Reductive C(sp2)-N elimination from isolated Pd(IV) amido aryl complexes prepared using H2O2 as oxidant

Article abstract of DOI:10.1021/jacs.6b12648

Di-2-pyridyl ketone (dpk)-supported amidoarylpallada(Il)cycles derived from various 2-(N-R-amino)biphenyls (R = H, Me, CF₃CO, MeSOj, CF₃SO₂) react with hydrogen peroxide in MeOH, THF, MeCN or AcOH to form the corresponding C-N coupled products, N-R-substituted carbazoles, in 82-98% yield. For R = MeS02 and CF₃SO₂, the corresponding reaction intermediates, amidoaryl Pd(IV) complexes were isolated and characterized by single crystal X-ray diffraction and/or NMR spectroscopy. For the first time, the C(sp²)-N reductive elimination from isolated amidoaryl Pd(lV) complexes has been studied in detail.

Full text of DOI:10.1021/jacs.6b12648

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Communication
2
Reductive C(sp)-N Elimination from Isolated Pd(IV)
2
2
Amido Aryl Complexes Prepared Using HO as Oxidant
Elikplim Abada, Peter Y. Zavalij, and Andrei N Vedernikov
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.6b12648 • Publication Date (Web): 03 Jan 2017
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Reductive C(sp2)-N Elimination from Isolated Pd(IV) Amido Aryl
Complexes Prepared Using H2O2 as Oxidant
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Elikplim Abada, Peter Y. Zavalij, and Andrei N. Vedernikov*
Department of Chemistry and Biochemistry, University of Maryland, College Park, MD 20742 (USA)
Supporting Information Placeholder
complexes 4a and 4b (Scheme 1, Fig. 1b) that can undergo
C(sp2)—N reductive elimination to form the
ABSTRACT: Di-2-pyridyl ketone (dpk) – supported ami-
doarylpallada(II)cycles derived from various 2-(N-R-
amino)biphenyls (R = H, Me, CF3CO, MeSO2, CF3SO2) react
with hydrogen peroxide in MeOH, THF, MeCN or AcOH to form
the corresponding C-N coupled products, N-R-substituted carba-
zoles, in 82-98% yield. For R = MeSO2 and CF3SO2 the corre-
sponding reaction intermediates, amidoaryl Pd(IV) complexes
were isolated and characterized by single crystal X-ray diffraction
and/or NMR spectroscopy. For the first time the C(sp2)-N reduc-
tive elimination from isolated amidoaryl Pd(IV) complexes has
been studied in detail.
1) Pd(OAc)2
2) dpk
H2O2
N
N
R
R
Pd
O
Pd
HN
N
N
N
N
R
1a-1c
2a-2c
HO OOH
3a-3c
- "(dpk)PdII"
OH
a: R=SO2CH3
N
R
Pd
O
b: R=SO2CF3
c: R=COCF3
N
R
N
N
5a-5c
Carbon-nitrogen coupling is a key product-forming step in tran-
sition metal-mediated syntheses of organic amines.1 The most
prominent example of this chemistry is the Buchwald – Hartwig
amination of electrophilic arene derivatives catalyzed by Pd
complexes.1-3 Palladium – catalyzed oxidative C-H amination of
arenes is an alternative approach to the synthesis of aromatic
amines which, depending on the nature of the oxidant, can be
quite atom-economical, as expected in the case of O2 or H2O2
used as oxidants. In fact, the existing protocols of palladium-
catalyzed oxidative C-H amination utilize a range of oxidants. In
particular, oxidative C(sp2)-H amination of 2-aminobiphenyls to
form carbazoles was reported using as an oxidant
OH
4a-4c
Scheme 1. Synthesis of neutral amido aryl palladium(IV) com-
plexes 4a-4c and their C(sp2)-N reductive elimination to form
carbazoles 5a-5c.
O2/Cu(OAc)2,4 PhI(OAc),5 Oxone6, or O2 in
a Cp*Ir(III)-
photoredox system.7 Oxidative CH amination of C(sp2)-H or
C(sp3)-H bonds was also performed using such oxidants as
10
K2S2O8,8
AgOAc,9
Ce(SO4)2
or
N-fluoro-2,4,6-
a)
b)
trimethylpyridinium triflate.10 Formation of high-valent Pd amido
hydrocarbyl intermediates and their C-N reductive elimination
were proposed as important steps in some of these reactions.5,6
Recently, a selective C(sp3)-N reductive elimination from an
isolated aryl alkyl Pd(IV) complex was reported by Sanford;11
no C(sp2)-N elimination was detected. It was concluded that the
C(sp3)-N coupling reaction at the Pd(IV) center operates an SN2
– type mechanism and is reminiscent of C(sp3)-N coupling of a
series of amido methyl Pt(IV) complexes studied by Goldberg.12
In turn, the formation of amido aryl Pd(IV) complexes and their
C(sp2)—N bond elimination were proposed by Gaunt5 to occur
in Pd-catalyzed oxidative C(sp2)-H amidation but no Pd(IV)
complexes have been observed in those experiments. In this
communication we report the first isolable amido aryl Pd(IV)
Figure 1. ORTEP plot (50% probability ellipsoids) for the amido
aryl Pd(II) complex 2b (a) and its Pd(IV) derivative 4b (b). Hy-
drogen atoms except those of the OH groups are omitted for
clarity.
corresponding N-substituted carbazoles 5 and discuss some
factors that affect the oxidative C(sp2)-N coupling. These re-
sults improve our understanding of the key steps of the latter
reaction involving high-valent Pd complexes, a synthetic meth-
od that has found important applications in modern organic
synthesis.5,6 Our results can also serve to improve our under-
standing of the reductive elimination reactivity of organotransi-
tion metal complexes, one of the fundamental organometallic
reactions.13 Notably, in this work the preparation of the amido
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aryl Pd(IV)complexes 4a and 4b could be achieved by oxida-
tion with H2O2 of the corresponding Pd(II) precursors 2 (e.g., 2b
in Fig. 1a or a MeOH adduct of 2c, 2c(MeOH) in Fig. S73) in a
range of solvents, MeOH, MeCN, AcOH and THF.
equiv. of H2O2 in either MeOH or AcOH solution was complete
within a few hours at 22 C to produce 5a in 88-92% yield; the
o
1
2
3
4
5
6
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formation of two or three major intermediates, depending on the
solvent, was evident. The oxidation of the electron-poorer N-
trifluoromethanesylfonyl (2b) and N-trifluoroacetyl (2c) analogs
to form the corresponding carbazoles 5 was only possible in
AcOH solutions at elevated temperatures; two major intermedi-
ates were involved in each case. The reactions required 30
o
minutes at 60 C for completion to produce 5b-5c in 91-98%
yield. When a different solvent, MeOH or MeCN, was used, 2c
reacted rapidly with H2O2 to form the hydroperoxoketal 3c that
decomposed cleanly back to 2c after 24 h at 22 oC with appar-
ent evolution of O2; no carbazole 5c was observed in these
experiments. A low solubility of 2b in MeOH precluded any H
NMR monitoring of this system.
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Scheme 2. Previous preparation of cationic isolable aryl
PdIV(X) complexes and their C(sp2)-X (X = OH, Cl, Br) reductive
elimination reactivity.14
Gratifyingly, the reactions of 2a and 2b with H2O2 could
be directed toward selective formation of the respective hy-
droperoxoketals
3 or amidoaryl Pd(IV) intermediates 4
(Scheme 1) via a correct choice of solvents favoring their crys-
tallization from reaction mixtures. The derived complexes 3 and
4 could be isolated and characterized as described below. The
use of THF as a solvent in the reaction of 2a and H2O2 led to a
pure colorless crystalline hydroperoxoketal adduct 3a (Scheme
1, Fig. 2) resulting from H2O2 addition across the dpk C=O bond
of 2a which was characterized by 1H NMR spectroscopy. In
THF-d8 solutions 3a exhibits two prominent singlets of the hy-
droperoxoketal OOH and OH groups at 11.91 and 8.31 ppm,
respectively. Based on crystallographic characterization of 3a,
the oxidizing hydroperoxo group is in the “endo” position, close
to the reducing Pd(II) center (Pd1-O42, 1.888 Å). In spite of the
proximity of these two redox active fragments, the complex is
slow to convert to its Pd(IV) isomer 4a at 22 oC which contribut-
ed to the success of its selective crystallization from the reac-
tion mixtures in THF. Interestingly, dissolution of 3a in MeCN
leads to its fast (<15 min) and quantitative (NMR) redox-
transformation to the Pd(IV) amido aryl complex 4a which could
be crystallized out of the solution. Alternatively, 4a could be
crystallized from a reaction mixture of 2a and 3 equiv. H2O2 in
1) Pd(OAc)2
2) dpk
H2O2
AcO
HN
Pd
N
R
HN
-"(dpk)PdII"
N
R
1d-1e
O
2d-2e
d: R=H
e: R=Me
N
R
5d-5e (89-95%)
Scheme 3. Oxidation of ionic amine aryl palladium(II) complex-
es 2d-2e to the corresponding carbazoles 5d-5e.
Previously we reported oxidation with H2O2 in water14,15 of
a series of di(2-pyridyl)ketone (dpk) - supported cationic aryl
Pd(II) complexes that could be cleanly converted to their isola-
ble Pd(IV) derivatives.16 The latter could be involved in various
reductive C(sp2)-X elimination reactions (X = OH, Cl, Br;
Scheme 2).
1
MeCN directly without isolation of the intermediate 3a. The H
Our attempts to extend the scope of this reaction to oxi-
dative C(sp2)-N coupling and the preparation of amido aryl
Pd(IV) complexes met initially with only partial success. Oxida-
tion with H2O2 (3 equiv.) of the cationic κ2-C,N-aminoaryl com-
plex 2d and its N-methyl derivative 2e (Scheme 3) in methanol-
ic solutions at 22 oC produced rapidly the corresponding carba-
zoles 5 in high 89-95% yields. This result confirmed the syn-
thetic utility of the dpk-enabled oxidative Pd(II)-C(sp2) bond
functionalization of amidoaryl Pd(II) complexes with H2O2 as
oxidant. At the same time, no noticeable accumulation of the
anticipated amido aryl Pd(IV) intermediates was observed
NMR spectra of 4a dissolved in DMSO-d6 feature two charac-
teristic OH group signals, a sharp singlet of the hydrated dpk
ligand at 8.22 ppm,14 and a broad singlet at 3.74 ppm assigned
to the Pd(IV)OH group. As expected, both signals disappear
upon addition of two drops of D2O to the solution.
o
when the reactions were monitored at 5 - 20 C. Hence, our
experiments with the substrates 2d-2e furnished no direct evi-
dence for the existence of the derived amidoaryl Pd(IV) inter-
mediates, most likely, because of the high reactivity of the an-
ticipated Pd(IV) species.
The use of the electron-poorer, weakly basic neutral ana-
logs of 2d-2e, the κ2-C,N-amidoaryl complexes 2a-2c (Scheme
1) showed a much more promising but also a more complex
behavior that depended on the identity of the complex and sol-
vent used. In the case of the most reactive N-
methanesylfonylamido Pd(II) derivative 2a its reaction with 3
Figure 2. ORTEP plot (50% probability ellipsoids) for the Pd(II)
hydroperoxoketal complex 3a. Hydrogen atoms except those of
the OH groups are omitted for clarity. Selected distances, Å:
Pd1-O42, 1.888.
The poor solubility of 4a in MeCN contributed to the suc-
cess of its isolation from this solvent but did not permit any
quantitative analysis of the kinetics its C-N reductive elimination
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tion from the ion pairs 4b(MeCN)+,BF4 predict the reaction
Gibbs activation energy of 20.2 kcal/mol in gas phase and 25.2
kcal/mol in CH2Cl2.17 As in the case of complex 4a, the DFT
suggests realization of a concerted C-N coupling from the 6-
coordinate cation 4b(MeCN)+.
-
to form carbazole 5a. In contrast, the solubility of 4a in MeOH is
1
2
3
sufficiently high and the C-N reductive elimination of 4a to pro-
duce the carbazole 5a and a soluble (dpk)Pd(II) complex17
(Scheme 1) is fast enough to allow the reaction monitoring by
1
means of H NMR spectroscopy.17 The reaction follows clean
4
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6
7
8
9
first order kinetics with the half-life of 37 1 min at 22 oC (ꢀG# =
22.0 kcal/mol). Our DFT calculations predict a reasonably close
Gibbs activation energy of 20.6 kcal/mol for this reaction in
MeOH and point to the realization of a concerted C-N coupling
mechanism directly from the 6-coordinate complex 4a.14,15,17
The successful isolation of the N-methanesulfonyl deriva-
tives 3a and 4a fueled our additional efforts at exploring the
reaction of H2O2 with an even less reactive N-
trifluoromethanesulfonyl complex 2b. The reaction run in MeCN
as a solvent produced a poorly soluble colorless crystalline 3b
in high yield. The latter was characterized by 1H NMR in
DMSO-d6 to reveal spectral parameters similar to those of 3a
and 3c.18 Remarkably, leaving a suspension of 3b in MeCN for
3-4 days at 22 oC led to its almost complete (>95%) conversion
to a poorly soluble orange crystalline amidoaryl Pd(IV) complex
4b which could be isolated in 83% yield. The transformation of
3b into 4b could also be monitored in solutions of 3b in DMSO-
d6 by means of 1H NMR spectroscopy. This reaction follows
To probe the reactivity in C-N coupling of 4b-derived cati-
onic aqua complex, 4b(H2O)+, the latter was prepared by com-
bining 4a suspended in CH2Cl2 with slightly less than 1 equiv.
of HBF4 (Scheme 4). The resulting purple solution was charac-
1
terized by ESI(+)/MS and H NMR spectroscopy. The solution
exhibited reactivity that is very similar to that of the MeCN ana-
log in CH2Cl2 to produce the carbazole 5b following first order
kinetics with half-life of 119 4 min at 22 oC (ꢀG# = 22.7
kcal/mol; compare with the DFT – predicted 21.9 kcal/mol for
the reaction involving ion pairs 4a(H2O)+,BF4- in gas phase and
22.7 kcal/mol in CH2Cl2 solution).
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Interestingly, the C-N coupling of 4b can be made even
faster when more than 1 equiv. of HBF4 is used. E.g., with 3
o
equiv. HBF4 the reaction was complete in 15 min at 22 C in
MeCN solution (97% yield of 5b) but the system exhibited a
much more complex kinetics behavior, possibly, because of a
parallel realization of several competing processes involving
different multiply protonated Pd(IV) species.
o
clean first-order kinetics with a half-life of 50 1 min at 22 C
(ꢀG# = 22.2 kcal/mol). The new amidoaryl Pd(IV) complex 4b
was characterized by single crystal X-ray diffraction (Fig. 1b)
and 1H NMR spectroscopy. Similar to 4a, in DMSO-d6 solutions
4b exhibits characteristic peaks of the hydrated dpk ligand14-15
and the Pd(IV)OH group at 8.33 and 3.71 ppm, respectively.
Notably, as compared to the methanesulfonyl analog 4a,
the trifluoromethanesulfonyl compound 4b is much less reac-
tive with respect to C-N reductive elimination. In particular, in
MeOH solutions the corresponding carbazole 5b formed only in
3.6% yield after 95h at 22 oC. Based on the pKa values of relat-
ed NH acids, N-R-phenylmethanesulfonylamides,20 the derived
amido ligand present in complex 4a is expected to be more
nucleophilic than its fluorinated analog in 4b. A similar trend,
slower C(sp2)-N reductive elimination from Pd(II) center of ami-
do aryl Pd(II) complexes having electron-poorer amido ligands
was recently explored by Buchwald.19
Notably, the C-N coupling of 4b to produce 5b and
(dpk)Pd(OAc)2 was found to be much faster in AcOH solutions.
Scheme 4. Transformations of 4b and formation of the corre-
sponding carbazole 5b in the presence of acid additives.
o
In this case 5b formed in 43% yield after 51h at 22 C along
with 53% of another product which was isolated and identified,
The results of our studies of reactivity of Pd(II) amido aryl
complexes 2a-2c with H2O2 can be summarized as follows. In
the multi-step reaction sequence 2-3-4-5 (Scheme 1) the for-
1
based on its H NMR spectral pattern and C-N reductive elimi-
nation reactivity, as the Pd(IV) acetato complex 4b(OAc)
(Scheme 4, top).17 The latter compound was also identified as
one of the major intermediates previously observed in the reac-
tion of 2b and H2O2 in AcOH, besides 3b and 4b.21
o
mation of hydroperoxoketals 3 is facile at 22 C for all of the
substrates and in all the solvents used in this work. The subse-
quent redox transformation of the hydroperoxoketals 3 to the
Pd(IV) amido aryls 4 and the C-N coupling of 4 are both much
slower, so that 3 and 4 can be observed in comparable
amounts in the reaction mixtures. The Pd(II)-to-Pd(IV) oxidation
step (3-to-4) is faster for the electron-richer N-
methanesulfonylamido derivative 3a, as compared to fluorinat-
ed compounds 3b and 3c; so is the C-N reductive elimination of
complex 4a, as compared to 4b. Notably, the C-N coupling of
4b can be accelerated dramatically in the presence of acid
additives that produce electron-poorer and much more reactive
cationic Pd(IV) species such as 4b(MeCN)+ and 4b(H2O)+. Fi-
nally, the cationic Pd(II) amino aryl complexes 2d-2e appear to
be the most electron rich of all the series of complexes 2 and
The accelerating effect of AcOH on the rate of the C-N
coupling of 4b prompted us to probe the effect of stronger acid
additives on this reaction. Addition of slightly less than 1 equiv.
of HBF4 to a suspension of 4b in MeCN produced purple solu-
tions of the corresponding cationic MeCN derivative
4b(MeCN)+, as confirmed by ESI(+)/MS and 1H NMR spectros-
copy (Scheme 4). The resulting MeCN solutions react following
a clean first order kinetics to form the carbazole 5b with half-life
o
of 104 1 min at 22 C (ꢀG# = 22.6 kcal/mol). The reaction of
-
4b(MeCN)+ isolated in the form of its BF4 salt is slightly faster
o
in CH2Cl2 with the half-life of 87 2 min at 22 C (ꢀG# = 22.5
kcal/mol). Our DFT calculations for the C-N reductive elimina-
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(5)
Jordan-Hore, J. A.; Johansson, C. C. C.; Gulias, M.; Beck, E. M.;
might also react via the intermediacy of the corresponding high-
ly reactive Pd(IV) species that remained undetected under our
reaction conditions.
Gaunt, M. J. J. Am. Chem. Soc. 2008, 130, 16184−16186.
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Youn, S. W.; Bihn, J. H.; Kim, B. S. Org. Lett. 2011, 13, 3738−3741.
Choi, S.; Chatterjee, T.; Choi, W. J.; You, Y.; Eun Jin Cho, E. J. ACS
Catal. 2015, 5, 4796−4802.
Combined with the ability to generate Pd(IV) amido aryl
complexes such as 4a and 4b using H2O2 these results may be
useful for the development of new green protocols for catalytic
oxidative C-H amination.
(8)
(9)
Thu, H. Y.; Yu, W. Y.; Che, C. M. J. Am. Chem. Soc. 2006, 128, 9048-
9049.
Neumann, J. J.; Rakshit, S.; Droge, T.; Glorius, F. Angew. Chem. Int.
Ed. 2009, 48, 6892-6895.
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(10) Mei, T. S.; Wang, X.; Yu, J.-Q. J. Am. Chem. Soc. 2009, 131, 10806-
10807.
ASSOCIATED CONTENT
Supporting Information
(11) (a) Perez-Temprano, M. H.; Racowski, J. M.; Kampf, J. W.; Sanford,
M. S. J. Am. Chem. Soc. 2014, 136, 4097−4100; (b) Pendleton, I. M.;
Perez-Temprano, M. H.; Sanford, M. S.; Zimmerman, P. M. J. Am.
Chem. Soc. 2016, 138, 6049-6060.
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The complete experimental and computational details (PDF),
crystallographic information for 2b, 3a, 4b and 2c(MeOH) (CIF).
This material is available free of charge on the ACS Publications
website.
(12)
Pawlikowski, A. V.; Getty, A. D.; Goldberg, K. I. J. Am. Chem. Soc.
2007, 129, 10382-10393.
(13) Hartwig, J. F. Reductive Elimination. In The Organotransition Metal
Chemistry: From Bonding to Catalysis; University Science Books:
Sausalito, CA, 2010; pp 321-348.
AUTHOR INFORMATION
(14) Oloo, W. N.; Zavalij, P. Y.; Zhang, J.; Khaskin, E.; Vedernikov, A. N. J.
Am Chem. Soc. 2010 132 14400-14402.
Corresponding Author
(15)
Oloo, W. N.; Zavalij, P. Y.; Vedernikov, A. N. Organometallics 2013,
32, 5601-5614.
* avederni@umd.edu
(16) See also
a most recent paper presenting the use of H2O2 for
PdII-C(sp2) oxidative functionalization with proposed PdIV intermedia-
cy: Behnia, A.; Boyle, P. D.; Blacquiere, J. M.; Puddephatt, R. J. Or-
ganometallics, 2016, 35, 2645–2654.
Notes
The authors declare no competing financial interests.
(17) See Supporting information for details.
ACKNOWLEDGMENT
(18) In particular, 1H NMR spectra of 3b in THF-d8 show two singlets at
11.03 and 8.40 ppm integrating as 1H each assigned to the
hydroperoxoketal OOH and OH groups, respectively. The
correspondign signals observed for 3c are at 14.12 and 8.76 ppm.
(19) Arrechea, P. L.; Buchwald, S. L. J. Am. Chem. Soc. 2016, 138,
12486−12493.
This work was supported by the US National Science Foundation
(CHE-1112019, CHE-1464772) and, in part, the US – Israel Bi-
national Science Foundation (grant 2010119).
REFERENCES
(20) The pKa values for PhNHSO2CF3 and PhNHSO2Me determined in 2:1
DMF:H2O mixture are 4.45 and 8.85, respectively: Trepka, R. D.;
Harrington, J. K.; Belisle, J. W. J. Org. Chem. 1974, 39, 1094-1098.
(21) Notably, oxidation with H2O2 in AcOH of the N-trifluoroacetyl analog
2c produces two major intermediates that could be assigned, based
on their 1H NMR spectral patterns, as 4c and 4c(OAc), the analogs of
the Pd(IV) hydroxo complex 4b and the Pd(IV) acetoxo complex
4b(OAc), respectively (Scheme 4, R = CF3CO). See SI for details.
(1)
Hartwig, J. F. In Handbook of Organopalladium Chemistry for Organic
Synthesis; Negishi, E. I., Ed.; Wiley-Interscience: New York, 2002;
Vol. 1, pp 1051-1058.
(2)
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Hartwig, J. F. Synlett 2006, 1283-1294.
Buchwald, S. L.; Mauger, C.; Mignani, G.; Scholz, U. Adv. Synth.
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(4)
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ACS Paragon Plus Environment