. G. Bertrand
UCR-CNRS Joint Research ChemistryLaboratory(UMI 2957)
Department of Chemistry
Universityof California
Riverside, CA 92521-0403 (USA)
Fax: (+1)951-827-2725
E-mail: gbertran@mail.ucr.edu
[**] We are grateful to the NIH (R01 GM 68825) and Rhodia for financial
support of this work.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
15.5 ppm, and by the deshielding of the N CH 13C and
=
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24 h at 708C are necessary to achieve complete conversion.
Not surprisingly, a limitation to the methodology was found
when an electron-donating tert-butyl group was placed on the
nitrogen atom. Here, because of the high basicity of the
nitrogen center, no trace of the cyclic iminium salt 4d was
detected when a toluene solution of 3d was heated at 1108C
for 24 h.
Use of alkenyl aldiminium salts 3a and 3e–g allowed
study of the influence of the substitution pattern of the
carbon–carbon double bond on the fate of the hydroiminium-
ation reaction, especially with regard to its regioselectivity
(Scheme 3). The temperature required for cyclization was
Scheme 2. Influence of the nature of the R and R1 substituents on the
rate of the hydroiminiumation reaction; Dipp=2,6-iPr2C6H3;
Mes=2,4,6-Me3C6H2. [a] Time and temperature required for complete
conversion of 3. [b] Yield of isolated product, without isolation of 3,
and using a twofold excess of HCl. [c] n.r.=no reaction.
1H NMR signals (2a: d = 173.6 and 7.6 ppm; 3a: d = 189.8
and 8.0 ppm). A single-crystal X-ray diffraction study unam-
biguously proved the alkenyl aldiminium structure of 3a
(Figure 1).[14] Pleasingly, it was noted that heating an aceto-
Scheme 3. Influence of the nature of the alkene substituents R1 and R2
on the rate and regioselectivityof the hydroiminiumation reaction.
Tos =toluene-4-sulfonyl. [a] Time and temperature required for com-
plete conversion of 3. [b] Yield of isolated product, without isolation of
3, and using a twofold excess of HCl. [c] The reaction did not go to
completion and was stopped after 72 h. [d] Yield as measured byNMR
spectroscopy.
Figure 1. Molecular structure of 3a in the solid state.
nitrile solution of 3a in a tube sealed by a teflon stopcock at
508C for 18 h afforded the desired cyclic iminium salt 4a in
88% yield. Obviously, the last two steps of the synthesis
(2a!4a) can be performed in situ, and the best results (88%
yield of isolated product) were obtained when a twofold
excess of HCl was used. The overall transformation (1a!4a)
can thus be done in 83% yield, which compares extremely
favorably with the previously reported method (48% yield);
moreover, the new route uses the same precursor 1a, but
avoids the use of the costly reagents 1,2-epoxy-2-methylpro-
pane and trifluoromethane sulfonic anhydride.
found to increase along the series 3a < 3e < 3 f < 3g. More
importantly, in all cases five-membered heterocycles 4
resulting from exo cyclization were obtained, with no trace
of the six-membered-ring isomers being detected. Strikingly,
the cyclization of 3g affords exclusively five-membered
heterocycle 4g (Figure 2), despite the presence of a phenyl
group at the terminal carbon atom of the olefin, which would
To study the influence of various steric and electronic
factors on the hydroiminiumation reaction, several different
alkenyl aldimines 2b–h were prepared (Schemes 2, 3, and 4).
Without exception, the protonation occurred smoothly at the
nitrogen atom, and the ensuing alkenyl aldiminium salts 3b–h
were obtained in good to excellent yields. The cyclization
process occurs slightly more easily when bulky substituents
are used on both sides of the NCC fragment (Scheme 2).
Indeed, when two methyl groups were used in place of the
cyclohexyl group of 3a, the formation of 4b required 24 h at
508C, whereas for derivative 3c (Ar= Mes, CR1R1 = CMe2)
Figure 2. Molecular structures of 3g (left) and 4g (right) in the solid
state.
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Chemie
=
1H, C CH2), 3.06 (sept, JHH = 6.8 Hz, 2H, CHCH3), 2.35 (s, 2H,
be expected to stabilize a benzylic carbocation intermediate.
Together these observations favor a mechanism in which the
proton would be transferred intramolecularly to the double
bond in the rate-determining step, similarly to the mechanism
proposed by Schlummer and Hartwig for the acid-catalyzed
hydroamination reaction.[7a] When compared to the latter
reaction involving alkenyl amine I, for which the formation of
the six-membered ring II was observed, our result suggests
=
CH2), 1.96 (m, 2H, H2CCy), 1.86 (s, 3H, CH3C ), 1.71–1.44 (m, 8H,
H2CCy), 1.21 ppm (d, JHH = 6.8, 12H, CHCH3); MS (EI): m/z: 32 6
[M+H]+.
Representative procedure for the synthesis of alkenyl iminium
salts 3: A solution of HCl in Et2O (1.54 mL, 2.0m, 3.1 mmol) was
added to a solution of alkenyl aldimine 2a (1.00 g, 3.1 mmol) in
hexane (10 mL) at ꢀ788C. Precipitation of a white powder was
immediately observed. After 15 minutes the mixture was warmed to
room temperature and stirring was continued for an additional
15 minutes. Filtration of the white precipitate, washing with hexanes
(2” 10 mL), and drying under vacuum afforded the alkenyl iminium
salt 3a in 92% yield. M.p. 83 8C (decomp); 13C{1H} NMR (75.1 MHz,
ꢀ
that the addition of N H across the double bond has a greater
“concerted” character[15] in the hydroiminiumation than in
the hydroamination reaction.[7a]
Six-membered heterocyclic aldiminium salts can also be
accessed as shown by the preparation of 4h (Scheme 4).
However, as observed in the hydroamination reaction,[7a] the
cyclization to 4h is more difficult than for the homologous
five-membered ring 4e.
=
=
CDCl3, 2 58C): d = 189.8 (NH CH), 143.0 (Cortho), 140.5 (C CH2),
=
135.4 (Cipso), 130.4 (Cpara), 124.5 (Cmeta), 117.0 ( CH2), 46.7 (CCy), 45.7
=
(CH2), 34.0 (H2CCy), 28.7 (CHCH3), 25.1 (CH3C ), 25.0 (H2CCy), 24.0
(CHCH3), 22.6 ppm (H2CCy); 1H NMR (300.0 MHz, CDCl3, 2 58C):
=
d = 15.50 (s, 1H, NH), 7.98 (s, 1H, CH N), 7.37 (t, JHH = 8.1 Hz, 1H,
para), 7.22 (d, JHH = 8.1 Hz, 2H, Hmeta), 4.99 (s, 1H, C CH2), 4.82(s,
=
H
=
1H, C CH2), 2.99 (sept, JHH = 6.8 Hz, 2H, CHCH3), 2.68 (s, 2H,
=
CH2), 2.42 (m, 2H, H2CCy), 1.90 (m, 2H, H2CCy), 1.84 (s, 3H, CH3C ),
1.73 (m, 2H, H2CCy), 1.58 (m, 4H, H2CCy), 1.24 ppm (d, JHH = 6.8 Hz,
12H, CHCH3); MS (FAB): m/z: 32 6 [M]+.
Representative procedure for the hydroiminiumation reaction
leading to 4: A solution of alkenyl iminium salt 3a (1.00 g, 2.8 mmol)
in acetonitrile (10 mL) in a tube sealed by a teflon stopcock was
heated at 508C for 18 h. The volatiles were removed under vacuum to
afford 4a as a white powder in 88% yield. Alternatively, a solution of
HCl in Et2O (3.08 mL, 2.0m, 6.2mmol) was added to a solution of
alkenyl aldimine 2a (1.00 g, 3.1 mmol) in acetonitrile (10 mL) at
ꢀ788C. The solution was warmed to room temperature and sealed
with a teflon stopcock, then heated at 508C for 18 h. The volatiles
were removed under vacuum to afford 4a as a white powder in 88%
yield. M.p. 1688C; 13C{1H} NMR (75.1 MHz, CDCl3, 2 58C): d = 193.0
Scheme 4. Synthesis of six-membered heterocyclic aldiminium salt 4h.
Besides the easy preparation of a wide variety of CAAC/
H+ compounds, the intramolecular hydroiminiumation
reported here features some distinct advantages when com-
pared to the intramolecular hydroamination reaction. The
resulting iminium ions are very reactive, potentially allowing
for the subsequent addition of a large range of nucleophiles,
and since they are often prochiral, this chemistry offers the
possibility of facile construction of a new stereogenic center a
to the nitrogen atom. The extension of this work to other
protonated sp2-nitrogen-containing species is under active
investigation.
=
(N CH), 144.6 (Cortho), 131.9 (Cpara), 129.0 (Cipso), 125.4 (Cmeta), 82.9
(CCH3), 53.6 (CCy), 45.6 (CH2), 33.8 (H2CCy), 30.0 (CHCH3), 29.1
(CH3), 26.8 (CH3), 24.2 (H2CCy), 22.3 (CH3), 21.3 ppm (H2CCy);
1
=
H NMR (300.0 MHz, CDCl3, 2 58C): d = 10.69 (s, 1H, CH N), 7.42
(t, JHH = 7.8 Hz, 1H, Hpara), 7.23 (d, JHH = 7.8 Hz, 2H, Hmeta), 2.57
(sept, JHH = 6.7 Hz, 2H, CHCH3), 2.37 (s, 2H, CH2), 1.80–1.34 (m,
10H, H2CCy), 1.47 (s, 6H, CCH3), 1.25 (d, JHH = 6.7 Hz, 6H, CHCH3),
1.13 ppm (d, JHH = 6.7 Hz, 6H, CHCH3); MS (FAB): m/z: 32 6 [M]+.
All spectroscopic data are comparable to those observed for the
corresponding triflate salt.[12a]
Experimental Section
Received: December 16, 2006
Published online: March 13, 2007
All manipulations were performed under argon by using standard
Schlenk techniques and oven-dried, argon-flushed glassware. Dry,
oxygen-free solvents were employed. H and 13C NMR spectra were
1
Keywords: aldiminium salts · carbenes · cyclization ·
recorded on Varian Inova 300 and Bruker Avance 300 spectrometers.
Representative procedure for the synthesis of alkenyl imines 2: A
solution of LDA (1.18 g, 11.0 mmol) in Et2O (20 mL), cooled to
ꢀ788C, was added to a solution of aldimine 1a (3.00 g, 11.0 mmol) in
Et2O (20 mL) at ꢀ788C. After 15 minutes the mixture was left to
warm to room temperature and stirring was continued for an
additional two hours. The volatiles were then removed under
vacuum to afford an oily yellow-orange residue, which was dissolved
in Et2O (30 mL) and cooled to ꢀ788C. 3-Bromo-2-methylpropene
(1.11 mL, 11.1 mmol) was then slowly added. After 15 minutes the
solution was warmed to room temperature and stirring was continued
for an additional 12h. Removal of the volatiles under vacuum and
extraction with hexanes afforded alkenyl aldimine 2a as a light-yellow
oil in 94% yield. 13C{1H} NMR (75.1 MHz, CDCl3, 2 58C): d = 173.6
.
hydroamination
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