α-Selective allylation of azo compounds using allylic barium reagents

Article abstract of DOI:10.1055/s-0032-1318306

The addition of allylic barium reagents to azo compounds was achieved with high α-regioselectivity. The double-bond geometry of allylic barium reagents was retained throughout the reaction at -78 °C and E- or Z-enriched allylic hydrazines were selectively

Full text of DOI:10.1055/s-0032-1318306

LETTER
▌635
lαet-teSr elective Allylation of Azo Compounds Using Allylic Barium Reagents
Selective Allylation of Azo Compounds
Akira Yanagisawa,* Takuya Jitsukawa, Kazuhiro Yoshida
Department of Chemistry, Graduate School of Science, Chiba University, Inage, Chiba 263-8522, Japan
Fax +81(43)2902789; E-mail: ayanagi@faculty.chiba-u.jp
Received: 26.12.2012; Accepted after revision: 01.02.2013
broad applicability to azo compounds; however, as far as
we know, there are no examples of use of the procedure
for the regioselective α-allylation of azo compounds.
First, we tried to perform a Barbier-type reaction of azo-
Abstract: The addition of allylic barium reagents to azo com-
pounds was achieved with high α-regioselectivity. The double-bond
geometry of allylic barium reagents was retained throughout the re-
action at –78 °C and E- or Z-enriched allylic hydrazines were selec-
tively obtained from the corresponding allylic barium reagents. An benzene with geranyl chloride promoted by reactive bari-
allylic hydrazine was efficiently converted into an allylic amine by
reductive N–N bond cleavage.
um at –78 °C; however, the major product was 1,2-
diphenylhydrazine and the yield of the desired adduct was
Key words: allylation, allylic amines, azo compounds, barium, hy-
drazine
low. We then took a Grignard-type approach in which
azobenzene was treated with a preformed geranylbarium
reagent. As a result, the targeted geranylated hydrazine
(α-adduct) was obtained in 77% yield and the correspond-
Organobarium reagents prepared from reactive Rieke ing γ-adduct was not detected at all (Scheme 2). In addi-
barium1 and allylic or propargylic halides are known to re- tion, the double-bond geometry of the geranylbarium
act with electrophiles with α-regioselectivity.2 We have reagent was retained at the reaction temperature.
previously reported that a Barbier-type propargylation of
azo compounds with propargylic halides proceeds
Ph
BaCl
γ
α
HN
N
smoothly in the presence of the reactive barium as a low-
valent metal to afford propargylic hydrazines (α-adducts)
not only from azobenzenes (diaryldiazenes) but also from
dialkyl azodicarboxylates.3 We report herein an α-selec-
tive Grignard-type allylation of azo compounds using al-
Ph
α
(2 equiv)
THF, –78 °C, 30 min
N
N
Ph
Ph
77% (α/γ > 99:1, E/Z = 99:1)
Scheme 2 Grignard-type reaction of geranylbarium reagent with
lylic barium reagents (Scheme 1). In addition, studies of azobenzene
the transformation of the products, allylic hydrazines, into
the corresponding allylic amines by reductive N–N bond
Subsequently, we investigated the reaction of various azo
cleavage of the former compounds are also described.
compounds with geranylbarium reagent; selected results
are presented in Table 1. The effect of azobenzene substit-
uents on the chemical yield was remarkable; a para-sub-
stituted electron-withdrawing group enhanced the
reaction rate (entry 6), whereas para-substituted electron-
donating groups reduced the reactivity of azobenzene (en-
tries 2–4). For ortho-substituted azobenzenes, the steric
hindrance of the substituent reduced the chemical yield of
the adduct (entries 1, 5, and 7). Although we also exam-
ined the reactivity of dialkyl azodicarboxylates, the reac-
tion of the electrophiles with geranylbarium reagent
resulted in the formation of complex mixtures. 1,2-Di-
tert-butyldiazene did not show any reactivity in the pres-
ent geranylation.
Ar
N
N
R1
Cl
R1
BaCl
γ
γ
α
α
Ar
Ba*
THF, –78 °C
R2
R2
–78 °C
Ar
Ar
Ar
Ar
HN
HN
N
R1
N
+
α
γ
R1 R2
R2
α/γ up to >99:1
Scheme 1 α-Selective allylation of azo compounds using allylic bar-
ium reagents
We further studied the allylation of azobenzene with a di-
verse range of allylic barium reagents under the optimized
reaction conditions (Table 2). Not only the geranylbarium
reagent, but also other γ-disubstituted allylic barium re-
agents showed exclusive α-selectivity (entries 1–3). The
γ-isomer was detected as a minor product in the reaction
of γ-monosubstituted allylic barium reagents (entries 5
and 6). The geometric purity of the α-adducts indicates
that geometric isomerization (E to Z or Z to E) of the al-
lylic barium reagents can be reduced to a minimum during
the allylation (entries 2, 3, 5, and 6). Thus, E-enriched α-
Several methods to synthesize allylic hydrazines starting
from azo compounds have been developed so far, includ-
ing allylation with allylic organometallics,4 ene reaction,5
and the Mitsunobu reaction.6 Among them, the first meth-
od seems to be the most synthetically useful from the
viewpoints of high reactivity of allylic metal reagents and
SYNLETT 2013, 24, 0635–0639
Advanced online publication: 27.02.2013
0
9
3
6
-
5
2
1
4
1
4
3
7
-
2
0
9
6
DOI: 10.1055/s-0032-1318306; Art ID: ST-2012-U1104-L
© Georg Thieme Verlag Stuttgart · New York

636
A. Yanagisawa et al.
LETTER
Table 1 Selective Geranylation of Azo Compoundsa
drazine synthesis; however, the α/γ ratio given by the
reaction of 3-chloro-1-butene (entry 8) was opposite to
that of trans-1-chloro-2-butene (entry 7), which means
that the initially generated secondary allylic barium re-
agent readily isomerizes to the corresponding primary al-
lylic barium reagent at –78 °C.
γ
BaCl
α
Ar
Ar
HN
N
Ar
(2 equiv)
N
N
α
THF, –78 °C, 1 h
Ar
α/γ > 99:1b,c
The high α-selectivity is a specific characteristic of the
barium reagent and other group 2 metal reagents, and al-
kali metal reagents derived from (E)-2-decenyl chloride
gave low α/γ regioselectivities, as shown in Table 3.
Among them, the magnesium reagent produced the oppo-
site γ-adduct preferentially (entry 2). The extraordinary α-
selectivity and stereospecificity of the azo addition of bar-
ium reagent provide an unprecedented route to allylic hy-
drazines.
Entry
Ar
Yield (%)b
E/Zb,c
1
2
3
4
5
6
7
2-MeC6H4
4-MeC6H4
4-MeOC6H4
4-PhOC6H4
2-FC6H4
55
69
41
67
62
88
45
98:2d
99:1
95:5
99:1
97:3d
99:1
98:2
To examine the electronic effect of the two nitrogen atoms
on the site selectivity, we performed the geranylation of
asymmetric azobenzene derivatives bearing an electron-
withdrawing group on one benzene ring and an electron-
donating group on the other benzene ring and, as a result,
a 64:36 mixture of two regioisomers A and B was ob-
tained as the product in the reaction of 2-(4-fluorophenyl)-
1-p-tolyldiazene with geranylbarium reagent (Table 4, en-
try 1). Similar site selectivity was observed for the reac-
tion with geranyllithium, although its chemical yield was
unsatisfactory (entry 2). Therefore, the difference in elec-
tron density between the two nitrogen atoms was found to
be small in these cases. A more satisfactory result was ob-
tained when the electron-donating group was replaced
with a p-methoxy group (entry 3). Site selectivity was re-
markably improved when a p-trifluoromethyl group was
4-FC6H4
2,4-F2C6H3
a Reaction conditions: geranyl chloride (2 equiv), reactive barium (2.4
equiv), azo compound (1 equiv), anhydrous THF, –78 °C, 1 h.
b Determined by 1H NMR analysis.
c Determined by HPLC analysis.
d Determined after N–N bond cleavage.
allylated hydrazines were selectively obtained from (E)-
allylic barium reagents (entries 2 and 5), whereas the cor-
responding (Z)-allylic barium reagents provided the Z-
isomers of the α-adducts predominantly (entries 3 and 6).
An allylic barium reagent, generated from a secondary al-
lylic chloride, was also applied to the present allylic hy-
Table 2 α-Selective Allylation of Azobenzene with Allylic Barium Reagentsa
R4
R1
BaCl
γ
α
Ph
Ph
Ph
Ph
R4 HN
R4 HN
Ph
R2
R3 (2 equiv)
N
N
R1
N
N
+
α
γ
THF, –78 °C, 30 min
Ph
R1 R2
R3
R2
R3
Entry
R1
R2
R3
R4
Yield (%)b
α/γb,c
E/Zb,c
1d
2
Me
Me
Me
H
H
H
H
H
H
H
H
74
77
63
68
88
53
65
51
>99:1
>99:1
>99:1
–
–
Me2C=CH(CH2)2
H
99:1
3:97
–
3
Me
Me2C=CH(CH2)2
H
4d
5d
6d
7d
8e
H
H
Me
H
n-C7H15
H
82:18
88:12
61:39
32:68
91:9
7:93
94:6
54:46
H
n-C7H15
H
Me
H
H
H
H
Me
H
a Reaction conditions (unless otherwise noted): allylic chloride (2 equiv), reactive barium (2.4 equiv), azobenzene (1 equiv), anhydrous THF,
–78 °C, 30 min.
b Determined by 1H NMR analysis.
c Determined by HPLC analysis.
d Allylic chloride (3 equiv) and reactive barium (3.3 equiv) were used.
e 3-Chlorobut-1-ene was used as allylic chloride.
Synlett 2013, 24, 635–639
© Georg Thieme Verlag Stuttgart · New York

LETTER
Selective Allylation of Azo Compounds
637
Table 3 Effect of Metals on the α/γ Regioselectivity of the Reaction of Azobenzene with (E)-2-Decenylmetal Reagentsa
γ
MX
α
n-C7H15
Ph
Ph
HN
N
Ph
Ph
Ph
(3 equiv)
THF, –78 °C, 30 min
N
N
HN
N
γ
+
n-C7H15
α
Ph
n-C7H15
α-Product
Entry
MX
Yield (%)b
α/γb,c
E/Zb,c
1
2
3
4
5
Lid
22
51
80
55
88
67:33
8:92
92:8
81:19
96:4
91:9
91:9
MgCle
CaCl
SrCl
BaCl
55:45
67:33
82:18
a Reaction conditions (unless otherwise noted): (E)-2-decenyl chloride (3 equiv), reactive metal (3.3 equiv), azobenzene (1 equiv), anhydrous
THF, –78 °C, 30 min.
b Determined by 1H NMR analysis.
c Determined by HPLC analysis.
d The reaction was carried out using lithium biphenylide (6.6 equiv) as reactive metal.
e The reaction was carried out using magnesium turnings (3.3 equiv) in place of reactive metal at –78 °C for 30 min and then at 0 °C for 1 h.
adopted as the electron-withdrawing group (entries 4 and (entry 5). Site-selective allylation of arylazocarboxylates
5). The highest site selectivity was attained using 2-[4- was also possible and, indeed, anticipated adducts of tert-
(trifluoromethyl)phenyl]-1-(4-methoxyphenyl)diazene as butyl phenylazocarboxylate with prenylbarium reagent
the substrate, whereby a 93:7 mixture of geranylated hy- were obtained with an A/B ratio of >99:1 (entry 6), al-
drazines A and B was furnished in 56% combined yield though the α/γ ratio of adduct A was 50:50, probably due
Table 4 Site-Selective Geranylation of Asymmetric Azo Compoundsa
γ
MX
Ar2
Ar1
α
Ar1
Ar2
HN
N
HN
N
Ar1
(2 equiv)
N
N
α
α
+
THF, –78 °C, 1 h
Ar2
A (α/γ > 99:1)b,c
B (α/γ > 99:1)b,c
Entry
Ar1
Ar2
MX
Yield (%)b
E/Zb,c
A/Bd
64:36
1
2
3
4
5
6
4-FC6H4
4-FC6H4
4-FC6H4
4-MeC6H4
4-MeC6H4
4-MeOC6H4
4-MeC6H4
4-MeOC6H4
Ph
BaCl
Lie
63
31
47
83
56
61
99.5:0.5
99:1
99:1
99:1
99:1
–
60:40
77:23
88:12
93:7
BaCl
BaCl
BaCl
BaClf
4-F3CC6H4
4-F3CC6H4
CO2t-Bu
>99:1b,g
a Reaction conditions: geranyl chloride (2 equiv), reactive barium (2.4 equiv), asymmetric azo compound (1 equiv), anhydrous THF, –78 °C, 1
h.
b Determined by 1H NMR analysis.
c Determined by HPLC analysis.
d Determined by 19F NMR analysis. The structure of the major isomer was determined by N–N bond cleavage.
e The reaction was carried out using lithium biphenylide (4.8 equiv) in place of reactive barium.
f The reaction was carried out using prenyl chloride in place of geranyl chloride.
g The α/γ ratio of adduct A was 50:50.
© Georg Thieme Verlag Stuttgart · New York
Synlett 2013, 24, 635–639

638
A. Yanagisawa et al.
LETTER
n-C7H15
γ
BaCl
α
ClBa
γ
Ph
Ph
N
N
ClBa
Ph
Ph
Ph
N
N
N
(3 equiv)
THF, –78 °C, 30 min
+
n-C7H15
α
N
Ph
n-C7H15
α/γ = 82:18 (determined after quenching at –78 °C)
1) r.t., 20 h
2) aq NH4Cl, 0 °C
Ph
Ph
Ph
HN
N
HN
N
+
γ
n-C7H15
α
Ph
n-C7H15
α/γ = 82:18
Scheme 3 Possibility of equilibrium between the α- and γ-adducts
to the reduced steric bulk of the tert-butoxycarbonyl exist in equilibrium between α-isomer 2 and γ-isomer 3.
group.
Accordingly, α-allylated hydrazines can be produced
from both isomers 2 and 3 by reacting them with an azo
compound through transition state structure 4 or 5. How-
ever, as the stereoisomerization of barium reagent 2 does
not occur at –78 °C,2b,c no γ-isomer 3 is formed at this
temperature. In contrast, γ-allylated hydrazines (γ-ad-
ducts) are accessible from 2 through an SE2′-type reaction
of 2 with the azo compound by way of six-membered cy-
clic transition state 6. However, 6 is destabilized by the
steric repulsion between an allylic alkyl group of the bar-
ium reagent and an aryl group of the azo compound. Thus,
allylic barium species 2 is considered to react selectively
at the α-carbon with an azo compound via four-membered
cyclic transition state 4,2b,c leading to the α-adduct with re-
tention of configuration of starting chloride 1. A longer
Ba–C bond9 of the allylic barium reagent 2 compared with
a M–C bond of other allylic group 2 metal reagents or al-
kali metal reagent is another possible reason for the pref-
erence for adoption of four-membered cyclic transition
state 4.
The possibility of an equilibrium between the α- and γ-ad-
duct was excluded by the following experiment (Scheme
3). Upon stirring at room temperature for 20 h and subse-
quent quenching with aqueous NH4Cl solution at 0 °C, no
change in the α/γ ratio was seen for the barium amide of
allylated hydrazine, which was prepared from azobenzene
and (E)-2-decenylbarium reagent at –78 °C. This result
indicates that an 82:18 mixture of the α- and γ-adducts is
kinetically produced and that the γ-adduct does not isom-
erize to the thermodynamically more stable α-adduct at
higher temperature. Hence, the E/Z stereochemistry of al-
lylated products well reflects the double-bond geometry
of the starting allylic barium reagents. This is another
unique feature of barium reagents.
The obtained allylic hydrazines can be effectively con-
verted into allylic amines through reductive N–N bond
cleavage.7 For example, treatment of a geranylhydrazine
derivative with an excess of Zn at room temperature gave
the corresponding geranylamine in 85% yield (Table 5). A
reduction method employing TiCl3 and NH4OAc was also
R1
Cl
γ
α
8
a promising alternative; however, samarium(II) iodide
did not promote N–N bond cleavage at all.
R2
1
–78 °C Ba*
Table 5 Reductive N–N Bond Cleavage of a Geranylhydrazine De-
rivative
BaCl
R1
R1
BaCl
γ
α
α
γ
R2
R2
3
–78 °C
Ar
2
Ph
Ar
HN
Ar
H
N
N
N
N
N
N
N
reduction
N
–78 °C
–78 °C
–78 °C
Ph
Ph
Ar
Ar
Ar
Ar
R1
Reduction
method
Solvent
THF
Temp Time
Yield
(%)
γ
N
N
N
Ar
Ar
N
ClBa
Ar
(°C)
(h)
Ar
BaCl
α
N
N
R2
R1
α
Ba
Cl
R1
Ar
SmI2, MeOH
TiCl3, NH4OAc
Zn
60
80
r.t.
1
3
4
<1
47
85
R2
R2
5
4
6
EtOH–DMF (1:1)
AcOH
Ar
Ar
Ar
Ar
HN
N
HN
N
R1
α
γ
R2
A probable reaction mechanism is shown in Scheme 4.
Two pathways can be considered for α-allylated hydra-
zines (α-adducts). An allylic barium reagent prepared
from allylic chloride 1 and reactive barium is assumed to
R1 R2
α-adduct (major)
γ-adduct (minor)
Scheme 4 Probable reaction pathways to α- and γ-adducts
Synlett 2013, 24, 635–639
© Georg Thieme Verlag Stuttgart · New York

LETTER
Selective Allylation of Azo Compounds
639
(5) (a) Rabjohn, N. J. Am. Chem. Soc. 1948, 70, 1181.
In conclusion, we have developed a novel Grignard-type
α-allylation of azo compounds with allylic barium re-
agents that are generated from allylic chlorides and reac-
tive barium. This method is synthetically useful in terms
of regioselectivity and stereoselectivity, and affords vari-
ous α-allylated hydrazines in moderate to high yields.10,11
In addition, the site-selective α-allylation of asymmetric
azo compounds has also been achieved to yield isomeric
ratios of up to 93:7. The utility of the α-allylated product
has been further demonstrated by its transformation into
an α-allylated amine through reductive N–N bond cleav-
age. Further studies on related reactions promoted by bar-
ium reagents are in progress.
(b) Brimble, M. A.; Heathcock, C. H. J. Org. Chem. 1993,
58, 5261. (c) Künneth, R.; Feldmer, C.; Knoch, F.; Kisch, H.
Chem. Eur. J. 1995, 1, 441. (d) Magnus, P.; Garizi, N.;
Seibert, K. A.; Ornholt, A. Org. Lett. 2009, 11, 5646.
(6) Brown, M. J.; Clarkson, G. J.; Inglis, G. G.; Shipman, M.
Org. Lett. 2011, 13, 1686.
(7) For a review of reductive N–N bond cleavage of hydrazines,
see: Gilchrist, T. L. In Comprehensive Organic Synthesis;
Vol. 8; Trost, B. M.; Fleming, I., Eds.; Pergamon: Oxford,
1991, 388.
(8) Zhang, Y.; Tang, Q.; Luo, M. Org. Biomol. Chem. 2011, 9,
4977.
(9) Kaupp, M.; Schleyer, P. v. R. J. Am. Chem. Soc. 1992, 114,
491.
(10) Carreira and co-workers have reported that the cobalt-
catalyzed hydrohydrazination of dienes provided allylic
hydrazines with good regioselectivity, see: Waser, J.;
González-Gómez, J. C.; Nambu, H.; Huber, P.; Carreira, E.
M. Org. Lett. 2005, 7, 4249.
Acknowledgment
We gratefully acknowledge financial support from the Iodine Re-
search Project in Chiba University led by Professor Hideo Togo.
(11) α-Selective Allylation: Synthesis of (E)-1-(3,7-
Dimethylocta-2,6-dien-1-yl)-1,2-diphenylhydrazine
(Scheme 2 and Table 2, Entry 2); Typical Procedure: To
a solution of biphenyl (370 mg, 2.4 mmol) in anhydrous
THF (5 mL) was added freshly cut lithium (16.7 mg, 2.4
mmol). Stirring the mixture at r.t. for 2 h gave a dark-blue
lithium biphenylide solution. Anhydrous BaI2 (470 mg, 1.2
mmol) was placed in a separate flask, and this was covered
with anhydrous THF (5 mL) and stirred at r.t. for 20 min. To
the solution of BaI2 in THF was added at r.t. a solution of
lithium biphenylide in THF under an argon stream. The
reaction mixture was stirred at r.t. for 20 min, then to the
resulting dark-brown suspension of reactive barium (1.2
mmol) in THF (10 mL) was added a solution of geranyl
chloride (0.185 mL, 1.0 mmol) in THF (2 mL) at –78 °C by
syringe pump (6 mL/h). After stirring for 20 min, the
mixture was treated with a solution of azobenzene (91.1 mg,
0.5 mmol) in THF (2 mL) at –78 °C (slow addition at 6
mL/h) and stirred for 10 min at this temperature. To the
mixture was added sat. aq NH4Cl (5 mL), and the aqueous
layer was extracted with Et2O (3 × 10 mL). The combined
organic extracts were washed with sodium thiosulfate (10%
w/v) and brine, dried over Na2SO4, and concentrated in
vacuo after filtration. The residual crude product was
purified by column chromatography on silica gel (hexane–
EtOAc, 20:1 then 80:1) to afford the allylic hydrazine. The
chemical yield was determined to be 77% by 1H NMR
spectroscopic analysis using 1,4-bis(trimethylsilyl)benzene
as an internal standard, and the α/γ and E/Z ratios were
determined to be >99:1 and 99:1, respectively, by 1H NMR
and HPLC analyses. 1H NMR (500 MHz, CDCl3): δ = 7.26–
7.20 (m, 2 H, ArH), 7.01 (dd, J = 7.7, 1.0 Hz, 2 H, ArH),
6.84–6.79 (m, 4 H, ArH), 5.57 (br s, 1 H, NH), 5.28 (dt,
J = 1.5, 6.8 Hz, 1 H, CH), 5.03 (tt, J = 8.0, 1.4 Hz, 1 H, CH),
4.10 (br s, 2 H, CH), 2.11–2.00 (m, 4 H, CH), 1.68 (d, J = 0.8
Hz, 3 H, CH), 1.59 (s, 3 H, CH), 1.58 (d, J = 0.85 Hz, 3 H,
CH); 13C NMR (125 MHz, CDCl3): δ = 150.2, 147.8, 141.3,
131.8, 129.3 (2C), 129.1 (2C), 123.9, 119.7, 118.7, 117.8,
113.5 (2C), 112.9 (2C), 48.1, 39.6, 26.3, 25.7, 17.7, 16.2; IR
(neat): 3321, 2965, 2922, 1600, 1495, 1596, 1254, 749, 692
cm–1; HRMS (ESI): m/z [M + H]+ calcd for [C22H29N2]+:
321.2325; found: 321.2322.
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Synlett 2013, 24, 635–639

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