Pd(II)-Catalyzed Aerobic Intermolecular 1,2-Diamination of Conjugated Dienes: A Regio- and Chemoselective [4 + 2] Annulation for the Synthesis of Tetrahydroquinoxalines

Article abstract of DOI:10.1021/acs.orglett.7b00919

A Pd(II)-catalyzed aerobic intermolecular 1,2-diamination of conjugated dienes was developed for the regio- and chemoselective preparation of a variety of functionalized tetrahydroquinoxalines, using simple sulfonyl protected o-phenylendiamines as a nitrogen source. This methodology provides a direct and efficient synthesis of tetrahydroquinoxalines. O₂ was used as the stoichiometric oxidant, and reaction conditions were applied to a series of o-phenylendiamines and conjugated dienes. 35 examples are described, and good yields and selectivities are obtained for the majority of the products.

Full text of DOI:10.1021/acs.orglett.7b00919

Letter
pubs.acs.org/OrgLett
Pd(II)-Catalyzed Aerobic Intermolecular 1,2-Diamination of
Conjugated Dienes: A Regio- and Chemoselective [4 + 2] Annulation
for the Synthesis of Tetrahydroquinoxalines
Zhengxing Wu, Ke Wen, Jingang Zhang, and Wanbin Zhang*
School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, P. R. China
S
* Supporting Information
ABSTRACT: A Pd(II)-catalyzed aerobic intermolecular 1,2-
diamination of conjugated dienes was developed for the regio-
and chemoselective preparation of a variety of functionalized
tetrahydroquinoxalines, using simple sulfonyl protected o-
phenylendiamines as a nitrogen source. This methodology
provides a direct and efficient synthesis of tetrahydro-
quinoxalines. O2 was used as the stoichiometric oxidant, and reaction conditions were applied to a series of o-phenylendiamines
and conjugated dienes. 35 examples are described, and good yields and selectivities are obtained for the majority of the products.
Scheme 1. Pd-Catalyzed Oxidative 1,2-Diaminations
icinal diamines are important compounds found in a wide
Vrange of biologically active compounds, synthetic inter-
mediates, and organocatalysts.1,2 The direct 1,2-diamination of
alkenes offers an especially efficient method for the synthesis of
vicinal diamines. In recent decades, several metal-mediated,1,3
metal-catalyzed,1,4−7 and metal-free1,8 methods have been
reported. Among these methods, Pd-catalyzed diamination is
one of the most important processes.5−7 Shi et al. carried out a
number of pioneering investigations concerning Pd(0)-catalyzed
1,2-diaminations.5 Several intra- and intermolecular Pd(II)-
catalyzed oxidative 1,2-diaminations using isolated alkenes have
also been reported (Scheme 1).6 Muniz et al. developed a series of
̃
intra- and intermolecular Pd(II)-catalyzed 1,2-diaminations for
isolated alkenes mainly using hypervalent iodine as an
oxidant.6a−c,g−j Michael et al. reported Pd(II)-catalyzed 1,2-
diaminations of isolated alkenes with N-fluorobenzene-
sulfonimide (NFSI) as an oxidant and a nitrogen source.6e,f,k
However, examples concerning Pd(II)-catalyzed oxidative 1,2-
diaminations with conjugated alkenes are uncommon. Lloyd-
Jones and Booker-Milburn reported a seminal Pd(II)-catalyzed
intermolecular diamination of conjugated alkenes using ureas as
nitrogen sources, obtaining five-membered N-heterocycles with
high yields using benzoquinone as an oxidant.7 However, it is still
generally difficult to use oxygen as an oxidant. Despite these
successful cases, for current Pd-catalyzed 1,2-diaminations, the
direct preparation of other valuable diamine products (such as
benzo-fused heterocycles) using simple nitrogen sources requires
further research; use of O2 as an oxidant in oxidative 1,2-
diaminations with alkenes also remains challenging due to
difficulties encountered in active and selective transformations.9
During our research concerning the Pd-catalyzed synthesis of
heterocycles,10 we developed a Pd(II)-catalyzed aerobic inter-
molecular 1,2-diamination of conjugated dienes using sulfonyl
protected o-phenylendiamines as nitrogen sources, for the
preparation of a variety of important functionalized tetrahydro-
quinoxalines (Scheme 1).2
We commenced our study by screening the reactions between
N-protected o-phenylendiamines 1 and isoprene 2a in the
presence of catalytic Pd(OAc)2 (10 mol %) and Cu(OAc)2 (20
mol %) under O2 in MeCN at 70 °C (Table 1, entries 1−5). To
our delight, for N-Ts protected o-phenylendiamine 1a, although
the reaction mixture was complex, the oxidative cyclization
proceeded to give the desired product 3aa in 23% yield, and the
Received: March 28, 2017
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.7b00919
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a
a
Table 1. Optimization of the Reaction Conditions
Table 2. Scope of Conjugated Dienes for 1,2-Diamination
diene
Pd(OAc)2 Cu(OAc)2
2a
yield
b
entry
1
R
(mol %)
(mol %)
(equiv)
oxygen
(3)
Ts (1a)
10
20
2
1 atm
balloon
23
(3aa)
2
3
4
5
Ms (1b)
Ac (1c)
Boc(1d)
Cbz (1e)
Ts
10
10
10
10
10
10
10
−
20
20
20
20
20
20
−
2
1 atm
17
(3ba)
balloon
2
1 atm
balloon
−
−
−
2
1 atm
balloon
2
1 atm
balloon
c
6
2
1 atm
balloon
trace
13
d
7
Ts
2
1 atm
balloon
8
9
Ts
2
1 atm
balloon
trace
−
Ts
20
20
20
−
2
1 atm
balloon
e
10
Ts
10
10
10
10
10
5
2
1 atm
balloon
27
e
11
Ts
10
10
10
5
1 atm
balloon
35
e
12
Ts
sealed
tube
82
e
13
Ts
20
−
sealed
tube
37
e
14
Ts
sealed
tube
63
e
15
Ts
−
10
sealed
tube
59
a
Unless otherwise noted, the reactions were carried out using 0.1
mmol of diamine 1, Pd(OAc)2, and isoprene 2a in MeCN (2 mL) at
a
Unless otherwise noted, the reactions were carried out using diamine
b
c
d
70 °C for 72 h under O2. Isolated yields. The solvent is DME. The
solvent is DMSO. 4 Å molecular sieves (40 mg) were added.
1a (0.1 mmol), Pd(OAc)2 (10 mol %), 4 Å molecular sieves (40 mg),
and diene 2 (10 equiv) in MeCN (2 mL) at 70 °C for 72 h. Method A:
In a sealed tube charged with O2 (starting at 1 atm). Method B:
e
b
Cu(OAc)2 (20 mol %) under O2 (1 atm balloon). Isolated yields.
1,4-diamination (regioselectivity) and 1,2-diamination at the
more substituted double bond (chemoselectivity) were not
detected (entry 1). The reaction of N-Ms protected o-
phenylendiamine 1b was also successful (entry 2); however, o-
phenylendiaminebearingotherAc,Boc, orCbzprotectinggroups
(1c−1e) did not undergo reaction (entries 3−5). A range of
solvents were then examined using o-phenylendiamine 1a.11 The
yields in DME or DMSO were no better than that in MeCN,
although product 3aa was also obtained (entries 6, 7 vs entry 1).
Control experiments were also carried out and compared to the
initialconditions(entries8, 9). Atraceamountofproduct3aawas
detected in the absence of Cu(OAc)2 (entry 8 vs entry 1), and
none of 3aa was obtained in the absence of Pd(OAc)2 (entry 9 vs
entry 1). These suggested that Pd(OAc)2 was essential for
diamination and that Cu(OAc)2 alone could not catalyze the
reaction. Addition of 4 Å molecular sieves or increasing the
amount of isoprene led to an increase in the yield of product 3aa
(entries 10, 11). Considering that the low boiling point of
isoprene 2a may reduce its concentration in the reaction system
and thus hinder the reaction, the reaction was thus carried out in a
sealedtube under O2 (starting at 1atm). Gratifyingly, thereaction
proceededwellandahighyield(product 3aaupto82%yield)and
selectivity were obtained (entry 12); the remaining material
consisted mostly of unreacted substrate. We proposed that the
c
d
e
Diene 2 (2 equiv) for 48 h. Diene 2 (2 equiv) for 12 h. Diene 2 (5
equiv) and Pd(OAc)2 (20 mol %).
increasedamountofisopreneinthereactionsystemusingasealed
tube may help promote the reaction and, more importantly, may
act as a ligand to prevent the precipitation of palladium. Addition
of Cu(OAc)2 (20 mol %) led to a series of complex side products
(entry 13 vs entry 12). When we decreased the amount of
isoprene (entry 14 vs entry 12) and Pd(OAc)2 (entry 15 vs entry
12) respectively, the corresponding yields were also reduced.
With the above optimized reaction conditions (Method A) in
hand (Table 1, entry 12), a variety of conjugated dienes were
surveyed to explore the scope of the reaction (Table 2). We
discovered that, in addition to isoprene 2a (Table 2, entry 1), 1,3-
butadiene 2b also underwent conversion with a good yield and
selectivity (entry 2). The reaction conditions also could tolerate
the alkenyl-substituted myrcene 2c, giving the corresponding
functionalized tetrahydroquinoxalines in high yields (entry 3).
The diene 2d bearing an epoxy group also underwent reaction in
high yield (entry 4). For 2-substituted conjugated diene 2e
bearing aryl group, the reaction proceeded well, and a high yield
and good selectivity could be obtained (entry 5). For terminal
methyl-substituted 1,3-diene 2f, the terminal diamination (3af)
B
DOI: 10.1021/acs.orglett.7b00919
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a
Scheme 2. Scope of Nitrogen Source for 1,2-Diamination
Scheme 3. Further Transformations of the Functionalized
Tetrahydroquinoxalines
3kb using Method B (the corresponding results using Method A
were also shown in the SI).11 For cyclic conjugated dienes 2g−2i,
the desired products could be obtained diastereoselectively using
Method B.12 A series of products (3fg−3kg) could be generated
using cyclopentadiene 2g (2−10 equiv). Most diaminations using
1,3-cyclohexadiene 2h (2 equiv) proceeded well with high yields
(3fh−3mh), regardless of the substitution pattern.12 For
reactions involving 1,3-cycloheptadiene 2i (2−5 equiv), a series
of products (3fi−3ki) could be obtained in high yields.
The functionalized tetrahydroquinoxalines generated via
intermolecular diamination could be transformed further. For
example, the free tetrahydroquinoxalines 4aa and 4ah can be
obtained from 3aa and 3ah by deprotection of the N-tosyl group
with a Na/naphthalene solution respectively, with moderate to
high yields (Scheme 3).
Aproposed catalytic cycle is shown in Scheme 4. For simplicity,
we chose 1a and diene 2h as representative examples. According
a
Unless otherwise noted, the reactions were carried out using diamine
1 (0.1 mmol), Pd(OAc)2 (10 mol %), 4 Å molecular sieves (40 mg),
and diene 2 (2 equiv) in MeCN (2 mL) at 70 °C for 12 h. Isolated
yields. Method A: In a sealed tube charged with O2 (starting at 1 atm).
Scheme 4. Proposed Catalytic Cycle
b
Method B: Cu(OAc)2 (20 mol %) under O2 (1 atm balloon). For 72
h. Diene 2 (10 equiv). The ratios arising from the asymmetrical
c
d
nature of the monosubstituted o-phenylendiamines were confirmed by
e
analysis of the 1H NMR spectrum of the crude reaction mixture. For
f
24 h. Diene 2 (5 equiv) for 72 h.
and internal diamination (3af′) of the diene occurred, giving the
corresponding product in a total yield of 42% yield (entry 6,
3af:3af′ = 1.2:1). We also used a terminal phenyl-substituted 1,3-
diene ((E)-buta-1,3-dienylbenzene) as a diene substrate;
however, no reaction occurred. For the acyclic dienes mentioned
above (entries 1−6), the reaction systems became complex and
the diaminations were unsatisfactory in the presence of
Cu(OAc)2 (Method B). The cyclic conjugated diene, cyclo-
pentadiene 2g, could be converted to its corresponding product
3ag using Method A or B, but both with a lower yield (entry 7).12
However, for other cyclic conjugated dienes such as 1,3-
cyclohexadiene 2h (entry 8) and 1,3-cycloheptadiene 2i (entry
9), Cu(OAc)2 was essentialtofacilitate theconversion despite the
appearance of some side products (Method B).
We next turned our attention to investigating the nitrogen
source scope of the 1,2-diamination. Scheme 2 shows that a range
of toluenesulfonyl protected o-phenylendiamines and conjugated
dienes are able to participate in the oxidative cyclization using
either Method A or B. The majority of the o-phenylendiamine
substrates bearing both electron-donating and -withdrawing
substituents were amenable to the reaction conditions using
isoprene 2a. The desired products (3fa−3ka) were obtained with
high yields and good selectivities using isoprene 2a (10 equiv);
Method B provided better results for most of o-phenylendiamine
substrates bearing electron-withdrawing substituents (3ha−3ja)
and a naphthalene substrate (3ka) (the corresponding results
using Method A were shown in the Supporting Information
(SI)).11,12 The annulation process also worked well when 1,3-
butadiene 2b (10 equiv) was subjected to reaction with various o-
phenylendiamines possessing both electron-donating and -with-
drawing groups (3fb−3kb), and the results were better for 3gb−
11,5g,6c
1
to in situ H NMR and HRMS studies,
substrate 1a and
Pd(OAc)2 first form the five-membered Pd(II) species C, which
may then be converted to the diene complex D via ligand
exchange. The π-allyl Pd intermediate E is formed from complex
D via an aminopalladation step (in previously reported work, the
sulfonyl-protected amines predominantly favored syn-amino-
palladation).13 Another potential pathway involving Pd(OAc)2,
1a, and 2h undergoing an anti-aminopalladation step to form π-
allyl Pd intermediate E′ cannot be excluded, due to the presence
of an equilibrium for the formation of species C from Pd(OAc)2
and 1a. The π-allyl Pd intermediate E and E′ may exist in
equilibrium.14 Finally, product3ahcanbeformedviatwopossible
amination processes, and anti-amination from intermediate E′ is
favored with attack on the π-allyl Pd intermediate by soft
nucleophiles such as sulfonamide.15 The resulting Pd(0) is
oxidized to Pd(II), which then participates in the next catalytic
cycle.
In conclusion, we have developed a general method for the
Pd(II)-catalyzed aerobic intermolecular 1,2-diamination of
conjugated dienes via [4 + 2] annulation. A class of important
tetrahydroquinoxalines were synthesized regio- and chemo-
selectively. This methodology provides a direct and efficient
synthesis in good yield using O2 as the stoichiometric oxidant.
C
DOI: 10.1021/acs.orglett.7b00919
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
(4) For selected examples of metal-catalyzed 1,2-diamination, for Rh:
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This methodology extends the significance of 1,2-diaminations,
allowing for the preparation of valuable diamine products using
simple nitrogen sources.
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ASSOCIATED CONTENT
* Supporting Information
■
A.; Muniz, K. Chem. - Eur. J. 2009, 15, 10563. Cu: (e) Sequeira, F. C.;
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TheSupportingInformationisavailablefreeofchargeontheACS
Publications website at DOI: 10.1021/acs.orglett.7b00919.
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H.; Zhao, B.; Shi, Y. J. Am. Chem. Soc. 2007, 129, 762. (b) Du, H.; Yuan,
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Experimental procedures; spectral data (PDF)
Crystallographic data for 3ah (CIF)
Crystallographic data for 3jh (CIF)
Crystallographic data for 3mh (CIF)
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: wanbin@sjtu.edu.cn.
ORCID
(6) For examples of Pd(II)-catalyzed 1,2-diaminations of isolated
alkenes: (a) Streuff, J.; Hovelmann, C. H.; Nieger, M.; Muniz, K. J. Am.
̈
̃
Chem. Soc. 2005, 127, 14586. (b) Muniz, K. J. Am. Chem. Soc. 2007, 129,
̃
14542. (c) Muniz, K.; Hovelmann, C. H.; Streuff, J. J. Am. Chem. Soc.
2008, 130, 763. (d) Muniz, K.; Hovelmann, C. H.; Campos-Gomez, E.;
Barluenga, J.; Gonzalez, J. M.;Streuff, J.; Nieger, M. Chem. -Asian J. 2008,
́
3, 776. (e) Sibbald, P. A.; Michael, F. E. Org. Lett. 2009, 11, 1147.
(f) Sibbald, P. A.; Rosewall, C. F.; Swartz, R. D.; Michael, F. E. J. Am.
Chem. Soc. 2009, 131, 15945. (g) Iglesias, A.; Per
Angew. Chem., Int.Ed. 2010,49,8109.(h)Muniz,K.;Kirsch,J.;Chavez,P.
Adv. Synth. Catal. 2011, 353, 689. (i) Martínez, C.; Muniz, K. Angew.
̃
̈
Wanbin Zhang: 0000-0002-4788-4195
Notes
̃
̈
The authors declare no competing financial interest.
́
́
ez, E. G.; Muniz, K.
̃
ACKNOWLEDGMENTS
■
́
̃
This work was partially supported by the National Natural
Science Foundation of China (Nos. 21232004, 21672142),
Program of Shanghai Subject Chief Scientists
(No.14XD1402300), and the Basic Research Foundation of
Shanghai Science and Technology Committee
(No.15JC1402200). We also thank the Instrumental Analysis
Center of Shanghai Jiao Tong University.
̃
Chem., Int. Ed. 2012, 51, 7031. (j) Chavez, P.; Kirsch, J.; Streuff, J.;Muniz,
́
̃
K. J. Org. Chem. 2012, 77, 1922. (k) Ingalls, E. L.; Sibbald, P. A.;
Kaminsky, W.; Michael, F. E. J. Am. Chem. Soc. 2013, 135, 8854.
(7) For Pd(II)-catalyzed 1,2-diamination of conjugated alkenes, see:
Bar, G. L. J.; Lloyd-Jones, G. C.; Booker-Milburn, K. I. J. Am. Chem. Soc.
2005, 127, 7308.
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Lett. 2000, 41, 8699. (b) Booker-Milburn, K. I.; Guly, D. J.; Cox, B.;
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