Practical regio- and stereoselective azidation and amination of terminal alkenes

Article abstract of DOI:10.1039/c8ob02734j

There is significant interest in developing more rapid and efficient production of nitrogen-containing allylic compounds, as widely used in various syntheses. This work reports a variety of allylic azides and allylic amines synthesized by an efficient, new one-pot protocol that employs readily available terminal alkenes as starting materials. This method is highly regio- and stereoselective, affording the linear (E)-isomer, under metal-free conditions. This process tolerates several functional groups including halogen-containing molecules; it is general for azides and amine nucleophiles; and, adducts were obtained in good yields.

Full text of DOI:10.1039/c8ob02734j

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Practical regio- and stereoselective azidation and
amination of terminal alkenes†
Cite this: Org. Biomol. Chem., 2018,
16, 9354
Olatunji S. Ojo, Octavio Miranda, Kyle C. Baumgardner and Alejandro Bugarin
*
Received 3rd November 2018,
Accepted 21st November 2018
DOI: 10.1039/c8ob02734j
rsc.li/obc
There is significant interest in developing more rapid and efficient In addition, the synthesis of allylic amines from alkenes has
production of nitrogen-containing allylic compounds, as widely been achieved via C–H oxidation. For example, Stahl reported
used in various syntheses. This work reports a variety of allylic the intramolecular amination of a few alkenes, employing pal-
azides and allylic amines synthesized by an efficient, new one-pot ladium catalysts in combination with molecular oxygen, with
protocol that employs readily available terminal alkenes as starting moderate yields obtained after 24 h reaction time.15 Most
materials. This method is highly regio- and stereoselective, recently, White and coworkers have developed an elegant
affording the linear (E)-isomer, under metal-free conditions. This method for the direct intermolecular amination of terminal
process tolerates several functional groups including halogen-con- alkenes, which utilize Pd(OAc)2, a bis(sulfoxide) ligand, and an
taining molecules; it is general for azides and amine nucleophiles; oxidant (e.g., benzoquinone).16 This method afforded highly
and, adducts were obtained in good yields.
stereoselective allylic amines and allylic sulfonamides directly
from terminal alkenes with moderate to good yields (51–95%),
albeit using stoichiometric amounts of oxidant and long reac-
tion times (24 h to 72 h). Although some of these methods have
advantages such as readily available starting materials, they
often produce mixtures of (E/Z) and linear/branched allylic com-
pounds, especially those that employ transition metal cataly-
sis.16a Furthermore, Topczewski has extensively studied allylic
azides rearrangements,17–21 and he has concluded that this
[3,3]-sigmatropic rearrangement22 provokes mixture of products,
although he also states that small differences in reactivity of the
isomers can result in formation of a single product.23
In light of the above considerations, and despite progress
achieved to date, it is reasonable to say that a milder, metal-
free, regio- and steroselective method to synthesis of nitrogen
containing allylic compounds is still desired. Herein, we report
transition metal-free and efficient azidation and amination
reactions that utilize readily available terminal olefins. Our
one-pot, two-step protocol is compatible with sodium azide
and primary, secondary, and tertiary amine nucleophiles.
Fig. 1 depicts the synthetic method employed for the regio-
and stereoselective synthesis of linear (E) allylic azides.
Among allylic compounds, nitrogen-containing molecules are
extensively employed in a variety of syntheses. This is particu-
larly notable in medicinal chemistry reflecting the abundant
presence of nitrogen moieties in natural products, as well as
numerous biological active compounds.1–3 As such, develop-
ment of a more general synthetic method of both azides and
amines remains an active area of research.1,4 Organic azides
and amines are very important starting materials, intermedi-
ates, and building blocks utilized in numerous organic
transformations.4–6 The present study offers an easier, new
approach to synthesize both allylic azides and amines directly
from terminal alkenes. Currently, allylic azides are typically
synthesized through substitution of alkyl halides with sodium
azide, but this method obviously requires prior synthesis of
the alkyl halides.2,4,7 Alternatively, a few reports have appeared
that employ allylic alcohols as starting materials, using tran-
sition metal catalysis (e.g., Pd,8 Ag,9 & Cu10), Lewis acids,11 or
triphenylphosphine12 to activate the allylic alcohols. Similar
approaches have been developed for synthesis of allylic
amines.13 In addition to the prior described methods, Toste
recently reported a well-designed hydroazidation of allenes to
produce allylic azides, albeit using precious gold as catalyst.14
Department of Chemistry and Biochemistry, University of Texas at Arlington,
Arlington, TX 76019, USA. E-mail: bugarin@uta.edu
†Electronic supplementary information (ESI) available: Experimental details,
procedures, 1H NMR
c8ob02734j
&
13C NMR spectra, and HRMS. See DOI: 10.1039/
Fig. 1 General method for synthesis of linear (E) allylic azides.
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Based on our unoptimized single report, where a vast solvents such as DMF, DMA, CH3CN, toluene, or THF (entries
excess of sodium azide was employed,24a and to test our 13 to 18). Further control experiments revealed that longer
hypothesis of a regio- and stereoselective azidation of olefins, reactions times, stronger bases, or higher temperatures, did
we used allylbenzene (1a) as the model substrate to optimize not increase the expected product yield and, even in some
the reactions conditions, as shown in Table 1. Initially, 1a was cases, diminished yields were observed, presumably due to
subjected to our previously optimized bromination condi- product decomposition and/or azide transposition.23
tions.24b After 10 minutes, 2.1 equivalents of sodium azide Consequently, we decided to employ additives to increase our
(NaN3) were added and the reaction mixture was stirred for 2 h reaction yields, while maintaining mild reaction conditions.
at room temperature. Unfortunately, the reaction did not Notably, addition of 1.0 equiv. of NaI significantly increased
produce the expected allylic azide product (3a) (Table 1, entry the yield of 3a to 69% (entry 20). Other additives such as KI,
1). Therefore, we decided to switch solvents for the second Me3NI, and TBAI promoted the reaction (entries 22 to 24), but
step. This time, the solvent (CHCl3) was removed under reduce the yields were low compared to NaI. Varying the stoichiometry
pressure and dimethylsulfoxide (DMSO) was added, followed of NaI was also studied (entries 20 & 21), however the highest
by the addition of 2.1 equivalents of sodium azide. To our yield (69%) was obtained with 1.0 equiv. of NaI (entry 20).
delight, cinnamyl azide (3a) was obtained in 34% (entry 2). Addition of more than 2.0 equiv. of NaI were not soluble in the
With this promising result on hand, we proceeded to screen reaction mixture and diminished the yield (not shown).
several inorganic bases (entries 3 to 6), and organic bases During reaction optimization, we observed formation of a DBU
(entries 7 to 12), all affording the expected product with yields salt in around 5–10% yield (see the ESI†), which was affecting
fluctuating from 7–51%, using only 1.2 equivalents of NaN3. the reaction yield. Therefore, we utilized the non-nucleophilic
The highest yield (51%) was observed when 1,8-diazabicyclo and expensive phosphazene P2-Et26 as base, with or without
[5.4.0]undec-7-ene (DBU)25 was employed as base (entry 11). NaI, and even for 24 h (entries, 12, 13, & 19). However, the
Next, solvent screening showed that DMSO is superior to other yields never surpassed those using DBU, and new, unidentified
byproducts were observed. Therefore, we decided to select the
low-cost DBU as the base for this transformation.
Having optimized reaction conditions for the direct conver-
sion of allylbenzene 1a to cinnamyl azide 3a, we next explored
the scope of the one-pot two-step protocol, as shown in
Table 2. A variety of terminal olefins (1a–1n) were initially
treated with bromine at 0 °C in chloroform.24 The reaction was
monitored by thin layer chromatography, and once it was
deemed complete (∼10 min), the volatiles were removed under
reduce pressure. Important to note that addition of 2.0 equiva-
lents of bromine were required for 2-allyl thiophene (1l) since,
it also underwent a selective aromatic bromination reaction to
afford a tribrominated adduct (Table 2, entry 12). In addition
to monosubstituted olefins, we employed the disubstituted
α-methylstyrene (1n) (Table 2, entry 14). In general, the dibro-
mination process was a straightforward transformation and
tolerated a comprehensive set of substrates. To the crude
dibrominated adducts, DMSO, NaN3, and DBU were added
sequentially and the reaction mixture was stirred for 2 h, at
room temperature, to further explore the scope of this one-pot,
two-step protocol (Table 2). All reactions were quenched after
2 h for comparative purposes, although small amounts of
dibrominated intermediate were still present. In all cases, the
reaction proceeds smoothly, in the presence of DBU and the
N-nucleophile (NaN3),27 to afford the corresponding allyl
azides adducts, in good yields. To make our study more rele-
vant, two sets of reaction conditions were investigated for every
substrate, one in the absence of NaI and the second with 1.0
equiv. of NaI. Both yields are presented in Table 2, wherein the
yields with NaI are reported within parenthesis. For allyl
benzene, the system is very efficient (entry 1). The reaction of
para halogenated allyl benzenes delivered the products in 62%
yield (F), 58% yield (Cl), and 50% yield (Br) (entries 2 to 4).
The adduct of electron-poor p-CF3 allyl benzenes is obtained
Table 1 Optimization of reactions conditionsa
NaN3
(equiv.)
Base
(1.1 equiv.)
Yieldb
(%)
Entry
Additive
Solvent
1
2
3
4
5
6
7
8
2.1
2.1
1.2
1.2
1.2
1.2
1.2
1.2
1.2
1.2
1.2
1.2
1.2
1.2
1.2
1.2
1.2
1.2
1.2
1.2
1.2
1.2
1.2
1.2
2.1
None
None
Na2CO3
K2CO3
Cs2CO3
KOtBu
Et3N
i-Pr2EtN
2,6-Lutidine
DMAP
DBU
P2-Et
P2-Et
DBU
DBU
DBU
DBU
DBU
P2-Et
DBU
DBU
DBU
DBU
DBU
None
None
None
None
None
None
None
None
None
None
None
None
None
None
None
None
None
None
NaId
CHCl3
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMF
0
34
28
30
35
7
35
28
28
34
51
50
61c
42
37
15
<5
0
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
DMA
MeCN
Toluene
THF
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
51
69
60
47
48
42
41
NaId
NaIe
KId
Me3NId
TBAId
NaId
None
a Reactions were carried out with 1a (0.4 mmol, 47 mg, 1 equiv.), Br2
(0.44 mmol, 70 mg, 23 μL, 1.1 equiv.), in 1.0 mL of CHCl3 at 0 °C for
10 min. Then, the volatiles were removed. To the crude reaction
mixture was added 1.0 mL of solvent, NaN3 (1.2 equiv.), and base (1.1
equiv.), at 22 °C and stirred for 2 h. b Isolated yields using silica gel
flash chromatography. c Stirred for 24 h. d 1.0 equiv. of additive was
employed. e 0.5 equiv. of NaI was employed.
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Table 2 Substrate scope of the one-pot protocola
Entry
Olefin #
Allyl azide #
Yieldb (%)c
1
2
3
4
5
6
7
8
1a
1b
1c
1d
1e
1f
R=
R=
H
F
Cl
Br
CF3
Me
Me
Et
3a
3b
3c
3d
3e
3f
51 (69)
53 (62)
47 (58)
40 (50)
53 (67)
44 (58)
53 (62)
49 (58)
1g
1h
3g
3h
9
1i
3i
45 (55)
10
11
1j
3j
35 (52)
42 (53)
1k
3k
12d
13
1l
1m
R=
H
Br
3l
3m
52 (61)
48 (57)
14
1n
3n
40 (47)
a Reactions were carried out with alkene 1 (0.4 mmol, 1 equiv.), bromine (0.44 mmol, 70 mg, 23 μL, 1.1 equiv.), in 1.0 mL of CHCl3 at 0 °C for
10 min. Then, the volatiles were removed. To the crude reaction mixture was added 1.0 mL of DMSO, NaN3 (0.48 mmol, 31.2 mg, 1.2 equiv.), and
DBU (0.44 mmol, 65.7 μL, 1.1 equiv.), at 22 °C and stirred for 2 h. b Isolated yields, based on 1, using silica gel flash chromatography. c Isolated
yields when 1.0 equiv. of NaI was added. d 2.0 equiv. of bromine were employed.
in 67% yield and p-Me in 58% (entries 5 and 6). The two ortho no other conditions were observed to be superior (Table 3). For
substituted derivatives afforded 62% and 58% yield (entries 7 instance, DBU was still required to produce higher yields, even
and 8). Allylpentafluorobenzene (1i) gave lower yield (55%), in the presence of excess of the amines. The scope for primary
perhaps due to steric effects of the two ortho groups. The meta amine nucleophiles included aliphatic and aromatic amines.
methoxy derivative showed a 52% yield (entry 10), while 1-allyl Hexyl amine gave the adduct 4a in 43% yield and 51% when
naphthalene afforded product 3k in 53% yield (entry 11). NaI was used (Table 3, entry 1). Electron-rich and electron-
Additionally, this NaN3/DBU/NaI mixture afforded good yield poor aromatic amines were also good substrates for this trans-
for the heteroaromatic (thiophene) halogenated molecules formation with yields up to 64% (Table 3, entries 2 to 5). In
(entries 12 and 13). An interesting example was the reaction of addition, secondary amines afforded their respective tertiary
α-methylstyrene (1n), which gave 47% yield. The lower yield allylamines in good yields, after 2 h (entries 6 to 11). Hindered
obtained in attributed to the lack of benzylic protons in the secondary amines28 such as diisopropylamine and isopropyl-
starting material (entry 14). It is especially interesting to note benzylamine were also tolerated and afforded the desired pro-
that functional groups, such as aryl halides, which may be pro- ducts in 56% and 50% yield, respectively (entries 6 and 9). To
blematic with palladium-catalyzed methods,13b are well toler- our delight, diallylamine was an excellent substrate with 61%
ated. Overall, the data provided in Table 2 provide solid evi- yield (entry 7). This helps to emphasize the wide-ranging scope
dence for the generality of our new approach.
of this method, which even provides easy access to triallyl-
In our search to expand our discovered method to other amine (4g) under mild reaction conditions. To our gratification,
nitrogen-containing allylic compounds. We next evaluated the N-methylaniline was also compatible and afforded the allylic
scope and synthetic utility of the DBU-mediated regio- and amination adduct in 59% yield and the electron-rich benzyl
stereoselective amination reaction of allylbenzene 1a with amine in 56% yield (entries 8 and 10). Likewise, and to further
primary and secondary amines, the results of which are pre- demonstrate the power of our strategy, 1-benzhydrylpiperazine
sented in Table 3. Although, we end up using the same was used as the amine nucleophile to synthesis the antihist-
optimal reaction condition from the above azidation reactions amine and calcium channel blocker; cinnarizine29 in 50%
(Table 2), it is important to note that an extensive optimization yield from commercially available starting materials, in a one-
was also conducted for this amination approach. Interestingly, pot procedure (entry 14).
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Table 3 Scope of aminesa
Entry
Amine
Product #
Yieldb (%)c
Primary amines
1
43 (51)
55 (64)
2
3
Scheme 1 Synthetic utility of two allylic azides.
45 (56)
49 (60)
36 (48)
4
5
quantitative conversion to 1,2-dibromohexane was observed as
expected. Unfortunately, upon addition of base (DBU) and
nucleophile (NaN3) in DMSO the expected product was not
detected, instead a mixture of 2-bromohexene and 2-azido-
hexene together with small amounts of unidentified DBU adducts
were observed.30 This implies that benzylic (acidic) protons are
required to favor the formation of allylic compounds.30
Secondary amines
6
47 (56)
50 (61)
To validate the synthetic utility and feasibility of this syn-
thetic protocol, reactions were carried out on gram-scales
using a one-pot procedure, for substrates 1a and 1b. To our
delight, the gram-scale procedure didn’t compromise the
expected yields, as the product 3a and 3b were isolated in 63%
and 54% yields (Scheme 1, middle). With large amounts of
cinnamyl azide 3a and 4-fluorocinamyl azide 3b on hand, we
select four different synthetic manipulations to highlight the
utility of the adducts produced by our reported DBU mediated
azidation reaction. First, both allylic azides 3a and 3b were
reduced to allylic amines 5a and 5b employing Lindlar catalyst
(Scheme 1, top-left).31 Second, allylic azides 3a and 3b were
reacted with dimethyl 2-butynedioate to afford the click
adducts 6a and 6b in excellent yields (Scheme 1, top-right).32
Third, the same 3a and 3b were chemically exposed to mCPBA
7
8
9
46 (59)
40 (50)
10
11
43 (56)
39 (50)
a Reactions conditions: Allylbenzene 1a (0.4 mmol, 1 equiv.), bromine to afford epoxides 7a and 7b (Scheme 1, bottom-left).33 Finally,
(0.44 mmol, 70 mg, 23 μL, 1.1 equiv.), in 1.0 mL of CHCl3 at 0 °C for
10 min. Then, the volatiles were removed. To the crude reaction
mixture was added 1.0 mL of DMSO, amine (0.48 mmol, 1.2 equiv.),
compounds 3a and 3b were reacted under reductive amination
conditions to afford the secondary aliphatic amines 8a and 8b
and DBU (0.44 mmol, 65.7 μL, 1.1 equiv.), at 22 °C and stirred for 2 h. in good yields, 82% and 80%, respectively (Scheme 1, bottom-
b Isolated yields, based on 1a, using silica gel flash chromatography.
right).34 These symbolic transformations demonstrate the
extensive versatility of allylic azides in organic synthesis.
c Isolated yields when 1.0 equiv. of NaI was added.
Azide precautions: Organic and inorganic azides are known
to be high-energy materials and explosions have been reported
On the basis of experimental observations30 and our pre-
with their use. All the azides reported herein were synthesized
vious work,24 we propose that after alkene dibromination, a
without incident; however, several precautions were taken.
First, all azides synthesized herein have a C/N ratio of ≥3 : 1.
Second, reactions with more than 1 mmol of azide were placed
stereoselective E2 elimination, enabled by DBU and followed
by a SN2 displacement, occurs. Additional details for the high
behind safety shields both in the fume hood and during rotary
regio- and stereoselectivity observed, together with the struc-
evaporation.4
tural elucidation of the byproducts obtained from the “nucleo-
philic” bases [DBU (S-2), DABCO (S-3), and pyridine (S-4)] and
limitations (i.e., use of 1-hexene as starting material) are
described in the ESI.† 30 For example, when DBU, DABCO, and
pyridine were reacted with trans-cinnamyl bromide, their
Conclusions
respective quaternary ammonium salts were obtained in 90%, In conclusion, we are reporting a milder new alternative to
98%, and 78% yield, respectively.30 On the other hand, when linear (E) allylic azides and amines obtained directly from
1-hexene was subjected to the standard reaction conditions, a terminal olefins. The method is general in scope for the syn-
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thesis of allylic azides, primary amines, secondary amines, ter- 12 L. Rokhum and G. Bez, J. Chem. Sci., 2012, 124, 687–691.
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products. We believe this new methodology provides a milder,
less expensive, more selective, and more straightforward
S. L. Shevick and F. D. Toste, Angew. Chem., Int. Ed., 2016,
55, 6079–6083.
alternative for rapidly building nitrogen-containing organic 15 A. B. Weinstein and S. S. Stahl, Angew. Chem., Int. Ed.,
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16 (a) S. A. Reed, A. R. Mazzotti and M. C. White, J. Am. Chem.
Soc., 2009, 131, 11701–11706; (b) R. Ma and M. C. White,
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Conflicts of interest
17 A. A. Ott, M. H. Packard, M. A. Ortuño, A. Johnson,
V. P. Suding, C. J. Cramer and J. J. Topczewski, J. Org.
Chem., 2018, 83, 8214–8224.
There are no conflicts to declare.
18 M. R. Porter, R. M. Shaker, C. Calcanas and
J. J. Topczewski, J. Am. Chem. Soc., 2018, 140, 1211–
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Acknowledgements
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We are grateful to the University of Texas at Arlington for sup-
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Donors of the American Chemical Society Petroleum Research
Fund for partial support of this research (grant # 58261-ND1).
Furthermore, the authors are thankful to the Shimadzu Center
for MS data collection and the NSF grants (CHE-0234811 and
CHE-0840509) for additional instrumentation.
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