Ferrocenyliminophosphites as easy-to-modify ligands for asymmetric catalysis

Article abstract of DOI:10.1002/ejoc.200700396

Several N,P bidentate phosphite-type ligands derived from readily available ferrocene-based iminoalcohols were successfully used in Rh-catalysed hydrogenations and Pd-catalysed allylic substitutions of a variety of substrates. Moderate-to-high catalytic a

Full text of DOI:10.1002/ejoc.200700396

FULL PAPER
DOI: 10.1002/ejoc.200700396
Ferrocenyliminophosphites as Easy-to-Modify Ligands for Asymmetric
Catalysis
[
a]
[a]
[a]
Konstantin N. Gavrilov,* Marina G. Maksimova, Sergey V. Zheglov,
[b]
[c]
[c]
[c]
Oleg G. Bondarev, Eduard B. Benetsky, Sergey E. Lyubimov, Pavel V. Petrovskii,
Anzhelika A. Kabro, Evamarie Hey-Hawkins, Sergey K. Moiseev, Valery N. Kalinin,
and Vadim A. Davankov[
[c]
[d]
[c]
[c]
c]
Keywords: Asymmetric catalysis / N,P ligands / Hydrogenation / Allylation
Several N,P bidentate phosphite-type ligands derived from
readily available ferrocene-based iminoalcohols were suc-
cessfully used in Rh-catalysed hydrogenations and Pd-cata-
lysed allylic substitutions of a variety of substrates. Moder-
ate-to-high catalytic activities under standard conditions
were observed, and the enantiomeric excess of the products
were up to 97%. Results obtained under systematic variation
of the ligand parameters indicate that the enantioselectivity
is largely determined by the nature of the phosphocentre and
also by the substituent in the C*HN-fragment.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2007)
Introduction
nitrogen), which allows one to modify accurately the chemi-
cal stability of the ligand, its donor–acceptor capability and
steric requirements. Most phosphites can be synthesised
rather simply and in high yield from a variety of optically
active precursors. Usually, it is possible to perform direct
one-pot phosphorylation of suitable chiral compounds,
whereas the synthesis of the corresponding phosphane de-
rivatives involves several steps. In addition, phosphites exhi-
bit higher oxidative stability because of the absence of P–C
bonds. In many cases, protocols for a catalytic process can
be developed that do not involve the use of a glove box,
including the ligand synthesis.
As for the variety of catalytic reactions, groups of ligands
that can be equally used in both Pd-catalysed asymmetric
allylic substitutions and Rh-catalysed asymmetric hydro-
genations are of special interest. Indeed, allylic substitution
is a versatile and widely used process in organic synthesis
that can result in the enantioselective formation of carbon–
carbon and carbon–heteroatom bonds; similarly, hydrogen-
ation is a highly attractive strategy for the synthesis of op-
tically active organic molecules that are of enormous aca-
demic and/or industrial interest. The search for multipur-
pose ligands has been especially fruitful among chiral bi-
The development of new groups of chiral ligands that
can provide high activity and enantioselectivity in a wide
range of asymmetric catalytic reactions remains a challenge
of high importance. The minimum requirements for such a
group of ligands include the following: broad reaction and
substrate scope, broad variability in both steric and elec-
tronic parameters of the ligands and the direct and simple
synthesis of ligands from inexpensive and readily available
starting materials in a one-pot synthesis.[1] A series of op-
tically active phosphites completely satisfies these criteria.
In general, the most important advantages of chiral phos-
phites include their pronounced π-acidity, their oxidation
stability, as well as their synthetic availability and low
[
2]
cost. Indeed, the induction ability of a common PPh2
fragment of phosphane-type systems can be modulated
only through the introduction of an electron-donating or
electron-withdrawing substituent into available positions of
the phenyl ring. As for the phosphites, they provide the pos-
sibility of using a much more efficient method, that is, the
replacement of carbon atoms in the first coordination
sphere of the phosphorus by heteroatoms (oxygen and/or
functional compounds incorporating P and N donor
[
a] Department of Chemistry, Ryazan State University,
[3]
groups, in particular, oxazolinophosphites.
In fact,
4
6 Svoboda str., Ryazan 390000, Russia
TADDOL- and diamine-based oxazolinophosphites proved
to be efficient in both Pd-catalysed allylations and Ir-cata-
Fax: +7-4912-775-498
E-mail: chem@rspu.ryazan.ru
[
[
b] Max-Planck-Institut für Kohlenforschung,
Kaiser-Wilhelm-Platz 1, 45470 Mülheim an der Ruhr, Germany
c] Institute of Organoelement Compounds, Russian Academy of
Sciences,
[4]
lysed hydrogenations.
Meanwhile, phosphites with a peripheral imino group
can be supposed to present another attractive alternative.
Like oxazolinophosphites, they contain a chiral unit with
28 Vavilova str., Moscow 119991, Russia
[d] Institut für Anorganische Chemie,
2
Johannisallee 29, 04103 Leipzig, Germany
the sp -hybridised donor nitrogen atom, but, unlike hy-
4940
© 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2007, 4940–4947

Ferrocenyliminophosphites as Ligands for Asymmetric Catalysis
droxyoxazolines, they can be produced from a more diverse several prochiral olefins and Pd-, Rh- and Ir-catalysed al-
and accessible class of synthons, namely, iminoalcohols. lylic substitution of mono- and disubstituted substrates
Among the iminophosphites, of special interest are those with different nucleophiles. The highly modular construc-
bearing a bulky ferrocenylidene substituent on the side arm. tion of the ferrocenyliminophosphites allows us to study its
The ferrocene nucleus makes the ligands more stable, struc- catalytic activity and selectivity on the basis of the contri-
turally rigid and easy to modify.[ In recent years, we have butions from the three main ligand components: (1) the ef-
designed a wide variety of chelating ferrocenyliminophos- fect of the configuration of the binaphthyl groups, (2) the
phites (Figure 1). Optimisation of their structures has pro- effect of substituents at the asymmetric carbon atom in the
duced good and excellent results in some Pd-catalysed al- ligand bridge and (3) the effect of substituents in the biaryl
lylic transformations.[ For example, the ligands afforded and phenyl groups. By carefully selecting these elements, we
up to 82% ee in the alkylation of ethyl 3-penten-2-yl car- achieved high enantioselectivities for several catalytic reac-
bonate (so-called “unmanageable” substrate) and up to tions.
5]
6]
97% ee in the alkylation of the well-known substrate 1,3-
diphenyl-2-propenyl acetate.
Results and Discussion
In addition to the previously reported (Sa)-1 and (Ra)-
6a]
,[ new ligands were prepared to enlarge the palette of
1
the ligand family (Scheme 1).
Direct phosphorylation of iminoalcohol 6 with reactant
5
(
readily provided ferrocenyliminophosphites (Sa)-7 and
Ra)-7. These compounds are orange-red powders and can
be exposed to the air for several months without any change
1
31
to their H or P NMR spectra or any loss in their catalytic
activity and enantioselectivity. It should be noted that com-
pound 6, similar to other iminoalcohols, exists in organic
solvents in equilibrium with its corresponding oxazolidine
[
6e,7]
tautomer.
Nevertheless, the phosphorylation reaction
proceeds selectively and no phospha derivatives of oxazolid-
ine were detected in the reaction products.
With the intent to use ligands (Sa)-7 and (Ra)-7 in cata-
lytic allylic substitution and hydrogenation reactions, we
first studied their interaction with the appropriate catalyst
precursors (Scheme 2).
As previously reported in the case of ligand (Ra)-1,[6a]
the reactions between [Pd(allyl)Cl]2 and stoichiometric
amounts of chiral iminophosphites (Sa)-7 or (Ra)-7 in the
Figure 1. Some efficient N,P-bidentate chiral ferrocenyliminophos-
phites.
In this paper, we describe in detail the application of presence of a chloride scavenger such as AgBF provided
4
these and some new ligands of the same type in other corresponding cationic complexes (Sa)-8 and (Ra)-8. Dupli-
benchmark tests, namely, Rh-catalysed hydrogenation of cation of peaks in the 31P NMR spectra of compounds
Scheme 1.
Scheme 2.
Eur. J. Org. Chem. 2007, 4940–4947
© 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
4941

K. N. Gavrilov et al.
FULL PAPER
(
Sa)-8 and (Ra)-8 [δ = 146.3 (28%) and 145.2 (72%); 141.8 stereocentre, which results in a matched combination for
P
(
36%) and 140.5 (64%) ppm, respectively, in CDCl ] indi- iminophosphites (Ra)-1 and (Ra)-7. Ligand (Ra)-7 with the
3
[6]
cates the presence of their exo and endo isomers. Similarly, tert-butyl substituent provided higher yields and ee’s of the
in the rhodium cationic complex (Ra)-9, iminophosphite product than its isomer (Ra)-1 (Table 1, Entries 2, 3 and 6,
(
Ra)-7 acts as a cis-chelating N,P ligand (Scheme 2). Thus, 7). Variation of the ligand-to-rhodium ratio from 1 to 2
31
the P NMR spectrum of (Ra)-9 (in CDCl ) shows a doub- shows that no excess of the ligand is needed to reach good
3
1
let at δ = 136.4 ppm with J
= 262.3 Hz. As shown by activities and enantioselectivities of the catalyst.
P
P,Rh
1
3C NMR spectroscopy (see Experimental Section), the 1,5-
cyclooctadiene ligand is a part of complex (Ra)-9. Note- Table 1. Rh-catalysed hydrogenation of α-dehydrocarboxylic acid
2 2 2
esters (CH Cl , 1.3 bar H , 20 °C, 20 h, molar ratio of substrate/
metal, 1000/1).
worthy is the asymmetric arrangement of this ligand, which
manifests itself in the fact that each carbon atom exhibits
its own signal. This seems to be due to the influence of the
bulky organophosphorus ligand, which causes a distortion
Entry Ligand
Substrate
L/Rh
Conv. [%]
ee [%]
1
2
3
4
5
6
7
8
9
1
(Sa)-1
(Ra)-1
(Ra)-1
(Sa)-7
(Sa)-7
(Ra)-7
(Ra)-7
3b
10
10
10
10
10
10
10
10
10
12
12
12
12
12
12
12
12
12
14
14
14
14
14
14
14
1
1
2
1
2
1
2
1
2
1
1
2
1
2
1
2
1
2
1
1
2
1
2
1
2
93
25
5
19
17
67
22
11
9
40
25
0
20
5
65 (S)
80 (R)
12 (R)
66 (S)
64 (S)
88 (R)
80 (R)
3 (R)
18 (R)
79 (R)
95 (S)
–
49 (R)
2 (R)
97 (S)
94 (S)
5 (R)
[
6f]
of the geometry of the metal complex. Comparison of
the 13C NMR spectral parameters of the free and coordi-
nated (Ra)-7 revealed substantial downfield coordination
shifts of the signals for the CH=N carbon atom (∆δC
=
1
2.1 ppm). As in the case of complexes (Sa)-8 and (Ra)-8,
chelate formation was also inferred from the MALDI TOF/
TOF MS spectrum, which showed the expected [M –
BF ] peak.
With ferrocenyliminophosphites in hand, we then exam- 12
ined the Rh-catalysed asymmetric hydrogenation of some 13
3b
0
(Sa)-1
(Ra)-1
(Ra)-1
(Sa)-7
(Sa)-7
(Ra)-7
(Ra)-7
3b
–
+
4
11
14
15
16
17
18
19
20
21
22
23
24
25
α-dehydrocarboxylic acid esters as standard substrates
Scheme 3).
81
31
5
(
3b
4
3 (R)
(Ra)-1
(Sa)-7
(Sa)-7
(Ra)-7
(Ra)-7
3b
62
17
12
85
95
5
86 (S)
63 (R)
50 (R)
92 (S)
92 (S)
5 (R)
3b
4
2 (R)
The same dependences were also observed in the Rh-
catalysed hydrogenation of methyl 2-acetamidoacrylate 12
Table 1, Entries 10–16). In this case, there is a matched
(
combination of the (R)-binaphthyl moieties in addition to
the (S)-C* stereocentre in the ligand backbone. The use of
iminophosphite (Ra)-7 showed an excellent optical yield
(97%, Table 1, Entry 15) for product 13.
Analogously, ligands with the (R)-binaphthyl moieties
Table 1, Entries 19–23) were found to be the best stereoin-
Scheme 3.
(
We first examined the transformation of itaconate 10 to ductors in the Rh-catalysed hydrogenation of methyl (Z)-
succinate 11. Ligands (Sa)-1, (Ra)-1 and (Sa)-7, (Ra)-7 con- 2-acetamido-3-phenylacrylate (14; Scheme 3). The highest
taining different enantiomerically pure binaphthyl moieties enantioselectivity, up to 92%, was shown by ligand (Ra)-7.
were tested in detail. The cationic rhodium complexes were Unfortunately, ferrocenyliminophosphite 3b without bi-
prepared in situ by mixing the well-known catalyst precur- naphthyl moieties produced the reaction products as an al-
sor [Rh(cod) ]BF with 1 or 2 equiv. of the ligand under an most racemic mixture of the enantiomers, along with very
2
4
atmosphere of argon in CH Cl . As shown in Table 1, the poor conversions of the starting materials.
2
2
configuration of the binaphthyl moieties determines the ab-
To further study the potential of ferrocenyliminophos-
solute configuration of product 11. Ligands 1 and 7 con- phites, we also tested them in the Pd-catalysed allylic alky-
taining (R)-binaphthyl moieties produced product (R)-11 lation of the monosubstituted substrate 1-(4-chlorophenyl)-
with enantioselectivity up to 88%, whereas the same ligands allyl methyl carbonate (16) with dimethyl malonate
containing (S)-binaphthyls produced product (S)-11 in (Scheme 4).
lower enantioselectivity (not higher than 66%). These re-
For such substrates, as well as for controlling the
sults indicate a cooperative effect between the configuration enantioselectivity of the process, the regioselectivity is also
of the binaphthyl moieties and the configuration of the C* of concern, because a mixture of regioisomers can be
4942
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© 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2007, 4940–4947

Ferrocenyliminophosphites as Ligands for Asymmetric Catalysis
Scheme 4.
formed. Most palladium catalysts developed to date favour Table 2. Pd, Rh and Ir-catalysed allylic alkylation of 16 with di-
methyl malonate (CH
substrate/metal, 25).
2
Cl
2
, BSA, KOAc, 20 °C, 48 h, molar ratio of
the formation of the achiral linear products rather than the
desired branched isomers. The development of highly regio-
and enantioselective palladium catalysts is still a chal- Entry Catalyst
L/M Isolated yield of 17/18 ee 17
[
8]
17 and 18 [%]
[%]
lenge. Our results are summarised in Table 2. The effi-
ciency of ferrocenyliminophosphites with binaphthyl moie-
ties in Pd-catalysed allylic alkylation differs from that for
Rh-catalysed hydrogenation. So, the Pd catalysts prepared
from the novel ligands (Sa)-7 and (Ra)-7 exhibited moder-
ate-to-good activity but only moderate regio- and enantio-
selectivity (up to 58%; Table 2, Entry 4), and (Sa)-7 was
more enantioselective. Cationic complex [Pd(allyl){(Sa)-1}]-
1
[Pd(allyl){(Sa)-1}]BF
[Pd(allyl){(Sa)-1}]BF
[Pd(allyl){(Ra)-1}]BF
[Pd(allyl)Cl]
[Pd(allyl)Cl]
(Sa)-8
[Pd(allyl)Cl]
[Pd(allyl)Cl]
(Ra)-8
[Rh(cod)Cl]
[Rh(cod)Cl]
(Ra)-9
[Ir(cod)Cl]
[Ir(cod)Cl]
4
1
1
1
1
2
1
1
2
1
1
2
1
1
2
1
2
1
2
1
1
2
1
96
69
92
62
85
71
92
57
91
15
34
10
52
88
92
84
37
78
86
69
78
62
21:79 75 (R)
17:83 4 (R)
56:44 47 (R)
32:68 58 (R)
25:75 19 (R)
26:74 53 (R)
52:48 34 (R)
44:56 18 (S)
44:56 43 (R)
[a]
2
3
4
5
6
7
8
9
1
1
4
4
2
/(Sa)-7
/(Sa)-7
2
2
/(Ra)-7
/(Ra)-7
2
BF with the sec-butyl substituent provided excellent chemi-
4
0
1
2
/(Ra)-7
/(Ra)-7
95:5
95:5
91:9
16 (R)
21 (R)
17 (R)
cal and good optical yields (96 and 75%, respectively;
Table 2, Entry 1), though with poor regioselectivity. Unlike 12
2
13
14
15
16
2
/(Ra)-7
/(Ra)-7
Ͼ99:1 51 (R)
99:1 45 (R)
this complex, its epimer, [Pd(allyl){(Ra)-1}]BF , provided
4
2
notably lower asymmetric induction (Table 2, Entry 1 vs. 3).
[Pd(allyl)Cl]
[Pd(allyl)Cl]
2
/2b
/2b
19:81 15 (R)
23:77 7 (R)
39:61 49 (R)
37:63 47 (R)
45:55 62 (R)
41:59 80 (R)
41:59 75 (R)
54:46 56 (R)
Excellent regioselectivity but low chemical and optical
2
yields were found in the Rh-catalysed allylic alkylation of 17
2
[Pd(allyl)Cl] /3a
[Pd(allyl)Cl]
[Pd(allyl)(3a)]BF
[Pd(allyl)Cl]
[Pd(allyl)Cl]
1
6 with the use of (Ra)-7 (Table 2, Entries 10–12). The Ir- 18
2
/3a
19
20
21
22
4
4
catalysed reactions led to the formation of isomer 17 with
a moderate enantioselectivity (up to 51%; Table 2, En-
tries 13 and 14).
2
/3b
/3b
2
[Pd(allyl)(3b)]BF
Iminophosphites 3a,b with 2,6-disubstituted phenyl moi-
eties were found to be more efficient in Pd-catalysed reac-
tions, and they showed good activity and moderate regiose-
[a] In THF.
lectivity. Enantioselectivity is directly governed by the sub- the cationic complex [Pd(allyl)(4a)]BF produced regiospe-
4
stituents at the ortho positions. Thus, 3a gave rise to ee’s up cific formation of linear isomer 18 in 90% yield. So, by vary-
to 62% and 3b up to 80% (Table 2, Entries 19 and 20). The ing the structure of the phosphocentre in ferrocenylimino-
nature of the counterion in the palladium complex also ex- phosphites the regioselectivity of the Pd-catalysed allylic al-
erts a substantial effect, and in fact, for 3a and 3b it is kylation of monosubstituted substrate 16 can be controlled.
opposite (Table 2, Entries 17 and 19, 20 and 22). Unfortu-
In general, an 80% optical yield of the product in the Pd-
nately, the presence of the biphenyl moieties in ferrocenyli- catalysed allylic alkylation of 1-(4-chlorophenyl)allyl methyl
minophosphite 2b negatively effected both the regio- and carbonate (16) is rather high. Indeed, only poor stereoselec-
enantioselectivity (Table 2, Entries 15 and 16). Interestingly, tivity was found in the Pd-catalysed allylic alkylation of 1-
Scheme 5.
Eur. J. Org. Chem. 2007, 4940–4947
© 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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4943

K. N. Gavrilov et al.
FULL PAPER
Table 3. Pd-catalysed allylic sulfonylation of 19 with pTolSO
2
Na (4-substituted phenyl)allyl acetates with the use of ox-
azolinophosphite ligands for substrates with the electron-
(
THF, 20 °C, 48 h, molar ratio of substrate/metal, 25), allylic alky-
2 2
lation of 19 with dimethyl malonate (CH Cl , BSA, KOAc, 20 °C,
withdrawing substituents in the para position of the phenyl
4
8 h, molar ratio of substrate/metal, 25) and amination of 19 with
[
9]
ring.
pyrrolidine (CH
2
Cl
2
, 20 °C, 48 h, molar ratio of substrate/metal, 25).
We also investigated the Pd-catalysed allylic substitution
of 1,3-diphenyl-2-propenyl acetate (19), which is widely
used as a model substrate, with S-, C- and N-containing
nucleophiles by using the chiral binol-based ferrocenyl-
iminophosphites (Scheme 5).
Moderate enantioselectivity was achieved in all cases (up
to 50–60%, Table 3). Ligand (Ra)-1 was found to be the
Allylic sulfonylation
Conv. [%][a]
Entry Catalyst
L/Pd
ee [%]
41[
a]
5 (S)
5 (S)
2 (S)
39 (S)
40 (S)
55 (S)
6 (S)
1
2
3
4
5
6
7
[Pd(allyl)Cl]
[Pd(allyl)Cl]
2
/(Sa)-1
/(Sa)-1
1
2
1
1
2
1
1
50[
a]
a]
a]
a]
a]
a]
2
[
[Pd(allyl){(Sa)-1}]BF
4
45
[Pd(allyl)Cl]
[Pd(allyl)Cl]
2
/(Ra)-1
/(Ra)-1
35[
53[
2
[
[Pd(allyl){(Ra)-1}]BF
(Sa)-8
4
72
12[
most efficient in the allylic sulfonylation with p-TolSO Na,
2
whereas ligand (Sa)-1 was the most efficient in both the
alkylation with dimethyl malonate and the amination with
pyrrolidine (Table 3, Entries 6, 8 and 22). In turn, ligands
Allylic alkylation
8
9
1
1
1
1
1
1
1
1
1
1
[Pd(allyl)Cl]
[Pd(allyl)Cl]
2
/(Sa)-1
/(Sa)-1
1
2
1
1
2
1
1
2
1
1
2
1
45
5
50 (S)
0
40 (R)
19 (R)
0
36 (R)
29 (R)
36 (R)
3 (S)
20 (S)
5 (S)
8 (S)
2
(Sa)-7 and (Ra)-7 are somewhat less efficient. In general,
0
1
2
3
4
5
6
7
8
9
[Pd(allyl){(Sa)-1}]BF
4
73
87
70
70
98
95
97
99
79
42
[Pd(allyl)Cl]
[Pd(allyl)Cl]
2
/(Ra)-1
/(Ra)-1
ferrocenyliminophosphites with binaphthyl moieties were
found to be less efficient than 2a–c with the biphenyl back-
bone. So, the amination with pyrrolidine with 2b gave ee
values up to 73% (Table 3, Entry 30), and the alkylation
2
[Pd(allyl){(Ra)-1}]BF
4
[Pd(allyl)Cl]
[Pd(allyl)Cl]
(Sa)-8
2
/(Sa)-7
/(Sa)-7
2
with dimethyl malonate with 2a–c produced ee values up to
[Pd(allyl)Cl]
2
/(Ra)-7
/(Ra)-7
[6c,6d]
88%.
Acyclic iminophosphites 3a,b and 4a,b were
[Pd(allyl)Cl]
2
found to be even more efficient; they gave rise to almost
quantitative chemical and optical yields in the enantioselec-
tive Pd-catalysed C*–C and C*–S bond formation reactions
(Ra)-8
Allylic amination
20
21
22
23
24
25
26
27
28
29
30
[Pd(allyl)Cl]
[Pd(allyl)Cl]
2
/(Sa)-1
/(Sa)-1
1
2
1
1
2
1
1
2
1
2
1
60
48
90
90
50
37
40
70
46
96
94
11 (R)
10 (R)
60 (R)
4 (R)
0
[6e,6f]
with substrate 19.
2
[Pd(allyl){(Sa)-1}]BF
4
[Pd(allyl)Cl]
[Pd(allyl)Cl]
2
/(Ra)-1
/(Ra)-1
[Pd(allyl){(Ra)-1}]BF
2
Conclusions
4
0
[Pd(allyl)Cl]
[Pd(allyl)Cl]
[Pd(allyl)Cl]
[Pd(allyl)Cl]
2
/(Sa)-7
/(Sa)-7
/(Ra)-7
/(Ra)-7
20 (R)
20 (R)
17 (R)
26 (R)
73 (R)
Ferrocenyliminophosphites provided good results in dif-
ferent types of catalytic reactions (Figure 2). Ligands with
binaphthyl moieties are obvious leaders in the Rh-catalysed
asymmetric hydrogenation of α-dehydrocarboxylic acid es-
ters. In the Pd-catalysed allylic alkylation of the monosub-
2
2
2
[Pd(allyl)(2b)]BF
4
[
a] Isolated yield of product 20.
Figure 2. Summarised catalytic results for most widely investigated groups of the ferrocenyliminophosphites.
944 Eur. J. Org. Chem. 2007, 4940–4947
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Ferrocenyliminophosphites as Ligands for Asymmetric Catalysis
4
stituted substrate 1-(4-chlorophenyl)allyl methyl carbonate distilled from KOH and then over a small amount of LiAlH before
use. Ligands (Sa)-1, (Ra)-1,[ 2b 3a, 3b, 4a and their cat-
6a]
[6c]
[6b]
[6f]
[6e]
and the unhindered linear substrate ethyl 3-penten-2-yl car-
bonate the binol- and phenol-based acyclic iminophos-
phites produced very similar results. At the same time, in
the Pd-catalysed allylic alkylation of 1,3-diphenyl-2-pro-
penyl acetate the ligands with 2,6-disubstituted phenyl moi-
eties were found to be much better stereoselectors and the
best one was ligand 3a.[ However, in contrast to the alky-
ionic cis-chelate complexes [Pd(allyl)(L)]BF
4
were prepared as de-
scribed earlier. Phosphorylating reagent (Sa)- or (Ra)-2-chloro-di-
naphtho[2,1-d:1Ј,2Ј-f][1,3,2]dioxaphosphepine (5)[
16]
and imino-
alcohol (2S)-2-(ferrocenylideneamino)-3,3-dimethylbutan-1-ol (6)
[6e]
were synthesised by using literature procedures. [Rh(cod)
2
]-
were prepared as de-
scribed earlier. Cationic palladium complexes (Sa)-8 and (Ra)-8
[17]
[18]
[19]
BF
4
,
[Rh(cod)Cl]
2
and [Pd(allyl)Cl]
2
6f]
lation of 19, for 1-(4-chlorophenyl)allyl methyl carbonate were synthesised in a manner analogous to the known pro-
6a]
(
16) enantioselectivities are higher when more sterically de- cedures.[ Catalytic experiments: allylic sulfonylation of substrate
manding substituents in the ortho positions of the phenyl 19 with sodium para-toluenesulfinate, allylic alkylation with di-
methyl malonate and allylic amination with pyrrolidine were per-
rings are present (iPr Ͼ Me). In addition, the iminophos-
phites with a biphenyl backbone 2a–c were discovered to
formed according to the appropriate procedures.[
20,21]
be less efficient than their binaphthyl analogues in the Pd- (Z)-Methyl 2-acetamido-3-phenylacrylate (14) was prepared as re-
[22]
catalysed allylic alkylation of the monosubstituted and un- ported previously. Starting substrates 16 and 19 were synthesised
as published.[
23,19]
Dimethyl itaconate (10), methyl 2-acetamidoac-
hindered linear substrates and more efficient in the Pd-cata-
lysed allylation of bulky substrate 19.[6c,6d,6f]
rylate (12), dimethyl malonate, BSA [N,O–bis(trimethylsilyl)acet-
amide] and sodium para-toluenesulfinate were purchased from Ald-
rich and Acros Organics and used without further purification.
In summary, we described the successful application of
various ferrocenyliminophosphites in Rh-catalysed asym-
metric hydrogenation and Pd-catalysed asymmetric allylic Preparation of Ligands. General Technique: A solution of imino-
alcohol 6 (1.096 g, 3.5 mmol) in benzene (15 mL) was added drop-
wise at 0 °C to a vigorously stirred solution of chlorophosphite
substitution reactions of several substrate types. To the best
of our knowledge, this is the first example of iminophos-
(
3
Sa)- or (Ra)-5 (1.228 g, 3.5 mmol) and Et N (0.49 mL, 3.50 mmol)
phite-type ligands used in enantioselective Rh-catalysed hy-
drogenations. Ferrocenyliminophosphites have the advan-
tage of being easily prepared in a few steps from inexpensive
amino acids. In addition, they can easily be tuned so that
the effect of varying the phosphocentre, different substitu-
ents and configurations of the binaphthyl moieties on the
catalytic performance can be explored and adjusted. Good
results can be achieved in different catalytic reactions by a
careful selection of these structural components.
in benzene (15 mL). The resulting mixture was warmed to 80 °C
for a short time, then cooled down to 20 °C and filtered. The sol-
vent was removed in vacuo (40 Torr), and the residue was extracted
with boiling heptane (3ϫ20 mL), filtered, evaporated and dried in
vacuo (1 Torr, 2 h).
(
2ЈS,Sa)-2-[2Ј-(Ferrocenylideneamino)-3Ј,3Ј-dimethylbutoxy]dinaph-
tho[2,1-d:1Ј,2Ј-f](1,3,2)dioxaphosphepine [(Sa)-7]: Yield: 1.603 g,
1
7
3 %, orange-red solid. M.p. 87–89 °C. H NMR (400.13 MHz,
CDCl ): δ = 0.89 (s, 9 H, CH ), 2.91 (m, 1 H, NCH), 4.05 (m, 1
H, OCH ), 4.19 (m, 1 H, OCH ), 4.24 (s, 5 H, C ), 4.42 (br. s,
), 4.75 (t, J = 1.2 Hz, 2 H, C ), 7.38 (m, 8 H, CHAr),
.94 (m, 4 H, CHAr), 8.13 (s, 1 H, HC=N) ppm. C NMR
3
3
2
2
5 5
H
3
2
7
H, C
5
H
4
5 4
H
Experimental Section
General Remarks: 31P, 13C and 1H NMR spectra were recorded
1
3
2
(100.6 MHz, CDCl ): δ = 161.7 (s, C=N), 149.0 (d, J = 5.3 Hz,
3
with a Bruker AMX 400 instrument. Complete assignment of all
OCipso), 147.7, 132.3, 131.4, 130.7, 130.0, 129.8, 128.3, 128.0, 126.7,
the resonances in 13C NMR spectra was achieved by DEPT tech-
126.4, 126.2, 124.9, 124.3, 124.1, 122.7, 122.6, 122.1, 121.9, 121.5
1
3
niques. Chemical shifts (ppm) are given relative to Me
4
Si ( H and
(CAr), 81.4 (s, CFc–ipso), 80.4 (d, J = 2.9 Hz, NCH), 70.8, 69.2,
13
31
2
C) and 85% H
3
PO
4
( P NMR). Mass spectra were recorded with 68.4, 67.7 (all s, CFc), 68.7 (s, C ), 64.9 (d, J = 6.6 Hz, OCH ),
Cp
2
3
1
a Varian MAT 311 spectrometer (EI), a Finnigan LCQ Advantage
spectrometer (electrospray ionisation technique, ESI) and a Bruker
Daltonics Ultraflex spectrometer (MALDI TOF/TOF). Elemental
analyses were performed at the Laboratory of Microanalysis (Insti-
tute of Organoelement Compounds, Moscow).
3 3
33.3 (s, C), 27.0 (s, CH ) ppm. P NMR (162.0 MHz, CDCl ): δ
+
= 143.4 ppm. MS (70eV, EI): m/z (%) = 627 (4) [M] , 543 (100)
+
+
[C H FeNO P] , 268 (29) [C H FeN] . C H FeNO P (627.2):
3
1
22
3
15 18
37 34
3
calcd. C 70.82, H 5.46, N 2.23; found C 70.97, H 5.63, N 2.15.
2ЈS,Ra)-2-[2Ј-(Ferrocenylideneamino)-3Ј,3Ј-dimethylbutoxy]dinaph-
tho[2,1-d:1Ј,2Ј-f](1,3,2)dioxaphosphepine [(Ra)-7]: Yield: 1.735 g,
(
Conversion of substrates 10, 12 and 14 and optical yields of prod-
ucts 11, 13 and 15 were determined by using a methylsilicon col-
umn and chiral GC (column G-TA, column Hydrodex-β-TBDAc)
and chiral HPLC (column Chiralpak-AD) according to the litera-
1
7
9 %, orange-red solid. M.p. 92–94 °C. H NMR (400.13 MHz,
CDCl ): δ = 0.91 (s, 9 H, CH ), 2.88 (m, 1 H, NCH), 4.09 (m, 1
H, OCH ), 4.21 (s, 5H. C ), 4.23 (m, 1 H, OCH ), 4.40 (br. s, 2
H, C ), 4.74 (t, J = 1.2 Hz, 2 H, C ), 7.38 (m, 8 H, CHAr),
.97 (m, 4 H, CHAr), 8.11 (s, 1 H, HC=N) ppm. C NMR
3
3
2
5
H
5
2
[
10]
ture. In the allylic alkylation of substrate 16, the regioselectivity
3
5
H
4
5 4
H
1
(
17/18 ratio) was measured by H NMR and optical yields of prod-
13
7
(
uct 17 were determined by HPLC (Daicel Chiralcel OD-H column)
2
100.6 MHz, CDCl
OCipso), 147.5, 132.3, 131.6, 130.4, 129.9, 129.7, 128.3, 127.8, 126.7,
26.6, 126.0, 124.6, 124.3, 123.9, 122.8, 122.4, 122.1, 121.9, 121.7
3
): δ = 162.0 (s, C=N), 148.9 (d, J = 5.6 Hz,
[
11]
according to the literature.
Absolute configuration was deter-
mined by comparison with published data.[ Optical yields of
12]
1
(
6
3
product 20 were determined by using HPLC [(R,R)-WHELK-01
3
CAr), 81.6 (s, CFc–ipso), 79.5 (d, J = 2.8 Hz, NCH), 70.9, 69.2,
[
13]
column] according to the literature.
Conversion of substrate
2
8.6, 68.1 (all s, CFc), 68.9 (s, CCp), 65.7 (d, J = 6.5 Hz, OCH
2
),
): δ
142.5 ppm. MS (70eV, EI): m/z (%) = 627 (2) [M] , 543 (100)
[
14]
[14]
[15]
19
and the optical purities of products 21 and 22 were de-
31
3 3
3.2 (s, C), 27.2 (s, CH ) ppm. P NMR (162.0 MHz, CDCl
termined by using HPLC (Daicel Chiralcel OD-H column) as de-
scribed previously.
+
=
+
+
[
C
31
H
22FeNO
3
P] , 268 (25) [C15H18FeN] . MS (MALDI TOF/
TOF): m/z (%) = 628 (81) [M + H] , 544 (7) [C31
+
+
All reactions were carried out under a dry argon atmosphere in
H23FeNO
3
P] ,
+
+
freshly dried and distilled solvents; Et
3
N and pyrrolidine were twice
314 (100) [C17
H
24FeNO] , 268 (18) [C15
H18FeN] . C37 34FeNO P
H
3
Eur. J. Org. Chem. 2007, 4940–4947
© 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
4945

K. N. Gavrilov et al.
Cl (5 mL)]. An allyl
FULL PAPER
(627.2): calcd. C 70.82, H 5.46, N 2.23; found C 71.02, H 5.59, N
chiral ligand (0.02 mmol) was dissolved in CH
2
2
2
.35.
substrate (0.50 mmol) was added to the solution, and the mixture
was stirred for 20 min followed by the addition of dimethyl malon-
ate (0.75 mmol), BSA (0.75 mmol) and KOAc (2.0 mg). The reac-
tion mixture was stirred for 48 h at 20 °C, and the solvent was
evaporated. The products were separated by column chromatog-
raphy (silica gel; 200ϫ17 mm; hexane/EtOAc, 9:1). The fractions
containing a mixture of regioisomers 17 and 18 were combined and
the solvents evaporated. The residue was used for determination
of the 17/18 ratio and optical purity of product 17 as mentioned
above.
Palladium and Rhodium Complexes
Pd(allyl){(Sa)-7}]BF [(Sa)-8]: Yield: 92%, red powder. M.p. 108–
10 °C (dec.). 13C NMR (100.6 MHz, CDCl
, selected data for al-
lylic part): δ = 121.3 [d, 2J = 9.2 Hz, CH (allyl)], 80.6 [d, 2J =
3.1 Hz, CH (allyl) trans-P], 54.1 [s, CH (allyl) trans-N] ppm. MS
4
ESI): m/z (%) = 774 (100) [M – BF –
[
1
4
3
4
(
2
2
+
4
] , 733 (31) [M – BF
+
allyl] . MS (MALDI TOF/TOF): m/z (%) = 774 (100) [M –
BF
+
+
4 4 4 3
] , 733 (67) [M – BF – allyl] . C40H39BF FeNO PPd (861.1):
calcd. C 55.75, H 4.56, N 1.63; found C 55.60, H 4.28, N 1.85.
[
1
Pd(allyl){(Ra)-7}]BF [(Ra)-8]: Yield: 89%, red powder. M.p. 114–
16 °C (dec.). 13C NMR (100.6 MHz, CDCl
, selected data for al-
lylic part): δ = 120.9 [d, 2J = 9.0 Hz, CH (allyl)], 80.8 [d, 2J =
3.5 Hz, CH (allyl) trans-P], 54.0 [s, CH (allyl) trans-N] ppm. MS
MALDI TOF/TOF): m/z (%) = 774 (100) [M – BF
4
Acknowledgments
3
The authors gratefully acknowledge receiving (S)-tert-leucinol from
Degussa AG (Germany), the chiral HPLC column (R,R)-WHELK-
4
2
2
+
(
4
] , 733 (51)
PPd (861.1): calcd. C 55.75,
H 4.56, N 1.63; found C 55.91, H 4.68, N 1.44.
01 from Regis Technologies (USA) and the chiral HPLC column
+
4 4 3
[M – BF – allyl] . C40H39BF FeNO
Chiralcel OD-H from Daicel Chemical Industries, Ltd. (Japan).
This work was partially supported by the Russian Foundation for
Basic Research (Grant No. 04–03–39017). M. G. M. thanks the
DAAD Foundation for a research fellowship. E. B. B. thanks the
Russian Science Support Foundation for a research fellowship.
(
(
Ra)-9: A solution of ligand (Ra)-7 (0.063 g, 0.1 mmol) in CH
5 mL) was added dropwise over 40 min to a vigorously stirred
]BF (0.041 g, 0.1 mmol) in the same solvent
5 mL) at 20 °C. The reaction mixture was stirred at 20 °C for 1 h,
2 2
Cl
solution of [Rh(cod)
(
2
4
concentrated under reduced pressure (40 Torr) to ca. 0.5 mL, and
precipitated with ether. The obtained precipitate was separated by
centrifugation, washed with diethyl ether (2ϫ10 mL) and dried in
air and in vacuo (1 Torr, 1 h).
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Rh(cod){(Ra)-7}]BF
4
[(Ra)-9]: Yield: 0.087 g, 94%, red-brown
13
powder. M.p. 124–126 °C (dec.). C NMR (100.6 MHz, CDCl ):
δ = 174.1 (s, C=N), 149.7 (d, J = 5.6 Hz, OCipso), 147.1, 132.4,
3
2
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1
(
d, JC,Rh = 10.8 Hz), 74.8 [d, JC,Rh = 10.4 Hz, CH=(cod) trans-
N], 69.9, 69.2, 68.4, 68.0 (all s, CFc), 68.8 (s, CCp), 66.0 (s, OCH ),
4.7 (s, C), 34.2, 29.8, 29.3, 27.9 [all s, CH (cod)], 28.2 (s, CH
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6
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(
5
4 4 3
100) [M – BF – cod] . C45H46BF FeNO
Catalytic Experiments
[
Rhodium-Catalysed Hydrogenation of α-Dehydrocarboxylic Acid
Esters. General Procedure: A dry 50-mL Schlenk flask under an
atmosphere of argon was charged with a mixture of a solution of
the ligand (1.7 µ in CH
Rh(cod) ]BF (2.0 µ in CH
mixture was stirred for 5 min at 20 °C. Then, a solution of the
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2
Cl
2
, 1.2 mL or 0.6 mL). A solution of
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2
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precatalyst (0.01 mmol) and the corresponding chiral ligand (0.02–
2 2
0.04 mmol) were dissolved in CH Cl (5 mL) and stirred at 20 °C
for 40 min [or a presynthesised transition metal complex with a
4946
www.eurjoc.org
© 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2007, 4940–4947

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Eur. J. Org. Chem. 2007, 4940–4947
© 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
4947