Palladium catalyzed C-H functionalization of O-arylcarbamates: Selective ortho -bromination using NBS

Article abstract of DOI:10.1021/jo300713h

A series of cyclometalated palladium complexes derived from O-phenylcarbamates has been synthesized by the reaction of the respective carbamates with Pd(OAc)₂ in the presence of acids, CF ₃CO₂H, CF₃SO₃

Full text of DOI:10.1021/jo300713h

Article
pubs.acs.org/joc
Palladium Catalyzed C−H Functionalization of O-Arylcarbamates:
Selective ortho-Bromination Using NBS
Alex John and Kenneth M. Nicholas*
Department of Chemistry & Biochemistry, Stephenson Life Sciences Research Center, University of Oklahoma, 101 Stephenson
Parkway, Norman, Oklahoma 73019, United States
S
* Supporting Information
ABSTRACT: A series of cyclometalated palladium complexes
derived from O-phenylcarbamates has been synthesized by the
reaction of the respective carbamates with Pd(OAc)2 in the
presence of acids, CF3CO2H, CF3SO3H, and p-TsOH. The
palladacycles were observed to coordinate amines and electron
t
rich anilines but not sulfonamides or carboxamides. Analysis of the Bu-NH2 adduct of the palladacycle 2b (2b·tBu-NH2) by
NMR spectroscopy (NOE) revealed a cis-coordination of the amine. However, the amine adducts failed to undergo ortho-
amination (C−N bond formation) under varied reaction conditions. Notably, the palladacycle 1d was found to react efficiently
with N-iodosuccinimide (NIS) to yield the ortho-iodinated carbamate, 1e. More significantly, this reaction can be extended to a
palladium-catalyzed ortho C−H bromination of aryl-O-carbamates even at 5 mol % loading of Pd(OAc)2 using N-
bromosuccinimide (NBS).
activated substrate is crucial for the subsequent oxidative C−N
bond formation reaction. An earlier study by the Che6 group
had suggested the incorporation of the amide reagent through
the intermediacy of a nitrene species.
INTRODUCTION
■
The emergence and development of aromatic C−H function-
alization chemistry for the formation of Ar−C and Ar−X (X =
halogen, O, S, N) bonds has relied heavily on Pd catalysis.1
However, it must be pointed out that unlike the more widely
encountered C−C or other C−heteroatom bond formation,
palladium catalyzed C−N bond-forming reactions have been
achieved with limited success.2 Many of the challenges
associated with development of synthetically useful C−H
amination reactions are reminiscent of those encountered
during the early progress of the Buchwald−Hartwig amination
reaction.3
Synthetic efforts to-date have focused on both the intra-
molecular as well as the intermolecular versions of the C−H
amination reaction.4,1d−f Significantly, intramolecular C−H
amination reactions have been demonstrated to efficiently
generate several key aromatic N-heterocycles.5 The intermo-
lecular reactions, on the other hand, count on employing
suitable chelating groups, like anilides,6 N-heterocycles,7
amides,8 ketones,9 and oximes7 to direct the C−H function-
alization. Interestingly, a plethora of N-reagents ranging from
simple amides7,9 to alkyl amines8 have been shown to
participate in the reaction and are used either with an internal
or external oxidant. From a mechanistic viewpoint, Pd(II)/
Pd(0),10 Pd(0)/Pd(II),6a or Pd(II)/Pd(IV)11 pathways have
been proposed to be operational in the reactions.
In keeping with our longstanding interest in C−H amination
reactions12 we became interested in exploring the Pd-catalyzed
C−H amination of the relatively little explored O-aryl
carbamates using amide N-reagents, which could produce
synthetically important o-aminophenol derivatives. To our
knowledge the only C−H functionalizations reported of O-aryl
carbamate substrates are Pd-13 and Rh-catalyzed14 ortho
arylation/olefination reactions, respectively. The O-aryl carba-
mates represent an interesting class of compounds since they
can undergo a variety of Ni-catalyzed transformations involving
cleavage of the C−O bond.15 More importantly, the carbamate
functionality also serves as a latent directing group, since it can
be removed post ortho-functionalization to generate the parent
arene.16
Herein we report the synthesis of cyclometalated palladium
complexes derived from O-phenyl carbamates and their amine
adducts. Attempts to achieve C−N bond formation from these
adducts under oxidative conditions have been unsuccessful.
However, the palladacycles were found to efficiently yield ortho-
halogenated products upon reaction with N-halosuccinimide
(NXS) reagents, and we were able to render the ortho-
bromination reaction catalytic under suitable conditions.
Among the N-reagents found useful in synthetically relevant
C−H amination reactions, the sulfonamides and carboxamides
deserve special mention since the resulting aminated products
can be readily converted to the respective free amines. In this
respect, it is worth pointing out that in a recent report on C−H
amidation of ketones, Liu9 et al. have indicated that an initial
coordination of the amide reagent to the Pd center in the C−H
RESULTS AND DISCUSSION
Toward Dehydrogenative Amination. A recent report of
palladium-catalyzed dehydrogenative arylation of O-phenyl-
■
Received: April 14, 2012
Published: June 4, 2012
© 2012 American Chemical Society
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Scheme 1. Synthesis of Carbamates and the Cyclometalated Palladium Complexes
carbamates13a by Dong et al. led us to reason that the
intermediate palladacycles could potentially react with a
suitable N-reagent to generate ortho-aminated carbamates.
However, we were disappointed that no amidation product
was observed upon reaction of 1a (Scheme 1) with either Tos-
NH2 or Nos-NH2 using a catalytic amount of Pd(OAc)2 (10
mol %) in the presence of TFA (50 mol %) as additive and
K2S2O8 as a stoichiometric oxidant. Furthermore, NMR analysis
of the reaction mixture revealed that the palladacycle (even in
stoichiometric amount) resulting from C−H activation of 1a
was not generated during the reaction. At this point, we
decided to synthesize the palladacycle and explore its reactivity
with various N-reagents. The cyclopalladated complex 1b was
prepared by a procedure analogous to that reported by Dong13a
Figure 1. Dimer to monomer transformation in CD3CN.
amine at room temperature (Figure 2) and was characterized
1
extensively by H, 13C{1H}, and ESI-MS (high-resolution).
1
et al. (Scheme 1) and was characterized by H, 13C{1H}, and
1
ESI-MS analysis. The H NMR spectrum of 1b exhibited a
characteristic13a downfield-shifted proton at 6.68 ppm, relative
to that at 7.10 ppm in 1a, corresponding to the ortho C−H of
the cyclometalated palladium complex. With palladacycle 1b in
hand, we set out to explore its reactivity with potential N-
reagents, including Tos-NH2, Nos-NH2, benzamide, and
trifluoroacetamide, under various oxidizing conditions
(K2S2O8, oxone, PhI(OAc)2, O2, N-fluorobenzenesulfonimide
(NFSI), Selectfluor, at 80−120 °C). NMR analysis of the
reaction mixtures revealed that the palladacycle was either
completely unreacted or underwent decomposition to 1a.
At this juncture, we note that Liu and co-workers9 recently
reported a palladium catalyzed intermolecular amidation of
aromatic ketones, in which tuning the electrophilicity of the
palladium center was found to be crucial for coupling,
presumably to enable coordination of the amide partner to
the palladium center. Accordingly, we synthesized the
palladium complexes 1c17 and 1d bearing triflate (OTf) and
tosylate (OTs) ligands, respectively (Scheme 1). However, no
amidation was observed even with these electronically
modulated palladacycles under conditions used with 1b, and
NMR analysis also revealed that the amide N-reagents do not
coordinate to the palladium center. It appears that all of the
1b−d complexes exist as bimetallic species in noncoordinating
solvents, which convert to the monomer in a donor solvent.
This is supported by the marked differences observed for the
1H NMR aromatic resonances in case of the palladacycle 1d
(Figure 1) recorded in CD3CN vs CDCl3 (see the Supporting
Information).
Figure 2. Synthesis of amine adduct (2c) of palladacycle 2b.
1
The H NMR spectrum of the amine adduct 2c exhibits a
characteristic resonance for the ortho C−H proton of the
palladacycle, slightly upfield shifted with respect to that in 2b.
The ESI mass spectrum of 2c has a molecular ion peak at m/z =
373 corresponding to [M − OTs]+. However, upon subjecting
the amine adducts of the palladium complexes to oxidative
coupling conditions (K2S2O8, oxone, PhI(OAc)2, O2, NFSI,
Selectfluor, etc., at 80−120 °C, up to 16 h), rapid
decomposition to either the respective carbamate or the
palladacycle was observed. To determine if the unreactivity of
the amine adducts could be the result of an unfavorable trans
geometry between the coordinated amine and the ortho-carbon,
NMR correlation spectroscopy experiments were carried out. In
fact, nuclear Overhauser effect (NOE) studies of 2c confirmed
the cis disposition of the amine and aryl group, since irradiation
of the t-butyl resonance at 1.20 ppm resulted in NOE
enhancement of the ortho C−H proton signal at 6.59 ppm
(see the Supporting Information).
Stoichiometric and Catalytic ortho-Halogenation.
While exploring the reactivity of the palladacycle 1d toward
several potential N-reagents, we were delighted to observe a
quantitative formation of an ortho-iodinated product, 1e, upon
reaction with N-iodosuccinimide (NIS).19 Though the reaction
exhibited similar efficiency starting from the O-phenyl-
Upon testing the reactivity of the palladacycles 1 with a range
of N-donors, it was observed that only aliphatic amines, like
tBu−NH2, coordinate to palladium in the cyclometalated
complexes. The amine adduct18 was synthesized by stirring a
dichloromethane solution of the palladium complex with the
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carbamate using stoichiometric Pd(OAc)2, efforts to render the
reaction catalytic (10 mol %) suffered from competing acid (p-
TsOH) promoted para-iodination. However, we soon realized
that this competing p-halogenation can be almost completely
eliminated by employing NBS to yield the corresponding ortho-
brominated O-phenylcarbamate, 1f. Specifically, when a mixture
of phenyl-O-carbamate (1a), NBS (1.1 equiv), and p-TsOH
(0.5 equiv) were heated in the presence of 10 mol % Pd(OAc)2
in 1,2-dichloroethane (DCE) at 60 °C, the ortho-brominated
product, 1f, was obtained in 95% isolated yield. Furthermore,
the reaction proceeded with negligible loss of efficiency even at
5 mol % of Pd loading. We note that Bedford20 and co-workers
recently reported an example of palladium-catalyzed ortho-
chlorination of a carbamate substrate, which proceeds under
solvent-free conditions at an elevated temperature of 120 °C.
Since electron-rich carbamates underwent selective uncata-
lyzed para-bromination to the respective electron-donating
groups, we focused on electron-poor carbamates for the
palladium-catalyzed reaction. A brief optimization study
suggested the need for higher temperature (ca. 90 °C) and
catalyst loading (10 mol %) to achieve efficient brominations
(Figure 3). With suitable reaction conditions at our disposal, we
Table 1. Palladium-Catalyzed ortho-Bromination of O-Aryl
Carbamates
a
a
O-Aryl carbamate (0.614 mmol), NBS (0.679 mmol), p-TsOH
(0.308 mmol), and Pd(OAc)2 (5 or 10 mol %) in DCE (ca. 2 mL) at
b
Figure 3. Palladium catalyzed ortho-bromination of aryl-O-carbamates.
60−90 °C for the indicated period of time. NMR yield, 3a and 3f
c
have similar Rf values. Isolated yield, 1H NMR conversion in
d
parentheses. Reaction carried out at 120 °C.
undertook a brief survey of the ortho-bromination reaction (see
Table 1). Significantly, the ortho-brominated carbamates were
obtained in moderate to good isolated yields. The low yield
observed with the highly electron-poor nitro carbamate 5a
(Table 1, entry 4) is apparently due to its reduced propensity to
undergo electrophilic activation. It is worth mentioning that for
the electron-rich carbamate, 6a, competitive formation of the
ortho-brominated product 6f was observed under palladium
catalysis (Scheme 2). Importantly, the inclusion of potentially
reactive groups like halogen (Table 1, entries 5, 6 and 8) and
carbomethoxy (Table 1, entry 7) further highlights the
functional group compatibility of the method. Interestingly,
for a carbamate bearing a meta chloro substituent (Table 1,
entry 8), a higher reaction temperature (ca. 120 °C) over a
longer period of time was needed to achieve good conversion.
We note that this palladium catalyzed ortho-bromination is
complementary to the widely used directed ortho-metalation
method, which uses highly reactive organolithium reagents
stoichiometrically to produce ortho-brominated aryl-O-carba-
mates.21,15a A plausible mechanism for the present ortho-
bromination reaction could involve Pd(II)/Pd(IV) catalysis.
Initial C−H activation of the carbamate substrate would
generate the palladacycle, which can then oxidatively add the
NBS to furnish the corresponding Pd(IV) intermediate. In this
respect, it must be mentioned that recent studies do support
the ability of NXS reagents to promote oxidation of Pd(II) to
Pd(IV) and subsequent reductive elimination from the Pd(IV)
manifold to release organic products.1b Alternatively, the
reaction may also proceed via a bimetallic Pd(III)−Pd(III)
pathway for halogenation of aromatics as identified by Ritter
and co-workers.22
CONCLUSION
■
In summary, we have synthesized and characterized amine
adducts of some cyclometalated palladium complexes derived
from the O-phenyl carbamate motif. NOE studies suggest a cis
disposition of the amine relative to the aryl moiety in these
amine adducts; however, they failed to undergo any C−N bond
formation under oxidative conditions. Significantly the cyclo-
metalated palladacycles undergo reaction with NXS reagents to
yield the corresponding ortho-halogenated carbamates; a
palladium catalyzed ortho-bromination reaction of O-aryl
carbamates has been achieved at 5−10 mol % loading of
Pd(OAc)2.
EXPERIMENTAL SECTION
■
General Procedures. All manipulations were carried out using
Schlenk tube techniques under an inert atmosphere. Pd(OAc)2,
dichloroethane, phenol(s), and dimethylcarbamoyl chloride were
purchased from commercial sources. Dichloroethane was distilled
over CaH2 and subsequently stored over activated 4 Å molecular
sieves. 1H and 13C{1H} NMR spectra were recorded either on a 300 or
400 MHz NMR spectrometer. H NMR peaks are labeled as singlet
(s), doublet (d), triplet (t), and multiplet (m). Mass spectra were
acquired in the ESI (+) mode.
General Procedure for the Synthesis of Carbamates. A
mixture of the phenol (10.0 mmol) and Me2NCOCl (1.38 mL, 14.9
mmol) and K2CO3 (2.07 g, 14.9 mmol) in 25 mL of CH3CN was
refluxed for 5 h. The reaction mixture was cooled to room temperature
and concentrated under a vacuum. The residue was dissolved in H2O
(ca. 50 mL) and extracted with Et2O (ca. 2 × 20 mL). The organic
fractions were combined and then washed successively with 1 M KOH
(ca. 25 mL) and water. Finally the organic layer was separated, dried
over anhydrous MgSO4, and concentrated under a vacuum to yield the
corresponding carbamates.
1
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Scheme 2. Temperature Effect on ortho/para-Halogenation Selectivity
Phenyl Dimethylcarbamate (1a). White crystalline solid (1.20 g,
73%):23 1H NMR (CDCl3, 300 MHz, 25 °C) δ 7.34 (t, 3JHH = 8.1 Hz,
2H), 7.17 (t, 3JHH = 8.1 Hz, 1H), 7.10 (d, 3JHH = 8.1 Hz, 2H), 3.09 (s,
3H), 3.00 (s, 3H).
3-Methoxyphenyl Dimethylcarbamate (2a). Pale yellow oil
(0.989 g, 65%):14a 1H NMR (CDCl3, 300 MHz, 25 °C) δ 7.24 (t, 3JHH
= 9.0 Hz, 1H), 6.76−6.68 (m, 3H), 3.79 (s, 3H), 3.09 (s, 3H), 3.00 (s,
3H); 13C{1H} NMR (CDCl3, 75 MHz, 25 °C) δ 160.5, 154.9, 152.6,
129.7, 114.1, 111.4, 107.8, 55.5, 36.8, 36.6.
Biphenyl-4-yl Dimethylcarbamate (3a). Off-white solid (1.59 g,
66%): mp 130−132 °C; 1H NMR (CDCl3, 300 MHz, 25 °C) δ 7.59−
7.56 (m, 4H), 7.43 (t, JHH = 7.5 Hz, 2H), 7.34 (t, JHH = 7.2 Hz, 1H),
7.18 (d, JHH = 8.1 Hz, 2H), 3.13(s, 3H), 3.04 (s, 3H); 13C{1H} NMR
(CDCl3, 75 MHz, 25 °C) δ 155.1, 151.1, 140.7, 138.4, 128.9, 128.1,
127.3, 127.3, 122.2, 36.9, 36.6; IR (KBr pellet, cm−1) 1730 (s), 1485
(m), 1393 (m), 1221 (s), 1196 (s), 1171 (s); LRMS (ESI) 242 [(M +
H)+]; HRMS (ESI) Calcd for C15H16NO2 (M + H)+ requires m/z =
242.1181, found m/z = 242.1177.
NMR (CDCl3, 300 MHz, 25 °C) δ 8.04 (d, JHH = 8.7 Hz, 2H), 7.19
(d, JHH = 8.7 Hz, 2H), 3.90 (s, 3H), 3.11 (s, 3H), 3.02 (s, 3H).
3-Chlorophenyl Dimethylcarbamate (10a). Clear liquid (1.31
g, 65%):13a 1H NMR (CDCl3, 300 MHz, 25 °C) δ 7.28 (t, JHH = 8.4
Hz, 1H), 7.19−7.15 (m, 2H), 7.02 (d, JHH = 8.4 Hz, 1H), 3.08 (s, 3H),
3.01 (s, 3H).
Synthesis of the Cyclopalladated Complexes 1b−d and 2b.
[(Phenyl Dimethylcarbamate)Pd(μ-TFA)]2 (1b). A mixture of
phenyl-O-carbamate (1a) (0.132 g, 0.800 mmol) and Pd(OAc)2
(0.150 g, 0.668 mmol) in DCE (ca. 2 mL) in a 2-dram vial was
stirred at room temperature for 5 min. To the stirred solution
trifluoroacetic acid (0.076 g, 0.667 mmol) was added dropwise, and
the reaction mixture was stirred for another 30 min. At the end of this
period, ca. 5 mL of hexanes were added to the stirred mixture, and the
palladacycle precipitated from the solution as a light gray solid (0.142
g, 69%): mp (decomp.) 200−202 °C; 1H NMR (CDCl3, 300 MHz, 25
3
°C) δ 7.10 (dt, JHH = 6.9 and 1.8 Hz, 1H), 7.04−6.40 (m, 2H), 6.68
3
(dd, JHH = 6.6 and 0.9 Hz, 1H), 2.72 (s, 3H), 2.31 (s, 3H); 13C{1H}
NMR (CDCl3, 75 MHz, 25 °C) δ 166.2 (q, J = 37.8 Hz), 154.4, 148.5,
133.2, 126.1, 124.1, 115.1 (q, J = 285 Hz), 115.0, 111.7, 36.8, 36.6;
LRMS (ESI) 270 [(M − CF3COO)+]; IR (KBr pellet, cm−1) 1676 (s),
1634 (s), 1450 (m), 1412 (m), 1202 (s); HRMS (ESI) Calcd for
C9H10NO2Pd (M − CF3COO)+ requires m/z = 269.9746, found m/z
= 269.9753.
4-Cyanophenyl Dimethylcarbamate (4a). White solid (1.29 g,
68%): mp 61−62 °C; 1H NMR (CDCl3, 300 MHz, 25 °C) δ 7.66 (d,
JHH = 8.1 Hz, 2H), 7.25 (d, JHH = 8.1 Hz, 2H), 3.11 (s, 3H), 3.02 (s,
3H); 13C{1H} NMR (CDCl3, 75 MHz, 25 °C) δ 155.0, 153.7, 133.6,
122.8, 118.6, 108.9, 36.9, 36.7; IR (KBr pellet, cm−1) 2228 (m), 1732
(s), 1389 (m), 1223 (s), 1159 (s); LRMS (ESI) 191 [(M + H)+];
HRMS (ESI) Calcd for C10H11N2O2 (M + H)+ requires m/z =
191.0821, found m/z = 191.0814.
4-Nitrophenyl Dimethylcarbamate (5a). Pale yellow solid (1.65
g, 78%): mp 104−105 °C; 1H NMR (CDCl3, 300 MHz, 25 °C) δ 8.24
(d, JHH = 8.7 Hz, 2H), 7.29 (d, JHH = 8.7 Hz, 2H), 3.12 (s, 3H), 3.03
(s, 3H); 13C{1H} NMR (CDCl3, 75 MHz, 25 °C) δ 156.5, 153.5,
144.7, 125.1, 122.3, 36.9, 36.6; IR (KBr pellet, cm−1) 1728 (s), 1520
(s), 1344 (m), 1227 (m); LRMS (ESI) 211 [(M + H)+]; HRMS (ESI)
Calcd for C9H11N2O4 (M + H)+ requires m/z = 211.0719, found m/z
= 211.0712.
2-Ethylphenyl Dimethylcarbamate (6a). Pale yellow oil (1.09 g,
57%):13a 1H NMR (CDCl3, 300 MHz, 25 °C) δ 7.25−7.11 (m, 3H),
7.08 (dd, JHH = 7.5 and 1.8 Hz, 1H), 3.13 (s, 3H), 3.03 (s, 3H), 2.61
(q, JHH = 7.5 Hz, 2H), 1.22 (t, JHH = 7.8 Hz, 3H); 13C{1H} NMR
(CDCl3, 75 MHz, 25 °C) δ 154.9, 149.6, 136.1, 129.3, 126.8, 125.6,
122.6, 36.8, 36.5, 23.4, 14.4.
4-Fluorophenyl Dimethylcarbamate (7a). Clear liquid (1.09 g,
60%):13a 1H NMR (CDCl3, 300 MHz, 25 °C) δ 7.06−7.02 (m, 4H),
3.09 (s, 3H), 3.00 (s, 3H); 13C{1H} NMR (CDCl3, 75 MHz, 25 °C) δ
161.5, 158.3, 154.9, 147.4, 147.3, 123.2, 123.1, 115.9, 115.6, 36.7, 36.4;
19F NMR (CDCl3, 282 MHz, 25 °C) δ −118.2.
4-Bromophenyl Dimethylcarbamate (8a). Clear oil (1.27 g,
53%):24 1H NMR (CDCl3, 300 MHz, 25 °C) δ 7.45 (d, JHH = 8.7 Hz,
2H), 7.00 (d, JHH = 8.7 Hz, 2H), 3.08 (s, 3H), 3.00 (s, 3H).
Methyl 4-(Dimethylcarbamoyloxy)benzoate (9a). To a stirred
suspension of NaH (0.394 g, 9.85 mmol, 60% in oil) in THF (ca. 25
mL) at ice bath temperature was added slowly methyl 4-
hydroxybenzoate (1.00 g, 6.57 mmol). The mixture was allowed to
stir for 30 min, followed by the addition of Me2NCOCl (0.66 mL, 7.16
mmol), and continued to stir for 8 h while allowing it to warm up to
room temperature. The reaction mixture was washed with cold water,
and the organics extracted into diethyl ether (ca. 20 mL). The organic
layer was dried over anhydrous Na2SO4 and concentrated under a
vacuum to yield the product as a white solid (1.93 g, 87%):15c 1H
[(Phenyl Dimethylcarbamate)Pd(μ-OTf)]2 (1c). A mixture of
phenyl-O-carbamate (1a) (0.110 g, 0.667 mmol) and Pd(OAc)2
(0.100 g, 0.445 mmol) in DCE (ca. 2 mL) in a 2-dram vial was
stirred at room temperature for 5 min. To the stirred solution was
added triflic acid (0.067 g, 0.446 mmol) dropwise, and the reaction
mixture was stirred for another 15 min. At the end of this period, ca. 5
mL of hexanes were added to the stirred reaction mixture, and the
palladacycle precipitated as a light gray solid (0.191 g, 67%): mp
(decomp.) 140−142 °C. The precipitated product was washed further
1
with hexanes (ca. 5 mL) and then dried under a vacuum: H NMR
3
(DMSO-d6, 300 MHz, 25 °C) δ 7.29 (d, JHH = 8.4 Hz, 1H), 7.11 (t,
3JHH = 7.5 Hz, 1H), 6.99 (t, 3JHH = 7.5 Hz, 1H), 6.85 (d, 3JHH = 8.1 Hz,
1H), 3.20 (s, 3H), 2.97 (s, 3H); 13C{1H} NMR (CDCl3, 75 MHz, 25
°C) δ 155.6, 152.8, 135.7, 129.9, 127.1, 125.8, 122.0, 37.0; IR (KBr
pellet, cm−1) 3287 (br), 1643 (s), 1414 (m), 1259 (br, s), 1176 (s),
1034 (s), 756 (m), 644 (s); HRMS (ESI) Calcd for C9H10NO2Pd (M
− OTf)+ requires m/z = 269.9746, found m/z = 269.9725.
[(Phenyl Dimethylcarbamate)Pd(μ-OTs)]2 (1d). A mixture of
phenyl-O-carbamate (1a) (0.215 g, 1.30 mmol) and Pd(OAc)2 (0.219
g, 0.975 mmol) in DCE (ca. 3 mL) in a 2-dram vial was stirred at room
temperature for 5 min. To the stirred solution was added p-TsOH
(0.188 g, 1.09 mmol), and the reaction mixture was continued to stir at
80 °C for another 30 min. At the end of this period, the reaction
mixture was cooled to room temperature and treated with hexanes (ca.
10 mL) while stirring, causing the palladacycle to precipitate as a light
gray solid (0.410 g, 82%): mp (dec.) 148−151 °C; 1H NMR (CDCl3,
300 MHz, 25 °C) δ 7.67 (d, 3JHH = 8.1 Hz, 2H), 7.37 (dd, 3JHH = 7.5
and 1.2 Hz, 1H), 7.10 (d, 3JHH = 8.4 Hz, 2H), 7.05 (dd, 3JHH = 8.7 and
1.2 Hz, 1H), 6.97 (dt, 3JHH = 8.1 and 1.2 Hz, 1H), 6.74 (dd, 3JHH = 8.7
1
and 1.2 Hz, 1H), 3.13 (s, 3H), 2.97 (s, 3H), 2.28 (s, 3H); H NMR
(acetonitrile-d3, 300 MHz, 25 °C) δ 7.67 (d, 3JHH = 8.1 Hz, 2H), 7.20
(d, 3JHH = 8.1 Hz, 2H), 7.15 (dt, 3JHH = 7.8 and 1.8 Hz, 1H), 7.07 (dd,
3JHH = 7.5 and 1.8 Hz, 1H), 6.97 (dt, 3JHH = 8.1 and 1.5 Hz, 1H), 6.85
3
(dd, JHH = 8.1 and 1.8 Hz, 1H), 3.19 (s, 3H), 3.02 (s, 3H), 2.35 (s,
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3H); 13C{1H} NMR (acetonitrile-d3, 75 MHz, 25 °C) δ 156.5, 151.1,
146.3, 140.2, 137.3, 134.4, 129.7, 128.0, 127.0, 125.9, 119.7, 37.9, 37.8,
21.6; IR (KBr pellet, cm−1) 1643 (s), 1408 (m), 1261 (s), 1126 (m),
1001 (m); HRMS (ESI) Calcd for C9H10NO2Pd (M − OTs)+ requires
m/z = 269.9746, found m/z = 269.9755.
(d, JHH = 9.1 Hz, 1H), 3.19 (s, 3H), 3.05 (s, 3H); 13C{1H} NMR
(CDCl3, 75 MHz, 25 °C) δ 153.7, 148.4, 138.8, 138.1, 132.0, 131.4,
128.7, 127.8, 126.9, 124.4, 122.2, 36.9, 36.6; IR (KBr pellet, cm−1)
2926 (w), 1720 (s), 1227(m), 1169(m); HRMS (ESI) Calcd for
C15H15BrNO2 (M + H)+ requires m/z = 320.0286, found m/z =
320.0289.
[(3-Methoxyphenyl Dimethylcarbamate)Pd(μ-OTs)]2 (2b). A
mixture of phenyl-O-carbamate (2a) (0.200 g, 1.02 mmol) and
Pd(OAc)2 (0.148 g, 0.659 mmol) in DCE (ca. 3 mL) in a 2-dram vial
was allowed to stir at room temperature for 5 min. To the stirred
solution was added p-TsOH (0.125 g, 0.727 mmol), and the reaction
mixture was continued to stir at room temperature for 10 min followed
by at 70 °C for another 30 min. At the end of this period, the reaction
mixture was cooled to room temperature, and then ca. 10 mL of
hexanes were added while stirring. The palladacycle then precipitated
from the solution as a light gray solid (0.271 g, 87%): mp (dec.) 122−
2-Bromo-4-cyanophenyl Dimethylcarbamate (4f). White solid
(0.133 g, 81%): mp 90−92 °C; 1H NMR (CDCl3, 300 MHz, 25 °C) δ
7.64 (d, JHH = 9 Hz, 1H), 7.53 (d, JHH = 2.4 Hz, 1H), 7.22 (dd, JHH
=
9.0 and 2.4 Hz, 1H), 3.09 (s, 3H), 3.03 (s, 3H); 13C{1H} NMR
(CDCl3, 75 MHz, 25 °C) δ 155.1, 153.8, 135.0, 133.7, 126.8, 121.5,
117.1, 112.4, 37.0, 36.7; IR (KBr pellet, cm−1) 2228 (m), 1734 (s),
1391 (m), 1225 (m), 1163 (m); HRMS (ESI) Calcd for
C10H9N2BrO2Na (M + Na)+ requires m/z = 290.9745, found m/z =
290.9743.
124 °C; 1H NMR (acetonitrile-d3, 300 MHz, 25 °C) δ 7.66 (d, 3JHH
=
8.1 Hz, 2H), 7.19 (d, 3JHH = 8.1 Hz, 2H), 6.92 (d, 3JHH = 9.0 Hz, 1H),
6.62 (dd, 3JHH = 8.7 and 3.0 Hz, 1H), 6.54 (d, 3JHH = 3.0 Hz, 1H), 3.75
(s, 3H), 3.17 (s, 3H), 3.02 (s, 3H), 2.35 (s, 3H); 13C{1H} NMR
(acetonitrile-d3, 75 MHz, 25 °C) δ 160.4, 156.2, 150.8, 145.4, 140.6,
137.0, 129.7, 126.9, 112.1, 104.9, 56.4, 37.9, 37.8, 21.5; IR (KBr pellet,
cm−1) 3152 (br), 1647 (s), 1472 (m), 1385 (m), 1200 (s), 1169 (s);
HRMS (ESI) Calcd for C10H12NO3Pd (M − OTs)+ requires m/z =
299.9852, found m/z = 299.9851.
2-Bromo-4-nitrophenyl Dimethylcarbamate (5f). Pale yellow
1
solid (0.074 g, 42%): mp 114−116 °C; H NMR (CDCl3, 300 MHz,
25 °C) δ 8.50 (d, JHH = 3 Hz, 1H), 8.21 (dd, JHH = 8.7 Hz and 2.4 Hz,
1H), 7.46 (d, JHH = 8.7 Hz, 1H), 3.19 (s, 3H), 3.06 (s, 3H); 13C{1H}
NMR (CDCl3, 75 MHz, 25 °C) δ 154.1, 152.7, 128.8, 124.4, 123.9,
116.9, 115.6, 37.2, 36.9; IR (KBr pellet, cm−1) 1744 (s), 1524 (m),
1348 (m), 1254 (m), 1161 (m), 1113 (m); HRMS (ESI) Calcd for
C9H10BrN2O4 (M + H)+ requires m/z = 288.9824, found m/z =
288.9818.
Synthesis of the Amine Adduct of the Cyclopalladated
Complex (2c). (3-Methoxyphenyl Dimethylcarbamate)Pd-
(OTs)(tBu-NH2) (2c). To a stirred suspension of 2b (0.051 g, 0.108
2-Bromo-6-ethylphenyl Dimethylcarbamate (6f) and 5-
1
Bromo-2-ethylphenyl Dimethylcarbamate (6f′). Data: H NMR
t
mmol) in dichloromethane (ca. 2 mL) was added Bu-NH2 (0.020 g,
(CDCl3, 300 MHz, 25 °C) δ 7.41 (d, JHH = 7.5 Hz, 0.6H), 7.35 (d, JHH
0.274 mmol), and the reaction mixture was allowed to stir at room
temperature for 1 h. The reaction mixture was subsequently filtered
and concentrated under a vacuum to yield the product as a dark gray
= 2.4 Hz, 1.1H), 7.30 (dd, JHH = 8.1 and 2.1 Hz, 1.5H), 7.18 (d, JHH
=
7.8 Hz, 0.5H), 7.03 (t, JHH = 7.8 Hz, 0.6H), 6.95 (d, JHH = 8.7 Hz,
1H), 3.18 (s, 1H), 3.11 (s, 3.9H), 3.04 (s, 1.6H), 3.01 (s, 4.0H), 2.55
(q, JHH = 7.8 Hz, 3.5H), 1.23−1.17 (m, 6.9H); HRMS (ESI) Calcd for
C11H15BrNO2 (M + H)+ requires m/z = 272.0286, found m/z =
272.0280.
2-Bromo-4-fluorophenyl Dimethylcarbamate (7f). Off-white
liquid (0.084 g, 52%): 1H NMR (CDCl3, 300 MHz, 25 °C) δ 7.31 (dd,
J = 7.5 and 3.0 Hz, 1H), 7.19−7.15 (m, 1H), 7.05−6.99 (m, 1H), 3.15
(s, 3H), 3.02 (s, 3H); 13C{1H} NMR (CDCl3, 75 MHz, 25 °C) δ
161.4, 158.1, 153.8, 145.3, 125.0, 124.9, 120.3, 119.9, 116.9, 116.7,
115.4, 115.1, 37.0, 36.7; 19F NMR (CDCl3, 282 MHz, 25 °C) δ
−115.4; IR (KBr pellet, cm−1) 1736 (s), 1599 (w), 1485 (m), 1387
(m), 1184 (m), 1159 (s); HRMS (ESI) Calcd for C9H10BrFNO2 (M +
H)+ requires m/z = 261.9879, found m/z = 261.9898.
1
solid (0.043 g, 73%): mp (dec.) 166−168 °C; H NMR (CDCl3, 300
3
3
MHz, 25 °C) δ 7.75 (d, JHH = 8.1 Hz, 2H), 7.19 (d, JHH = 8.1 Hz,
3
3
2H), 7.10 (d, JHH = 8.4 Hz, 1H), 6.59 (dd, JHH = 8.4 and 2.4 Hz,
1H), 6.40 (d, 3JHH = 1.8 Hz, 1H), 3.73 (s, 3H), 3.39 (d, 3JHH = 9.0 Hz,
1H), 3.33 (s, 3H), 3.13 (s, 3H), 2.93 (d, 3JHH = 11.1 Hz, 1H), 2.36 (s,
3H), 1.20 (s, 9H); 13C{1H} NMR (CDCl3, 75 MHz, 25 °C) δ 158.4,
157.0, 154.7, 141.7, 140.6, 138.4, 129.0, 126.0, 118.0, 111.5, 107.7,
55.3, 53.1, 37.1, 36.9, 31.6, 21.5; IR (KBr pellet, cm−1) 3277 (br), 3225
(br), 3144 (br), 2968 (m), 1703 (s), 1375 (m), 1180 (br), 1005 (m);
LRMS (ESI) 373 [(M − OTs)+]; HRMS (ESI) Calcd for
C14H23N2O3Pd (M-OTs)+ requires m/z = 373.0744, found m/z =
373.0747.
2-Iodophenyl Dimethylcarbamate (1e).22 To a stirred solution
of 1d (0.032 g, 0.072 mmol) in CH2Cl2 (ca. 1 mL) was added N-
iodosuccinimide (0.016 g, 0.071 mmol), and the reaction mixture was
allowed to stir at room temperature for 8 h. The reaction mixture was
filtered through a silica pad, and the filtrate was concentrated under a
2,4-Dibromophenyl Dimethylcarbamate (8f). Off-white liquid
(0.091 g, 46%): 1H NMR (CDCl3, 300 MHz, 25 °C) δ 7.71 (d, JHH
=
2.4 Hz, 1H), 7.42 (dd, JHH = 8.7 and 2.4 Hz, 1H), 7.10 (d, JHH = 8.4
Hz, 1H), 3.14 (s, 3H), 3.02 (s, 3H); 13C{1H} NMR (CDCl3, 75 MHz,
25 °C) δ 153.4, 148.1, 135.3, 131.4, 125.4, 118.7, 117.4, 36.9, 36.6; IR
(KBr pellet, cm−1) 1734 (s), 1466 (m), 1389 (m), 1225 (m), 1159
(m); HRMS (ESI) Calcd for C9H1081Br79BrNO2 (M + H)+ requires
m/z = 323.9058, found m/z = 323.9054.
1
vacuum to yield 1e as an off-white solid (0.020 g, 95%): H NMR
(CDCl3, 300 MHz, 25 °C) δ 7.79 (d, 3JHH = 6.9 Hz, 1H), 7.35 (t, 3JHH
3
3
= 7.5 Hz, 1H), 7.18 (d, JHH = 7.8 Hz, 1H), 6.95 (t, JHH = 8.1 Hz,
1H), 3.19 (s, 3H), 3.05 (s, 3H).
General Procedure for Palladium Catalyzed ortho-Bromina-
tion of O-Arylcarbamates. To a stirred solution of the substituted
carbamate (0.614 mmol) in dichloroethane (ca. 2 mL) was added N-
bromosuccinimide (0.121 g, 0.679 mmol), p-TsOH (0.053 g, 0.308
mmol), and Pd(OAc)2 (5−10 mol %). The reaction mixture was
stirred at 60−90 °C for the stipulated period of time. After solvent
evaporation, silica gel column chromatography of the reaction residue
using ether/hexanes yielded the pure ortho-brominated products.
2-Bromophenyl Dimethylcarbamate (1f).25 Off-white solid
(0.133 g, 89%): 1H NMR (CDCl3, 300 MHz, 25 °C) δ 7.58 (dd, 3JHH
Methyl 3-Bromo-4-(dimethylcarbamoyloxy)benzoate (9f).
Clear liquid (0.115 g, 62%): H NMR (CDCl3, 300 MHz, 25 °C) δ
1
8.25 (d, JHH = 1.5 Hz, 1H), 7.97 (dd, JHH = 8.4 and 1.5 Hz, 1H), 7.30
(d, JHH = 9.0 Hz, 1H), 3.89 (s, 3H), 3.15 (s, 3H), 3.02 (s, 3H);
13C{1H} NMR (CDCl3, 75 MHz, 25 °C) δ 165.3, 153.0, 152.4, 134.5,
129.7, 128.5, 123.9, 116.3, 52.4, 36.9, 36.6; IR (KBr pellet, cm−1) 1732
(s), 1599 (w), 1385 (m), 1283 (s), 1250 (s), 1157 (s), 1111 (m), 762
(m); HRMS (ESI) Calcd for C11H13BrNO4 (M + H)+ requires m/z =
302.0028, found m/z = 302.0022.
2-Bromo-5-chlorophenyl Dimethylcarbamate (10f). Off-white
liquid (0.121 g, 71%): 1H NMR (CDCl3, 300 MHz, 25 °C) δ 7.49 (d,
JHH = 8.7 Hz, 1H), 7.26 (d, JHH = 2.4 Hz, 1H), 7.07 (dd, JHH = 8.7 Hz
and 2.4 Hz, 1H), 3.15 (s, 3H), 3.03 (s, 3H); 13C{1H} NMR (CDCl3,
75 MHz, 25 °C) δ 153.3, 149.4, 133.7, 133.6, 127.1, 124.8, 114.7, 37.0,
36.7; IR (KBr pellet, cm−1) 1738(s), 1466(m), 1377(m), 1157(m);
HRMS (ESI) Calcd for C9H10BrClNO2 (M + H)+ requires m/z =
277.9583, found m/z = 277.9576.
4
3
4
= 7.5 and JHH = 1.2 Hz, 1H), 7.31 (dt, JHH = 6.9 Hz and JHH = 1.2
3
4
Hz, 1H), 7.21 (dd, JHH = 8.4 Hz and JHH = 1.8 Hz, 1H), 7.08 (dt,
4
3JHH = 8.1 Hz and JHH = 1.8 Hz, 1H), 3.17 (s, 3H), 3.04 (s, 3H);
13C{1H} NMR (CDCl3, 75 MHz, 25 °C) δ 153.7, 148.8, 133.1, 128.3,
126.7, 124.2, 116.4, 36.8, 36.6.
3-Bromobiphenyl-4-yl Dimethylcarbamate (3f). Off-white
1
solid: mp 78−82 °C; H NMR (CDCl3, 300 MHz, 25 °C) δ 7.79
(d, JHH = 2.4 Hz, 1H), 7.54−7.50 (m, 3H), 7.46−7.36 (m, 3H), 7.28
5604
dx.doi.org/10.1021/jo300713h | J. Org. Chem. 2012, 77, 5600−5605

The Journal of Organic Chemistry
Article
K. M. J. Org. Chem. 2010, 75, 7644−7650. (e) Barman, D. N.; Liu, P.;
Houk, K. N.; Nicholas, K. M. Organometallics 2010, 29, 3404−3412.
(13) (a) Zhao, X.; Yeung, C. S.; Dong, V. M. J. Am. Chem. Soc. 2010,
132, 5837−5844. (b) Bedford, R. B.; Webster, R. L.; Mitchell, C. J.
Org. Biomol. Chem. 2009, 7, 4853−4857.
ASSOCIATED CONTENT
■
S
* Supporting Information
Spectral data for all new compounds reported. This material is
available free of charge via the Internet at http://pubs.acs.org.
(14) (a) Gong, T.-J.; Xiao, B.; Liu, Z.-J.; Wan, J.; Xu, J.; Luo, D.-F.;
Fu, Y.; Liu, L. Org. Lett. 2011, 13, 3235−3237. (b) Feng, C.; Loh, T.-
C. Chem. Commun. 2011, 47, 10458−10460.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: knicholas@ou.edu. Tel.: +1-(405)-325-3696. Fax: +1-
(405)-325-6111.
(15) (a) Mesganaw, T.; Silberstein, A. L.; Ramgren, S. D.; Nathel, N.
F. F.; Hong, X.; Liu, P.; Garg, N. K. Chem. Sci. 2011, 2, 1766−1771.
(b) Quasdorf, K. W.; Antoft-Finch, A.; Liu, P.; Silberstein, A. L.;
Komaromi, A.; Blackburn, T.; Ramgren, S. D.; Houk, K. N.; Snieckus,
V.; Garg, N. K. J. Am. Chem. Soc. 2011, 133, 6352−6363. (c) Xu, L.; Li,
B.-J.; Wu, Z.-H.; Lu, X.-Y.; Guan, B.-T.; Wang, B.-Q.; Zhao, K.-Q.; Shi,
Z.-J. Org. Lett. 2010, 12, 884−887.
(16) Sengupta, S.; Leite, M.; Raslan, D. S.; Quesnelle, C.; Snieckus,
V. J. Org. Chem. 1992, 57, 4066−4068.
(17) A related palladacycle has been isolated. Xiao, B.; Fu, Y.; Xu, J.;
Gong, T.-J.; Dai, J.; Yi, J.; Liu, L. J. Am. Chem. Soc. 2010, 132, 468−
469.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We are grateful for financial support provided by the National
Science Foundation. A.J. would like to thank Dr. Steven Foster
and Dr. Susan L. Nimmo for help with ESI mass spectra and
NMR NOE acquisition, respectively.
(18) Other amines included cyclohexylamine and electron rich
aromatic amines, like aniline and toluidine, etc.
(19) For recent examples of Pd-catalyzed ortho-halogenation, see:
(a) Bedford, R. B.; Haddow, M. F.; Mitchell, C. J.; Webster, R. L.
Angew. Chem., Int. Ed. 2011, 50, 5524−5527. (b) Dudnik, A. S.;
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