Fine tuning of the structure of phosphine-phosphoramidites: Application for rhodium-catalyzed asymmetric hydrogenations

Article abstract of DOI:10.1016/j.tetasy.2012.11.013

Diastereomers of (4-(diphenylphosphino)pentan-2-yl)-N-isopropyl- {dinaphtho[2,1-d:1′,2′-f][1,3,2]dioxa-phosphepin-2-yl}-4-amine, (S)-(2S,4S)-1, and (S)-(2R,4R)-3; the octahydro derivative 4 of (S)-(2S,4S)-1, and derivative 2 of (S)-(2S,4S)-1 containing a 1,3-propanediyl backbone, have been synthesized and used for rhodium-catalyzed asymmetric hydrogenations of prochiral olefins in order to study the role of the stereogenic elements in the backbone and in the terminal moiety. The central chirality in the bridge has been found to determine the configuration of the product with a cooperative effect from the terminal groups. Excellent ee's (up to 99.9%) were obtained in the hydrogenation of methyl (Z)-α-acetamidocinnamate using a rhodium complex with the matched arrangement (S)-(2S,4S)-1. The hydrogenation is accomplished in a highly enantioselective manner by using green solvents such as propylene carbonate.

Full text of DOI:10.1016/j.tetasy.2012.11.013

Tetrahedron: Asymmetry 24 (2013) 66–74
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Fine tuning of the structure of phosphine–phosphoramidites: application
for rhodium-catalyzed asymmetric hydrogenations
a
b
c
c
a,
⇑
Szabolcs Balogh a, , Gergely Farkas , Áron Szöllosy , Ferenc Darvas , László Ürge , József Bakos
}
a Department of Organic Chemistry, University of Pannonia, 8200 Veszprém, Egyetem u. 10, Hungary
b
}
Department of Inorganic and Analytical Chemistry, University of Technology and Economics, 1111 Budapest, Muegyetem rkp. 3-9, Hungary
c ThalesNano Nanotechnology Inc., 1031 Budapest, Záhony u. 7, Graphisoft Park, Hungary
a r t i c l e i n f o
a b s t r a c t
Diastereomers of (4-(diphenylphosphino)pentan-2-yl)-N-isopropyl-{dinaphtho[2,1-d:10,20-f][1,3,2]dioxa-
phosphepin-2-yl}-4-amine, (S)-(2S,4S)-1, and (S)-(2R,4R)-3; the octahydro derivative 4 of (S)-(2S,4S)-1,
and derivative 2 of (S)-(2S,4S)-1 containing a 1,3-propanediyl backbone, have been synthesized and used
for rhodium-catalyzed asymmetric hydrogenations of prochiral olefins in order to study the role of the
stereogenic elements in the backbone and in the terminal moiety. The central chirality in the bridge
has been found to determine the configuration of the product with a cooperative effect from the terminal
Article history:
Received 22 October 2012
Accepted 16 November 2012
groups. Excellent ee’s (up to 99.9%) were obtained in the hydrogenation of methyl (Z)-a-acetamidocinna-
mate using a rhodium complex with the matched arrangement (S)-(2S,4S)-1. The hydrogenation is
accomplished in a highly enantioselective manner by using green solvents such as propylene carbonate.
Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction
terminal moieties, predominantly diphenylphosphine and diox-
aphosphepine (Fig. 1).
Asymmetric hydrogenation of olefins is an appropriate method
in terms of selectivity, activity, and economic viability for prepar-
ing optically active compounds.1 Chiral bidentate ligands with
two phosphorus donor atoms are the most widely used com-
pounds in asymmetric hydrogenation reactions.2 It is well known
that hetero-bidentate ligands provide overall excellent selectivi-
ties.3 Among them, phosphine–phosphoramidites4 have emerged
over the last decade as a new class of ligands suitable for asymmet-
ric catalytic syntheses including asymmetric hydrogenations,5 Cu-
catalyzed asymmetric additions of diethylzinc to enones,6 and
asymmetric hydroformylations.7 They form five to seven-mem-
bered chelate rings with the metal and some of these catalysts pro-
vide high selectivity in asymmetric transformations.
While in the review of Crévisy, phosphine–phosphoramidites
are divided into four classes based on their amine building blocks:
cyclic amines, ferrocene, benzyl/aryl-amines, and chiral-pool
diphenylphosphino-amines,8 Reek et al. have introduced an alter-
native classification based on the linker unit between the phos-
phine and phosphoramidite. The linker can be a rigid cyclic
amine (class 1), a rigid (bi)cyclic moiety (class 2), or a flexible chain
containing at least one non-cyclic sp3-hybridized carbon atom
(class 3).9 All of the phosphine–phosphoramidites have the same
Leitner et al. reported the first example of a phosphine–phospho-
ramidite ligand, QUINAPHOS, and demonstrated that this ligand is
highly enantioselective for the Rh-catalyzed asymmetric hydroge-
nation of benchmark substrates such as dimethyl itaconate
(ee = 99%) and methyl 2-acetamidoacrylate (ee = 98%) or in the Ru-
catalyzed hydrogenation of ketones with ee’s of up to 94%. (Ra,Rc)-
and (Ra,Sc)-QUINAPHOS yield 4.8% (S) and 74% (S) ee’s in the asym-
metric hydroformylation of styrene, respectively.4 A pronounced
matched/mismatched effect was observed for the configuration of
the two stereocenters. Zheng et al. showed, that (Sc,Sa)- and (Sc,Ra)-
PEAPhos give 99.1% (R) and 48.5% (R) in the asymmetric hydrogena-
tion of methyl (Z)-a-acetamidocinnamate, and 99.5% (R) and 73.9%
(R) in the asymmetric hydrogenation of N-(1-phenylvinyl)acetam-
ide, respectively.10 These results indicate that the absolute configu-
ration of the product is controlled by the chirality of the bridge.
The same research group introduced the ferrocene based PPFA-
Phos ligand containing three chiral elements.11This ligand pro-
vided ee’s of 99.6% (S) and 82.6% (R) with (Sc,Sp,Ra) and (Sc,Sp,Sa)
configurations, respectively, in the asymmetric hydrogenation of
N-(1-phenylvinyl)acetamide. The results clearly indicate that the
configuration of the product and the selectivity of the hydrogena-
tion are mainly determined by the axial chirality of the BINOL
backbone. These examples show that the properly chosen chiral
bridge associated with the matching terminal chiral moieties can
lead to viable ligands in asymmetric transformations.
⇑
Corresponding author. Tel.: +36 88624355; fax: +36 88624469.
E-mail address: bakos@almos.vein.hu (J. Bakos).
A review on the application of organic carbonates as solvents in
Current address: MR laboratory, University of Pannonia, Institute of Chemistry,
8200 Veszprém, Egyetem u. 10, Hungary.
catalysis has recently been published.12 Until now, organic cyclic
0957-4166/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetasy.2012.11.013

S. Balogh et al. / Tetrahedron: Asymmetry 24 (2013) 66–74
67
n-Bu
O
O
N
O
O
N
P
N
P
P
O
O
PPh2
PPh2
Fe
PPh2
(Sc,Sp,Sa)-PPFAPhos
(Ra,Sc)-nBu-QUINAPHOS
(Sc,Sa)-PEAPHOS
R
O
O
O
H
O
H
N
P
N
P
N
P
O
O
PPh2
PPh2
PPh2
(S)-HY-Phos
(S)-AnilaPhos
(R,R)-THNAPhos
derivatives
R2
P(R1)2
O
O
N
O
P
N
P
O
PPh2
R2
Crévisy's ligand
(S)-indolphos
with one stereogenic center
derivatives
Figure 1. Selected phosphine–phosphoramidite ligands.
carbonates have not played a major role as solvents for asymmetric
hydrogenations.13 The exceptions in homogeneous catalysis in-
clude the platinum-catalyzed hydrosilylations of unsaturated fatty
acids investigated by Behr et al.;14 regioselective, rhodium-cata-
lyzed hydroformylations;15 iridium- and rhodium-catalyzed asym-
metric hydrogenations and palladium-catalyzed asymmetric allylic
alkylations by Börner et al.;16 palladium-catalyzed Heck reaction
by Reetz et al.,17 and rhodium-catalyzed intermolecular alkyne
hydroacylations by Willis et al.18
Recently, we reported a new phosphine–phosphoramidite li-
gand containing an aliphatic chiral linker, which provided excel-
lent selectivity and activity in the asymmetric hydrogenation of
challenging substrates.19 These promising catalytic properties in-
spired us to study the role of the stereogenic elements in the
bridge. Herein our aim was the development of effective chiral li-
gands that could be easily prepared and successfully applied in
asymmetric synthesis using alkylene carbonates as green solvents.
Sharpless’ method.20 The ring opening of a cyclic sulfate with an
amine takes place smoothly, with complete inversion at the stere-
ogenic center to give the sulfated amine (Scheme 1). Subsequent
reaction with lithium-diphenylphosphide gave the desired amin-
oalkylphosphine with complete inversion at the stereogenic center
and with good yield.
These valuable intermediate aminoalkylphosphines
A
(Scheme 2) can be combined with chlorophosphites to obtain
phosphoramidite products. Despite the relatively strong basicity
of aminoalkylphosphines, the reaction only takes place if the amine
contains the 1,3-propanediyl backbone (S)-2 (Fig. 2). In the case of
aminoalkylphosphines with a 2,4-pentanediyl backbone 1, 3, and
4, the reaction did not take place, presumably due to steric hin-
drance around nitrogen.
In the next set of experiments, the aminoalkyl-phosphine was
treated with PCl3 in the presence of triethylamine, which led to
amino-dichlorophosphine B. After the addition of the correspond-
ing diol in the presence of triethylamine, the desired compound D
was obtained with up to 32% yield. The diastereomers 1 and 3 were
readily prepared by the reaction of (S)-1,10-bi-2-naphthol with
(2S,4S)- and (2R,4R)-amino-dichlorophosphine B, respectively.
Next we attempted to increase the yield of the product by the
activation of the amine. Lithium amide C of the corresponding
amine was formed with Li metal in the presence of styrene.21
2. Results and discussion
2.1. Ligand synthesis methods
Chiral aminoalkylphosphines can be conveniently prepared
from cyclic sulfate esters, which are synthesized according to the
+H2N
HN
LiPPh2
i-Pr-NH2
O
O
S
-
OSO3
PPh2
O
O
Scheme 1. Synthesis of aminoalkylphosphines from cyclic sulfates.

68
S. Balogh et al. / Tetrahedron: Asymmetry 24 (2013) 66–74
Li / styrene
BuLi
PCl3
HN
R
Cl2P
N
R
Li
N
R
NEt3, THF
PPh2
PPh2
PPh2
R
R
R
A
B
C
O
O
DMAP
NEt3, THF
44 %
NEt3, THF
P
Cl
O
OH
OH
NEt3, THF
20 %
NEt3, THF
15-32 %
P
Cl
O
O
P
N
O
R
PPh2
R
D
Scheme 2. Strategies for the synthesis of phosphine–phosphoramidite ligands.
O
O
P
N
P
N
O
O
PPh2
PPh2
(S)-2
(S)-(2S,4S)-1
O
O
O
P
O
P
N
N
PPh2
PPh2
(S)-(2R,4R)-2
(S)-(2S,4S)-4
Figure 2. Phosphine–phosphoramidite ligands with an aliphatic bridge.
The lithiated amine readily reacts with the chloro compound but
side reactions take place, which make the work-up and purification
of the ligand difficult, resulting in the product being obtained in
low yield (20%).
phorus in the phosphine moiety (PC). From the d(PN) data it can be
concluded that ligand 4 possessing an H8-biaryl moiety has a phos-
phoramidite group with a stronger
their H0-analogues.
r-donor ability compared to
In order to improve the yield of product D, a new methodology
was developed using 4-dimethylaminopyridine (DMAP) as the cat-
alyst.22 The role of DMAP is to activate the chlorophosphite by the
formation of a labile ion pair between the chloride and the phos-
phityl-pyridinium cation. The addition of 10 mol % of DMAP as cat-
alyst and triethylamine as the proton acceptor to the reaction
mixture improved the yield of the desired compound to 44%.
The coordination chemistry of the rhodium complexes of li-
gands 1–4 was investigated because of its relevance to the asym-
metric catalysis described below. The products formed upon
treatment of the chiral ligands with 1 equiv of [Rh(COD)2]BF4 were
evaluated by 31P{1H} NMR spectroscopy. At a 1:1 ligand:metal ra-
tio, two double doublets were achieved indicating the formation of
a seven-membered chelate ring in the [Rh(COD)(PP0)]BF4 com-
plexes. The coordination chemical shifts
(Dd = d(com-
2.2. Stereoelectronic characterization of the ligands
plex) ꢀ d(ligand)) were found to be negative for the PN and
positive for PC moieties, as expected for a good
p-acceptor and a
The new ligands 2–4 (Fig. 2) are considerably air- and moisture-
stable white solids and have been characterized by 1H-, 13C- and
31P NMR spectroscopy. Their 31P{1H} NMR spectra were particu-
larly informative with regards to the purity (Table 1): in each case,
two singlets in the expected range were observed, one for the
phosphorus in the phosphoramidite unit (PN) and one for the phos-
good d-donor moiety, respectively.
The
r-donor ability of the phosphorus atoms in phosphine–
phosphoramidites 1–4 has been evaluated by measuring the mag-
nitude of the 1J(77Se–31P) coupling constant in the 77Se isotopomer
of the corresponding seleno-phosphate-amide and phosphine–sel-
enide. This method is one of the simplest and most reliable ones for

S. Balogh et al. / Tetrahedron: Asymmetry 24 (2013) 66–74
69
Table 1
31P NMR data of ligands 1–4 and the corresponding [Rh(COD)(PP0)]BF4 complexesa
Ligand
d (PN) (ppm)
1J(RhPN) (Hz)
d (PC) (ppm)
1J(RhPC) (Hz)
1J(PNPC) (Hz)
dlig(PN) (ppm)
dlig(PC) (ppm)
D
d (PN) (ppm)
Dd (PC) (ppm)
1
2
3
4
137.3
139.2 (broad)
148.8
242.8
234.8
235.5
240.5
19.2
7.9
18.1
19.4
140.5
141.2
135.7
140.9
40.1
45.7
34.1
40.0
153.5
151.1
152.9
146.0
3.0
ꢀ16.6
0.0
ꢀ16.2
ꢀ11.9
ꢀ4.1
16.2
24.5
18.1
17.6
132.9
1.8
ꢀ13.1
a
All spectra measured in CDCl3 at 20 °C. PN is related to the phosphoramidite group and PC is related to the phosphino group. The coordination chemical shift was
calculated as follows:
d = d(complex) ꢀ d(ligand).
D
assessing the donating ability of phosphorus donors. The magni-
tude of lJ(Se–P) is very much dependent upon the nature of the or-
ganic groups bonded to the phosphorus: electron-withdrawing
groups cause the coupling constant to increase whereas electron-
donating and bulky groups cause it to decrease.23
The synthesis of diselenides was performed by the in situ NMR
tube reaction of the corresponding ligand with elemental sele-
nium. According to the preparation method, the reaction proceeds
in a stepwise manner. The phosphine site readily reacts with sele-
nium powder after some minutes while the formation of disele-
nides takes only one hour in toluene at room temperature
(Table 2).
a significantly larger difference (
properties of the phosphine and phosphite than in the case of the
phosphine–phosphoramidite J = 239.7 Hz). Our ligands de-
scribed herein are more electron-donating than the phosphites be-
cause the electronegativity of nitrogen (3.04) is less than that of
oxygen (3.44); the donor character is further increased by the
hyperconjugation effect of the isopropyl group connected to nitro-
gen. It is interesting to note that the diselenide of 2 (Fig. 4) is char-
acterized by the largest, and at the same time with the lowest
coupling constant among the compounds investigated herein, indi-
DJ = 287.8 Hz) in the electronic
(D
cating the lowest and the highest
r-donor capability of phospho-
ramidite and phosphine. For the sake of comparison, a typical
phosphoramidite such as MonoPhos 6, is much more p-acidic than
any of our ligands has a 1J(77Se–31P) value as high as 971.1 Hz. It
should be noted that the electron density at the P-donor of a
diphosphine, such as the BDPP ligand 7, is very similar
(1J(77Se–31P) = 722 Hz) to that obtained for the most basic phos-
phine site of 2 (1J(77Se–31P)B = 722.8 Hz).
Table 2
31P–77Se coupling constants in diselenides of ligands 1–7a
O
O
P
Se
P
Ph2P
Ph2P
N
O
N
O
Se
Se
2.3. Catalytic experiments
Ligand
1J(77Se–31P)A (Hz)
1J(77Se–31P)B (Hz)
Recently, we demonstrated the superior performance of ligand 1
in Rh-catalyzed asymmetric hydrogenations of a wide range of ole-
fin substrates.15 The free ligand and the catalyst was not just stable
in green solvents such as alkylene carbonates or protic solvents such
as methanol, even the selectivity and activity were better in these
solventscompared to dichloromethane. The catalyst providedexcel-
lent ee’s of up to 99.0% in propylene carbonate for the asymmetric
(S)-(2S,4S)-1
(S)-2
(S)-5
(S)-(2R,4R)-3
(S)-(2S,4S)-4
(S)-6
959.8
962.5
1038.3b
955.0
947.1
971.124
—
732.8
722.8
750.5
742.5
728.2
—
(2S,4S)-7
72225
a
All spectra measured in toluene, at 20 °C. 1J(77Se–31P)A is the coupling constant
hydrogenation of the
L-DOPA precursor (3-methoxy-4-acetoxy-
in the phosphoramidite group and 1J(77Se–31P)B in the phosphino group.
(Z)- -acetamidocinnamic acid). In addition, similarly high ee’s were
a
1J(77Se–31P)A is the coupling constant in the phosphite group.
b
achieved in methanol solution for acetamidocinnamic ester and its
ortho- and para-methoxy derivatives.
Our structure offers flexible opportunities for derivatization for
more precise control over the positioning of the steric bulk. Herein
we have tested ligands 1–4 in asymmetric hydrogenation (Table 1).
All reactions were run until full conversion. Ligand 2, without ste-
reogenic elements in the bridge showed low selectivities for all
substrates (entries 3, 4, 10, and 14). In addition, ligand 2 gave
the weakest performance among the ligands. Crévisy et al. reported
on a similar ligand, which does not contain a stereogenic center in
the linker; however, no application of this ligand in catalysis has
been reported on. Another phosphine–phosphoramidite with one
stereogenic center in the bridge (Fig. 1) introduced by Crévisy
From the data of Table 2, it can be seen that the electronic effect
of a minor change in the backbone or in the dioxaphosphepine
moiety is effectively transmitted to the P-donors. The partially
hydrogenated ligand 4 is slightly more basic then the binaphthyl
derivatives 1–3. Thus, the coupling constants of the diselenide de-
rived from partially saturated binaphthol derivative 4 are 12.7 and
4.6 Hz smaller than those derived from the related binaphthol 1.
It is also important to note that the electronic changes of the
phosphorus functionalities of the same type in the diastereomers
1 and 3 show opposite trends. The 1J(77Se–31P) of an analogous
phosphine–phosphite 526 (Fig. 3) 1038.3 Hz and 750.5 Hz reflects
O
O
P
P
P
N
P
O
O
O
PPh2
(S)-5
(2S,4S)-BDPP 7
(S)-MonoPhos 6
Figure 3.

70
S. Balogh et al. / Tetrahedron: Asymmetry 24 (2013) 66–74
Figure 4. 31P{1H} NMR spectrum (161 MHz, CDCl3) of diselenide-2.
and Mauduit was evaluated in the copper-catalyzed conjugate
addition of dietylzinc to enones.6 The best ee’s of up to 53% were
obtained when cyclohex-2-enone was used as the substrate. This
catalyst was not tested in the catalytic asymmetric hydrogenation
reaction. To the best of our knowledge, our results are the first
examples that give valuable information on the role of the stereo-
genic center in the bridge when the linker is an aliphatic chain.
One of the key principles of green chemistry is the elimination
of solvents in chemical processes or the replacement of hazardous
solvents with environmentally friendly ones. Alkylene carbonates
have recently emerged as green solvents. The JEFFSOLÒ carbonates
can be used as safe and environmentally friendly solvents and re-
place the commonly used methylene chloride, acetone, aromatic
solvents, and other highly volatile and hazardous solvents. They
also have a low vapor pressure which allows their easy application
at higher temperature (See Table 3).
We found that the catalyst generated by mixing [Rh(COD)2]BF4
with ligand (S)-(2S,4S)-1 in propylene carbonate or methanol un-
der a hydrogen pressure of 5 bar led to full conversion of the ace-
tamidocinnamic ester with ee values of 99.2% and 99.9%,
respectively. In contrast, ligand (S)-(2R,4R)-3 with the (2R,4R)-pen-
tanediyl fragment displayed low enantioselectivity and favored a
hydrogenation product with the opposite configuration. Enantio-
meric excesses of 55.8% (S) and 77.2% (S) were measured in propyl-
ene carbonate and in methanol, respectively, for methyl (Z)-a-
acetamidocinnamate (entries 5 and 6 compared to entries 1 and
2). The same change could be observed for methyl 2-acetamidoac-
rylate (compare entries 9 and 11) and dimethyl itaconate (compare
entries 13 and 15). It is known that the bulky chiral BINOL and
octahydro-BINOL moieties are generally good chiral information
transmitters but in this case, the 2,4-pentanediyl moiety has more
influence in determining the absolute configuration of the predom-
inant hydrogenation product.
The properly selected (2S,4S)-configurations in the backbone
lead to enhanced enantioselectivities (1, matched arrangements)
in contrast to the results displayed by the mismatched arrange-
ments (S)-(2R,4R)-3.
Table 3
Asymmetric hydrogenation with Rh(I) catalysts of chiral phosphine–phosphoramidite
ligandsa
The high selectivity obtained is the result of a combined action
between the chiral BINOL phosphite moiety and the ligand back-
bone chirality. It is most likely that in the matched arrangement
the methyl groups dispose the terminal phenyl rings and the naph-
thyl moieties into a chiral arrangement (alternating edge-face and/
or equatorial-axial). By replacing the H0-BINOL moiety with its par-
tially hydrogenated and bulkier H8 analogue, no significant selec-
tivity change was observed when compared to ligand 1.
A highly desirable goal of green chemistry is to replace organic
solvents in chemical reactions with water. An alternative is to carry
out the reaction without any solvent. In our case we were able to
carry out the hydrogenation of dimethyl itaconate without any sol-
vent. The hydrogenated product was obtained at 50 °C under
50 bar of hydrogen pressure with 99.5% enantioselectivity at a sub-
strate/catalyst molar ratio of 2500 (entry 17). The excellent cata-
lytic activity was also demonstrated by the use of a low catalyst
loading. The reduction of the catalyst loading, and the elevated
temperature and pressure did not affect the conversion and
enantioselectivity.
Entry
Substrate
Ligand
Reaction
timeb (h)
ee (%)
1
COOMe
NHAc
(S)-(2S,4S)-1
(S)-2
2.8
2.0
99.2 (R)
99.9 (R)
2c
3
2.3
0.5
4.1
2.2
2.0
1.5
44.5 (R)
44.8 (R)
55.8 (S)
77.2 (S)
98.8 (R)
99.7 (R)
4c
5
(S)-(2R,4R)-3
(S)-(2S,4S)-4
6c
7
8c
9
10
11
12
13
14
COOMe
NHAc
(S)-(2S,4S)-1
(S)-2
(S)-(2R,4R)-3
(S)-(2S,4S)-4
(S)-(2S,4S)-1
(S)-2
2.2
1.8
2.5
2.5
3.0
3.4
97.7 (S)
24.2 (S)
60.4 (R)
95.1 (S)
99.5 (R)
77.5 (R)
15
16
(S)-(2R,4R)-3
(S)-(2S,4S)-4
(S)-(2S,4S)-1
2.5
2.3
1.0
49.0 (S)
99.6 (R)
99.5 (R)
17d
MeOOC
COOMe
a
S/Rh = 125, P/Rh = 1.1, Solvent: 5 mL propylene carbonate, Pressure: 5 bar H2,
RT.
b
The catalytic application mentioned above might confirm that
updating this synthetic approach could provide access to useful li-
gands. We are currently following this approach and are compiling
Time for 100% conversion.
Reaction carried out in MeOH.
S/Rh = 2500, P/Rh = 1.1, no solvent, Pressure: 50 bar H2, 50 °C.
c
d

S. Balogh et al. / Tetrahedron: Asymmetry 24 (2013) 66–74
71
a pool of hybrid phosphine–phosphoramidites of a fine scale of
4.3. Synthesis
electronic and steric nature. This set of ligands should support
our efforts to develop efficient and environmentally benign cata-
lytic systems.
4.3.1. (11bS)-N-((2S,4S)-4-(Diphenylphosphino)pentan-2-yl)-N-
isopropyl-{dinaphtho[2,1-d:10,20-f][1,3,2]dioxaphosphepin-2-
yl}-4-amine 1
A typical synthesis using DMAP follows: (2S,4S)-2-diphenyl-
phosphino-4-i-propylamino-pentane (1.25 g, 4.0 mmol) and tri-
3. Conclusion
ethylamine (670 lL, 5.0 mmol) were dissolved in 30 mL of THF.
In conclusion, we have synthesized a series of phosphine–
phosphoramidite ligands to investigate the role of the linking
bridge between the two phosphorus atoms. Our phosphine–
phosphoramidite ligands represent a new class of ligands: the
linker unit between the phosphine and phosphoramidite con-
tains three sp3-hybridized carbon atoms. Ligand 2, which does
not have a stereogenic element in the 1,3-propanediyl bridge,
showed much lower ee’s than ligand 1. Moreover, the selectivity
and the configuration of the product changed from 99.9% (R) to
77.2% (S) when ligand 3 was used in the asymmetric hydrogena-
4-Dimethylaminopyridine (36 mg, 0.3 mmol) was added to the
solution and then cooled to approximately ꢀ30 °C. At this temper-
ature (S)-chloro-binaphtho[2,1-d:10,20-f][1,3,2]dioxaphosphepine
(1.5 g, 4.5 mmol) in 20 mL of THF was added over 30 min. The for-
mation of a white precipitate was observed. The mixture was stir-
red at RT for 2 h. The reaction was followed by silicagel TLC plate
with chloroform/methanol = 4/1 eluent. Rf for (2S,4S)-2-diphenyl-
phosphino-4-i-propylamino-pentane is 0.5. After consumption of
the substrate, the suspension was filtered through a short pad of
Al2O3. After evaporation of the volatiles, 1.1 g of a white foam
(yield: 44.1%) was obtained.
tion of methyl (Z)-a-acetamidocinnamate in methanol. These re-
sults indicate that despite the excellent chiral information
transmission ability of the BINOL moiety, the high selectivity
achieved is the result of a combined action between the chiral
BINOL phosphite moiety and the ligand backbone chirality. We
assume that the methyl groups dispose the chelate conforma-
tion, terminal phenyl rings, and the naphthyl moieties into a chi-
ral arrangement. To the best of our knowledge, our results are
the first example that gives valuable information on the role of
the stereogenic center in the bridge when the linker is an ali-
phatic chain.
4.3.2. Diselenide of 1
A mixture of 1 (40 mg, 0.064 mmol) and elemental selenium
(15 mg, 0.19 mmol) in toluene (2.5 mL) was stirred for 24 h. The
unreacted selenium was left to settle and the required quantity
of the solution was added into an NMR tube. 31P{1H} NMR
(121 MHz, toluene): d = 84.0 (d, 1J(Se–P) = 959.8 Hz), 49.1 ppm (d,
1J(Se–P) = 732.8 Hz).
4.3.3. 1-i-Propylamino-3-sulfato-propane
2,2-Dioxide-1,3,2-dioxathian (5.35 g, 38.7 mmol) and i-propyl-
amine (3.17 mL, 38.7 mmol) were stirred in 40 mL of ether for 24 h
at RT. The reaction mixture was filtered and the solid was washed
three times with ether. The remaining volatiles were evaporated to
give 5.7 g of white powder (Yield: 74.6%). Mp: 181–183 °C. Anal.
Calcd for C6H15NO4S: C, 36.53%; H, 7.66%; N, 7.10%; S, 16.26%. Found:
C, 36.83%; H, 7.76%; N, 7.56%; S, 15.86%. 1H NMR (400 MHz, DMSO):
d = 8.15 (s, 2H, NH2+), 3.84 (t, J = 6.06 Hz, 2H, CH2), 3.28 (m,
J = 6.51 Hz, 1H, CH), 2.96 (t, J = 7.38 Hz, 2H, CH2), 1.84 (m,
J = 6.73 Hz, 2H, CH2), 1.19 (d, J = 6.52 Hz, 6H, CH3), 13C {1H} NMR
(75 MHz, DMSO): d = 63.04 (s), 49.49 (s), 41.73 (s), 26.16 (s), 18.66
(s).
4. Experimental
4.1. General
All reactions and manipulations were carried out under argon
atmosphere using standard Schlenk techniques. Methanol, THF,
and diethyl ether were distilled and dried before use. JEFSOLÒ Pro-
pylene carbonate was provided by Huntsman Corporation and
used without drying or degassing (H2O content: 0.05 m/m%). The
substrates methyl (Z)-a-acetamidocinnamate and methyl 2-acet-
amidoacrylate used for the asymmetric hydrogenations were syn-
thesized according to the literature.27 All other chemicals were
obtained commercially. 31P{1H}-, 13C{1H}- and 1H NMR spectra
were recorded on a Bruker Avance 400 spectrometer in the NMR
Laboratory at the University of Pannonia in Veszprém, or on a VAR-
IAN UNITY 300 or Bruker DRX-500 spectrometer at the University
of Technology and Economics in Budapest.
4.3.4. 1-Diphenylphosphino-3-isopropylamino-propane
At first, 1-i-propylamino-3-sulfato-propane (3.7 g, 19.8 mmol)
was added dropwise to a solution of LiPPh2⁄1,4-dioxane (10.6 g,
37.8 mmol) in 60 mL of abs. THF under argon and stirred for
2 h. The original color of the reaction mixture turned brown
and at the end of the reaction green. After evaporation of the sol-
vent, 80 mL of distilled oxygen-free water and 60 mL of ether
were added to the residue and the mixture was stirred until
the two phases became clear solutions. The pH was then set to
1 with 10% solution of oxygen free HCl. The phases were then
separated and the water phase was washed three times with
30 mL portions of ether. The pH was then set to around 9–10
4.2. GC analyses
4.2.1. Methyl (Z)-acetamidocinnamate
CHIRASIL-L-VAL column (25 m ꢁ 0.25 mm, df = 0.12
lm). Tem-
perature program: 2 min at 140 °C; 2 °C/min from 140 °C to 180 °C;
40 min at 180 °C. Retention times were 12.3 min for (R), 13.2 min
for (S)-product, and 22.1 min for (Z)-a-acetamidocinnamic acid
methyl ester.
by the dropwise addition of Na2CO3.
A white precipitation
formed which was filtered, dissolved in dichloromethane, and
dried over MgSO4. The solution was evaporated to give 2.25 g
of a white solid (yield: 63.6%). Mp: 181 °C. Anal. Calcd for
4.2.2. Methyl 2-acetamidoacrylate
b-Dex 255 column (30 m ꢁ 0.25 mm, df = 0.25
lm). Retention
C18H24NP: C, 75.76%; H, 8.48%; N, 4.91%; P, 10.85%. Found: C,
75.49%; H, 8.31%; N, 5.00%. 1H NMR (300 MHz, CDCl3): d = 9.52–
8.14 (s, 1H, NH), 7.46–7.37 (m, 4H, aromatic), 7.36–7.27 (m,
6H, aromatic), 3.23 (q, J = 6.31 Hz, 1H, CH), 2.93 (t, J = 7.16 Hz,
2H, CH2), 2.20–2.09 (m, 2H, CH2), 2.09–1.93 (m, 2H, CH2), 1.37–
1.30 (d, J = 6.3 Hz, 6H, CH3), 31P{1H} NMR (121 MHz, CDCl3)
d = ꢀ17.30 (s). 13C{1H} NMR (75 MHz, CDCl3): d = 138.04 (d,
J = 12.7 Hz), 132.76 (d, J = 18.7 Hz), 128.80 (s), 128.60 (d,
times at 140 °C isotherm were 6.7 min for (S), 7.4 min for (R) -prod-
uct, and 5.9 min for acetamidoacrylic acid methyl ester.
4.2.3. Dimethyl itaconate
b-Dex 255 column (30 m ꢁ 0.25 mm, df = 0.25
lm). Retention
times at 85 °C isotherm were 18.8 min for (R), 20.0 min for (S) -
product, and 27.6 min for dimethyl itaconate.

72
S. Balogh et al. / Tetrahedron: Asymmetry 24 (2013) 66–74
J = 6.7 Hz), 49.94 (s), 45.19 (d, J = 14.3 Hz), 25.51 (d, J = 13.1 Hz),
22.82 (d, J = 18.2 Hz), 19.22 (s).
of the solvent 80 mL of distilled oxygen free water and 60 mL
of ether were added to the residue and the mixture was stirred
until the two phases became clear solutions. The pH was then
set to 1 with a 10% solution of oxygen-free HCl. The phases were
then separated and the water phase was washed three times with
30 mL portions of ether. The pH was then set to around 9–10 by
the dropwise addition of Na2CO3. The product was then extracted
four times with 30 mL portions of ether. After drying over MgSO4
the mixture was filtered and evaporated to give 4.95 g of a clear
4.3.5. (11bS)-N-(3-(Diphenylphosphino)propyl)-N-isopropyl-
{dinaphtho[2,1-d:10,20-f][1,3,2]dioxaphosphepin-2-yl}-4-amine
2
1-Diphenylphosphino-3-isopropylamino-propane
(2.25 g,
7.89 mmol) and triethylamine (1.7 mL, 11.83 mmol) were dis-
solved in 40 mL of abs. THF under argon. A solution of (S)-2-
chloro-binaphtho[2,1-d:10,20-f][1,3,2]dioxaphosphepine
(3.46 g,
or pale yellow oil (Yield: 74.2%). ½a D20
¼ þ74:3 (c 1, CH2Cl2). Anal.
ꢂ
9.86 mmol) in 20 mL of abs. THF was then added to the reaction
mixture via canulla under vigorous stirring at 0 °C. After being
stirred at RT for one hour the reaction mixture was filtered
through a short pad of Al2O3 and then the solvent was evapo-
rated. The remaining white foam was further purified with flash
chromatography over silicagel using diisopropyl-ether/petrole-
ther = 1/1 eluent to give 1.95 g of white powder (yield: 41.2%).
Calcd for C8H28NP C, 76.64; H, 9.00; N, 4.47. Found: C, 76.62; H,
8.99; N, 4.47. 1H NMR (500 MHz, CDCl3): d = 7.44–7.32 (m, 4H,
aromatic), 7.25–7.16 (m, 6H, aromatic), 2.86–2.71 (m, 2H, CH),
2.35–2.24 (m, 1H, CH), 1.44–1.33 (m, 1H, CH2), 1.26–1.15 (m,
1H, CH2), 0.98–0.85 (m, 12H, CH3). 31P{1H} NMR (121 MHz,
CDCl3): d = ꢀ0.03 (s). 13C {1H} NMR (75 MHz, CDCl3): d = 137.06
(dd, J = 14.4 Hz, J = 7.5 Hz), 133.72 (dd, J = 19.1 Hz, J = 5.8 Hz),
128.75 (s), 128.34 (t, J = 6.6 Hz), 47.69 (d, J = 12.7 Hz), 45.27 (s),
40.86 (d, J = 17.0 Hz), 27.31 (d, J = 9.7 Hz), 23.25 (d, J = 24.8 Hz),
20.20 (s), 16.30 (d, J = 15.5 Hz).
½
a 2D0
ꢂ
¼ þ336:4 (c 0.99, CH2Cl2), mp: 60–62 °C. Anal. Calcd for
C38H35NO2P2: 76.11; H, 5.88; N, 2.34. Found: C, 76.27; H, 5.58;
N, 2.27. 1H NMR (500 MHz, CDCl3): d = 8.03–7.56 (m, 4H, aro-
matic), 7.53–7.26 (m, 18H, aromatic), 3.57–3.44 (m, 1H, CH),
2.96–2.73 (m, 2H, CH2), 1.91–1.44 (m, 4H, CH2), 1.20 (d,
J = 6.60 Hz,3H, CH3), 1.15 (d, J = 6.60 Hz, 3H, CH3). 31P{1H} NMR
(121 MHz, CDCl3): d = 151.10 (s), ꢀ16.63 (s). 13C {1H} NMR
(75 MHz, CDCl3): d = 150.30 (d, J = 5.2 Hz), 149.78 (s), 138.77
(dd, J = 26.9 Hz, J = 13.0 Hz), 132.86 (m), 132.55 (s), 131.39 (s),
130.64 (s), 130.22 (s), 129.78 (s), 128.41 (m), 127.07 (d,
J = 6.5 Hz, 126.03 (d, J = 2.8 Hz), 124.60 (d, J = 23.2 Hz), 124.09
(d, J = 5.1 Hz), 122.25 (d, J = 1.7 Hz), 122.05 (s), 47.80 (d,
J = 23.3 Hz), 43.89 (dd, J = 14.5 Hz, J = 13.5 Hz), 28.42 (dd,
J = 15.8 Hz, J = 2.8 Hz), 25.46 (d, J = 11.4 Hz), 23.28 (dd,
J = 13.3 Hz, J = 7.1 Hz).
4.3.9. (11bS)-N-((2R,4R)-4-(Diphenylphosphino)pentan-2-yl)-N-
isopropyl-{dinaphtho[2,1-d:10,20-f][1,3,2]dioxaphosphepin-2-
yl}-4-amine 3
Compound 3 was synthesized with the lithiation method
(Scheme 2). (2R,4R)-2-Diphenylphosphino-4-i-propylamino-pen-
tane (2.5 g, 7.9 mmol), Li metal (111.7 mg, 15.9 mmol), THF
(550 lL, 6.8 mmol) and styrene (457 lL, 4.0 mmol) were stirred
vigorously in 40 mL of hexane under argon for 3 h at RT and
for 1 h at 45 °C. A yellow solution formed with Li metal particles
in the solvent. After filtration of the excess Li metal, the yellow
´ ´
filtrate was added to
a
(S)-2-chloro-binaphtho[2,1-d:1,2-
f][1,3,2]dioxaphosphepine (4.2 g, 12.0 mmol) in 50 mL of ether
solution and cooled to about ꢀ10 °C for 30 min under argon.
The cooling bath was removed and the reaction mixture was
stirred for 2 h on RT. The white precipitates were filtered and
washed with ether. After evaporation of the filtrate 4.5 g of
white foam was obtained. Further purification was performed
with flash chromatography on silica gel with the toluene/n-hex-
ane = 1/1 (Rf. 0.7) to give the pure product as a white powder
4.3.6. Diselenide of 2
This compound was prepared by following the procedure de-
scribed for the preparation of diselenide of 1. 31P{1H} NMR
(121 MHz, toluene): d = 88.2 (d, 1J(Se–P) = 962.5 Hz), 33.8 ppm (d,
1J(Se–P) = 722.8 Hz).
4.3.7. (2R,4S)-2-i-Propylamino-4-sulfato-pentane
At first, (4S,6S)-4,6-dimethyl-2,2-dioxide-1,3,2-dioxathiane
(4 g, 24.07 mmol) and i-propylamine (6.2 mL, 72.2 mmol) were
stirred in 80 mL of THF for 48 h during which time a white precip-
itate was formed. The solvent was then evaporated after which
60 mL of ether was added to the residue. The suspension was stir-
red for 30 min and filtered. The solid was washed two times with
ether, and dried with azeothropic distillation of toluene. The sol-
vent was evaporated in vacuo to give 4.80 g of a white powder
(Yield: 88.5%). Mp: 248 °C. Anal. Calcd for C8H19NO4S: C, 42.65;
H, 8.50; N, 6.22; S, 14.23. Found: C, 42.69; H, 8.31; N, 6.28; S,
14.58. 1H NMR (500 MHz, DMSO): d = 8.00 (s, 2H, NH2+), 4.26–
4.09 (m, 1H, CH), 3.40–3.21 (m, 2H, CH), 1.73–1.59 (m, 1H, CH2),
1.51–1.38 (m, 1H, CH2), 1.24–0.90 (m, 12H, CH3). 13C {1H} NMR
(75 MHz, DMSO): d = 70.95 (s), 49.23 (s), 46.64 (s), 22.56 (s),
19.84 (s), 18.59 (s), 16.81 (s).
(1.0 g, yield: 20.0%). ½a D20
¼ þ299:5 (c 1, CH2Cl2), mp: 63–64 °C.
ꢂ
Anal. Calcd for C40H39NO2P2: C, 76.54; H, 6.26; N, 2.23. Found:
C, 76.55; H, 6.21; N, 2.17. 1H NMR (500 MHz, CDCl3): d = 8.00–
7.87 (m, 4H, aromatic), 7.50–7.14 (m, 18H, aromatic), 3.44–
3.19 (m, 2H, CH), 2.22–2.05 (m, 1H, CH), 1.76–1.51 (m, 2H,
CH2), 1.34–1.08 (m, 9H, CH3), 0.69–0.52 (m, 3H, CH3). 31P{1H}
NMR (121 MHz, CDCl3): d = 152.90 (s), 0.00 (s), 13C {1H} NMR
(75 MHz, CDCl3): d = 150.42 (d, J = 6.82 Hz), 150.10 (s), 137.22
(d, J = 14.27 Hz), 136.91 (d, J = 14.89 Hz), 133.85 (d,
J = 19.23 Hz), 133.34 (d, J = 18.61 Hz), 132.84 (s), 130.89 (d,
J = 62.03 Hz), 129.75 (d, J = 62.03 Hz), 129.09 (s), 128.65 (d,
J = 11.79 Hz), 128.26 (d, J = 6.82 Hz), 128.08 (s), 126.48 (d,
J = 84.98 Hz), 124.48 (d, J = 19.85 Hz), 123.90 (d, J = 4.96 Hz),
122.35 (s), 121.95 (d, J = 2.48 Hz), 47.29 (d, J = 10.54 Hz), 47.12
(d, J = 9.30 Hz), 45.19 (d, J = 14.27 Hz), 27.43 (d, J = 10.54 Hz),
24.85 (d, J = 13.65 Hz), 21.20 (d, J = 15.61 Hz), 15.25 (d,
J = 14.89 Hz),. 15.3 (d, J = 14.9 Hz).
4.3.8. (2R,4R)-2-Diphenylphosphino-4-i-propylamino-pentane
At first, LiPPh2ꢃ1,4-dioxane (20.9 g, 74.6 mmol) was dissolved
in 40 mL of abs. THF under an argon atmosphere and then cooled
to about ꢀ30 °C. Next, (2R,4S)-2-i-propylamino-4-sulfato-pentane
(4.8 g, 21.3 mmol) was added drop wise to the red solution for
over 10 minutes. The cooling bath was removed, the reaction
mixture was allowed to warm up to RT, and stirred for one hour.
The color of the reaction mixture remains red. After evaporation
4.3.10. Diselenide of 3
This compound was prepared by following the procedure de-
scribed for the preparation of the diselenide of 1. 31P{1H} NMR
(121 MHz, toluene): d = 84.5 (d, 1J(Se–P) = 955 Hz), 49.4 ppm (d,
1J(Se–P) = 742.5 Hz).

S. Balogh et al. / Tetrahedron: Asymmetry 24 (2013) 66–74
73
4.3.11. Preparation of (11bS)-N-((2S,4S)-4-(diphenylphosphino)-
4.5. Typical procedure to obtain [Rh(COD)(1-4)]BF4 NMR
samples
pentan-2-yl)-N-isopropyl-{8,9,10,11,12,13,14,15-octahydrodina-
phtho[2,1-d:10,20-f][1,3,2]dioxaphosphepin-2-yl}-4-amine 4
(2S,4S)-4-(Diphenylphosphino)-N-isopropyl-pentan-2-yl-amine
(2.1 g, 6.7 mmol) in 60 mL of abs. THF was added dropwise to a
stirred solution of PCl3 (2.45 mL, 26.8 mmol) and triethylamine
(3.71 mL, 26.8 mmol) in 15 mL THF over 20 min at –20 °C temper-
ature. The mixture was then left to warm to room temperature and
was stirred over 1 h. The solvent was evaporated at reduced pres-
sure and the remaining white solid was suspended in 80 mL of
ether. After filtering of triethylamine-hydrochloride and the excess
amount of triethylamine-trichlorophosphine adduct, the organic
phase was evaporated to give a clear oil. This was then dissolved
in 60 mL of abs. THF. Next, triethylamine (1.80 mL, 13.0 mmol)
and a solution of (S)-5,50,6,60,7,70,8,80-octahydro-1,10-binaphtha-
lene-2,20-diol (1.43 g, 4.86 mmol) in 10 mL of abs. THF was added
to the reaction mixture over 10 min at ꢀ20 °C. The reaction mix-
ture was then evaporated at reduced pressure. The remaining
white solid was filtered on a short pad of activated Al2O3 with tol-
uene or chloroform. The product precipitates in ether, is partially
soluble in toluene, and very soluble in chloroform or dichlorometh-
ane. Final purification can be performed with flash chromatogra-
phy under ambient conditions with silicagel as the stationary
phase and commercial chloroform as the eluent (Rf: 0.9) to give
Compound 2 (17.99 mg, 0.03 mmol) in CH2Cl2 (5 mL) was added
dropwise to a solution of [Rh(COD)2]BF4 (12.18 mg, 0.03 mmol) in
CH2Cl2 (5 mL). The resulting orange solution was stirred for
20 min, concentrated, and dissolved in CDCl3 (0.6 mL).
Acknowledgments
The authors thank the National Office for Research and Technol-
ogy (KMOP 1.1.4) for financial support as well as the Huntsman
Corporation for providing samples of their green cyclic carbonate
solvents. This article was partly made under the project TÁMOP-
4.2.1/B-09/1/KONV-2010-0003 and TÁMOP-4.2.2/B-10/1-2010-
0025. These projects are supported by the European Union and
co-financed by the European Social Fund. Special thanks are due
to Béla Édes for the professional technical assistance.
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1.37 g of a white powder (Yield: 32.3%). ½a D20
¼ 60:1 (c 1, CH2Cl2),
ꢂ
mp: 155 °C. Anal. Calcd for C40H47NO2P2: C, 75.57; H, 7.45; N,
2.20. Found: C, 75.84; H, 7.72; N, 2.22. 1H NMR (500 MHz, DMSO):
d = 7.49–7.41 (m, 4H, aromatic), 7.36–7.27 (m, 6H, aromatic), 6.97
(dd, 2H, J = 28.21 Hz, J = 8.20 Hz, aromatic), 6.75 (dd, 2H,
J = 33.21 Hz, J = 7.92 Hz, aromatic), 3.44–3.30 (m, 1H, CH), 3.21–
3.09 (m, 1H, CH), 2.85-2.71 (m, 3H, CH, CH2), 2.71–2.50 (m, 3H,
CH, CH2), 2.31–2.10 (m, 3H, CH, CH2), 1.81–1.64 (m, 7H, CH2 (6H
in H8 binaphthalene, 1H in pentane bridge), 1.54–1.40 (m, 3H,
CH2 (2H in H8 binaphthalene, 1H in pentane bridge), 1.17 (d, 3H,
J = 6.60 Hz, CH3), 1.09 (d, 3H, J = 6.88 Hz, CH3), 1.01 (d, 3H,
J = 7.16 Hz, CH3), 0.77 (dd, 3H, J = 14.68 Hz J = 6.60 Hz, CH3).
31P{1H} NMR (121 MHz, CDCl3): d = 146.03 (s), 1.81 (s). 13C {1H}
NMR (75 MHz, CDCl3): d = 149.49 (s), 148.26 (d, J = 4.96 Hz),
137.95 (d, J = 1.86 Hz), 137.32 (s), 137. 05 (d, J = 4.97 Hz), 136.86
(d, J = 5.58 Hz), 133.86 (d, J = 19.23 Hz), 133.87 (d, J = 1.86 Hz),
133.45 (d, J = 17.98 Hz), 132.53 (s), 129.36 (d, J = 4.34 Hz), 129.16
(d, J = 13.65 Hz), 128.69 (d, J = 8.68 Hz), 128.39 (d, J = 3.09 Hz),
128.30 (d, J = 3.10 Hz), 127.59 (d, J = 1.86 Hz), 118.87 (d,
J = 2.48 Hz), 118.62 (s), 46.35 (d, J = 13.03 Hz), 46.18 (d,
J = 13.03 Hz), 45.12 (d, J = 10.55 Hz), 29.15 (d, J = 11.79 Hz), 27.77
(d, J = 18.61 Hz), 27.24 (d, J = 10.55 Hz), 24.30 (d, J = 8.69 Hz),
22.70 (d, J = 19.23 Hz), 22.76 (d, J = 1.86 Hz), 21.27 (d, J = 8.07 Hz),
14.98 (d, J = 13.64 Hz).
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This compound was prepared by following the procedure de-
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4.4. Typical procedure for the asymmetric hydrogenation
In
a Schlenk tube, a mixture of [Rh(COD)2]BF4 (4.1 mg,
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pylene-carbonate under argon and stirred for 20 min. Methyl (Z)-
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(100 mL internal volume) equipped with a glass test tube. The mix-
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