Catalytic asymmetric hydrogenation using a [2.2]paracyclophane based chiral 1,2,3-triazol-5-ylidene-Pd complex under ambient conditions and 1 atmosphere of H2

Article abstract of DOI:10.1039/c5ra03021h

Chiral 1,2,3-triazol-5-ylidene-Pd complexes with the planar chiral [2.2]paracyclophane wing tip group have been synthesized and structurally characterized. The complex with a labile acetonitrile co-ligand is an excellent catalyst for chemoselective hydrogenation of alkynes and alkenes and enantioselective hydrogenation of prochiral alkenes at ambient conditions and 1.0 atmosphere of H₂.

Full text of DOI:10.1039/c5ra03021h

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Catalytic asymmetric hydrogenation using a
[2.2]paracyclophane based chiral 1,2,3-triazol-5-
ylidene–Pd complex under ambient conditions and
1 atmosphere of H2†
Cite this: RSC Adv., 2015, 5, 21558
Received 4th February 2015
Accepted 17th February 2015
*
Ayan Dasgupta, Venkatachalam Ramkumar and Sethuraman Sankararaman
DOI: 10.1039/c5ra03021h
www.rsc.org/advances
Chiral 1,2,3-triazol-5-ylidene–Pd complexes with the planar chiral has been investigated. Structures of complexes 1 and 2 are
[2.2]paracyclophane wing tip group have been synthesized and shown in Scheme 1 and were readily synthesized from the
structurally characterized. The complex with a labile acetonitrile co- corresponding triazolium iodide 3 in both racemic (rac) and
ligand is an excellent catalyst for chemoselective hydrogenation of enantio pure (Sp) forms. Compound 3 was synthesized in ve
alkynes and alkenes and enantioselective hydrogenation of prochiral steps from [2.2]paracyclophane (ESI†). Treatment of 3 with
alkenes at ambient conditions and 1.0 atmosphere of H2.
silver oxide43–45 gave the corresponding silver complex 4 which
was characterized by NMR and HRMS data. Reaction of 4 with
PdCl2(MeCN)2 in reuxing acetonitrile yielded 1 in 91% yield.
Further, treatment of 1 with PPh3 gave complex 2. Structures
of 1, 2 and 3 were conrmed by spectroscopic and single crystal
XRD data. In both the complexes the geometry around Pd atom
is distorted square planar (Fig. 1). However, the stereochemistry
of the two chlorine atoms around Pd in 1 is trans and that of 2 is
cis. Also the orientation of the triazolylidene–PdCl2 unit with
respect to the PC scaffold is very different in these complexes
(Fig. 1).
Catalytic enantioselective transformations (CET) mediated by
N-heterocyclic carbenes (NHCs)1–4 and their transition metal
complexes5–9 have gained importance in asymmetric organic
synthesis.10–14 Compared to the conventional phosphane
ligands, chiral NHC ligands are relatively easier to synthesize by
appending a suitable chiral auxiliary group to the N-heterocycle
scaffold.1,2,4,15–17 Mono substituted [2.2]paracyclophane (PC)
derivatives are planar chiral and they are congurationally very
robust.18–21 Several NHC–metal complexes bearing chiral PC
wing tip groups have been reported and their utility in CET has
been demonstrated.22–26 The vast majority of these belong to
imidazol-2-ylidene-metal (normal NHC–metal) complexes. PC
based chiral mesoionic carbene (MIC)–metal complexes are
unknown and hence their potential as catalyst for CET remains
unexplored. In particular the application of MIC–Pd complexes
Complexes rac-1 and rac-2 (racemic form) were screened as
catalysts for the hydrogenation of E-stilbene. The reactions were
ꢀ
carried out in methanol at room temperature (30–35 C) with
2 mol% catalyst loading under 1.0 atm of H2 (795 mm Hg of H2).
for catalytic asymmetric hydrogenation (CAH),27
a trans-
formation of high industrial importance, has not been investi-
gated. Earlier reports of CAH using NHC–metal complexes
involved Ir,28–36 Rh37–39 and Ru40 metals and generally at high
pressures of H2. Herein we report the design, synthesis and
structural characterization of PC based 1,2,3-triazol-5-ylidene–
Pd complexes.5,41,42 Application of these complexes as catalysts
for chemoselective hydrogenation of substituted alkenes and
alkynes and for the CAH of prochiral alkenes at 1.0 atm of H2
Department of Chemistry, Indian Institute of Technology Madras, Chennai 600036,
India. E-mail: sanka@iitm.ac.in; Fax: +91 44 2257 0545
† Electronic supplementary information (ESI) available: Synthesis and
characterization data of all compounds and CIF les of complexes 1, 2 and 3.
CCDC 1000089, 891212 and 915053. For ESI and crystallographic data in CIF or
other electronic format see DOI: 10.1039/c5ra03021h
Scheme 1 Synthesis of NHC complexes 1 and 2 from rac-3 and Sp-3.
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Table 1 Hydrogenation of styrene derivatives
R1
X
R2
Y
Time (h)
Yield (%)
1
2
3
4
5
6
7
8
Ph
H
H
H
H
CHO
H
H
NO2
NO2
NO2
NO2
Ph
H
H
H
H
CHO
H
H
NO2
NH2
NO2
NH2
7
8
8
8
8
8
8
8
18
5
100
100
100
100
95
90
97
76
88
CO2H
CO2Me
CHO
CO2H
COMe
COPh
Ph
Ph
COPh
COPh
CO2Me
CO2Me
CH2OH
CO2Me
COMe
COPh
Ph
Ph
COPh
COPh
Fig. 1 Ellipsoid (50% probability) representation of the structures of
complexes Sp-1 (left) and rac-2 (right) in the crystal.
9
10
11
With complex 1 as the catalyst the reaction mixture turned from
yellow to black and E-stilbene was converted to 1,2-diphenyl-
ethane within 7 h in quantitative yield (Scheme 2). With
complex 2 the reaction mixture remained yellow throughout
and no reaction was observed over 24 h. With E-1,2-diphenyl-
propene as the substrate the reaction was complete to give
1,2-diphenylpropane in 20 h with 1 and no reaction was
observed with 2. Hydrogenation of E-stilbene did not occur with
complex 1 in the presence of 4 mol% of Ph3P indicating inhi-
bition of the reaction in the presence of Ph3P due to the
formation of complex 2. This is in sharp contrast to the stabi-
lization of the catalytically active NHC–Rh species by Ph3P
reported in literature.38 The difference in the reactivity between
1 and 2 can be attributed to the liability of acetonitrile ligand in
1 compared to the triphenylphosphane in 2. Also the Pd atom in
2 is sterically more hindered than that in 1 thus reducing the
catalytic activity of the former compared to the latter. The visual
color changes observed in case of 1 and the lack of it in case of 2
indicates the formation of active catalyst from 1 and not from 2.
Control experiments clearly revealed that 2 could be recovered
quantitatively aer workup of the reaction. The apparent cata-
lytic turnover number (TON) aer 13 consecutive cycles of
hydrogenation of trans-stilbene with 1 in MeOH was 640 and the
overall yield of 1,2-diphenylethane was 98%. Hydrogenation of
stilbene did not proceed in CH2Cl2, THF and CH3CN when
either 1 or 2 was used as the catalyst. Thus, a combination of 1
as the catalyst and MeOH as solvent under 1.0 atm of H2 at room
temperature was found to be most effective for the catalytic
hydrogenation of alkenes and alkynes.
70
90
18
corresponding alcohol under the reaction conditions (entry 5).
Benzalacetone and benzalacetophenone were reduced to the
corresponding saturated ketones leaving the keto group intact
(entry 6 and 7). Alkenes can be selectively reduced in the pres-
ence of nitro groups. Thus 4-nitrostilbene and 4-nitro-
benzalacetophenone were rst reduced to 1-(4-nitro-
phenyl)-2-phenylethane and 3-(4-nitrophenyl)-1-phenylpropan-1-
one (entry 8 and 10), respectively, which were further reduced
to 1-(4-aminophenyl)-2-phenylethane and 3-(4-aminophenyl)-1-
phenylpropan-1-one, respectively, upon prolonged hydrogena-
tion (entry 9 and 11). Similarly 4-nitrotolan was rst reduced to 1-
(4-nitrophenyl)-2-phenylethane in 60% isolated yield. Prolonged
hydrogenation led to the formation of 1-(4-aminophenyl)-2-
phenylethane in 91% isolated yield (Scheme 3). The supposed
intermediate, namely 4-nitrostilbene was not observed by TLC
and NMR analysis of the reaction mixture at various stages of the
reaction. Thus C–C double and triple bonds were reduced much
faster than the nitro group. Cinnamic acid and methyl cinnamate
were reduced to methyl 3-phenylpropionate (entry 2 and 3).
Under the reaction conditions acids were converted to the cor-
responding methyl esters in MeOH. CAH of prochiral alkenes
shown in Scheme 4 using Sp-1 gave excellent yields and good
enantioselectivities of the corresponding alkanes. 1,1-Disubsti-
tuted alkenes reacted faster than trisubstituted alkenes. Assign-
ment of absolute conguration of the products is based on
correlation of the observed optical rotation with the literature
(ESI†).46,47 Methyl a-acetamidoacrylate was hydrogenated to the
corresponding saturated ester with 81% ee and in near quanti-
tative yield.
Complex 1 was screened for the hydrogenation of a variety of
alkenes and alkynes to establish its reactivity and chemo-
selectivity. The results are summarized in Table 1 and
Scheme 3.
Cinnamaldehyde was quantitatively converted to cinnamyl
alcohol (entry 4). Aromatic aldehyde was not reduced to the
Scheme 2 Comparison of catalytic activities of 1 and 2.
Scheme 3 Hydrogenation of tolan derivatives.
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PC based chiral 1,2,3-triazol-5-ylidene–Pd complexes 1 and 2
were synthesized and structurally characterized by single crystal
XRD. While complex 1 showed excellent activity towards cata-
lytic hydrogenation of alkenes and alkynes, no reaction was
observed with 2 under identical conditions. Strong coordina-
tion of PPh3 compared to acetonitrile and steric hindrance
could be responsible for the difference in reactivity between
these two complexes for catalytic hydrogenation. For the
hydrogenation of E-stilbene 1 was found to be active up to 13
cycles with a TON of 640. Enantio pure Sp-1 was found to be an
excellent catalyst for the CAH of prochiral alkenes in near
quantitative yields and good enantioselectivity. Further inves-
tigations on the synthetic utility of these Pd complexes are in
progress. Finally it is noteworthy that Pd instead of Ir and Rh as
metal for hydrogenation and that hydrogenation is carried out
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Acknowledgements
Research grants from DST and CSIR, New Delhi are gratefully
acknowledged. ADG thanks CSIR, New Delhi for a fellowship.
We thank Prof. H. Hopf, Institute of Organic Chemistry, TU-BS,
´
¨
Germany, for useful discussions; Prof. A. Lutzen, Kekule Institut
¨
fur Organische Chemie und Biochemie, University of Bonn,
Germany and Prof. T. Lindel, Institute of Organic Chemistry,
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