Highly diastereoselective dihydride formation by activation of methanol with IrCl{(S)-binap}(PPh3)

Article abstract of DOI:10.1039/b102395k

Reaction of [IrCl{(S)-binap}(PPh₃)] {(S)-3} with methanol gave one of the diastereomers of the cis, mer-dihydride, cis,mer-OC-6-44-A-[IrCl(H₂){(S)-binap}(PPh₃)] {(S)-4a} stereoselectively, the structure of which was determined crystallographically, whereas the reaction of (S)-3 with H₂ produced a 1:1 mixture of the diastereomers of the cis,mer-dihydride, (S)-4a and cis,mer-OC-6-44-C-[IrCl(H₂){(S)-bi-nap}(PPh₃)] {(S)-4b}.

Full text of DOI:10.1039/b102395k

Highly diastereoselective dihydride formation by activation of
methanol with IrCl{(S)-binap}(PPh3)
Kazuhide Tani,* Kouji Nakajima, Aika Iseki and Tsuneaki Yamagata
Department of Chemistry, Graduate School of Engineering Science, Osaka University, Toyonaka, Osaka
560-8351, Japan. E-mail: tani@chem.es.osaka-u.ac.jp; Fax: +81-6-6850-6245
Received (in Cambridge, UK) 13th March 2001, Accepted 13th July 2001
First published as an Advance Article on the web 3rd August 2001
Reaction of [IrCl{(S)-binap}(PPh3)] {(S)-3} with methanol
gave one of the diastereomers of the cis,mer-dihydride,
cis,mer-OC-6-44-A-[IrCl(H2){(S)-binap}(PPh3)]
{(S)-4a}
stereoselectively, the structure of which was determined
crystallographically, whereas the reaction of (S)-3 with H2
produced a 1+1 mixture of the diastereomers of the cis,mer-
dihydride, (S)-4a and cis,mer-OC-6-44-C-[IrCl(H2){(S)-bi-
nap}(PPh3)] {(S)-4b}.
Although the stereochemistry for the formation of the dihy-
drides of rhodium and iridium carrying chiral ligands is of great
interest in connection with their potential as asymmetric
hydrogenation and its mechanism,1 only very rare cases of
detection of diastereoselective formation of dihydrides have
been reported.2–6 Even in such cases the absolute configuration
of the hydride was only estimated based on spectroscopic
results. Here we report a highly diastereoselective formation of
the dihydride (S)-4a by reaction of [IrCl{(S)-binap}(PPh3)]
{(S)-3}7 with methanol, the absolute configuration of which
was determined by X-ray analysis. Oxidative addition of
dihydrogen to (S)-3 gave also the same cis,mer-dihydrides 4 but
as a 1+1 mixture of the diastereomers 4a and 4b.
Fig. 1 View of (R)-3 showing the labeling of the heteroatoms. Displacement
ellipsoids are drawn at the 50% probability level. Selected bond distances
(Å) and angles (°): Ir–P(1) 2.1998(19), Ir–P(2) 2.277(2), Ir–P(3) 2.305(2),
Ir–Cl 2.394(2); P(1)–Ir–P(2) 91.58(7), P(1)–Ir–P(3) 99.20(8), P(2)–Ir–P(3)
157.11(8), P(1)–Ir–Cl 159.23(10), P(2)–Ir–Cl 87.76(9), P(3)–Ir–Cl
89.09(9).
Recently we have reported that iridium( ) complexes bearing
I
peraryldiphosphines [IrCl(diphosphine)]2 (1) can activate O–H
bonds easily at room temperature. For example, [IrCl(binap)]2
(1a) reacted easily with methanol at room temperature to give
the hydrido(methoxo) complex, [{Ir(H)(binap)}2(m-OMe)2(m-
Cl)]Cl (2) in good yield,8 which becomes an efficient catalyst
precursor for transfer hydrogenation of alkynes using methanol
as a hydrogen source.9 During the studies on the reactivity of
complex 1, we have found that [IrCl(binap)]2 (1a) reacted with
2 equiv. of PPh3 to give a square-planar complex, [IrCl(bi-
nap)(PPh3)] (3) selectively as a red solid.7 The molecular
structure of (R)-3 is shown in Fig. 1.† In contrast to complex 1a,
complex 3 was fairly stable in the solid state. Heating a toluene
solution of complex (S)-3 with large excess of methanol at 70
°C during 6 h, however, gave pale-red precipitates (67% yield)
of the dihydride (S)-4a10 as a single diastereomer. The crude
reaction mixture obtained after removal of the all solvents
showed almost quantitative and selective formation of the
dihydride 4 containing a small amount of the other diastereomer
(S)-4b11 {(S)-4a+(S)-4b = 92+8}. The reaction of (S)-3 with
methanol proceeded even at room temperature in toluene and
after 45 h gave selectively the cis,mer-dihydride (S)-4a in about
90% yield {(S)-4a : (S)-4b = 96+4} (Scheme 1). The reaction,
however, is very much suppressed when excess triphenylphos-
phine is present. Complex (S)-4a was fully characterized by
coordinate to the central atom in a mer configuration. The
dihydride complex showed two Ir–H stretchings (Nujol) at 2207
(trans to Cl) and 2083 cm21 (trans to P). Suitable crystals for X-
ray analysis were obtained by recrystallization from toluene-
methanol. The absolute configuration of the dihydride was
concretely determined by X-ray crystallography.† The ORTEP
drawing is depicted in Fig. 2. The predominant diastereomer
(S)-4a obtained by the reaction of (S)-3 with methanol was
revealed to be the (S)-OC-6-44-A isomer. The long bond
distance (2.363(2) Å) of Ir–P(2) indicates a strong trans
influence of the hydride ligand. Similarly, the Ir(III)–Cl bond
distance (2.505(3) Å) is long and even longer than the Ir(
distance (2.394(2) Å) in the Ir( ) complex (R)-3. The acute P(1)–
I
)–Cl
I
Ir–P(3) angle (167.30(9)°) reflects also the small hydride
ligand. The same diastereomeric dihydride (S)-4a was also
obtained selectively by the reaction of (S)-2 with 2 equiv. of
PPh3 in a mixed solvent of methanol and toluene under reflux
1
elemental analyses and spectroscopic studies. The H NMR
spectrum showed two characteristic hydride signals at d 219.56
and –9.97; the latter signal showed one large P–H coupling
indicating that the hydride is situated trans to one phosphorus
atom while the former signal does not show any couplings
larger than ca. 20 Hz, indicating the hydride situates cis to all
the phosphorus atoms. The 31P NMR spectrum measured at 35
°C showed three phosphorus signals at d 24.7 (br m), 14.9 (dd,
J = 16, 358 Hz) and 19.6 (dd, J = 11, 358 Hz) in a 1+1+1 ratio;
the signal pattern indicates that the three phosphorus atoms
Scheme 1 Reagents and conditions: i, MeOH–toluene, room temp.; ii, H2 (1
atm)/toluene, room temp.
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Chem. Commun., 2001, 1630–1631
This journal is © The Royal Society of Chemistry 2001
DOI: 10.1039/b102395k

Notes and references
† Crystal data for (R)-3: C62H47ClIrP3, M = 1112.56, trigonal, space group
P31 (no. 144), a = 18.579(5), b = 18.579(5), c = 13.368(6) Å, a = 90, b
= 90, g = 120°, U = 3996(2) Å3, T = 296(2) K, Z = 3, Dc = 1.387 Mg
m23, l(Mo-Ka)
=
0.71069 Å, m
=
2.684 mm21, 2qmax
0.2958, Tmax 0.4998), a linear
= 55.0°,
absorption corrections14 (Tmin
=
=
correction was applied (24.9% decay), 12250 unique reflections including
Friedel pairs (Rint = 0.0426), direct methods (SIR97),15 full-matrix least-
squares methods (SHELXL-97),16 refined on F2. The aromatic H-atoms
were included in the refinement on calculated positions riding on their
carrier atoms (C(sp2)–H = 0.93 Å, Uiso(H) = 1.2Ueq(C) Å2). R1/wR2 (for
9697 reflections with I > 2.0s(I)) = 0.0455/0.1288, R1/wR2 (for 12550
reflections with all data)
restraints, c (Flack parameter) = 20.005(7), GOF = 1.040, Dr (max./
min.) = 1.513/20.745 e Å23
= 0.0739/0.1429 for 581 parameters and 5
.
For (S)-4a: C62H49ClIrP3, M = 1114.57, monoclinic, space group P21
(no. 4), a = 11.070(3), b = 21.979(4), c = 11.788(3) Å, b = 96.50(2)°, U
= 2849.7(12) Å3, T = 296(2) K, Z = 2, Dc = 1.299 Mg m23, l(Mo-Ka)
= 0.71069 Å, m = 2.509 mm21, 2qmax = 65.0°, absorption corrections14
(Tmin = 0.4023, Tmax = 0.8168), a linear correction was applied (9.2%
decay), 21894 reflections measured, 20525 unique reflections including
Friedel pairs (Rint = 0.0693), direct methods (SIR97),15 full-matrix least-
squares methods (SHELXL-97),16 refined on F2. Non-hydrogen atoms were
anisotropically refined. The aromatic H-atoms were included in the
refinement on calculated positions riding on their carrier atoms (C(sp2)–H
= 0.93 Å, Uiso(H) = 1.2Ueq(C) Å2). Probable hydride-ligand positions
were calculated at the minima of the potential energy by the program
HYDEX17 and were included as fixed contributions. R1 = 0.0578, wR2 =
0.1483, (for 8479 reflections with I > 2.0s(I)), R1 = 0.2305, wR2 =
0.1953, (for 20525 reflections with all data), parameters = 603, c (Flack
parameter) = 20.050(9), GOF = 0.964.
Fig. 2 View of (S)-4a showing the labeling of the heteroatoms and the
hydride hydrogen atoms. Displacement ellipsoids are drawn at the 50%
probability level. Selected bond distances (Å) and angles (°): Ir–P(1)
2.299(3), Ir–P(3) 2.326(3), Ir–P(2) 2.363(2), Ir–Cl 2.505(3); P(1)–Ir–P(3)
167.30(9), P(1)–Ir–P(2) 92.70(16), P(3)–Ir–P(2) 100.00(16), P(1)–Ir–Cl
88.81(10), P(3)–Ir–Cl 88.38(10), P(2)–Ir–Cl 102.98(10).
but accompanied by small amounts of several unidentified
hydrides [eqn. (1)]. Hydrogenation of complex (S)-3 with
dihydrogen in toluene at room temperature also yielded the
same cis,mer-dihydride 4 but as an almost 1+1 mixture of the
two diastereomers (S)-4a and (S)-4b. These dihydrides are
obtained by a concerted cis addition of H2 along the P–Ir–Cl
axis of the square planar complex (S)-3. In the reaction products
obtained by oxidative addition of H2 to (S)-3 substantial
amounts of cis,fac-isomer 5 resulted from cis addition of H2
along the P–Ir–P axis of complex (S)-3 and the other possible
dihydride 6 could not be detected.
CCDC reference numbers 168686 and 168687.
See http://www.rsc.org/suppdata/cc/b1/b102395k/ for crystallographic
data in CIF or other electronic format.
1 P. P. Deutsch and R. Eisenberg, Chem. Rev., 1988, 88, 1147.
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3381.
Although the detailed mechanism for diastereoselective
formation of (S)-4a by the reaction of (S)-3 with methanol is not
clear at present, the large difference of the selectivity between
methanol and hydrogen may be explained as follows. Dihydride
formation from the reaction of 3 with methanol could be
explained by b-hydrogen elimination from the Ir–OMe group of
the initial oxidative addition product of methanol, a hydrido-
(methoxo) complex, ‘IrCl(H)(OMe)(binap)(PPh3)’, which is
not detected in the reaction mixture. Such iridium dihydride
formation from the H–Ir–OMe species has been reported.12
Because methanol is much larger than dihydrogen, methanol
approaches also only along the P–Ir–Cl axis to complex (S)-3
and in addition can discriminate efficiently between the two
9 K. Tani, A. Iseki and T. Yamagata, Chem. Commun., 1999, 1821.
10 OC-6-44-A isomer 4a: 1H NMR (C6D6, 35 °C): d 219.56 (m, 1H),
29.97 (dddd, J = 5.2, 11.2, 22.1, 132.9 Hz, 1H), 6.3–8.1 (m, 44H), 8.20
(t, J = 8.4 Hz, 1H), 8.87 (t, J = 8.4 Hz, 2H). 31P NMR (C6D6, 35 °C):
d 24.7 (br m, 1P), 14.9 (dd, J = 16, 358 Hz, 1P), 19.6 (dd, J = 11, 358
Hz). IR(Nujol): 2207 (nIr–H), 2083 cm21 (nIr–H), Anal. Calc. for
C62H49ClIrP3: C, 66.81; H, 4.43. Found: C, 66.73; H, 4.54%.
11 OC-6-44-C isomer 4b: 1H NMR (C6D6, 35 °C): d 220.03 (m, 1H),
210.56 (dddd, J = 5.1, 16.9, 22.2, 136.8 Hz, 1H); 31P NMR (C6D6, 35
°C): d 1.0 (m, 1P), 4.6 (dd, J = 13, 356 Hz, 1P), 7.0 (dd, J = 16, 356
Hz). IR(Nujol): 2274 (nIr–H), 2119 cm21 (nIr–H).
12 O. Blum and D. Milstein, Angew. Chem., Int. Ed. Engl., 1995, 34, 229;
O. Blum and D. Milstein, J. Am. Chem. Soc., 1995, 117, 4582.
13 As one referee pointed out, a possibility that the high diaster-
eoselectivity arises during the b-hydrogen elimination from methoxide
cannot be excluded at present.
14 A. C. T North, D. C. Phillips and F. S. Mathews, Acta. Crystallogr.,
Sect. A, 1968, 24, 351.
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Crystallogr., 1999, 32, 115.
16 G. M. Sheldrick, , SHELXL 97, Programs for Crystal Structure Analysis
(Release 97-2), University of Go¨ttingen, Germany, 1997.
17 A. G. Orpen, J. Chem. Soc., Dalton Trans., 1980, 2509.
diastereotopic planes of (S)-3, above and below the Ir( ) square
I
plane. By inspection of a CPK model, approach of methanol
from above the square plane of (S)-3 described in Scheme 1 is
more preferable and leads to the highly stereoselective forma-
tion of dihydride (S)-4a.13 In contrast, the small dihydrogen
molecule can not efficiently discriminate between the diaster-
eotopic planes.
This work was partly supported by the Grant-in Aid for
Scientific Research from the Ministry of Education, Science,
Sports, and Culture of Japan.
Chem. Commun., 2001, 1630–1631
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