Oxidative dehydrocoupling of N-H bonds using a redox-active main group superbase

Article abstract of DOI:10.1039/c1cc00053e

Reaction of the redox-active base Sn(NMe₂)₂/ ⁿBuLi with o-phenylene diamine leads to oxidative dehydrocoupling and rearrangement into the triazolyl anion.

Full text of DOI:10.1039/c1cc00053e

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COMMUNICATION
Oxidative dehydrocoupling of N–H bonds using a redox-active main
group superbasew
Robert J. Less, Vesal Naseri,* Mary McPartlin and Dominic S. Wright*
Received 4th January 2011, Accepted 1st April 2011
DOI: 10.1039/c1cc00053e
n
Reaction of the redox-active base Sn(NMe ) / BuLi with
2
2
o-phenylene diamine leads to oxidative dehydrocoupling and
rearrangement into the triazolyl anion.
The applications of superbases in a range of deprotonation
reactions are long-established in organic chemistry, such as the
t
1
well known Schlosser base consisting of BuOK and RLi. The
past two decades has seen a growing interest in synergic bases of
this type, in which the presence of two metals results in enhance-
ment in the basicity and specificity of deprotonation over and
Scheme
ꢀ
3 2 3
Cp*ZrH ] (top) and Sb(NMe ) / BuLi (bottom).
2 2 6 4
1 Dehydrocoupling reactions of 1,2-(PH ) -C H with
2
above that found for the separate components. The use of a
n
[
range of synergic bases, followed by the addition of an electro-
phile, is increasingly being employed as a standard methodology
in the modification of organic molecules. Mulvey in particular
has focused on the relationship between the structures of an
extensive series of heterometallic amide bases, revealing remarkable
reactivity and selectivity towards various organic and inorganic
then undergoes reductive C–P bond cleavage to finally produce
ꢀ
8
6 4 3
the 1,2,3-triphospholide anion [C H P ] (Scheme 1, bottom).
Given the broad applications of dehydrocoupling reactions to
6
a wide variety of bonds, it is surprising that dehydrocoupling of
amines (formally 2N–H - N–N + H–H) has attracted so little
3
species which can be tuned by the choice of metals present.
9
10
11
attention. Recently, Corma et al. and Zhang and Jiao have
made significant breakthroughs in this area by showing that
catalytic oxidative coupling of a range of aromatic amines
II
/RM (E = Sn ,
Heterometallic p-block reagents E(NMe
2
)
n
III
As , Sb , Bi ; M = alkali metal) also function as superbases,
III
III
for example, in the quadruple deprotonation of 2-amino-phenyl-
4
phosphine. However, the presence of redox-active metals (E)
(
ArNH ) into diazo compounds (RNQNR) can be achieved
2
I
using a Cu Br/pyridine complex or Au nanoparticles as cata-
means that reactions may not simply stop at deprotonation:
subsequent bond-forming and bond-breaking reactions can also
occur. The resulting ability for p-block bases of this type to
dehydrocouple P–H bonds into P–P bonds is similar to the
lysts, with air as the oxidant. We show in this communication
n-
for the first time that the redox-active superbase Sn(NMe
2 2
) /
BuLi is also useful in the oxidative coupling of amines.
The reaction of o-phenylene diamine [1,2-(NH
1H ) with a 1 : 2 stoichiometric mixture of Sn(NMe
BuLi in thf at reflux (16 h) gives a deep, blood-red solution
2 2 6 4
) -C H ]
5,6
activity of a range of transition metal organometallics. A case
in point are reactions involving the diphosphine 1,2-(PH ) -C H .
(
n
4
2 2
) and
2
2
6
4
Stephan and coworkers have shown that the dehydrocoupling
1
together with tin metal.z The H NMR spectrum of the
CH Cl extract of the hydrolysed mixture showed that benzo-
triazole [C H N H] (2H) (Fig. 1) is the main product, together
ꢀ
reaction with the single-site catalyst [Cp*
dimeric tetraphosphane [(C P)PH]
then couples into the cyclic octamer [C
2
ZrH
3
]
gives the
2
2
6
H
4
2
as an intermediate, which
7
6
4
3
6
H
4
P
2
]
8
(Scheme 1, top).
with a minor amount of 2,3-diamino-phenazene (3) (Fig. 1)
ESIw). The formation of 3 was also confirmed by the isolation
and structural characterization of the crystalline hydrochlor-
Under the more basic and reducing conditions involved, the
analogous reaction involving the mixed-metal main group system
(
n
/ BuLi initially gives the dianion [(C
2ꢀ
Sb(NMe
2
)
3
6
H
4
P)P]
2
, which
+
ꢀ
ide salt [3][3H] Cl ꢁ7H
2
O,y obtained by storage of the acid-
ified aqueous extract (ESIw).
Chemistry Department, Cambridge University, Lensfield Road,
Cambridge CB2 1EW, UK. E-mail: vn226@cam.ac.uk,
dsw1000@cam.ac.uk
w Electronic supplementary information (ESI) available: Details of
+
ꢀ
the structural solutions and X-ray data for [3][3H] Cl ꢁ7H
Sn (1H) (1) and [(1H)SnLiꢁPMDETA] . CCDC
][2ꢁLiPMDETA]
96653 ([Sn (1H) (1)
2
O,
[
7
7
2
4
2
2
7
2
4
][2ꢁLiPMDETA]
2
), 796654 ([(1H)SnLiꢁPMDETA]
2
)
and 796655 (3). For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/c1cc00053e
4
Fig. 1 The products 2 and 3 generated from reaction of 1H .
This journal is c The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 6129–6131 6129

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The formation of 3 in this reaction is readily understood in
terms of the redox-active, superbase properties of the hetero-
metallic reagent employed. It can be noted that 3 is also formed
II
in the reaction of o-phenylene diamine with Cu in air and a
1
2
similar mechanism is probably involved here (ESIw). Notably,
however, there are no reports of the formation of 2H in this or
1
3
related reactions involving transition metals.
Although the reaction producing 2H is clearly closely related
to that seen previously in the 1,2,3-triphospholide (Scheme 1),
the formation of N–N bonds at the expense of seemingly more
thermodynamically stable C–N bonds is a surprising feature of
this transformation. In order to understand more about the
mechanism of formation of 2H, we next attempted to charac-
terize intermediates in the reaction. The reaction in thf was
repeated as before, but this time at room temperature (14 h).
Layering with PMDETA [=(Me NCH CH ) NMe] at room
2
2
2 2
temperature for a week led to the gradual deposition of yellow
crystals of [(1H)SnLiꢁPMDETA] (Fig. 2), together with a
small number of orange crystals of the salt [Sn (1H) (1) ][2
2
7
2
4
(
LiꢁPMDETA)
2
]
2
(Fig. 3).z More of this second product is
generated if the reaction is stirred for longer at room tempera-
ture prior to layering with PMDETA.
The centrosymmetric dimer [(1H)SnLiꢁPMDETA]
2
(Fig. 2)
3
ꢀ
contains the triply-deprotonated [1H] trianion.y The imido
nitrogen of each trianion (–N ) is involved in bonding within
2ꢀ
ꢀ
the central Sn N ring unit, while the amido nitrogen (–NH )
2
2
II
chelates the Sn centres exo to the ring. The bonding of the
+
Li counterions to the N-atoms of the Sn N units is augmented
2
2
2
ꢀ
by a relatively short p-arene interaction to the a-C atoms of the
Fig. 3 Structures of (a) the Sn dianion [Sn (1H) (1) ] and (b) one of the
7 7 2 4
14
+
˚
˚
C H
6 4
group [Liꢁ ꢁ ꢁC(6) 2.708(7) A]. This gives Li a nominal
two independent [2(LiꢁPMDETA)
2
]
cations. Selected bond lengths (A)
ꢀ
dianion, Sn(1)–N range 2.131(4)–
2
7 2 4
and angles (1); in the [Sn (1H) (1) ]
five-coordination number.
2
.215(4), Sn(2,3,4,5,6,7)–N range 2.130(5)–2.273(5), Sn(2,5)ꢁꢁꢁN(4,7)
.655(5)–2.819(6), N(10,80)–Sn(5,2) range 2.281(6)–2.287(5), N–SN–N
(core) range 78.0(2)–91.1(1), SN–N–Sn range 92.2(2)–108.9(2); in the
The solid-state structure of [Sn (1H)
7
2
(1)
4
][2(LiꢁPMDETA)
2 2
]
2
7
contains a heptanuclear Sn dianion, in which both the trianion
3ꢀ
4ꢀ
[
1H] and tetraanion [1] are present (Fig. 3a), and two
+
...
[
2(LiꢁPMDETA)
2
]
cations, NꢀN range 1.328(8)–1.375(8), N(triazolyl)–Li
crystallographically-independent cations, in which two PMDETA-
+
solvated Li ions coordinate a triazolyl anion, [2] (Fig. 3b).
... ...
range 1.92(2)–2.01(1), NꢀNꢀN range 116.0(8)–116.1(6).
ꢀ
2ꢀ
The double-cubane arrangement of the [Sn (1H) (1) ] dianion
7
halves of the dianion together, with the remaining metal atoms
II
all being Sn . There is a very broad range of SN–N bond lengths
˚
within the SN–N core, ranging from 2.130(5)–2.273(5) A for the
II
majority of the Sn /Sn –N bonds to 2.655(5)–2.819(6) A for the
2
4
1
5
is reminiscent of neutral [Sn (NR) ] cages. Like these neutral
8
7
IV
cages, the central Sn centre [Sn(1)] bridges the two cubane
IV
˚
very weak Sn(2,5)ꢁ ꢁꢁN(4,7) interactions [trans to the chelate
+
bonding to N(10) and N(80)]. The [2ꢁ(LiꢁPMDETA)
2
]
cations
are similar conceptually to other inverse sandwich cations of this
+ 16
type, such as [PMDETAꢁLi–(m-Cl)–LiꢁPMDETA] , while the
...
˚
... ...
NꢀN bond lengths [range 1.328(8)–1.375(8) A] and NꢀNꢀN
angles [range 116.0(8)–116.1(6)1] in this unit are typical of other
structurally characterized triazolyl complexes.
17
Given the combined experimental observations, one potential
ꢀ
mechanism for the formation of the triazolyl anion of [2] is
outlined in Scheme 2 in which the formation of a diazo inter-
mediate occurs via oxidation of the highly electron-rich imido N
ꢀ
atoms of the [1H] anion. It is noteworthy in this regard that we
2 2
find that the room-temperature reaction of Sn(NMe ) alone
with PhNH produces PhNQNPh (m/z = 182.1). The simplest
way by which this oxidation can be achieved is through
2
Fig. 2 Structure of dimeric molecules of [(1H)SnLiꢁPMDETA]
2
. Selected
˚
bond lengths (A) and angles (1); Sn(1)–N(1) 2.132(4), Sn(1)–N(2) 2.165(3),
Sn(1)–N(2A) 2.188(3), N(2)–Li(1) 1.995(7), Li(1)–N(PMDETA) range
II
0
IV
2
Sn + 2e - 2Sn . However, the observation of Sn in the
structure of [Sn (1H) (1) (Fig. 3) suggests
][2(LiꢁPMDETA)
that more complicated redox chemistry may be operating,
2 2
]
2.105(8)–2.18(1), C(6)ꢁꢁꢁLi(1) 2.708(7), N(1)–Sn(1)–N(2) 78.16(12),
7
2
4
N(1)–Sn(1)–N(2A) 100.4(1), N(2)–Sn(1)–N(2A) 85.1(1), Sn(1)–N(2)–
Sn(1A) 94.9(1).
II
0
IV
IV
II
e.g., 2Sn - Sn + Sn , Sn + 2e - Sn . The potential
6
130 Chem. Commun., 2011, 47, 6129–6131
This journal is c The Royal Society of Chemistry 2011

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further 14 h, PMDETA (3 mL) was layered carefully onto the reaction
mixture. After 1 week, large yellow-orange, block-shaped crystals of
[
(1H)SnLiꢁPMDETA]
crystals of [Sn (1H) (1)
pure samples of [(1H)SnLiꢁPMDETA]
2
had formed, in which a few yellow-orange
7
2
4
][2(LiPMDETA)
2
]
2
are also present. Analytically-
can be manually separated.
2
(
Elemental analysis, found C 43.9, H 6.9, N 16.3, calcd for [(1H)SnLiꢁ
PMDETA]
C 44.6, H 7.0, N 17.3.) However, crystalline [(1H)SnLiꢁ
PMDETA] and [Sn (1H) (1) ][2(LiPMDETA) proved to be too
2
2
7
2
4
2 2
]
1
13
insoluble to obtain solution H or C NMR spectra (only hydrolysis
products were observed).
+
ꢀ
y Crystal data: [3][3H] Cl ꢁ7H
2 8 7
O, C24H35ClN O , M = 583.05,
ꢀ
triclinic, space group P1, Z = 2, a = 7.0826(2), b = 12.5439(4),
˚
c = 16.0868(6) A, a = 82.669(1), b = 78.708(2), g = 84.163(2)1,
3
ꢀ1
ꢀ3
,
˚
V = 1385.75(8) A , m(Mo–Ka) = 0.196 mm , rcalc = 1.397 Mg m
T = 180(2) K. Total reflections 14 267, unique 6208 (Rint = 0.053).
= 0.076 [I > 2s(I)] and wR
= 0.237 (all data). [(1H)SnLiꢁ
PMDETA] : C30 56Li 10Sn , M = 808.11, monoclinic, space group
P21/n, Z = 2, a = 13.9707(3), b = 9.3155(2), c = 14.8029(3) A,
R
1
2
Scheme 2 Proposed mechanism of formation of the triazolyl anion
+
from o-phenylene diamine (the Li counterions have been omitted for
2
H
2
N
2
˚
clarity).
3
ꢀ1
˚
b = 106.888(1)1, V = 1843.43(7) A , m(Mo–Ka) = 1.389 mm
,
ꢀ3
r
3
[
calc = 1.456 Mg m , T = 180(2) K. Total reflections 23029, unique
760 (Rint = 0.060). R = 0.036 [I > 2s(I)] and wR = 0.087 (all data).
Sn (1H) (1) : C78 28Sn , M = 2310.63,
1
2
7
2
4
][2ꢁLiPMDETA]
2
H122Li
4
N
7
ꢀ
triclinic, space group P1, Z = 2, a = 13.86730(10), b = 18.7128(2),
c = 20.1729(3) A, a = 97.4860(10), b = 104.4300(10), g = 103.9620(10)1,
˚
3
ꢀ1
ꢀ3
,
˚
V = 4817.83(9) A , m(Mo–Ka) = 1.839 mm , rcalc = 1.593 Mg m
T = 180(2) K. Total reflections 41 657, unique 13 382 (Rint = 0.045).
= 0.056 [I > 2s(I)] and wR = 0.185 (all data). Nonius KappaCCD
diffractometer, solved by direct methods and refined by full-matrix least
R
1
2
2
¨
squares on F (G. M. Sheldrick, SHELX-97, Gottingen, 1997). CCDC
Fig. 4 Heterocycles that are potentially accessible by oxidative
796653, 796654 and 796655.
dehydrocoupling.
1
2
L. Lochmann, Eur. J. Inorg. Chem., 2000, 115.
R. E. Mulvey, F. Mongin, M. Uchiyama and Y. Kondo, Angew.
Chem., Int. Ed., 2007, 46, 3802.
2ꢀ
intermediacy of a tetraazopentalyl dianion [(C H N)N]
2
(the
6
4
3
4
R. E. Mulvey, Organometallics, 2006, 25, 1060; R. E. Mulvey, Acc.
Chem. Res., 2009, 42, 743.
F. Garcıa, S. M. Humphrey, R. A. Kowenicki, E. J. L. McInnes,
´
C. M. Pask, M. McPartlin, J. M. Rawson, M. L. Stead, A. D. Woods
and D. S. Wright, Angew. Chem., Int. Ed., 2005, 44, 3456.
R. J. Less, R. Melen, V. Naseri and D. S. Wright, Chem. Commun.,
N analogue of the tetraphosphane dianion in Scheme 1) cannot
be discounted in this case but does not appear to be necessary
ꢀ
for the formation of the [2] anion.
In summary, we have revealed for the first time the potential
5
6
2 n
of redox-active main group E(NMe ) /RM superbases to
2
009, 4929.
oxidatively dehydrocouple N–H bonds. In the current study
this provides access to 1,2,3-triazole through a series of reactions
which is unknown with transition metal counterparts. This new
methodology might well be useful for other ring systems such as
imidazoles (A), oxadiazoles (B) and diazaphospholes (C) (Fig. 4)
and we are exploring this potential.
S. Greenberg and D. W. Stephan, Chem. Soc. Rev., 2008, 37, 1482;
D. W. Stephan, Angew. Chem., Int. Ed., 2000, 39, 314; T. J. Clark,
K. Lee and I. Manners, Chem.–Eur. J., 2006, 12, 8634;
R. Waterman, Dalton Trans., 2009, 18; R. Waterman, Curr. Org.
Chem., 2008, 12, 1322.
(a) N. Etkin, M. C. Fermin and D. W. Stephan, J. Am. Chem. Soc.,
1997, 119, 2954; (b) A. J. Hoskin and D. W. Stephan, Angew.
Chem., Int. Ed., 2001, 40, 1865.
7
8
We gratefully acknowledge the EPSRC (D.S.W., V.N.) and
The Leverhulme Trust (R.J.L.) for financial support. We also
thank Dr J. E. Davies (Cambridge) for collecting crystal data
F. Garcıa, R. J. Less, V. Naseri, M. McPartlin, J. M. Rawson,
´
M. Sancho Tomas and D. S. Wright, Chem. Commun., 2008, 859;
J. M. Goodman, R. J. Less, V. Naseri, E. J. L. McInnes,
R. E. Mulvey and D. S. Wright, Dalton Trans., 2008, 6454.
Y.-K. Lim, K.-S. Lee and C.-G. Cho, Org. Lett., 2003, 5, 979.
on [(1H)SnLiꢁPMDETA]
2
and [Sn (1H)
7
2
(1)
4
][2ꢁLiPMDETA]
2
.
9
1
1
´
0 A. Grirrane, A. Corma and H. Garcıa, Science, 2008, 322, 1661.
1 C. Zhang and N. Jiao, Angew. Chem., Int. Ed., 2010, 49, 6174.
Notes and references
z Synthesis of 2H: a mixture of o-phenylene diamine (208 mg, 2.0 mmol)
and Sn(NMe (414 mg, 2.0 mmol) was dissolved in 30 mL thf at
5 1C and stirred for 20 min. To the orange solution at 25 1C was
added BuLi (1.7 mol L in heptane, 2.4 mL, 4.0 mmol). The mixture
was brought to reflux (16 h), turning deep blood red (after ca. 1 h).
The mixture was cooled to room temperature and the solvent removed
under vacuum and replaced with DCM (30 mL). This solution was
hydrolyzed with 30 mL of distilled water and dilute HCl(aq) added until
the aqueous phase was of pH 8. The organic phase was separated and
12 S.-M. Peng and D.-S. Liaw, Inorg. Chim. Acta, 1986, 113, L11;
S. K. Brownstein and G. D. Enright, Acta Crystallogr., Sect. C:
Cryst. Struct. Commun., 1995, 51, 1579; S. Stankovic and D. Lazar,
´
2 2
)
2
t
ꢀ1
Acta Crystallogr., Sect. C: Cryst. Struct. Commun., 1995, 51, 1581.
13 For example, M. G. Miles and J. D. Wilson, Inorg. Chem., 1975,
14, 2357.
+
14 Within the range of ion–dipole areneꢁ ꢁ ꢁLi interactions: Search of
the Cambridge Crystallographic Data Base (Aug. 2010), range
˚
2.241–3.442 A (mean 2.47).
15 F. Benevelli, E. L. Doyle, E. A. Harron, N. Feeder, E. A. Quadrelli,
dried with MgSO
H NMR spectroscopy (see ESIw) showed that this consists of a
4
. Removal of the solvent gave a yellow semi-solid.
1
D. Sa
D. R. Armstrong, F. Benevelli, A. D. Bond, N. Feeder, E. A. Harron,
A. D. Hopkins, M. McPartlin, D. Moncrieff, D. Saez, E. A. Quadrelli
and D. S. Wright, Inorg. Chem., 2002, 41, 1492.
´
ez and D. S. Wright, Angew. Chem., Int. Ed., 2000, 39, 1501;
+
ꢀ
mixture of 1H
were deposited from the acidified aqueous residue after storage for
week at room temperature. [(1H)SnLiꢁPMDETA] ; Sn(NMe
1 mol L solution in toluene, 0.45 mL, 0.45 mmol) was added
dropwise to a solution of o-phenylene diamine (45 mg, 0.42 mmol) in
4
, 2H and 3. Deep red crystals of [3][3H] Cl ꢁ7H
2
O
´
1
(
2
2 2
)
ꢀ1
16 N. H. Buttrus, C. Eaborn, P. B. Hitchcock, J. D. Smith,
J. G. Stamper and A. C. Sullivan, Chem. Commun., 1986, 969.
17 Search of the Cambridge Crystallographic Data Base (Dec. 2010),
ꢀ1
1.5 mL thf at room temperature and stirred for 1.5 h. nBuLi (1.6 mol L
.
..
˚
... ...
in hexanes, 0.62 mL, 1.0 mmol) was then added dropwise at room
temperature, producing a clear, orange solution. After stirring for a
NꢀN range 1.275–1.388
A
(mean 1.33), NꢀNꢀN range
105.0–117.81 (mean 111.61).
This journal is c The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 6129–6131 6131