1,4-Palladium migration via C-H activation, followed by arylation: Synthesis of fused polycycles

Article abstract of DOI:10.1021/ja035121o

A novel palladium migration/arylation methodology for the synthesis of complex fused polycycles has been developed, in which one or more sequential Pd-catalyzed intramolecular migration processes involving C-H activation are employed. The chemistry works best with electron-rich aromatics, which is in agreement with the idea that these palladium-catalyzed C-H activation reactions parallel electrophilic aromatic substitution. Copyright

Full text of DOI:10.1021/ja035121o

Published on Web 08/28/2003
1,4-Palladium Migration via C-H Activation, Followed by Arylation: Synthesis
of Fused Polycycles
Marino A. Campo, Qinhua Huang, Tuanli Yao, Qingping Tian, and Richard C. Larock*
Department of Chemistry, Iowa State UniVersity, Ames, Iowa 50011
Received March 12, 2003; E-mail: larock@iastate.edu
Table 1. Synthesis of Polycycles via Pd-Catalyzed
The ability of palladium to activate C-H bonds has been used
Migration/Arylationa
extensively in organic synthesis.1 In recent years, palladium-
catalyzed C-H activation has received considerable attention due
to the wide variety of reactions this metal will catalyze.2 For
instance, catalytic amounts of Pd salts have been used to activate
the addition of C-H bonds of electron-rich arenes to alkenes and
alkynes, and to effect carbonylation.3,4
We have previously reported that intramolecular C-H activation
in organopalladium intermediates derived from o-halobiaryls leads
to a 1,4-palladium migration and have shown that such intermediates
can be trapped by Heck reactions.5 We have recently explored
sequential migration/arylation via through-space C-H activation
and its synthetic potential, and we now wish to report a convenient
synthesis of fused polycycles using this novel rearrangement. Our
strategy involves the use of palladium C-H activation to catalyze
a 1,4-palladium migration within biaryls generating key arylpalla-
dium intermediates such as A, which subsequently undergo C-C
bond formation by intramolecular arylation producing fused poly-
cycles (Scheme 1). This through-space migration of the metal
Scheme 1
moiety between the ortho positions of biaryls amounts to an overall
1,4-palladium shift, which could possibly involve an intermediate
hydridopallada(IV)cycle generated by insertion of palladium into
a neighboring C-H bond. This process represents a very powerful
new tool for the preparation of complex molecules, which might
be difficult to prepare by any other present methodology.
a The reaction was carried out under our standard reaction conditions
employing 0.25 mmol of the aryl halide, 5 mol % Pd(OAc)2, 5 mol %
dppm, and 2 equiv of CsO2CCMe3 in DMF (4 mL) at 100 °C unless
otherwise noted. b The yield in parentheses corresponds to a GC yield of
product in which the C-I bond has been reduced to a C-H bond. c The
reaction temperature was increased to 110 °C. d The yield was determined
By using our previously developed optimal migration conditions
[0.25 mmol of aryl halide, 5 mol % Pd(OAc)2, 5 mol % diphenyl-
phosphinomethane (dppm), and 2 equiv of CsO2CCMe3 in DMF
(4 mL) at 100 °C],5 we have explored this 1,4-palladium migration/
arylation process using a variety of substrates carefully selected to
study the generality of the process, to understand the migration
behavior, and to establish its applicability to commonly encountered
synthetic problems (Table 1). We began by allowing 3′-benzyl-2-
iodobiphenyl (1) to react under our standard reaction conditions at
100 °C, but after 2 d this substrate failed to react. However, by
simply increasing the reaction temperature to 110 °C, we were able
to obtain the desired compound 2 in a 40% isolated yield (entry
1). The disappointingly low yield obtained with this substrate might
be explained by the poor reactivity of the benzyl moiety as an intra-
molecular trap. To test this idea, we carried out a reaction with the
1
by H NMR spectroscopy.
more electron-rich 2-iodo-3′-phenoxybiphenyl (3) and obtained the
desired 4-phenyldibenzofuran (4) in an impressive 89% isolated
yield (entry 2). Clearly, these results indicate that the electron-rich
oxygen-substituted phenyl ring is superior as an arylating agent.
Our finding that electron-rich arenes are superior to electron-neutral
arene traps is consistent with literature reports indicating that the
ease of C-H activation by palladium parallels electrophilic aromatic
substitution.6
We proceeded to investigate the sequential migration/arylation
reaction of more complex polyaromatic compounds. In theory,
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10.1021/ja035121o CCC: $25.00 © 2003 American Chemical Society

C O M M U N I C A T I O N S
Scheme 2
Scheme 3
5-phenoxybiphenyl (15) was allowed to react under our migration
conditions, and an 88% yield of double migration product 4 was
isolated (entry 8, Scheme 3). Mechanistically, the palladium first
inserts into the aryl iodide bond to form intermediate B, which
migrates to the phenyl unit by through-space C-H activation. The
metal moiety in the first migration intermediate C can return to the
original aromatic ring in either the position from which it originally
migrated (B) or the position ortho to the phenoxy group (D), where
it can be trapped by arylation. Note that the yield for this double
migration chemistry is very similar to that from the single migration
chemistry (entry 2) and the success of this double palladium migra-
tion indicates that multiple migration processes are entirely feasible.
In conclusion, we have developed a novel palladium migration/
arylation methodology for the synthesis of complex fused poly-
cycles, which employs one or more sequential Pd-catalyzed
intramolecular migration processes involving C-H activation. The
chemistry developed here works best with electron-rich aromatics,
which is in agreement with the idea that these palladium-catalyzed
C-H activation reactions parallel electrophilic aromatic substitution.
We are presently examining a wide variety of palladium migration
processes and their synthetic applications.
Acknowledgment. We thank the donors of the Petroleum
Research Fund, administrated by the American Chemical Society,
and the National Science Foundation, for partial support of this
research. We are also grateful to Johnson Matthey, Inc., and
Kawaken Fine Chemicals Co., for donating the palladium salts.
Supporting Information Available: General experimental proce-
dures and spectroscopic characterization of all new products (PDF).
This material is available free of charge via the Internet at http://
pubs.acs.org.
2-iodo-1-phenylnaphthalene (5) should afford fluoranthene (6) using
our methodology. Mechanistically, the palladium must undergo a
1,4-palladium migration from the 2-position of the naphthalene to
the o-position of the phenyl substituent, followed by arylation at
the 8-position of the naphthalene. Although the reaction did not
proceed at 100 °C, at 110 °C the desired compound 6 was produced
in an 81% yield (entry 3).
An interesting example of this migration involves the rearrange-
ment of easily prepared 9-iodo-10-phenylphenanthrene (7) to benz-
[e]acephenanthrylene (8), and the reaction proceeded at 110 °C to
generate the desired migration product in a 78% yield (entry 4).
We have also studied the regioselectivity of this migration by using
an m-tolyl moiety in the 10-position of the 9-iodophenanthrene (entry
5). Compound 9 has two available positions for palladium migra-
tion, the more sterically congested neighboring 2-position or the
remote 6-position of the phenyl ring. The palladium-catalyzed cycli-
zation of compound 9 generated compound 10 exclusively in a 56%
yield. This result indicates that palladium migration/arylation occurs
exclusively at the less sterically congested 6-position of the phenyl
moiety and that either the presence of a methyl group apparently
completely inhibited migration to the more hindered 2-position or
elseringclosureatthatmorehinderedpositioniscompletelyinhibited.
To confirm our suspicion that the palladium prefers to migrate
to a more electron-rich position, because of the relatively easy
activation of an electron-rich C-H bond,5,6 compound 11 was
allowed to react under our migration conditions, and indole 12 was
produced in a 92% yield in 1 d at 100 °C (entry 6). From the results
of entries 1 and 6, it appears that the high efficiency of palladium
migration to a relatively electron-rich position allows the sequential
migration/arylation to proceed smoothly at a lower temperature and
in a shorter reaction time, although the benzyl group is not a
particularly good arylating agent.
We have also carried out the palladium-catalyzed sequential
migration/alkyne insertion/arylation of aryl halide 13 in the hope
that the arylpalladium intermediate generated by a 1,4-Pd shift via
through-space C-H activation could be trapped by alkyne insertion-
annulation chemistry described earlier by us (Scheme 2).7 The reac-
tion was carried out under our standard migration conditions, and
carbazole 14 was isolated in a 65% yield (entry 7). It is important
to note that this reaction was complete in 0.5 d at 100 °C, consistent
with the particularly facile migration of Pd to the electron-rich
indole ring system. This successful alkyne insertion chemistry sug-
gests that there is the exciting possibility of trapping aryl- and other
organopalladium intermediates generated by a 1,4-Pd shift by many
other synthetically useful palladium methodologies, such as ami-
nation and annulation. We are currently examining this possibility.
A mechanistically interesting question is whether the arylpalla-
dium intermediate can migrate more than once and still effect
synthetically useful chemistry. To examine this possibility, 2-iodo-
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