Pyridine-enhanced head-to-tail dimerization of terminal alkynes by a rhodium-N-heterocyclic-carbene catalyst

Article abstract of DOI:10.1002/chem.201302079

A general regioselective rhodium-catalyzed head-to-tail dimerization of terminal alkynes is presented. The presence of a pyridine ligand (py) in a Rh-N-heterocyclic-carbene (NHC) catalytic system not only dramatically switches the chemoselectivity from alkyne cyclotrimerization to dimerization but also enhances the catalytic activity. Several intermediates have been detected in the catalytic process, including the π-alkyne-coordinated Rh^I species [RhCl(NHC)(η²-HC ≡CCH₂Ph)(py)] (3) and [RhCl(NHC){η²-C(tBu) ≡C(E)CH=CHtBu}(py)] (4) and the Rh^III-hydride-alkynyl species [RhClH{-C ≡CSi(Me) ₃}(IPr)(py)₂] (5). Computational DFT studies reveal an operational mechanism consisting of sequential alkyne Ci￡ H oxidative addition, alkyne insertion, and reductive elimination. A 2,1-hydrometalation of the alkyne is the more favorable pathway in accordance with a head-to-tail selectivity. Control plan: Addition of pyridine to rhodium-N-heterocyclic- carbene catalysts not only switches the chemoselectivity from alkyne cyclotrimerization to dimerization, but also enhances the catalytic activity for the formation of 1,3-enynes (see figure). A 2,1-hydrometalation of the alkyne is the more favorable pathway calculated by DFT.

Full text of DOI:10.1002/chem.201302079

DOI: 10.1002/chem.201302079
Pyridine-Enhanced Head-to-Tail Dimerization of Terminal Alkynes by a
Rhodium–N-Heterocyclic-Carbene Catalyst
Laura Rubio-Pꢀrez,[a] Ramꢁn Azpꢂroz,[a] Andrea Di Giuseppe,[a] Victor Polo,[b]
Ricardo Castarlenas,*[a] Jesffls J. Pꢀrez-Torrente,[a] and Luis A. Oro*[a]
À ꢀ
Abstract: A general regioselective rho-
dium-catalyzed head-to-tail dimeriza-
tion of terminal alkynes is presented.
The presence of a pyridine ligand (py)
in a Rh–N-heterocyclic-carbene (NHC)
catalytic system not only dramatically
switches the chemoselectivity from
alkyne cyclotrimerization to dimeriza-
tion but also enhances the catalytic ac-
tivity. Several intermediates have been
detected in the catalytic process, in-
ACHTUGTNREN{NGU C CSi(Me)3}ACHTUNGTREN(NUGN IPr)(py)2] (5). Compu-
tational DFT studies reveal an opera-
tional mechanism consisting of sequen-
cluding the p-alkyne-coordinated RhI
2
ꢀ
species [RhCl
A
G
{h2-C
tial alkyne C H oxidative addition,
alkyne insertion, and reductive elimina-
tion. A 2,1-hydrometalation of the
alkyne is the more favorable pathway
in accordance with a head-to-tail selec-
tivity.
À
(NHC)
C(E)CH=CHtBu}(py)] (4) and the
RhIII–hydride–alkynyl species [RhClH-
À
Keywords: alkynes · carbenes · C
C coupling · dimerization · rhodium
Introduction
Conjugated 1,3- or 1,4-disubstituted enynes are key subunits
of a variety of biologically active molecules,[1] polymers,[2] or
photoactive derivatives,[3] as well as building blocks for fur-
ther structural elaborations.[4] The catalytic C C coupling re-
À
action of two alkynes is a powerful atom-economical syn-
thetic approach,[5] although chemo-, regio-, and stereoselec-
tivity control still remains a major challenge. Competitively
to the formation of head-to-head (E/Z) and head-to-tail
(gem) enynes, other products such as butatrienes,[6] diynes,[7]
dieneynes,[8] cyclotrimers,[9] oligomers,[10] or polymers[11] can
also be obtained, depending on the catalyst and reaction
conditions (Scheme 1). A broad range of catalysts based on
Pd,[12] Ru,[13] Rh,[14] Ni,[15] Ir,[16] Fe,[17] Au,[18] Co,[19] Os,[20]
Ti,[21] Zr,[22] Re,[23] Y,[24] Sc,[25] Hf,[26] Cr,[27] lanthanides,[28]
actinides,[29] or main group elements[30] can promote alkyne
dimerization, albeit with different grades of success with
regard to selectivity. In particular, preferential preparation
À
Scheme 1. Organic products arising from C C coupling of terminal al-
kynes.
of head-to-tail enynes has been disclosed for aro
ACHTUNGTRENNUNGmat-
ic[13i,14e,16b,24a] or aliphatic[12d,14a,c,g,k,22a,24c,26,29b] alkynes, al-
though examples of selective initiators regardless of the sub-
stituent on the alkyne are limited to the catalysts of Naka-
mura (Ti),[21a] Trost (Pd),[12b] Eisen (Al),[30b] and Zhang
(Au)[18] and their respective co-workers.
In order to control the selectivity of a catalytic transfor-
mation, an in-depth knowledge of mechanistic issues is es-
sential. With regard to alkyne dimerization, the alkyne plays
[a] Dr. L. Rubio-Pꢀrez, Dipl.-Chem. R. Azpꢁroz, Dr. A. Di Giuseppe,
Dr. R. Castarlenas,+ Prof. J. J. Pꢀrez-Torrente, Prof. L. A. Oro
Departamento Quꢁmica Inorgꢂnica
Instituto Sꢁntesis Quꢁmica y Catꢂlisis Homogꢀnea
Universidad de Zaragoza—CSIC
C/Pedro Cerbuna 12, 50009 Zaragoza (Spain)
E-mail: rcastar@unizar.es
À
a dual role: the rather acidic terminal C H proton of an
oro@unizar.es
alkyne is activated and subsequently added across the triple
bond of a second alkyne molecule. Two main mechanisms
for this transition-metal-mediated dichotomous behavior
have been proposed: a) oxidative addition of the terminal
C H bond of one alkyne to generate a hydride–alkynyl spe-
cies, followed by migratory insertion of the other alkyne
[b] Dr. V. Polo
Departamento Quꢁmica Fꢁsica e
Instituto de Biocomputaciꢃn y Fꢁsica de Sistemas Complejos (BIFI)
Universidad de Zaragoza
À
C/Pedro Cerbuna 12, 50009 Zaragoza (Spain)
[+] ARAID Foundation Researcher.
À
À
into M H (hydrometalation) or M C (carbometalation)
bonds, and subsequent reductive elimination, or b) forma-
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201302079.
15304
ꢄ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2013, 19, 15304 – 15314

FULL PAPER
phines in organometallic cata-
lysts by NHCs has enhanced, or
sometimes complemented, the
catalytic activity and extended
the scope of many transition-
metal-mediated
transforma-
tions.[33] Alkyne dimerization is
not an exception and, in fact, it
provides
a test ground for
NHC-based catalysts.[12d,e,13g,l,16h]
On the other hand, our re-
search group has been studying
for some time the chemistry of
rhodium–NHC complexes for
À
À
new C C and C X bond form-
ing reactions.[34] We have found
that the selectivity in alkyne hy-
drothiolation can be dramati-
cally switched to the formation
of branched vinyl sulfides by
the simple addition of pyridine
to dinuclear catalysts of the
Scheme 2. General overview for the formation of conjugated 1,3- or 1,4-disubstituted enynes.
tion of a metal–vinylidene intermediate, successive nucleo-
philic attack of the hydride or alkynyl group on the electro-
philic Ca atom of the cumulene moiety, and elimination.
The vinylidene pathway produces E/Z-enynes and is gener-
ally proposed for ruthenium catalysts. In contrast, the migra-
tory insertion route generates E- or gem-enynes, depending
type [{RhACTHNGUTRENUNG ACHTUGNTRENNNUG
(m-Cl)(h2-olefin)(NHC)}2].[34e] It has been demon-
strated that pyridine coordination trans to the NHC in the
active species directs the coordination of the incoming
alkyne cis to the carbene. The higher trans influence of a hy-
dride ligand paves the way to a cis thiolate–alkyne disposi-
tion, which favors thiolate insertion. In view of these results,
we became interested in broadening the scope of pyridine-
modified Rh–NHC catalysts to other types of addition reac-
tions, such as the dimerization of terminal alkynes. Herein,
we disclose that pyridine-based catalysts not only switch the
chemoselectivity from cyclotrimerization to the regioselec-
tive formation of 1,3-substituted enynes, but they also en-
hance the catalytic activity. The formation of head-to-tail
products regardless of the electronic nature of the alkyne
substituent has no precedent for rhodium catalysts. We have
studied the catalytic process by means of low-temperature
NMR spectroscopy experiments and DFT theoretical calcu-
lations.
À
À
on the bond that the alkyne is inserted into (M C or M H)
and the orientation of insertion (1,2 or 2,1; Scheme 2).
Very commonly, pathway a is simplified if only an initiator
ligand is present. It is encountered if the putative metal–hy-
dride moiety from the alkyne oxidative addition has been
removed, or even never formed, by the action of an exter-
nal[13e,16c] or internal[10b,12b,25] base, which results in an M-al-
kynyl as the sole bond the alkyne can insert into. In contrast
to metal–alkynyl species, metal–hydrides are poor initiators
for the dimerization process. Therefore, the metal–hydride
can be converted into a metal–alkynyl species by protona-
À
tion by the acidic C H proton of the alkyne with concomi-
tant release of molecular hydrogen, or, alternatively, metal–
vinyl intermediates can be formed by insertion of the triple
À
bond into the M H ligand. Furthermore, metal–vinyl spe-
Results and Discussion
cies have been proposed as propagating species for alkyne
polymerization.[11a,b] In spite of such potential deactivation
processes, catalytic pathways mediated by metal–hydride–al-
kynyl species have been revealed to be operative.[12e,14i,15a,16e]
Particularly, the formation of gem-enynes may arise from
carbometalation through the 1,2-insertion pathway[12b,18,21a,30]
or hydrometalation through the 2,1-insertion route.[7d, 13h,14f]
Although the regioselectivity depends intrinsically on the in-
sertion stage, the reductive elimination can be decisive if it
is the rate-determining step and the migratory insertion is
reversible.[16d,31]
Catalytic alkyne dimerizations: The catalysts used in this
study are the dinuclear monolefin complex [{Rh
AHCTUNGTRENNUNG
ACHUTGTNREN(NUG m-Cl)ACHTUNGTRENNUNG(IPr)-
(h2-coe)}2] (1; coe: cyclooctene; IPr: 1,3-bis-(2,6-diisopropyl-
phenyl)imidazol-2-carbene), previously prepared by James
and co-workers,[35] and the mononuclear derivative [RhCl-
(h2-coe)(py)] (2; py: pyridine), recently synthesized in
ACHUTNGREN(NUG IPr)ACHTUNGTRENNUGN
our laboratories by simple chlorido-bridge cleavage of 1 by
pyridine.[34e] Scheme 3 displays the structure of both cata-
lysts and the general structures of the products that result
from the catalytic transformations.
The advent of N-heterocyclic carbenes (NHCs) as ligands
for transition-metal complexes has had a big impact on ho-
mogeneous catalysis.[32] Substitution of the ubiquitous phos-
A preliminary catalytic test with phenylacetylene by using
a 5 mol% loading of 1 showed that, in contrast to the ex-
pected enynes, aromatic products resulting from cyclotrime-
Chem. Eur. J. 2013, 19, 15304 – 15314
ꢄ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
15305

R. Castarlenas, L. A. Oro et al.
1,2,4-triphenylbenzene (products a and b, respectively) were
formed in an almost quantitative total yield in a 13:87 ratio.
Interestingly, the chemoselectivity was reversed dramatically
when catalyst 2 was used. Besides cyclotrimers a and b, the
conjugated 1,3- or 1,4-disubstituted enynes (products c and
d, respectively) and a gem-dienyne (e), resulting from trime-
rization, were the significant products detected, with the
head-to-tail dimer as the major isomer. Moreover, addition
of ten equivalents of pyridine to 2 increases both the catalyt-
ic activity and the selectivity towards the gem-dimer up to
92% and suppresses the formation of cyclotrimers
(Scheme 4; Table 1, entry 4). Treatment of catalytic samples
of 1 with ten equivalents of NEt3 did not cause a similar
effect (Table 1, entry 5), which indicates that the role of pyr-
idine is different from simply acting as a base for the depro-
tonation of the terminal alkyne but rather it is engaged in
the coordination to the metallic center.[34e,36] The introduc-
tion of an electron-donating group on the nitrogenated aro-
matic ring does not significantly modify the “pyridine
effect” (Table 1, entry 6), whereas the presence of 2-ethyl-
pyridine reduced the catalytic activity, probably because of
hindered coordination of the N-donor ligand (Table 1,
entry 7).[34e] Addition of ten equivalents of acetonitrile does
not favor the formation of head-to-tail dimers but instead
slightly modifies the catalytic outcome obtained with dimer-
ic species 1, with formation of the cyclotrimers as the main
products.
Scheme 3. Rh–NHC catalysts and the products obtained from the catalyt-
ic reactions.
The addition of pyridine has a strong impact, not only on
selectivity but also on catalytic activity. Figure 1 shows the
monitoring of alkyne dimerization promoted by 2 and by
2+pyridine (10 equiv) for phenylacetylene, 3-phenyl-1-pro-
pyne, and 1-hexyne. In all cases, the reaction rate was in-
creased when pyridine was added, more markedly with the
aliphatic alkynes. After an overnight reaction at 408C, pyri-
dine-added catalytic systems showed more than 80% yield
of the gem-enynes, whereas only 42 and 10% yields were at-
tained with 1-hexyne and 3-phenyl-1-propyne, respectively,
when the reaction was catalyzed by 2 in the absence of
added pyridine.
Scheme 4. Chemoselectivity of the Rh–IPr-mediated catalytic coupling of
phenylacetylene.
[a]
À
Table 1. Phenylacetylene coupling promoted by Rh IPr catalysts.
Entry
Catalyst
Additive[b]
T
t
Conversion
[%]
Product
The scope of the catalytic system was evaluated
[8C]
[h]
a
b
c
d
e
for various terminal alkynes under the optimized
reaction conditions of 2+pyridine (10 equiv) at
408C (Table 2). As a general trend, aromatic al-
kynes reacted faster than aliphatic ones. High se-
lectivity for gem-enynes was observed regardless of
the electronic nature of the substituent on the
alkyne, except with bulky substituents such as tert-
butyl or trimethylsilyl, for which the (E)-1,4-disub-
stituted enyne is the main product. Phenylacety-
lene was almost completely transformed after 6 h
at 408C, with a turnover frequency at 50% conver-
sion (TOF1/2) of 12.5 hÀ1 (Table 2, entry 1). The in-
troduction of an electron-withdrawing group at the
para position of the aromatic ring resulted in a rate
increase, whereas the rate was diminished when an
1
2
3
1
2
2
–
–
–
40
25
40
16
11
8
99
99
99
13
1
1
87
10
10
–
78
82
–
2
2
–
8
4
4
5
2
1
40
40
6
99
95
–
–
92
–
2
–
6
–
NEt3
16
12
88
6
1
40
9
98
–
–
94
4
2
7
8
1
1
40
40
13
8
65
95
–
–
95
9
3
–
2
–
CH3CN
12
78
[a] Reaction in C6D6 (0.5 mL) with 5 mol% of catalyst and [substrate]=0.4m.
[b] 10 mol per mol of metal.
rization of the alkyne were obtained (Scheme 4; Table 1,
entry 1). After an overnight reaction at 408C, 1,3,5- and
electron-donating group was incorporated at the para posi-
tion (Table 2, entries 2 and 3). In contrast, the meta-substi-
15306
www.chemeurj.org
ꢄ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2013, 19, 15304 – 15314

Dimerization of Terminal Alkynes by a Rhodium Catalyst
FULL PAPER
merized in 8 h with 89% selectivity for the gem-enyne
(Table 2, entry 9). However, propargyl methyl ether reacted
very slowly at 408C, but it was smoothly transformed after 2
days at 608C, albeit with significant formation of the trimer-
ic dienyne species, up to 14% (Table 2, entry 10). Highly en-
cumbered alkynes such as tert-butylacetylene and trimethyl-
silylacetylene reacted even more slowly. After an overnight
reaction at 608C, acceptable conversions were obtained but
there was a preference for the trans head-to-head enynes
(Table 2, entries 11 and 12). Finally, enynes such as 2-
methyl-1-buten-3-yne and 1-cyclohexenylacetylene were ef-
ficiently dimerized to furnish gem trienynes selectively
(Table 2, entries 13 and 14). Interestingly, the dimerization
reaction can be carried out on a preparative scale (1 mmol).
The dimerization of phenylacetylene, 1-hexyne, and cyclo-
propylacetylene was performed in toluene by using
Figure 1. Monitoring of the dimerization of alkynes at 408C in C6D6.
Table 2. Dimerization of terminal alkynes catalyzed by 2 and pyridine (10 equiv).[a]
a catalytic system comprising 5 mol% of 2 and ten
[b]
Entry
Substrate
t
Conversion
[%]
gem
E-enyne Trimer
TOF1/2
[hÀ1
equivalents of pyridine. The corresponding gem
enynes were isolated as colorless oils in 85, 73, and
61% yields, respectively, after purification by
column chromatography (silica gel, 70–230 mesh)
with n-hexane as the eluent.
[h]
G
]
1
2
3
6
4
99
92
92
98
92
2
–
2
6
2
5
12.5
14.3
5.5
13 88
The dimerization of terminal alkynes by the cat-
alytic system of 2 with pyridine (10 equiv) also
gave variable amounts of gem-dienyne trimers,
which were identified by NMR spectroscopy and
MS. These species were mainly formed at the end
of the catalytic reaction when the amount of head-
to-tail isomers surpassed 80%. Interestingly, we
have been able to characterize the gem-dienyne
derived from methyl propargyl ether as the (Z)-(2-
alkynyl)-(1,3-disubstituted)-1,3-butadiene deriva-
tive (see the Supporting Information for further
details). The structure of this isomer was unambig-
4
3
96
99
–
1
18.5
5
6
5
99
96
84
–
4
3
14.8
3.9
13 90
13 87
13 93
13
7
94
4
2
1.7
8
92[c]
89
–
11
–
–
1.9
5.0
–
9
8
90
10
11
12
48[d] 98
13[d] 89
13[d] 75
77
9
14
5
1
1
uously determined by H,13C-APT, H–13C HSQC,
42
53
–
1
HMBC, and H–1H NOE NMR experiments; thus,
14
51/19[e]
15
–
a similar structure was assumed for the other dien-
ynes.
13
14
13 98
93
91
7
9
–
–
5.4
5.0
13[d] 98
Mechanistic studies: In order to clarify the mecha-
nism operating in our catalytic system, a series of
stoichiometric low-temperature reactions of pre-
cursors 1 and 2 with alkynes have been performed
(Scheme 5). Treatment of complex 1 with benzyla-
[a] Reaction in C6D6 (0.5 mL) with 2 (0.01 mmol), pyridine (0.1 mmol), and [sub-
strate]=0.4m at 408C. [b] TOF1/2: turnover frequency at 50% conversion. [c] Plus un-
identified products. [d] Performed at 608C. [e] cis isomer.
tuted ethynylanisole displays the highest TOF1/2 value
(18.5 hÀ1) of the alkynes studied, with complete selectivity
(Table 2, entry 4). The catalytic outcome did not change sig-
nificantly for the more hindered 4-tert-butylphenylacetylene
(Table 2, entry 5). Aliphatic alkynes needed an overnight re-
action at 408C to attain good conversion and selectivity to-
wards head-to-tail dimers (Table 2, entries 6–8). No isomeri-
zation of the double bond of the enynes from the terminal
to internal position was detected in any case. Cyclopropyla-
cetylene was converted selectively into the head-to-tail
enyne for the first time that we are aware of. The presence
of a heteroatom in the alkyne substituent did not hamper
catalytic activity. Thus, 1-dimethylamine-2-propyne was di-
cetylene, tert-butylacetylene, or trimethylsilylacetylene in
the absence of pyridine gave rise to a mixture of unidenti-
fied products. In contrast, addition of benzylacetylene to
a [D8]toluene solution of 2 at À408C led immediately to the
2
ꢀ
formation of [RhCl
(IPr)(h -HC CCH2Ph)(py)] (3) through
an exchange reaction between cyclooctene and the alkyne.
1
The H NMR spectrum of 3 displayed the typical signals for
coordinated pyridine, in addition to a broad signal at d=
ꢀ
3.52 ppm ( CH) and two doublets at d=2.45 and 2.04 ppm
(CH2), which correspond to the h2-alkyne.[37] The 13C{1H}-
APT NMR experiment at low temperature corroborates the
assigned structure for 3. The carbene carbon atom signal ap-
peared as a doublet at d=182.8 ppm with a JCÀ value of
Rh
Chem. Eur. J. 2013, 19, 15304 – 15314
ꢄ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
15307

R. Castarlenas, L. A. Oro et al.
lytic dimerization of tert-butylacetylene (Table 2, entry 11).[14
h,i,38]
The 1H NMR spectrum of 4 showed a coupling of
15.9 Hz for the olefinic protons, which indicates an E config-
uration of the double bond of the 1,4-disubstituted enyne.
The h2-alkyne coordination of the enyne to rhodium is re-
flected in the 13C{1H} NMR spectrum, which showed two
À
doublets for the Rh Csp carbon atoms at d=98.6 (JCÀ
=
Rh
17.8 Hz) and 74.2 ppm (JCÀRh =16.8 Hz). The IPr ligand
signal appeared as a doublet at d=181.8 ppm with a JCÀ
Rh
value of 56.7 Hz. After extraction of the NMR data, heating
of the mixture containing 2, 4, and tert-butylacetylene at
608C showed a smooth appearance of the signals corre-
sponding to free trans head-to-head and head-to-tail coupled
dimers.
The different behavior of benzylacetylene and tert-butyla-
cetylene towards 2 is rather surprising. A complex bearing
an h2-alkyne ligand was observed for benzylacetylene,
whereas a similar species was not detected for tert-butylace-
tylene, for which coordination of the enyne resulting from
alkyne dimerization was observed. Taking into account the
presence of the highly encumbered IPr ligand, the steric
properties of the precursor and the enyne formed might
play an important role in coordination to metal center.
Thus, benzylacetylene is bound tighter than the more bulky
tert-butylacetylene, whereas the head-to-head enyne has less
steric demand than the gem-dimer arising from benzylacety-
lene dimerization. In addition, the strong coordination of
the enyne to the metal center also explains the higher tem-
perature required for the catalytic dimerization of tert-buty-
lacetylene.
Scheme 5. Detected intermediates relevant to the mechanism of alkyne
dimerization.
53.4 Hz, whereas the Csp carbon atom signals were observed
at d=90.2 and 66.4 ppm, as doublets with JCÀ values of
Rh
16.4 and 15.8 Hz, respectively.
The formation of 3 is in accordance with a plausible first
step consisting of coordination of the triple bond to the met-
The reaction of 2 with trimethylsilylacetylene followed
a different route. In accordance with the steric demand, and
similarly to tert-butylacetylene, formation of the p-alkyne
was not observed. Heating of a mixture of 2, pyridine, and
trimethylsilylacetylene (1:5:5) at 308C resulted in the slow
formation of rhodium–hydride species that reached approxi-
mately 45% yield after 2 h. Low-temperature 1H, 13C{1H},
and 15N NMR data then evidenced the formation of the hy-
À ꢀ
À
allic center (see below). The alkyne must then undergo C
H oxidative addition to generate a hydride–alkynyl species
or rearrange to form a metal–vinylidene moiety. Warming of
an NMR sample of 3 to room temperature gave rise to
a small doublet in the hydride region (d=À17.36 ppm, JHÀ
H =25.3 Hz) with concomitant rapid formation of free gem-
enyne and a mixture of unidentified metal complexes. Un-
fortunately, it was not possible to unequivocally ascribe the
hydride signal to a rhodium–hydride–alkynyl complex. Vi-
nylidene species were not detected.
dride–alkynyl complex [RhClHACHTNUTRGENN{GU C CSi(Me)3}ACHTUNGTRNEN(UGN IPr)(py)2]
[39]
À
(5), which arises from alkyne C H oxidative addition and
is stabilized by coordination of two molecules of pyridine.
The H NMR spectrum of 5 at À508C displayed a doublet
1
In view of the fact that putative hydride intermediates de-
rived from benzylacetylene react very quickly, we envisage
that a less reactive alkyne, such as tert-butylacetylene or tri-
methylsilylacetylene, could be more informative. Thus, treat-
ment of 2 with five equivalents of both tert-butylacetylene
and pyridine at À408C did not lead to the formation of the
expected p-alkyne complex, most probably because of the
bulky tert-butyl substituent, which should reduce the stabili-
ty of this species. However, heating of the sample at 308C
for 1 h gave rise to the observation of a small hydride spe-
cies (d=À16.93 ppm, JHÀH =21.5 Hz) and the formation of
a new complex in around 30% yield. Characterization by
NMR spectroscopy at low temperature revealed the new de-
ꢀ
at d=À16.47 ppm (JHÀRh =21.6 Hz), which corresponds to
the hydride ligand. The coordination of two molecules of
pyridine in 5 at low temperature is reflected in the deshield-
ed ortho-pyridine region. Two doublets appeared at d=9.35
and 9.29 ppm, each integrating 1:1 with respect to the hy-
dride signal and corresponding to the pyridine located cis to
the IPr ligand, which has hindered rotation, whereas a dou-
blet at d=8.76 ppm, which integrates by two, was ascribed
to the pyridine that is trans to the IPr ligand and that freely
rotates. At 258C, the cis-IPr pyridine decoordinates from
the complex (see the Supporting Information). Analogous
behavior has been previously described for the related Rh–
rivative to be [RhCl
(IPr)
E
G
(IPr)(py)2].[34e] The correla-
thiolate complex [RhClHACTHNGURTENNU(G SPh)CAHTUNGTRENNNUG
tion 15N–1H NMR spectrum confirmed the hindered rotation
for the cis-IPr pyridine ligand in 5; hence, the two ortho-pyr-
15308
www.chemeurj.org
ꢄ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2013, 19, 15304 – 15314

Dimerization of Terminal Alkynes by a Rhodium Catalyst
FULL PAPER
idine signals correlate with the same nitrogen atom signal at
d=278.5 ppm (Figure 2). The resonances at d=256.5 and
273.2 ppm correspond to the trans-IPr pyridine and the pyri-
dine of the starting complex 2, respectively. Free pyridine
Figure 3. DFT-computed structures and energies for possible stereoiso-
mers of 5.
Complex 5a is the catalytic intermediate resulting from
À
C H oxidative addition before insertion of the alkyne. The
fact that this species could be only detected with trimethyl-
silylacetylene should be ascribed to the slightly greater acid-
ity of the alkyne with regard to the aliphatic ones[40] and to
its high steric demand, which hinders coordination to the
metal center and subsequently the insertion step; this is in
agreement with the low reactivity of this alkyne (Table 2,
entry 12).
In view of the stoichiometric data, a plausible mechanism
for the formation of head-to-tail enyne dimerization prod-
ucts is depicted in Scheme 6. The first step consists of the
substitution of the cyclooctene ligand by the alkyne, as re-
Figure 2. 1H–15N NMR correlation spectrum in the ortho-pyridine region
for 2 and 5 at À508C.
À
was observed at 318.3 ppm (out of the figure). Finally, the
13C{1H} NMR spectrum confirmed the presence of both al-
kynyl (d, d=139.3 ppm, JCÀRh =51.3 Hz) and IPr (d, d=
174.9 ppm, JCÀRh =51.5 Hz) ligands. Treatment of 2 with tri-
methylsilylacetylene in the absence of pyridine resulted in
decomposition products. DFT calculations on the stability of
different possible stereoisomers with the hydride (5a), al-
kynyl (5b), or chloride (5c) ligands trans to the pyridine
have been performed (Figure 3). In accordance with the
greater trans influence of the hydride ligand, isomer 5a is
9.3 and 31.3 kcalmolÀ1 more stable than 5b and 5c, respec-
tively.
flected by the formation of 3. Then, C H oxidative addition
of the alkyne affords an RhIII hydride–alkynyl species simi-
lar to 5. At this point, two alternative pathways for the for-
mation of head-to-tail enynes are possible: carbometalation
through 1,2-insertion (path I)[12b,18,21a,30] or hydrometalation
through 2,1-insertion (path II).[7d,13h,14f] Finally, reductive
elimination gives rise to the enyne and regenerates the RhI–
alkyne active species. The role of the pyridine ligands may
be in the stabilization of the hydride–alkynyl intermediate
species, which thereby increases catalytic activity and, at the
same time, prevents cyclotrimerization by blocking vacant
sites.
Scheme 6. Mechanistic proposal for the formation of: a) head-to-tail enynes, and b) gem-dienynes.
Chem. Eur. J. 2013, 19, 15304 – 15314
ꢄ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
15309

R. Castarlenas, L. A. Oro et al.
A mechanistic proposal for
the formation of the minor
gem-dienyne products is also
shown in Scheme 5. The ap-
pearance of this product at the
end of the catalytic reaction
strongly suggests that the
enyne product competes with
the alkyne to enter into the
same catalytic cycle and later
evolve similarly. Thus, inser-
tion of the alkyne moiety of
the gem-dienyne into the al-
kynyl or hydride ligands (Path-
s III and IV, respectively) fol-
lowed by reductive elimination
fully accounts for the forma-
tion of the observed (Z)-(2-al-
kynyl)-(1,3-disubstituted)-1,3-
butadiene compound. On the
other hand, alkyne insertion
into the vinyl moiety of the
Rh–alkynyl–vinyl intermediate
is less likely because it has
been proposed as the propa-
gating step in alkyne polymeri-
zation, which was not observed
in our case.[11]
Figure 4. DFT-calculated DE values [kcalmolÀ1] along the energy surface of formation of enynes through
path I. Structures I-D to I-H correspond to head-to-tail enynes (R1: H; R2: Ph), and structures I-D’ to I-H’
lead to head-to-head enynes (R1: Ph; R2: H).
DFT calculations on the
alkyne dimerization mecha-
nism: In order to support the
mechanistic picture proposed
in Scheme 6, a detailed compu-
tational study with DFT calcu-
lations (B3LYP approach) has
been carried out. Due to the
role of steric hindrance on the
reaction regioselectivity, full
IPr and pyridine ligands have
been considered explicitly in
the calculations. This mecha-
nistic study starts at complex 3
(structure A) and includes
both paths I and II, which cor-
respond to carbometalation
and hydrometalation, respec-
Figure 5. DFT-calculated DE values [kcalmolÀ1] along the energy surface of formation of enynes through
path II. Structures II-D to II-H correspond to head-to-tail enynes (R1: H; R2: Ph), and structures II-D’ to II-H’
lead to head-to-head enynes (R1: Ph; R2: H).
tively, and the formation of head-to-head and head-to-tail
enynes for each path. The energetic profiles for these four
reactions are presented in Figure 4 (path I) and Figure 5
stabilized, although substitution of pyridine by an alkyne is
energetically allowed and the catalytic cycle may be contin-
ued.
À
(path II). In both cases, the initial step is the C H bond oxi-
Following path I, a second alkyne can be coordinated into
the vacant position cis to the alkyne ligand with two possible
orientations. These two complexes, I-D and I-D’, will lead to
the final head-to-tail and head-to-head enynes, respectively.
Due to the high trans influence of the hydride ligand, the
alkyne coordination is very weak. Insertion of the alkyne
dative addition of the p-coordinated alkyne, with an ener-
getic barrier (B-TS) of 14.5 kcalmolÀ1 leading to hydride-al-
kynyl intermediate C, which is only 0.7 kcalmolÀ1 less stable
than A. Coordination of a second molecule of pyridine in
the vacant site of C, resembling 5, stabilizes the complex by
8.1 kcalmolÀ1. This value indicates that the resting state is
À
into the Rh C bond is a very exothermic step, and it takes
15310
www.chemeurj.org
ꢄ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2013, 19, 15304 – 15314

Dimerization of Terminal Alkynes by a Rhodium Catalyst
FULL PAPER
place through the transition states I-E-TS and I-E’-TS with
activation barriers of 42.2 and 37.3 kcalmolÀ1, respectively,
with respect to the resting state. Thus, the formation of the
head-to-tail coupling product is more favorable due to the
lower energy barrier. The formation of the enynes is com-
TS, the phenyl group of the coordinated alkyne is parallel to
the bulky isopropyl substituents of the NHC. In fact, steric
hindrance affects II-E’-TS more markedly than II-E-TS.
Indeed, in structure II-F, there are steric interactions be-
tween the phenyl group of the vinyl and isopropyl groups,
which become more important for II-G-TS due to the geo-
À
pleted by C H reductive elimination through the Y-shape
À
transition states I-G-TS and I-G’-TS. The energetic barriers
are considerably smaller than in the previous step, and the
overall reaction is highly exothermic.
metric requirements for the formation of the new C C
bond. In this case, the vinyl ligand must place the phenyl
group just in between both isopropyl substituents. Therefore,
À
In the hydrometalation path, the hydride ligand must be
located cis to the vacant site, which exerts a considerable
trans influence to the chlorido ligand. This situation is ener-
getically more unfavorable than that in path I, and there-
fore, the relative energies of II-D and II-Dꢃ are higher: 14.5
and 11.4 kcalmolÀ1, respectively. The alkyne insertion into
for alkynes with bulky substituents, the C C reductive elimi-
nation step should be very unfavorable. This finding is con-
sistent with the lack of selectivity found for bulky alkynes,
as shown in entries 11 and 12 of Table 2.
À
the Rh H bond occurs though the II-E-TS and II-E’-TS
Conclusion
transition states, with calculated activation barriers of 29.6
and 31.7 kcalmolÀ1, respectively, with respect to the resting
state. These energies are much more affordable than the
(h2-coe)(py)] (2) with pyri-
The catalytic system [RhClACTHNUGRTENNUG(IPr)HCATUNGTRENNGUN
dine efficiently catalyzed the head-to-tail dimerization of
terminal alkynes. For the first time for a rhodium catalyst,
a varied range of terminal alkynes with substituents of dif-
ferent electronic character has been converted into the cor-
responding 1,3-disubstituted enynes under mild conditions
with selectivities higher than 90% in most cases. Dimeriza-
tion of alkynes with bulky substituents was unselective and
gave the E-1,4-disubstituted enynes as the major products.
The presence of pyridine in the catalytic system not only
switches the chemoselectivity from alkyne cyclotrimerization
to dimerization but also enhances the catalytic activity. Pyri-
dine coordination to the metallic center prevents cyclotrime-
rization and stabilizes the intermediate species of the cata-
lytic cycle.
À
alkyne insertion into the Rh C bond, which leads to inter-
mediates I-F and I-F’. The transition-state structures for the
À
C C coupling also present a Y-shape, which is typical for
enyne reductive elimination,[41] with activation energies of
25.0 and 17.6 kcalmolÀ1 for II-G-TS and II-G’-TS, respec-
tively.
In all cases, the migratory insertion step is higher in
energy than the corresponding reductive elimination step.
Therefore, the overall picture shows that the regioselectivity
and reaction rate are controlled by the alkyne insertion into
À
the Rh H bond, with the barrier for the head-to-tail ar-
rangement being 2.1 kcalmolÀ1 smaller than that for the
head-to-head. The molecular geometry of the intermediates
and transition-state structures along this path are shown in
Figure 6. It can be observed that in structures II-D and II-E-
À
The proposed dimerization mechanism involves C H
alkyne oxidative addition, alkyne insertion, and reductive
elimination steps. Several intermediates participating in the
catalytic cycle have been detected, which have included
RhI–p-alkyne and p-enyne derivatives and the RhIII–hy-
dride–alkynyl species. Coordination of two molecules of pyr-
idine aids stabilization of the hydride–alkynyl intermediates
and causes an interesting fluxional behavior involving the
dissociation of a pyridine ligand, which has been studied by
low-temperature H, 13C{1H}, and 15N NMR spectra.
1
Computational DFT studies support the proposed mecha-
nism with 2,1-hydrometalation of the alkyne as the determi-
nant for the selectivity and the rate-determining step. The
higher trans influence of the hydride related to the alkynyl
ligand suggests that carbometalation could be kinetically fa-
vored because the alkyne coordinates preferentially cis to
the alkynyl moiety;[34e] however, the C C bond formation
À
step following this pathway represents a higher hurdle than
À
the formation of a C H bond in the hydrometalation route
through a less stable mutually cis hydride–p-alkyne inter-
mediate.
Further work on the application of this catalytic system to
cross-dimerization reactions and the study of the “pyridine
effect” in related Rh–NHC-promoted addition reactions is
currently being undertaken in our laboratories.
Figure 6. DFT-optimized structures and distances [ꢅ] for intermediates
and transition states for alkyne dimerization by following path II.
Chem. Eur. J. 2013, 19, 15304 – 15314
ꢄ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
15311

R. Castarlenas, L. A. Oro et al.
Ph), 7.20 (d, JHÀH =7.2 Hz, 4H, Hm-Ph), 6.90, 6.67 (both d, JHÀH =1.7 Hz,
2H, =CHN), 6.72 (overlapped, Hp-py(b)), 6.43 (t, JHÀH =6.5 Hz, 2H, Hp-
Experimental Section
py(a)), 6.41, 6.27 (both dd, JHÀH =6.3, 5.2 Hz, 2H, Hm-py(b)), 6.08 (dd, JHÀ
6.5, 5.4 Hz, 2H, Hm-py(a)), 3.93, 3.89, 3.84, 3.65 (all sept, JHÀH =6.5 Hz, 4H,
CHMeIPr), 1.78, 1.76, 1.56, 1.53, 1.23, 1.19, 1.10, 1.08 (all d, JHÀH =6.5 Hz,
=
H
General: All reactions were carried out with rigorous exclusion of air by
using Schlenk tube techniques. Alkynes were purchased from commercial
sources and were used as received, except for phenylacetylene, which
was distilled under argon and stored over molecular sieves. Organic sol-
vents were dried by standard procedures and distilled under argon prior
to use or obtained oxygen- and water-free from a Solvent Purification
24H, CHMeIPr), 0.43 (s, 9H, HSiÀMe), À16.47 ppm (d, JRh H =21.6 Hz, 1H,
À
Rh H); 13C{1H}-APT NMR (100.5 MHz, [D8]toluene, 223 K): d=174.9
À
À
(d, JCÀRh =51.5 Hz, Rh CIPr), 153.1 (s, Co-py(a)), 151.3, 151.0 (both s, Co-
À
py(b)), 147.5, 146.6, 146.2, 145.8 (all s, Cq-IPr), 139.3 (d, JRh C =51.3 Hz, Rh
À
System (Innovative Technologies). The organometallic catalysts [{Rh
ACHTUNGERTN(NUNG m-
ꢀ
C C), 139.6, 137.5 (both s, CqN), 135.2 (s, Cp-py(a)), 134.7 (s, Cp-py(b)), 129.5,
Cl)(IPr) (IPr)
G
E
N
ACHTUNGTRENNUNG
127.9, 124.8, 122.3 (all s, CPh), 124.4, 123.5 (both s, =CHN), 122.6, 122.5,
pared as previously described in the literature. 1H, 13C{1H}, and HSQC
1H–15N spectra were recorded either on Varian Gemini 2000 300 MHz,
Bruker ARX 300 MHz, or Bruker Avance 400 MHz instruments. Chemi-
cal shifts (expressed in parts per million) are referenced to residual sol-
vent peaks (1H, 13C{1H}), or external liquid NH3 (15N). Coupling con-
stants, J, are given in Hz. Spectral assignments were achieved by a combi-
nation of 1H–1H COSY, 13C{1H}-APT, and 1H–13C HSQC/HMBC experi-
ments. GC-MS analyses were recorder on an Agilent 5973 mass selective
detector interfaced to an Agilent 6890 series gas chromatograph system,
by using an HP-5MS 5% phenyl methyl siloxane column (30 mꢆ250 mm
with a 0.25 mm film thickness).
À ꢀ
122.4 (all s, Cm-py), 104.0 (d, JRh C =9.7 Hz, Rh C C), 28.8, 28.6, 28.4,
À
28.1 (all s, CHMeIPr), 26.7, 26.2, 26.1, 25.7, 24.2, 23.4, 23.1, 22.9 (all s,
CHMeIPr), 2.21 ppm (s, CSiÀMe); 15N–1H HMQC (53 MHz, [D8]toluene,
223 K): d=318.3 (free py), 278.5 (Npy(b)), 273.2 (py, 2), 256.5 (Npy(a)),
196.1, 191.2 (IPr), 192.5 ppm (IPr, 2).
Catalytic alkyne dimerization reactions: An NMR tube containing a solu-
tion of catalyst (0.01 mmol) in C6D6 (0.5 mL) was treated with alkyne
(0.20 mmol) and pyridine (0.1 mmol) and heated at 408C. The reaction
course was monitored by 1H NMR spectroscopy, and the conversion was
determined by integration of the corresponding resonances of the alkyne
and the products.
Computational details: The geometry of all structures was optimized
with the G09 program package[42] at the DFT level by using the B3LYP
approximation[43] combined with the 6-31G
ACTHNUTRGNE(NUG d,p) basis set for H, C, N, Cl,
2
ꢀ
In situ preparation of [RhCl
(IPr)(h -HC CCH2Ph)(py)] (3): A solution
and Si atoms[44] and the SDD pseudo-potential[45] for the Rh atoms. The
nature of the stationary points was confirmed by frequency analysis, and
the intrinsic reaction paths were traced by connecting the transition
structures with the respective minima.
of 2 (40 mg, 0.064 mmol) in [D8]toluene (0.5 mL, NMR tube) was treated
with pyridine (15 mL, 0.188 mmol) and 3-phenyl-1-propyne (23 mL,
0.188 mmol) at 233 K. The 1H NMR spectrum was immediately recorded
1
at low temperature: H NMR (400 MHz, [D8]toluene, 233 K): d=8.40 (d,
JHÀH =4.6 Hz, 2H, Ho-py), 7.2–6.9 (11H, HPh), 6.64, 6.48 (both d, JHÀ
=
H
1.6 Hz, 2H, =CHN), 6.56 (t, JHÀH =6.5 Hz, 1H, Hp-py), 6.14 (dd, JHÀH =6.5,
4.6 Hz, 2H, Hm-py), 4.60, 3.59, 3.58, 2.17 (all sept, JHÀH =6.7 Hz, CHMeIPr),
ꢀ
ꢀ
3.52 (br, 1H, HC C), 2.45, 2.04 (both d, JHÀH =18.2 Hz, 2H, CCH2),
2.12, 1.71, 1.28, 1.20, 1.17, 1.16, 1.06, 1.03 ppm (all d, JHÀH =6.7 Hz,
CHMeIPr); 13C{1H}-APT NMR (100.5 MHz, [D8]toluene, 233 K): d=182.8
Acknowledgements
À
(d, JCÀRh =53.4 Hz, Rh CIPr), 151.5 (s, Co-py), 148.0, 147.7, 145.8, 145.5 (all
s, Cq-IPr), 138.7 (s, Cq-Ph), 137.6, 136.0 (both s, CqN), 135.2 (s, Cp-py), 130–
Financial support from the Spanish Ministerio de Economꢁa y Competiti-
vidad (MEC/FEDER) Projects (CTQ2010-15221, CTQ2012-35665), the
Diputaciꢃn General de Aragꢃn (E07), the ARAID Foundation under
the program “Jꢃvenes Investigadores”, and CONSOLIDER INGENIO-
2010 under the Project MULTICAT (CSD2009-00050) is gratefully ac-
knowledged. L.R.-P. thanks CONACyT (Mexico, 186898) for a postdoc-
toral fellowship.
122 (CHPh), 123.3, 122.7 (both s, =CHN), 122.4 (s, Cm-py), 90.2 (d, JCÀ
=
Rh
ꢀ
ꢀ
ꢀ
16.4 Hz, HC C), 66.4 (d, JCÀRh =15.8 Hz, HC C), 31.2 (s, CCH2), 29.2,
28.7, 28.6, 28.5 (all s, CHMeIPr), 27.0, 25.6, 25.5, 24.5, 24.3, 23.7, 22.9,
22.7 ppm (all s, CHMeIPr).
{h2-C
N
ꢀ
In situ preparation of [RhCl
(IPr)
A solution of 2 (30 mg, 0.048 mmol) in [D8]toluene (0.5 mL, NMR tube)
was treated with pyridine (19 mL, 0.235 mmol) and tert-butylacetylene
(29 mL, 0.235 mmol) and heated at 303 K for 2 h (approximately 30%
[1] a) K. C. Nicolaou, W.-M. Dai, S.-C. Tsay, V. A. Estevez, W. Wrasi-
dlo, Science 1992, 256, 1172; b) D. J. Faulkner, Nat. Prod. Rep. 2001,
18, 1; c) M. Dçrfler, N. Tschammer, K. Hamperl, H. Hꢇbner, P.
Gmeiner, J. Med. Chem. 2008, 51, 6829; d) T. H. Jones, R. M. M.
Adams, T. F. Spande, H. M. Garrafo, T. Kaneko, T. R. Schultz, J.
Nat. Prod. 2012, 75, 1930.
[2] a) Y. Takayama, C. Delas, K. Muraoka, M. Uemura, F. Sato, J. Am.
Chem. Soc. 2003, 125, 14163; b) H. Katayama, M. Nakayama, T.
Nakano, C. Wada, K. Akamatsu, F. Ozawa, Macromolecules 2004,
37, 13; c) Z. Liu, I. Schmidt, P. Thamyongkit, R. S. Loewe, D.
Syomin, J. R. Diers, Q. Zhao, V. Misra, J. S. Lindsey, D. F. Bocian,
Chem. Mater. 2005, 17, 3728; d) M. Gholami, R. R. Tykwinski,
Chem. Rev. 2006, 106, 4997.
[3] a) N. N. P. Moonen, C. Boudon, J.-P. Gisselbrecht, P. Seiler, M.
Gross, F. Diederich, Angew. Chem. 2002, 114, 3170; Angew. Chem.
Int. Ed. 2002, 41, 3044; b) Y. Liu, M. Nishiura, Y. Wang, Z. Hou, J.
Am. Chem. Soc. 2006, 128, 5592; c) Y. Zhao, N. Zhou, A. D. Slep-
kov, S. C. Ciulei, R. McDonald, F. A. Hegmann, R. R. Tykwinski,
Helv. Chim. Acta 2007, 90, 909; d) W. P. Forrest, Z. Cao, H. R. Ham-
brick, B. M. Prentice, P. E. Fanwick, P. S. Wagenknecht, T. Ren, Eur.
J. Inorg. Chem. 2012, 5616.
yield): 1H NMR (300 MHz, [D8]toluene, 273 K): d=8.50 (d, JHÀ
=
H
5.2 Hz, 2H, Ho-py), 7.4–6.3 (Ph, py), 6.53, 6.49 (both d, JHÀH =2.0 Hz, 2H,
=CHN), 5.91 (d, JHÀH =15.9 Hz, 1H, =CHtBu), 5.24 (dd, JHÀH =15.9 Hz,
JHÀRh =1.2 Hz, 1H, CCH=), 4.08, 3.81, 3.49, 3.37 (all sept, JHÀH =6.5 Hz,
ꢀ
4H, CHMeIPr), 1.82, 1.81, 1.56, 1.54, 1.19, 1.16, 1.12, 1.05 (all d, JHÀ
=
H ꢀ
6.5 Hz, 24H, CHMeIPr), 1.09 (s, 9H, =CHCMe3,), 0.92 ppm (s, 9H,
CCMe3); 13C{1H}-APT NMR (75.0 MHz, [D8]toluene, 273 K): d=181.8
À
(d, JCÀRh =56.7 Hz, Rh CIPr), 152.4 (s, Co-py), 148.0, 146.4, 144.3, 143.9 (all
s, Cq-IPr), 143.3 (d, JCÀRh =1.5 Hz, =CHtBu), 138.8, 138.4 (both s, CqN),
135–122 (Ph, py), 124.5, 124. 3 (both s, =CHN), 113.8 (d, JCÀRh =1.5 Hz, =
CHCMe3), 98.6 (d, JCÀRh =17.8 Hz, Rh h -C
Rh =16.8 Hz, Rh h -CACHTUNGTRENNUNG(tBu) CCH=), 33.5 (s, =CHCMe3), 30.3 (s,
CCMe3), 29.9 (d, JCÀRh =1.8 Hz, CCMe3), 28.7 (s, =CHCMe3), 28.7, 28.6,
28.4, 28.3 (all s, CHMeIPr), 26–22 ppm (CHMeIPr).
2
À
ꢀ
U
2
À
ꢀ
ꢀ
ꢀ
À ꢀ
In situ preparation of [RhClHACHTUNGRNEUNG{ C CSi(Me)3}ACHTUGNRETN(NUGN IPr)(py)2] (5): A solution
of 2 (30 mg, 0.048 mmol) in [D8]toluene (0.5 mL, NMR tube) was treated
with pyridine (19 mL, 0.235 mmol) and trimethylsilylacetylene (33 mL,
0.235 mmol) and heated at 303 K for 2 h (approximately 45% yield):
1H NMR (400 MHz, [D8]toluene, 223 K): d=9.35, 9.29 (both d, JHÀ
=
H
5.2 Hz, 2H, Ho-py(b)), 8.76 (d, JHÀH =5.4 Hz, 2H, Ho-py(a)), 7.41 (m, 2H, Hp-
15312
www.chemeurj.org
ꢄ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2013, 19, 15304 – 15314

Dimerization of Terminal Alkynes by a Rhodium Catalyst
FULL PAPER
[4] a) B. Ochiai, Y. Tomita, T. Endo, Macromolecules 2001, 34, 1634;
b) B. Kang, D.-H. Kim, Y. Do, S. Chang, Org. Lett. 2003, 5, 3041;
c) P. Wessig, G. Mꢇller, Chem. Rev. 2008, 108, 2051; d) N. Li, Z. Shi,
Y. Tang, J. Chen, X. Li, Beilstein J. Org. Chem. 2008, 4, 48; e) J. B.
Werness, W. Tang, Org. Lett. 2011, 13, 3664.
[5] a) C. S. Yi, Synlett 1999, 281; b) B. M. Trost, F. D. Toste, A. B. Pin-
kerton, Chem. Rev. 2001, 101, 2067; c) V. Ritleng, C. Sirlin, M. Pfeff-
er, Chem. Rev. 2002, 102, 1731; d) H. Katayama, F. Ozawa, Coord.
Chem. Rev. 2004, 248, 1703; e) M. Nishiura, Z. Hou, J. Mol. Catal. A
Herrmann, P. Rigo, J. Schwarz, J. Organomet. Chem. 2000, 593, 489;
h) K. Melis, D. De Vos, P. Jacobs, F. Verpoort, J. Organomet. Chem.
2002, 659, 159; i) Y. Gao, R. J. Puddephatt, Inorg. Chim. Acta 2003,
350, 101; j) H. Katayama, H. Yari, M. Tanaka, F. Ozawa, Chem.
Commun. 2005, 4336; k) X. Chen, P. Xue, H. H. Y. Sung, I. D. Wil-
liams, M. Peruzzinni, C. Bianchini, G. Jia, Organometallics 2005, 24,
4330; l) A. Prades, M. Viciano, M. Sanaffl, E. Peris, Organometallics
2008, 27, 4254; m) M. Jimꢀnez-Tenorio, M. C. Puerta, P. Valerga, Or-
ganometallics 2009, 28, 2787; n) C.-K. Chen, H.-C. Tong, C.-Y. C.
Hsu, C.-Y. Lee, Y. H. Fong, Y.-S. Chaung, Y.-H. Lo, Y.-C. Lin, Y.
Wang, Organometallics 2009, 28, 3358; o) A. Coniglio, M. Basseti,
S. E. Garcꢁa-Garrido, J. Gimeno, Adv. Synth. Catal. 2012, 354, 148.
[14] a) H. Singer, G. Wilkinson, J. Chem. Soc. A 1968, 849; b) L. Carlton,
G. Read, J. Chem. Soc. Perkin Trans. 1 1978, 1631; c) S. Yoshikawa,
J. Kiji, J. Furukawa, Makromol. Chem. 1977, 178, 1077; d) I. P. Kova-
lev, K. V. Yevdakov, Y. A. Strelenko, M. G. Vinogradov, G. I. Nikish-
in, J. Organomet. Chem. 1990, 386, 139; e) W. T. Boese, A. S. Gold-
man, Organometallics 1991, 10, 782; f) M. Schꢈfer, J. Wolf, H.
Werner, Organometallics 2004, 23, 5713; g) C.-C. Li, Y.-C. Lin, Y.-H.
Liu, Y. Wang, Organometallics 2005, 24, 136; h) M. Schꢈfer, J. Wolf,
H. Werner, Dalton Trans. 2005, 1468; i) W. Weng, C. Guo, R. Cele-
nligil-Cetin, B. Foxman, O. Ozerov, Chem. Commun. 2006, 197; j) T.
Katagiri, H. Tsurugi, T. Satoh, M. Miura, Chem. Commun. 2008,
3405; k) H. M. Peng, J. Zhao, X. Li, Adv. Synth. Catal. 2009, 351,
1371; l) H.-D. Xu, R.-W. Zhang, X. Li, S. Huang, W. Tang, W.-H.
Hu, Org. Lett. 2013, 15, 840.
À
2004, 213, 101; f) E. Bustelo, P. H. Dixneuf in Handbook of C H
Transformations, Vol. 1 (Ed.: G. Dyker), Wiley-VCH, Weinheim,
2005, Chapter II.
[6] a) Y. Wakatsuki, H. Yamazaki, N. Kumegawa, T. Satoh, J. Y. Satoh,
J. Am. Chem. Soc. 1991, 113, 9604; b) M. A. Esteruelas, J. Herrero,
A. M. Lꢃpez, M. Olivꢂn, Organometallics 2001, 20, 3202; c) M.
Schꢈfer, J. Wolf, H. Werner, Dalton Trans. 2005, 1468; d) L. Leroyer,
V. Maraval, R. Chauvin, Chem. Rev. 2012, 112, 1310.
[7] a) P. Siemsen, R. C. Livigston, F. Diederich, Angew. Chem. 2000,
112, 2740; Angew. Chem. Int. Ed. 2000, 39, 2632; b) L. Fomina, B.
Vazquez, E. Tkatchouk, S. Fomine, Tetrahedron 2002, 58, 6741; c) K.
Kamata, S. Yamaguchi, M. Kotani, K. Yamaguchi, N. Mizuno,
Angew. Chem. 2008, 120, 2441; Angew. Chem. Int. Ed. 2008, 47,
2407; d) T.-H. Hsiao, T.-L. Wu, S. Chatterjee, C.-Y. Chiu, H. M. Lee,
L. Bettucci, C. Bianchini, W. Oberhauser, J. Organomet. Chem.
2009, 694, 4014; e) S. Tokuji, H. Yorimitsu, A. Osuka, Angew. Chem.
2012, 124, 12523; Angew. Chem. Int. Ed. 2012, 51, 12357; f) H.-Y.
Gao, H. Wagner, D. Zhong, J.-H. Franke, A. Studer, H. Fuchs,
Angew. Chem. 2013, 125, 4116; Angew. Chem. Int. Ed. 2013, 52,
4024.
[8] a) A. Haskel, J. Q. Wang, T. Straub, T. G. Neyroud, M. S. Eisen, J.
Am. Chem. Soc. 1999, 121, 3025; b) M. V. Jimꢀnez, E. Sola, F. J.
Lahoz, L. A. Oro, Organometallics 2005, 24, 2722; c) Y.-T. Wu, W.-
C. Lin, C.-J. Liu, C.-Y. Wu, Adv. Synth. Catal. 2008, 350, 1841; d) K.
Ogata, Y. Atsuumi, S.-I. Fukuzawa, Org. Lett. 2011, 13, 122.
[9] a) W. Reppe, W. J. Sweckendiek, Justus Liebigs Ann. Chem. 1948,
560, 104; b) S. Saito, Y. Yamamoto, Chem. Rev. 2000, 100, 2901;
c) J. A. Varela, C. Saa, Chem. Rev. 2003, 103, 3787; d) J. C. Santos,
V. Polo, J. Andres, Chem. Phys. Lett. 2005, 406, 393; e) V. Cadierno,
S. E. Garcꢁa-Garrido, J. Gimeno, J. Am. Chem. Soc. 2006, 128,
15094; f) B. R. Galan, T. Rovis, Angew. Chem. 2009, 121, 2870;
Angew. Chem. Int. Ed. 2009, 48, 2830; g) A. Dachs, S. Osuna, A. Ro-
glans, M. Solꢉ, Organometallics 2010, 29, 562; h) G. Domꢁnguez, J.
Pꢀrez-Castells, Chem. Soc. Rev. 2011, 40, 3430; i) B. ꢊ. ꢊztꢇrk, S.
Karabulut, Y. Imamoglu, Appl. Catal. A 2012, 433–434, 214.
[10] a) M. B. Sabarde, M. F. Farona, Polym. Bull. 1987, 18, 441; b) P. M.
Byers, I. V. Alabugin, J. Am. Chem. Soc. 2012, 134, 9609.
[15] a) S. Ogoshi, M. Ueta, M.-A. Oka, H. Kurosawa, Chem. Commun.
2004, 2732; b) N. Matsuyama, H. Tsurugi, T. Satoh, M. Miura, Adv.
Synth. Catal. 2008, 350, 2274.
[16] a) C.-H. Jun, Z. Lu, R. H. Crabtree, Tetrahedron Lett. 1992, 33,
7119; b) C. S. Chin, G. Wong, J. Song, Bull. Korean Chem. Soc. 1994,
15, 961; c) T. Ohmura, S.-I. Yorozuya, Y. Yamamoto, N. Miyaura,
Organometallics 2000, 19, 365; d) R. Ghosh, X. Zhang, P. Achord,
T. J. Emge, K. Krogh-Jespersen, A. S. Goldman, J. Am. Chem. Soc.
2007, 129, 853; e) K. Ogata, A. Toyota, J. Organomet. Chem. 2007,
692, 4139; f) M. Ciclosi, F. Estevan, P. Lahuerta, V. Pasarelli, J.
Pꢀrez-Prieto, M. Sanaffl, Adv. Synth. Catal. 2008, 350, 234; g) M. Ez-
Zoubir, F. Le Boucher dꢋHerouville, J. A. Brown, V. Ratovelomana-
na-Vidal, V. Michelet, Chem. Commun. 2010, 46, 6332; h) C. D. For-
ysth, W. J. Kerr, L. C. Paterson, Synlett 2013, 24, 587.
[17] G. C. Midya, S. Paladhi, K. Dhara, J. Dash, Chem. Commun. 2011,
47, 6698.
[18] S. Sun, J. Kroll, Y. Luo, L. Zhang, Synlett 2012, 23, 54.
[19] G. Hilt, W. Hess, T. Vogler, C. Hengst, J. Organomet. Chem. 2005,
690, 5170.
[20] a) P. Barbaro, C. Bianchini, M. Peruzzini, A. Polo, F. Zanobini,
Inorg. Chim. Acta 1994, 5, 220; b) M. A. Esteruelas, L. A. Oro, N.
Ruiz, Organometallics 1994, 13, 1507; c) J. Alꢃs, T. BolaÇo, M. A.
Esteruelas, M. Olivꢂn, E. OÇate, M. Valencia, Inorg. Chem. 2013,
52, 6199.
[11] a) Y. Kishimoto, P. Eckerle, T. Miyatake, M. Kainosho, A. Ono, T.
Ikariya, R. Noyori, J. Am. Chem. Soc. 1999, 121, 12035; b) I. Saeed,
M. Shiotsuki, T. Masuda, Macromolecules 2006, 39, 8567; c) K.
Akagi, Chem. Rev. 2009, 109, 5354; d) J. Liu, J. W. Y. Lam, B. Z.
Tang, Chem. Rev. 2009, 109, 5799; e) M. V. Jimꢀnez, J. J. Pꢀrez-Tor-
rente, M. I. Bartolomꢀ, E. Vispe, F. J. Lahoz, L. A. Oro, Macromole-
cules 2009, 42, 8146; f) Z. Ke, S. Abe, T. Ueno, K. Morokuma, J.
Am. Chem. Soc. 2011, 133, 7926; g) M. A. Casado, A. Fazal, L. A.
Oro, Arab. J. Sci. Eng. 2013, 38, 1631.
[12] a) B. M. Trost, C. Chan, G. Ruther, J. Am. Chem. Soc. 1987, 109,
3486; b) B. M. Trost, M. T. Sorum, C. Chan, A. E. Harms, E. Rꢇhter,
J. Am. Chem. Soc. 1997, 119, 698; c) M. Rubina, V. Gevorgyan, J.
Am. Chem. Soc. 2001, 123, 11107; d) C. Yang, S. P. Nolan, J. Org.
Chem. 2002, 67, 591; e) C. Jahier, O. V. Zatolochnaya, N. V. Zvya-
gintsev, V. P. Ananikov, V. Gevorgyan, Org. Lett. 2012, 14, 2846.
[13] a) C. Bianchini, M. Peruzzini, F. Zanobini, P. Frediani, A. Albinati,
J. Am. Chem. Soc. 1991, 113, 5453; b) A. M. Echavarren, J. Lꢃpez,
A. Santos, J. Montoya, J. Organomet. Chem. 1991, 414, 393; c) C. S.
Yi, N. Liu, Organometallics 1996, 15, 3968; d) C. Slugovc, K. Mereit-
er, E. Zobetz, R. Schmid, K. Kirchner, Organometallics 1996, 15,
5275; e) C. S. Yi, N. Liu, Organometallics 1998, 17, 3158; f) M. Bas-
setti, S. Marini, F. Tortorella, V. Cadierno, J. Dꢁez, M. P. Gamasa, J.
Gimeno, J. Organomet. Chem. 2000, 593, 292; g) W. Baratta, W. A.
[21] a) M. Akita, H. Yasuda, A. Nakamura, Bull. Chem. Soc. Jpn. 1984,
ˇ
ˇ
57, 480; b) K. Mach, R. Gyepes, M. Horꢂcek, L. Petrusova, J. Kubis-
ta, Collect. Czech. Chem. Commun. 2003, 68, 1877; c) G. V. Oshov-
sky, B. Hessen, J. N. H. Reek, B. de Bruin, Organometallics 2011, 30,
6067.
[22] a) A. D. Horton, J. Chem. Soc. Chem. Commun. 1992, 185; b) R. H.
Platel, L. L. Schafer, Chem. Commun. 2012, 48, 10609.
[23] A. Kawata, Y. Kuninobu, K. Takai, Chem. Lett. 2009, 38, 836.
[24] a) K. H. den Haan, Y. Wielstra, J. H. Teuben, Organometallics 1987,
6, 2053; b) R. Duchateau, C. T. van Wee, J. H. Teuben, Organometal-
lics 1996, 15, 2291; c) K. Komeyama, T. Kawabata, K. Takehira, K.
Takaki, J. Org. Chem. 2005, 70, 7260.
[25] M. E. Thompson, S. M. Baxter, A. R. Bulls, B. J. Burger, M. C.
Nolan, B. D. Santarsiero, W. P. Schaefer, J. E. Bercaw, J. Am. Chem.
Soc. 1987, 109, 203.
[26] M. Yoshida, R. F. Jordan, Organometallics 1997, 16, 4508.
[27] N. Hagihara, M. Tamura, H. Yamazaki, M. Fujigara, Bull. Chem.
Soc. Jpn. 1961, 34, 892.
Chem. Eur. J. 2013, 19, 15304 – 15314
ꢄ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
15313

R. Castarlenas, L. A. Oro et al.
[28] a) M. Nishiura, Z. Hou, Y. Wakatsuki, T. Yamaki, T. Miyamoto, J.
Am. Chem. Soc. 2003, 125, 1184; b) S. Ge, V. F. Quiroga Norambue-
na, B. Hessen, Organometallics 2007, 26, 6508; c) R. M. Gauvin, L.
Delevoye, R. A. Hassan, J. Keldenich, A. Mortreux, Inorg. Chem.
2007, 46, 1062.
[29] a) T. Straub, A. Haskel, M. S. Eisen, J. Am. Chem. Soc. 1995, 117,
6364; b) J. Q. Wang, A. K. Dash, J. C. Berthet, M. Ephritikhine,
M. S. Eisen, Organometallics 1999, 18, 2407; c) A. K. Dash, I. Gour-
evich, J. Q. Wang, J. Wang, M. Kapon, M. S. Eisen, Organometallics
2001, 20, 5084; d) J. Q. Wang, M. S. Eisen, J. Organomet. Chem.
2003, 670, 97.
Palacios, A. Di Giuseppe, A. Opalinska, R. Castarlenas, J. J. Pꢀrez-
Torrente, F. J. Lahoz, L. A. Oro, Organometallics 2013, 32, 2768.
[35] X.-Y. Yu, B. O. Patrick, B. R. James, Organometallics 2006, 25, 4870.
[36] a) D. J. Berrisford, C. Bolm, K. B. Sharpless, Angew. Chem. 1995,
107, 1159; Angew. Chem. Int. Ed. Engl. 1995, 34, 1059; b) C. C.
Rom¼o, F. E. Kꢇhn, W. A. Herrmann, Chem. Rev. 1997, 97, 3197;
c) E. M. Vogl, H. Grçger, M. Shibasaki, Angew. Chem. 1999, 111,
1672; Angew. Chem. Int. Ed. 1999, 38, 1570; d) T. Nishimura, T.
Onoue, K. Ohe, S. Uemura, J. Org. Chem. 1999, 64, 6750; e) E. M.
Ferreira, B. M. Stoltz, J. Am. Chem. Soc. 2003, 125, 9578; f) J. Na-
sielski, N. Hadei, G. Achonduh, E. A. B. Kantchev, C. J. OꢋBrien, A.
Lough, M. G. Organ, Chem. Eur. J. 2010, 16, 10844; g) A. J. Pardey,
C. Longo, Coord. Chem. Rev. 2010, 254, 254; h) S. R. Neufeldt, M. S.
Sanford, Acc. Chem. Res. 2012, 45, 936.
[30] a) J. Barluenga, J. M. Gonzꢂlez, I. Llorente, P. J. Campos, Angew.
Chem. 1993, 105, 928; Angew. Chem. Int. Ed. Engl. 1993, 32, 893;
b) A. K. Dash, M. S. Eisen, Org. Lett. 2000, 2, 737.
[31] a) I. F. D. Hyatt, H. K. Anderson, A. T. Morehead, Jr., A. L. Sar-
gent, Organometallics 2008, 27, 135; b) C. Gonzꢂlez-Rodrꢁguez, R. J.
Pawley, A. B. Chaplin, A. L. Thompson, A. S. Weller, M. C. Willis,
Angew. Chem. 2011, 123, 5240; Angew. Chem. Int. Ed. 2011, 50,
5134.
[37] a) H. Werner, M. Schꢈfer, J. Wolf, K. Peters, H. G. von Schnering,
Angew. Chem. 1995, 107, 213; Angew. Chem. Int. Ed. Engl. 1995, 34,
191; b) A. lvarez, R. Macias, J. Bould, C. Cunchillos, F. J. Lahoz,
L. A. Oro, Chem. Eur. J. 2009, 15, 5428; c) K. Okamoto, Y. Omoto,
K. Ohe, Dalton Trans. 2012, 41, 10926.
[32] a) W. Herrmann, Angew. Chem. 2002, 114, 1342; Angew. Chem. Int.
Ed. 2002, 41, 1290; b) S. Wꢇrtz, F. Glorius, Acc. Chem. Res. 2008,
41, 1523; c) J. M. Praetorius, C. M. Crudden, Dalton Trans. 2008,
4079; d) S. Dꢁez-Gonzꢂlez, N. Marion, S. P. Nolan, Chem. Rev. 2009,
109, 3612; e) A. J. Arduengo III, L. I. Iconaru, Dalton Trans. 2009,
6903; f) J. A. Mata, M. Poyatos, Curr. Org. Chem. 2011, 15, 3309;
g) E. D. Velazquez, F. Verpoort, Chem. Soc. Rev. 2012, 41, 7032.
[33] a) M. S. Sanford, J. A. Love, R. H. Grubbs, J. Am. Chem. Soc. 2001,
123, 6543; b) S. Lee, J. F. Hartwig, J. Org. Chem. 2001, 66, 3402;
c) R. Castarlenas, M. A. Esteruelas, E. OÇate, Organometallics 2005,
24, 4343; d) W. A. Herrmann, K. ꢊfele, S. K. Schneider, E. Herdt-
weck, S. D. Hoffmann, Angew. Chem. 2006, 118, 3943; Angew.
Chem. Int. Ed. 2006, 45, 3859; e) Y. Zhu, Y. Fan, K. Burgess, J. Am.
Chem. Soc. 2010, 132, 6249; f) H. A. Malik, G. J. Surmunen, J. Mont-
gomery, J. Am. Chem. Soc. 2010, 132, 6304; g) C.-Y. Ho, L. He,
Angew. Chem. 2010, 122, 9368; Angew. Chem. Int. Ed. 2010, 49,
9182; h) K. Gao, N. Yoshikai, J. Am. Chem. Soc. 2011, 133, 400;
i) M. R. Harris, L. E. Hanna, M. A. Greene, C. E. Moore, E. R.
Jarvo, J. Am. Chem. Soc. 2013, 135, 3303.
[34] a) M. V. Jimꢀnez, J. J. Pꢀrez-Torrente, M. I. Bartolomꢀ, V. Gierz,
F. J. Lahoz, L. A. Oro, Organometallics 2008, 27, 224; b) A. Di Giu-
seppe, R. Castarlenas, J. J. Pꢀrez-Torrente, F. J. Lahoz, V. Polo, L. A.
Oro, Angew. Chem. 2011, 123, 4024; Angew. Chem. Int. Ed. 2011,
50, 3938; c) L. Palacios, X. Miao, A. Di Giuseppe, S. Pascal, C. Cun-
chillos, R. Castarlenas, J. J. Pꢀrez-Torrente, F. J. Lahoz, P. H. Dix-
neuf, L. A. Oro, Organometallics 2011, 30, 5208; d) I. Mena, M. A.
Casado, P. Garcꢁa-OrduÇa, V. Polo, F. J. Lahoz, A. Fazal, L. A. Oro,
Angew. Chem. 2011, 123, 11939; Angew. Chem. Int. Ed. 2011, 50,
11735; e) A. Di Giuseppe, R. Castarlenas, J. J. Pꢀrez-Torrente, M.
Crucianelli, V. Polo, R. Sancho, F. J. Lahoz, L. A. Oro, J. Am. Chem.
Soc. 2012, 134, 8171; f) R. Castarlenas, A. Di Giuseppe, J. J. Pꢀrez-
Torrente, L. A. Oro, Angew. Chem. 2013, 125, 223; Angew. Chem.
Int. Ed. 2013, 52, 211; g) R. Azpꢁroz, A. Di Giuseppe, R. Castarle-
nas, J. J. Pꢀrez-Torrente, L. A. Oro, Chem. Eur. J. 2013, 19, 3812;
h) G. Lꢂzaro, M. Iglesias, F. J. Fernꢂndez-Alvarez, P. J. Sanz-Miguel,
J. J. Pꢀrez-Torrente, L. A. Oro, ChemCatChem 2013, 5, 1133; i) L.
[38] a) C. S. Yi, N. Liu, Organometallics 1997, 16, 3910; b) H. Werner, F.
Kukla, P. Steinert, Eur. J. Inorg. Chem. 2002, 1377.
[39] a) Y. Wakatsuki, N. Koga, H. Werner, K. Morokuma, J. Am. Chem.
Soc. 1997, 119, 360; b) J. M. Lynam, Chem. Eur. J. 2010, 16, 8238.
[40] A. J. Kresge, P. Pruszinsky, P. J. Stang, B. L. Williamson, J. Org.
Chem. 1991, 56, 4808.
[41] V. P. Ananikov, D. G. Musaev, K. Morokuma, Organometallics 2005,
24, 715.
[42] Gaussian 09, Revision A.1, M. J. Frisch, G. W. Trucks, H. B. Schlegel,
G. E. Scuseria, M. A. Robb, J. R. Cheeseman, G. Scalmani, V.
Barone, B. Mennucci, G. A. Petersson, H. Nakatsuji, M. Caricato, X.
Li, H. P. Hratchian, A. F. Izmaylov, J. Bloino, G. Zheng, J. L. Son-
nenberg, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa,
M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, T. Vreven,
J. A. Montgomery, Jr., J. E. Peralta, F. Ogliaro, M. Bearpark, J. J.
Heyd, E. Brothers, K. N. Kudin, V. N. Staroverov, R. Kobayashi, J.
Normand, K. Raghavachari, A. Rendell, J. C. Burant, S. S. Iyengar,
J. Tomasi, M. Cossi, N. Rega, J. M. Millam, M. Klene, J. E. Knox,
J. B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R. E.
Stratmann, O. Yazyev, A. J. Austin, R. Cammi, C. Pomelli, J. W.
Ochterski, R. L. Martin, K. Morokuma, V. G. Zakrzewski, G. A.
Voth, P. Salvador, J. J.Dannenberg, S. Dapprich, A. D. Daniels, ꢊ.
Farkas, J. B. Foresman, J. V. Ortiz, J. Cioslowski, D. J. Fox, Gaussian,
Inc., Wallingford, CT, 2009.
[43] a) A. D. Becke, J. Chem. Phys. 1993, 98, 1372; b) C. Lee, W. Yang,
R. G. Parr, Phys. Rev. B 1988, 37, 785; c) A. D. Becke, J. Chem.
Phys. 1993, 98, 5648.
[44] a) W. J. Hehre, R. Ditchfield, J. A. Pople, J. Chem. Phys. 1972, 56,
2257; b) P. C. Hariharan, J. A. Pople, Theor. Chim. Acta 1973, 28,
213.
[45] a) D. Andrae, U. Haussermann, M. Dolg, H. Stoll, H. Preuss, Theor.
Chim. Acta 1990, 77, 123; b) M. Dolg, H. Stoll, H. Preuss, R. M.
Pitzer, J. Phys. Chem. 1993, 97, 5852.
Received: May 30, 2013
Published online: September 23, 2013
15314
www.chemeurj.org
ꢄ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2013, 19, 15304 – 15314