Exploiting Alkylquinone Tautomerization: Amine Benzylation

Article abstract of DOI:10.1021/acs.orglett.6b01629

A general protocol for the synthesis of benzylic amines via side-chain amination of alkylquinones is reported. The reactions are initiated by the tautomerization of an alkylquinone to the corresponding quinone methide, which is subsequently trapped in situ by an amine nucleophile. This process is promoted by tertiary amines in protic solvents under mild conditions and is compatible with many functional groups. 1,2- and 1,4-benzoquinones, as well as naphthoquinones, participate in this reaction using a wide range of primary and secondary amines/anilines. The synthetic utility of this transformation is also explored.

Full text of DOI:10.1021/acs.orglett.6b01629

Letter
pubs.acs.org/OrgLett
Exploiting Alkylquinone Tautomerization: Amine Benzylation
Luis M. Mori-Quiroz and Michael D. Clift*
Department of Chemistry, The University of Kansas, 2010 Malott Hall, 1251 Wescoe Hall Drive, Lawrence, Kansas 66045, United
States
S
* Supporting Information
ABSTRACT: A general protocol for the synthesis of benzylic
amines via side-chain amination of alkylquinones is reported. The
reactions are initiated by the tautomerization of an alkylquinone to
the corresponding quinone methide, which is subsequently trapped
in situ by an amine nucleophile. This process is promoted by tertiary
amines in protic solvents under mild conditions and is compatible with many functional groups. 1,2- and 1,4-benzoquinones, as
well as naphthoquinones, participate in this reaction using a wide range of primary and secondary amines/anilines. The synthetic
utility of this transformation is also explored.
Scheme 1. Quinone Side-Chain Diamination by Todd14 and
enzylic amines are key constituents of pharmaceuticals,
This Work
Bagrochemicals, and materials and are commonly found as
intermediates in modern organic chemistry.1 Accordingly, a wide
variety of methods for their synthesis have been developed.
Traditional methods for amine synthesis, such as nucleophilic
addition to imines2 and nucleophilic substitution of benzylic
electrophiles,3,4 typically require prefunctionalization of the
benzylic coupling partner and/or nucleophile. Transition-metal-
catalyzed reactions,4−8 such as the amination of styrene
derivatives,5 the direct benzylic C−H amination of alkylarenes,6
the α-arylation of aliphatic amines,7 and imine addition
reactions,8 offer several advantages over traditional methods,
including mild reaction conditions and increased functional
group tolerance; however, their application is limited by the need
to employ expensive and sometimes toxic metal catalysts. Other
methods based on the benzylic amination of alkyl-9 or vinyl-
substituted10 arenes often require superstoichiometric quantities
of oxidants or reagents. Therefore, the development of new
technologies for the synthesis of benzylic amines that do not
depend on the prefunctionalization of reaction partners, or the
need for transition-metal catalysts or superstoichiometric
reagents, is highly desirable. The utilization of unfunctionalized
substrates to access reactive intermediates capable of intercepting
primary and secondary amines would allow rapid and direct
access to benzylic amines. Herein we report a straightforward and
efficient method for benzylic amine synthesis based on the direct
coupling of alkylquinones with primary and secondary amines.
This new protocol for benzylic amine synthesis proceeds by way
of alkylquinone tautomerization11 to generate a highly electro-
philic quinone methide intermediate that is, in turn, trapped by
an amine coupling partner.12,13
The “side-chain” amination of alkylquinones was first reported
by Todd et al. in 1964 and involved a sequence of amination/
oxidation events, leading to doubly aminated hydroquinones in
modest yields (Scheme 1, eq 1).14 Based on the related addition
of sodium malonate to duroquinone reported decades earlier by
Smith et al.,15 Todd proposed that tautomerization of the
alkylquinone generated a key quinone methide intermediate that
immediately reacted with the amine nucleophile. Since then,
isolated reports on the amination of alkylquinones have appeared
in the literature.16 However, none of them have provided a
general method for efficient benzylic amine synthesis. Given the
limited understanding of this potentially powerful trans-
formation, we set out to develop a general protocol for quinone
side-chain monoamination and perform a detailed examination
of its utility (Scheme 1, eq 2).
Our optimization studies focused on the reaction between 2,6-
dimethylquinone and dibenzylamine (Table 1). Examination of
various solvents at 70 °C under a nitrogen atmosphere for 24 h
revealed that the reaction failed to proceed in nonpolar solvents
(entry 1). Only small amounts of the desired benzylamine
derivative 3, together with hydroquinone 4, were ever obtained
in DME (entry 2).17 Because hydroquinone 4 is presumably
generated via single-electron transfer (SET) and hydrogen atom
abstraction from dibenzylamine,18 we reasoned that the use of
Received: June 6, 2016
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.6b01629
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a
Table 1. Summary of Optimization Studies
Scheme 2. Scope Evaluation with Respect to the Amine
a
a
entry solvent
catalyst
time (h) yield of 3 (%) yield of 4 (%)
1
PhMe
none
none
none
none
none
none
none
none
24
24
24
24
24
24
24
24
2
0
6
0
7
2
DME
3
i-PrOH
EtOH
66
65
43
52
48
85
94
85
92
47
25
25
80
23
27
9
4
5
MeCN
DMF
6
18
29
9
7
DMSO
Et3N
8
b
9
i-PrOH
i-PrOH
i-PrOH
i-PrOH
i-PrOH
i-PrOH
i-PrOH
Et3N
6
b
10
11
12
13
14
15
i-Pr2NEt
2
7
b
Bu3N
2
8
b
DMAP
2
nd
5
b
pyridine
none
2
2
5
c
Et3N
2
7
a
1
Yields determined by H NMR analysis using 1,3,5-trimethoxyben-
b
c
zene as an internal standard. 1 equiv used. 0.2 equiv used. nd = not
determined.
protic solvents might attenuate this undesired reaction by
accelerating quinone tautomerization. Indeed, the use of protic
solvents improved both the reaction efficiencies and product
ratios (entries 3 and 4). Highly polar, aprotic solvents provided
product 3 in diminished yields and led to increased quinone
reduction (entries 5−7). Surprisingly, carrying out the reaction
in neat triethylamine provided benzylic amine 3 in 85% yield with
only a small amount of byproduct 4 (9%, entry 8). Guided by
these results, we examined the use of 1 equiv of triethylamine in
2-propanol and observed significant rate acceleration, affording
benzylic amine 3 in 94% yield in just 2 h with only a trace quantity
of hydroquinone 4 (entry 9). Other tertiary amines provided
similar yields (entries 10 and 11). DMAP provided only a modest
yield of product 3 (entry 12, 47%), while pyridine was an
ineffective catalyst (entry 13, 25%) and was comparable to the
reaction without any additive (entry 14, 25%). Although the
majority of our optimization studies employed a stoichiometric
amount of an amine catalyst, the catalytic activity was evidenced
by the use of a substoichiometric quantity of triethylamine (entry
15, 80%). The combination of an external base and protic solvent
presumably aids in quinone tautomerization, thereby accelerat-
ing the desired reaction.
With optimized conditions in hand, we next examined a
diverse range of primary and secondary amine reaction partners
using 2,6-dimethyl-1,4-benzoquinone (1) as a representative
quinone (Scheme 2). Model substrate 3 was isolated in 92%
yield. N-Allyl-N-homobenzylamine 5 was obtained in good yield
(89%). Less hindered secondary amines also participated in this
reaction, providing products 6−8 in moderate to good yields
(59−85%). When volatile amines such as diethylamine and N-
methylallylamine were used, the reactions were carried out at
room temperature, providing 7 and 8 in good yields (65% and
59% respectively). N-Methylcyclohexylamine afforded product 9
in 85% yield, but the more sterically encumbered dicyclohexyl-
amine and diisopropylamine led to lower yields of products 10
and 11 (44% and 41%, respectively, not shown) accompanied by
a significant amount of hydroquinone 4. Fortunately, we found
a
b
Reaction ran at room temperature for 24 h. Reaction ran in PhMe/i-
c
d
PrOH (4:1) with 5 equiv of Et3N. Reaction ran for 3 h. Amine
hydrochloride used; reaction ran with 2 equiv of Et3N.
that the addition of a nonpolar cosolvent (toluene) led to a
significant improvement in reaction efficiency, delivering 10 and
11 in good yields (75% and 63%, respectively).
Cyclic amines such as morpholine, N-Boc-piperazine, and L-
proline methyl ester provided the corresponding benzylic amines
in good yields (12−14, 70−79%). N-Benzyl-O-methylhydroxyl-
amine provided the product in low yield (15, 47%), while the
demethylated analogue failed to provide any of the desired
product (not shown). N-Alkylanilines provided the correspond-
ing benzylic amines in moderate yields (16−18, 40−63%).
Notably, competitive amination of the quinone ring was detected
as a byproduct accompanying 16.18 Ortho-substitution of the
arene ring did not substantially change the yield of the reactions
(17, 63% and 18, 40%). Employing a dibenzoazepine
nucleophile led to the desired benzylic amine 19 in modest
yield (45%), perhaps due to the electronic deactivation of this
amine by the two aromatic substituents. Primary amines such as
( )-α-methylbenzylamine and α-amino esters reacted in
moderate yields (20−22, 43−53%) due to competitive
B
DOI: 10.1021/acs.orglett.6b01629
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
amination of the quinone ring.18 N-Benzyl protection of L-
leucine methyl ester provided the desired product in good yield
(23, 71%). It is important to point out that the benzylation of
enantiopure amino acids that might be subject to epimerization
provided products 14, 22, and 23 with little to no change in
enantiomeric purity. However, amino acids bearing a more acidic
α-proton, such as phenyl glycine, provided the corresponding
benzylic amine product with reduced enantiopurity (21, 39% ee).
We also explored the scope of the benzylation reaction with
respect to the alkylquinone using N-allyl-N-homobenzylamine as
the nucleophile (Scheme 3). Methoxy-substituted quinones
quinone delivered diamine 32 in 42% yield. To further explore
regiocontrol in these reactions, a nonsymmetrical dimethylqui-
none was tested, leading to regioisomeric benzylic amines 33a
and 33b in excellent overall yield (89%), albeit with modest
regioselectivity (2:1). Surprisingly, this reaction proceeds
through nucleophilic attack at the more hindered position to
provide amine 33a as the major product. Lastly, a methyl
naphthoquinone underwent amination in low yield due to
competitive reduction of the reacting naphthoquinone and
oxidation of the product, affording naphthoquinone 34 in low
yield (17%).
Given that the reaction allows the preparation of enantiopure
benzylic amines (e.g., 14, 22, and 23, Scheme 2), we wondered if
using a chiral amine would induce the diastereoselective
amination of a suitable quinone. Thus, we evaluated the reaction
of 2,6-diethyl-1,4-benzoquinone with ( )-α-methylbenzylamine
(Scheme 4). The reaction proceeded efficiently, but the
Scheme 3. Scope Evaluation with Respect to the Quinone
Scheme 4. Evaluation of Diastereoselectivity
diastereoselectivity was poor (dr = 1.3:1.0). Lowering the
reaction temperature did not significantly improve the
diastereoselectivity (dr = 1.8:1.0 at 0 °C, not shown).
To highlight the utility of this method, we subjected
pharmaceutical agent idebenone to our amination reaction
with morpholine (Scheme 5). Notably, regioselective amination
Scheme 5. Formal C−H Amination of Idebenone
a
b
c
Reaction ran at 80 °C for 6 h. Reaction ran for 3 h. Reaction ran for
4 h.
took place at the less hindered position (methyl substituent) to
afford benzylic amine 36 in 74% yield (>20:1 rr). Benzylic amine
36 could be oxidized under mild conditions to provide aminated
idebenone derivative 37. This two-step procedure provides a
useful protocol for carrying out the formal C−H amination of
alkylquinones.
In conclusion, we have developed a general method for the
efficient synthesis of benzylic amines via alkylquinone amination.
This transformation enables the synthesis of complex benzylic
amines in a single step under mild conditions. We expect this
protocol to be adopted as a useful tool to rapidly access benzylic
amines and prepare aminated derivatives of quinone-containing
natural products and pharmaceuticals.
afforded the amination products 24 and 25 in good yields (92%
and 83%, respectively) without substitution of the methoxy
substituents. On the other hand, employing a chloroquinone
provided amine 26 in modest yield (35%) presumably due to
competitive chloride substitution.18
Surprisingly, 2,3-dimethyl-1,4-benzoquinone, which is un-
hindered at the 5- and 6-positions and therefore potentially
susceptible to nucleophilic attack,18 provided benzylic amine 27
in 58% yield. Duroquinone required extended reaction time and
higher temperature, affording monoaminated benzylic amine 28
in good yield (80%), as opposed to the diaminated product
analogous to those described by Todd.14 o-Quinone 3-chloro-4-
methyl-1,2-benzoquinone afforded the corresponding benzylic
amine 29 in low yield (22%) accompanied by a significant
quantity of the substitution product. Thymoquinone, bearing
methyl and isopropyl substituents, underwent regioselective
amination at the methyl position to give 30 in 93% yield. This
side-chain amination is not limited to methylquinones, as 2,6-
diethyl-1,4-quinone afforded the corresponding α-methylbenzyl-
amine 31 in 81% yield. Employing an N-Boc-(2-aminoethyl)-
ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.6b01629.
Additional optimization studies, control experiments,
1
general experimental procedures, and H, 13C, and 19F
NMR spectra (PDF)
C
DOI: 10.1021/acs.orglett.6b01629
Org. Lett. XXXX, XXX, XXX−XXX

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Letter
12394−12397. (f) Zhu, Y.; Buchwald, S. L. J. Am. Chem. Soc. 2014, 136,
4500−4503.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: mclift@ku.edu.
Notes
(9) (a) Leathen, M. L.; Peterson, E. A. Tetrahedron Lett. 2010, 51,
2888−2891. (b) Fan, R.; Li, W.; Pu, D.; Zhang, L. Org. Lett. 2009, 11,
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The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
(c) Martínez, C.; Muniz, K. Adv. Synth. Catal. 2014, 356, 205−211.
̃
Financial support from the NSF (EPS-0993806) and The
University of Kansas is gratefully acknowledged. Support for
NMR instrumentation was provided by NIH Shared Instru-
mentation Grants Nos. S10OD016369 and S10RR024664 and
NSF Major Research Instrumentation Grant No. 0320648.
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DOI: 10.1021/acs.orglett.6b01629
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