Metal-free α-amination of secondary amines: Computational and experimental evidence for azaquinone methide and azomethine ylide intermediates

Article abstract of DOI:10.1021/jo400483h

We have performed a combined computational and experimental study to elucidate the mechanism of a metal-free α-amination of secondary amines. Calculations predicted azaquinone methides and azomethine ylides as the reactive intermediates and showed that iminium ions are unlikely to participate in these transformations. These results were confirmed by experimental deuterium-labeling studies and the successful trapping of the postulated azomethine ylide and azaquinone methide intermediates. In addition, computed barrier heights for the rate-limiting step correlate qualitatively with experimental findings.

Full text of DOI:10.1021/jo400483h

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Article
Metal-free alpha-Amination of Secondary Amines:
Computational and Experimental Evidence for
Azaquinone Methide and Azomethine Ylide Intermediates
Arne Dieckmann, Matthew T Richers, Alena Yu. Platonova, Chen Zhang, Daniel Seidel, and K. N. Houk
J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/jo400483h • Publication Date (Web): 21 Mar 2013
Downloaded from http://pubs.acs.org on March 30, 2013
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Metal-free α-Amination of Secondary Amines: Computational and
Experimental Evidence for Azaquinone Methide and Azomethine Ylide
Intermediates
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Arne Dieckmann,† Matthew T. Richers,‡ Alena Yu. Platonova,‡,§ Chen Zhang,‡ Daniel Seidel,*,‡
and K. N. Houk*,†
† Department of Chemistry and Biochemistry, University of California, Los Angeles, California
90095, United States
‡ Department of Chemistry and Chemical Biology, Rutgers, The State University of New Jersey,
Piscataway, New Jersey 08854, United States
§ Department of Organic Synthesis Technology, Ural Federal University, Yekaterinburg, 620002,
Russia
*Email: houk@chem.ucla.edu, seidel@rutchem.rutgers.edu
Abstract
We have performed a combined computational and experimental study to elucidate the
mechanism of a metal-free α-amination of secondary amines. Calculations predicted azaquinone
methides and azomethine ylides as the reactive intermediates and showed that iminium ions are
unlikely to participate in these transformations. These results were confirmed by experimental
deuterium labeling studies and the successful trapping of the postulated azomethine ylide and
azaquinone methide intermediates. In addition, computed barrier heights for the rate-limiting step
correlate qualitatively with experimental findings.
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Introduction
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Aminal substructures1 are present in a number of natural products,2 which makes simple synthetic
procedures to their precursors and analogues important to the organic chemist. Recently, one of
our groups developed an efficient route to ring-fused aminals3,4 by metal-free, redox-neutral5 C–H
functionalization of cyclic amines (Scheme 1).6,7 The procedure is straightforward and only
requires heating an aminobenzaldehyde with an excess of amine in ethanol to afford the aminal in
one step. Most methods that involve the functionalization of relatively nonreactive C–H bonds
require the use of transition metal catalysts, often in combination with (super)stoichiometric
amounts of oxidant.8 Here we report the results of a computational and experimental study aimed
at delineating the mechanistic pathways of this practical and convenient transformation. The
mechanism was predicted by an extensive exploration of possible pathways using density
functional theory (DFT) calculations based on the original experimental results3,4 and is in line
with subsequently performed deuterium labeling and trapping experiments.
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Some of the key findings of the initial investigation are summarized in eqs 1–4. The scope of the
aminal formation includes different cyclic secondary amines and electron-deficient ortho-
aminobenzaldehydes were found to work best. Interestingly, not only the electronic structure, but
also the geometry of the amines has a profound effect on reactivities and yields. Pyrrolidine
gives excellent yields with electron-poor aminobenzaldehydes such as 1a (eq 1). Good yields can
also be obtained with more electron-rich aminobenzaldehydes (e.g., 1b), although extended
reaction times are required. Even with the highly reactive aminobenzaldehyde 1a, piperidine
requires prolonged reaction times at elevated temperatures and the yield drops significantly (eq
2). Morpholine is even less reactive. Cyclic amines with benzylic α-C–H bonds such as 1,2,3,4-
tetrahydroisoquinoline (THIQ) are excellent substrates (eq 3). In contrast, no product could be
obtained with 1,2,3,4-tetrahydroquinoline (THQ) under a variety of conditions (eq 4).
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Scheme 1. Potential Mechanistic Pathways for the Redox-Neutral Aminal Formation. Blue
Arrows Refer to the Lowest Energy Pathway as Elucidated by DFT Calculations.
CHO
+
NH
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NH2
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NH2
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H
OH
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N
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N
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–B
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–HB
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–OH
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NH
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NH
H
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NH2
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6b
8b
9b
3b
–H
–H
H
–H2O
N
N
B
–HB
N
H
NH
2b
7b
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Various potential mechanisms have been considered for these transformations, all of which are in
line with experimental conditions. Using the reaction of 1b and pyrrolidine as a prototypical
example, a number of potential mechanistic pathways are summarized in Scheme 1. All start
with the formation of hemiaminal 3b that should be formed rapidly upon mixing of the aldehyde
and amine. Afterwards, 3b can eliminate hydroxide to form iminium ion 4b, which can undergo
a variety of reactions. Deprotonation by an external base either leads to ortho-aza-quinone
methide 5b,9 or azomethine ylide 6b.10,11 Aza-quinone methide 5b can also be obtained by a
direct dehydration of hemiaminal 3b (vide infra). Alternatively, the protonated azomethine ylide
10b can be formed by an internal proton transfer12 and is likely to undergo another proton transfer
resulting in iminium species 8b. In addition to the rather unlikely pathway involving 10b as an
intermediate, iminium ion 8b can be obtained from 5b via azomethine ylide 6b. The latter could
be formed from 5b either by a 1,6-hydride shift13 or a 1,6-proton transfer.12 Subsequent
protonation of azomethine ylide 6b, e.g. by solvent molecules, results in 8b. The ring closure can
either proceed via iminium ion 8b or zwitterion 7b. An intramolecular attack of the amino group
nitrogen on the iminium moiety in 8b leads to the protonated product 9b, while the formation of
7b by a (solvent-mediated) proton transfer and a subsequent intramolecular attack leads to the
neutral product 2b. The direct transformation of 4b to 8b via 1,3-hydride shift was not
considered.14
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Overall, there are several plausible and interconnected mechanisms leading to products 2 that
differ with respect to the intermediates involved and their protonation states. As a consequence, a
purely experimental mechanistic elucidation of this reaction is likely to be extremely challenging.
In order to discriminate between the different mechanistic possibilities, we undertook a detailed
computational study based on DFT and arrived at a consistent, but partly unexpected mechanism.
In addition, new experimental data were obtained on selectivities and reactivities of different
substrates, and deuterium-labeling studies were performed that provide evidence that supports the
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computational results. Further support was obtained by trapping of an azomethine ylide and an
azaquinone methide.
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For the sake of clarity, we will first provide our new experimental results. Afterwards, we will
discuss our calculations and rationalize the experimental findings based on the predicted
mechanism.
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Experimental Results and Discussion
Evidence for the intermediacy of azaquinone methides. In order to support or rule out the
mechanistic pathways presented in Scheme 1, we designed a number of experiments with the goal
to trap some of the proposed intermediates, in particular ortho-azaquinone methides (e.g., 5b) and
azomethine ylides (e.g., 6b). After a series of failed attempts to trap the proposed quinoidal
intermediates via intermolecular hetero-Diels-Alder reactions, we explored the possibility of
tethering a dienophile to one of the reactants. To this end, we prepared aminobenzaldehyde 11
bearing an α,β-unsaturated ester attached to nitrogen via a four-carbon alkyl chain linker (Scheme
2). Upon exposure of 11 to standard aminal forming conditions with excess pyrrolidine, we
recovered compound 12 in 7% yield, the apparent product of an endo-selective hetero-Diels-
Alder reaction (see structure 17). Another product that was isolated from the reaction mixture is
compound 13 (7%), possibly formed upon elimination of pyrrolidine from compound 12. In
addition, we obtained conjugate addition product 14 (42%), aminal 15 (20%),15 and recovered
starting material 11 (9%). While these results are consistent with an ortho-azaquinone methide
intermediate, we needed to rule out alternative reaction pathways for the formation of 12 that do
not involve a [4+2] cycloaddition.
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Scheme 2. Capture of an ortho-azaquinone methide intermediate via intramolecular [4+2]
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cycloaddition and relevant control experiments.
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Potentially, tricycle 13 could be formed directly in a Baylis-Hillman-like reaction,16 and a
conjugate addition of pyrrolidine to 13 could result in the formation of apparent Diels-Alder
product 12. We tested for this possibility in a series of experiments (Scheme 2). Heating 11 in
the absence of any additives did not lead to formation of 13. Since pyrrolidine could simply act
as a base to catalyze cyclization of tethered alkene 11 to yield cyclization product product 13, we
also performed the reaction in the presence of Hünig's base (similar pKaH to pyrrolidine) and N-
methylpyrrolidine. No reaction was observed in either case, and starting material 11 was
recovered quantitatively. Furthermore, to ensure that the apparent Diels-Alder product 12 is not
the product of conjugate addition of pyrrolidine to tricycle 13, the latter was exposed to
pyrrolidine in refluxing ethanol for 48 hours. No reaction was observed in this instance. This
strongly suggests that 12 is not a conjugate addition product, but rather that 13 results from the
elimination of pyrrolidine from 12.
An aza-Baylis-Hillman-type pathway16 (e.g., structure 17 in Scheme 2) would also account for
the formation of 12. However, given the unlikeliness of iminium ion formation under the
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reaction conditions (see computational results), this pathway was not considered further.
Interestingly, the analogous reaction of 11 with piperidine only led to conjugate addition product
18 in 47% yield, in addition to recovered starting material. The lack of formation of 19 or the
corresponding aminal product can be attributed to an increased difficulty of accessing the
required ortho-azaquinone methide or azomethine ylide intermediates. Another possible
pathway, namely pyrrolidine acting as a nucleophilic Lewis base catalyst in an intramolecular
Baylis-Hillman reaction was ruled out on the basis that this would require the formation of an
intermediate with a ten-membered ring (not shown).
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Evidence for the intermediacy of azomethine ylides. Aldehydes are known to act as potent
dipolarophiles in reactions with azomethine ylides.17 In cases where azomethine ylides are
formed from amino acids and aldehydes in the presence of other dipolarophiles, these
cycloadditions can become unintended side reactions. We decided to exploit this reactivity
pattern to establish the intermediacy of azomethine ylides in the aminal formation. In order to
promote intermolecular [3+2] cycloaddition and hopefully suppress aminal formation, pyrrolidine
was allowed to react with two equivalents of aminobenzaldehyde 1a (eq 5). The reaction was
performed in ethanol solution fourfold more concentrated than under standard conditions. A
microwave reactor was used to facilitate product formation. Following a reaction time of 30 min
at 150 °C, cycloaddition product 20 was isolated in 18% yield along with aminal 2a (74%).
When toluene was used as the solvent under otherwise identical conditions, the yield of the [3+2]
product 20 increased to 27%, while aminal 2a was recovered in 58% yield. This increase in yield
in an apolar solvent is consistent with a reduced quantity of proton sources available to protonate
the azomethine ylide. In both solvents, 20 was obtained as a single diastereomer. The relative
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stereochemistry of 20 matches that of the major products previously reported in analogous [3+2]
reactions.17 These observations strongly support the intermediacy of an azomethine ylide.
Deuterium-labeling studies. A number of deuterium-labeling experiments were performed in
order to obtain further insights into the mechanism of the aminal formation. When a reaction of
aminobenzaldehyde 1a and pyrrolidine was conducted in EtOD, aminal 2a was obtained with
close to 100% incorporation of one deuterium atom, distributed approximately equally over the
two diastereotopic benzylic protons (eq 6).18 To confirm that deuteration occurred during aminal
formation, non-deuterated 2a was exposed to identical reaction conditions (reflux in EtOD for 48
h in presence of two equivalents of pyrrolidine). No trace of deuterium incorporation was
observed in this case. These results are consistent with an azomethine ylide intermediate related
to 6b being protonated by solvent to form an iminium ion of type 8b. The corresponding
experiment was also performed with THIQ (eq 7). Interestingly, in this case partial deuterium
incorporation was observed for all three benzylic protons with a total deuterium incorporation of
~ 100%. The observation of deuterium incorporation at the aminal carbon likely reflects a
difference in charge distributions of the azomethine ylides derived from pyrrolidine vs. THIQ.19
However, the fact that substantially less than one deuterium atom was incorporated into the two
diastereotopic benzylic positions of the dibromoaniline ring seemed at odds with the proposed
mechanism. One possible explanation would be that the protonation step exhibits a relatively
large kinetic isotope effect. The two starting materials could serve as a source of protons. In
order to minimize the total number of protons available in the system, we repeated this
experiment with substrates in which the exchangeable protons had been replaced with deuterium
(eq 8). Indeed, in the event, substantially increased deuterium incorporation was observed in the
benzylic position of the dibromoaniline ring. Interestingly, the recovered THIQ was found to be
partially deuterated, indicating the reversibility of the early reaction steps. Deuteration of the
benzylic position of THIQ requires the presence of 1a (i.e., heating of THIQ in EtOD under
reflux for 16 h did not lead to any incorporation of deuterium into the benzylic position of THIQ).
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Deuterium labeling experiments were also used to potentially gain some insights into the nature
of the rate limiting step of the reaction by measuring the kinetic isotope effect (KIE). As the
relatively long reaction times and high temperatures required for aminal formation would make
spectroscopic monitoring of the progress rather difficult, we chose to measure isotope effects with
PH/PD values from competition experiments rather than determining KH/KD from reaction rates.20
A reaction of aminobenzaldehyde 1a and pyrrolidine was conducted in a 1:1 mixture of EtOH
and EtOD (eq 9). A PH/PD value of 2.1 was observed, which would be consistent with the
protonation step being rate determining. A similar outcome was observed in the corresponding
experiment with THIQ (eq 10). However, calculation of a meaningful PH/PD value is complicated
by the above mentioned complexities (see eqs 7 and 8). Regardless, there appears to be a
substantial KIE.21
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The relative rates of C–H vs. C–D functionalization were probed with partially deuterated amine
substrates (eqs 11–13). A reaction of pyrrolidine-2,2-d2 with 1a resulted in the formation of
partially deuterated 2a in 77% yield (eq 11). The observed PH/PD value of 3.5 is consistent with
the C–H functionalization step being rate determining. A substantially lower PH/PD value of 1.9
was observed in the corresponding reaction with THIQ-1-d (eq 12). A related competition
experiment with a 1:1 mixture of THIQ and THIQ-1,1-d2 also gave rise to a PH/PD value of 1.9
(eq 13). The experiments in eqs 11–13 conclusively rule out the intervention of a 1,3-hydride
shift, as no measurable amount of deuterium was incorporated into the benzylic position of the
dibromoaniline ring. Overall, the isotopic labeling experiments outlined in eqs 6–13 do not rule
out azomethine ylide protonation or C–H functionalization as the rate limiting step.
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Regioselectivity of the aminal formation for non-symmetrical amines. Insights into the
mechanism of the aminal formation may also be obtained from nonsymmetrical amines that
could, at least in principle, give rise to different regioisomeric products. As shown earlier, the
reaction of THIQ and aminobenzaldehyde 1b under standard conditions gave rise to product 2g in
high yield, resulting from exclusive functionalization of a benzylic C–H bond (eq 3). This
outcome is entirely anticipated based on the generally observed greater reactivity of benzylic over
aliphatic C–H bonds. We were thus surprised to observe trace amounts of regioisomeric product
2h when this reaction was first conducted under microwave conditions with the initial goal of
simply enhancing the reaction rate. Closer inspection revealed that substantial amounts of
product 2h can be obtained at higher temperatures (eq 14). Specifically, a reaction of 1b and
THIQ, conducted under microwave irradiation at 250 °C for 30 min, gave rise to 2h in 16% yield
in addition to the expected product 2g which was isolated in 72 % yield. Moreover, extending the
reaction time from 30 min to 2 h led to the formation of 2h as the major product in 47% yield,
without significantly affecting the combined yield of 2g and 2h. These observations suggest that
aminal 2g is in fact the kinetic product of this transformation whereas 2h represents the
thermodynamically more stable aminal product. Furthermore, there appears to be a pathway for
product isomerization. Prompted by this discovery, we decided to investigate the reaction of 2-
methylpyrrolidine with aminobenzaldehyde 1b (eq 15). Interestingly, for this particular substrate
combination, virtually identical product ratios were obtained under a variety of conditions.
Aminal 2j was consistently obtained as the major product, illustrating the preferential
functionalization of a tertiary over a secondary C–H bond. These results are consistent with our
previous findings in a reaction of 2-methylpyrrolidine with 1a which was conducted under
reflux.3a
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Computational Methods
Geometry optimizations were performed with the meta-hybrid density functional M06-2X22 and a
6-31+G(d,p) basis set. Solvation by ethanol was taken into account by the SMD solvent model,23
which was applied to both optimizations as well as frequency calculations. It was recently shown
that the presence of a polarizable continuum model does not have a great impact on frequencies,
while it might be mandatory to locate certain transition states that only exist in polar media.24
Thermal corrections were calculated from unscaled harmonic vibrational frequencies at the same
level of theory for a standard state of 1 mol L−1 (17.12 mol L−1 for ethanol) and 298.15 K, as the
experimental conditions of refluxing ethanol and high pressure in sealed tubes cannot be
reproduced. The resulting free energies refer to Gibbs free energies. Free energies as well as
enthalpies are corrected for zero-point vibrational energy. All stationary points were characterized
and confirmed by vibrational analysis. An ultrafine grid corresponding to 99 radial shells and 590
angular points was used throughout this study for numerical integration of the density. Natural
population analyses25 used the NBO program (version 3.1) as implemented in Gaussian 09. All
calculations were performed with Gaussian 09.26
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Computational Results and Discussion
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The general mechanism. At the outset of our computational study we considered all mechanisms
depicted in Scheme 1. In the following, the mechanism that was predicted to be the most
favorable is discussed with the prototypic reaction of amino aldehyde 1b and pyrrolidine (Scheme
3). A matching free energy profile is shown in Figure 2.
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The first step in the reaction cascade is the formation of hemiaminal 3b, which is exothermic, but
endergonic according to our calculations. To obtain an iminium ion as suggested in Scheme 1,
hydroxide needs to be eliminated. Upon elimination, hydroxide spontaneously abstracts the amine
hydrogen leading to a set of two quinoidal intermediates, cis-5b and trans-5b (Figure 1). We
could also locate transition states trans-TS-3b and cis-TS-3b, directly connecting hemiaminal 3b
with trans-5b and cis-5b by a concerted elimination of water (Scheme 3 and Figure 2). Both
transition states are lower in terms of enthalpy and free energy than the corresponding iminium
ion, suggesting that trans-5b and cis-5b are formed directly from 3b and not via iminium species
4b as assumed before (Scheme 2). As a consequence, pathways involving the iminium ion do not
warrant further consideration.
It must be noted that computed enthalpies and as a consequence free energies are overestimated
particularly for TS-3b, as this transition state benefits greatly from hydrogen bonding of solvent
molecules to the leaving water molecule. As a consequence, we consider TS-3b (24.7 kcal mol–1)
to be always lower in enthalpy and free energy than TS-5b (15.9/16.9 kcal mol–1), which is in
perfect agreement with experimental data.
trans-TS-3b and cis-TS-3b differ with respect to the geometry of substituents at one exocyclic
double bond. While cis-5b allows an abstraction of the α-hydrogens of the heterocycle by the
imine nitrogen via TS-5b, an intramolecular reaction is impossible in trans-5b. trans-TS-3b and
trans-5b are 1 kcal mol–1 lower in energy than their corresponding cis-isomers due a greater
planarity of the resulting exocyclic π-system (Figure 1), corresponding to a reduced A1,3-strain
interaction.
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Scheme 3. The general mechanism for the α-amination of nitrogen heterocycles is exemplified
7
8
with the prototypic reaction of 1b and pyrrolidine leading to product 2b.
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trans-5b
N
H
trans-TS-3b
N
H
N
HN
O
isomerization
OH
–H2O
NH2
H
N
H
1b
N
3b
H
N
cis-TS-3b
H
cis-5b
TS-5b
H
N
N
N
TS-6b
TS-7b
EtOH
N
H
NH
NH
H
2b
7b
6b
cis-5b:
trans-5b:
H
H
N
H
N
N
N
H
NH
NH
N
H
NH
2 3
4
2
4
1
1
3
ꢀ
1234 = 31º
ꢀ
1234 = 9º
–0.84
–0.91
Figure 1. Structures of quinoidal intermediates cis-5b and trans-5b. Charges for nitrogen atoms
were obtained from a natural population analysis. The dihedral angle θ is a measure for the
planarity of the exocyclic π-system (0° corresponds to a perfectly flat geometry).
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A highly negative charge on the primary nitrogen obtained from a natural population analysis in
5b indicates a significant contribution from a zwitterionic resonance-structure involving an
iminium ion at the heterocycle, which restores the aromaticity of the system. Although the trans-
geometry is slightly preferred, the cis/trans energy difference is quite small and dihedral scans
proved the barrier for isomerization to be lower than the barrier for intramolecular proton transfer
(TS-5b), so that trans-5b can be directly converted to cis-5b. Furthermore, up to this point all
steps are reversible so that trans-5b may be recycled to cis-5b. The transition state for an
intramolecular proton transfer TS-5b has a free energy barrier of 12.7 kcal mol−1 relative to cis-
5b and is likely to be the rate-determining step. While a 1,6-hydride shift has been considered
before, the substantial negative charge on the nitrogen in 5b precludes this mechanistic
alternative. The intrinsic reaction coordinate associated with TS-5b leads to azomethine ylide 6b
(Scheme 3). A natural population analysis of 6b shows the negative charge resides mainly on the
exocyclic methine carbon, which is rapidly protonated by ethanol (TS-6b). Experimental
deuterium labeling studies with EtOD show deuterium incorporation at this position, supporting
our proposed mechanism (vide supra). While the enthalpic barrier of TS-6b is negative, the free
energy barrier calculated for an ethanol concentration of 17.12 mol L−1 has a value of 5.9 kcal
mol−1 with respect to 6b. Although we attempted to correct the free energy for the large excess of
solvent molecules, it is still substantially overestimated as the entropic penalty for this step can be
assumed to be negligible.
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The protonation of 6b is directly followed by deprotonation of the primary amino group by the
coordinated ethoxide, which proceeds without a barrier as the resulting zwitterion 7b is
resonance-stabilized. Finally, ring-fused aminal 2b is formed from 7b by intramolecular
nucleophilic attack on the iminium ion. The free energy barrier for this step is very small (3.3
kcal mol−1), resulting in a very short lifetime of 7b. Product formation is substantially exergonic
(−9.4 kcal mol−1) and probably irreversible under the experimental conditions.
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ꢀG/kcal mol–1
7
8
9
29.6
(25.7)
29.7
(16.4)
25.7
(14.4)
25.8
(20.4)
30
20
24.7
(13.1)
trans-
TS-3b
23.8
(21.2)
TS-5b
TS-6b
22.5
(19.0)
cis-TS-3b
TS-7b
16.9
(14.4)
6b
7b
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15.9
(12.7)
cis-5b
4.7
(–7.7)
trans-5b
10
3b
0.0
(0.0)
0
1
1.36
–9.4
(–13.5)
1.43
1.24
1.26
TS-6b
–10
2b
trans-TS-3b
TS-5b
2.34
1.68
–20
1.05
2.30
2.38
TS-7b
cis-TS-3b
1.73
1.05
Figure 2. Gibbs free energy profile for the reaction depicted in Scheme 3. Free energies and
enthalpies in parentheses are given in kcal mol−1 and bond lengths in Å.
Table 1: Free energies (and enthalpies in parentheses) in kcal mol−1 for all intermediates and
transition states (M06-2X/6-31+G(d,p)/SMD(Ethanol)).
product
2x
trans-
TS-3x
cis-
trans-
5x
3x
3.4
TS-3x
cis-5x
11.8
TS-5x
25.1
6x
TS-6x
23.9
7x
TS-7x
17.7
2x
a
b
c
d
e
f
21.9
(10.3)
24.7
22.6
(11.0)
25.7
11.1
(8.8)
15.9
18.1
12.9
-8.6
(-9.1)
4.7
(9.8)
16.9
(21.5)
29.6
(15.8)
23.8
(13.6)
28.0
(11.9)
22.5
(14.2)
25.8
(-17.0)
-9.4
(-7.7)
2.9
(13.1)
29.2
(14.4)
29.2
(12.7)
13.3
(14.4)
15.2
(25.7)
32.1
(21.2)
23.5
(16.4)
26.8
(19.0)
15.9
(20.4)
17.2
(-13.5)
-7.8
(-8.4)
2.9
(18.2)
31.4
(18.8)
30.8
(11.3)
16.2
(11.3)
16.8
(29.9)
32.9
(23.5)
26.0
(17.9)
32.1
(14.9)
21.4
(14.3)
21.9
(-11.4)
-4.6
(-8.0)
0.0
(20.0)
25.3
(20.4)
25.5
(14.1)
15.1
(15.5)
16.3
(30.5)
23.1
(24.6)
14.4
(22.0)
28.5
(19.9)
14.0
(19.0)
15.7
(-7.8)
-11.0
(-15.9)
-13.8
(-17.9)
-6.9
(-11.4)
2.4
(16.2)
25.5
(16.6)
24.2
(13.8)
16.2
(15.0)
17.4
(20.1)
29.8
(12.7)
23.3
(14.8)
32.2
(13.6)
16.4
(12.3)
19.7
(-10.7)
4.7
(16.5)
30.6
(16.7)
31.5
(14.1)
20.3
(15.3)
21.2
(26.2)
27.5
(21.3)
19.0
(18.0)
29.1
(16.0)
22.7
(16.0)
24.1
g
h
i
(-7.3)
4.7
(19.2)
30.6
(20.2)
31.4
(18.7)
20.7
(19.6)
22.3
(24.1)
33.9
(16.9)
28.9
(19.1)
32.8
(20.7)
25.7
(20.4)
27.6
(-11.0)
-9.2
(-7.3)
4.6
(19.4)
32.2
(20.2)
34.1
(19.2)
20.1
(20.0)
21.1
(30.5)
35.6
(26.9)
32.5
(23.2)
37.5
(24.0)
25.4
(24.2)
27.4
(-13.4)
-9.3
(-6.7)
(21.4)
(22.6)
(17.1)
(17.9)
(31.2)
(29.4)
(25.3)
(22.7)
(22.8)
(-13.8)
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Reactions involving pyrrolidine, piperidine and morpholine. Inspection of the reactions of
pyrrolidine with aldehydes 1a and 1b (Scheme 1) reveals dibromo-substitution of the aldehyde to
give better yields after shorter reaction times. A comparison of the calculated free energies
profiles for both reactions (Table 1) shows the reaction of 1a and pyrrolidine to proceed via lower
lying intermediates and transition states. The phenyl ring of aldehyde 1a is electron-deficient and
induces a better charge delocalization into the aromatic system in all intermediates and transition
states following 3a. This effect is most pronounced in 7a, which is stabilized by 9.6 kcal mol−1
relative to 7b. The formation of hemiaminal 3a is also more favorable by 1.3 kcal mol−1 than the
formation of 3b owing to the more electrophilic character of the carbonyl group in 1a.The free
energy difference between the rate-determining transition states TS-5a and TS-5b is 4.5 kcal
mol−1, which is exclusively caused by the change in electronic structure and explains the higher
yield of the reaction involving aldehyde 2a. Piperidine requires higher reaction temperatures and
gives slightly lower yields than pyrrolidine while morpholine gives low yields even at elevated
temperatures (eq 2).
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TS-5a
1.42
1.23
TS-5c
1.43
1.23
Figure 3. An overlay of the geometries of cis-5a and cis-5c (sticks) with transition states TS-5a
and TS-5c (balls and sticks).
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The formation of quinoidal intermediates 5c and 5d is disfavored in comparison to 5a. 5c and 5d
also partly restore the aromaticity of the aryl-ring by adopting a zwitterionic resonance structure,
which involves an exocyclic double bond at the iminium ion. The formation of the latter is less
favorable in six-membered than in five-membered rings (see SI for calculations on model
systems). Free energies of TS-5c and TS-5d are higher than TS-5a, because 5c and 5d require
more distortion to adopt the transition state geometries (Figure 3). This does explain the better
experimental performance of pyrrolidine; however no significant discrimination can be made
between piperidine and morpholine based on the energies of the rate-limiting steps TS-5c and
TS-5d.
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Reactions involving THIQ and THQ. Our experimental results indicate that products 2e and 2g
are obtained under kinetic control, while 2f and 2h represent the thermodynamically stable
products. Transition state energies for TS-5e and TS-5g are lower by 6.4 and 6.7 kcal mol−1 than
those of TS-5f and TS-5h, respectively, confirming the experimental results. This stabilization is
caused by the location of the proton to be abstracted in THIQ, which allows an effective
delocalization of the resulting charge into the aromatic ring in 6e and 6g (Figure 4). However,
products 2e and 2g are less stable than 2f and 2h, respectively, which explains their isomerization
at prolonged reaction times. Furthermore, 2g is predicted to be less stable by 4.1 kcal mol−1 than
2e and thus allows a more facile isomerization.
No product could be obtained at all when THQ was used as an amine instead of THIQ. The high
barrier of TS-5i is in good agreement with this finding and is partly caused by a substantial
distortion required to transform cis-5i to TS-5i. In addition, the reactions to obtain intermediate
cis-5i have a strongly positive reaction free energy (21.1 kcal mol−1) as iminium-like structures
involving THQ are energetically disfavored (see SI), probably due to the conjugation of the
nitrogen lone pair with the aromatic ring.
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–0.17
–0.29
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–0.09
–0.09
6f
6e
–0.28
–0.14
–0.15
–0.06
6g
6h
Figure 4. Structures and carbon charges of THIQ azomethine ylides.
Trapping of 6a by a 1,3-dipolar cycloaddition. The azo-methine ylide 6a could be trapped
experimentally by a 1,3-dipolar cycloaddition with aldehyde 1a. Not surprisingly, the
cycloaddition of these highly polar reactants involves a stepwise mechanism with a zwitterionic
intermediate 21 (Figure 5). Transition state TS-8 for the first bond formation features a distance
of 2.28 Å between the reaction centers while the oxygen and iminium carbon are well separated
(2.86 Å). The calculated barrier of 1.6 kcal mol–1 is significantly lower than any barrier for the
amination reaction cascade and indicates that this reaction is essentially diffusion-controlled.
However, the rate is limited by the low concentration of azomethine ylide 6a, which is readily
protonated by ethanol being present in huge excess. The formation of the zwitterionic
intermediate 21 is exergonic by –8.1 kcal mol–1 and followed by a fast intramolecular ring closure
via TS-9. The total cycloaddition reaction is exergonic by –35.6 kcal mol–1.
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2.86
2.28
TS-8
2.90
1.59
21
2.22
TS-9
Figure 5. Transition states TS-8 and TS-9 and zwitterionic intermediate 21 for the [3+2]
cycloaddition between 1a and 6a (see equation 5). The total reaction is exergonic by –35.6 kcal
mol–1 relative to 1a and 6a.
Conclusions
We have derived a mechanism for the α-amination of nitrogen heterocycles by density functional
theory calculations involving an unanticipated direct transition of hemiaminals 3 to quinoidal
intermediates 5. Our computations are supported by experimental studies including deuterium
labeling and trapping of the predicted azaquinone methide and azomethine ylide intermediates.
According to our calculations, the rate-limiting step of the entire reaction cascade is an
intramolecular proton transfer TS-5; the barrier of this step correlates qualitatively with
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experimental results. Experimental work towards extending the scope of this reaction in
combination with computational predictions is in progress and will be reported in due course.
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Experimental Section
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General Information: Microwave reactions were carried out in a CEM Discover reactor using
sealed 10 mL reaction vessels and temperatures were measured with an infrared temperature
sensor. Silicon carbide (SiC) passive heating elements were purchased from Anton Paar.
Purification of reaction products was carried out by flash column chromatography using Sorbent
Technologies Standard Grade silica gel (60 Å, 230–400 mesh). Analytical thin layer
chromatography was performed on EM Reagent 0.25 mm silica gel 60 F254 plates. Visualization
was accomplished with UV light, potassium permanganate and Dragendorff-Munier stains
followed by heating. Proton nuclear magnetic resonance spectra (1H-NMR) are reported in ppm
using the solvent as an internal standard (CDCl3 at 7.26 ppm, (CD3)2CO at 2.04 ppm). Data are
reported as app = apparent, s = singlet, d = doublet, t = triplet, q = quartet, m = multiplet, comp =
complex, br = broad; and coupling constant(s) in Hz. Proton-decoupled carbon nuclear magnetic
resonance spectra (13C-NMR) are reported in ppm using the solvent as an internal standard
(CDCl3 at 77.0 ppm).
Aminal 2a: A 10 mL round bottom flask was charged with 2-amino-3,5-dibromobenzaldehyde
(0.279 g, 1.0 mmol), absolute ethanol (4 mL) and pyrrolidine (0.246 mL, 3.0 mmol). The mixture
was stirred at reflux under nitrogen for 23 h. After this time the reaction solvent was removed
under reduced pressure and the residue was purified by silica gel chromatography. 2a was
recovered as a white solid in 92% yield (0.305 g) (Rf = 0.19 in hexanes/EtOAc 60:40 v/v); mp:
122–124 ºC; IR (KBr) 3403, 3052, 2971, 2938, 2907, 2839, 1768, 1692, 1575, 1438, 1349, 1258,
1
1119, 980, 927, 861, 747, 722, 637 cm-1; H NMR (500 MHz, CDCl3) 7.37 (d, J = 1.7 Hz, 1H),
6.99 (d, J = 0.9 Hz, 1H), 4.37 (ddd, J = 5.2, 2.8, 0.8 Hz, 1H), 4.23 (br s, 1H), 4.09 (d, J = 16.2 Hz,
1H), 3.78 (d, J = 16.2 Hz, 1H), 2.82–2.75 (comp, 2H), 2.20–2.11 (m, 1H), 2.04–1.87 (comp, 2H),
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1.73 (dddd, J = 12.6, 9.9, 4.2, 2.8 Hz, 1H); 13C NMR (125 MHz, CDCl3) δ 139.6, 132.5, 129.2,
121.7, 109.0, 108.3, 71.3, 49.9, 49.6, 32.7, 21.7; m/z (ESI–MS) 333.0 [M+H]+.
3
4
5
6
7
8
9
Aminal 2b: A 10 mL round bottom flask was charged with 2-aminobenzaldehyde (0.121 g, 1.0
mmol), absolute ethanol (4 mL) and pyrrolidine (0.246 mL, 3.0 mmol). The mixture was stirred
at reflux under nitrogen for 72 h. After this time the reaction solvent was removed under reduced
pressure and the residue was purified by silica gel chromatography. 2b was recovered as a white
solid in 73% yield (0.127 g) (Rf = 0.25 in EtOAc/MeOH 95:5 v/v); mp: 63–64 ºC; IR (KBr)
3246, 2966, 2826, 1608, 1585, 1478, 1383, 1255, 749 cm-1; 1H NMR (500 MHz, CDCl3) 7.02
(app t, J = 7.6 Hz, 1H), 6.95 (app d, J = 7.4 Hz, 1H), 6.70 (app dt, J = 7.4, 0.9 Hz, 1H), 6.54 (app
d, J = 7.9 Hz, 1H), 4.17–4.13 (m, 1H), 4.04 (d, J = 15.6 Hz, 1H), 3.90 (d, J = 15.6 Hz, 1H), 3.67
(br s, 1H), 3.03 (app dt, J = 8.8, 5.5 Hz, 1H), 2.68 (app dt, J = 8.8, 5.5 Hz, 1H), 2.18–2.09 (m,
1H), 1.97–2.07 (m, 1H), 1.96–1.87 (m, 1H), 1.66 (app tdd, J = 12.3, 10.2, 4.4 Hz, 1H); 13C NMR
(125 MHz, CDCl3) δ 137.6, 133.3, 128.6, 126.0, 125.5, 125.2, 124.3, 120.0, 118.9, 115.2, 72.4,
51.9, 50.9, 31.9, 21.3; m/z (ESI–MS) 175.1 [M+H]+.
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Aminal 2c: To a stirred solution of 2-amino-3,5-dibromobenzaldehyde (0.279 g, 1.0 mmol) in
isopropanol (4 mL) was added piperidine (0.297 mL, 3.0 mmol). The mixture was heated to 140
°C for 48 h in a sealed tube. After this time the reaction solvent was removed under reduced
pressure and the residue was purified by silica gel chromatography. 2c was recovered as a white
solid in 67% yield (0.232 g) (Rf = 0.28 in Hex/EtOAc 70:30 v/v); mp: 89–92 ºC; IR (KBr) 3405,
2936, 2853, 2771, 1596, 1561, 1486, 1442, 1370, 1351, 1294, 1272, 1190, 1119, 856, 713 cm-1;
1H NMR (500 MHz, CDCl3) 7.36 (d, J = 2.1 Hz, 1H), 6.96 (d, J = 1.4 Hz, 1H), 4.22 (s, 1H), 3.79
(br s, 1H), 3.72–3.59 (comp, 2H), 2.96–2.88 (m, 1H), 2.25–2.15 (m, 1H), 1.95–1.87 (m, 1H), 1.76
(app tt, J = 10.1, 4.9 Hz, 1H), 1.71–1.64 (comp, 2H), 1.63–1.54 (m, 1H), 1.50–1.41 (m, 1H); 13C
NMR (125 MHz, CDCl3) δ 139.1, 132.5, 128.7, 122.3, 108.5, 108.3, 70.2, 56.0, 51.5, 31.9, 25.6.
21.3; m/z (ESI–MS) 347.0 [M+H]+.
Aminal 2d: To a stirred solution of 2-amino-3,5-dibromobenzaldehyde (0.279 g, 1.0 mmol) in
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isopropanol (4 mL) was added morpholine (0.260 mL, 3.0 mmol). The mixture was heated to
140 °C for 48 h in a sealed tube. After this time the reaction solvent was removed under reduced
pressure and the residue was purified by silica gel chromatography. 2d was recovered as a light
brown solid in 15% yield (0.052 g) (Rf = 0.15 in hexanes/EtOAc 80:20 v/v); mp: 156–157 ºC;
IR (KBr) 3344, 2982, 2937, 2901, 2855, 1590, 1492, 1464, 1342, 1315, 1280, 1140, 1121, 1079,
1041, 861, 756, 730 cm-1; 1H NMR (500 MHz, CDCl3) 7.42 (s, 1H), 7.00 (s, 1H), 4.25 (s, 1H),
4.05 (br s, 1H), 3.97 (app d, J = 15.2 Hz, 1H), 3.91–3.77 (comp, 3H), 3.72–3.61 (comp, 2H),
2.91–2.84 (m, 1H), 2.42–2.36 (m, 1H); 13C NMR (125 MHz, CDCl3) δ 138.5, 132.5, 128.8,
121.4, 109.3, 108.9, 69.2, 67.0, 66.9, 54.7, 48.3; m/z (ESI–MS) 349.0 [M+H]+.
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Aminal 2e: To a 10 mL round bottom flask with magnetic stir bar was added 2-amino-3,5-
dibromobenzaldehyde (0.279 g, 1.0 mmol), absolute ethanol (4 mL) and 1,2,3,4-
tetrahydroisoquinoline (0.381 mL, 3.0 mmol). The mixture was stirred at reflux under nitrogen
for 16 h. After this time the solvent was removed under reduced pressure and the residue was
purified by silica gel chromatography. 2e was recovered as a white solid in 96% yield (0.378 g)
(Rf = 0.43 in hexanes/EtOAc 80:20 v/v); mp: 145–147 ºC; IR (KBr) 3408, 3065, 2934, 2899,
2846, 1590, 1480, 1334, 1280, 1234, 1117, 1006, 991, 865, 772, 735, 721, 685 cm-1; 1H NMR
(500 MHz, CDCl3) 7.43 (d, J = 1.7 Hz, 1H), 7.37–7.27 (comp, 3H), 7.22 (app d, J = 7.4 Hz, 1H),
7.07 (s, 1H), 5.28 (d, J = 2.3 Hz, 1H), 4.39 (d, J = 16.2 Hz, 1H), 4.31 (s, 1H), 3.81 (d, J = 16.2
Hz, 1H), 3.19–3.02 (comp, 2H), 2.97–2.86 (m, 1H), 2.77–2.66 (m, 1H); 13C NMR (125 MHz,
CDCl3) δ 138.7, 134.7, 134.5, 132.4, 129.2, 128.8, 128.3, 126.5, 126.4, 121.7, 109.0, 108.7, 69.1,
55.3, 44.5, 29.1; m/z (ESI–MS) 395.0 [M+H]+.
Aminal 2g: To a 10 mL round bottom flask with a magnetic stir bar was added 2-
aminobenzaldehyde (0.121 g, 1.0 mmol), absolute ethanol (4 mL) and 1,2,3,4-
tetrahydroisoquinoline (0.381 mL, 3.0 mmol). The mixture was stirred at reflux under nitrogen
for 48 h. After this time the solvent was removed under reduced pressure, and the residue was
purified by silica gel chromatography. 2g was recovered as a yellow oil in 96% yield (0.227 g)
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(Rf = 0.33 in hexanes/EtOAc 70:30 v/v); IR (KBr) 3387, 3024, 2916, 2837, 2791, 2740, 1725,
1606, 1583, 1487, 1424, 1339, 1305, 1249, 1112, 1044, 1021, 936, 749 cm-1; 1H NMR (500
MHz, CDCl3) 7.36 (dd, J = 7.2, 1.7 Hz, 1H), 7.30–7.23 (comp, 2H), 7.20 (dd, J = 7.2, 1.2 Hz,
1H), 7.07 (app t, J = 7.6 Hz, 1H), 7.01 (app d, J = 7.5 Hz, 1H), 6.77 (app dt, J = 7.4, 1.1 Hz, 1H),
6.58 (app d, J = 8.0 Hz, 1H), 5.16 (d, J = 3.2 Hz, 1H), 4.35 (d, J = 15.8 Hz, 1H), 3.87 (d, J = 15.8
Hz, 1H), 3.86 (br s, 1H), 3.21 (ddd, J = 11.4, 8.3, 4.8 Hz, 1H), 3.06 (ddd, J = 14.0, 8.3, 5.7 Hz,
1H), 2.98 (app td, J = 16.4, 4.8 Hz, 1H), 2.72 (app td, J = 10.9, 5.3 Hz, 1H); 13C NMR (125
MHz, CDCl3) δ 142.3, 135.8, 134.9, 129.3, 128.1, 127.5, 127.3, 126.5, 126.4, 119.8, 118.7,
115.6, 69.7, 56.0, 45.5, 29.4; m/z (ESI–MS) 237.1 [M+H]+.
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Aminal 2h: A 10 mL microwave reaction tube was charged with a 10 x 8 mm SiC passive
heating element, 2-aminobenzaldehyde (0.121 g, 1.0 mmol), n-BuOH (4 mL) and 1,2,3,4-
tetrahydroisoquinoline (0.254 mL, 2.0 mmol). The reaction tube was sealed with a Teflon-lined
snap cap and heated in a microwave reactor at 250 °C (200 W, 80–120 psi) for 2 h. After cooling
with compressed air flow, the reaction solvent was removed under reduced pressure and the
residue was purified by silica gel chromatography. 2h was recovered as a yellow solid in 47%
yield (0.111 g) in addition to 2g (38% yield, 0.089 g). Characterization data for 2h: (Rf = 0.14 in
hexanes/EtOAc 80:20 v/v); mp: 151–153 ºC; IR (KBr) 3356, 3032, 2894, 2750, 1612, 1591,
1491, 1452, 1437, 1390, 1368, 1270, 1141, 1125, 1093, 1020, 746, 723 cm-1; 1H NMR (500
MHz, CDCl3) 7.20–7.11 (comp, 3H), 7.07–7.00 (comp, 2H), 6.98 (app d, J = 7.5 Hz, 1H), 6.73
(app dt, J = 7.5, 1.1 Hz, 1H), 6.50 (dd, J = 8.0, 0.9 Hz, 1H), 4.75–4.68 (m, 1H), 4.38 (d, J = 16.1
Hz, 1H), 4.03 (d, J = 15.1 Hz, 1H), 3.85 (d, J = 16.1 Hz, 1H), 3.78–3.67 (comp, 2H), 3.33 (dd, J
= 16.8, 4.6 Hz, 1H), 2.81 (dd, J = 16.8, 3.1 Hz, 1H); 13C NMR (125 MHz, CDCl3) δ 142.8,
134.0, 130.4, 128.8, 127.5, 127.2, 126.5, 126.3, 126.0, 118.7, 118.4, 114.8, 65.7, 54.9, 49.6, 34.8;
m/z (ESI–MS) 237.1 [M+H]+.
Aminal 2j: A 10 mL microwave reaction tube was charged with a 10 x 8 mm SiC passive
heating element, 2-aminobenzaldehyde (0.121 g, 1.0 mmol), n-BuOH (4 mL) and 2-
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methylpyrrolidine (0.306 mL, 3.0 mmol). The reaction tube was sealed with a Teflon-lined snap
cap and heated in a microwave reactor at 250 °C (200 W, 100–150 psi) for 15 minutes. After
cooling with compressed air flow, the reaction solvent was removed under reduced pressure and
the residue was purified by silica gel chromatography. 2j was isolated as a yellow oil in 66%
yield (0.124 g) (Rf = 0.27 in EtOAc); IR (KBr) 3397, 2970, 1647, 1609, 1493, 1457, 1414, 1354,
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1271, 1215, 1131, 1036, 747 cm-1; H NMR (500 MHz, CDCl3) 7.01 (app t, J = 7.8 Hz, 1H), 6.95
(app d, J = 7.4 Hz, 1H), 6.64 (app t, J = 7.4 Hz, 1H), 6.43 (app d, J = 7.8 Hz, 1H), 4.23 (d, J =
17.0 Hz, 1H), 3.75 (d, J = 17.0 Hz, 1H), 3.59 (br s, 1H), 3.01 (app td, J = 8.4, 4.4 Hz, 1H), 2.75
(app q, J = 8.4 Hz, 1H), 1.98–1.75 (comp, 4H), 1.41 (s, 3H); 13C NMR (125 MHz, CDCl3) δ
142.0, 127.4, 127.1, 117.0, 116.6, 114.0, 73.1, 50.8, 45.3, 39.8, 25.5, 19.8; m/z (ESI–MS) 189.0
[M+H]+.
In addition, compound 2k was isolated as a yellow oil as a mixture of diastereomers in 26% yield
(0.049 g), dr = 54:46 as determined by integration of one set of 1H NMR signals (δmajor 1.26 ppm,
δminor 1.16 ppm) (Rf = 0.45 in EtOAc); IR (KBr) 3386, 2961, 2870, 1608, 1494, 1375, 1302,
1262, 1154, 1041, 747 cm-1; 1H NMR of major diastereomer (500 MHz, CDCl3) 7.08–6.98
(comp, 2H), 6.77 (app dt, J = 7.4, 1.1 Hz, 1H), 6.70–6.64 (comp, 1H), 4.07 (d, J = 13.9 Hz, 1H),
3.99 (br s, 1H), 3.65–3.57 (m, 1H), 3.46 (d, J = 13.9 Hz, 1H), 2.49–2.39 (m, 1H), 2.25–1.97
(comp, 2H), 1.74–1.48 (comp, 2H), 1.26 (d, J = 6.1 Hz, 3H); 13C NMR (125 MHz, CDCl3) δ
143.1, 143.0, 127.4, 127.1, 127.0, 121.7, 119.2, 117.6, 117.2, 116.8, 113.6, 74.2, 70.8, 58.6, 53.3,
52.6, 45.7, 31.0, 30.7, 29.7, 28.8, 19.5, 18.6; m/z (ESI–MS) 189.0 [M+H]+.
Synthesis of aminoaldehyde 11: To a 25 mL round bottom flask with fitted with a magnetic stir
bar was added 2-aminobenzyl alcohol (0.246 g, 2.00 mmol), methanol (6.25 mL), (E)-ethyl 7-
oxohept-2-enoate27a (0.374 g, 2.20 mmol) and acetic acid (0.321 mL, 5.6 mmol). The resulting
solution was cooled to 0 °C in an ice bath and sodium cyanoborohydride (0.189 g, 3.00 mmol)
was added. The solution was allowed to warm to room temperature and was stirred for 1 h, after
which time the reaction was quenched with 5 mL of 5% aq. KHSO4 solution. The product was
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extracted with EtOAc (2 x 10 mL) and the extract was washed with sat. NaHCO3 (1 x 10 mL)
followed by brine (1 x 10 mL). The organic layer was dried over sodium sulfate, filtered and
dried in vacuo. The crude product was purified by silica gel chromatography and ethyl 7-((2-
(hydroxymethyl)phenyl)amino)hept-2-enoate (11') was obtained as a colorless oil in 91% yield
(0.503 g) as a mixture of stereoisomers; ratio E/Z = 3.55:1 (Rf = 0.23 in hexanes/EtOAc 80:20
v/v); Characterization data of the E isomer: IR (KBr) 3391, 2931, 1716, 1652, 1607, 1520, 1456,
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1312, 1192, 1038, 927, 822, 748 cm-1; H NMR (500 MHz, CDCl3) 7.21 (app td, J = 7.8, 1.6 Hz,
1H), 7.04 (dd, J = 7.8, 1.3 Hz, 1H), 6.96 (app dt, J = 15.6, 6.9 Hz, 1H), 6.67–6.62 (comp, 2H),
5.83 (dt, J = 15.6, 1.5 Hz, 1H), 4.63 (s, 2H), 4.17 (q, J = 7.1 Hz, 2H), 3.15 (t, J = 6.9 Hz, 2H),
2.25 (app qd, J = 7.2, 1.4 Hz, 2H), 1.73–1.65 (comp, 2H), 1.64–1.57 (comp, 2H), 1.28 (t, J = 7.1
Hz, 3H); 13C NMR (125 MHz, CDCl3) δ 166.7, 148.7, 147.6, 129.5, 129.0, 124.2, 121.6, 116.2,
110.4, 64.7, 60.2, 43.1, 31.8, 28.8, 25.5, 14.2; m/z (ESI–MS) 278.1 [M+H]+.
A 10 mL round bottom flask with a stir bar was charged with 11' (0.277 g, 1 mmol, ratio of
stereoisomers (E/Z) = 3.55:1), dichloromethane (3.57 mL) and manganese dioxide (0.522 g, 6.00
mmol), and the resulting solution was stirred at room temperature for 20 h. The reaction mixture
was filtered through a pad of celite and rinsed with dichloromethane (3 x 20 mL). The solvent
was removed in vacuo and the residue was purified by silica gel chromatography, yielding both E
and Z isomers. Pure E-isomer 11 was obtained as a bright yellow oil in 62% yield (0.198 g) (Rf =
0.31 in hexanes/EtOAc 90:10 v/v); IR (KBr) 3331, 2984, 2745, 1647, 1521, 1457, 1265, 1040,
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981, 870, 749 cm-1; H NMR (500 MHz, CDCl3) 9.81 (s, 1H), 8.31 (br s, 1H), 7.46 (dd, J = 7.9,
1.4 Hz, 1H), 7.42–7.35 (m, 1H), 6.95 (app dt, J = 15.6, 6.9 Hz, 1H), 6.75–6.63 (comp, 2H), 5.87–
5.81 (m, 1H), 4.18 (q, J = 7.1 Hz, 2H), 3.33–3.19 (m, 2H), 2.36–2.21 (m, 2H), 1.81–1.68 (m, 2H),
1.67–1.56 (m, 2H), 1.28 (t, J = 7.1 Hz, 3H); 13C NMR (125 MHz, CDCl3) δ 193.9, 166.5, 150.7,
148.3, 136.7, 135.8, 121.8, 118.3, 114.7, 110.7, 60.2, 42.1, 31.8, 28.5, 25.5, 14.2; m/z (ESI–MS)
276.3 [M+H]+.
Compound 12: To a 5 mL round bottom flask was added aldehyde 11 (0.25 mmol, 0.069 g),
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absolute ethanol (1 mL) and pyrrolidine (0.75 mmol, 0.062 mL). The resulting mixture was
stirred at reflux for 48 h. The reaction mixture was cooled to room temperature and solvent was
removed in vacuo. The residue was purified via silica gel chromatography (hexanes/EtOAc
80:20 v/v – EtOAc/MeOH/NEt3 74:25:1 v/v/v). Racemic compound 12 was obtained as a tan oil
in 7% yield (0.0060 g) (Rf = 0.44 in hexanes/EtOAc 80:20 v/v); Relative stereochemistry was
determined using 2D NMR and J–coupling analysis; IR (KBr) 3329, 2933, 1717, 1654, 1577,
1522, 1458, 1338, 1160, 1041, 751 cm-1; 1H NMR (500 MHz, (CD3)2CO) 7.08 (app td, J = 7.3,
1.7 Hz, 1H), 6.94 (dd, J = 7.3, 1.7 Hz, 1H), 6.78 (app d, J = 8.2 Hz, 1H), 6.53 (app td, J = 7.3, 3.3
Hz, 1H), 4.28–4.20 (m, 1H), 4.15–4.01 (comp, 3H), 3.47 (app td, J = 10.7, 2.2 Hz, 1H), 2.87 (app
t, J = 12.8, 1H), 2.62–2.54 (dd, J = 10.7, 4.7 Hz, 1H) 2.54–2.47 (m, 2H), 2.40–2.29 (m, 2H),
1.99–1.93 (m, 1H), 1.87–1.81 (m, 1H), 1.71–1.65 (m, 1H), 1.62–1.46 (comp, 6H), 1.25 (t, J = 7.1
Hz, 3H), 1.09–0.99 (m, 1H); 13C NMR (125 MHz, CDCl3) δ 172.2, 145.5, 130.1, 128.7, 115.3,
111.5, 109.7, 60.2, 59.5, 54.7, 51.8, 50.9, 48.0, 33.5, 25.4, 24.9, 23.4, 14.2; m/z (ESI–MS) 327.5
[M–H]+.
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In addition, compound 13 was isolated as a yellow oil in 7% yield (0.0044 g) (Rf = 0.47 in
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hexanes/EtOAc 90:10 v/v); IR (KBr) 3419, 2360, 2090, 1649, 1559, 1540, 1507, 1457 cm-1; H
NMR (500 MHz, CDCl3) 7.32 (s, 1H), 7.18–7.14 (m, 1H), 7.01 (dd, J = 7.4, 1.2 Hz, 1H), 6.65–
6.57 (comp, 2H), 4.45 (dd, J = 10.8, 1.9 Hz, 1H), 4.30–4.19 (m, 2H), 3.94 (app d, J = 13.6 Hz,
1H), 3.07–2.97 (m, 1H), 1.85–1.79 (m, 1H), 1.78–1.65 (comp, 3H), 1.54–1.44 (comp, 2H), 1.33
(t, J = 7.1 Hz, 3H); 13C NMR (125 MHz, CDCl3) δ 165.6, 145.5, 134.9, 132.1, 130.1, 124.5,
120.6, 116.7, 111.2, 60.4, 58.2, 46.7, 28.9, 25.0, 22.1, 14.3; m/z (ESI–MS) 256.3 [M–H]+.
In addition, compound 14 was isolated as a tan oil in 42% yield (0.0370 g) (Rf = 0.20 in
hexanes/EtOAc 70:30 v/v); IR (KBr) 3447, 2936, 2870, 2115, 1732, 1652, 1578, 1521, 1459,
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1200, 1160, 1039, 751 cm-1; H NMR (500 MHz, CDCl3) 9.80 (s, 1H), 8.29 (br s, 1H), 7.46–7.41
(m, 1H), 7.39–7.33 (m, 1H), 6.69–6.62 (comp, 2H), 4.12 (q, J = 7.1 Hz, 2H), 3.29–3.15 (comp,
2H), 3.02–2.93 (m, 1H), 2.64–2.48 (comp, 5H), 2.32 (ddd, J = 14.7, 7.3, 2.3 Hz, 1H), 1.80–1.63
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(comp, 6H), 1.63–1.44 (comp, 4H), 1.24 (t, J = 7.1 Hz, 3H); 13C NMR (125 MHz, CDCl3) δ
193.8, 172.9, 150.8, 136.7, 135.7, 118.2, 114.5, 110.7, 60.3, 58.6, 49.5, 42.4, 36.4, 32.6, 29.2,
23.5, 23.2, 14.2; m/z (ESI–MS) 347.2 [M+H]+.
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In addition, compound 15 was isolated as a tan oil in 22% yield (0.0228 g) (Rf = 0.09 in i-
PrNH2/MeOH/EtOAc 1:25:74 v/v/v); IR (KBr) 3421, 2931, 1733, 1654, 1497, 1458, 1374, 1033
cm-1; 1H NMR (500 MHz, CDCl3) 7.08 (app t, J = 7.8 Hz, 1H), 6.91 (app d, J = 7.3 Hz, 1H),
6.66–6.59 (comp, 2H), 4.13 (q, J = 7.1 Hz, 2H), 3.89 (app t, J = 5.7 Hz, 1H), 3.85 (d, J = 14.6 Hz,
1H), 3.79 (d, J = 14.6 Hz, 1H), 3.34–3.26 (m, 1H), 3.14–3.01 (comp, 2H), 2.99–2.92 (m, 1H),
2.61–2.51 (comp, 5H), 2.34 (dd, J = 14.8, 7.3 Hz, 1H), 2.14–2.06 (m, 1H), 1.99–1.80 (comp, 4H),
1.78–1.72 (comp, 4H), 1.66–1.48 (comp, 4H), 1.43–1.33 (comp, 2H), 1.25 (t, J = 7.1 Hz, 3H);
13C NMR (125 MHz, CDCl3) δ 173.0, 144.7, 144.6, 127.4, 126.9, 121.0, 120.9, 116.6, 112.0,
60.3, 58.8, 52.3, 51.6, 49.6, 47.6, 47.5, 36.5, 36.4, 32.7, 30.6, 27.4, 27.3, 23.5, 23.4, 20.6, 14.2;
m/z (ESI–MS) 400.2 [M+H]+.
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Aminoaldehyde 18: To a 5 mL round bottom flask was added aldehyde 11 (0.25 mmol, 0.069
g), absolute ethanol (1 mL) and piperidine (0.75 mmol, 0.074 mL). The resulting mixture was
stirred at reflux for 96 h. The reaction mixture was cooled to room temperature and solvent was
removed in vacuo. The residue was purified via silica gel chromatography (hexanes/EtOAc
80:20 v/v – EtOAc/MeOH/NEt3 74:25:1 v/v/v). 18 was obtained as an orange oil in 47% yield
(0.0421 g) (Rf = 0.32 in hexanes/EtOAc 50:50 v/v); IR (KBr) 3328, 2933, 2854, 2740, 1731,
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1651, 1610, 1580, 1520, 1462, 1335, 1234, 1159, 1113, 1038, 877, 750, 663 cm-1; H NMR (500
MHz, CDCl3) 9.79 (s, 1H), 8.30 (br s, 1H), 7.44 (app d, J = 7.8 Hz, 1H), 7.36 (app t, J = 7.8 Hz,
1H), 6.75–6.70 (comp, 2H), 4.11 (q, J = 7.1 Hz, 2H), 3.21 (dd, J = 12.7, 6.6 Hz, 2H), 3.02–2.91
(m, 1H), 2.52 (dd, J = 14.2, 6.8 Hz, 1H), 2.49–2.43 (comp, 2H), 2.42–2.35 (comp, 2H), 2.15 (dd,
J = 14.2, 6.8 Hz, 1H), 1.74–1.63 (comp, 2H), 1.61–1.28 (comp, 10H), 1.24 (t, J = 7.1 Hz, 3H);
13C NMR (125 MHz, CDCl3) δ 193.7, 173.3, 150.8, 136.6, 135.7, 118.2, 114.5, 110.7, 61.6, 60.1,
49.4, 42.4, 35.1, 30.7, 28.9, 26.5, 24.9, 24.2, 14.2; m/z (ESI–MS) 361.2 [M+H]+.
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Page 29 of 41
The Journal of Organic Chemistry
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N,O–Acetal 20: A 10 mL microwave reaction tube was charged with a 10 x 8 mm SiC passive
heating element, 2-amino-3,5-dibromobenzaldehyde (0.279 g, 1.0 mmol), PhMe (1 mL) and
pyrrolidine (0.041 mL, 0.5 mmol). The reaction tube was sealed with a Teflon-lined snap cap and
heated in a microwave reactor at 150 °C (200 W, 30–60 psi) for 30 minutes. After cooling with
compressed air flow, the reaction mixture was loaded directly onto a column and purified by
silica gel chromatography. Racemic compound 20 was obtained as a tan solid in 27% yield
(0.0809 g) in addition to 2a (58% yield, 0.0957 g). Characterization data for 20: (Rf = 0.53 in
hexanes/EtOAc 60:40 v/v). Relative stereochemistry was determined using 2D NMR and J–
coupling analysis; mp: 153–156 ºC; IR (KBr) 3438, 3393, 3344, 2961, 1607, 1577, 1570, 1507,
1484, 1458, 1379, 1340, 1286, 1264, 1195, 1170, 1050, 865, 739 cm-1; 1H NMR (500 MHz,
CDCl3) 7.59 (d, J = 2.2 Hz, 1H), 7.39 (d, J = 2.1 Hz, 1H), 6.71 (d, J = 2.2 Hz, 1H), 5.88 (d, J =
2.1 Hz, 1H), 5.01 (br s, 2H), 4.76 (app d, J = 4.6 Hz, 1H), 4.39 (d, J = 9.8 Hz, 1H), 4.34 (br s,
1H), 4.15 (d, J = 9.8 Hz, 1H), 3.11 (app td, J = 8.8, 3.2 Hz, 1H), 2.69 (app q, J = 8.8 Hz, 1H),
2.28–2.17 (m, 1H), 2.08–1.94 (comp, 2H), 1.93–1.84 (m, 1H); 13C NMR (125 MHz, CDCl3) δ
142.6, 137.9, 134.2, 133.2, 132.9, 132.2, 124.5, 117.8, 111.5, 108.5, 108.2, 107.2, 77.3, 64.2,
58.3, 50.0, 33.1, 20.8; m/z (ESI–MS) 611.8 [M+H]+.
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Aminal 2a (partially deuterated according to eq 6): To a 10 mL round bottom flask fitted with
a magnetic stir bar was added 2-amino-3,5-dibromobenzaldehyde (0.279 g, 1.0 mmol), EtOD (4
mL) and pyrrolidine (0.246 mL, 3.0 mmol). The resulting mixture was stirred at reflux for 24 h.
After this time the solvent was removed under reduced pressure and the product was dissolved in
EtOAc (10 mL). This solution was washed with distilled water (3 x 10 mL), dried over sodium
sulfate, filtered and concentrated in vacuo. The resultant residue was purified by silica gel
chromatography. 2a was recovered as a white solid in 77% yield (0.257 g) (Rf = 0.33 in
hexanes/EtOAc 60:40 v/v); IR (KBr) 3404, 3053, 2971, 2937, 2903, 2839, 1591, 1482, 1333,
1277, 1239, 1222, 1132, 880, 724 cm-1; 1H NMR (500 MHz, CDCl3) 7.39 (d, J = 2.1 Hz, 1H),
7.02–6.99 (m, 1H), 4.47–4.36 (m, 1H), 4.24 (br s, 1H), 4.15–4.06 (comp, 1H, 50% D), 3.84–3.75
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