Pseudo enantiomeric carbohydrate olefin ligands - Case study and application in kinetic resolution in rhodium(I)-catalysed 1,4-addition

Article abstract of DOI:10.1002/adsc.201400459

In order to investigate significant differences in asymmetric induction for pseudo enantiomeric carbohydrate olefin ligands in rhodium(I)-catalysed 1,4-addition reactions, we designed a set of new olefin ligands differing in relative configuration and pyranoside conformation. With these, we have successfully elucidated structural requirements for metal binding and also identified an improved alternative for one pseudo enantiomer. Furthermore, we report the efficient kinetic resolution of a racemic 4-hydroxycyclopentenone derivative by 1,4-addition.

Full text of DOI:10.1002/adsc.201400459

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DOI: 10.1002/adsc.201400459
pseudo Enantiomeric Carbohydrate Olefin Ligands – Case Study
and Application in Kinetic Resolution in Rhodium(I)-Catalysed
1,4-Addition
Holger Grugel,a Fabian Albrecht,a and Mike M. K. Boysena,*
a
Institute of Organic Chemistry, Leibniz University of Hannover, Schneiderberg 1B, 30167 Hannover, Germany
E-mail: mike.boysen@oci.uni-hannover.de
Received: May 6, 2014; Published online: && &&, 0000
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201400459.
Abstract: In order to investigate significant differ-
ences in asymmetric induction for pseudo enantio-
meric carbohydrate olefin ligands in rhodium(I)-
catalysed 1,4-addition reactions, we designed a set
of new olefin ligands differing in relative configura-
tion and pyranoside conformation. With these, we
have successfully elucidated structural requirements
for metal binding and also identified an improved
alternative for one pseudo enantiomer. Further-
more, we report the efficient kinetic resolution of
a racemic 4-hydroxycyclopentenone derivative by
1,4-addition.
catalysis is an active research field. Chiral olefin li-
gands,[1] introduced independently by Hayashi[2] and
Carreira,[3] are the ligands of choice for rhodium(I)-
mediated 1,4-additions of boronic acids to enones.[4]
Carbohydrates are attractive, inexpensive starting
materials for the design of novel ligands.[5] With
gluco-enoPhos (1a, 1aa)[6] and galacto-enoPhos (ps-
ent-1a),[7] we have introduced a set of pseudo enantio-
meric[8] olefin-phosphinite ligands derived from d-glu-
cose and d-galactose (Scheme 1), giving 1,4-addition
products in opposite configurations and typically
above 90% ee. Significantly lower selectivity was ob-
served for the gluco-ligands 1a and 1aa with bulky ar-
ylboronic and alkenylboronic acids, while high ees
were retained with galacto-ligand ps-ent-1a.[7] This
finding prompted us to investigate the origin of the
differing asymmetric induction exerted by 1a and 1aa
on the one hand and ps-ent-1a on the other hand, as
well as to develop an improved alternative to gluco-li-
Keywords: asymmetric catalysis; carbohydrates;
conjugate addition; kinetic resolution; ligand design
Due to the increasing demand for chiral building gands 1a and 1aa.
blocks in natural product and medicinal chemistry,
An obvious explanation for the different efficiency
the design of new chiral ligands for asymmetric metal of the gluco-ligands (1a, 1aa) and galacto-ligand (ps-
Scheme 1. Ligands gluco-enoPhos (1a and 1aa) and galacto-enoPhos (ps-ent-1a): proposed coordination to Rh(I).
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Holger Grugel et al.
Scheme 2. Preparation of fuco-enoPhos (1b), bicyclo-gluco-enoPhos (1c) and rhamno-enoPhos (ps-ent-1b).
ent-1a) may lie in the relative configuration of their gands shown in Scheme 2. The 4,5-cis configured
pyranose scaffolds – 4,5-trans for 1a and 1aa but 4,5- ligand fuco-enoPhos (1b) is closely related to galacto-
cis for ps-ent-1a – which will impact the preferred ligand ps-ent-1a but lacking the trityloxy group.
conformations of the free ligands (cf. Scheme 1). In Ligand bicyclo-gluco-enoPhos (1c) is 4,5-trans config-
5
general, 2,3-unsaturated pyranosides prefer half chair ured but permanently locked in a HO conformation
5
1
conformations OH5 and HO, namely.[9] The H NMR with an axial phosphinite. Ligand rhamno-enoPhos
O
data for free 1a (4,5-trans) prove a H5 conformation (ps-ent-1b) is 4,5-trans configured but has no trityloxy
(3J4,5 =9.5 Hz, diaxial H-4 and H-5) with equatorial 4- group. Syntheses of the new ligands started from gly-
phosphinite and 5-trityloxymethyl groups.[6a] In free cals 2,[10] 3 and 4[11] (Scheme 2) which are accessible
ps-ent-1a (4,5-cis) either the trityloxymethyl residue from l-fucose and l-rhamnose (2, 4) or commercially
or the phosphinite is in an unfavourable axial posi- available (3). Ligands 1b and ps-ent-1b were prepared
tion, thus neither OH5 nor HO is specially favoured. via Ferrier rearrangement, deacetylation and phos-
5
Bidentate coordination to Rh(I) requires proximity of phinite introduction, and for the synthesis of bicyclic
olefin and phosphinite, and we reasoned that only 1c, glucal 3 was subjected to 6-O deacetylation,[12] in-
conformers with axial phosphinites will effectively tramolecular Ferrier rearrangement[13] and phos-
5
bind to Rh(I). Therefore, we expected a HO confor-
ACHTUNGTRENNUNG
mation for 1a in its rhodium complex with both trity-
loxymethyl group and phosphinite forced into unfav- Phos (ps-ent-1b) and its rhodium complex gave direct
ourable axial positions (Scheme 1). Ligand ps-ent-1a evidence for a major conformational change upon
with no significant conformational preference in its metal binding. After addition of a Rh(I) salt, ps-ent-
free state can be expected to adopt a OH5 conforma- 1b showed all characteristics of bidentate coordina-
tion in its Rh(I) complex with the bulky trityloxy- tion: the H-2/H-3 multiplet experienced an upfield
methyl residue in a favourable equatorial position shift in the 1H NMR, while the 31P NMR showed
(Scheme 1). These putative differences in the coordi- a downfield shift for the phosphinite signal, which
nation mode may result in reduced complex stability was split into
a
doublet by 31P, 103Rh coupling
and efficiency for 1a and 1aa. Bidentate coordination (Table 1). This is in accordance with our previous fin-
of 1a was unequivocally established in 1H and dings[6a] and Rh(I) complexes of other ligands.[14] The
31P NMR studies of its Rh(I) complex,[6a] but overlap- H-5 signal hardly shifted upon complexation, but the
1
ping signals in the H NMR spectrum prevented eluci- vicinal H-5/H-4 coupling changed drastically. Free ps-
5
dation of the pyranose conformation, and no single ent-1b adopts a HO conformation with equatorial ori-
crystals of the complex could be obtained.
entation of phosphinite and methyl group (3J4,5
=
In order investigate the interplay of relative pyra- 7.6 Hz), but binding to Rh(I) induced a ring flip to
O
noside configuration and conformation of our ligands the H5 conformer with diaxial orientation of the sub-
in Rh(I) complexes, we prepared the new olefin li- stituents (3J4,5 <1.0 Hz) (Table 1). This finding verifies
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pseudo Enantiomeric Carbohydrate Olefin Ligands – Case Study
Table 1. NMR studies on ps-ent-1b and its Rh(I) complex.
that complexation will trigger a ring flip to conform-
ers with axial phosphinites, even if this forces other
bulky residues into axial positions.
Next, we evaluated the new ligands in 1,4-addition
reactions and compared the results to those obtained
with gluco- and galacto-enoPhos[7] (Table 2). In the
benchmark reaction with cyclohexenone (5a) and
phenylboronic acid (6a), 1b, 1c and ps-ent-1b gave ex-
cellent ee values (entries 1, 2, and 4) just like 1a and
ps-ent-1a (entries 3 and 5). Therefore, the bulky trityl-
ps-ent-1b
d [ppm] J [Hz]
[Rh(ps-ent-1b)ACTHUNGETRNNU(G acac)]
d [ppm] J [Hz]
AHCTUNGTREGoNNUN xy residue of 1a, 1aa and ps-ent-1a, is not a prerequi-
H-2[a] 5.75–
5.86
n.d.
4.46–
4.55
4.08–
4.15
n.d.
n.d.
site for efficient asymmetric induction.[15] The direc-
tion of asymmetric induction is determined by the ori-
entation of the 4-phosphinite relative to the plane of
the pyranoside. Ligands with a phosphinite oriented
below the pyranoside plane (fuco-ligand 1b, gluco-li-
gands 1c and 1a) produced R-7aa (entries 1–3),
rhamno- and galacto-ligands ps-ent-1b and ps-ent-1a
with a phosphinite above the pyranoside plane yield-
ed S-7aa (entries 4 and 5).
H-3[a]
H-5[a] 3.59
(qd)
3J4,5 =7.6,
3J5,6 =6.2
–
3.77 (q) 3J4,5 <1.0,
3J5,6 =6.2
Ph2P[b] 111.56
(s)
157.03
(d)
1JP, Rh =194.9
[a]
[b]
1
From H NMR (400 MHz).
From 31P NMR (162 MHz).
Table 2. 1,4-Additions of boronic acids to enones in the presence of 1a–c and ps-ent-1a, 1b.
Entry
Ligand
Enone 5
R2 in 6
Adduct 7
No.
Yield [%][a]
ee [%][b]
1
1b
1c
1a
ps-ent-1b
ps-ent-1a
1b
1c
1aa
92
80
80
88
92
96
78
94
80
96
98
54
88
90
76
37
49
77
97 R
98 R
99 R
98 S
99 S
95 R
87 R
60 R
2
3[c]
4
6a
6b
7aa
5[c]
6
7
5a
8[c]
9
7ab
ps-ent-1b
ps-ent-1a
1b
66 S
93 S
10[c]
11
12[c]
13
14[c]
15
16[c]
17
18[c]
86 (À)
78 (À)
80 (+)
96 (+)
92 (+)
89 (+)
90 (À)
94 (À)
92 R
79 R
93 S
92 S
1a
6c
7ac
ps-ent-1b
ps-ent-1a
1b
1a
ps-ent-1b
ps-ent-1a
6d
7bd
5b
19
1b
1a
ps-ent-1b
ps-ent-1a
63
34
30
66
20[c]
21
6e
7be
22[c]
[a]
Isolated yield.
[b]
[c]
Determined by GC.
Results from ref.[7]
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Table 3. 1,4-Addition of boronic acid 6a to enone 8.
Entry
Ligand
Rh(I) salt
Adduct 9
Yield [%][a]
cis/trans[b]
1[c]
2
–
1b
1a
ps-ent-1b
ps-ent-1a
[Rh
A
G
93
92
78
79
86
20:80
95:5
89:11
7:93
3[c]
4
5[c]
6:94
[a]
Combined yield of cis- and trans-9.
[b]
[c]
1
Determined by H NMR.
Results from ref.[7]
Striking differences were seen for sterically encum- material giving trans- and cis-configured products in
bered 2-methoxyphenylboronic acid (6b), for which moderate to high ee. We started our studies with
gluco-ligand 1aa (4,5-trans) had led to 60% ee. The enantiopure enone 8,[7,19] (Table 3). Under substrate
fuco-ligand 1b (4,5-cis) gave 95% ee, while 4,5-trans control, the addition of boronic acid 6a predominantly
configured but conformationally locked gluco-ligand yielded trans-9 (entry 1). The fuco-ligand 1b as well as
1c led to a smaller yet significant improvement (87% gluco-ligand 1a favour formation of cis-9, leading to
ee) (entries 6–8). On the other hand, rhamno-ligand a mismatched combination of substrate and catalyst
ps-ent-1b (4,5-trans) turned out to be inferior (66% control. Both ligands override substrate control but
ee) to 4,5-cis configured galacto-ligand ps-ent-1a (93% fuco-ligand 1b proved yet again to be superior by sig-
ee) (entries 9 and 10). Similar trends were observed nificantly improving the cis/trans ratio (entries 2 and
with alkenylboronic acid 6c (entries 11–14). 1,4-Addi- 3). Reaction in the presence of ps-ent-1a and ps-ent-
tion of boronic acids 6d and 6e to cyclopentenone 5b 1b constitutes a matched combination of reagent and
in the presence of 1a had resulted in modest yields catalyst control. While the cis/trans ratio of 9 was
(entries 16 and 20). These were improved using fuco- almost identical for both ligands, galacto-enoPhos (ps-
ligand 1b (entries 15 and 19), while rhamno-ligand ps- ent-1a) gave a higher yield (entries 4 and 5).
ent-1b was inferior to ps-ent-1a (entries 17 and 18 as
well as 21 and 22).
Next, we investigated O-TBS-protected 4-hydroxy-
cyclopentenone (10), an attractive material in sustain-
These results clearly show that fuco- and galacto-li- able chemistry, prepared from furfuryl alcohol,[20]
gands 1b and ps-ent-1a with 4,5-cis pyranoside scaf- which is accessible from hemicellulose of agricultural
folds are superior to 4,5-trans configured gluco- and wastes.[21] Non-racemic 10, an important building
rhamno-ligands 1a, 1aa and ps-ent-1b. The example of block in the synthesis of many bioactive com-
bicyclic 1c highlights the complex interplay of pyrano- pounds,[22] is mainly prepared from cis-3,5-dihydroxy-
side configuration and ligand conformation affecting cyclopentene via enzymatic desymmetrisation in six
asymmetric induction. While 4,5-trans configured 1a steps.[23,24] Methods for kinetic resolution of rac-10 by
and ps-ent-1b have to undergo conformational enzymatic and Pd-catalysed processes have recently
changes upon complexation, 4,5-trans configured 1c is received attention,[25] but 1,4-addition has as yet not
locked in a favourable conformation for Rh(I) bind- been explored for this purpose.[26]
ing. This results in improved stereoselectivity com-
pared to 1a and ps-ent-1b. Yet 1c is inferior to 4,5-cis marised in Table 4. Reaction with boronic acid 6a in
configured 1b and ps-ent-1a. the presence of an achiral Rh catalyst and CsF as the
The results on kinetic resolution of rac-10 are sum-
With improved pseudo enantiomers 1b and ps-ent- base yielded exclusively known trans-configured com-
1a, we turned to 1,4-additions involving chiral enones. pound 11.[27] After reacting rac-10 with 0.6 equiv. of
These may either yield diastereomeric products, with 6a in the presence of 1a, the remaining R-10 was iso-
stereoselectivity depending on the relative efficiency lated in excellent ee and 36% yield, while ps-ent-1a
of substrate and catalyst control or result in kinetic yielded S-10 in modest ee (entries 1 and 2). Apart
resolution[16] of racemic enones. Previous studies with from addition products 11, we observed 12, formed
racemic 6-alkyl[17] and 5-TMS[18] cyclohexenones by via a Heck-type side reaction.[28] Using 0.4 equiv. of
others had led to complete conversion of the starting 6a, ligands 1a, and ps-ent-1a gave products S,R-11 and
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pseudo Enantiomeric Carbohydrate Olefin Ligands – Case Study
Table 4. Kinetic resolution of rac-10 by 1,4-addition.
Ligand
Base
Boronic Acid 6a
[equiv.]
Enone 10
Yield [%]
Adduct 11
Yield [%]
s[e]
G
ee[a]
ee[a]
1[b]
2[b]
3[b]
4[b]
5[c]
6[c]
7[c]
8[c]
1a
ps-ent-1a
1a
ps-ent-1a
1b
ps-ent-1a
1b
CsF[d]
CsF[d]
CsF[d]
CsF[d]
Et3N
Et3N
Et3N
Et3N
0.6
0.6
0.4
0.4
0.6
0.6
0.4
0.4
36
37
26
79
40
40
42
40
98 R
38 S
n.d.
39
20
29
19
52
55
39
38
n.d.
n.d.
–
–
–
–
31
44
57
57
88 S,R
92 R,S
72 S,R
79 R,S
86 S,R
92 R,S
n.d.
99 R
99 S
86 R
78 S
ps-ent-1a
[a]
[b]
[c]
[d]
[e]
Determined by GC.
Yields after separation of 10 and 11.
1
Yields determined from the 10/11 ratio in the H NMR spectrum of the product mixture.
12 in 4–20% yield.
Calculation of s value according to ref.[29]
R,S-11 respectively in good ee but unsatisfactory Experimental Section
yields (entries 3 and 4). To overcome low yields and
by-product formation, we re-investigated the reaction Synthesis of Enantioenriched R-10
conditions and substituted ligand 1b for 1a. To our de-
In a glove box [RhACHTNUTRGNE(UNG C2H4)2Cl]2 (2.7 mg, 7.1 mmol) and fuco-
light, Et3N as a base improved yield and ee and sup-
pressed formation of 12. A yield of 40% and 99% ee
for R-10 and S-10 were achieved using 1b and ps-ent-
1a with 0.6 equiv. of 6a (entries 5 and 6), while de-
creasing the amount of 6a to 0.4 equiv. yielded addi-
tion products S,R-11 and R,S-11 in fair ee (entries 7
and 8).
In conclusion, we have elucidated the interplay of
pyranoside configuration and conformation in Rh(I)
complexes of carbohydrate olefin ligands and its con-
sequences for asymmetric induction in 1,4-additions.
During our studies, we have identified fuco-ligand 1b
as an improved alternative to 1a, leading to substan-
tially increased ee values. The pseudo enantiomeric
ligand pair 1b and ps-ent-1a now gives consistently
high ee values for both achiral enones and bulky bor-
onic acids and chiral enones. Furthermore, we suc-
cessfully employed our ligands in the kinetic resolu-
tion of a racemic O-TBS-protected 4-hydroxycyclo-
pentenone, allowing the recovery of the starting mate-
rial in excellent ee.
enoPhos (1b) (4.6 mg, 15.5 mmol) were placed into a flame-
dried Schlenk flask. Under a nitrogen atmosphere, degassed
1,4-dioxane (1 mL) was added and the solution was stirred
for 15 min at room temperature. An aqueous solution of
Et3N (65 mL, 471 mmol in 0.5 mL degassed H2O) was added
and the mixture was stirred for additional 15 min. Enone
rac-10 (100 mg, 471 mmol, 1.0 equiv.) was added and the re-
action mixture was stirred for additional 15 min. Next phe-
nylboronic acid 6a (34.5 mg, 283 mmol, 0.6 equiv.) and de-
gassed 1,4-dioxane (1 mL) were added and the resulting
mixture was stirred for 2 h at room temperature. After com-
pletion of the reaction Et2O (10 mL) and H2O (10 mL) were
added and the aqueous layer was extracted with Et2O (2ꢁ,
15 mL). The organic phase was dried over Na2SO4, the sol-
vent was removed under reduced pressure and the residue
was purified by flash chromatography on silica gel (PE/
EtOAc, 10:1). The products (113 mg) were isolated in one
fraction, comprising enantioenriched enone R-10 and 1,4-ad-
dition product S,R-11. The product ratio was determined
from the 1H NMR spectrum: R-10/S,R-11=43:57, R-10
(188 mmol, 40% yield; S,R-11: 248 mmol, 52% yield). Enan-
tiomeric excesses were determined from the product mix-
ture in a single run: R-10 99% ee; S,R-11 72% ee.
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[13] H. B. Mereyala, K. C. Venkataramanaiah, V. S. Dalvoy,
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Acknowledgements
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Financial support by the DFG (BO 1938/5-1), Rodos BioTar-
get GmbH and a Heisenberg Fellowship for MMKB (BO
1938/4-1) is gratefully acknowledged. We thank Prof. Oliver
Reiser, University of Regensburg, Germany, for donation of
the precursor for rac-10.
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[1ÀC
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