P-stereogenic secondary iminophosphorane ligands and their rhodium(I) complexes: Taking advantage of NH/PH tautomerism

Article abstract of DOI:10.1002/anie.201201031

Have a good SIP: P-stereogenic secondary iminophosphorane (SIP) ligands with a sulfonyl group attached to nitrogen have been prepared. In the presence of rhodium, the tautomeric equilibrium is shifted from the favored PH tautomer towards the P^III tautomer, thereby allowing coordination of the SIP ligand through the P and O atoms. The resulting Rh complexes are effective in the [2+2+2] cycloaddition of enediynes with terminal alkynes. Copyright

Full text of DOI:10.1002/anie.201201031

Angewandte
Chemie
DOI: 10.1002/anie.201201031
Phosphorus Ligands
P-Stereogenic Secondary Iminophosphorane Ligands and Their
Rhodium(I) Complexes: Taking Advantage of NH/PH Tautomerism**
Thierry Leꢀn, Magda Parera, Anna Roglans, Antoni Riera,* and Xavier Verdaguer*
Phosphane ligands play a key role in metal catalysis. Among
these species, bulky electron-rich P-stereogenic phosphanes
show high efficiency in asymmetric hydrogenation and other
industrially relevant applications.[1,2] An important drawback
for this class of compounds is that they are sensitive to
oxidation, and therefore must be handled under a strict inert
atmosphere. Protection of the phosphorus lone pair with BH3
is extremely useful in the synthesis of P-stereogenic com-
The analogous equilibrium between aminophosphanes
and secondary iminophosphoranes (SIPs) has not been
exploited in catalysis.[5] Very recently, Reek and co-workers
described the use of phosphinosulfonamide ligands 1 and 2 in
rhodium-catalyzed asymmetric hydrogenation reactions
(Figure 2).[6] In solution, 1 was found as 1:1.2 mixture of the
corresponding aminophosphane/SIP tautomers showing a low
pounds; however,
a final deprotection/isolation of the
oxygen-sensitive free phosphane is required before complex-
ation with a metal.[3] Such a procedure is not ideal for routine
screening and scale-up. Secondary phosphane oxides (SPOs)
do not have this limitation, because they are resistant to
oxidation and inert to water.[4] Their potential as ligands arises
from the tautomeric equilibrium of these species, as shown in
Figure 1. The pentavalent phosphane oxides are in equilibri-
Figure 2. Secondary iminophosphoranes as potential ligands.
exchange rate on the NMR timescale (RT, CDCl3). Under the
same conditions, ligand 2 was present only in its NH form. The
authors showed that ligands 1 and 2 exhibit rich coordination
chemistry and that, depending on the reaction conditions,
these ligands could lead to either mono- or dinuclear Rh
species, as confirmed by NMR and DFT studies.
Figure 1. Phosphinous acid/SPO tautomerism (left) and aminophos-
phane/SIP tautomerism (right).
um with the trivalent phosphinous acids. The equilibrium is
usually shifted towards the PV form, but it can be shifted to the
phosphinous acid form by coordination of phosphorus to
a metal. Furthermore, P-stereogenic SPOs are configuration-
ally stable and thus chiral information is preserved through
the PH/OH equilibrium.
We recently reported the synthesis of P-stereogenic
aminophosphane 3 (Figure 2) and showed its potential in
the synthesis of the ligand maxphos, which forms an efficient
PnP* hydrogenation catalyst in conjunction with rhodium.[7]
We considered 3 to be highly suitable for the synthesis of
electron-rich P*–sulfonyl iminophosphorane ligands and felt
that we could benefit from NH/PH tautomerism to render
these ligands stable to oxidation. Thus, herein we report on
the synthesis of bulky P-stereogenic iminophosphoranes
derived from 3, and show how these compounds can serve
as convenient preligands in asymmetric catalysis. The X-ray
structure of the resulting monomeric rhodium complexes and
their application in the asymmetric [2+2+2] cycloaddition of
endiynes are also described.
[*] Dr. T. Leꢀn, Prof. A. Riera, Prof. X. Verdaguer
Unitat de Recerca en Sꢁntesi Asimꢂtrica (URSA)-PCB
Institute for Research in Biomedicine (IRB Barcelona) and
Departament de Quꢁmica Orgꢃnica, Universitat de Barcelona
c/Baldiri Reixac 10, 08028 Barcelona (Spain)
E-mail: antoni.riera@irbbarcelona.org
xavier.verdaguer@irbbarcelona.org
Homepage: http://www.ursapcb.org
Starting from 3, anion formation with NaH and reaction
with commercially available sulfonylchlorides provided the
corresponding borane-protected phosphinosulfonamides 4–7
(Table 1). Bulky sulfonylchlorides provided the desired
products in excellent yields (Table 1, entries 1–3). The syn-
thesis of 7, bearing a small, electron-withdrawing CF3SO2N
group, proved troublesome and under the strongly basic
conditions used could not be accomplished using TfCl or Tf2O
(Tf = trifluoromethanesulfonyl). However, this compound
M. Parera, Prof. A. Roglans
Departament de Quꢁmica, Universitat de Girona
Campus Montilivi s/n, 17071 Girona (Spain)
[**] We thank MICINN (CTQ2011-23620 and 23121), IRB Barcelona,
and the Generalitat de Catalunya (2009SGR 00901 and 00637) for
financial support. T.L. and M.P. thank the Generalitat de Catalunya
for an FI fellowship.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201201031.
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Table 1: Synthesis of borane-protected phosphinosulfonamides.
inant, the secondary iminophosphorane form was almost
exclusively the tautomer found in solution for ligands 8–11.
The relative basicity of the phosphorus lone pair can explain
this behavior. Electron-rich groups on the P atom increase the
basic character of the lone pair, thus favoring the formation of
the PH tautomer. Likewise, electron-withdrawing groups on
nitrogen make the NH group more acidic, again favoring the
PH form.[10]
Most interestingly, the SIPs 8–11 are stable to oxidation
and can be stored for months under air without noticeable
decomposition. They are also configurationally stable, as
confirmed by [a]D analysis in chloroform.[11] From a practical
point of view, SIPs 8–11 are of interest because they can be
considered a new class of P-stereogenic preligands, like the
SPOs. As such, they are unique in the sense that they bear
a tert-butylmethylphosphino fragment that is crucial in the
construction of highly effective ligands such as quinoxP*,[12]
tcfp,[13] and maxphos.[7a]
Entry
R
Product
t [h]
Yield [%]
1
2
Mes
Tripp
p-Tol
CF3
4
5
6
7
5
7
8
4
85
85
78
95
3
4[a]
[a] N-phenyl-bis(trifluoromethanesulfonimide) was used instead of the
corresponding sulfonylchloride. Mes=2,4,6-trimethylphenyl,
Tripp=2,4,6-tris(isopropyl)phenyl.
was smoothly prepared in a 95% yield using N-phenyl-
bis(trifluoromethanesulfonimide) (Table 1, entry 4).[8]
Borane deprotection of compounds 4–7 was conducted
under basic conditions in the presence of an excess of 1,4-
diazabicyclo[2.2.2]octane (DABCO). Heating was required to
attain satisfactory conversions in a reasonable time frame. In
this manner, phosphino arylsulfonamides were deprotected
readily in excellent yields (Table 2, entries 1–3). Finally,
borane removal of the triflamide derivative 7 was not viable
We next studied the capacity of SIP ligands for coordina-
tion to rhodium (Scheme 1). Ligands 8, 9, and 10, which were
conveniently scaled up and easily purified, were chosen for
Table 2: Removal of the borane group.
Entry
R
Borane
T [8C]
t [h]
Product
Yield [%]
1
2
Mes
Tripp
p-Tol
CF3
4
5
6
7
80
100
80
2
2
5
6
8
9
10
11
89
92
78
38
Scheme 1. Synthesis of cationic RhI complexes with SIP ligands.
Cod =1,5-cyclooctadiene, Mes=2,4,6-trimethylphenyl, Tripp=2,4,6-
tris(isopropyl)phenyl.
3
4[a]
50
[a] Deprotection was conducted under acidic conditions (HBF4 in
MeOH). DABCO=1,4-diazabicyclo[2.2.2]octane, Mes=2,4,6-trimethyl-
phenyl, Tripp=2,4,6-tris(isopropyl)phenyl.
this purpose.[14] The reaction of bulky iminophosphoranes
with [Rh(cod)2]BF4 resulted in the exchange of a single 1,5-
cyclooctadiene (cod) unit, as determined by H NMR spec-
1
under basic conditions. We attributed this lack of reactivity to
the increased acidity of the NH group. In the presence of
DABCO, this group is deprotonated, thereby making the
troscopy. The addition of an excess of the SIP ligand did not
lead to a dimeric species; we propose that the steric bulk of
the ligand is responsible for this behavior. Complexes 12 and
13 were isolated as air-stable yellow solids.[15] In contrast, 14
resulted in an oil that slowly decomposed to an unidentified
mixture of complexes and, in our hands, could not be purified.
Upon coordination to rhodium, the tautomeric equilibri-
um is effectively displaced towards the phosphinosulfona-
mide form. This leaves a relatively acidic NH group in place
that shows a resonance around 8 ppm in the 1H NMR
spectrum (CDCl3). Accordingly, we attempted to prepare
the analogous neutral complexes in the presence of a base.
These were expected to be similar from a structural point of
view, but very different electronically. The reaction of SIP
ligands 8 and 9 with [Rh(cod)2]BF4 in the presence of
triethylamine provided the neutral, highly apolar complexes
15 and 16, which were then purified by either extraction with
hexanes or filtration through alumina to separate them from
the salt byproduct (Scheme 2).
À
neighboring phosphane even more electron rich. R3P BH3
complexes of electron-rich phosphanes cannot be deprotected
by borane exchange using an amine.[9] Instead, compound 7
was deprotected under acidic conditions (HBF4, MeOH) to
afford the desired compound 11 in 38% yield (Table 2,
entry 4).
1H NMR analysis of compounds 8 and 9 in CDCl3 showed
a single set of signals and the characteristic resonance of a PH
group, with couplings of 452 Hz (JP) and 4 Hz (JH). In
contrast, compound 10, which bears a TsN group (Ts =
p-toluenesulfonyl), showed two sets of signals in a 1:8 ratio
1
in the 31P and H NMR spectra (CDCl3, RT), with the PH
tautomer being the most abundant in solution. Finally, ligand
11, which bears an electron-withdrawing TfNH group, was
present only as the PH tautomer, as determined by NMR
spectroscopy in CDCl3. In contrast with the ligands reported
by Reek and co-workers, where the NH form was predom-
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center and the hemilabile sulfonyl group, could provide
good results in the cyclization of this type of substrates.
Thus, we started studying the use of cationic complexes 12
and 13 as catalysts in the cyclization of the oxygen-tethered
enediyne 17 in refluxing CH2Cl2 (Table 3, entries 1 and 2).
Under these conditions, the reaction took place cleanly and
with complete conversion to afford diene 20 as the sole
product with up to 63% enantiomeric excess. Catalyst 13,
Scheme 2. Preparation of neutral RhI complexes with SIP ligands.
Cod =1,5-cyclooctadiene, Mes=2,4,6-trimethylphenyl, Tripp=2,4,6-
tris(isopropyl)phenyl.
Table 3: [2+2+2] Intramolecular cycloaddition of terminal enediynes
catalyzed by SIP–Rh complexes.
Given their novelty and the lack of structural studies for
this class of ligands, we sought a definitive structural
elucidation for the corresponding rhodium complexes. In
this regard, we succeeded in growing crystals suitable for
X-ray analysis of cationic complex 12 (Figure 3).[16] The
structure of 12 showed that the ligand coordinates to the
Entry
X
Catalyst Conditions
Conv. [%][a] ee [%][b]
1
2
3
4
5
6
7
8
O
O
O
O
12
13
13
15
16
13
13
13
reflux, 6 h
reflux, 6 h
RT, 26 h
reflux, 7 h
reflux, 7 h
reflux, 8 h
RT, 15 h
reflux, 3 h,
10 mol% HBF4
RT, 24 h,
10 mol%, HBF4
RT, 24 h,
99
99
98[c]
0
59
63
69
–
O
0
–
NTs
NTs
NTs
99
99
95[c]
67
67
85
9
NTs
NTs
13
13
99
99
90
81
Figure 3. a) Ortep plot for the X-ray structure of 12 with thermal
ellipsoids shown at 50% probability; anion is omitted for clarity.
b) Metal and ligand assembly showing the pseudo-C2 disposition of
bulky groups around the metal. Selected bond distances (ꢄ): O(1)–S
1.419(7), O(2)–S 1.456(6), S–N 1.603(7).
10
100 mol% HBF4
reflux, 24 h
reflux, 24 h
10 mol% HBF4
reflux, 24 h
10 mol% TFA
11
12
NSO2Mes 13
NSO2Mes 13
99
68
83
91[c]
13
NSO2Mes 13
99
94
14
15
NSO2Mes binap[d] reflux, 24 h
NSO2Mes binap[d] reflux, 24 h
10 mol% TFA
45[c]
29[c]
54
52
metal center through the phosphorus atom and the O2 of the
sulfone group. The steric repulsions between the bulky tert-
butyl group attached to phosphorus and the aromatic group of
the sulfone force them to adopt a trans orientation and so
direct the coordination of the sulfone. In this scenario, the
sulfur atom of the sulfone group becomes stereogenic and
adopts an R configuration. Interestingly, the bulky trans
groups are in similar proximity to the metal center (3.57
and 3.58 ꢀ), thereby creating a pseudo-C2 symmetry.
[a] Selectivity was 100% when using catalysts 12 and 13. Conversion was
determined by 1H NMR spectroscopy. [b] In all cases the major product
was the levorotatory one. [c] Yield of isolated product. [d] A mixture of
[Rh(cod)2]BF4/(R)-binap (1:1) was used as catalyst. Binap=2,2’-bis(di-
phenylphosphino)-1,1’-binaphthyl, Mes=2,4,6-trimethylphenyl, TFA=
trifluoroacetic acid, Ts=p-toluenesulfonyl.
We have recently reported the use of N-phosphinosulfin-
amide ligands (PNSO) in the [2+2+2] intramolecular cyclo-
addition of enediynes with terminal alkynes.[17] Enediynes
with terminal alkynes are challenging substrates to cyclize.[18]
For instance, Tanaka reported the Rh-catalyzed cyclization of
17 with 78% yield and 48% ee using the tolbinap/[Rh-
(cod)2]BF4 system (tolbinap = 2,2’-bis[di(p-methylphenyl)-
phosphino]-1,1’-binaphthyl).[19] The PNSO ligands showed
an acceptable reactivity profile, but afforded low selectivity
(up to 32% ee). The low selectivity was attributed to the fact
that the chiral information in these ligands is located at the
hemilabile sulfinyl group. At this point, we thought that the
rhodium–SIP complexes 12–16, with the chiral tert-butylme-
thylphosphino moiety strongly attached to the rhodium
which contains the bulky Tripp group, was more selective than
the mesityl complex 12. Using 13, the selectivity was
improved to 69% ee when the reaction was run at room
temperature (Table 3, entry 3). In contrast, the neutral
complexes 15 and 16, with a more electron-rich metal
center, did not provide any cyclization product and were
inactive in the [2+2+2] cyclization of enediynes (Table 3,
entries 4 and 5). The N-tethered substrate 18 was cyclized
with similar levels of selectivity (67% ee) at both room
temperature and at reflux (Table 3, entries 6 and 7). However,
after some experimentation, we found that the addition of
a catalytic amount of acid enhanced the selectivity with this
substrate. The addition of HBF4 (10 mol%) allowed an
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increase to 90% ee (Table 3, entries 8 and 9). This behavior
was also observed for substrate 19, which, upon addition of
trifluoroacetic acid (TFA; 10 mol%), provided the corre-
sponding diene 22 in 94% ee (Table 3, entry 13). Finally, for
comparison, enediyne 19 was submitted to cyclization using
the standard binap/[Rh(cod)2]BF4 catalytic system (binap =
2,2’-bis(diphenylphosphino)-1,1’-binaphthyl) to afford 22 in
only 45% yield and 54% ee (Table 3, entries 14 and 15).[20]
Cyclization of enediynes 17–19 can occur through path-
ways I or II (Scheme 3). It has been postulated that pathway I,
which includes an early-stage stereodetermining step, pro-
vides a high level of stereocontrol, while pathway II leads to
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a lower enantiomeric bias.[18a] On the other hand, DFT
calculations indicated that enediynes with terminal alkynes
cyclize preferentially through initial alkyne–alkyne coupling
(pathway II), usually leading to low enantiomeric excess.[21]
Most remarkably, the selectivities found in this study of new
SIP ligands are the highest reported for substrates that cyclize
through pathway II, and thus highlight the utility of ligands
with PH/NH tautomerism in catalysis.
In summary, herein we disclose P-stereogenic N-sulfonyl
secondary iminophosphorane ligands as a novel class of
preligands in metal catalysis. These compounds are configura-
tionally stable and, thanks to NH/PH tautomerism, they do
not undergo oxidation in air. We have shown that the
tautomeric equilibrium is displaced towards the NH form in
the presence of a rhodium source, thus allowing efficient P,O
coordination with the metal center. Upon complexation, the
bulky groups on the sulfone and the phosphane are placed
trans to each other, thereby creating a pseudo-C2 symmetry
around the metal center. Finally, we have demonstrated that
these complexes are efficient and selective catalysts for the
[2+2+2] cycloaddition of enediynes with terminal alkynes. In
this regard, our system provides satisfactory results where
other methods fall short.
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Received: February 7, 2012
Revised: April 12, 2012
Published online: && &&, &&&&
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Keywords: asymmetric catalysis · cycloaddition reactions ·
ligand design · phosphorus · rhodium
.
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Chemie
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[14] Ligands 8–10 were prepared on a gram scale. 1.66 g of 9 was
prepared in one batch, see the Supporting Information for
details.
[15] Complexes 12, 13, 15, and 16 were found to be bench-stable
compounds for a period of weeks. Although they were isolated
as a pure compounds based on NMR spectroscopy, they failed to
provide correct elemental analysis.
[16] CCDC 865072 (12) contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
from The Cambridge Crystallographic Data Centre via www.
ccdc.cam.ac.uk/data_request/cif.
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[10] DFT calculations at the B3LYP/6-31G* level do not reproduce
the observed stability of the SIP form with respect the NH form.
However, the position of the NH/PH equilibrium can be
rationalized by invoking the basic character of the phosphorus
atom. The preference for the PH tautomer when electron rich
groups are attached to the P atom is well documented for
aminodiphosphanes; see: a) A. M. Caminade, E. Ocando, J. P.
Majoral, M. Cristante, G. Bertrand, Inorg. Chem. 1986, 25, 712 –
714; b) T. Chivers, D. J. Eisler, J. S. Ritch, H. M. Tuononen,
Angew. Chem. 2005, 117, 5033 – 5036; Angew. Chem. Int. Ed.
2005, 44, 4953 – 4956; c) J. S. Ritch, T. Chivers, D. J. Eisler, H. M.
Tuononen, Chem. Eur. J. 2007, 13, 4643 – 4653.
[11] A sample of SIP 8 was left in a chloroform solution for two
weeks and showed a stable [a]D. In solution, compounds 8 and 9,
which bear bulky sulfonyl groups, are stable to both oxidation
and hydrolysis. Compounds 10 and 11 were found to be stable to
oxidation, but they were slowly hydrolyzed (3 days) in non-
anhydrous solvents.
[19] K. Tanaka, G. Nishida, H. Sagae, M. Hirano, Synlett 2007, 1426 –
1430.
[12] T. Imamoto, K. Sugito, K. Yoshida, J. Am. Chem. Soc. 2005, 127,
11934 – 11935.
[13] a) G. Hoge, H. Wu, W. S. Kissel, D. A. Pflum, D. J. Greene, J.
Bao, J. Am. Chem. Soc. 2004, 126, 5966 – 5967; b) I. D. Gridnev,
T. Imamoto, G. Hoge, M. Kouchi, H. Takahashi, J. Am. Chem.
Soc. 2008, 130, 2560 – 2572.
[20] Monophos, dipamp, and maxphos (a trichickenfootphos analog)
were inactive in this process and showed no conversion.
[21] a) A. Dachs, A. Roglans, M. Solꢅ, Organometallics 2011, 30,
3151 – 3159; b) A. Dachs, A. Pla-Quintana, T. Parella, M. Solꢅ,
A. Roglans, Chem. Eur. J. 2011, 17, 14493 – 14507.
Angew. Chem. Int. Ed. 2012, 51, 1 – 6
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!

.
Angewandte
Communications
Communications
Phosphorus Ligands
Have a good SIP: P-stereogenic secon-
dary iminophosphorane (SIP) ligands
with a sulfonyl group attached to nitrogen
have been prepared for the first time. In
the presence of rhodium, the tautomeric
equilibrium is shifted from the favored
PH tautomer towards the PIII tautomer,
thereby allowing coordination of the SIP
ligand through the P and O atoms. The
resulting Rh complexes are effective in
the [2+2+2] cycloaddition of enediynes
with terminal alkynes.
T. Leꢁn, M. Parera, A. Roglans, A. Riera,*
X. Verdaguer*
&&&& — &&&&
P-Stereogenic Secondary
Iminophosphorane Ligands and Their
Rhodium(I) Complexes: Taking
Advantage of NH/PH Tautomerism
6
www.angewandte.org
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2012, 51, 1 – 6
These are not the final page numbers!