Catalytic asymmetric intramolecular hydroacylation with rhodium/phosphoramidite-alkene ligand complexes

Article abstract of DOI:10.1002/anie.201104595

Give me a P: An asymmetric intramolecular Rh-catalyzed hydroacylation reaction of pent-4-enals for the preparation of functionalized cyclopentanones in good yield and high enantioselectivity is described (see scheme, DCE=dichloroethane). This process uses rhodium complexes featuring novel modular phosphoramidite-alkene ligands and achiral phosphine coligands. Copyright

Full text of DOI:10.1002/anie.201104595

Communications
DOI: 10.1002/anie.201104595
Asymmetric Catalysis
Catalytic Asymmetric Intramolecular Hydroacylation with Rhodium/
Phosphoramidite–Alkene Ligand Complexes**
Thomas J. Hoffman and Erick M. Carreira*
The rational design and development of novel transition-
metal catalysts bearing diolefin[1,2] and phosphine–olefin
ligands[3] has recently gained attention for the promotion of
catalytic enantioselective reactions such as conjugate and
imine additions, as well as the cyclization of ynals.[4] The
heteroleptic complexes generated from phosphine–alkene
ligands can be particularly useful as they include at least two
donors with distinct steric and electronic properties. Phos-
phine–alkene ligands featuring dibenzo[b,f]azepine[5] motifs
have previously been reported in enantioselective allylic
displacement[3j,6] and conjugate addition[7] reactions. As the
exploration of these and related ligand types continues to
evolve, their use in novel processes will increase. Herein, we
report an asymmetric intramolecular Rh-catalyzed hydro-
acylation[8] reaction of pent-4-enals for the preparation of
cyclopentanones [Eq. (1)]. Two key features of the catalytic
system are noteworthy: this is the first time phosphoramidite–
importance. Subsequent investigations with isotopic labelling
have also been undertaken to shed light on the mechanistic
details.[10,15] It has been suggested that the benefits of ethylene
in the reaction mixture, mentioned in the early reports, arise
from the formation of a coordinatively saturated cationic
rhodium species stabilized against decomposition.[10,11] This
aspect of using ethylene piqued our interest and led us to
examine the use of donor ligands incorporating an olefin.
Additionally, we envisioned the implementation of combina-
torial catalysis[16] involving heteroleptic complexes generated
in situ, an approach that is highly rewarding as illustrated by
the observations of Reetz et al.,[17] Shibasaki and co-work-
ers,[18] and Ding and co-workers.[19]
In prospecting experiments we examined pentenal 1a, as
the prototypical substrate, under various reaction conditions
with complexes generated in situ from [{RhCl(C2H4)2}2] and
phosphoramidite ligands (S)-L1[6] and (R,R,R)-L2[20] in the
presence of AgI (Table 1). These reaction conditions failed to
provide cyclopentanone. Interestingly, the introduction of
Ph3P (8 mol%) into the reaction mixture, which included (S)-
L1 (8 mol%), [{Rh(C2H4)2}] (4 mol%), and AgSbF6 (8
mol%), led to formation of 2a in 52% yield and 66% ee
(Table 1, entry 3).[21] This result from a reaction involving the
addition of an achiral ligand is intriguing and was unexpected.
The inclusion of a second equivalent of PPh3, relative to (S)-
L1, slowed the reaction and resulted in lower enantioselec-
tivity (21% yield, 40% ee; Table 1, entry 4). When ligand L2
was tried under similar reaction conditions no product was
observed (Table 1, entry 5). After the initial results with
ligand (S)-L1, the addition of several phosphine coligands was
investigated.[22] The use of P(2-furyl)3 shut down catalysis
altogether (Table 1, entry 6) and AsPh3 did not promote the
reaction efficiently (18% yield, 64% ee; Table 1, entry 7).
Furthermore, employing P(C6F5)3 provided 2a in high selec-
tivity, albeit in poor yields (20% yield, 90% ee; Table 1,
entry 8) and attempts with P(o-tol)3 (44% yield, 64% ee,
Table 1, entry 9) and P(2,6-OMePh)3 (36% yield, 80% ee;
Table 1, entry 10) were unsuccessful in improving upon the
initial result. Additionally, alkyl-substituted phosphine
ligands MePPh2 (40% yield, 66% ee; Table 1, entry 11) and
PCy3 (60% yield, 50% ee; Table 1, entry 12) only gave 2a
with modest yields and selectivity. However, the use of the
bulky, electron-rich P(tBu)3 greatly increased the reaction
selectivity (94% ee; Table 1, entry 13); when the less bulky
MeP(tBu)2 was investigated, both the reaction yield and
selectivity were improved (78% yield, 95% ee; Table 1,
entry 14). As a control, the use of (S)-L3, which lacks the
olefin donor, had a negative impact on the reaction perfor-
mance (33% yield, 64% ee; Table 1, entry 15). Additionally,
the use of P(tBu)3 (25% conversion, 34% ee; Table 1,
alkene ligands have been used for this reaction type and the
incorporation of an achiral phosphine coligand is necessary to
promote enantioselective catalysis.
After the seminal report in 1972[9] by Sakai et al., in which
stoichiometric RhI was used, Miller and co-workers[10] and
Larock et al.[11] showed that substituted g-pentenals undergo
hydroacylative cycloisomerization using [Rh(PPh3)3Cl]. Their
protocol featured solvent saturated with ethylene and neces-
sitated high catalyst loading (up to 50 mol%); additionally,
they noted the formation of considerable amounts of side
products from competitive decarbonylation pathways. Bos-
nich and co-workers[12] and Sakai et al.[13] independently
reported catalytic enantioselective intramolecular hydroacy-
lation with cationic rhodium perchlorate catalysts prepared
from binap or Me-DuPhos.[14] These studies showed that to
obtain good product selectivity the matching of the diphos-
phine ligand to the pentenal substrates was of the utmost
[*] Dr. T. J. Hoffman, Prof. Dr. E. M. Carreira
Laboratorium fꢀr Organische Chemie, ETH Zꢀrich
8093 Zꢀrich (Switzerland)
E-mail: carreira@org.chem.ethz.ch
Homepage: http://www.carreira.ethz.ch
[**] We thank Dr. Marc LaFrance for his assistance in ligand synthesis.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201104595.
10670
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2011, 50, 10670 –10674

Table 1: Initial hydroacylation reaction condition screening.
Entry Ligand
Additive
None
Conv.[%][a] Yield [%] ee [%][b]
1[c]
2[c]
3
(S)-L1
(R,R,R)-L2 None
(S)-L1
(S)-L1
(R,R,R)-L2 PPh3
–
–
–
–
–
–
[d]
[d]
PPh3
PPh3
>95
52
21
–
66
40
–
4[e]
5
36
[d]
–
[d]
6
7
8
9
10
11
12
13
14
15
16
17[e]
(S)-L1
(S)-L1
(S)-L1
(S)-L1
(S)-L1
(S)-L1
(S)-L1
(S)-L1
(S)-L1
(S)-L3
(S)-L3
(S)-L3
P(2-furyl)3
AsPh3
P(C6F5)3
P(o-tol)3
P(2,6-OMePh)3
MePPh2
PCy3
P(tBu)3
MeP(tBu)2
MeP(tBu)2
P(tBu)3
–
–
–
>95
>95
>95
75
>95
>95
80
>95
63
25
18
20
44
36
40
60
58
78
33
–
64
90
64
80
66
50
94
95
64
34
66
Scheme 1. Phosphoramidite–alkene ligand screening.
preparation of which may be cumbersome and produce waste.
By contrast, hydroacylations may be considered ideal[33] in so
far as all of the atoms of the starting material are found in the
product.
With the optimized reaction conditions established, a
series of pent-4-enal substrates was prepared and examined.
Alkyl-functionalized pent-4-enals (1b–d) were studied first,
and they cleanly provided cyclopentanone products 2b (72%
yield, 95% ee; Table 2, entry 1), 2c (80% yield, 92% ee;
Table 2, entry 2), and 2d (75% yield, 96% ee; Table 2,
entry 3) in good yield and excellent selectivity. Next, a
series of aryl-substituted pentenals were examined beginning
with electron-rich aryl substrates 1e and 1 f, which furnished
the cyclized products 2e (62% yield, 96% ee; Table 2,
entry 4) and 2 f (54% yield, 90% ee; Table 2, entry 5),
respectively. Pentenal systems with electron-poor aryl rings
substituted with halogen (1g), trifluoromethyl (1h), and
carbonyl (1i) groups provided products 2g (56% yield,
90% ee; Table 2, entry 6), 2h (68% yield, 97% ee; Table 2,
entry 7), and 2i (54% yield, 90% ee; Table 2, entry 8), in high
optical purity. Interestingly, we observed that when the alkyl
and aryl pentenal substrates were submitted to identical
reaction conditions, the configuration of the cyclopentanones
isolated is opposite for b-alkyl- and b-aryl-substituted prod-
ucts. When the hydroacylation method was applied to the 3-
substituted pent-4-enal substrate (3R)-1j using (S)-L6 ligand,
the syn-hydroacylated product 2j was readily obtained (68%
yield, 80% ee, syn/anti = 98:2; Table 2, entry 9). Indeed, a
similar outcome was observed for (3R)-1k, as cyclopentanone
2k was isolated in good yield and diastereoselectivity (65%
yield, 90% ee, syn/anti = 94:6; Table 2, entry 10).
In summary, we have shown that cationic rhodium
complexes featuring novel phosphoramidite–alkene ligands
and an achiral phosphine coligand effectively catalyze the
intramolecular hydroacylation of pent-4-enal substrates, thus
providing b-substituted cyclopentanones, which incorporate
alkyl and arene groups, in good yield and excellent selectivity.
This report represents an expansion of the types of reactions
using phosphoramidite–olefin ligands. The fact that the
reactivity of the complexes derived from phospine–alkene
ligands can be modulated or fine-tuned by the addition of a
second achiral phosphine may provide additional avenues for
further developments involving olefins as ligands.
PMe(tBu)3
16
–
[a] Conversion was determined by 1H NMR spectroscopy. [b] The
ee value was measured by supercritical fluid chromatography (SFC)
analysis (Chiralpak OJ-H). [c] Also performed using 16 mol% of ligand.
[d] No reaction occured. [e] Reaction was carried out using 16 mol% of
the coligand additive. Cy=cyclohexyl, DCE=dichloroethane.
entry 16) or 16 mol% of MeP(tBu)2 (16% conversion,
66% ee; Table 1, entry 17) in combination with (S)-L3
failed to improve the process.
A novel set of phosphine–alkene ligands ((S)-L4, (S)-L5,
and (R)-L6) featuring three distinct chiral constructs was
prepared and then examined. This study commenced with
phenanthrol-based (S)-L4 but use of the resulting complex
gave only a modest outcome for 2a (75% yield, 70% ee;
Scheme 1). The vanol-derived ligand (S)-L5 led to the
product 2a but only slightly improved the selectivity (66%
yield, 96% ee; Scheme 1) However, the spinol-derived[23]
ligand (R)-L6 gave the best outcome, as 2a was isolated in
90% yield and 97% ee (Scheme 1).[24]
It is worthwhile to provide a context for the development
of an approach to b-substituted cyclopentanones by hydro-
acylation, because conjugate addition approaches for their
preparation in high optical activity and yield have been
reported. However, alkyl-substituted cyclopentanones, other
than those incorporating the most rudimentary of substitu-
ents, are not always easily accessed. Moreover, conjugate
additions require the stoichiometric use of metal organic
reagents (that is, Grignard,[25] as well as organotin,[26] zinc,[27]
aluminium,[28] lithium,[29] and copper[30] species), or metalloid
reagents (that is, organoboron,[31] and organosilicon[32]), the
Angew. Chem. Int. Ed. 2011, 50, 10670 –10674
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www.angewandte.org 10671

Communications
Table 2: Pent-4-enal substrate scope.[a]
(57 mg, 0.16 mmol) and heated at 808C for 15 h.
Upon completion, the reaction mixture was cooled
to 238C, diluted with Et2O, and filtered through a
pad of silica gel. Removal of the solvent under
reduced pressure followed by purification of the
residue by column chromatography on silica gel (n-
pentane/Et2O; 95:5) gave 2a (51 mg, 90% yield,
97% ee) as a clear oil.
Entry
Pentenal
Ligand
(R)-L6
(R)-L6
Product
Yield ee
[%] [%][b]
Received: July 4, 2011
Published online: September 16, 2011
1
1b
1c
2b 72
95
92
Keywords: asymmetric catalysis ·
.
cyclopentanone · hydroacylation · P ligands ·
rhodium
2
3
2c 80
2d 75
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1g
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Experimental Section
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