Rhodium-catalyzed branched-selective alkyne hydroacylation: A ligand-controlled regioselectivity switch

Article abstract of DOI:10.1002/anie.201100956

It's all in the ligand: By choice of the appropriate diphosphine ligand a previously linear-selective alkyne hydroacylation process can be "switched" to be highly branched-selective (see scheme, l=linear, b=branched). Structural data for the ortho-iPr-dpp

Full text of DOI:10.1002/anie.201100956

Communications
DOI: 10.1002/anie.201100956
Regioselective Hydroacylation
Rhodium-Catalyzed Branched-Selective Alkyne Hydroacylation:
A Ligand-Controlled Regioselectivity Switch**
Carlos Gonzꢀlez-Rodrꢁguez, Rebekah J. Pawley, Adrian B. Chaplin, Amber L. Thompson,
Andrew S. Weller,* and Michael C. Willis*
With the identification of new catalysts[1] and the develop-
ment of strategies such as chelation control,[2] transition
metal-catalyzed intermolecular alkene and alkyne hydro-
acylation reactions are becoming useful methods for the
preparation of ketones.[3] Although substrate limitations still
Scheme 1. Catalyst-controlled linear- and branched-selective alkene and
alkyne hydroacylation reactions.
exist, the current methods allow the preparation of a broad
range of functionalized products. Consequently, it is perhaps
not surprising that several recent reports have begun to
address the control of reaction selectivity. In particular, a
number of enantioselective processes have recently been
described.[4] Control of regioselectivity in hydroacylation
reactions has also been explored, but usually with the
caveat that a particular set of reaction substrates lead to a
fixed regiochemical outcome. Linear selectivity represents
the most common reaction pathway for both alkene and
alkyne hydroacylation reactions, and although several
branched-selective reactions are known, they usually require
a specific combination of substrates. For example, provided
that aromatic aldehydes and terminal alkyl alkynes are
employed, Jun et al. have utilized a Rh/picoline cocatalytic
system to report a highly branched-selective alkyne process;[5]
Krische et al.[1d] and Ryu et al.[1c] have both reported
branched-selective Ru-catalyzed diene hydroacylations; Sue-
mune et al.[2a,4d] and Dong et al.[4g] have exploited chelating
alkenes to achieve branched-selectivity, and Krische et al.
have described a branched-selective reductive hydroacylation
protocol employing styryl substrates.[1a] The ability to selec-
tively access branched products from alkene hydroacylation
presents opportunities for asymmetric synthesis,[4d,g] while the
alkyne adducts are synthetically useful exo-methylene enones.
However, as far as we are aware, there are no examples of
catalyst or ligand control being used to switch between linear
and branched products for a given substrate combination
(Scheme 1).[6]
During our studies on the use of b-S-substituted aldehydes
in chelation-controlled intermolecular alkene and alkyne
hydroacylation reactions we have observed almost complete
selectivity for the linear adducts.[7,8] However, in the context
of exploring an asymmetric process we observed that when a
DIPAMP-derived Rh-catalyst was employed for the combi-
nation of aldehyde 1a and phenylacetylene (2a) the ketones
(3a + 4a) were obtained as a 1:4 mixture of branched and
linear regioisomers (Table 1, entry 1). This is in contrast with
the use of the parent dppe-derived catalyst, which delivers the
same products in a 1:10 branched:linear ratio (Table 1,
entry 2). Intrigued by this observation we now report the
use of related ortho-substituted dppe-derived ligands, 5, in the
development of branched-selective alkyne hydroacylation
reactions. These ligands have previously been employed in
alkene/CO copolymerization chemistry, where structure–
activity relationships have been outlined.[9]
Table 1: Ligand effects on the Rh-catalyzed hydroacylation reaction of
aldehyde 1a and phenylacetylene.[a]
Entry
Ligand (R)
Conversion [%][b]
3a:4a[b]
1
2
3
4
5
6
7
8
DIPAMP
dppe
100
100
100
100
100[c]
100
45
1:4
1:10
1:2.5
1:1
3:1
2.5:1
1:3
5a (o-Me)
5b (o-Et)
5c (o-iPr)
5d (o-Cy)
5e (o-tBu)
5 f (p-iPr)
[*] Dr. C. Gonzꢀlez-Rodrꢁguez, R. J. Pawley, Dr. A. B. Chaplin,
Dr. A. L. Thompson, Prof. A. S. Weller, Dr. M. C. Willis
Department of Chemistry, University of Oxford
Chemistry Research Laboratory
Mansfield Road, Oxford, OX1 3TA (UK)
Fax: (+44)1865-28-5002
100
1:7
[a] Conditions: aldehyde (0.3 mmol), alkyne (0.45 mmol), [Rh(nbd)2]-
[BF4] (10 mol%), ligand (10 mol%), acetone (2 mL), 508C, 12 h. nbd=
norbornadiene. [b] Determined by 1H NMR analysis of the crude reaction
mixtures. [c] 79% yield of isolated product.
E-mail: michael.willis@chem.ox.ac.uk
Homepage: http://mcwillis.chem.ox.ac.uk/MCW/Home.html
[**] We thank the EPSRC and Xunta de Galicia (Angeles Alvariꢂo
contract and an Estadꢁas: 2009/188 and 2010/163 to CGR) for
support of this work. Additional thanks to Diamond Light Source for
beamtime on I19 (MT1858), and Kirsten E. Christensen and
David R. Allan for assistance.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201100956.
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Angew. Chem. Int. Ed. 2011, 50, 5134 –5138

Table 1 shows that there is a significant ligand-based
ortho-substituent effect observed (entries 3–7) for the combi-
nation of aldehyde 1a and phenylacetylene (2a). Increasing
the size of the ortho-substituent results in an increase in the
relative amount of the branched isomer produced, with the
isopropyl-substituted ligand (5c) being most selective, deliv-
ering a 3:1 ratio in favor of the branched isomer (Table 1,
entry 5). The tBu-substituted ligand did not follow the above
trend and resulted in the formation of a considerably less
active catalyst (Table 1, entry 7). To confirm the importance
of the ortho-substituents the corresponding para-isopropyl
ligand (5 f) was also evaluated, and in line with use of the
parent dppe ligand, delivered the linear isomer as the major
product (Table 1, entry 8). This initial ligand analysis demon-
strated that for the particular hydroacylation reaction studied,
i.e., 1a + 2a, it was possible to turn a 1:10 linear-selective
system (dppe) into a branched-selective process with a
selectivity of 3:1 (5c).
Performing the same coupling reactions but employing the
catalyst incorporating the parent ligand, dppe, delivers the
linear isomers as the major products, but with lower
selectivities (e.g. 3d 1:4.5 branched:linear, 3e 1:2).
We next explored variation of the aldehyde reaction
component; the bis-CF3-substituted arylalkyne (2e), in com-
bination with ligand 5c, was employed throughout (Table 3).
Table 3: Scope of the intermolecular branched-selective hydroacylation
employing alkyne 2e and varied aldehydes.[a]
Entry
1
Aldehyde
Product
Yield [%][b]
83
3:4[c]
3l
20:1
With the viability of a ligand-controlled selectivity switch
established we next explored the generality of the process.
Table 2 documents the reactions between aldehyde 1a and a
2
3m
93
>20:1
Table 2: Scope of the intermolecular branched-selective hydroacylation
employing aldehyde 1a and substituted alkynes.[a]
3
4
3n
3o
82
89
15:1
>20:1
Entry
R
Product
Yield [%][b]
3:4[c]
5[d,e]
6[e]
3p
3q
3r
58
75
80
76
10:1
11:1
13:1
10:1
1
2
3
4
5
6
Bu
3b
3c
3d
3e
3 f
3g
3h
3i
81
79
90
92
65
82
63
82
53
76
2.5:1
2.5:1
8:1
>20:1
8:1
3-thienyl
4-CF3-C6H4
3,5-CF3-C6H3
4-NO2-C6H4
4-CO2Me-C6H4
CO2Me
CO2Et
COPh
(CF2)5CF3
7[e]
4:1
7[d]
8[d]
9[d]
10[d]
>20:1
>20:1
>20:1
11:1
8[e]
3s
3j
3k
[a] See Table 2 for conditions. [b] Yields of isolated products. [c] Deter-
mined by 1H NMR analysis of the crude reaction mixtures. [d] Substrate
used as a 4:1 mixture of diastereomers. [e] Reactions performed at 508C.
[a] Conditions: aldehyde 1a (0.3 mmol), alkyne (0.45 mmol), [Rh(nbd)2]-
[BF4] (10 mol%), ligand 5c (10 mol%), acetone (2 mL), RT, 2–48 h.
[b] Yields of isolated products. [c] Determined by 1H NMR analysis of the
crude reaction mixtures. [d] Reactions performed at 508C.
A variety of aromatic aldehydes featuring both electron-
donating and electron-withdrawing substituents delivered the
branched enone products with excellent yields and selectiv-
ities (Table 3, entries 1–3). The b-S-enal employed in entry 4,
used for the first time in hydroacylation chemistry, performed
equally well, allowing the corresponding bis-enone to be
isolated in high yield. The final four entries demonstrate that
a range of substituted alkyl aldehydes are also good substrates
for the reaction, although a slightly elevated reaction temper-
atures of 508C were required (Table 3, entries 5–8).
range of alkynes, all employing catalysts based on the iPr-
substituted ligand 5c. Both the butyl- and 3-thienyl-substi-
tuted alkynes performed similarly to phenylacetylene, deliv-
ering the branched isomer as the major product with
relatively modest selectivities (Table 2, entries 1 and 2).
However, reactions employing electron-deficient arylalkynes
were significantly more selective (Table 2, entries 3–6). For
example, the use of 4-CF3-substituted phenylacetylene
increased the branched:linear ratio to 8:1 and the corre-
sponding 3,5-di-CF3-substituted alkyne to > 20:1 (Table 2,
entries 3 and 4). Non-aromatic alkynes also performed well,
with ester- and ketone-substituted examples delivering sim-
ilarly high selectivities (Table 2, entries 7–9). The final
example demonstrates the efficient and selective incorpora-
tion of a highly fluorinated alkylalkyne (Table 2, entry 10).
To probe the factors behind this switch in selectivity we
have studied the structures of the active catalysts in solution
and the solid state, as well as closely related model complexes.
A suitable pre-catalyst [Rh(5c)(acetone)2][BArF ] (6; ArF =
4
C6H3(CF3)2)[10] is formed from hydrogenation of [Rh(5c)-
(nbd)][BArF ] in acetone.[11] A solid-state structure of this
4
complex is shown in Figure 1 and confirms a square-planar
RhI center coordinated with two acetone ligands. The phenyl
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5135

Communications
31P environments that show coupling to a RhI center [J-
(RhP) = 189, 172, J(PP) = 32 Hz]; and separate correlations
(31P-1H HMBC) between these and either the aldehdye or the
SMe resonances. The RhIII oxidative-addition product, 8, is
characterized at 200 K by a hydride environment that shows
coupling to a trans phosphine [d = À7.04 ppm, dd, J(PH) =
169, J(RhH) = 16 Hz], a SMe environment that shows a
strong correlation in the 31P/1H-HMBC experiment, and
inequivalent phosphorus environments by 31P{1H} NMR
spectroscopy, one of which shows a much reduced value for
J(RhP) compared with the other (87 versus 160 Hz) identify-
ing it as being opposite the high trans influence hydride.[7e]
This places the acyl ligand trans to the vacant site (or at most,
a weakly bound acetone ligand). Similar structures for
rhodium acyl hydrides have been noted previously using the
DPEphos ligand,[7e] and calculated for the intramolecular
hydroacylation of 4-pentenals.[13]
Stoichiometric addition of aldehyde 1a to 6 followed by
one equivalent of alkyne 2e leads to the spectroscopically
characterized product complex [Rh(5c){k2(O,S)-CCH2(C6H3-
Figure 1. Solid-state structures of 6 and 11. Thermal ellipsoids at the
50% probability level (left) and Van der Waals radii (right). Phenyl
rings for which the dihedral angle has been calculated are highlighted
in red. Anion and hydrogen atoms are omitted for clarity.
(CF3)2)(OC)C6H4SMe}][BArF ] (9). Addition of MeCN to 9
4
liberates branched product 3e alongside a species character-
ized as [Rh(5c)(MeCN)2][BArF ]; while addition of a further
4
2 equivalents of aldehyde 1a to 9 generates a mixture of 7, 8,
and 9 alongside free product, 3e, thus demonstrating turn-
over. Catalysis (10 mol%, 0.02 molLÀ1 2e) is relatively rapid
(TOF 60 hÀ1) and at the end complex 9 is observed. Addition
of a further 10 equivalents each of 1a and 2e restarted
catalysis.
groups are orientated so that the bulky iPr groups point away
from the metal,[9a] although they are still able to adopt a
variety of conformations as measured by the dihedral angle of
À
the arenes with respect to the Rh P bond, for example, Rh1-
P2-C27-C32, À74.9(3)8; Rh1-P1-C9-C10, 7.7(3)8.
Crystalline material from the mixture of 7 and 8 was not
obtained, and to probe in detail the solid-state structures of
these intermediates we turned to using the acyl chloride 10.
This underwent clean oxidative addition with 6 to give 11,
Addition of aldehyde 1a to complex 6 results in an
equilibrium mixture of RhI substrate-bound complex [Rh-
(5c){k2(O,S)-(OHC)C6H4SMe}][BArF ] (7)[12] and the product
4
of oxidative cleavage, [Rh(5c)(H){k2(C,S)-(CO)C6H4SMe}]-
[Rh(5c)(Cl){k2(C,S)-(OC)C6H4SMe}][BArF ], in quantitative
4
[BArF ] (8; Scheme 2). Complexes 7 and 8 are in exchange
yield (by NMR spectroscopy) as a single isomer. A solid-
state structure of 11 (as the [BArCl4]À salt,[14] Figure 1, ArCl =
3,5-Cl2C6H3) shows an arrangement of ligands exactly as
inferred from the spectroscopic characterization of 8 at low
temperature, namely a five-coordinate RhIII complex with an
acyl group trans to a vacant site and the chloride (that is,
hydride in 8) trans to a phosphine. Solution NMR data are
fully consistent with this description. This structure also gives
pointers to the underlying reasons behind the linear/branched
selectivity observed in the catalytic process. The oxidative
addition of aldehyde to form a RhIII pseudo-octahedral center
results in a conformation of the phenyl groups that is far more
directional than in precursor 6, with the arenes pointing
directly towards the vacant site on the metal; as shown by the
relevant dihedral angles: Rh51-P63-C91-C92, À13.4(5)8 and
Rh51-P60-C73-C74, 9.7(5)8.
4
with one another at room temperature, and progressive
cooling to 200 K resolves separate sharp resonances in the 1H
and 31P{1H} NMR spectra. The ratio of 7:8 at 200 K is 3:1,
which changes to 1:1 at 250 K. At 200 K the spectroscopic
markers that identify 7 are a peak centered at d = 10.25 ppm
[d, J(PH) = 6.1 Hz] shifted 0.15 ppm to high frequency from
free 1a, which is assigned to the bound aldehdye; inequivalent
Direct structural comparison between 11 and its dppe
analogue would help underscore this idea, as a more flexible
orientation of the phenyl groups would correlate with the
lower linear/branched selectivities with dppe. Addition of 10
to [Rh(dppe)(acetone)2][BArCl4] gave a product spectroscopi-
cally identified as [Rh(dppe)(Cl){k2(C,S)-(OC)C6H4SMe}]-
[BArCl4] (12), but unfortunately crystals suitable for a X-ray
diffraction have proved elusive. However 13C{1H} NMR data
for 12 demonstrate free-rotation of the phenyl groups at
Scheme 2.
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Angew. Chem. Int. Ed. 2011, 50, 5134 –5138

293 K, a situation that would not be expected for ligand 5c.[9b]
Restricted rotation in complexes with chelate ligands closely
related to 5c has been noted;[9c] while inspection of the space
filling diagram of closely related [Rh(dppe)(I)2(COMe)][15]
(Supporting Information) shows significantly twisted phenyl
groups (dihedral À70.88 and À24.78) compared with 11
suggesting a less rigid conformation for the phenyl groups in
dppe. Overall these observations support the idea that the
phenyl-group orientation in complex 8, and in turn the
increased selectivity, comes from a minimization of steric
strain between the iPr groups and the bound aldehyde at the
RhIII center.
That the bulkier product (i.e., branched) is favored with
the bulky ligand 5c suggests that it is the irreversible rate-
limiting[16] reductive elimination of the product, for example,
3e, that controls the final regiochemistry, as the preced-
ing[1a,b,3a,13,16] hydride insertion step would likely favor a linear
product when using a bulkier ligand. The progressive increase
in selectivity on changing the arene functionalization, H <
Me < Et < iPr (Table 1), is consistent with this idea. This
scenario requires that the insertion step is fast and reversi-
ble.[1b] Reversible insertion of hydrides into alkynes, although
rare, has recently been reported on IrIII centers.[17] We have
recently suggested a similar scenario in explaining product
distributions in competitive alkene versus aldehyde hydro-
acylation reactions on changing the backbone of a simple
chelate ligand;[7h] while faster reductive elimination of
branched isomers, and equilbria between linear and branched
tion illustrates the utility of the exo-methylene products in a
simple application to a one-pot three-component coupling;
rather than isolate the branched adduct obtained from the
union of aldehyde 1a and alkyne 2e, reaction with indole in
1,2-dichloroethane (DCE) at 808C results in direct conjugate
addition and allows substituted ketone 13 (see the Supporting
Information) to be isolated in 88% yield.
In conclusion, we have developed a system that provides
control over regioselectivity in the hydroacylation of alkynes,
especially electron-poor derivatives, in which a bulky ortho-
iPr-dppe-derived catalyst promotes the formation of the
branched adduct. In this regard these results contrast to the
closely related alkene hydroformylation, in which bulky
ligands promote linear products.[18] This may be related to
the suggested rate-limiting step in each: reductive elimination
in olefin hydroacylation and hydride insertion in hydro-
formylation. Future contributions will explore and exploit the
mechanism in more detail.
Received: February 7, 2011
Published online: April 19, 2011
Keywords: alkynes · hydroacylation · regioselectivity ·
.
rhodium catalysis
[1] Recent examples: a) Y.-T. Hong, A. Barchuk, M. J. Krische,
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=
intermediates, has been noted in [Cp*Rh(H2C CHSiMe3)2]
(Cp* = h5-C5Me5) catalyst systems.[1b] The experimental
observation that electron-poor alkynes give the best selectiv-
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reductive elimination associated with these alkynes.[1b]
[2] Recent examples: a) M. Imai, M. Tanaka, K. Tanaka, Y.
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Org. Lett. 2007, 9, 1215.
Scheme 3 demonstrates the utility of our developed
methodology. The first two reactions illustrate the switch in
regioselectivity now achievable by the correct choice of
ligand; the combination of aldehyde 1q and alkyne 2e using
the ortho-iPr-dppe-derived catalyst selectively delivers the
branched adduct (3q, 1:11 linear/branched). Employing the
catalyst incorporating dppe delivers the linear isomer as the
major product with 2:1 selectivity. However, use of a
DPEphos-derived catalyst[7d,e] leads to efficient formation of
the linear isomer with 20:1 selectivity. The third transforma-
[3] a) M. C. Willis, Chem. Rev. 2010, 110, 725; b) C.-H. Jun, E.-A. Jo,
J.-W. Park, Eur. J. Org. Chem. 2007, 1869.
[4] a) R. T. Stemmler, C. Bolm, Adv. Synth. Catal. 2007, 349, 1185;
b) J. D. Osborne, H. E Randell-Sly, G. S. Currie, A. R. Cowley,
M. C. Willis, J. Am. Chem. Soc. 2008, 130, 17232; c) Y. Shibata,
K. Tanaka, J. Am. Chem. Soc. 2009, 131, 12552; d) Y. Inui, M.
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114, 2250; Angew. Chem. Int. Ed. 2002, 41, 2146.
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H. E. Randell-Sly, R. L. Woodward, G. S. Currie, Org. Lett. 2005,
7, 2249; c) M. C. Willis, H. E. Randell-Sly, R. L. Woodward, S. J.
McNally, G. S. Currie, J. Org. Chem. 2006, 71, 5291; d) G. L.
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Scheme 3.
Angew. Chem. Int. Ed. 2011, 50, 5134 –5138
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www.angewandte.org
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Communications
ꢀ
A. S. Weller, M. C. Willis, Angew. Chem. 2006, 118, 7780; Angew.
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Z’ = 2), a = 12.8020(16) ꢂ, b = 18.794(2) ꢂ, c = 31.915(4) ꢂ,
a = 99.370(2)8,
b = 93.1700(10)8,
g = 104.344(2)8.
V=
7303.3(16) ꢂ3, T= 150(2) K, 23810 unique reflections [R(int) =
0.112]. Final R1 = 0.0621 [I > 2s(I)]. CCDC 804185 (6),
CCDC 804186 (11), and CCDC 815141 (13) contain the supple-
mentary crystallographic data for this paper. These data can be
obtained free of charge from The Cambridge Crystallographic
Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
[8] Specific substrate combinations lead to branched products:
[12] Complex 7 is formed as an approximate 1:0.37 mixture of two
isomers, see Figure S-1 (Supporting Information). Both show
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the 31P{1H} NMR spectrum and appear as broad doublets (CHO)
in the 1H NMR spectrum at 200 K. Both complexes are
consumed on addition of alkyne 2e. We suggest that these
isomers are due to restricted rotation of the bulky arene groups
and/or restricted inversion at sulfur.
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son, R. L. Wingad, Dalton Trans. 2005, 659; c) S. J. Dossett, A.
Gillon, A. G. Orpen, J. S. Fleming, P. G. Pringle, D. F. Wass,
M. D. Jones, Chem. Commun. 2001, 699.
[10] We find no significant difference in selectivity with the use of
[BF4]À and [BArF ]À anions for the conditions reported in
4
Tables 1 and 2.
[11] Crystallographic data. 6: C76H72BF24O2P2Rh, M = 1649.00,
monoclinic, P21/n (Z = 4), a = 19.8241(2) ꢂ, b = 20.2882(2) ꢂ,
c = 21.4016(2) ꢂ, b = 111.5098(4)8. V= 1043.25(7) ꢂ3, T=
150(2) K, 16254 unique reflections [R(int) = 0.0255].
[16] D. P. Fairlie, B. Bosnich, Organometallics 1988, 7, 946.
[17] R. Ghosh, X. Zhang, P. Achord, T. J. Emge, K. Krogh-Jespersen,
A. S. Goldman, J. Am. Chem. Soc. 2007, 129, 853.
Final
R1 = 0.0510
[I > 2s(I)].
11·0.89acetone:
[18] Z. Freixa, P. W. N. M. van Leeuwen, Dalton Trans. 2003, 1890.
5138
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