C-H activations on a 1H-1,4-benzodiazepin-2(3H)-one template

Article abstract of DOI:10.1016/j.tet.2008.01.059

Air stable benzodiazepine containing palladacycles were synthesized by a C-H activation reaction and studied by mass spectrometry and X-ray crystallography. Catalytic C-H functionalizations of 1-methyl-5-phenyl-1H-1,4-benzodiazepin-2(3H)-one with diphenyliodonium hexafluorophosphate led to a mixture, which included the starting material and the expected product 1-methyl-5-(2′-biphenyl)-1H-1,4-benzodiazepin-2(3H)-one.

Full text of DOI:10.1016/j.tet.2008.01.059

Available online at www.sciencedirect.com
Tetrahedron 64 (2008) 6082e6089
www.elsevier.com/locate/tet
CeH activations on a 1H-1,4-benzodiazepin-2(3H)-one template
a
a
a
John Spencer a, , Babur Z. Chowdhry , Anthony I. Mallet , Rajendra P. Rathnam ,
*
Trushar Adatia b, Alan Bashall b, Frank Rominger c
a School of Science, University of Greenwich at Medway, Central Avenue, Chatham, Kent ME4 4TB, UK
b Department of Health and Human Sciences, London Metropolitan University, North Campus, 166-220 Holloway Road, London N7 8DB, UK
c Organisch-Chemishes Institut der Ruprecht-Karls-Universita¨t Heidelberg, Im Neuenheimer Feld 270, D-69120 Heidelberg, Germany
Received 10 September 2007; received in revised form 16 November 2007; accepted 20 November 2007
Available online 25 January 2008
Abstract
Air stable benzodiazepine containing palladacycles were synthesized by a CeH activation reaction and studied by mass spectrometry and
X-ray crystallography. Catalytic CeH functionalizations of 1-methyl-5-phenyl-1H-1,4-benzodiazepin-2(3H)-one with diphenyliodonium
hexafluorophosphate led to a mixture, which included the starting material and the expected product 1-methyl-5-(20-biphenyl)-1H-1,4-benzodi-
azepin-2(3H)-one.
Ó 2008 Elsevier Ltd. All rights reserved.
Keywords: Benzodiazepine; CeH activation; Palladium; Catalysis
1. Introduction
nature of this Symposium-in-Print, we will initially provide
an overview of selected publications in the burgeoning area
of palladacycle mediated CeH activation chemistry, which
will highlight these synthetically relevant transformations,
benefiting from the unique properties of palladacycles includ-
ing their facile synthesis by chelation-assisted CeH activation,
their ease of handling/air stability as well as their well-defined
chemistry and use as tools for the mechanistic investigations
of carbonecarbon bond forming reactions.
Ambient temperature ortho-alkenylations and arylations of
anilines and anilides catalyzed by palladium have been re-
cently reported and are likely to involve palladacycle interme-
diates given the high degree of regioselectivity observed.4 A
number of catalytic alkenylations use an additional 1 equiv
of a silver(I) salt and tolerate bromo substituents on the aryl
coupling partner (Scheme 1).
Palladacycles are a class of air stable organometallic com-
plexes with interesting properties and applications in many
areas of organic synthesis, especially in high turnover catalytic
CeC bond forming reactions, such as Heck or Suzukie
Miyaura couplings. Palladacycles are typically five- or six-
membered ring systems and are formed by a CeH activation
reaction, assisted by coordination of the heteroatom of the
ligand to the electrophilic Pd(II) species.1 They are now estab-
lished intermediates or precursors in atom economical CeH
activations,2 whereby a CeH bond is involved as one coupling
partner as opposed to a CeX (X¼Cl, Br, I) bond. Intramolec-
ular coordination to the metal and the formation of pallada-
cycles as either precatalysts or intermediates can promote
the high ortho-regioselectivity observed in these synthetically
powerful processes.3
We will report hereafter our recent findings on the stoichio-
metric and catalytic CeH activation of 1-methyl-5-phenyl-1H-
1,4-benzodiazepin-2(3H)-one derivatives. In line with the
NHCOR1
NHCOR1
CO2R2
CO2R2
"Pd"
+
X
R1=Me, Ph
X=Br, H
* Corresponding author. Tel.: þ44 0208 3318215; fax: þ44 0208 3319805.
E-mail address: j.spencer@gre.ac.uk (J. Spencer).
Scheme 1. Catalytic palladium mediated vinylation reactions.
0040-4020/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2008.01.059

J. Spencer et al. / Tetrahedron 64 (2008) 6082e6089
6083
O
Related palladium catalyzed ortho-arylations of benzyl-
amines tolerate an iodide group on the benzylamine coupling
partner or a bromide on the aryl halide coupling partner
(Scheme 2).5 The activation of sp3 CeH bonds is also possible
by the use of a pyridine or quinoline tether, which coordinates
to the metal and directs the CeH activation. In both instances
(Scheme 3) a Pd(II)ePd(IV) pathway is thought to operate.
NHCOCH3
O
N
cat. Pd(OAc)2
cat. Cu(OAc)2
O2
120 °C
O
O
O
via
NHAc
Pd
OAc
O
cat. Pd(OAc)2
+
2
4-MePhI
AgOAc, CF3CO2H
ii) (CF3CO)2O
I
I
NHCOCF3
O
NH2
A
Scheme 5. Carbazoles via CeH activation and CeN bond formation.
Ar
NHCOCy
cat. Pd(OAc)2
AgOAc,
O
N
+
Ar-I
The regioselective synthesis of carbazoles involving CeH
activation and CeN bond formation has recently been reported
(Scheme 5). These processes are believed to involve a pallada-
cycle intermediate such as A, which loses HOAc prior to a
reductive elimination (CeN bond formation) step.7
Palladium catalyzed CeH activation, transmetallation pro-
cesses have also been reported. A cyclic transition state (B,
Scheme 6), involving a palladacycle intermediate has been
postulated.8
NH
Ar
N
Scheme 2. Palladium mediated arylation of benzylamines.
A range of CeH activation/arylation processes have been
developed, which employ a palladium catalyst with a stoichio-
metric amount of hypervalent iodine(III) arylating derivative.
The proposed mechanism involves a chelation-assisted CeH
activation (Y¼coordinating group), oxidation of Pd(II) to
Pd(IV) by the iodine(III) agent, and reductive elimination to
furnish the arylated product.6
2. Results and discussion
2.1. Stoichiometric CeH activation
The Sanford group has also disclosed cross-couplings of
aromatics via two CeH activations, involving palladacycles,
(Scheme 4). No homocoupling products were observed for
these reactions, which require stoichiometric amounts of silver
salts and several equivalents of arene coupling partner.6e
We recently disclosed the synthesis of a range of benzodiaz-
epine-based palladacycles 2 and 3 by CeH activation and these
were found to be effective precatalysts in a number of Heck and
Suzuki reactions (Scheme 7).9 The air stable dimeric complex
N
N
Pd(OAc)2
Ph2IBF4
+
general mechanism
C
Ph2I+
C
Ph
(IV)
C
C
H
(II)
Pd
Pd(II)
Pd
Y
Y
Y
Y
CH activation
palladacycle
reductive elimination
Scheme 3. CeH activation/arylations with iodonium salts.
O
F
F
cat. Pd(OAc)2
2 equiv. Ag2CO3
N
F
+
+
N
O
F
Scheme 4. Cross-couplings involving double CeH activations and a postulated palladacycle intermediate.

6084
J. Spencer et al. / Tetrahedron 64 (2008) 6082e6089
CH3
B
O
B
N
N
via
CH3
Pd(OAc)2
O
B
+
(MeBO)3
O
CH3
Pd
N
B
N
N
N
Pd(OAc)2
+
+
(MeBO)3
50%
20%
Scheme 6. CeH activation/transmetallation via a palladacycle.
CH3
N
CH3
O
O
CH3
N
N
O
R2
2
R2
PdCl4
-
L
R2
N
N
N
Cl
Pd
Cl
Pd
L
2
2
for 2a
(R2=H)
1
3
1a: R2=H.
1b: R2=Me.
1c: R2=i-Pr
CH3
N
B(OH)2
O
L=PPh3, pyridine
N
4a
Scheme 7. Reactions of benzodiazepine containing palladacycles.
2a reacted with triphenylphosphine to afford the monomeric
complex 3a (L¼PPh3) and underwent a stoichiometric reaction
with an arylboronic acid to afford the ortho-substituted benzo-
diazepine 4a.10
We were aware of two major limitations to the chemistry
depicted in Scheme 7.
recryztallisation afforded X-ray quality crystals, which were
found to be PdCl2(PPh3)2 indicating that the original CeH acti-
vation reaction had not taken place and a coordination complex
of the type (LeH)2PdCl2, where LeH¼1b, had been formed.
Treatment of the purported coordination complex with PPh3
presumably leads to displacement of LeH from palladium.
The attempted CeH activation of 1c using these conditions
was also unsuccessful although using Pd(OAc)2 as the metallat-
ing agent led presumably to 2c. The latter was not isolated as
such dimeric complexes are usually insoluble for NMR mea-
surements and exist as mixtures of isomers in solution. Treat-
ment of the purported complex 2c with either PPh3 or
pyridine led to the palladacycles 3c and 3d, respectively
(Scheme 8). Unfortunately, the treatment of 1b with Pd(OAc)2
did not lead to the expected 2b. Complex 3c was characterized
in solution by 1H and 31P spectroscopy and in the solid phase by
X-ray diffraction (Fig. 1). X-ray analysis of crystals of 3d ob-
tained from CH2Cl2/hexane led to the characterization of the
trans-isomer (Fig. 2). Bond lengths and angles (Tables 1 and
2) are comparable to similar complexes already reported in
the literature.9
(i) The ligands 1 and complexes 2 and 3 are mainly derived
from glycine and are hence achiral (i.e., 1ae3a, R2¼H).
With the plethora of amino acid starting materials avail-
able from the chiral pool we wanted to synthesize benzo-
diazepine containing ligands as well as palladacycles
containing a stereogenic centre i.e., with R2sH.
(ii) The synthesis of 4 involved stoichiometric amounts of
the palladacycle 2a. We wished to synthesize analogues
related to 4 employing CeH activation chemistry.
To address (i), we synthesized the ligands 1b and 1c using
standard synthetic protocols and alanine and valine-based start-
ing materials, respectively.11 The attempted CeH activation of
1b with Na2PdCl4, followed by treatment with PPh3 and

J. Spencer et al. / Tetrahedron 64 (2008) 6082e6089
6085
CH3
N
CH3
O
O
CH3
N
N
O
Pd(OAc)2
L
N
N
N
OAc
Pd
OAc
Pd
L
2
3c: L=PPh3
3d: L=pyridine
1c
2c
Scheme 8. Formation of palladacycles containing a remote stereogenic centre.
Figure 1. Ortep plot of complex 3c.
Figure 2. Ortep plot of complex 3d.

6086
J. Spencer et al. / Tetrahedron 64 (2008) 6082e6089
Table 1
Selected bond lengths (A) and angles ( ) for 3c
further potential side products have thus far proven to be in-
ꢀ
˚
separable by standard column chromatography. Therefore,
the synthesis and separation of 4b and analogues are ongoing
theme in our laboratory.
PdeC(21)
PdeN(5)
2.015(6)
2.085(4)
2.102(4)
PdeO(22)
PdeP
2.2620(16)
81.1(2)
C(21)ePdeN(5)
C(21)ePdeO(22)
N(5)ePdeO(22)
C(21)ePdeP
N(5)ePdeP
O(22)ePdeP
C(32)ePePd
C(26)ePePd
C(38)ePePd
3. Conclusion
171.10(18)
90.94(17)
96.30(15)
173.21(13)
92.08(12)
115.3(2)
Chelation-assisted CeH activations of an aryl group is
readily achieved using palladium in both stoichiometric and
catalytic amounts. Mechanistically relevant palladacycles are
easily isolated and can be characterized in solution and in
the solid state.
114.8(2)
110.98(19)
Symmetry transformations used to generate equivalent atoms.
4. Experimental section
Table 2
Selected bond lengths (A) and angles ( ) for 3d
4.1. General
ꢀ
˚
PdeC(21)
PdeN(5)
1.980(4)
2.025(3)
2.058(4)
2.117(3)
80.90(15)
94.77(17)
175.67(15)
173.08(14)
92.18(12)
92.15(14)
129.2(3)
113.2(3)
124.6(3)
118.5(4)
122.3(4)
All reactions were carried out in air and commercial grade
solvents and materials were used except where specified.
NMR spectra were measured on a Jeol EX270 spectrometer
at 270 MHz (1H) and 109 MHz (31P). Electrospray Ionization
(ESI) mass spectra were obtained on a model 300MSMS (Ap-
plied Biosystems, Warrington, UK) instrument. A TurboIon
source was employed in positive ion mode with minimum
heating. Samples containing 1 mg/ml were introduced as loop
injections into a flow of 50:50 v/v acetonitrile/0.05% aqueous
formic acid. The spectra and the theoretical isotope distribu-
tions were obtained with MassLynx v3.5 (Waters Associates,
UK). The X-ray structure data for 3c and 3d have been depos-
ited at the Cambridge Crystallographic Data Centre (CCDC
658227: 3c; CCDC 658228: 3d).
PdeN(26)
PdeO(22)
C(21)ePdeN(5)
C(21)ePdeN(26)
N(5)ePdeN(26)
C(21)ePdeO(22)
N(5)ePdeO(22)
N(26)ePdeO(22)
C(20)eC(21)ePd
C(16)eC(21)ePd
C(23)eO(22)ePd
C(31)eN(26)ePd
C(27)eN(26)ePd
Symmetry transformations used to generate equivalent atoms.
The analysis of palladacycles and their fragmentation by
mass spectrometry is a promising area of current interest.12
The spectra for complexes 3a and 3c are shown in Figure 3.
Species containing acetate and chloride residues both lose
these as anions and were detected as the counter cations.
Moreover, an acetonitrile adduct was observed for the cation
of 3a. An apparent sensitivity of the compounds to elevated
temperatures meant that the ion source conditions were main-
tained at a value as low as possible.
4.2. 3-Isopropyl-5-phenyl-1H-1,4-benzodiazepin-2(3H)-one12
2-Aminobenzophenone(1.97 g, 10.00 mmol), EEDQ(2.47 g,
10.00 mmol) and (L)-BoceGlyeOH (2.17 g. 10.00 mmol) were
combined in CH2Cl2 (20 ml) and stirred overnight at room tem-
perature. The organic layer was washed successively with HCl
(20 ml, 10% solution) and Na2CO3 (50 ml, saturated), dried
over MgSO4, filtered, and concentrated. The crude product was
dissolved in TFA (30 ml) and DCM (20 ml) and allowed to stir
for 2.5 h at room temperature. After concentration, the resulting
oil was dissolved in CH2Cl2 (20 ml) and reconcentrated then re-
dissolved in CH2Cl2 (20 ml). The organic layer was washed with
Na2CO3 (50 ml, saturated), dried (MgSO4 as above) and concen-
trated. Thereafter, to the resulting oil were added ammonium
acetate (4.00 g, 51.89 mmol) and AcOH (30 ml) and the reaction
mixture was stirred at room temperature overnight then concen-
trated under reduced pressure to afford an oil. The latter was dis-
solved in ethyl acetate (30 ml) and washed with a saturated
solution of Na2CO3 (3ꢁ30 ml). The organic layer was dried
over MgSO4 and filtered. After concentration of the solvent
and flash chromatography (10:1:1 DCM/ethyl acetate/hexane)
2.2. Catalytic CeH activation
Given the impressive use of palladacycles in catalytic CeH
activation chemistry, we have undertaken preliminary steps to-
wards the synthesis of the ortho-arylated benzodiazepine 4b
employing hypervalent iodonium salts as the arylating agent
(Table 3). A typical reaction was performed at 100 ꢀC for
12 h, using 1.5 equiv Ph2IBF4, 5 mol % Pd(OAc)2 in HOAc
in a sealed tube and the worked-up reaction mixture was ana-
lyzed by 1H NMR. The best conditions were found to be when
using a large excess of iodonium salt (2.5 equiv) or long reac-
tion times (64 h) (entries 2e4) where conversions reached ca.
60%. The use of microwave energy (entry 5) or additives such
as silver salts3a (entries 7 and 8) or PPh3 (entry 6) did not im-
prove the yield. However, the product, starting material and
1
a yellow solid was obtained (1.97 g, 71%). H NMR (CDCl3)
d 9.58 (1H, br s), 7.50e7.24 (9H, m), 3.12 (1H, d), 2.75 (1H,
m), 1.17 (3H, d), 1.08 (3H, d).

J. Spencer et al. / Tetrahedron 64 (2008) 6082e6089
6087
+
N
O
N
Pd
Ph3P
C34H28N2OPPd
100
617.1
Theoretical Isotope distribution
619.1
616.1
%
621.1
615.1
622.1
0
617.3
619.3
100
616.3
%
0
621.3
615.3
622.4
659.6
661.4
658.6
633.6
m/z
595 600 605 610 615 620 625 630 635 640 645 650 655 660 665 670 675
+
N
O
N
Pd
Ph3P
C37H34N2OPPd
659.1
100
%
Theoretical Isotope distribution
661.1
658.1
663.1
657.1
664.1
0
659.5
661.5
100
658.6
663.3
657.6
%
664.4
m/z
0
595 600 605 610 615 620 625 630 635 640 645 650 655 660 665 670 675
Figure 3. ESI of palladacycles 3a and 3c, showing both theoretical and experimentally derived values for the isotopomeric cluster of Pdþ ions.

6088
J. Spencer et al. / Tetrahedron 64 (2008) 6082e6089
Table 3
Attempted ortho-arylation of 1a
over MgSO4 then filtered over Celite. Hexane was allowed to
slowly diffuse into the CH2Cl2 solution and black crystals
1
were obtained (0.054 g, 56%). H NMR (CDCl3) d 9.07 (2H,
CH3
O
CH3
N
O
N
d), 7.8e7.24 (8H, m), 6.21 (1H, d), 4.95 (1H, d), 3.40 (4H, m),
1.04 (3H, d), 0.76 (3H, d). Found: C, 57.74; H, 5.02; N, 7.63.
(C26H27N3O3Pd$0.5H2O requires C, 57.31; H, 5.18; N, 7.71).
Pd(OAc)2
IPh2BF4
N
N
4.6. Reaction of 2c with triphenylphosphine, 3c
4b
1a
Compound 2c was prepared on a 0.68 mmol scale and treated
with triphenylphosphine (178 mg, 0.68 mmol) and stirred over-
night in CH2Cl2 (10 ml). Thereafter, the reaction mixture was
concentrated. Orange crystals were obtained from dichlorome-
thane/hexane (196 mg, 42%). 1H NMR (CDCl3): d 7.79 (6H, m),
7.56 (2H, m), 7.36 (11H, m), 7.10 (1H, m), 6.83 (1H, m), 6.55
(2H, m), 4.99 (1H, d), 3.46 (3H, s), 1.56 (3H, br s), 1.20 (1H,
d), 0.96 (3H, d), 0.72 (3H, d). 31P (CDCl3): d 40.69. Found: C,
62.11; H, 5.07; N, 3.51. (C39H37N2O3PPd$0.5CH2Cl2 requires
C, 62.29; H, 5.03; N, 3.68).
Entry
Ph2IBF4 (equiv)
Time (h)
Temp (ꢀC)
Conversion (%)
1
1.5
2.5
1.5
2.5
1.5
1.5
1.5
1.5
12
12
64
60
100
100
100
100
200
100
100
120
40
59
2
3
61
61
4
5a
6b
7c
8d
a
0.25
<10
<10
<10
29
12
12
48
In microwave.
b
c
d
PPh3 added (1 equiv).
AgBF4 added (1 equiv).
AgOAc added (1 equiv).
4.7. Arylation reactions: attempted formation of 4b
A mixture of 1a (78 mg, 0.33 mmol), Ph2IBF4 (180 mg,
0.49 mmol)13 and Pd(OAc)2 (3 mg, 5 mol %) in AcOH
(5 ml) was stirred for 64 h at 100 ꢀC. Thereafter, the cooled re-
action mixture was filtered over Celite and concentrated under
reduced pressure. The resulting crude product was dissolved in
CH2Cl2, washed with Na2CO3 then brine. The separated or-
ganic layer was dried over MgSO4, filtered and concentrated
in vacuo. (Selected regions) 1H NMR for 1a (CDCl3)
4.3. 1-Methyl-3-(isopropyl)-5-phenyl-1,3-dihydro-2H-1,4-
benzodiazepin-2-one, 1c
This was made on 1.19 mmol scale by stirring the product
from previous step (297 mg, 1.19 mmol) with NaH (60 mg,
1.5 mmol of a 60% suspension in mineral oil) in DMF for
30 min under Ar. Methyl iodide (132 ml, 2.11 mmol) was
added to the reaction mixture, which was stirred overnight at
room temperature. The reaction mixture was diluted with ethyl
acetate (20 ml) and washed with brine (3ꢁ20 ml), dried over
MgSO4, filtered and concentrated under reduced pressure.
Compound 1c was obtained as yellow solid (314 mg, 74%).
1H NMR (CDCl3) d 7.59e7.24 (9H, m), 3.41(3H, s), 3.08
(1H, d), 1.16 (3H, d), 1.02 (3H, d). 13C (CDCl3) d 169.6,
167.6, 143.9, 138.9, 131.1, 130.2, 130.2, 129.6, 129.1,
128.1, 123.6, 121.3, 69.7, 35.0, 29.5, 20.3, 19.2. HRMS calcd
291.1492; C19H20N2O requires 291.1490.
1
d 4.76(1H, d), 3.69(1H, d), 3.39 (NMe). H NMR for 1a/4b
mixture (CDCl3) d 4.11 (1H, d), 3.65 (1H, d), 3.12 (NMe) (as-
sumed to be 4b); 4.76 (1H, d), 3.69 (1H, d), 3.39 (NMe) (1a).
EI m/z mixture of 326 and 250 amu for 4b and 1a, respec-
tively. HRMS calcd 327.1492; C22H18N2O requires 327.1495.
Acknowledgements
Johnson Matthey is thanked for a generous loan of Pd salts.
Dr. Matthew Gaunt (University of Cambridge) is thanked for
useful discussions and for providing an initial sample of
IPh2BF4. The EPSRC Mass Spectrometry Service Centre
(Swansea) is thanked for the HRMS measurements.
4.4. Palladacycle 2c
Palladacycle 2c was made from 1c on a 0.18 mmol scale by
stirring with Pd(OAc)2 (40 mg, 0.18 mmol) in AcOH (5 ml)
for 1 h under reflux. The reaction was cooled, filtered (Celite)
and the solvent was concentrated in vacuo. The resulting solid
was dissolved in CH2Cl2 (10 ml) and the reaction mixture was
reconcentrated. This was repeated twice. After reconcentration
with diethyl ether, a red solid was obtained.
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ter 2 h, the organic layer was washed with water (5 ml), dried

J. Spencer et al. / Tetrahedron 64 (2008) 6082e6089
6089
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