Carbometalated Complexes Possessing Tripodal Pseudo-C3-Symmetric Triptycene-Based Ligands

Article abstract of DOI:10.1021/acs.organomet.7b00907

New air-stable tripodal triphosphine ligands 1,8,13-tris(diisopropylphosphino)-2,3-dimethoxytriptycene (P₃T1) and 1,8,13-tris(diphenylphosphino)-2,3-dimethoxytriptycene (P₃T2) are presented and discussed. Their synthesis is based on

Full text of DOI:10.1021/acs.organomet.7b00907

Communication
Cite This: Organometallics XXXX, XXX, XXX−XXX
Carbometalated Complexes Possessing Tripodal
Pseudo‑C ‑Symmetric Triptycene-Based Ligands
3
Ilya Kisets and Dmitri Gelman*
Institute of Chemistry, Edmond J. Safra Campus, The Hebrew University of Jerusalem, Jerusalem 91904, Israel
*
S Supporting Information
ABSTRACT: New air-stable tripodal triphosphine ligands 1,8,13-tris-
(
(
diisopropylphosphino)-2,3-dimethoxytriptycene (P T1) and 1,8,13-tris-
3
diphenylphosphino)-2,3-dimethoxytriptycene (P T2) are presented and discussed. Their
3
synthesis is based on the first practical synthesis of 1,8,13-tribromotriptycene. Reaction with
palladium leads to the formation of new pseudo-C -symmetric carbometalated complexes:
3
namely, Pd(P T1)Cl and Pd(P T2)Cl. The catalytic activity of the new ligands and
3
3
complexes was tested in the palladium-catalyzed chemoselective transfer hydrogenation of
α,β-unsaturated ketones.
fter more than 50 years of intensive research, numerous
catalytic transformations have become routine for
cycloaddition methodology, which guarantees facile access to a
A
readily modifiable platform with tailored steric and electronic
7
manufacturing fine and bulk chemicals on both academic and
industrial scales. The breakthrough toward popularizing catalysis
in organic synthesis came three decades ago after understanding
that ligands can strongly affect and modify the intrinsic catalytic
performance of late transition metals (Rh, Ru, Pd, Ir, Pt, and
others). Since then, thousands of structurally different (mainly
multidentate) ligands have been synthesized and tested for
numerous metal-catalyzed reactions. However, despite the
structural diversity of commercially available and reported
properties. In addition, the rigidity of the frame and the lack of
available labile α- and β-hydrogens can be translated into
robustness and conformational stability, along with exceptional
3
σ-donation of the metalated bridgehead sp -hybridized carbon.
Following this initial interest and keeping in mind that
triptycene-based ligands, in general, have gained a high
8
reputation in catalysis, extending the bis(dialkylphosphino)-
tripticene system to the symmetrically substituted tris-
(dialkylphosphino)tripticene was quite natural and logical.
Herein, we report the synthesis and characterization of a new
scaffolds, C -symmetric ligands are still quite rare and, therefore,
3
1
less explored.
class of triptycene-based triphosphines (P T1 and P T2), their
3 3
Most of the C -symmetric ligands known to date possess hard
coordination to Pd(II), and the benchmarking catalytic tests in
the Pd(II)-catalyzed chemoselective transfer hydrogenation of
α,β-unsaturated ketones.
3
donors (N- or O-based) and, therefore, are more suitable for the2
synthesis of main-group- or early-transition-metal complexes.
Examples of ligands coordinating late transition metals through
phosphorus and their applications in chiral molecular recog-
The ligands P T1 and P T2 were prepared in three steps from
3
3
the readily accessible starting materials, as presented in Scheme
. Initially, 1,8-dibromoanthracene (1) is converted into 1,13-
1
nition and catalysis are less common, but some have appeared in
3
dibromo-6,7-dimethoxytriptycene (2) using a slightly modified
the literature. However, metalated C -symmetric complexes
3
9
3
literature protocol in 63% yield. A successive electrophilic
possessing a direct C/Si/N(sp )−M bond have been even less
studied. These compounds often possess unique reactivity,
4
bromination of 2 results in the formation of a 1:1 mixture of the
syn- and anti-substituted tribromide 3. Fortunately, the desired
3syn can be selectively crystallized out in 42% yield, obviating
tedious chromatographic isolation.
arguably because of the singular electronic and steric properties
of the metal site originating from the strong σ-donating character
3
of the anionic C(sp )-hybridized carbon in combination with the
5
Surprisingly, an extensive literature search revealed that
syntheses of symmetrically halogenated triptycenes are ex-
tremely rare and generally report on the synthesis of inseparable
coordination sphere of unusual geometry.
Our group has a longstanding interest in developing catalysts
3
based on carbometalated PC(sp )P pincer ligands, especially
10
6
mixtures of isomers. Thus, the reported protocol represents the
those derived from a dibenzobarrelene-based scaffold. Our
first practical synthesis of 1,8,13-halogenated triptycenea
interest in these compounds stems from the fact that, unlike the
3
case for many known classes of PC(sp )P pincer ligands,
synthesis of dibenzobarrelene derivatives is very modular because
Received: December 25, 2017
it is accomplished through the use of reliable Diels−Alder
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.organomet.7b00907
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Communication
Scheme 1. Synthesis of the New Pseudo-C -Symmetric
space coupling in the corresponding free ligands (JPa−Pb = 212 Hz
3
Triptycene-Based Ligands and Complexes
in Pd(P T1)Cl (6) versus J = 10 Hz for P T1 and J = 218 Hz
a b
P −P
3
3
in Pd(P T2)Cl (7) versus J = 18 Hz for P T2 that correspond to
3
3
Δν/J = 20 for 6 and Δν/J = 5 for 7). In both cases, the triplet
signal assigned to the phosphine in the substituted ring (P ) of
a
the free ligand now appears as a multiplet arising from the
second-order splitting consisting of four transition lines centered
at δ 55.76 ppm (JPa = 212 Hz) for 6 and at δ 30.92 ppm
P
−P
b
P
(
JPa−Pb = 218 Hz) for 7. The parent doublet signal ascribed to the
phosphines on the unsubstituted rings of P T1 and P T2 is now
3
3
situated at δ 34.8 ppm for 6 and 24.4 ppm for 7 and is also seen
P
as a four-transition-line multiplet. The absence of the front
methine hydrogen and the perfectly resolved isopropyl methyl
1
groups in the H NMR of 6 confirm their rigid (i.e., metalated)
structure.
Despite the unambiguous identification of the new compound,
single crystals of Pd(P T1)Cl (6) and Pd(P T2)Cl (7) (CCDC
3
3
1
578676 and CCDC 1578674, correspondingly), grown by the
slow diffusion of hexane into their saturated solutions in
chloroform at room temperature, were subjected to X-ray
diffraction analysis. Selected crystallographic parameters and the
ORTEP drawingare presented in Figure 1. As expected, the
11
valuable synthetic intermediate. Finally, phosphination of 3syn
was carried out by sequential treatment with n-BuLi/TMEDA
and the corresponding chlorodialkylphosphine. Both P T1 (4)
3
and P T2 (5) were synthesized in approximately equal yields
3
(
33−35%).
NMR spectroscopy unequivocally validates the desired
3
1
structure. For example, P NMR of P T1 displays two
3
resonances for two different phosphine types: a doublet centered
at −8.27 ppm (J = 10 Hz) for phosphines installed onto
unsubstituted rings and a triplet appearing at 6.17 ppm (J = 10
Hz) for the MeO substituents neighboring the phosphine (AB
2
1
2
system). Since six bonds separate all phosphorus nuclei, the
splitting must originate from through-space interactions of the
phosphines if they still converge at the same point, apparently in
proximity to the central methine hydrogen. Indeed, the lack of
Figure 1. ORTEP drawing (50% probability ellipsoids) of Pd(P T2)Cl
3
(7). Hydrogen atoms and solvent molecules are omitted for clarity.
1
free rotation can be clearly concluded from the H NMR by the
perfectly resolved anisochronous methyl groups within each of
the isopropylphosphine substituents (please see the Supporting
3
palladium center in C(sp )-metalated complexes has almost a
1
3
Information). Furthermore, whereas peripheral methine
hydrogen appears as an expected singlet at 5.25 ppm, the front
hydrogen is shifted strongly downfield (7.93 ppm) and is split
virtually into a quartet, which supports this assessment. Both of
these phenomena have been described for bis-phosphine
perfect trigonal-bipyramidal shape. For example, for Pd(P T2)-
3
15
Cl, the observed Cl1−Pd1−C1 angle is 176.6° and for P1−
Pd1−P2, P2−Pd1−P3, and P1−Pd1−P3 the angles range from
118.6 to 122.8°. As expected, the Pd−Cl bond in Pd(P T2)Cl is
3
slightly longer than the corresponding bond in the previously
reported anthracene-based two-dimensional pincer complexes
14
compounds of this type. NMR spectra of the ligand P T2 are
3
3
less indicative, however, but exhibit similar patterns and are
presented in the Supporting Information.
owing to a stronger trans influence exerted by the sp -hybridized
7b,16
carbon.
With the ligands at hand, we performed a series of
Chemoselective transfer hydrogenation of α,β-unsaturated
complexation studies. Thus, for example, when P T1 (4) and
ketones was chosen to benchmark the catalytic activity of the new
3
17
P T2 (5) react stoichiometrically with PdCl (MeCN) in
complexes. We chose this particular transformation because it
is mediated by Pd-hydride species whose stability/reactivity
balance is responsible for the higher efficiency and selectivity of
the catalysts. We hypothesized that chemoselectivity will greatly
benefit from the structural and conformational stability of the
tripodal triptycene-based complexes. On the other hand, the
electron-rich nature of the palladium centers will enhance the
reactivity of the Pd-hydride intermediates. Moreover, this
3
2
2
toluene at 100 °C for 48 h, the carbometalated Pd(P T1)Cl
3
(
6) and Pd(P T2)Cl (7) form as orange solids in 83−89%
3
31 1
isolated yields. P{ H} NMR spectra taken from the new
compounds 6 and 7 displayed an expected AB pattern, but with
2
well-expressed secondary effects. The stronger influence of the
second-order splitting in 6 and 7 is due to a stronger through-
metal P −P coupling in the complexes versus a weak through-
a
b
B
DOI: 10.1021/acs.organomet.7b00907
Organometallics XXXX, XXX, XXX−XXX

Organometallics
transformation is of great importance in organic synthesis. It is
Communication
18
CH CH CH OD as a hydrogen source afforded an α-deuterated
3
2
2
19
often carried out in the presence of silicon or boron hydrides,
product, whereas the reaction using CD CD CD OH afforded
3 2 2
1
8b,20
21
hydrogen,
or hydrogen surrogates such as alcohols and
the product exclusively deuterated at the β-position (eq 1)the
2
2
formates. However, of all these alternatives, the use of alcohol
as an environmentally friendly, safe, and cost-efficient hydrogen
source is the most important one.
The catalysts Pd(P T1)Cl and Pd(P T2)Cl were initially
3
3
tested in the conjugate reduction of benzalacetophenone ((E)-
chalcone). Brief experimentation showed that the primary
alcohols were generally superior over the secondary alcohols as
solvents for the described transformation (e.g., 1-butanol or 1-
propanol versus 2-butanol or 2-propanol), whereas ethanol and
methanol led to no conversion. For example, 68% of the
corresponding saturated product in >99% selectivity forms in
refluxing 1-PrOH in the presence of 1 mol % of Pd(P T1)Cl and
3
1
0 mol % of K PO as a base. A significantly lower yield, yet with
3 4
excellent selectivity (25% and >99%, respectively), were
observed under the same substrate:catalyst:base ratio in 2-
propanol (Table 1, entries 1 and 2). We assume that the Pd−H
selectivity cannot be achieved if the process is mediated by the
ligand-free particle catalysts. Finally, employment of the
previously reported anthracene-based two-dimensional pincer
complexes of palladium led to diminished conversion and poor
selectivity under the same reaction conditions (eq 2).
Table 1. Representative Results of the Transfer
Hydrogenation of Enones Catalyzed by Pd(P T1)Cl
3
To conclude, we have disclosed for the first time synthesis and
coordination studies of unprecedented air-stable tripodal ligands
a
b
entry
conditions
6 1 mol % K PO , n-PrOH
Ar
phenyl
yield (sel), %
(
P T1 and P T2) based on a pseudo-C -symmetric triptycene
3 3 3
1
2
3
4
5
6
68 (99)
25 (99)
89 (99)
92 (99)
73 (99)
72 (99)
3
4
derivative. The whole synthesis relies on the first practical
synthesis of 1,8,13-tribromotriptycene. The catalytic activity of
the new palladium complexes was tested in palladium-catalyzed
chemoselective transfer hydrogenation of α,β-unsaturated
ketones showing uniqueness of the reported scaffold. Further
coordination and catalytic studies of this new family of tripodal
ligands, including those that are chiral and enantiopure, will be
reported soon.
6 1 mol % K PO , i-PrOH
phenyl
3
4
6 0.5 mol % K PO , n-PrOH
phenyl
3
4
6 0.5 mol % K PO , n-PrOH
4-MeO-phenyl
1-naphthyl
2-furyl
3
4
6 0.5 mol % K PO , n-PrOH
3
4
6 0.5 mol % K PO , n-PrOH
3
4
a
Other conditions: 0.5 mol % of Pd(P T1)Cl, 10 mol % of K CO in
refluxing n-butanol (or as stated in the table). Isolated yield;
selectivity according to NMR integration.
3
2
3
b
ASSOCIATED CONTENT
bond formation is more difficult when sterically demanding
alcohols interact with a cribbed palladium center. On the other
■
*
S
Supporting Information
hand, the excellent chemoselectivity of the presumed Pd(P T1)-
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.7b00907.
3
H species also originates from the singular steric environment
around the catalytic site. The diminished activity of the lower
primary alcohols can be rationalized by the lower reaction
temperature. Further improvement was achieved using boiling n-
butanol in the presence of 10 mol % of K CO and 0.5 mol % of
Experimental procedures, X-ray crystallographic data for
all structures, and spectroscopic data (PDF)
2
3
equally reactive Pd(P T1)Cl or Pd(P T2)Cl (Table 1, entry 3).
3
3
Accession Codes
It is noteworthy that this reaction does not require oxygen,
moisture-free conditions, or special equipment; therefore, the
procedure can be performed in air using the standard grade
solvents.
CCDC 1578674 and 1578676 contain the supplementary
crystallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
To test the general applicability of the new Pd complexes, a
limited number of various α,β-unsaturated ketones were allowed
to react under the optimized conditions. Substituted and
unsubstituted, aromatic, and heteroaromatic enones were
transfer-hydrogenated to provide the desired products in high
yields and with excellent selectivity (Table 1, entries 4−6).
In order to confirm that the reaction is indeed mediated by the
presumed Pd−H species and not by colloidal palladium that can,
in principle, form, upon decomposition of the complexes, we
carried out a mercury test as well as two deuterium-labeling
experiments for the transfer hydrogenation of chalcone. Thus,
the presence of metallic mercury in the reaction vessel showed no
influence on the outcome. On the other hand, the use of
AUTHOR INFORMATION
■
Corresponding Author
*D.G.: e-mail, dmitri.gelman@mail.huji.ac.il; fax, +972-2-
6585279.
ORCID
Dmitri Gelman: 0000-0001-7185-1345
Notes
The authors declare no competing financial interest.
C
DOI: 10.1021/acs.organomet.7b00907
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Communication
Kadosaka, T.; Sakata, Y.; Misumi, S. Bull. Chem. Soc. Jpn. 1971, 44,
ACKNOWLEDGMENTS
■
1
(
649−1652.
The studies were supported by the ISF (Israel Science
Foundation) Grant Nos. 917/15 and 1384/13. We thank Dr.
Benny Boguslavsky for solving X-ray structures.
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DOI: 10.1021/acs.organomet.7b00907
Organometallics XXXX, XXX, XXX−XXX