Intermolecular hydroamination versus stereoregular polymerization of phenylacetylene by rhodium catalysts based on N-O bidentate ligands

Article abstract of DOI:10.1016/j.inoche.2013.11.034

N-O bidentate ligands, such as 8-quinolinol and aminoacids, in combination with the dinuclear precursor [{Rh(μ-OMe)(COD)}₂] are versatile catalytic systems. Thus, stereoregular polymerization of phenylacetylene (PA) is observed in the presence

Full text of DOI:10.1016/j.inoche.2013.11.034

Inorganic Chemistry Communications 40 (2014) 78–81
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Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
Intermolecular hydroamination versus stereoregular polymerization of
phenylacetylene by rhodium catalysts based on N–O bidentate ligands
a
a,b,
E.A. Jaseer a, Miguel A. Casado b, , Abdulaziz A. Al-Saadi , Luis A. Oro
⁎
⁎
a
Center of Research Excellence in Petroleum Refining and Petrochemicals and Department of Chemistry, King Fahd University of Petroleum and Minerals, Dhahran 31261, Saudi Arabia
Department of Inorganic Chemistry, Instituto de Síntesis Química y Catálisis Homogénea, Universidad de Zaragoza-CSIC, 50009 Zaragoza, Spain
b
a r t i c l e i n f o
a b s t r a c t
Article history:
N–O bidentate ligands, such as 8-quinolinol and aminoacids, in combination with the dinuclear precursor
[{Rh(μ-OMe)(COD)}2] are versatile catalytic systems. Thus, stereoregular polymerization of phenylacetylene
(PA) is observed in the presence of secondary amines. Interestingly, the outcome of the catalysis changes drasti-
cally on addition of strong coordinating phosphines, giving the product of the intermolecular anti-Markovnikov
hydroamination of phenylacetylene.
Received 22 October 2013
Accepted 23 November 2013
Available online 3 December 2013
Keywords:
Rhodium
© 2013 Elsevier B.V. All rights reserved.
Hydroamination
Polymerization
Phenylacetylene
Aminoacid
8-quinolinol
The large majority of rhodium catalyzed organic reactions use a va-
riety of monodentate or bidentate phosphine ligands [1]. In contrast,
rhodium complexes with mixed anionic N–O bidentate ligands have
been scarcely used, in spite that some of them are readily available
and relatively inexpensive, such as 8-quinolinol, one of the earliest ana-
lytical reagents, and α-aminoacids. Rhodium complexes with these li-
gands have been reported many years ago [2,3] but some decades
elapsed until some applications in catalytic reactions were described.
In particular, the interest on the investigation of the catalytic properties
of rhodium aminoacidate complexes was driven by the chirality of these
complexes, both on the ligand and the metal with the aim to achieve
asymmetric catalytic transformations [4]. Concerning 8-quinolinolato
rhodium complexes, only very recently some interesting catalytic reac-
tions have been described [5,6]. In particular, Kakiuchi group has shown
the capability of rhodium (I) quinolinolato complexes to promote sever-
al catalytic coupling reactions involving terminal alkynes [6]. Very re-
cently our group has reported some phenylacetylene regioselective
catalytic reactions promoted by rhodium (I) complexes with N-
heterocyclic carbenes [7], as well as the stereoregular polymerization
of phenylacetylene by rhodium (I) complexes with hemilabile phos-
phine ligands [8]. In this communication we report on the catalytic ac-
tivity of rhodium (I) complexes with 8-quinolinolato and aminoacidate
ligands in phenylacetylene transformations.
capable of taking a proton from any anionic acid bidentate ligand
whose pKa is stronger than that of methanol [9–11]. The resulting com-
plexes can be isolated or simply used in situ. This is the case with the
bidentate ligands used in this work. Thus, we have realized that the in
situ reaction of [{Rh(μ-OMe)(COD)}2] [9] (COD = 1,5-cyclooctadiene)
with 8-quinolinol (HQ) afforded quantitatively the known complex
[Rh(Q)(COD)] [2a,d] (1) upon release of methanol (NMR evidence, see
Supporting Information). The yellow solutions obtained by following
this protocol were utilized as such in the study of both the reactivity
and catalytic performance of these species.
Interaction of 1 with phenylacetylene (PA) did not transform the al-
kyne in any observable way under regular conditions. However, when a
secondary amine (such as piperidine) was present in the medium with
catalytic amounts of solutions of 1, we observed the gradual consump-
tion of PA at room temperature; however, we did not observe the pres-
ence of any organic compound by GC measurements. Work-up of the
catalytic solutions by adding methanol afforded orange solids that
were further characterized as stereoregular polyphenylacetylene
(PPA) by using size-exclusion chromatography and NMR techniques.
Given the unexpected nature of these catalytic reactions, we studied
them in some detail. Table 1 shows the data obtained in the stereoreg-
ular polymerization of PA by using [{Rh(μ-OMe)(diolefin)}2]/HQ/
amine (diolefin = COD, NBD, TFB; NBD = 2,5-norbornadiene; TFB =
tetrafluorobenzobarrelene) catalytic mixtures in toluene.
It has been reported that the bridging methoxo group of [{Rh(μ-
OMe)(diolefin)}2] starting complexes (diolefin = COD, NBD, TFB) is
As it can be deduced from Table 1, the use of the methoxo- or the
parent amido-bridged [12] COD-containing complexes as catalytic pre-
cursors does not have any significant effect in terms of catalytic perfor-
mance (entries 1 and 3), since both afford polymers with comparable
properties. In this line, the nature of the amine does not seem to
⁎
Corresponding authors at: Department of Inorganic Chemistry, University of Zaragoza,
50009, Zaragoza, Spain.
E-mail addresses: mcasado@unizar.es (M.A. Casado), oro@unizar.es (L.A. Oro).
1387-7003/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.inoche.2013.11.034

E.A. Jaseer et al. / Inorganic Chemistry Communications 40 (2014) 78–81
79
influence on the polymer properties (entry 2, pyrrolidine used as
amine). The addition of bulky phosphines (PCy3, P(o-OMeC6H4)3) to
the in situ preformed [(Q)Rh(COD)] complex also rendered polymeric
PPA under the same conditions (entries 4 and 5, respectively), although
in less yields than those obtained with the phosphine-free ternary cata-
lytic systems. Changing of the diene in the methoxo starting dinuclear
complexes [{Rh(μ-OMe)(diolefin)}2] from COD to other known diole-
fins with higher π-acidity such as NBD or TFB did have a pronounced ef-
fect on the outcome of the catalytic reactions. As a matter of fact the
polymerization reactions proceeded much faster with the NBD and
TFB-containing complexes than those with the system containing
COD, as confirmed by direct comparison of the kinetic experiments per-
formed with these precursors by monitoring PA consumption (see
Figure S1). These observations, in line with some reports from Masuda
and coworkers on some related cationic rhodium complexes [13,14], in-
directly point out that the diene ligand should be present in the active
catalytic rhodium species, since it affects both the efficiency of the cata-
lytic process and the nature of the PPAs obtained (Mw and Mn). More
precisely, the NBD- and TFB-containing catalytic systems increase the
efficiency indexes of the reactions (Ieff; entries 6 and 7, respectively),
which in general are modest and comparable to those obtained with
other reported rhodium catalytic systems based on chelate ligands
[15]. Interestingly, the polydispersity ratios observed do not seem to de-
pend on the nature of the diene, and were found to be in general quite
good (1.3–2.0). The analysis of the orange polymers by NMR spectrosco-
py showed them to be stereoregular (see Figure S2); in all cases they
showed a significant sharp singlet at 5.84 ppm in their 1H NMR spectra
and peaks at 131.9 and 139.2 ppm in the 13C{1H} NMR spectra corre-
sponding to the “CH_C” functional groups.
hydroamination [6b], cycloaddition [6c] and dimerization [6d] that
showed an unexpected activity of these quinolato rhodium complexes.
Therefore we carried out our own study focused on assessing
the catalytic performance of the in situ ternary systems composed of
[{Rh(μ-OMe)(COD)}2]/HQ/PR3 (ratio 1:2:4) in this catalytic transforma-
tion that accounts for an 100% atom economy and affords enamines,
which are themselves a valuable source to be used for further trans-
formation to other N-containing compounds. NMR experiments
showed that replacement of the diene takes place fast and renders
the bis(phosphine) complexes of the type [Rh(Q)(PR3)2] quantitatively
(see Supporting Information). Table 2 shows the available catalytic data
obtained from the testing of HQ and 5-chloro-8-hydroxyquinoline (5-
Cl-HQ) derivatives as bidentate ligands for the rhodium-catalyzed
hydroamination of PA with piperidine.
Table 2
Catalytic data in intermolecular hydroamination of PA with piperidine.a
Entry
Catalytic system
T
(°C)
Time
(h)
Conv.
(%)
TOF (h−1
(%)b
)
1
2
3
4
5
6
7
8
9
[{Rh(μ-OMe)(COD)}2]/HQ/PR3
[{Rh(μ-NH2)(COD)}2]/HQ/PR3
[{Rh(μ-OMe)(COD)}2]/HQ/PR3
[{Rh(μ-OMe)(COD)}2]/HQ/PR3
[{Rh(μ-OMe)(COD)}2]/HQ/PR3
[{Rh(μ-OMe)(COD)}2]/HQ/PR3
[{Rh(μ-OMe)(COD)}2]/HQ/PR3
[{Rh(μ-OMe)(COD)}2]/HQ/PR3
[{Rh(μ-OMe)(COD)}2]/5-Cl-HQ/PR3
[{Rh(μ-OMe)(COD)}2]/5-Cl-HQ/PR3
RT
RT
RT
40
RT
RT
RT
RT
RT
40
24
24
24
24
24
24
48
48
24
8
100
100
92c
100
33d
63e
43f
1.3 (51%)
1.2 (47%)
1.3 (52%)
4.4 (44%)
0.1 (33%)
0.4 (46%)
0.2 (41%)
0.3 (28%)
2.3 (35%)
9 (45%)
28g
100
100
Table 1
10
Catalytic data in stereoregular in Rh-catalyzed polymerization of PA
a
Conditions: 0.05 mmol (10 mol%) [Rh], 0.1 mmol HQ (or 5-Cl-HQ), 1 mmol PA,
3 mmol piperidine, 0.2 mmol P(p-OMeC6H4)3, toluene (2 mL), mesitylene (1.8 mmol)
as internal standard.
.
b
TOF = Mol PA/mol Rh/h, calculated at the indicated PA consumption (%).
1 mmol Cs2CO3 added.
0.2 mmol PPh3.
0.2 mmol P(p-FC6H4)3.
THF used as solvent.
2% mol [Rh].
c
d
a
Entry Catalytic system
Mn
Mw
Mw/Mn Conv. (%) Ieff
e
1
2
3
4
5
6
7
[{Rh(OMe)(COD)}2]/HQ 79,600 123,000 1.55
[{Rh(OMe)(COD)}2]/HQ 96,800 170,000 1.76
81
90b
88
1.02
0.93
1.05
6.56
3.72
1.26
1.56
f
g
[{Rh(NH2)(COD)}2]/HQ
84,150 147,800 1.76
[{Rh(OMe)(COD)}2]/HQ 15,250
[{Rh(OMe)(COD)}2]/HQ 26,900
38,960 2.55
71,100 2.64
39c
86d
100e
100f
The reactions proceeded at room temperature and in all cases they
were regioselective rendering the E-enamine showed above, the prod-
uct of an anti-Markovnikov amine addition. Several trends were
established upon changing key catalytic parameters in the model reac-
tion. The need of using a loading of 10% [Rh] of catalyst became evident
from the first screening experiments in order to achieve quantitative
conversions to the enamine (compare entries 1 and 8). On the other
hand, the use of PPh3 or P(p-FC6H4)3 as external ligands in these reac-
tions gave only moderate yields of the enamine after 24 h at room tem-
perature (entries 5 and 6, respectively). The catalytic performance was
found to be substantially increased by shifting to the more basic P(p-
OMeC6H4)3, which was then chosen as the added phosphine in the ca-
talyses. Again, the use of the amido precursor [{Rh(μ-NH2)(COD)}2] or
the methoxo-bridged analog in combination with HQ/P(p-OMeC6H4)3
in these catalytic processes did not change the activity of the resulting
catalytic species in terms of yields of enamine (entries 1 and 2), which
indirectly proves that the bridging ligands (NH2 and OMe) are the key
for deprotonating the HQ derivatives and subsequently coordinating
them to rhodium. On the other hand, the use of an external weak base
(such as Cs2CO3) does not really affect the catalytic performance, al-
though slight less yields of the enamine are obtained (entry 3). In this
line the shifting to more polar THF as solvent in the catalytic reactions
was not beneficial in terms of yield, since a drop to 43% was achieved
(compare entries 1 and 7). A subtle change in the HQ skeleton does
have a noticeable positive effect in catalysis; more specifically, the
shifting from HQ to 5-Cl-HQ provides quantitative conversions to the
[{Rh(OMe)(NBD)}2]/HQ 79,600 167,000 2.10
[{Rh(OMe)(TFB)}2]/HQ 64,000 83,120 1.30
Conditions: RT, in toluene, 24 h, 10% mol [Rh], piperidine (3 mmol), except in entry 2,
where pyrrolidine was used.
a
Initiation efficiency, Ieff = Mtheor / Mn × 100; where Mtheor = [PA]0 / [Rh] ×
MWphenylacetylene × polymer yield [18].
b
Pyrrolidine.
Two molar-equiv. of PCy3 added.
Two molar-equiv. of P(o-OMeC6H4)3 added.
Reaction time 3.5 h.
c
d
e
f
Reaction time 0.5 h.
As mentioned earlier, the addition of steric-demanding phosphines
to the ternary catalytic mixture composed of [{Rh(μ-OMe)(COD)}2]/
HQ/amine did not have apparently any effect on the polymer structures.
The yields of these polymerization reactions were however relatively
low, particularly in the case of the sterically demanding P(o-
OMeC6H4)3 phosphine, in which the o-methoxo substituent precludes
an easy coordination to the metal center. The situation changes drasti-
cally when strong-coordinating phosphines such as the ubiquitous
PPh3 or P(p-OMeC6H4)3 are present in the formation of the catalytic pre-
cursors: under given conditions, piperidine adds to PA affording the cor-
responding enamine through an intermolecular hydroamination
catalytic process and no polymerization was observed. In this line, re-
cent reports by Kakiuchi and coworkers showed that the isolated
[Rh(Q)(PR3)2] mononuclear complexes were active species in catalytic
transformations of terminal alkynes such as hydroalkoxylation [6a],

80
E.A. Jaseer et al. / Inorganic Chemistry Communications 40 (2014) 78–81
E-enamine (see entries 9 and 10), as does the HQ-containing system;
however an increase in the TOF (measured at approximately 50% con-
version of PA) is observed (compare entries 1 and 9). Another parame-
ter that has an impact in the TOF values is the temperature; in this way,
when the catalytic reactions are carried out at 40 °C, the TOF increases
from 1.3 to 4.4 h−1 (HQ-based system, entries 3 and 4) and from 2.3
to 9 h−1 (5-Cl-HQ-based system, entries 9 and 10), without the loss of
the regio- and chemoselectivity.
the process). These unsaturated and electronic rich species may be in
principle able to bind PA by π-coordination, resembling one of
the proposed first steps in the polymerization. However in the
bis(phosphine) metallic fragment, a possible nucleophilic attack of the
amine to this intermediate (or rather to its vinylidene-rhodium isomer)
cannot be excluded.
In summary, we have shown in this piece of work the activity of N–O
rhodium-based systems for catalytic processes of different nature. We
were able to tailor the reaction conditions in order to favor one or the
other catalytic outcome. In this way, polymerization is operative in the
presence of an amine and the absence of phosphines at room tempera-
ture. Thus, the use of these P-donor ligands at 100 °C, on the other hand,
inhibits polymerization giving place to the hydroamination product in a
regioselective way. Further work designed to assess the scope of this
addition reactions is currently under study.
Following our interest on the catalytic activity of transition metal
complexes with aminoacidato ligands [4a,16], we screened some
natural-occurring aminoacids as N–O scaffolds for active rhodium spe-
cies under the conditions optimized as described above for intermolec-
ular hydroamination (Table 2). In all cases, the pre-formation of the
catalytic system was achieved in situ by mixing complex [{Rh(μ-
OMe)(COD)}2] and the corresponding aminoacid and the phosphine
P(p-OMeC6H4)3, which resulted to be better ligand for hydroamination
than PPh3 in terms of catalytic activity. During the course of this work,
we realized however that the influence of both the temperature and
the nature of the aminoacidate ligand were crucial for the outcome of
the catalytic reactions. Exploratory studies by using the aminoacid L-
proline as bidentate ligand in the Rh-catalyzed hydroamination of PA
with piperidine (chosen as the model catalytic reaction throughout
this report) allowed us to realize that both polymerization and intermo-
lecular hydroamination were competitive processes within a given
range of temperatures and conditions. This is true for all the α-
aminoacid tested, although this competition was observed at different
rates for each specific aminoacid. Several tuning experiments led us to
detect, however, that the increasing of the temperature inhibited poly-
mer formation favoring in turn alkyne hydroamination (see figure S3).
We performed several catalytic experiments to find out that in gen-
eral, working at 100 °C suppresses completely the polymerization
mechanism allowing the intermolecular hydroamination to be the
only operative process, maintaining under these conditions the anti-
Markovnikov regioselectivity observed at RT in the Q-based systems
(in comparison, working at less temperatures led to polymer forma-
tion). In this way, the aminoacids L-phenyl glycine and L-phenyl alanine
are the less suitable ligands for hydroamination, since under optimum
conditions (100 °C and a [PA]/[piperdine] = 1/3) polymer formation
cannot be inhibited (12% and 44% polymer obtained, respectively).
However, the choice of these conditions with L-threonine, L-alanine,
L-leucine, L-proline and L-histidine as N–O ligands gave excellent cata-
lytic performance in the addition reaction and no polymer was formed.
The reactions were fast and regioselective, affording in all cases the
corresponding anti-Markovnikov E-enamine in quantitative yields. The
calculation of the TOF values at this temperature proved to be very dif-
ficult to achieve because of the high activity and reaction rates showed
by the catalysts. We were able, however, to calculate the TOF value for
the L-histidine aminoacid, which was found to be very high (510 h−1
at 85% of PA conversion). This value is around 40 times superior to
that observed with the 5-Cl-HQ systems (TOF = 9 h−1 at 40 °C),
which is indicative of a more productive catalytic behavior.
Acknowledgments
The authors would like to acknowledge the support from the Minis-
try of Higher Education, Saudi Arabia, in establishment of the Center of
Research Excellence in Petroleum Refining & Petrochemicals at King
Fahd University of Petroleum and Minerals (KFUPM). The support of
the KFUPM under the KACST funded project (T-K-11-630) and the
KFUPM-University of Zaragoza research agreement are also highly ap-
preciated. The author (MAC) thankfully acknowledges the support
from the project CTQ2012-35665.
Appendix A. Supplementary material
Supplementary data to this article can be found online at http://dx.
doi.org/10.1016/j.inoche.2013.11.034.
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