Skeletal change in the PNP pincer ligand leads to a highly regioselective alkyne dimerization catalyst

Article abstract of DOI:10.1039/b511148j

A Rh complex of a bulky diarylamino-based PNP pincer ligand is a robust catalyst for the dimerization of terminal alkynes and highly selective for the trans-enyne product. The Royal Society of Chemistry 2006.

Full text of DOI:10.1039/b511148j

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Skeletal change in the PNP pincer ligand leads to a highly regioselective
alkyne dimerization catalyst{
Wei Weng, Chengyun Guo, Remle C¸ elenligil-C¸ etin, Bruce M. Foxman and Oleg V. Ozerov*
Received (in Berkeley, CA, USA) 5th August 2005, Accepted 7th October 2005
First published as an Advance Article on the web 16th November 2005
DOI: 10.1039/b511148j
be efficiently converted to the dihydrides (PNP)RhH2 (3a–c) by
i
A Rh complex of a bulky diarylamino-based PNP pincer ligand
is a robust catalyst for the dimerization of terminal alkynes and
highly selective for the trans-enyne product.
action of NaBH4 in PrOH.
We have previously discussed that the ‘‘tied’’ TPNP ligand, such
as in 3a, exerts a greater degree of steric pressure than the ‘‘untied’’
MePNP ligand, such as in 3b (or FPNP in 3c).8 Here this disparity
leads to a qualitative difference in the solid state structures of 3a
and 3b.{ 3a is a monomer in the solid state, but 3b is a dimer,
loosely bound by bridging hydrides (Fig. 1). The nature of the
bridge-bonding is outside the scope of this report, but the
resistance to dimerization by TPNP is in accord with its greater
steric bulk. Notably, the P–Rh–P angle in 3b (156.58(16)u) is much
smaller than that in 3a (169.11(6)u). This contraction in 3b is in
part necessary to accommodate the dimeric solid state structure.
The smaller P–Rh–P angle corresponds to a greater twist of the
PNP backbone resulting in a larger angle between the two
aromatic rings. Such a distortion is barred in 3a by the CH2CH2
Alkyne dimerization is an attractive, atom-economical method of
synthesizing conjugated enynes.1 Conjugated enynes are attractive
as building blocks in organic synthesis and components of
biologically active molecules.2 The challenge is tuning the
selectivity to the desired enyne isomer (A, B or C, Scheme 1).3–7
Additionally, dimerization to butatrienes, alkyne cyclotrimeriza-
tion, oligo-, and polymerization are often viable competing
processes. Few examples of highly selective catalysis have been
reported.3–7 Lanthanide catalysts are capable of producing head-
to-head cis-enynes (C, Scheme 1) with high selectivity.5 Methly
alumoxane has been shown to catalyze selective alkyne dimeriza-
tion to the gem-enynes B.6 Pd and Ni catalysts have been shown to
produce the trans-enynes A,7 but the chemo- and the regioselec-
tivity, as well as the breadth of their scope, are not ideal.
T
linker connecting the two aromatic rings in PNP.
In solution, the hydrides of each of 3a–c give rise to a single
resonance, split into a doublet of triplets (JRh–H # 21 Hz, JH–P
#
We have been engaged in the transition metal chemistry of the
diarylamido-based PNP pincer ligands (such as those in 2 and 3,
Scheme 1).8 Related ligands have been used by other groups as
well.9 We have recently described that N–C oxidative addition (to
give 2a–c) provides a way to introduce anionic diarylamido-PNP
ligands into the coordination sphere of Rh.8 Compounds 2a–c can
9 Hz) at similar chemical shifts in their 1H NMR spectra. The 31
P
NMR resonances of 3a–c possess similar chemical shifts as well.
The unsymmetric dimer was not observed upon co-dissolution of
3b and 3c. All of this is indicative of a monomeric structure for 3a–
c in solution. Presumably, 3a–c in solution adopt the Y-shaped
5-coordinate geometry that is also observed for Cl(PiPr3)2RhH2.10
Pertinent analysis of the structural preferences of 5-coordinate d6
complexes can be found elsewhere.11
All three compounds 3a–c are active as alkyne dimerization
catalysts (Scheme 1, Table 1). None of the catalysts give rise to
isomer C. 3a is both more active and more selective for the
formation of A than 3b or 3c. 3a is capable of the essentially
regiospecific production of A for a variety of different 1-alkynes.
Dimerization of HCMCCH2NMe2 is an exception, perhaps owing
to the possibility of coordination of the amino group.12 While a
fewothercatalystshavebeenreportedtodisplayselectivityforA,7
none match 3a in both scope and selectivity. Moreover, few
Scheme 1
Department of Chemistry, MS 015, Brandeis University, 415 South
Street, Waltham, Massachusetts, USA. E-mail: ozerov@brandeis.edu
{ Electronic Supplementary Information (ESI) available: Experimental
and characterisation details for the compounds prepared. See DOI:
10.1039/b511148j
Fig. 1 POV-Ray renditions§1 of the X-ray solid state structures of 3a
i
(left) and 3b (right). The Me groups of the Pr groups and those on the
aromatic ring in 3a, as well as all H atoms, are omitted for clarity.
Chem. Commun., 2006, 197–199 | 197
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Table 1 Alkyne dimerization with catalysts 3a–ca
# R
Catalyst Time/h A : B : Xb
% Conversionc
1
2
3
4
5
6
7
8
9
Ph
3a
3a
3a
3a
3a
3a
3a
3a
8
7
7
7
99 : 1 : 0
98 : 2 : 0
99 : 1 : 0
98
98 (98)e
99
4-MeC6H4
4-FC6H4
nBu
99 : ,1 : 0 97
nPr
3
99 : 1 : 0
90 : 10 : 0
100
63
Me2NCH2
tBu
Me3Si
120
120
13
13
3
120
120
120
120
120
120
120
120
120
120
120
120
58 : 0 : 35d 89
90 : ,1 : 10 92
Me3SiOCH2 3a
98 : 2 : 0
97 : 3 : 0
99 : 1 : 0
98 : 2 : 0
98 : 2 : 0
68 : 32 : 0
66 : 34 : 0
74 : 26 : 0
78 : 0 : 22
90
10 HOCH2
11 Ph
3a
3b
3b
3b
3b
3b
3b
3b
3b
98 (96)e
57
85
12 4-MeC6H4
13 4-FC6H4
14 nBu
Scheme 2 Proposed mechanism of alkyne dimerization.
60
73
62
40
15 nPr
cannot be produced when insertion of an alkyne into a Rh–H
bond is involved (5 A 6). The choice between A and B is made at
the same insertion step (5 A 6). Positioning of the non-H
substituent away from Rh should be preferred for steric reasons.
The more sterically imposing TPNP should therefore lead to higher
preference for A, as is the case.
16 Me2NCH2
17 tBu
18 Me3Si
14
83 : 2 : 11d 87
19 Me3SiOCH2 3b
50 : 50 : 0
70 : 30 : 0
99 : 1 : 0
60 : 40 : 0
16
65
64
44
20 HOCH2
21 Ph
3b
3c
3c
22 nPr
a
Reactions performed at 100 uC in 0.5 mL of C6D6, 3.29 mmol
A closely related mechanistic proposal has been put forth
recently by Ogoshi and Kurosawa for the Ni0/PtBu3 system to
account for a preference for A.7 Goldman et al. and Ishikawa et al.
proposed a similar mechanism for alkyne dimerization by the
[(Me3P)2RhCl]2 and the (Ph3P)3RhCl fragment, respectively.4d,14
The selectivity in the catalysis provided by (Ph3P)3RhCl and
[(Me3P)2RhCl]2 varied greatly depending on the alkyne, presum-
ably because of the lack of steric enforcement of A over B (but C
was not observed).4d,14 Our PNP ligand family was designed,
among other reasons, to serve as a chelate analogue of the
ubiquitous mer-Cl(R3P)2 motif.8 Other Rh compounds have been
shown to catalyze alkyne dimerization, but with inferior
selectivities, requiring additives such as MeI or bases, and often
by mechanisms other than the one proposed here.3,4,14
alkyne and 0.5 mol% catalyst. By 1H NMR integration, ¡2%;
GC-MS was used to verify the identity of each. Fraction of
b
c
consumed alkyne by 1H NMR integration vs. an internal integration
standard (1,4-dioxane), ¡2%. Balance tetramer. Isolated yield in
parentheses.
d
e
catalysts are this selective for any enyne isomer. In contrast to
many other examples,3–7 catalysis by 3a–c does not require
additives (e.g., donor solvents or bases) and is also not plagued by
cyclotrimerization and oligomerization. Small amounts of tri-
and tetramers were observed only for the bulkiest (and slowest to
react) alkynes. Products of cyclotrimerization were not detected
for any of the substrates in Table 1, although the electron-
deficient alkyne HCMCCO2Et gave primarily cyclotrimerization
products (see ESI{).
The catalysts employed in this study operate at a relatively high
temperature, however, this corresponds to a low (0.5%) catalyst
loading. With 1-pentyne as substrate, the catalyst (from 3a) was
active at the end of a standard reaction and continued functioning
upon addition of further substrate with the same high regioselec-
tivity (.600 turnovers were achieved). Catalysis by 3a–c is not
sensitive to hydroxyl groups in the substrate, is tolerant of water
and, to some degree, also to air.{ Such tolerance by 3a–c is in
contrast with the lanthanide catalysts which, albeit more active, are
notoriously air and moisture sensitive.5
Preparation of three carbonyl derivatives (8a–c, Table 2)
allowed us to assess the electronic properties of the three PNP
ligands. Based on this analysis, 3b is closer to 3a electronically,
while nearly identical to 3c sterically. Since the selectivity of 3b as a
catalyst differs greatly from that of 3a but is almost identical to
that of 3c, the selectivity most likely arises from their steric
differences."
We were able to isolate the enyne complex 7a-Ph (R = Ph,
TPNP ligand). It is the terminal organometallic product of the
reaction of 3a with PhCMCH. Solution NMR studies indicate
coordination of Rh to the triple bond, which acts as a 2-electron
donor15 (13C{1H} NMR: d 93.9 (dt, JC–Rh = 7 Hz, JC–P = 4 Hz),
We tentatively propose the mechanism outlined in Scheme 2 to
account for our observations. One equivalent of alkyne serves to
strip the Rh center of two hydrogens via hydrogenation of the
triple bond, producing an alkene by-product (observed by NMR
in the reaction mixtures). The RhI alkyne complex 4 may undergo
C–H oxidative addition to give the RhIII hydrido–alkynyl complex
5. Insertion of the second equivalent of alkyne is then possible into
the Rh–H bond. C–C reductive coupling from 6 gives the enyne
complex 7. Displacement of the enyne by alkyne closes the
catalytic loop. Werner et al. recently demonstrated the relevance of
similar steps to alkyne dimerization.13
1
86.8 (br d, JC–Rh = 12 Hz); H NMR: d 7.74 and 6.96 (both: d,
16 Hz, olefinic CH)). 7a-Ph catalyzes the dimerization of PhCMCH
with the same selectivity (and very similar activity) as 3a.{
Addition of excess enyne A-Ph (R = Ph, Scheme 2) had no
discernible effect on the rate of dimerization of PhCMCH.
Table 2 Comparative electronic and catalytic properties of PNP
complexes
(PNP)Rh2 catalyst
3a
3b
3c
PrCCH dimer, selectivity A : B
nCO of (PNP)Rh(CO) (8)/cm21
99 : 1
1943
66 : 34
1945
60 : 40
1950
The proposed mechanism is consistent with the observed
relative selectivity. The cis-isomer C is not observed with 3a–c. C
198 | Chem. Commun., 2006, 197–199
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§ Created using Persistence of Vision Ray Tracer (POV-Ray, http://
www.povray.org/) and Ortep-3 for Windows.16
Rw = 0.0534, CCDC 281663. For both 3a and 3b the bridging H atoms
(2 per Rh) were not located. For crystallographic data in CIF or other
electronic format see DOI: 10.1039/b511148j
" The TPNP complexes also possess a different twist angle of the amido
plane with respect to the coordination square plane of the metal.8 We
cannot exclude the possibility that this contributes to the difference in
reactivity.
Scheme 3
We also designed an experiment (inspired by a literature
example)14 to test for the intermediacy of a vinylidene complex.
Cross-dimerization of PhCMCH and nPrCMCH produced only one
isomer of the cross-dimer and the homodimer of PhCMCH. The
analogous reaction between PhCMCD and nPrCMCH has led only
to a single isotopomer of 9, shown in Scheme 3. Had a vinylidene
formed from either of the alkynes, the two substituents on the
triple bond (Ph and D or Pr and H) would have been found on the
same carbon in the product. As this was not the case, a vinylidene
can be ruled out as an intermediate.
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3277.
In summary, we are reporting a regiospecific, moisture- and
somewhat air-tolerant alkyne dimerization catalyst, as well as the
mechanistic proposals. The increased selectivity of the TPNP-based
catalyst is believed to arise from skeletal adjustment in the pincer
ligand that does not affect the substitution on the atoms directly
bound to the metal. The robustness of the PNP catalysts
presumably originates from the insensitivity of a late metal, such
as Rh, to O-based functionalities and impurities, as well as the
rigid and strong binding of Rh by the PNP ligand. The latter
factor defines the coordination sphere and restricts the reactivity of
the remaining coordination sites.
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Acknowledgment is made to Brandeis University and to the
donors of the Petroleum Research Fund, administered by the
American Chemical Society, for support of this research.
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12 Interestingly, HCMCCH2NMe2 also displayed decreased selectivity in a
case of a highly 1,2,4-selective Ti-based alkyne cyclotrimerization
catalyst, see: O. V. Ozerov, B. O. Patrick and F. T. Ladipo, J. Am.
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Notes and references
{ Crystal data 3a: C26H40NP2Rh, M = 531.46, orthorhombic, space group:
P212121, a = 10.3888(4), b = 12.6906(13), c = 20.1427(8) s, U =
2655.6(3) s3, Z = 4, rcalc = 1.329 g cm23, T = 294 K, m(Mo-Ka) =
0.779 mm21, plate-like habit, red-orange, 0.29 6 0.36 6 0.50 mm. Data
collected on an Enraf-Nonius CAD-4U diffractometer, 4468 unique data,
2818 [I > 1.96s(I)], 271 parameters, R = 0.0489, Rw = 0.0505, Flack X =
0.37(6), CCDC 281662. Crystal data for 3b: C26H42NP2Rh, M = 533.48,
tetragonal, space group: I41/a, a = 18.053(14), b = 18.053(14), c =
16.603(5) s, U = 5411(6) s3, Z = 8, rcalc = 1.310 g cm23, T = 294 K, m(Mo-
Ka) = 0.765 mm21, diamond-like habit, red-orange, 0.14 6 0.29 6
0.29 mm. Data collected on an Enraf-Nonius CAD-4 Turbo diffract-
ometer, 2050 unique data, 913 [I > 1.96s(I)], 137 parameters, R = 0.0578,
13 M. Schaefer, J. Wolf and H. Werner, Dalton Trans., 2005, 1468.
14 W. T. Boese and A. S. Goldman, Organometallics, 1991, 10, 782.
15 B. C. Ward and J. L. Templeton, J. Am. Chem. Soc., 1980, 102, 1532.
16 L. Farugia, J. Appl. Crystallogr., 1997, 30, 565.
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Chem. Commun., 2006, 197–199 | 199