Oxidant-free direct coupling of internal alkynes and 2-alkylpyridine via double C-H activations by alkylhafnium complexes

Article abstract of DOI:10.1021/ja1100118

We have developed a novel oxidant-free direct cross-coupling reaction of 2,6-lutidine and internal alkynes leading to five-membered carbocyclic compounds mediated by nonmetallocene cationic hafnium alkyl complexes. Mechanistic studies of the coupling reaction showed that the reaction begins with C(sp ³)-H bond activation via σ-bond metathesis, after which the coordinatively unsaturated hafnium center mediates further insertion, migration, and β-H elimination reactions to give five-membered carbocycles from readily available substrates.

Full text of DOI:10.1021/ja1100118

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pubs.acs.org/JACS
Oxidant-Free Direct Coupling of Internal Alkynes and 2-Alkylpyridine
via Double C-H Activations by Alkylhafnium Complexes
Hayato Tsurugi, Koji Yamamoto, and Kazushi Mashima*
Department of Chemistry, Graduate School of Engineering Science, Osaka University, Toyonaka, Osaka 560-8531, Japan, and CREST,
JST, Japan
S Supporting Information
b
Scheme 1. Synthesis of Five- and Six-Membered Carbocycles
Using Readily Available Alkynes
ABSTRACT: We have developed a novel oxidant-free
direct cross-coupling reaction of 2,6-lutidine and internal
alkynes leading to five-membered carbocyclic compounds
mediated by nonmetallocene cationic hafnium alkyl com-
plexes. Mechanistic studies of the coupling reaction showed
that the reaction begins with C(sp3)-H bond activation via
σ-bond metathesis, after which the coordinatively unsaturated
hafnium center mediates further insertion, migration, and
β-H elimination reactions to give five-membered carbo-
cycles from readily available substrates.
he development of new methods for selective syntheses of
T
functionalized carbocycles with a controlled configuration
has been extensively studied because of the presence of these
skeletons in biologically relevant compounds.1,2 Transition-metal-
mediated intermolecular cyclization reactions of readily available
starting materials, such as simple alkynes, are a straightforward
method for constructing small carbocycles in comparison with the
intramolecular cyclization of complexed dienes, enynes, or diynes.
The [2 þ 2 þ 2] cyclotrimerization of alkynes is an attractive
and elegant synthetic method for producing polysubstituted arenes,
and a large number of excellent procedures have been developed
(Scheme 1a).3 As an exceptional strategy for constructing five-
membered carbocyclic compounds, the transition-metal-catalyzed
Pauson-Khand cyclization reaction produces a wide variety of
five-membered cyclopentenones or cyclopentadienones from al-
kynes, alkenes, and carbon monoxide (Scheme 1b).4 Another notable
method of forming five-membered carbocycles is the five-mem-
bered zirconametallacycle-mediated coupling reaction with alde-
hydes, acid chlorides, or 1,1-dihalo compounds (Scheme 1c).5
Although multiple activation of C(sp2)-H bonds followed by a
cyclization reaction resulting in the homologation of aromatic
compounds(Scheme1d) hasrecentlyattracted interest,6 the direct
cross-coupling reaction of alkanes with 2 equiv of alkynes involving
double C(sp3)-H bond activation is an efficient and favor-
able pathway for preparing five-membered cyclic compounds
(Scheme 1e). Catalytic and stoichiometric direct cross-coupling
approaches require oxidants to eliminate the preactivation of the
substrates.6-8 From the point of view of environmentally benign
synthesis, the oxidant-free catalytic direct cross-coupling reaction
is highly desirable; however, the developmentofacatalyticprotocol
for such a transformation remains challenging, although a stoichio-
metric and stepwise oxidant-free direct cross-couplingreaction by a
cationic zirconocene-mediated method has been reported.9,10
We recently developed a rational strategy for synthesizing
coordinatively unsaturated group 4 metal alkyl complexes via an
insertion reaction of the metal-carbon bond into the CdN
bond of R-diimine ligands.11 As an extension of this methodology
to N,N0-bis(arylmethylene)ethylenediamine (β-diimine), we stu-
died the reaction of the β-diimine ligands with Hf(CH2Ph)4
and found that alkylation of one of the two CdN bonds as well as
ortho C(sp2)-H bond activation of one of two aromatic rings of
the ligand afforded a dibenzylhafnium complex with a β-diimine-
derived dianionic (N,N,C)-tridentate ligand. Such an electron-
deficient, coordinatively unsaturated metal center was anticipated
to serve as a necessary platform to assist σ-bond metathesis
and direct cross-coupling reactions. Herein we report the first
example of an oxidant-free catalytic direct cross-coupling reac-
tion, in which the hafnium complex bearing the (N,N,C)-
tridentate ligand catalyzes the coupling reaction of 2 equiv of various
alkynes with a 2-alkylpyridine derivative to give five-membered
carbocyclic compounds (Scheme 1e). Subsequent acid-catalyzed
isomerization to give the pentasubstituted cyclopentadienes
clearly revealed the usability of this novel direct cross-coupling
reaction for the synthesis of multisubstituted cyclopentadienes as
precursors of various cyclopentadienyl ligands. The mechanism
of this novel catalytic transformation was revealed by isolation of
the hafnium complex corresponding to each reaction step.
Received: November 8, 2010
Published: December 22, 2010
r
2010 American Chemical Society
732
dx.doi.org/10.1021/ja1100118 J. Am. Chem. Soc. 2011, 133, 732–735
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COMMUNICATION
Scheme 2. Direct Cross-Coupling Reaction of 2,6-Lutidine
and Internal Alkynes
Figure 1. Molecular structure of complex 2. Hydrogen atoms have been
omitted for clarity. Selected bond distances (Å) and angles (deg): Hf-N1,
2.271(8); Hf-N2, 2.072(7); Hf-C5, 2.338(9); Hf-C28, 2.244(10);
Hf-C29, 2.739(11); Hf-C35, 2.253(11); Hf-C36, 2.810(12); C28-
Hf-C35, 110.2(4); Hf-C28-C29, 91.8(6); Hf-C35-C36, 95.8(7).
Scheme 3. Reactions of 2 with Heteroaromatic Substrates
Treatment of Hf(CH2Ph)4 with β-diimine ligand 1 in toluene
produced dibenzyl complex 2 in 80% yield (eq 1):
During the course of the reaction, two consecutive reactions
proceeded to give complex 2 with an (N,N,C)-tridentate ligand:
(1) the insertion reaction of one benzyl group into one of the two
CdN moieties of 1 to form a hafnium-amido bond and (2)
cyclometalation of one aromatic ring bound to the nitrogen atom of
the ligand via activation of the C-H bond with release of toluene,
giving a Hf-C(aromatic) bond. Such an intramolecular C(sp2)-H
bond activation at one of the ortho positions of the aromatic
substituent by an early-transition-metal alkyl group has been
reported previously.12 In the 1H NMR spectrum of 2, the methylene
proton signals of HfCH2Ph were observed as two ABq resonances at
δH 2.34 and 2.88 (2J = 10.7 Hz) and δH 2.42 and 2.80 (2J = 11.3
Hz), and ABX signals corresponding to the methylene and methine
protons of the NCHCH2Ph moiety were observed at δH 3.55, 3.73,
1
methyl}hafnium complex 4. In the H NMR spectrum, the
resonance for the imine proton was not present, and signals
assignable to two NCHCH2Ph groups were observed. Also, the
signal for the cyclometalated carbon disappeared, and a signal for
Hf-CH2(C5H3N-CH3-6) appeared at δC 62.0. Similarly, heat-
ing a toluene solution of 2 in the presence of 2-phenylpyridine at
60 °C for 22 h gave cyclometalated product 5 in 94% yield through
the formation of an dianionic (N,N)-bidentate ligand. X-ray
analysis of 5 revealed the overall molecular structure (Figure 2a),
indicating a large structural difference between the starting
complex 2 and the product 5: the Hf-C(aromatic) bond of 2
was cleaved by the abstraction of the H atom of the ortho C-H
bond of phenylpyridine via σ-bond metathesis to form {η2-(C,
N)-2-pyridylphenyl}hafnium,andaspontaneousmigrationinsertion
of one benzyl group bound to the Hf atom in the CdN moiety of
the ligand backbone afforded a dianionic (N,N0)-diamide ligand
of 5. In contrast, treatment of 2 with excess pyridine afforded 6, in
which pyridine is simply coordinated to the metal center, and
no C-H bond activation was observed (Figure 2b).
Pyridine-directed C-H bond-activated complexes 4 and 5
were used as catalysts for the coupling reaction. Both 4 and 5
were inactive for the reaction involving further alkyne insertion;
however, the combination of complex 4 and B(C6F5)3, which
was used as a cocatalyst for generating cationic alkyl species,
became an active catalyst for the coupling reaction. Thus, we
examined the 1:1 reaction of 4 with B(C6F5)3, which resulted in
cationic complex 7. The molecular structure determination of 7
revealed that B(C6F5)3 abstracted the bridged carbon atom of the
{η2-(C,N)-(6-methylpyrid-2-yl)methyl}hafnium moiety to form a
borate anion coordinated to the cationic hafnium center through the
nitrogen atom of the pyridine ring (Figure 3). The hafnium atom
3
and 5.30 (2J = 12.6 Hz, J = 3.6 and 10.4 Hz). The 13C NMR
spectrum of 2 displayed a signal assignable to one CdN group at δC
175.0. It was noteworthy that a downfield-shifted resonance due
to the cyclometalated carbon was observed at δC 199.5.12,13 The
molecular structure of 2 was clarified by X-ray analysis (Figure 1).
The system of 2 and B(C6F5)3 catalyzed the coupling reaction
of 1 equiv of 2,6-lutidine with 2 equiv of the internal alkynes
3-hexyne, 4-octyne, and 5-decyne in chlorobenzene at 100 °C for
24 h to give the corresponding products 3a-c (Scheme 2),
accompanied by the release of H2. This is the first example of an
oxidant-free catalytic direct cross-coupling reaction, in sharp
contrast to the previously reported catalytic direct cross-coupling
reactions catalyzed by late-transition-metal catalysts, which re-
quire the addition of oxidants or bases to the reaction medium.7,8
To elucidate the reaction mechanism, we performed the
reaction of 2 with several heteroaromatic substrates (Scheme 3).
Heating a mixture of 2 and 2,6-lutidine in toluene at 60 °C for 24 h
resulted in selective activation of one benzylic C(sp3)-H bond
of 2,6-lutidine to form the {η2-(C,N)-(6-methylpyrid-2-yl)-
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COMMUNICATION
Scheme 4. Reaction of 7 with 3-Hexyne
Figure 2. Molecular structures of complexes (a) 5 and (b) 6. Hydrogen
atoms have been omitted for clarity. Selected bond distances (Å) and angles
(deg) for 5: Hf-N1, 2.043(3); Hf-N2, 2.050(4); Hf-N3, 2.349(4); Hf-
C35, 2.339(6); Hf-C52, 2.381(3); Hf-C35-C36, 116.5(4). For 6: Hf-
N1, 2.304(3); Hf-N2, 2.075(3); Hf-N3, 2.463(3); Hf-C28, 2.302(3);
Hf-C35, 2.280(3); Hf-C28-C29, 119.6(2); Hf-C35-C36, 104.9(2).
{η2-(C,N)-(6-methylpyrid-2-yl)methyl}hafnium (A) and alkenyl-
borate. Second, the alkyne reacts with A to form a dienylhafnium
intermediate B. The Hf atom migrates to the benzylic position
via σ-bond metathesis to give C, and subsequent insertion of the
Hf-C bond into the tethered alkene moiety produces the cyclo-
pentenylhafnium intermediate D. The Hf atom shifts to the
2-position of the cyclopentenyl ring to form another cyclopen-
tenylhafnium intermediate E through the η3 coordination mode,
and the coupling product 3 is obtained after β-H elimination.
Finally, the hafnium hydride F reacts with 2,6-lutidine via σ-bond
metathesis to regenerate the catalytically active {η2-(C,N)-
(6-methylpyrid-2-yl)methyl}hafnium species A along with H2.
Cyclopentene 3a was successfully converted to pentasubstituted
cyclopentadiene 11a bytheadditionofconcentratedHCl(aq) (eq2):
Figure 3. Synthesis and molecular structure of zwitterionic complex 7.
Hydrogen atoms have been omitted for clarity. Selected bond distances
(Å) and angles (deg): Hf-N1, 1.998(7); Hf-N2, 2.007(6); Hf-N3,
2.282(5); Hf-C35, 2.287(12); Hf-C35-C36, 115.2(8).
adopts a tetrahedral geometry involving coordination by two nitrogen
atoms of the diamido ligand, one nitrogen atom of the pyridine unit,
and one carbon atom of the benzyl group. The Hf-N1 and Hf-N2
distances are 1.988(7) and 2.007(6) Å, respectively, which are typical
for observed σ-bonded hafnium-nitrogen bonds, whereas the
Hf-N3 bond length of 2.282(5) Å is consistent with the coordina-
tion of the pyridyl nitrogen atom to the hafnium atom.12c,14 In the 19F
NMR spectrum of 7, resonances of the fluorine atoms of the C6F5
group were observed at δ -131.7, -165.1, and -167.9, indicating
the formation of the four-coordinate borate anion in solution.
We conducted further controlled experiments by hydrolysis.
The reaction of 7 with 3-hexyne at room temperature was quenched
by hydrolysis after 24 h, and the products were analyzed by
GC-MS and NMR measurements, which revealed the formation
of 2-methyl-6-[(2Z,4E)-2,3,4-triethylhepta-2,4-dienyl]pyridine (8), a
1:2 coupling product of 2,6-lutidine and 3-hexyne, and (E)-(2-
ethylpent-2-enyl)benzene (9), which is the product of 1:1
coupling of the benzyl group and 3-hexyne (Scheme 4a). In contrast,
when the same reaction of 7 and 3-hexyne was conducted at
60 °C for 24 h, hydrolysis afforded 9 and the cyclized product 10
(Scheme 4b). Quenching the reaction mixture with D2O resulted
in the formation of 9-d1 and 10-d1 (Scheme 4c). The formation
of 10-d1 indicates that the hafnium atom was attached to the
2,3,4,5-tetraethylcyclopent-2-enyl ring at the 4-position.
Cyclopentadienes are valuable precursors for generating cyclo-
pentadienyl anions, which are widely used as supporting ligands
in d-element, f-element, and main-group organometallic complexes.15
Thus, this coupling methodology is a candidate method for rapid
access to multisubstituted cyclopentadienyl ligands.
We have demonstrated a coupling reaction of 2,6-lutidine
and internal alkynes leading to five-membered carbocyclic com-
pounds mediated by a nonmetallocene cationic alkylhafnium
complex as a first example of an oxidant-free direct cross-coupling
reaction. Formally, the methyl group of 2,6-lutidine becomes a
C1 source of the [2 þ 2 þ 1] cyclization reaction through the
activation of two C-H bonds of the methyl group, and the
double C-H activation on the same carbon atom is a new
strategy for generating C1 sources for various coupling reactions.
Mechanistic studies of the coupling reaction have suggested that
C(sp3)-H bond activation via σ-bond metathesis is the first step
of the reaction. The coordinatively unsaturated metal center can
mediate further insertion, migration, and β-H elimination reac-
tions to produce five-membered carbocycles from readily available
substrates. Further applications of early-transition-metal alkyl
complexes for C-H bond activation and direct cross-coupling
reactions are being developed in our laboratory.
On the basis of the experimental results outlined in Figure 3 and
Scheme 4, we propose the catalytic cycle shown in Scheme 5. In the
first step, 1 equiv of alkyne inserts into the Hf-C bond of 7
to form a vinylhafnium intermediate, which is converted to
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Scheme 5. Plausible Reaction Mechanism for the Direct Cross-Coupling Reaction of 2,6-Lutidine and Internal Alkynes
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’ AUTHOR INFORMATION
Corresponding Author
mashima@chem.es.osaka-u.ac.jp
’ ACKNOWLEDGMENT
H.T. acknowledges financial support by a Grant-in-Aid for
Young Scientists (B), General Sekiyu Research & Development
Encouragement & Assistance Foundation, and the Multidisciplinary
Research Laboratory System of the Graduate School of Engi-
neering Science, Osaka University. This work was supported
by the Core Research for Evolutional Science and Technology
(CREST) Program of the Japan Science and Technology Agency.
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