MaxPHOS ligand: PH/NH tautomerism and rhodium-catalyzed asymmetric hydrogenations

Article abstract of DOI:10.1002/adsc.201300662

MaxPHOS is an active and robust P-stereogenic ligand for asymmetric catalysis. The presence of an -NH- bridge between the two phosphine moieties allows the NH/PH tautomerism to take place. The neutral ligand, in which the NH form predominates, is an air-sensitive compound. However, protonation of MaxPHOS leads to the stable PH form of the ligand, in which the overall positive charge is distributed on both P centers. This protonation turns the MaxPHOS×HBF₄ salt 3 into an air-stable compound both in the solid state and in solution. The salt 3 is also a convenient precursor for the preparation of rhodium(I) complexes by direct ligand exchange with the complex [Rh(acac)(cod)]. Finally, the corresponding rhodium(I)-MaxPHOS complex was tested in the asymmetric hydrogenation of a wide range of substrates. The complex proved to be a highly selective and robust system in these reactions.

Full text of DOI:10.1002/adsc.201300662

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DOI: 10.1002/adsc.201300662
MaxPHOS Ligand: PH/NH Tautomerism and Rhodium-
Catalyzed Asymmetric Hydrogenations
Edgar Cristꢀbal-Lecina,a Pablo Etayo,b Sꢁan Doran,a Marc Revꢁs,a
Pablo Martꢂn-Gago,a Arnald Grabulosa,c Andrea R. Costantino,a,d
Anton Vidal-Ferran,b,e Antoni Riera,a,f,* and Xavier Verdaguera,f,*
a
Institute for Research in Biomedicine (IRB Barcelona), c/Baldiri Reixac 10, E-08028, Barcelona Spain
Fax : (+34)-93-403-7095; phone: (+34)-93-403-4813; e-mail: antoni.riera@irbbarcelona.org or
xavier.verdaguer@irbbarcelona.org
Institute of Chemical Research of Catalonia (ICIQ), Avgda. Paꢃsos Catalans 16, E-43007 Tarragona, Spain
Departament de Quꢂmica Inorgꢄnica, Universitat de Barcelona, Martꢂ i Franquꢅs 1, E-08028 Barcelona, Spain
On leave from: CONICET, Argentina, Instituto de Quꢂmica del Sur (INQUISUR), Departamento de Quꢂmica,
b
c
d
Universidad Nacional del Sur, Av. Alem 1253, 8000 Bahꢂa Blanca, Argentina
Catalan Institution for Research and Advanced Studies (ICREA), Passeig Lluꢂs Companys 23, E-08010 Barcelona, Spain
Departament de Quꢂmica Orgꢄnica, Universitat de Barcelona, Martꢂ i Franquꢅs 1, E-08028 Barcelona, Spain
e
f
Received: September 6, 2013; Revised: December 16, 2013; Published online: March 3, 2014
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201300662.
Abstract: MaxPHOS is an active and robust P-ste- The salt 3 is also a convenient precursor for the
reogenic ligand for asymmetric catalysis. The pres- preparation of rhodium(I) complexes by direct
À
À
ence of an NH bridge between the two phosphine ligand exchange with the complex [RhACTHNUTRGENN(UG acac)ACHTUNTGREN(NUGN cod)].
moieties allows the NH/PH tautomerism to take Finally, the corresponding rhodium(I)-MaxPHOS
place. The neutral ligand, in which the NH form pre- complex was tested in the asymmetric hydrogenation
dominates, is an air-sensitive compound. However, of a wide range of substrates. The complex proved to
protonation of MaxPHOS leads to the stable PH be a highly selective and robust system in these reac-
form of the ligand, in which the overall positive tions.
charge is distributed on both P centers. This protona-
tion turns the MaxPHOS·HBF4 salt 3 into an air-
stable compound both in the solid state and in solu- Keywords: asymmetric catalysis; hydrogenation; P li-
tion.
gands; rhodium; tautomerism
Introduction
the pentavalent iminophosphorane and the corre-
sponding aminophosphane (Figure 1).[4] This equilibri-
Chiral phosphines make a critical contribution to the um is usually shifted towards the P(V) form, which
achievement of high activity and selectivity in asym- makes SPOs and SIPs stable to oxidation. However,
metric catalysis.[1] Consequently, much research effort in the presence of a metal source, coordination to the
has been devoted to developing a wide array of effi- metal efficiently shifts the equilibrium towards the
cient phosphine ligands for a range of catalytic pro-
cesses. Among these, P-stereogenic electron-rich alk- configurationally stable through the PH/NH tauto-
ACHTUNGTRENNUNGylphosphines are highly proficient in asymmetric hy-
PACHTNGUTERN(UNG III) form. We and others have shown that SIPs are
drogenation and other industrially relevant process-
es.[2] A critical disadvantage of this class of com-
pounds is that some of them are prone to oxidation
when exposed to air and therefore have to be handled
under a strictly inert atmosphere.[3] P-stereogenic sec-
ondary phosphine oxides (SPOs) and P-stereogenic
secondary iminophosphoranes (SIPs) do not have this
Figure 1. Phosphinous acid/SPO tautomerism (left) and ami-
limitation as there is tautomeric equilibrium between nophosphine/SIP tautomerism (right).
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Edgar Cristꢀbal-Lecina et al.
Scheme 1. Synthesis of MaxPHOS·HBF4 salt S-3.
Figure 2. Resonance descriptors for MaxPHOS·HBF4, S-3.
meric equilibrium, thus proving that P-stereogenic with JH,P values of 458 and 477 Hz, respectively, sug-
SIPs are valuable pre-ligands for asymmetric cataly- gesting that the tautomeric equilibrium is fully dis-
sis.[5,6]
placed towards the PH form. In this scenario, two
We recently described the synthesis of the P-stereo- electron distributions (I and II) are feasible for Max-
genic aminophosphine building block S-1 and its ap- PHOS·HBF4 (Figure 2). To shed light on the electron-
plicability in the synthesis of HBF4 salt of MaxPHOS ic and structural features of S-3, we sought to eluci-
S-3 (Scheme 1).[7,8] S-1 is a convenient P-stereogenic date the corresponding solid-state structure by X-ray
building block. Both enantiomers can be prepared in diffraction analysis. Suitable crystals of S-3 were ob-
an optically pure form on a multigram scale using tained by crystallization from hot ethyl acetate. The
either enantiomer of cis-1-amino-2-indanol as chiral solved crystal structure showed two independent and
auxiliary.[9] Deprotonation of S-1 with sodium hydride very similar salt pairs in the unit cell (Figure 3).[10]
À
À
and reaction with (t-Bu)2PCl provides the intermedi- The almost identical P(1) N(1) and P(2) N(1) bond
ate iminophosphorane S-2, in which the tautomeric distances found in the X-ray structure suggest that the
equilibrium is completely shifted towards P(V). Re- resonance-hybrid III is the most accurate descriptor
moval of the borane group is achieved by heating S-2 for the [MaxPHOS·H]+ cation and that the positive
with HBF4 in MeOH to afford the phosphonium salt charge is evenly distributed between the two P atoms.
À
À
S-3 in 79% yield (2 steps). The occurrence of the PH/ The angle (134.6/137.38) of P(1) N(1) P(2) is notice-
NH equilibrium in S-3 turns this compound into an
air-stable phosphine pre-ligand.
The promising reactivity of MaxPHOS rhodium
complexes and the interesting structure of the puta-
tive free ligand, which can have various resonance de-
scriptors, led us to further address their structure and
to explore the scope of the asymmetric hydrogenation
reaction catalyzed by the Rh-MaxPHOS system. Here
we report on the chemical properties of MaxPHOS.
Convenient ligand exchange reactions for the prepa-
ration of its corresponding Rh(I) complexes along
with a complete study of its use in asymmetric hydro-
genation are also provided.
Results and Discussion
Figure 3. X-ray structure of (S)-MaxPHOS·HBF4 salt S-3.
Only one of the two independent salt pairs in the unit cell is
shown. ORTEP drawing showing 50% probability ellipsoids.
Chemical Properties of MaxPHOS·HBF4 and
MaxPHOS
À
Selected bond distances (ꢇ) and angles: P(1A) N(1A)
1.594; P(2A) N(1A) 1.585; P(1B) N(1B) 1.585; P(2B)
1
The H NMR spectrum of MaxPHOS·HBF4 salt S-3
À
À
À
in CDCl3 is consistent with a single compound present
in solution. Two distinct resonances for the non-equiv-
alent PH groups were visible at 6.57 and 6.83 ppm
À
À
À
À
N(1B) 1.585; P(1A) N(1A) P(2A) 134.68; P(1B) N(1B)
À
À
P(2B) 137.38; H
(1PA) P(1A) P
G
ACHTUNGTREN(NGNU 2PA) 67.78; and
À
À
À
HACTHNUGTRNE(NUNG 1PB) P(1B) P(2B) HACTHUNGTRENNNUG
796
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MaxPHOS Ligand: PH/NH Tautomerism
ably distant from the 1188 found in the structurally
[11]
À
À
related diphosphinoamine ligand Ph2P NH PPh2.
This observation is possibly attributable to the steric
strain imposed by the three bulky tert-butyl groups. In
the solid state, the [MaxPHOS·H]+ adopts a distorted
À
C2v conformation, with the P H bonds pointing in the
À
same direction. The torsion angle between H(1P)
À
À
P(1) P(2) H(2P) shows a deviation of 67.7/66.18
from planarity. This deviation again can be attributed
to the bulkiness of the groups attached to the phos-
phorus atoms.
Figure 4. Neutral MaxPHOS and its three tautomeric forms.
Isolation of MaxPHOS in its salt form (S-3) has Synthesis of MaxPHOS-Derived Rh(I) Complexes
several advantages over unprotected electron-rich
phosphines. S-3 is a shelf-stable crystalline solid that The stability of the MaxPHOS·HBF4 salt made this
can be stored indefinitely in air without noticeable de- compound an ideal precursor for the reliable prepara-
composition or oxidation. This salt is soluble in high tion of Rh(I) complexes. Our initial procedure used
and medium polarity organic solvents (e.g., MeOH, [RhACTHNUTRGNEU(NG cod)2]BF4 as source of metal (Scheme 3, method
CH2Cl2, EtOAc). In solution, S-3 is also stable to- A).[7] In these conditions, an equivalent of base
wards oxidation in non-deoxygenated solvents due to (NaHCO3) was required to neutralize the evolving
its low acidity. For example, no noticeable decomposi- acid, and the resulting NaBF4 salts had to be selec-
tion of this salt was observed when solved for tively precipitated and filtered to isolate pure [Rh(S-
a month in non-deoxygenated CDCl3. In the presence MaxPHOS)ACHTUNGTRENNUNG
of 2 equivalents of NEt3, no more than 5% of S-3 was solid. The use of [Rh
G
ACHTUNGTRENNUNG
deprotonated, as monitored by 1H NMR (CDCl3), proved and more expedient synthesis of complex S-5
thereby suggesting that the salt is less acid that the (Scheme 3, method B). In this procedure the depart-
corresponding [HNEt3]+, which has a pKa =9 (in ing acetylacetonate ligand quenched the H+, and no
water).
salt by-products were formed. This approach allowed
Neutral MaxPHOS (S-4) was prepared by deproto- the direct isolation of complex S-5 by crystallization
nation of S-3 with n-BuLi at À788C and treatment of in a simple and high-yielding reaction. Using this
the resulting mixture with deoxygenated i-PrOH to same procedure, the analogous norbornadiene com-
ensure the neutrality of the resulting aminophosphine plex S-6 was synthesized in 77% yield using
product (Scheme 2). This approach allowed us to iso- [RhACTHNUTRGENN(GU acac)ACHTUTGNREN(NUGN nbd)] as a metal source. Complex S-6 is
late S-4 as an air-sensitive oil with 31P NMR resonan- also an orange crystalline solid; however, switching
1
ces at 79.5 and 52.8 ppm (C6D6). The H NMR of S-4 the 1,5-cyclooctadiene (cod) moiety for the 2,5-norbo-
À
in C6D6 did not show the characteristic H P resonan- nadiene (nbd) ligand decreased the stability of the re-
ces, thus indicating that the only species in solution sulting Rh complex. Complex S-5 showed high stabili-
for the neutral ligand is the NH tautomer (Figure 4). ty both in solid state and in solution. After one week
This observation contrasts with the related (t- a solution of S-5 in non-deoxygenated CDCl3 did not
Bu)2PNHPACHTUNGTRENNUNG
(t-Bu)2 ligand, which is present as a 70:30 show noticeable decomposition by 1H and
mixture of NH/PH tautomers in toluene solution.[12] 31P NMR.[14] In contrast, complex S-6 showed poor
The preference of one tautomer over the other can be bench stability, and when left in the open air for sev-
explained in terms of the relative basicity of the lone eral hours it quickly decomposed.
pair on phosphorus.[13] Electron-releasing groups on P
Suitable crystals of R-6 for X-ray analysis were ob-
enhance its basicity, thus favoring the corresponding tained upon layering diethyl ether over a solution of
PH tautomer. In this respect, the experimental results Rh complex in CH2Cl2 (Figure 5).[10] The solid state
indicate that switching a single tert-butyl group to structure of R-6 is very similar to that previously re-
a methyl is enough to drive the tautomeric equilibri- ported for complex S-5.[7] The Rh-coordinated Max-
À À
um towards the NH form.
PHOS ligand shows a bite angle of 70.18 and a P N
P angle of 101.98. In this respect, it is interesting to
note the difference in P N P angles between the un-
À À
coordinated MaxPHOS·HBF4 salt (137–1348) and the
coordinated ligand (101.98). When not coordinated,
À À
the P N P angle widens 32–358 to accommodate the
steric bulk on the phosphorus atoms.
Scheme 2. Preparation of the neutral MaxPHOS ligand.
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Scheme 3. Preparation of MaxPHOS-derived Rh(I) complexes.
tron-donating diphosphine ligands provide highly
active and selective catalysts for Rh-catalyzed asym-
metric hydrogenations.[15] In this regard, we compared
the electronic nature of MaxPHOS ligand with the
corresponding carbon-bridged analog trichickenfoot-
phos (TCFP) ligand.[16] With this purpose, we
prepared square
the
corresponding
planar
[Rh(MaxPHOS)(CO)2]BF4 and [RhACTHNGUTERNN(UG TCFP)(CO)2]BF4
G
through carbonylation of the bis-olefin complexes
under CO atmosphere and compared the resultant
symmetric IR stretching frequency for the coordinat-
ed CO ligands (Table 1). This method has been used
by Tolman and others to quantify the donor-acceptor
capacity of phosphines.[17] We also prepared the corre-
sponding diselenides of MaxPHOS and TCFP by
direct treatment with elemental selenium (see the
Supporting Information). Despite the presence of
steric interference, 31P, 77Se coupling constants on
phosphine selenides have also been used to compare
the s-donor capacity of phosphorus ligands.[18] For ex-
ample, PPh3 shows a 31P, 77Se coupling constant of
760.3 Hz, while the more electron-donating PCy3 has
a J (31P, 77Se) of 712 Hz.
Figure 5. X-ray structure of [Rh(R-MaxPHOS)ACTHNURGTNEUNG(nbd)]BF4
complex R-6. ORTEP drawing showing 50% probability el-
lipsoids. Counter ion was omitted for clarity. Selected bond
À
À
distances (ꢇ) and angles: P(1) N(1), 1.706; P(2) N(1),
À
À
À
À
1.697; P(1) Rh, 2.317; P(2) Rh, 2.284; P(1) N(1) P(2),
À
À
101.98; and P(1) Rh N(1), 70.18.
The data in Table 1 indicate that MaxPHOS ligand
is overall less electron-rich that the all-carbon analog
Evaluation of the Electronic Properties of MaxPHOS
TCFP. stretching
The
CO
symmetric
for
[Rh(MaxPHOS)(CO)2]BF4 and [RhACTHNGUTERNN(UG TCFP)(CO)2]BF4
G
Assessment of the steric and electronic properties of were found at 2091 and 2085 cmÀ1, respectively, thus
a particular ligand contributes to understanding and suggesting a more electron-rich metal in the case of
building structure-activity relationships. Bulky elec- the TCFP ligand. Also, the larger 31P, 77Se coupling
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MaxPHOS Ligand: PH/NH Tautomerism
Table 1. IR stretching frequencies and selenide 31P, 77Se cou-
pling constants for MaxPHOS and TCFP ligands.
control (99% ee) within 10 min for both catalysts, as
monitored by H2 uptake (Table 2, entries 1 and 2).
The results at lower catalyst loading (S/C=10,000)
confirmed a similar reactivity for both catalytic sys-
tems in the reduction of MAA (Table 2, entries 3 and
4). The reaction of the trisubstituted Z-MAC sub-
strate was somewhat slower for the S-5 catalyst and
took 30 min to reach full conversion while the TCFP
catalyst reached full conversion in only 15 min
(Table 2, entries 5 and 6). An interesting feature of
the present ligand is that the catalyst system can be
easily prepared in situ by mixing the air-stable Max-
Ligand
nCO symmetric (cmÀ1)
J (31P, 77Se) (Hz)
[Rh(L)(CO)2]BF4
Diselenide
MaxPHOS 2091
TCFP 2085
776, 740
723, 713
PHOS·HBF4 salt (S-3) and [Rh
ACHUTGTNREN(UNG acac)HCATUNGTRENN(UGN cod)] in
MeOH. The in situ generation of catalyst provided
constants for the MaxPHOS diselenide indicate lower similar results in terms of activity and selectivity to
s-donor capacity than the all-carbon analog. Thus, we those achieved with the pre-formed catalyst (Table 2,
conclude that the electron-withdrawing inductive entry 7). Finally, hydrogenation of Z-MAC with S-5
effect of the nitrogen atom in MaxPHOS prevails catalyst was carried out at S/C=10,000 and complete
over its mesomeric effect, and that the switch from conversion and selectivity were achieved in 43 h
À
À
À
À
a
CH2 to an NH bridge produces a less electron- (Table 2, entry 8).
rich ligand.
To further confirm the capacity of S-5 in the asym-
metric hydrogenation, we addressed the reduction of
a series of a-amino acid precursors (Table 3). Sub-
strates with either alkyl or aryl side-chains were hy-
drogenated in high enantiomeric excess. The catalyst
system is compatible with the use of Cbz and Boc
MaxPHOS in Rh-Catalyzed Asymmetric
Hydrogenation
To evaluate the performance of MaxPHOS, we first groups as protective and chelating moieties on the
undertook the hydrogenation of the benchmark sub- amine function (Table 3, entries 1–3).[19] Aromatic
strates methyl a-acetamidoacrylate (MAA) and (Z)- side chains with electron-donating substituents pro-
methyl a-acetamidocinnamate (Z-MAC) using [Rh(S- duced the reduction products with near to perfect se-
MaxPHOS)
ACHTUNGTRENNUNG
pared it to that of the carbon analog [RhACHTUNGTRENNNUG
[16]
ACHTUNGTRENNUNG(cod)]BF4
MeOH at S/C=100 took place with complete stereo- tuted substrate in 98% ee (Table 3, entry 5) while the
Table 2. Hydrogenation of MAA and Z-MAC using MaxPHOS and TCFP ligands.
Entry
R
Cat.
S/C
H2 [bar]
Time[a]
Conversion [%][b]
ee [%][c]
1
2
3
4
5
6
7
8
H
H
H
H
Ph
Ph
Ph
Ph
S-5
100[f]
3
3
5
5
3
3
3
5
10 min
10 min
16 h
>99
>99
>99
>99
>99
>99
>99
>99
99
99
99
99
99
99
99
99
TCFP[d]
S-5
100[f]
10,000[g]
10,000[g]
100[f]
TCFP[d]
S-5
16 h
30 min
15 min
30 min
43 h
TCFP[d]
in situ[e]
S-5
100[f]
100[f]
10,000[h]
[a]
Reaction time monitored by means of hydrogen uptake.
Conversion was determined by H NMR of the crude reaction mixture.
[b]
[c]
[d]
[e]
[f]
1
Enantiomeric excess determined by chiral GC or HPLC.
(cod)]BF4 provided by Stremꢈ was used as a catalyst.
Commercial [RhACTHUNRTGENNUG(S-TCFP)ACHTUNGTRENNUGN
Catalyst was generated in situ by mixing the ligand salt S-3 and [Rh
Reactions carried out with 0.4 g of substrate and 5 mL of MeOH.
Reaction carried out with 10 g of MAA and 10 mL of MeOH.
Reaction carried out with 2 g of Z-MAC and 7 mL of MeOH.
ACHUTGTNRENN(GU acac)ACHUTNGTREN(NUGN cod)] in MeOH.
[g]
[h]
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Table 3. Asymmetric hydrogenation of a-amino acid precursors using S-5 as catalyst.
Entry
R
R’
H2 [bar]
Cat. [mol%]
Solvent
Conv. [%][a]
ee [%][b]
1
2
3
H
Me
Me
Cbz
Boc
Cbz
3
3
3
0.3
0.3
0.3
MeOH
MeOH
MeOH
>99
>99
>99
99
97
99
4
5
6
7
Ac
Ac
Ac
Ac
3
0.3
MeOH
MeOH
MeOH
MeOH
>99
>99
>99
> 99
99
98
99
99
1
1
16
2
3
1
Ac
Ac
60
5
3
1
MeOH
MeOH
>99[c]
97
92
8
99[d]
Ac
Ac
3
3
1
0.3
THF
MeOH
> 99
>99
99
98
9
10
Ac
3
3
0.3
0.3
MeOH
MeOH
>99
>99
98
96
11
12
Boc
Ac
Ac
3
0.3
3
MeOH
MeOH
>99
>99
96
99
13[21]
15
[a]
Reactions were left to react overnight at room temperature unless otherwise specified. Conversion was determined by
1H NMR of the crude reaction mixture.
Enantiomeric excess was determined by chiral HPLC or GC.
Reaction time 48 h at room temperature.
[b]
[c]
[d]
Reaction time 72 h at 758C.
mesityl-substituted acrylate was reduced in 97% ee useful high enantiomeric excess (Table 4, entries 1
(Table 3, entry 8). Substrates with electron-withdraw- and 2). While (E)-methyl 3-acetamido-2-butenoate
ing groups on the aromatic ring provided somewhat was reduced in complete selectivity using MeOH, the
lower enantiomeric excesses, ranging from 96% to reduction of the Z isomer was less selective in this
99% ee (Table 3, entries 9–12).[20] In some cases, for solvent. Reduction of (Z)-methyl 3-acetamido-2-but-
these substrates, switching the solvent from MeOH to
ACHTUNGTRENUNeNG noate in MeOH afforded the product in 89% ee;
THF enhanced reaction selectivity (Table 3, entry 9). however, switching the solvent to THF increased the
Finally, hydrogenation of dehydroamino acid sub- enantiomeric excess to 96%.[22] b-Keto enamides were
strate containing a 3’-quinolyl side chain proceeded also reduced to the corresponding acetamido ketones
with complete selectivity (Table 3, entry 13).[21]
with selectivities ranging from 92 to 97% ee (Table 4,
We also tested other types of substrate, the results entries 3–5). The reaction in this case had to be care-
of which are reported in Table 4. Both (E)- and (Z)- fully monitored and stopped when the first enamide
b-acetamido esters were reduced in synthetically function was reduced. 1,3-Acetamido alcohols were
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MaxPHOS Ligand: PH/NH Tautomerism
Table 4. Asymmetric hydrogenation of miscellaneous substrates using complex S-5 as catalyst.[a]
Entry
Substrate
H2 [bar]
Cat. [mol%]
Solvent
Conv. [%][b]
ee [%][c]
99 (S)
1[7]
2
3
0.3
MeOH
>99
3
3
1
1
MeOH
THF
>99
>99
89 (S)
96 (S)
3
3
1
1
1
MeOH
MeOH
MeOH
>99[d]
>99[d]
>99[d]
92 (S)
96 (S)
97 (S)
4
5
1.5
1.5
6
7
3
3
0.3
1
MeOH
MeOH
>99
95 (R)
87 (R)
>99[e]
8
9
3
3
0.3
0.3
MeOH
MeOH
>99
>99
97 (S)
97 (S)
10
11
12
2
1
1
3
MeOH
MeOH
MeOH
>99
>99
>99
75 (S)
33 (S)
99 (S)
14
12
13
12
1
MeOH
>99
99 (S)
[a]
Hydrogenation reactions were carried out at room temperature unless otherwise specified.
Reactions were left to react overnight unless specified. Conversion was determined by H NMR of the crude reaction
[b]
1
mixture.
[c]
Enantiomeric excess was determined by chiral HPLC or GC. Absolute configuration of the products is shown in brack-
ets.
[d]
[e]
Reaction time between 0.8 and 1.5 h. Longer reaction times provided conversion to the 1,3-acetamido alcohols.
Reaction was carried out at 08C.
produced when the hydrogenation was left to pro- acetamido-3,4-dihydronaphthalene was hydrogenated
ceed.[23] Dimethyl itaconate and the precursor of the in 75% ee and the corresponding 2-acetamido deriva-
Roche ester were reduced in 95 and 87% ee, respec- tive was hydrogenated at 14 bar with 33% ee (Table 4,
tively (Table 4, entries 6 and 7). Simple vinyl acet- entries 10 and 11).[24,25] Finally, vinyl acetamides con-
amides were also tested, producing mixed results. taining fluoropyridinyl and fluoropyrimidinyl substitu-
Open-chain phenyl and 2-naphthyl vinylamides were ents were hydrogenated in 99% ee using the Rh-Max-
reduced in 97% ee (Table 4, entries 8 and 9). Howev- PHOS system (Table 4, entries 12 and 13). These sub-
er, cyclic enamides provided poorer selectivity; 1- strates are precursors for a variety of highly active
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Edgar Cristꢀbal-Lecina et al.
tered through a cannula to another flask, and crystallization
was then induced by adding anhydrous Et2O (500 mL) to
afford S-5 as an orange crystalline solid; yield: 59.3 g (95%).
kinase inhibitors currently under clinical development
for cancer therapy.[26]
IR (film): nmax =3277, 2946, 1475, 1056 cmÀ1
;
1H NMR
(400 MHz, CDCl3): d=1.21 (d, J=16 Hz, 9H), 1.38 (d, J=
15 Hz, 9H), 1.40 (d, J=14 Hz, 9H), 1.77 (dd, J=8 and 1 Hz,
3H), 2.10–2.30 (m, 4H), 2.36–2.57 (m, 4H), 5.11 (br, 2H),
5.39 (br, 1H), 5.54 (m, 2H); 13C NMR (100 MHz, CDCl3):
d=13.6 (d, JP,C =20 Hz, CH3), 26.4 (d, JP,C =5 Hz, 3CH3),
28.8 (CH2), 28.9 (CH2), 29.0 (d, JP,C =6 Hz, 3CH3), 29.1 (d,
Conclusions
In summary, we have shown that the P-stereogenic
aminodiphosphine MaxPHOS is
system with interesting properties. The presence of
a nitrogen linkage between the two phosphine moiet-
ies makes MaxPHOS a less electron-rich ligand than
a novel ligand
J
P,C =6 Hz, 3CH3), 31.3 (CH2), 31.9 (CH2), 35.8 (d, JP,C =
26 Hz, C), 38.8 (d, JP,C =12 Hz, 2 C), 91.1 (dd, JP,C =10 and
7 Hz, CH), 91.7 (t, JP,C =8 Hz, CH), 95.5 (dd, JP,C =9 and
6 Hz, CH), 98.1 (dd, JP,C =9 and 7 Hz, CH); 31P NMR
(121 MHz, CDCl3): d=47.8 (dd, J=128 and 48 Hz), 70.1
(dd, J=128 and 49 Hz); 19F NMR (376 MHz, CDCl3): d=
À
the all-carbon analog TCFP. The presence of the
À
NH bridge between the two P-centers allows the
NH/PH tautomerism to take place. For the neutral
ligand, the NH form predominates; however, protona-
tion of MaxPHOS leads to a particular stable form of
the ligand in which the two phosphorus atoms are
linked to a hydrogen atom and the overall positive
charge is distributed on both P centers through reso-
nance. Thus, NH/PH tautomerism makes the corre-
sponding MaxPHOS·HBF4 salt 3 completely air-stable
since both phosphorus atoms are protected from oxi-
dation. Also, we have shown that the Max-
PHOS·HBF4 salt 3 is a convenient precursor for the
preparation of Rh(I) complexes by direct ligand ex-
À151.3;
HR-MS-ESI:
m/z=474.1919,
calcd.
for
C21H43NP2Rh [MÀBF4]+: 474.1920.
[Rh(S-MaxPHOS)ACHTNUGRTENUNG(nbd)]BF4 (S-6)
[RhACTHNUGRTENNUG(acac)ACHUTNGTRENN(GUN nbd)] (0.227 g, 0.941 mmol) and (S)-Max-
PHOS·HBF4 S-3 (0.350 g, 0.996 mmol) were stirred in
a flame-dried Schlenk tube under N2 in MeOH (6 mL) for
2 h. The solvent was removed under vacuum. The residue
obtained was dissolved in the minimum amount of CH2Cl2
(1.6 mL) in a Schlenk tube before being frozen in liquid N2.
An excess of Et2O was added, and the tube was placed in
the fridge overnight to allow the solid to slowly melt and to
permit slow diffusion of the two solvent layers to form crys-
change with [Rh
strated that MaxPHOS is a selective and robust
ACHTUNGTRENNUNG(acac)ACHTUNGTRNE(NUGN cod)]. Finally, we have demon-
ligand system for the asymmetric Rh(I)-catalyzed hy- tals. The next day the product was isolated as orange crys-
tals; yield: 0.330 g (77%). IR (KBr): nmax =3273, 2950, 2868,
drogenation of a wide range of substrates.
1
2348, 1048 cmÀ1; H NMR (400 MHz, CDCl3): d=5.80 (br s,
1H), 5.59 (br s, 1H), 5.46 (br s, 1H), 5.39 (br s, 2H), 4.22 (d,
J=10.6 Hz, 2H), 1.72 (s, 2H), 1.67 (dd, J=1.4, 7.6 Hz, 3H),
1.37 (d, J=16.3 Hz, 9H), 1.33 (d, J=15.0 Hz, 9H), 1.14 (d,
J=15.0 Hz, 9H); 13C NMR (101 MHz, CDCl3): d=89.8 (dd,
Experimental Section
Neutral (S)-MaxPHOS (S-4)
J
P,C =9.7, 6.3 Hz, CH), 86.6 (dd, JP,C =11.0, 6.1 Hz, CH), 82.6
[7]
(dd, JP,C =9.7, 7.5 Hz, CH), 82.4 (dd, JP,C =8.9, 7.2 Hz, CH),
71.2 (CH2), 56.3 (CH), 55.9 (CH), 38.8 (d, JP,C =12.4 Hz, C),
38.5 (d, JP,C =14.4 Hz, C), 35.7 (d, JP,C =26.9 Hz, C), 29.0 (d,
(S)-MaxPHOS·HBF4 (176 mg, 0.5 mmol) was dissolved in
10 mL of anhydrous, deoxygenated THF and cooled to
À788C in a CO2/acetone bath. n-BuLi (0.32 mL of a 1.6M
solution, 0.5 mmol) was carefully added using a syringe, and
the mixture was left stirring for 30 min. After this period,
the cooling bath was removed and 1 mL of deoxygenated i-
PrOH was added. The solvents were removed under
vacuum, and the residue was extracted with thoroughly de-
oxygenated CH2Cl2/water. The combined organic extracts
were dried over Na2SO4 and filtered. The solvent was re-
moved under vacuum to give the title product as a colorless
J
J
P,C =6.8 Hz, 3CH3), 28.9 (d, JP,C =6.7 Hz, 3CH3), 26.4 (d,
P,C =6.3 Hz, 3CH3), 13.0 (dd, JP,C =19.0, 2.1 Hz, CH3);
31P NMR (121 MHz, CDCl3): d=76.3 (dd, J=136.0,
54.6 Hz), 52.52 (dd, J=138.2, 54.6 Hz); HR-MS-ESI: m/z=
+
À
458.1604, calcd. for [M BF4] (C22H45NP2Rh): 458.1607.
Representative Rh-Catalyzed Asymmetric
Hydrogenations
1
air-sensitive oil. Yield: 105 mg (80%). H NMR (400.1 MHz,
(S)-Methyl 2-acetamido-3-(3,4,5-trimethylphenyl)propano-
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
C6D6): d=1.09 (d, J=10.8 Hz, 9H), 1.05 (d, J=10.8 Hz,
9H), 1.00 (d, J=6.0 Hz, 3H), 0.98 (d, J=11.6 Hz, 9H);
31P NMR (121 MHz, C6D6): d= +79.5 (d, J=179.1 Hz, 1P),
+52.8 (d, J=179.1 Hz, 1P).
0.018 mmol) were weighed and placed in an Aceꢈ glass pres-
sure tube equipped with a pressure gauge and valve. The
vessel was introduced into the glove box, and anhydrous de-
oxygenated MeOH (5 mL) was added to the reaction mix-
ture. The pressure tube was removed from the glove box
and connected to a hydrogen manifold. With stirring, the
vessel was then purged with the aid of vacuum and hydro-
gen, and finally charged at 2 bar of hydrogen gas. The vessel
was then removed from the hydrogen manifold, and the
mixture was left under stirring to react overnight at room
[Rh(S-MaxPHOS)ACHTNUGRTENUNG(cod)]BF4 (S-5)
[RhACHTUNGTRENNUNG(acac)ACHTUNGTRENNUNG(cod) (34.54 g, 111.36 mmol), (S)-MaxPHOS·HBF4
S-3 (39.49 g, 112.47 mmol) and anhydrous MeOH (800 mL)
were stirred in a round-bottomed flask under N2 atmosphere
overnight. The MeOH was removed under vacuum and
solved again with CH2Cl2 (140 mL). The solution was fil-
802
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MaxPHOS Ligand: PH/NH Tautomerism
temperature in a fume hood. The tube was then depressur-
ized and the reaction mixture was filtered through a short
pad of SiO2 and subsequently eluted with EtOAc. The re-
sulting solution was concentrated under vacuum to afford
the desired compound as a white solid; yield: 464 mg (98%).
[2] For examples of electron-rich P-stereogenic phosphines
in catalysis see: a) F. Maienza, F. Spindler, M. Thom-
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2002, 67, 5239; b) M. Stankevic, K. M. Pietrusiewicz, J.
Org. Chem. 2007, 72, 816; c) H. Geng, W. Zhang, J.
Chen, G. Hiou, L. Zhou, Y. Zou, W. Wu, X. Zhang,
Angew. Chem. 2009, 121, 6168; Angew. Chem. Int. Ed.
2009, 48, 6052; d) D. Noh, H. Chea, J. Ju, J. Yun,
Angew. Chem. 2009, 121, 6178; Angew. Chem. Int. Ed.
2009, 48, 6062; e) A. Taylor, R. A. Altman, S. L. Buch-
wald, J. Am. Chem. Soc. 2009, 131, 9900; f) Y. Shibata,
K. Tanaka, J. Am. Chem. Soc. 2009, 131, 12552; g) I.
Chen, L. Yin, W. Itano, M. Kanai, M. Shibasaki, J. Am.
Chem. Soc. 2009, 131, 11664; h) K. Tamura, M. Sugiya,
K. Yoshida, A. Yanagisawa, T. Imamoto, Org. Lett.
2010, 12, 4400; i) D. R. Fandrick, K. R. Fandrick, J. T.
Reeves, Z. Tan, W. Tang, A. G. Capacci, S. Rodriguez,
J. J. Song, H. Lee, N. K. Yee, C. H. Senanayake, J. Am.
Chem. Soc. 2010, 132, 7600; j) A. Yanagisawa, S. Take-
shita, Y. Izumi, K. Yoshida, J. Am. Chem. Soc. 2010,
132, 5328; k) H. Ito, T. Okura, K. Matsuura, M. Sawa-
mura, Angew. Chem. 2010, 122, 570; Angew. Chem. Int.
Ed. 2010, 49, 560; l) W. Tang, A. G. Capacci, A. White,
S. Ma, S. Rodriguez, B. Qu, J. Savoie, N. D. Patel, X.
Wei, N. Haddad, N. Grinberg, N. K. Yee, D. Krishna-
murthy, C. H. Senanayake, Org. Lett. 2010, 12, 1104;
m) J. Granander, F. Secci, S. J. Canipa, P. OꢋBrien, B.
Kelly, J. Org. Chem. 2011, 76, 4794.
1
Conversion was determined by H NMR (>99%) and enan-
tiomeric excess by HPLC. [a]D: +103.7 (c 0.5, CDCl3); mp
88–918C. IR (KBr): nmax =3267(br), 2936, 1759, 1637 cmÀ1
;
1H NMR (400 MHz, CDCl3): d=6.72 (s, 2H, 2 CH Ar), 5.88
(d, J=7 Hz, NH), 4.82 (dt, J=8.0, 6.0 Hz, CHa), 3.74 (s,
3H, OCH3), 3.01 (qd, J=14.0, 6.0 Hz, 2H, CH2b), 2.24 (s,
6H, 2 CH3), 2.13 (s, 3H, CH3), 1.98 [s, 3H, C(O)CH3];
13C NMR (100 MHz, CDCl3): d=172.6 (C, NHCO), 170.0
(C, CO), 136.8 (2C, Ar), 134.0 (C, Ar), 132.7 (C, Ar), 128.6
(2CH, Ar), 53.5 (CH), 52.4 (OCH3), 37.5 (CH2), 23.3 [C(O)-
CH3], 20.8 (2Ar-CH3), 15.3 (Ar-CH3); HR-MS: m/z=
264.1601, calcd. for C15H21NO3 +H+ [M+H]+: 264.1600;
HPLC
[Chiralpakꢈ
IA,
95:5
heptane/2-propanol,
1 mLminÀ1, 254 nm]: tR(S)=10 min (major peak), tR(R)=
12 min.
(S)-Methyl 2-acetamidopropanoate:[27] Methyl 2-acetami-
doacrylate (10.0 g, 69.8 mmol) and S-5 catalyst (3.9 mg,
0.0069 mmol) were weighed and placed in a Bꢉchiꢈ 250
Miniclave. At this point the vessel was introduced in the
glove box, and anhydrous deoxygenated MeOH (10 mL)
was added to the reaction mixture. The vessel was removed
from the glove box, and the pressure vessel was connected
to a hydrogen manifold. With stirring, the vessel was then
purged with the aid of vacuum and hydrogen, and finally
charged at 5 bar of hydrogen gas. The vessel was then re-
moved from the hydrogen manifold, and the mixture was
left under stirring to react at room temperature for 16 h in
a fume hood. The autoclave was then depressurized. The re-
action mixture was filtered through a short pad of SiO2 and
subsequently eluted with EtOAc. The resulting solution was
concentrated under vacuum to afford the desired compound
as a colorless oil; yield: 9.8 g (97%). Conversion was deter-
mined by 1H NMR (>99%) and enantiomeric excess by
chiral GC. [a]D:À91.2 (c 1.0, H2O), 1H NMR (400 MHz,
CDCl3) d=1.31 (d, J=7.3 Hz, 3H), 1.94 (s, 3H), 3.67 (s,
3H), 4.6 (p, 7.2 Hz, 1H), 6.06 (br s, 1H); 13C NMR
(100 MHz, CDCl3): d=18.2 (CH3), 22.9 (CH3), 58.0 (CH),
52.3 (CH3), 169.9 (C=O), 173.6 (C=O); GC [Supelco Beta
DEXTM 120 (30 mꢊ0.25 mmꢊ0.25 mm), isothermal 908C, 15
psi He]: tR(S)=59.5 min (major peak), tR(R)=60.5 min.
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Acknowledgements
We thank the Spanish Ministerio de Economia y Competitivi-
dad (MEC) (CTQ2011-23620, CTQ2010/15292 and
CTQ2011-28512), IRB Barcelona, the Generalitat de Catalu-
nya (2009SGR 00901 and 2009SGR 00623), the ICIQ Foun-
dation, and Enantia for financial support. M. R., P. M.-G.
and E. C.-L. thank the MEC for fellowships. S. D. thanks
“La Caixa Foundation” for a fellowship.
[5] T. Leꢀn, M. Parera, A. Roglans, A. Riera, X. Verda-
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Adv. Synth. Catal. 2014, 356, 795 – 804
ꢆ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
asc.wiley-vch.de
803

FULL PAPERS
Edgar Cristꢀbal-Lecina et al.
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A
R
ACHTUNGTRENNUNG
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