Allylic C–H Activation of Olefins by a TpMe2IrIII Compound

Article abstract of DOI:10.1002/ejic.201501253

The Ir^IIIcompound [Tp^Me2Ir(C₆H₅)₂(N₂)] (1) [Tp^Me2= hydridotris(3,5-dimethylpyrazolyl)borate] reacts with 1-hexene, propene, α-methylstyrene, and 2,3-dimethylbutadiene to yield organometallic products that derive from allylic C–H activations (complexes 3 from 1-hexene, 2 from propene, 5 from α-methylstyrene, and 7, 8, and 9 from 2,3-dimethylbutadiene), in all cases along with organic products formed in catalytic (Ir-induced) dehydrogenative coupling of benzene (the solvent of the reaction) and the corresponding olefin, with the latter also acting as the hydrogen scavenger in each case. Differently, complex 1 reacts with (E)-β-methylstyrene and cyclohexadiene to yield complex 2 and the known (η⁴-cyclohexadiene)Ir^Iderivative 6, respectively. Finally, compound 1 reacts under mild conditions with cyclopentadiene and methylcyclopentadiene with the generation of phenyl derivatives 11 and 12 in which the corresponding cyclopentadienyl ligand adopts the η⁵coordination and forces the Tp^Me2ligand to coordinate in the κ²mode.

Full text of DOI:10.1002/ejic.201501253

DOI: 10.1002/ejic.201501253
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CLUSTER
ISSUE
C–H Activation
Allylic C–H Activation of Olefins by a TpMe2IrIII Compound
Crispín Cristóbal,[a] Laura L. Santos,[a] Rubén Gutiérrez-González,[b] Eleuterio Álvarez,[a]
Margarita Paneque*[a] and Manuel L. Poveda*[a]
Abstract: The IrIII compound [TpMe2Ir(C6H5)2(N2)] (1) [TpMe2
=
acting as the hydrogen scavenger in each case. Differently, com-
plex 1 reacts with (E)-ꢀ-methylstyrene and cyclohexadiene to
yield complex 2 and the known (η4-cyclohexadiene)IrI deriva-
tive 6, respectively. Finally, compound 1 reacts under mild con-
ditions with cyclopentadiene and methylcyclopentadiene with
the generation of phenyl derivatives 11 and 12 in which the
corresponding cyclopentadienyl ligand adopts the η5 coordina-
tion and forces the TpMe2 ligand to coordinate in the κ2 mode.
hydridotris(3,5-dimethylpyrazolyl)borate] reacts with 1-hexene,
propene, α-methylstyrene, and 2,3-dimethylbutadiene to yield
organometallic products that derive from allylic C–H activations
(complexes 3 from 1-hexene, 2 from propene, 5 from α-methyl-
styrene, and 7, 8, and 9 from 2,3-dimethylbutadiene), in all
cases along with organic products formed in catalytic (Ir-in-
duced) dehydrogenative coupling of benzene (the solvent of
the reaction) and the corresponding olefin, with the latter also
Introduction
TpMe2IrI centers [TpMe2 = hydridotris(3,5-dimethylpyrazolyl)bo-
rate][1] are more prone to C–H vinylic activation of olefins such
as propene than the corresponding allylic process. This is clearly
demonstrated by the thermal behavior of [TpMe2Ir(C2H3R)2] (R =
H, Me) to give the hydrides [TpMe2Ir(H)(CH=CHR)(C2H3R)], where
in fact the methyl substituent greatly facilitates the transoid vin-
ylic C–H cleavage.[2] TpMe2IrIII unsaturated fragments behave dif-
ferently in their interaction with substituted olefins that have
allylic C–H bonds. One of the first reactions carried out with the
IrIII dinitrogen complex [TpMe2Ir(C6H5)2(N2)] (1) was with ethyl-
ene (C6H6, 60 °C), which gave an ethylene adduct of the iridacy-
cle shown in Scheme 1 [Reaction (a)]. As can be easily deduced,
the metallacycle part of this product is the formal result of the
insertion of C2H4 into one of the Ir–Ph bonds followed by the
ortho-metalation of the resulting CH2CH2Ph ligand with con-
comitant formation of a molecule of benzene.[3] The first step
of the reaction is the extrusion of the N2 ligand in 1, a process
that is known to be reversible.
Scheme 1. Reactions of complex 1 with (a) ethylene and (c) propene, and
(b) of the pyridylidene derivative shown with propene.
More recently, we have reported the reactivity of pyridyl-
idenes of TpMe2IrIII, and specifically we have studied the reaction
of the diphenyl species derived from 2-methylpyridine (shown
in Scheme 1) with propene.[4] As also described in Scheme 1
[Reaction (b)], the result was a hydride with a chelating ligand
that, to form, requires the activation of one of the C–H bonds
of the methyl group of propene. The somewhat harsh condi-
tions of the reaction (C6H6, 120 °C) are those needed for the N–
H activation of the pyridylidene ligand. We then reasoned that
the same product might be alternatively obtained by the reac-
tion of propene with complex 1 and found that this was the
case when the same experimental conditions were employed
[Scheme 1, Reaction (c)]. Here a mechanism identical to that
already described for the ethylene was invoked but with the
incorporation of a ꢀ-hydrogen elimination step and other sub-
tleties that will be detailed in the following section.
[a] Instituto de Investigaciones Químicas, Departamento de Química
Inorgánica,
Centro de Innovación en Química Avanzada (ORFEO-CINQA), Consejo
Superior de Investigaciones Científicas (CSIC)
Universidad de Sevilla,
Avenida Américo Vespucio 49, 41092 Sevilla, Spain
E-mail: paneque@iiq.csic.es
mpoveda@iiq.csic.es
www.iiq.csic.es
[b] Departamento de Química Inorgánica, Orgánica y Bioquímica,
Centro de Innovación en Química Avanzada (ORFEO-CINQA),
Universidad de Castilla-La Mancha,
13071 Ciudad Real, Spain
Supporting information and ORCID(s) from the author(s) for this article are
available on the WWW under http://dx.doi.org/10.1002/ejic.201501253.
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3
On the basis of these results, it is clear that allylic C–H activa- particularly the JH,H coupling constant of the olefin protons,
tion is more favorable than its vinylic counterpart, which in fact which amounts to 10.9 Hz, and from the NOESY spectrum. It
has not been observed in the TpMe2IrIII system.[5] As allylic C–H was also confirmed by X-ray analysis (see Figure S2 in the Sup-
activations are of significant interest, we decided to carry out a porting Information).
more general study of the reactivity of complex 1 towards a
series of olefins with allylic C–H bonds including monosubsti-
tuted terminal olefins, terminal or internal disubstituted olefins
that contain a Ph group, and conjugated dienes (either open or
cyclic).
Results and Discussion
Reactions of Complex 1 with Propene and 1-Hexene
The previously reported synthesis[3] of complex 2 from 1 and
propene was revisited, and it was found that it cleanly took
place at 60 °C (C6H6, 1 atm) with no intermediates observed at
this lower temperature. Also, the structure of compound 2 was
unequivocally characterized by means of single-crystal X-ray
analysis (Figure 1), and this study confirmed the stereochemis-
try proposed in the literature for this species.[3]
Scheme 2. Reaction of complex 1 with propene (C6H6, 60 °C) and the mecha-
nism proposed for the formation of 3.
The mechanism proposed for the formation of complex 3 is
also shown in Scheme 2. Following dinitrogen dissociation and
coordination of the olefin, the formed species A evolves
through regio- and stereoselective insertion of the double bond
into one of the Ir–Ph bonds to generate the unsaturated inter-
mediate B. This species then experiences ortho-metalation of
the phenyl substituent of the alkyl ligand to give C, which, in
turn, undergoes a stereoselective ꢀ-hydrogen elimination to
give the unobserved hydride D. Eventually, this latter species
gives the final, thermodynamically more stable stereoisomer by
a simple change of the coordinated face of the olefin.
When the reaction of complex 1 with 1-hexene was carried
out under harsher conditions (C6H6, 5 equiv. of 1-hexene, ca.
0.4 M
, 120 °C, 18 h), the 1H and 13C NMR spectra of the resulting
crude product showed, in addition to complex 3, the presence
of an equimolecular amount of an approximately 5:1 mixture
(i.e., the thermodynamic ratio) of (E)- and (Z)-1-phenylhexene[6]
(4; Scheme 3). The ratio of 3/4 increased to approximately 1:6
under more forceful conditions (C6H6, 20 equiv. of 1-hexene, ca.
Figure 1. Structural view of complex 2 showing 30 % thermal ellipsoids (most
hydrogen atoms omitted for clarity). Selected bond lengths [Å]: Ir1–C16
2.022(6), Ir1–C24 2.137(7), Ir1–C23 2.172(6), C23–C24 1.397(11).
The related reaction of complex 1 with an excess amount of
1-hexene gave complex 3, a species with a structure similar to
that found for 2 but with one of the terminal =CH2 hydrogen
1.6
M, 150 °C, 18 h).
The formation of olefins (E)- and (Z)-1-phenylhexene (4) is
atoms of the bonded olefin replaced with an nPr group. Com- the result of a dehydrogenative catalytic coupling of benzene
plex 3 was obtained as a single stereoisomer (Scheme 2), the and the terminal vinylic C–H bonds (Scheme 4a), and whereas
one that presents the (E) configuration of the olefin moiety. This the formation of 3 results from an allylic C–H activation, only
stereochemistry is easily inferred from NMR spectroscopic data, vinylic hydrogen atoms are involved in the generation of the
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with respect to the hydride was deduced by the NOESY spec-
trum. Formally, the reaction observed can be described as the
result of two consecutive steps: First, the plausible formation of
E by allylic C–H activation of the Me group of a molecule of α-
methylstyrene (with formation of C6H6), and then the – very
unlikely – interchange Ir–Ph + =C–H → Ir–H + =C–Ph between
the latter species E and the less-congested olefinic C–H bond
of a second molecule of the organic reagent. Therefore, this
mechanism is not realistic, and the proposed one is presented
in Scheme 6.
Scheme 3. Reaction of complex 1 with an excess amount of propene at
120 °C (C6H6).
dehydrogenative coupling products, thereby resulting in an
arylation of the olefin.[7] The arylation of olefins by means of a
dehydrogenative process is very interesting for synthetic pur-
poses, in particular if the withdrawing agent for the activated
hydrogen atoms is oxygen.[8] In our system, the olefin used was
at the same time the sacrificial agent, and analysis of the vola-
tile portion of the reaction revealed the formation of a stoichio-
metric amount of n-hexane (Scheme 4a). In view of these re-
sults, we studied the reaction of 1 with propene in benzene at
120 °C and observed that under these conditions a thermody-
namic mixture of the (E) and (Z) isomers of 1-phenylpropene
was also formed (Scheme 4b).
Scheme 5. Reaction of complex 1 with α-methylstyrene.
Scheme 4. Phenylation of (a) 2-hexene and (b) propene by dehydrogenative
coupling of benzene with these olefins mediated by complex 1 (C6H6, 120–
150 °C).
Although these and all other olefin phenylation processes
reported herein are conceptually very simple reactions, their
mechanism must be very complex, and a detailed investigation
is out of the scope of this work.
Reaction of 1 with α-Methylstyrene
The reaction of complex 1 with an excess amount of α-methyl-
styrene (C6H6, 120 °C) is interesting in the sense that, instead
of the expected product E (i.e., the result of the simple activa-
tion of one of the C–H bonds of the methyl substituent), it
generated an approximately 1:1 mixture of the hydride allylic
species 5 and (E)-α-methylstilbene (Scheme 5). Complex 5 was
characterized by the usual techniques (see the Experimental
Section), and in particular the conformation of the allyl ligand
Scheme 6. Mechanism proposed for the formation of compound 5 from 1
and α-methylstyrene.
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Initial coordination of the olefin could be followed by its Reaction of Complex 1 with Cyclohexadiene
regioselective insertion into one of the Ir–Ph bonds, thus lead-
ing to the intermediate species G, which, instead of experienc-
ing ortho-metalation, as was the case for the related intermedi-
ate B invoked in the reaction of 1 with 1-hexene (see
Scheme 2), evolves through a stereospecific ꢀ-H elimination, to
give the olefin adduct H. Then olefin substitution generates the
observed (E)-α-methylstilbene and intermediate J, in which a
final C–H activation of the methyl group gives the observed
organometallic product 5. If this proposed mechanism were
true, it is certainly surprising that this latter C–H activation takes
place in intermediate J and not in the related species F or H,
and this might be another example of the high process selectiv-
ity typically observed in late third-row organometallic systems.
Complex 1 also acts as a precatalyst for the dehydrogenative
coupling of α-methylstyrene and benzene with formation of
equimolecular amounts of (E)-α-methylstilbene and cumene
(Scheme 7).
The reaction of 1 with a large excess amount of 1,3-cyclohexa-
diene (20 equiv., C6H6, 150 °C) took place with quantitative for-
mation of the known compound 6[9] shown in Scheme 9, in
which the cyclic diene is η4-coordinated to the IrI center. A small
amount (ca. 20 %) of biphenyl was also formed. This reaction
implies a reduction from TpMe2IrIII to TpMe2IrI, but the amount
of biphenyl formed, well below the stoichiometric, rules out a
simple reductive elimination process triggered by the presence
of the diene. On the other hand, the formation of complex 6
in this reaction is not totally surprising since we have already
observed similar Tp′IrIII-to-Tp′IrI reductions in cases when an η4-
diene product was formed,[10,5] although it is also true that this
is the first case in which biphenyl is produced from complex 1.
Scheme 9. Reaction of complex 1 with an excess amount of cyclohexadiene
(C6H6, 150 °C).
Scheme 7. Phenylation of α-methylstyrene by dehydrogenative coupling of
benzene with that olefin mediated by complex 1 (C6H6, 120 °C).
Reaction of Complex 1 with 2,3-Dimethylbutadiene (DMB)
Reaction with (E)-ꢀ-Methylstyrene
The reaction of 1 with DMB is quite complex, since it depends
not only on the solvent used [cyclohexane (a) or benzene (b)]
but also on the concentration of the diene and the temperature
of the reaction, as will be discussed in detail in the following
sections.[11]
When complex 1 was exposed to an excess amount of (E)-ꢀ-
methylstyrene (C6H6 or C6H12, 120 °C), a clean reaction took
place in which this olefin – surprisingly and in spite of contain-
ing an allylic methyl group – did not behave like propene and
1-hexene or α-methylstyrene, and instead the already described
product 2 was formed (i.e., the outcome of the reaction of com-
plex 1 with propene) (Scheme 8). From the experimental point
of view, it is worth mentioning that no phenylation of (E)-ꢀ-
methylstyrene was observed under the conditions of Scheme 7,
and this is clearly due to the steric constrains associated with
this particular olefin. In addition, we were not able to disclose
satisfactorily the fate of the Ph ligands of precursor 1 and hence
the stoichiometry of the reaction. Neither in C6H6 nor in C6H12
did we detect the presence of biphenyl, and only in benzene
were we able to identify approximately 20 % of the olefin
PhCH=CHCH2Ph as well as (in both solvents) an analogous
amount of a non-volatile organic byproduct with a Ph group,
which we were unable to purify and identify. A similar problem
was encountered in the reaction of complex 1 with cyclohexa-
diene reported below.
(a) Reaction in Cyclohexane
The reaction of complex 1 with a large excess amount of DMB
in cyclohexane (20 equiv., 1.5
M, 120 °C, 16 h) was not very
clean because of the low solubility of complex 1 in this solvent
and yielded the hydride-allyl species 7 as the major product
(ca. 40 %) along with the equivalent amount of the phenylated
diene (Scheme 10) that resulted from the stereospecific substi-
tution of an exo-methylene H atom of DMB by a phenyl group.
Scheme 10. Reaction of complex 1 with a large excess amount of DMB in
Scheme 8. Reaction of complex 1 with (E)-ꢀ-methylstyrene.
cyclohexane (120 °C).[11]
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The reaction balance requires the generation of 1 equiv. of when it is carried out in C6H12 at 120 °C with only 1 equiv. of
benzene and resembles the reaction of 1 with α-methylstyrene, substrate.
also in the sense that the corresponding phenyl-allyl derivative
K (i.e., the result of the straightforward C–H activation of a
methyl group of DMB) was not formed.
The mechanism proposed for the reaction of complex 1 with
an excess amount of DMB in C6H12 is related to that for the
reaction of 1 with α-methylstyrene (Scheme 6) and no further
Complex 7 was characterized by means of NMR spectroscopy discussion is needed.
among other techniques. Along with a characteristic signal at
(b) Reaction in Benzene
δ = –28.16 ppm for the hydride ligand, in the 1H NMR spectrum
The reaction of complex 1 with a large excess amount of DMB
in benzene (20 equiv., 1.5 , C6H6, 120 °C, 16 h) was even more
the resonances for the allyl protons are located at δ = 3.57 and
2.46 ppm (2JH,H = 2.3 Hz), whereas the noncoordinated olefinic
protons resonate at δ = 5.48 and 5.0 ppm. In the 13C{1H} NMR
spectrum, the noncoordinated CH2 carbon nucleus is responsi-
ble for a signal at δ = 114.2 ppm (1JC,H = 156 Hz), whereas the
two equivalent methylene groups of the allyl moiety resonate
at δ = 17.5 ppm (1JC,H = 153 Hz).
Interestingly, the outcome of the reaction of complex 1 with
DMB changed when it was conducted under the same condi-
tions but using only 1 equiv. of the diene (Scheme 11). In this
case, the hydride-allyl compound 8, which contains a Ph group
in one of the two positions of the pendant olefin specifically
located in a (Z) disposition with respect to the Me substituent
(NOESY evidence), was obtained in almost quantitative yield.
M
complex, because, in addition to the same products that
formed in cyclohexane, a new cyclometallated allylic species 9
was observed along with other organic products derived from
the phenylation of DMB (Scheme 13). The new products were
all characterized, mainly by NMR spectroscopy, but not all of
them could be purified by column chromatography (e.g., com-
plex 9, which was characterized in admixture with complex 7).
Scheme 11. Reaction of complex 1 with 1 equiv. of DMB in cyclohexane
(120 °C).
Scheme 13. Reaction of complex 1 with a large excess amount of DMB in
benzene (120 °C).
Logically it was expected that allyl compound 8 would react
with DMB under the conditions of Scheme 10 to yield complex
7 with concomitant generation of the phenylated diene [i.e.,
As a result of this complexity, we decided to study this reac-
(Z)-2,3-dimethyl-1-phenylbutadiene], and this was indeed the tion in detail and did so by changing the reaction time, temper-
case (Scheme 12). However, this reaction is deceptively simple: ature, ratio of 1/DMB, and so forth. After analyzing all the exper-
In fact, monitoring of the reaction of Scheme 10 in C6D12 with imental data obtained, the results indicate the following:
3 equiv. of DMB showed that the reaction started at approxi-
mately 80 °C and yielded the species reported in Scheme 10,
but compound 8 was not observed as an intermediate. In addi-
tion, 8 did not react with the excess amount of DMB under
these mild conditions, and hence it can be suggested that the
reaction shown in Scheme 12 is reversible and that complex 8
is the thermodynamic product of the reaction of 1 and DMB
(1) At 80 °C the first process was the generation of the phe-
nylated product (Z)-2,3-dimethyl-1-phenylbutadiene and the
concomitant formation of the known hydrido-phenyl derivative
10, which in fact exists in solution as a mixture of this species
and the two dinuclear stereoisomers that contain a bridging
dinitrogen ligand[9] (Scheme 14).
Scheme 14. First observed reaction of complex 1 with a large excess amount
of DMB in benzene (80 °C; short reaction times).
(2) On raising the temperature to 100 °C, species 10 gradu-
ally disappeared to afford allyl compounds 7 (major species at
Scheme 12. Reaction of complex 8 with a large excess amount of DMB in
cyclohexane (120 °C).
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early reaction times), 8, and 9 with an increase in the phenyl- position of the diene over time under the conditions specified
ated diene already present in the mixture. In addition, the other in the latter scheme (C6D6, 0.01 equiv. of 1, 80 °C). As can be
three organic products shown in Scheme 13 started to appear seen, the deuteration of the vinylic positions is much faster than
(2,3-dimethyl-1-butene was detected and characterized in the the deuteration of the Me groups, which only transformed onto
volatile fraction). All these organic products must be the result CD0.6H2.4 counterparts after 7 d under these conditions.
of the catalytic processes shown in Scheme 15, and among
them the dimethyl-dihydronaphthalene should be the result of
the transformation of the purported phenylated species (E)-2,3-
dimethyl-1-phenylbutadiene, an isomer of the observed (Z) spe-
cies, but probably more reactive towards an intramolecular sub-
sequent arylation process that prevents its detection in the re-
action mixture.
Scheme 17. Deuteration of DMB in C6D6 mediated by compound 1.
Figure 2. Deuteration [%] of the different positions of DMB (C6D6, 0.01 equiv.
1, 80 °C) versus time.
Scheme 15. Catalytic processes proposed for the formation of the phenylated
olefins formed in the reaction of Scheme 13.
Reactions of Complex 1 with Cyclopentadienes
(3) The isomeric species 7 and 9 did not interconvert under
their formation conditions.
Two final types of olefin with allylic hydrogen atoms studied in
(4) Remarkably, an increase in the temperature up to 120 °C
this work are cyclopentadiene and methylcyclopentadiene.
favored the catalytic processes shown in Scheme 15, and as the
These reagents possess particularly reactive allylic hydrogen at-
concentration of (Z)-2,3-dimethyl-1-phenylbutadiene increased,
oms, and in fact they reacted with complex 1 under very mild
it reacted with 7 to form allyl compound 8 (Scheme 16), thus
conditions (C6H6, 60 °C) with formation of derivatives 11 and
12, which contain η5-cyclopentadienyl ligands (Scheme 18).
This type of coordination forces the TpMe2 ligand to become
bidentate to maintain the 18-electron count.[13] At odds with
the reactions previously mentioned in this paper, one of the Ph
ligands remains in the final compounds, and as a consequence
no organic products derived from coupling processes were ob-
served in the reactions.
following a process that is the reverse of the one shown in
Scheme 12. This suggests that in this system reversible transfor-
mations that involve dienes and allyl moieties take place, al-
though we have not carried out studies to determine the equi-
librium constant.
Scheme 16. Transformation of 7 into 8 by reaction with (Z)-2,3-dimethyl-1-
phenylbutadiene.
(5) Finally, the complexity of the system is evidenced by the
fact that the reaction of allyl species 8 with DMB under the
reaction conditions of Scheme 13 (20 equiv. DMB, 1.5
120 °C, 16 h) affords very similar results.
Some of the studies carried out to investigate the issues
above were performed in C6D6, and this allowed us to observe
the deuteration[12] of the excess amount of DMB used
M, C6H6,
Scheme 18. Reactions of complex 1 with cyclopentadiene and methylcyclo-
pentadiene.
Compounds 11 and 12 were unequivocally characterized by
(Scheme 17); Figure 2 shows the relative deuteration of each single-crystal X-ray diffraction (Figure 3 and Figure S4 in the
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22 spectrometer. The NMR instruments were Bruker DRX-500, 400/
R, and DPX-300 spectrometers. Spectra were referenced to external
SiMe4 (δ = 0 ppm) by using the residual protio solvent peaks as
internal standard (1H NMR spectroscopic experiments) or the char-
acteristic resonances of the solvent nuclei (13C NMR spectroscopic
experiments). Spectral assignments were made by means of routine
one- and two-dimensional NMR spectroscopic experiments where
appropriate. HRMS data were obtained with a JEOL JMS-SX 102A
mass spectrometer by the Mass Spectrometry Services of the Uni-
versity of Seville (CITIUS). All manipulations were performed under
dry, oxygen-free dinitrogen and by applying conventional Schlenk
techniques. Compound 1 was obtained according to a published
procedure.[3] Compounds 2[4] and 6[9] and the organic products
formed were identified by comparison of their NMR spectroscopic
data with those of the literature. Owing to the small scale used for
the reactions with iridium compounds as the starting material and
the mixtures obtained in most of the reactions, which required col-
umn chromatography to separate the different products, we were
unable to run satisfactory elemental analyses for compounds 3, 5,
7, 8, and 9 (which was identified by NMR spectroscopy on a mixture
with 7 since they eluted together). 13C{1H} NMR spectra for these
compounds are included in the Supporting Information (Fig-
ures S5–S9). CCDC 1434248 (for 2), 1434249 (for 3), 1434250 (for
11), and 1434251 (for 12) contain the supplementary crystallo-
graphic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre.
Supporting Information, respectively), and the results clearly
show the three-legged piano-stool structure, with η5 coordina-
tion of the cyclopentadienyl ligands and κ2 bonding mode for
the TpMe2 ligand. The NMR spectroscopic data are also in agree-
ment with this structure; the five CH moieties of the Cp ligand
in compound 11 are equivalent [δ(1H) = 5.07 ppm and
δ(13C{1H}) = 80.9 ppm].
Compound 3: Compound 1 (0.20 g, 0.3 mmol) was dissolved in
C6H6 (0.5 mL), 1-hexene (180 μL, 1.45 mmol) was added, and the
solution was stirred at 120 °C for 18 h. The solvent was removed
under reduced pressure, and an 1H NMR spectrum of the crude
reaction mixture revealed the presence of compound 3 in approxi-
mately 50 % yield along with 1-phenylhexene. Following column
chromatography on silica gel (1:50 → 1:20, Et2O/hexane), the or-
ganic product and compound 3 were isolated.
Figure 3. Structural view of complex 11 showing 30 % thermal ellipsoids
(most hydrogen atoms omitted for clarity). Selected bond lengths [Å]: Ir1–
C16 2.078(3), Ir1–C24 2.164(3), Ir1–C23 2.195(3), Ir1–C25 2.173(3), Ir1–C26
2.196(3), Ir1–C22 2.257(3), Ir1–N1 2.069(2), Ir1–N3 2.072(2).
Conclusion
The IrIII derivative [TpMe2Ir(C6H5)2(N2)] (1) selectively activates
allylic C–H bonds of a variety of olefins. Monosubstituted termi-
nal olefins provide phenyl complexes substituted in the ortho
position by a carbon chain with a coordinated olefin, whereas
the disubstituted terminal olefins (including dienes) form Ir–
allyl derivatives. Finally, allylic C–H bonds of cyclopentadienes
are easily activated and form η5-cyclopentadienyl complexes
with a κ2-Tp ligand. Although these organometallic compounds
are formed as the result of C–H allylic cleavages, vinylic C–H
bonds might also be activated in these systems, as inferred
1H NMR (500 MHz, CDCl3, 25 °C): δ = 6.99 (d, 1 H, C3H), 6.73 (t, 1 H,
C4H), 6.52 (t, 1 H, C5H), 6.48 (d, 1 H, C6H), 5.81, 5.78, 5.69 (s, 1 H
3
each, 3 CHpz), 5.04 (dd, JH,H = 10.9, 5.5 Hz, 1 H, HC), 3.70, 3.57 (dd,
m, 1, 2JH,H = 16.0 Hz, 3JH,H = 5.6 Hz, H each, CHAHB), 3.57 (m, 3JH,H
=
10.9, 3.6 Hz, 1 H, HD), 2.49, 2.39, 2.37, 2.22, 1.80, 1.56 (s, 3 H each,
6 Mepz), 1.28 (m, 2 H, CH2Me), 1.85, 1.11 (m, 1 H each, CHEHF), 0.80
(t, 3 H, Me), –17.39 (s, 1 H, Ir–H) ppm. 13C{1H} NMR (125 MHz, CDCl3,
from 1 acting as a (pre)catalyst in the full deuteration of the 25 °C): δ = 150.9 (C2), 151.5, 150.3, 150.1, 143.7, 143.4, 143.1 (Cqpz),
137.8 (C1), 136.8 (C6), 123.2 (C5), 122.3 (C3), 121.4 (C4), 108.2, 106.6,
olefinic CH2 moieties of 2,3-dimethylbutadiene in C6D6 in the
presence of 1, and in the formation of organic products that
result from the dehydrogenative coupling of benzene (the sol-
vent) and the corresponding alkene. In these couplings, the ole-
fin also acts as the hydrogen scavenger.
106.3 (CHpz), 71.1 (C8), 61.2 (C9), 42.8 (C7), 34.1 (C10), 24.9 (C11), 16.3,
16.2, 14.0, 13.1, 13.0, 12.9 (Mepz), 14.4 (Me) ppm. IR (Nujol): ν = (B–
˜
H) 2519, ν(Ir–H) 2189 cm–1. HRMS (FAB): calcd. for C27H38BIrN6 [M]+
650.2880; found 650.2882.
Compound 5: Compound 1 (0.20 g, 0.30 mmol) was dissolved in
C6H6 (4 mL), and α-methylstyrene (0.194 mL, 1.5 mmol) was added.
The solution was stirred at 120 °C for 12 h, and after this period of
time, the solvent was removed under reduced pressure. 1H NMR
spectra of the crude reaction mixture revealed the presence (1:1) of
(E)-α-methylstilbene, and compound 5 (ca. 50 % yield). Compound
5 was purified by column chromatography on silica gel by using a
Experimental Section
General Considerations: Microanalyses were performed by the Mi-
croanalytical Service of the Instituto de Investigaciones Químicas
(Sevilla, Spain). Infrared spectra were obtained with a Bruker Vector
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1:20 mixture of Et2O/hexane as eluent, whereas (E)-α-methylstil- Data for 8
bene was separated by using a 1:50 mixture of Et2O/hexane as elu-
ent, and cumene was detected after condensation of the volatile
fraction of the reaction.
1H NMR (500 MHz, CDCl3, 25 °C): δ = 7.35 (m, 4 H, o-, m-, 4 CHPh),
7.23 (t, 1 H, p-CHPh), 7.12 (s, 1 H, CH), 5.81, 5.66 (s, 2:1, 3 CHpz), 3.66,
2.51 (d, 2JH,H = 2.3 Hz, 2 H each, CH2 allylic), 2.40, 2.39, 2.07, 2.06 (s,
1:2:1:2, 6 Mepz), 2.15 (s, 3 H, Me), –27.91 (s, 1 H, Ir–H) ppm. 13C{1H}
NMR (125 MHz, CDCl3, 25 °C): δ = 151.1, 151.0, 143.5, 143.2 (1:2:1:2,
1H NMR (500 MHz, CDCl3, 25 °C): δ = 7.74, 7.33, 7.23 (d, t, t, 2:2:1, 5
H, o-, m-, p-CHPh, respectively), 5.81, 5.68 (s, 2:1, 3 CHpz), 3.89, 2.81
3
(d, JH,H = 2.8 Hz, 2 H each, 2 HA and 2 HB, respectively), 2.41, 2.39,
2.09, 2.05 (s, 1:2:1:2, 6 Mepz), –27.80 (s, 1 H, Ir–H) ppm. 13C{1H} NMR
(125 MHz, CDCl3, 25 °C): δ = 151.0, 150.9, 143.5, 143.3 (1:2:1:2, Cqpz),
144.9 (CqPh), 128.0, 127.8, 127.6 (2:1:2, m-, p-, o-, CHPh), 108.7, 105.8
(1:2, CHpz), 95.1 (C1), 20.6 (CHAHB), 16.1, 14.8, 13.1, 12.9 (2:1:1:2,
Cqpz), 141.2 (CqMe), 138.8 (CqPh), 128.5 (CH), 129.6, 128.2, 126.5
(2:2:1, o-, m-, p-, CHPh), 108.8, 105.8 (1:2, CHpz), 99.2 (Cq allylic), 17.48
(1JC,H = 153 Hz, CH2 allylic), 14.3 (Me), 16.0, 14.7, 13.1, 12.8 (2:1:1:2,
Mepz) ppm. IR (Nujol): ν = (B–H) 2525, ν(Ir–H) 2216 cm–1. HRMS
˜
Mepz) ppm. IR (Nujol): ν = (B–H) 2526, ν(Ir–H) 2172 cm–1. HRMS
˜
(FAB): calcd. for C27H35BIrN6 [M – H]+ 647.2646; found 647.2605.
(FAB): calcd. for C24H32BIrN6 [M]+ 606.2254; found 606.2274.
Data for 9
Reaction of Complex 1 with (E)-ꢀ-Methylstyrene: Compound 1
(0.20 g, 0.3 mmol) was dissolved in C6H6 (0.5 mL), (E)-ꢀ-methylsty-
rene (196 μL, 1.5 mmol) was added, and the solution was stirred at
120 °C for 18 h. The solvent was removed under reduced pressure,
and the 1H NMR spectra of the crude reaction mixture revealed the
presence of compound 2 as the major product.
Reaction of Complex 1 with Cyclohexadiene: 1,3-Cyclohexadiene
was added (0.09 mL, 0.8 mmol) to a solution of 1 in C6H6 (0.03 g,
0.04 mmol; 0.5 mL), and the mixture was stirred at 150 °C for 16 h.
After removing the solvent under reduced pressure, quantitative
conversion into product 6 was ascertained by 1H NMR spectroscopy.
The spectrum also showed the presence of biphenyl (ca. 20 %). The
solid residue was washed with cold pentane to yield 6 as beige
solid.
1H NMR (500 MHz, CDCl3, 25 °C): δ = 5.85, 5.81, 5.55 (s, 1 H each, 3
CHpz), 3.88 and 2.53, 3.81 and 2.48 (s, 1 H each, 2 CH2 allylic), 3.07
(m, 1 H, CHMe), 2.46, 2.40, 2.36, 2.32, 1.86 (s, 2:1:1:1:1, 6 Mepz), 1.11
(d, JH,H = 7.0 Hz, 3 H, CHMe), 0.85, 0.42 (m, t, JH,H = 7.7 Hz, 1 H
each, Ir–CH2) ppm. 13C{1H} NMR (125 MHz, CDCl3, 25 °C): δ = 151.5,
151.4, 150.7, 143.4, 143.2, 143.0 (Cqpz), 108.0, 107.7, 107.5 (CHpz),
82.3 (Cq allylic), 37.1 (CHMe), 30.2, 20.3 (1:1, CH2 allylic), 24.6 (CHMe),
15.6, 15.1, 15.0, 13.1, 13.0, 12.8 (Mepz), –42.7 (1JC,H = 156 Hz, Ir–CH2)
ppm.
3
3
Compounds 7, 8, and 9: Compound 1 (0.20 g, 0.3 mmol) dissolved
in C6H6 (4 mL) was treated with 2,3-dimethylbutadiene (0.673 mL,
5.95 mmol), and the resulting solution was stirred at 120 °C for 16 h.
The solvent was then removed under vacuum, and NMR spectro-
scopic monitoring of the crude product revealed the formation of
a mixture of 7 (20 %), 8 (45 %), and 9 (30 %) together with 2,3-
dimethyl-1-phenylbutadiene, 2,3-dimethyl-1,4-diphenylbutadiene,
and 2,3-dimethyl-1,4-dihydronaphthalene. Separation of 7 and 8
was achieved by column chromatography on silica gel (1:19, Et2O/
pentane), and 9 eluted in a fraction together with part of 7, whereas
a 1:200 mixture of Et2O/pentane was used as eluent for the separa-
tion of the organic products. The product 2,3-dimethyl-1,4-diphen-
ylbutadiene was isolated as a white solid by crystallization from
methanol. Finally, 2,3-dimethyl-1-butene was identified by means
Compound 11: Compound 1 (0.083 g, 0.04 mmol) was dissolved
in C6H6 (0.5 mL), and recently distilled cyclopentadiene (0.05 mL,
0.8 mmol) was added. The solution was stirred at 60 °C for 16 h,
and the solvent was then evaporated under vacuum. 1H NMR spec-
troscopic monitoring of the crude product revealed the formation
of 11 in almost quantitative yield. The solid residue was washed
with cold pentane to yield 11 as a green solid (53 % yield).
1
of H NMR spectroscopic analysis of the volatile fraction.
Data for 7
1H NMR (500 MHz, CDCl3, 25 °C): δ = 6.83–6.73 (m, 5 H, Ir–C6H5),
5.88, 5.75 (s, 2:1, 3 CHpz), 5.07 (s, 5 H, C5H5), 2.56, 2.19, 1.60, 1.47 (s,
2:1:2:1, 6 Mepz) ppm. 13C{1H} NMR (125 MHz, CDCl3, 25 °C): δ =
152.9, 147.9, 147.6, 142.1 (2:2:1:1, Cqpz), 138.1, 126.9, 121.9 (br. s, br.
s, s, 2:2:1, C6H5), 136.8 (Ir–CPh), 107.7, 106.4 (2:1, CHpz), 80.9 (C5H5),
1H NMR (500 MHz, CDCl3, 25 °C): δ = 5.80, 5.64 (s, 2:1, 3 CHpz), 5.48
(s, 1 H, HA), 5.00 (s, 1 H, HB), 3.57, 2.46 (d, JH,H = 2.3 Hz, 2 H each,
2
16.1, 14.0, 14.0, 10.8 (2:2:1:1, Mepz) ppm. IR (Nujol): ν = (B–H)
˜
CH2 allylic), 2.40, 2.39, 2.05, 2.03 (s, 1:2:2:1, 6 Mepz), 2.08 (s, 3 H, Me),
–28.16 (s, 1 H, Ir–H) ppm. 13C{1H} NMR (125 MHz, CDCl3, 25 °C): δ =
151.0, 143.2 (1:1, Cqpz) 147.7 (CqMe), 114.2 (1JC,H = 156 Hz, CHAHB),
2477 cm–1. C26H32BIrN6 (631.61): calcd. C 49.4, H 5.1, N 13.3; found
C 49.1, H 5.1, N 13.4.
108.7, 105.8 (1:2, CHpz), 96.6 (Cq allylic), 18.8 (Me), 17.5 (1JC,H
=
Compound 12: Compound 1 (0.20 g, 0.30 mmol) was dissolved in
153 Hz, CH2 allylic), 16.0, 14.6, 13.1, 12.8 (2:1:1:2, Mepz) ppm.
C6H6, and methylcyclopentadiene (0.51 mL, 5.9 mmol) was added.
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The solution was stirred at 60 °C for 12 h, and after that period of
time, the solvent was removed under reduced pressure. Compound
12 was crystallized from pentane/CH2Cl2 mixture by cooling to
–20 °C, and the brown precipitate obtained was removed by filtra-
tion and washed with cold pentane (–20 °C, 5 mL) (44 % yield).
Keywords: C–H activation · Iridium · Scorpionates · C–C
coupling · Olefination · Organometallic compounds
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1H NMR (500 MHz, CDCl3, 25 °C): δ = 7.91, 6.80, 6.71 (br. s, t, br. s,
2:1:2, Ir–C6H5), 5.86, 5.76 (s, 2:1, 3 CHpz), 5.25 (s, 2 H, 2 CHA), 4.25 (s,
2 H, 2 CHB), 2.54, 2.16, 1.57 (s, 2:1:2, 5 Mepz), 1.45 (s, 6 H, Mepz and
C5H4Me) ppm. 13C{1H} NMR (125 MHz, CDCl3, 25 °C): δ = 152.2,
147.9, 147.2, 142.1 (2:2:1:1, Cqpz), 139.6, 137.2, 128.0, 125.6, 121.8
(C6H5), 137.1 (Ir–CPh), 114.4 (CqCp), 107.8, 106.5 (2:1, CHpz), 82.4
(CHA), 72.3 (CHB), 16.0, 14.1, 13.9, 10.8 (2:2:1:1, Mepz) 11.8 (C5H4Me)
ppm. IR (Nujol): ν = (B–H) 2477 cm–1. C27H34BIrN6 (645.64): calcd. C
[9] O. Boutry, M. L. Poveda, E. Carmona, J. Organomet. Chem. 1997, 528,
143–150.
˜
50.2, H 5.3, N 13.0; found C 50.2, H 5.4, N 13.1.
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Supporting Information (see footnote on the first page of this
article): Complete crystallographic data and technical details for the
X-ray diffraction studies; copies of the 13C{1H} NMR spectra for com-
pounds 3, 5, 7, 8, and 9.
Acknowledgments
Financial support (FEDER contribution) from the Spanish Minis-
try of Science (grant nos. CSD2007-0006 and CTQ2014-51912-
REDC) and the Junta de Andalucía (grant no. FQM-119 and
project no. P09-FQM-4832) is acknowledged.
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ton, A. H. White, Organometallics 2003, 22, 2820–2826.
Received: October 30, 2015
Published Online: January 19, 2016
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© 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim