Intermolecular Hydroamination of 1,3-Dienes to Generate Homoallylic Amines

Article abstract of DOI:10.1021/jacs.7b09188

We report a Rh-catalyzed hydroamination of 1,3-dienes to generate homoallylic amines. Our work showcases the first case of anti-Markovnikov selectivity in the intermolecular coupling of amines and 1,3-dienes. By tuning the ligand properties and Br?nsted acid additive, we find that a combination of rac-BINAP and mandelic acid is critical for achieving anti-Markovnikov selectivity.

Full text of DOI:10.1021/jacs.7b09188

Communication
pubs.acs.org/JACS
Intermolecular Hydroamination of 1,3-Dienes To Generate
Homoallylic Amines
Xiao-Hui Yang, Alexander Lu, and Vy M. Dong*
Department of Chemistry, University of California, Irvine, Irvine, California 92697, United States
*
S Supporting Information
Our laboratory recently rendered the hydroamination of
,3-dienes enantio- and regioselective. In this previous study,
ABSTRACT: We report a Rh-catalyzed hydroamination
of 1,3-dienes to generate homoallylic amines. Our work
showcases the first case of anti-Markovnikov selectivity in
the intermolecular coupling of amines and 1,3-dienes. By
tuning the ligand properties and Brønsted acid additive, we
find that a combination of rac-BINAP and mandelic acid is
critical for achieving anti-Markovnikov selectivity.
1
we generated allylic amines by using a [Rh(COD)OMe] /
2
6
m,11
JoSPOphos catalyst and triphenylacetic acid additive.
In this communication, we focused on achieving a 1,2-anti-
Markovnikov addition for the synthesis of homoallylic amines.
To begin this study, we chose indoline and isoprene as
coupling partners. Isoprene, a common building block for
12
polymers, derives from thermal cracking of naphtha/oil
13
and biosynthesis by many organisms. We aimed to transform
this feedstock into the corresponding homoallylic amine by
studying the effects of bidentate phosphine ligand and acid
combinations. Table 1 summarizes our most relevant findings.
Because of its use in alkyne hydroamination, phthalic acid
was chosen as the initial acid for screening different
oined in the 19th century, the concept of Markovnikov
selectivity remains relevant today in classifying and
1
C
developing regioselective transformations of olefins, including
2
hydroamination. Though hydroamination of alkenes typically
3
furnishes the Markovnikov product, there have been break-
14
throughs in reversing the regioselectivity to obtain the anti-
bisphosphine ligands. When using dppm, a small bite angle
4
15
Markovnikov isomer. However, the intermolecular hydro-
ligand, we observe 1,2-Markovnikov addition product A as the
5
amination of 1,3-dienes has been limited to producing allylic
major product (entry 1). Increasing the bite angle of the ligand
decreases the reactivity and regioselectivity, leading to mixtures
of A, B, C, and D (entries 2−4). Switching to a ferrocene-
backbone ligand, dppf, we observed homoallylic amine C as
the major product in 33% NMR yield (entry 5). When using
rac-BINAP as ligand, the yield and regioselectivity of homoallylic
amines through 1,2-addition or 1,4-addition, presumably due to6
the intermediacy of a stabilized π-allyl-metal (Figure 1A, top).
1
amine C increases to 74% and 19:1 rr by H NMR analysis
(
entry 6). Rh(COD) SbF and [Rh(COD)OMe] precatalyst
2 6 2
give similar results with [Rh(COD)Cl] (entry 7 and 8).
2
In accordance with Hartwig’s observations, the use of palladium
as the catalyst yields 1,4-addition product D as the major isomer
6b
(
entry 9). These results showcase an intermolecular hydro-
amination of isoprene where regioselectivity is controlled by
16
catalyst choice.
With the optimal ligand in hand, we investigated the effect of
the acid additive. No product was observed in the absence of acid
(
entry 10). Diphenyl phosphate was successfully used to form
17
C−C bonds in the hydrofunctionalization of alkynes. There-
fore, acids with similar pK to diphenyl phosphate and phthalic
acid were examined. As pK values are solvent dependent, the
a
Figure 1. Inspiration for anti-Markovnikov hydroamination of
18
a
1
,3-dienes for synthesis of homoallylic amines.
pK of the acids in DCE were determined using UV−vis
a
19
spectroscopy. In general, higher yields are obtained with
Homoallylic amines are valuable synthetic building blocks
that are typically generated by coupling imines to organometallic
reagents (Figure 1B). As a complement to using stoichiomet-
ric organometallics, we aimed to use simple dienes by develop-
ing a catalytic and regioselective hydroamination (Figure 1A,
bottom). Herein, we disclose the first intermolecular hydro-
amination of 1,3-dienes to proceed with anti-Markovnikov
selectivity. Our Rh-catalyst transforms conjugated dienes
into homoallylic amines with high regiocontrol and atom
stronger acids (c.f., entries 11−13); mandelic acid (A3) gives
20b
7
81% yield and the best regioselectivity.
With this protocol, we examined the coupling of various
1
,3-dienes 1 with indoline 2a (Table 2). In general, the hydro-
amination proceeds with 18:1 to >20:1 regioselectivities and
2−83% yields for various 2-aryl-1,3-butadienes (3aa−3ia).
5
8
,9
Received: August 29, 2017
1
0
economy.
©
XXXX American Chemical Society
A
DOI: 10.1021/jacs.7b09188
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
Table 1. Catalyst and Acid Effects on Regioselective
Hydroamination of Isoprene
Table 2. Anti-Markovnikov Hydroamination of Various
a
1,3-Dienesa
yield
rr
b
c
d
entry
precatalyst
ligand
dppm
acid
(%)
(A:B:C:D)
1
2
3
4
5
6
7
8
9
[Rh(COD)Cl]2
[Rh(COD)Cl]2
[Rh(COD)Cl]2
[Rh(COD)Cl]2
[Rh(COD)Cl]2
[Rh(COD)Cl]2
Rh(COD) SbF
A2
A2
A2
A2
A2
71
20
14
15:<1:<1:1
2:2:1:4
dppe
dppp
dppb
dppf
2:1:1:3
<10
33
ND
<1:<1:5:1
<1:<1:19:1
1:1:>20:1
1:1:>20:1
1:1:1:>20
ND
rac-BINAP A2
rac-BINAP A2
74
71
2
6
[Rh(COD)OMe]2 rac-BINAP A2
69
Pd(PPh3)4
none
A2
84
10
11
12
13
[Rh(COD)Cl]2
[Rh(COD)Cl]2
[Rh(COD)Cl]2
[Rh(COD)Cl]2
rac-BINAP none
rac-BINAP A1
rac-BINAP A3
rac-BINAP A4
NR
17
1:1:>20:1
1:1:>20:1
<1:<1:13:1
81
79
a
Reaction conditions: 1 (0.3 mmol), 2a (0.2 mmol), [Rh(COD)Cl]2
a
Reaction conditions: indoline (0.1 mmol), isoprene (0.5 mmol),
M] (5 mol %), ligand (5 mol %), acid (50 mol %), DCE (0.2 mL),
(2.5 mol %), rac-BINAP (5 mol %), A3 (50 mol %), DCE (0.4 mL),
[
80 °C, 15 h. Isolated yields. Regioselectivity ratio (rr) is the ratio of
b
c
1
1
24 h. pK in DCE and pK in water in parentheses. H NMR yields
3 to 4, which is determined by H NMR analysis of reaction mixture.
a
a
b
of major isomer using 1,3,5-trimethoxybenzene as the internal
standard. Regioselectivity determined by H NMR analysis of
Using Rh(COD) SbF (5.0 mol %) instead of [Rh(COD)Cl] .
2
6
2
d
1
reaction mixture.
Substrates with an electron-donating group on the aryl ring show
slightly higher reactivity than those with an electron-with-
drawing group. When the aryl ring is ortho-substituted, the yield
is lowered to 60% (3ba). In the case where the aryl ring contains
an electron-withdrawing trifluoromethyl group (3ha), the regio-
selectivity is 18:1 rr. The transformation also proceeds with
alkyl substituted dienes. 2-Cyclohexyl-1,3-butadiene undergoes
hydroamination with 73% yield (3ja). Myrcene, a readily avail-
able monoterpene, furnishes the homoallylic amine (3ka) in
Next, we examined the hydroamination of 2-phenyl-1,3-
butadiene 1a with different amines 2 (Table 3). Indolines
provide the hydroamination products (3ab−3ai) with 62−89%
yield and 8:1 to >20:1 anti-Markovnikov selectivity. Indolines
with electron-donating groups show higher regioselectivities
than those with electron-withdrawing groups. The hydro-
amination of sterically hindered 2-methyl-indoline gives
homoallylic amine (3ab) in 62% yield and 10:1 regioselectivity.
Anilines bearing either electron-donating or electron-with-
drawing groups both undergo hydroamination with 40−63%
yields (3aj−3al). Hydroamination of the acyclic amine
N-methylaniline yields the product 3am in 47% yield with
3:1 regioselectivity. 1,2,3,4-tetrahydroquinoline can be used
for hydroamination with 45% yield and 4:1 selectivity for the
anti-Markovnikov product (3an). Under our hydroamination
conditions, morpholine is an effective amine partner and pro-
vides the homoallylic amine 3ao in 79% yield with >20:1 rr.
Identifying a catalyst that can enable coupling with more
challenging amines (such as primary alkyl amines and acyclic
78% yield and >20:1 rr. The hydroamination proceeds efficiently
for substrates bearing ether and amide groups and provides the
corresponding products in 74% yield (3la) and 87% yield (3ma)
with >20:1 regioselectivities.
Transformations of 1,2-disubstituted dienes also proceed
well. Hydroamination of the fused ring substrates provide
the homoallylic amines in 70% and 73% yields (3na and 3oa)
with >20:1 regioselectivities. In addition, we obtain the
homoallylic amines 3pa and 3qa from acyclic 1,2-disubstituted
dienes in 73% and 53% yields with >20:1 regioselectivities
by using Rh(COD) SbF as precatalyst. When using the mixture
2
6
of (Z)- and (E)-β-ocimene isomers, we observe that only the
E)-β-ocimene is transformed to give homoallylic amine (3ra)
in 51% yield (7 equiv diene used); (Z)-β-ocimene is unreactive
eq 1).
(
(
B
DOI: 10.1021/jacs.7b09188
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Journal of the American Chemical Society
Communication
Table 3. Anti-Markovnikov Hydroamination Using Various
Aminesa
Figure 2. Proposed mechanism for anti-Markovnikov selectivity.
hydroamination (eq 1), we propose that s-cis conformation of
23
the 1,3-diene is necessary for coordination to the catalyst.
Because of the steric clash between the R substituent and the
catalyst, the hydride insertion is disfavored. In line with this idea,
we observe only Markovnikov addition using substrates bearing
hydrogen at the 2-position, regardless of the catalyst used (eq 2).
When the hydroamination was performed with deuterated
indoline d-2a, the deuterium is incorporated into the 3-position
of product 3, not the recovered diene (eq 3). This isotopic
labeling result helps us rule out mechanisms that involve
reversible diene insertion into the Rh-hydride.
a
Reaction conditions: 1a (0.3 mmol), 2 (0.2 mmol), [Rh(COD)Cl]2
(2.5 mol %), rac-BINAP (5 mol %), A3 (50 mol %), DCE (0.4 mL),
8
0 °C, 15 h. Isolated yields. Regioselectivity ratio (rr) is the ratio of
1
3
to 4, which is determined by H NMR analysis of reaction mixture.
b
Using Rh(COD) SbF (5 mol %) instead of [Rh(COD)Cl] .
2
6
2
dialkyl amines) is still an ongoing challenge that needs to be
2
0c
solved.
We reason that the anti-Markovnikov selectivity results from a
synergy between the substrate and catalyst structure. To probe
the mechanism, we studied the kinetic profile for coupling
1
,3-diene 1a and indoline 2a. When using [Rh(COD)Cl]2
precatalyst, we observed a half-order dependence on rhodium
precatalyst, which suggests that breaking up the Rh-dimer is a
slow step under our optimal conditions. Because this catalyst
resting state is off-cycle, we switched to using monomeric
Rh(COD) SbF precatalyst to probe the mechanism further.
2
6
On the basis of both literature precedence and the following
mechanistic studies, we propose the mechanism depicted in
Figure 2. Rhodium precatalyst reacts with BINAP ligand to form
the active catalyst I, which is the resting state as determined by
In accordance with anti-Markovnikov hydroamination of
24
alkenes using directing groups, we imagine that the amine
attacks the activated 1,3-diene from the less substituted position
to generate Rh-π-allyl intermediate V. Reductive elimination
of V at the 3-position gives homoallylic amine 3. This step is
related to Rh-catalyzed alkyne or allene hydrofunctionalizations,
1
31
H and P NMR. The amine and acid undergo proton transfer
−
to form ammonium-ion 5 and carboxylate anion (X ). Using
2
1
HypNMR, the equilibrium constant for this process was
which tend to favor the branched product after reductive
18
14,17,25
measured to be K = 10.5. Ligand exchange of COD with car-
elimination.
We reason that electron rich amines improve
eq
boxylate anion results in complex II. In the turnover-limiting
step, the ammonium-ion 5 oxidatively adds to the Rh(I)-
complex II to generate Rh(III)-hydride III. In support of the
proposed turnover-limiting step, we observe a first order
dependence on the Rh-catalyst and a first order dependence
on the ammonium-ion 5. Moreover, we see a zeroth-order
dependence on the diene, which means its involvement occurs
regioselectivity by promoting reductive elimination at the
3-position rather than 1-position.
Hydroamination of conjugated dienes represents an attrac-
tive way to transform dienes into amine building blocks, but
achieving regioselective hydroamination is challenging given
the extended π-system. To date, there are few reports of func-
tionalizing the terminal carbon of 1,3-dienes through 1,2-hydro-
20d
5b
after the turnover-limiting step.
The 1,3-diene can exchange with the amine to form η -Rh-
hydride intermediate IV, which is analogous to a Co-hydride
functionalization. Modification of the substrate structure has
4
been used to achieve challenging transformations, such as the
2
6
regiodivergent arylboration of 1,3-dienes. Through key
interactions between the catalyst and the 2-substitution of the
substrate, we were able to achieve a 1,2-anti-Markonvikov
22
intermediate implicated in the hydrovinylation of 1,3-dienes.
Because (E)-β-ocimene (and not the Z isomer) undergoes
C
DOI: 10.1021/jacs.7b09188
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Journal of the American Chemical Society
Communication
hydroamination of dienes to access homoallylic amines. Future
studies are warranted to better understand the origin of this
regiocontrol. Insights from this communication will guide the
invention of new diene hydrofunctionalizations.
F.; Yoshifuji, M. Angew. Chem., Int. Ed. 2001, 40, 4501. (d) Pawlas, J.;
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(
́
ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/jacs.7b09188.
■
10, 2919. (i) Brinkmann, C.; Barrett, A. G. M.; Hill, M. S.; Procopiou, P.
A. J. Am. Chem. Soc. 2012, 134, 2193. (j) Banerjee, D.; Junge, K.; Beller,
M. Angew. Chem., Int. Ed. 2014, 53, 1630. (k) Goldfogel, M. J.; Roberts,
C. C.; Meek, S. J. J. Am. Chem. Soc. 2014, 136, 6227. (l) Banerjee, D.;
Junge, K.; Beller, M. Org. Chem. Front. 2014, 1, 368. (m) Yang, X.-H.;
Dong, V. M. J. Am. Chem. Soc. 2017, 139, 1774. For select examples of
intramolecular hydroamination of 1,3-dienes, see: (n) Hong, S.;
Kawaoka, A. M.; Marks, T. J. J. Am. Chem. Soc. 2003, 125, 15878.
(o) Shapiro, N. D.; Rauniyar, V.; Hamilton, G. L.; Wu, J.; Toste, F. D.
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*
S
Experimental procedures and spectral data for all new
compounds (PDF)
AUTHOR INFORMATION
Corresponding Author
dongv@uci.edu
■
*
(
7) For select reviews on homoallylic amine synthesis by imine
allylation, see: (a) Denmark, S. E.; Fu, J. Chem. Rev. 2003, 103, 2763.
(b) Yus, M.; Gonzalez-Gomez, J. C.; Foubelo, F. Chem. Rev. 2011, 111,
774.
8) For intramolecular hydroamination of 1,3-dienes giving
ORCID
Vy M. Dong: 0000-0002-8099-1048
Notes
The authors declare no competing financial interest.
́
́
7
(
homoallylic amines, see: (a) Stubbert, B. D.; Marks, T. J. J. Am.
Chem. Soc. 2007, 129, 4253. (b) Yamamoto, H.; Sasaki, I.; Shiomi, S.;
Yamasaki, N.; Imagawa, H. Org. Lett. 2012, 14, 2266. (c) Pierson, J. M.;
Ingalls, E. L.; Vo, R. D.; Michael, F. E. Angew. Chem., Int. Ed. 2013, 52,
13311.
ACKNOWLEDGMENTS
Funding provided by UC Irvine, the National Institutes of
Health (GM105938) and the National Science Foundation
■
(
CHE-1465263). We thank Materia Inc. for providing Grubbs
(9) In general, functionalization of the distal carbon through 1,2-
addition into terminal 1,3-dienes is rare, and Markovnikov selectivity is
useful for distinguishing between these two modes of 1,2-addition.
catalyst C627, which was used to prepare 1,3-dienes 1a−1j.
(
(
10) Trost, B. M. Science 1991, 254, 1471.
11) For a complementary approach using Pd-catalysis, see: Adamson,
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DOI: 10.1021/jacs.7b09188
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