Highly diastereoselective oxidative addition of alkyl halides to rhodium and iridium carbonyl complexes having an η5:η1-(Ind-P)n ligand ((Ind-P)nH = C9H7(CH2)nPR2; R = Ph, Cy; n = 2-4)

Article abstract of DOI:10.1021/om9800997

Highly diastereoselective oxidative addition of alkyl halides to {η⁵:η¹-(Ind-P)_n}RhCO ((Ind-P)_n = C₉H₆(CH₂)_n PR₂; n = 2 (1), 3 (2), 4 (3); R = Ph (a), Cy (b), Cy = cyclohexyl) has been achieved and the stereo-selectivity was governed by the length of spacer of the (Ind-P)_n ligand and the kind of alkyl halide; reaction of {η⁵-(Ind-P)_n=2}RhCO (1a) with EtI in CH₂Cl₂ gave (R*,R*)-{η⁵:η¹-(Ind-P) _n=2}RhI(COEt) (7a) in 98% yield with 92% de, whereas {η⁵:η¹-(Ind-P)_n=4}RhCO (3a) gave the other diastereomer, (R*,S*)-{η⁵:η¹-(Ind-P) _n=4}RhI-(COEt), in 97% yield with 96% de. Reaction of {η⁵:η¹-(Ind-P)ⁿ⁼⁴}Ir(CO) (8a) with MeI gave a diastereomer mixture of [{η⁵:η¹-(Ind-P)ⁿ⁼⁴}Ir(Me)(CO)]I (9a) in a 92:8 ratio, which did not afford the corresponding acetyl complex upon heating under reflux in CH₂Cl₂.

Full text of DOI:10.1021/om9800997

4338
Organometallics 1998, 17, 4338-4340
High ly Dia ster eoselective Oxid a tive Ad d ition of Alk yl
Ha lid es to Rh od iu m a n d Ir id iu m Ca r bon yl Com p lexes
Ha vin g a n η5:η1-(In d -P )n Liga n d ((In d -P )n H )
C9H7(CH2)n P R2; R ) P h , Cy; n ) 2-4)
Yasutaka Kataoka,* Atsushi Shibahara, Yoshinori Saito,
Tsuneaki Yamagata, and Kazuhide Tani*
Department of Chemistry, Graduate School of Engineering Science, Osaka University,
Toyonaka, Osaka 560-8531, J apan
Received February 17, 1998
Ch a r t 1
Summary: Highly diastereoselective oxidative addition
of alkyl halides to {η5:η1-(Ind-P)n}RhCO ((Ind-P)n
)
C9H6(CH2)nPR2; n ) 2 (1), 3 (2), 4 (3); R ) Ph (a ), Cy
(b), Cy ) cyclohexyl) has been achieved and the stereo-
selectivity was governed by the length of spacer of the
(Ind-P)n ligand and the kind of alkyl halide; reaction of
{η5:η1-(Ind-P)n)2}RhCO (1a ) with EtI in CH2Cl2 gave
(R*,R*)-{η5:η1-(Ind-P)n)2}RhI(COEt) (7a ) in 98% yield
with 92% de, whereas {η5:η1-(Ind-P)n)4}RhCO (3a ) gave
the other diastereomer, (R*,S*)-{η5:η1-(Ind-P)n)4}RhI-
(COEt), in 97% yield with 96% de. Reaction of {η5:η1-
(Ind-P)n)4}Ir(CO) (8a ) with MeI gave a diastereomer
mixture of [{η5:η1-(Ind-P)n)4}Ir(Me)(CO)]I (9a ) in a 92:8
ratio, which did not afford the corresponding acetyl
complex upon heating under reflux in CH2Cl2.
The hybrid ligands (Cp′-P)nH, which contain both a
cyclopentadienyl or an indenyl group and a tertiary
phosphine connected by an appropriate spacer, should
have the combined characteristics of their components
(Chart 1).1-3 Therefore, their transition-metal com-
plexes would have a great possibility of inducing some
unexpected stoichiometric or catalytic reactions. More-
over, although optically active transition-metal com-
plexes bearing a Cp′-P ligand are expected to exhibit
potential activity for asymmetric catalysis,3a,b synthetic
procedures for obtaining such optically active complexes
remain relatively undeveloped. Some chiral transition-
metal complexes having a stereogenic center on the
metal have been prepared and applied to asymmetric
synthesis,4-7 but the controlled generation or the reso-
lution of the chiral metal center is still one of the
challenging projects. Thus, if the method of controlling
generation of a chiral metal center is available, a new
way to obtain optically active complexes having a chiral
metal center would be opened. On the basis of these
concepts we have performed oxidative addition of alkyl
halides to rhodium and iridium complexes having an
η5:η1-(Ind-P)n ligand ((Ind-P)n ) C9H6(CH2)nPR2; C9H6
) 1-indenyl; n ) 2 (1), 3 (2), 4 (3); R ) Ph (a ), Cy (b);
Cy ) cyclohexyl) in order to examine the ability of the
“planar chirality” (indenyl-based chirality) controlling
the stereochemistry of the “central chirality” (metal-
centered chirality) arising at the metal.
We have recently disclosed the preparation of several
types of achiral or chiral (Cp′-P)H ligands and the
synthesis of rhodium(I) and ruthenium(II) complexes
with their anions, such as {η5:η1-(Ind-P)n}RhCO or {η5:
η1-(Ind-P)n}Ru(PPh3)Cl.3 It is well-known that the
oxidative addition of an alkyl or an aryl halide R′X to
(η5-C5H5)Rh(CO)PPh3 affords the rhodium(III) acyl
complexes (η5-C5H5)RhX(COR′)PPh3.8 If this oxidative
addition is applied to {η5:η1-(Ind-P)n}RhCO (1-3), the
resulting acyl complexes exhibit both planar chirality
and central chirality. We have found that the oxidative
(1) (a) Charrier, C.; Mathey, F. J . Organomet. Chem. 1979, 170, C41.
(b) Kauffmann, T.; Olbrich, J . Tetrahedron Lett. 1984, 25, 1967. (c)
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New J . Chem. 1990, 14, 599. (e) Butenscho¨n, H.; Kettenbach, R. T.;
Kru¨ger, C. Angew. Chem., Int. Ed. Engl. 1992, 31, 1066. (f) Barthel-
Rosa, L. P.; Catalano, V. J .; Nelson, J . H. J . Chem. Soc., Chem.
Commun. 1995, 1629. (g) Barthel-Rosa, L. P.; Catalano, V. J .; Maitra,
K.; Nelson, J . H. Organometallics 1996, 15, 3924.
(2) Lee, I.; Dahan, F.; Maisonnat, A.; Poilblanc, R. Organometallics
1994, 13, 2743.
(3) (a) Saito, Y.; Yamagata, T.; Kataoka, Y.; Tani, K. The 41st
Symposium on Organometallic Chemistry, J apan, Kinki Chemical
Society: Osaka, J apan, 1994; Abstract PA.109. (b) Kataoka, Y.; Saito,
Y.; Nagata, K.; Kitamura, K.; Shibahara, A.; Tani, K. Chem. Lett. 1995,
833. (c) Kataoka, Y.; Saito, Y.; Shibahara, A.; Tani, K. Chem. Lett. 1997,
621.
(4) For an example from the Davies lab, see: Davies, S. G. Ald-
richim. Acta 1990, 23, 31 and references therein.
(5) For an example from the Brookhart lab, see: Brookhart, M.; Liu,
Y.; Goldman, E. W.; Timmers, D. A.; Williams, G. D. J . Am. Chem.
Soc. 1991, 113, 927.
(6) For an example from the Faller lab, see: Faller, J . W.; Chase,
K. J .; Mazzieri, M. R. Inorg. Chim. Acta 1995, 229, 39 and references
therein.
(7) For an example from the Gladysz lab, see: Cagle, P. C.; Meyer,
O.; Vichard, D.; Weickhardt, V. K.; Arif, A. M. Gladysz, J . A.
Organometallics 1996, 15, 194 and references therein.
(8) (a) Oliver, A. J .; Graham, W. A. G. Inorg. Chem. 1970, 9, 243.
(b) Hart-Davis, A. J .; Graham, W. A. G. Inorg. Chem. 1970, 9, 2658.
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S0276-7333(98)00099-5 CCC: $15.00 © 1998 American Chemical Society
Publication on Web 09/04/1998

Communications
Organometallics, Vol. 17, No. 20, 1998 4339
and the acetyl moiety is situated just below the benzene
ring of the indenyl group. Therefore, the higher field
shift of the acetyl methyl proton of the major isomer
may be ascribed to the shielding effect of the aromatic
ring current of the indenyl group. In contrast, the 31P
NMR signal of the major isomer appears at lower field
than that of the minor isomer.
Some representative results on the reaction of {η5:
η1-(Ind-P)n}RhCO with alkyl halides are shown in Table
1. The reaction of {η5:η1-(Ind-P)n)2}RhCO (1a ), which
has a shorter alkylene spacer, with MeI in CH2Cl2 at
20 °C for 1 h also afforded the rhodium acetyl complex
{η5:η1-(Ind-P)n)2}RhI(COMe) (6a )11 but with greatly
inferior stereoselectivity, a diastereomer mixture of 33:
67 (run 1). The major isomer was found to be the other
diastereomer, R*,R*, different from the case of 5a by
1
X-ray analysis.11 Consistently, H NMR spectra of 6a
F igu r e 1. Crystal structure of (R*,S*)-{η5:η1-C9H6(CH2)3-
PPh2}RhI(COMe) ((R*,S*)-5a ; 50% probability ellipsoids).
showed a tendency in the chemical shift values opposite
to that of the acyl complex 5a . The acetyl methyl proton
of the major isomer of 6a was observed at a lower field
addition of an alkyl halide to {η5:η1-(Ind-P)n}RhCO
occurs with high stereoselectivity induced by the en-
cumbrance of the benzene ring of the indenyl moiety;
i.e., the planar chirality can control the formation of the
central chirality (eq 1).
1
(δ 3.10) than that of the minor isomer (δ 2.39). The H
NMR and the 31P NMR spectra of the acetyl complexes
obtained from 3a , which has a longer alkylene spacer
of n ) 4, showed a tendency similar to that of 5a ; thus,
the major isomer was assigned to be R*,S* (run 6). The
diastereoselectivity was also on a level similar to that
found for n ) 3 but much higher than that of n ) 2
(run 1 vs runs 5 and 6).
Small differences in the bulkiness of R′ in the alkyl
halide influence the stereoselectivity of the oxidative
addition considerably. For example, the oxidative ad-
dition of EtI toward 1a proceeded slowly compared to
that of MeI but with much higher diastereoselectivity,
92% de, for the production of the acyl complex {η5:η1-
(Ind-P)n)2}RhI(COEt) (7a )11 (run 3). The highest se-
lectivity, R*,R*:R*,S* ) 98:2 (96% de), has been accom-
plished for the reaction of 3a with EtI. The structure
of the major isomer has also been confirmed to be R*,R*
by X-ray crystallography.11,12 The reactivity for the
oxidative addition was also influenced by the length of
the spacer; 1a showed a higher reactivity than 3a (run
2 vs run 7). The angle Ind(centroid)-Rh-P in 1a
should become smaller compared to that of 3a due to
the shorter spacer. There is more reaction space around
the rhodium in 1a compared to that in 3a . The easier
access of EtI to the rhodium center should enhance the
reaction rates, but at the same time the diastereoselec-
tivity decreases due to less influence of the steric
hindrance not only of the bulky indenyl group but also
of the phosphorus substituents.
The {η5:η1-(Ind-P)n}RhCO complexes were readily
prepared from [Rh(CO)2Cl]2 and a lithium salt of (Ind-
P)nH in THF.3 Reaction of {η5:η1-(Ind-P)n)3}RhCO (2a )
with excess MeI in CH2Cl2 at 20 °C for 1 h gave a red
solution, from which the acetyl rhodium(III) complex
{η5:η1-(Ind-P)n)3}RhI(COMe) (5a ) was obtained in 97%
yield as orange powders which were purified by silica
gel column chromatography (hexane:AcOEt ) 5:1). The
acetyl complex was fully characterized by the usual
spectroscopic methods as well as elemental analyses.9
1H and 31P NMR spectra showed that the acetyl complex
5a was a mixture of two diastereomers (major:minor )
89:11, 78% de). The 1H NMR (C6D6) spectrum of 5a
displayed an acetyl methyl proton of the major isomer
at δ 2.21 and that of the minor one at δ 2.85. The
structure of the major isomer has been finally confirmed
to be the R*,S* isomer10 by X-ray crystallography.
Crystals of the major isomer suitable for X-ray analysis
were obtained by recrystallization of the diastereomer
mixture from a toluene-hexanes solution. The ORTEP
drawing of (R*,S*)-5a is presented in Figure 1.11 The
acetyl complex has a monomeric three-legged piano-stool
structure. The iodide resides at the less hindered site,
When a bulkier alkyl halide such as i-PrI or t-BuI was
employed, the {η5:η1-(Ind-P)n}RhCO was consumed very
(11) The reflection data for X-ray analyses were collected on a
Rigaku AFC-7R diffractometer equipped with a Rotaflex rotating anode
X-ray generator (50 kV, 200 mA) with graphite-monochromated Mo
KR radiation (0.710 69 Å). Crystal data for (R*,S*)-5a : red prismatic
crystal, 0.3 × 0.2 × 0.2 mm; P1h (No. 2), a ) 10.946(8) Å, b ) 12.800(6)
Å, c ) 9.184(5) Å, R ) 101.84(4)°, â ) 112.44(4)°, γ ) 91.29(5)°, V )
1157(1) Å3, Z ) 2, Dcalcd ) 1.764 g/cm3, µ ) 21.30 cm-1, 23 °C, ω-2θ
scan, 5594 measured reflections with 3.0° < 2θ < 55.0° collected, 5315
(9) 5a : The ratio of the diastereomer mixtures was determined by
1H NMR and 31P NMR (R*,R*: R*,S* ) 11:89). Mp: 147.0-150.0 °C
dec. The analytical and spectral data are provided in the Supporting
Information.
(10) The first chirality symbol shows the planar chirality, and the
second one indicates the central chirality around the rhodium. The
priorities of substituents around the metal were determined according
to the rules reported by Tirouflet et al.; see: Lecomte, C.; Dusausoy,
Y.; Protas, J .; Tirouflet, J .; Dormond, A. J . Organomet. Chem. 1974,
73, 67.
unique reflections, Rint ) 0.022, direct method (SHELXS-86), Tmin
)
0.570, Tmax ) 0.652, 4320 reflections with Fo > 3.0σ(Fo) used refine-
ments (Imoto, H., ANYBLK program for Least-Squares refinement.
Department of Chemistry, School of Science, The University of Tokyo,
Hongo, Tokyo, 113, J apan), R ) 0.0342, Rw ) 0.0303, w ) 1/σ2(F),
∆/σ(max) ) 0.000, GOF ) 3.173, ∆Fmin,
) -0.85, 0.97 e/Å3. Crystal
max
data for (R*,R*)-6a and (R*,R*)-7a are provided in the Supporting
Information.

4340 Organometallics, Vol. 17, No. 20, 1998
Ta ble 1. Rea ction of {η5:η1-(In d -P )}Rh CO w ith R′Xa
{η5:η1-(Ind-P)n}RhX(COR′)
time (h)
yield (%)b
Communications
run no.
{η5:η1-(Ind-P)n}RhCO
R′I
R*,S*:R*,R*c
1
2
3
4
5
6
7
8
9
1a
1a
1a
1a
2a
3a
3a
3a
1b
1b
MeI
EtI
EtI
EtBr
MeI
MeI
EtI
EtI
MeI
EtI
1
13
51
72
1
90 (6a )
33:67d
4:96d
4:96d
1:>99f
89:11d
85:15
98:2
98:2
34:66
6:94
83 (convn)e
98 (7a )
40
97 (5a )
92
1
13
66
1
77 (convn)e
97
97 (6b)
10
13
53 (convn)e
a
b
{η5:η1-(Ind-P)n}RhCO was treated with excess R′X in CH2Cl2 at 20 °C. Isolated yield. c The structures of diastereomers and their
ratio were estimated by 1H and 31P NMR except as otherwise noted. The structures of the major isomers were determined by X-ray
d
analysis. e Conversion was determined by 1H and 31P NMR. f After purification, the ratio changed to 33:67.
slowly and the acyl complex was scarcely obtained. In
contrast, the reactivity of benzyl bromide or allyl
bromide toward {η5:η1-(Ind-P)n}RhCO was so high that
several side reactions occurred concomitantly. In the
case of the reaction of 1a with EtBr, though the reaction
did not go to completion even after 72 h under reflux
conditions, the acyl complex {η5:η1-(Ind-P)n)2}RhBr-
(COEt) obtained in the reaction was only the R*,R*-
isomer, but it easily epimerized to give a diastereomer
mixture (R*,R*:R*,S* ) 67:33) during the purification
by silica gel column chromatography (run 4). The other
rhodium acyl complexes examined did not show such
epimerization during similar purification by column
chromatography as monitored with NMR spectra. The
reason only the complex {η5:η1-(Ind-P)n)2}RhBr(COEt)
undergoes epimerization during the purification is not
clear at the moment. When the diphenylphosphino
group was changed to a dicyclohexylphosphino group
in the (Ind-P)n ligands, the diastereoselectivity of the
oxidative addition was not affected (runs 1 and 2 vs runs
9 and 10) but the reactivity diminished apparently (run
2 vs run 10).
(major:minor ) 92:8) (eq 2). However, the iridium
cationic complex was too inert to be transformed to the
corresponding acetyl complex even under a prolonged
reaction time or under reflux conditions.12
To clarify why the diastereoselectivity was changed
by the different length of the spacer in {η5:η1-(Ind-P)n}-
RhCO, we are now investigating the preparation of the
cationic rhodium(III) complexes by changing the coun-
teranion from iodide to a more weakly coordinating
anion such as PF6 or BF4 and their reactivity, including
the stereoselectivity.
Reaction of the optically active complex {η5:η1-(Cp′-
P)*}RhCO (4), which has stereogenic centers in the
spacer of a Cp′-P ligand, with MeI in CH2Cl2 at 20 °C
for 1 h afforded the corresponding acetyl complex in 99%
isolated yield as a diastereomer mixture (major:minor
) 64:36). Compared to the reaction of 3a , which has
the same length of the spacer, the diastereoselectivity
was low. In the oxidative addition of alkyl halides to
the Cp′-P rhodium complex, the planar chirality can
better control the chiral center arising at rhodium than
the chiral center in the backbone. This is probably due
to the fact that the planar chirality is situated much
closer to the reaction center compared to the chiral
center in the spacer.
Ack n ow led gm en t. This work was partly supported
by a Grant-in-Aid for a Scientific Research from the
Ministry of Education, Science, and Culture of J apan.
Y. K. thanks the foundation from Teijin Award in
Synthetic Organic Chemistry of J apan.
Su p p or tin g In for m a tion Ava ila ble: Text giving syn-
thetic experimental details and tables giving full crystal-
lographic data, positional parameters, bond lengths, and bond
angles and ORTEP drawings for (R*,S*)-5a , (R*,R*)-6a , and
(R*,R*)-7a (44 pages). Ordering information is given on any
current masthead page.
As an intermediate in the oxidative addition of an
alkyl halide, R′X, to (η5-C5H5)RhCO(PPh3), a cationic
rhodium(III) complex, [(η5-C5H5)Rh(CO)R′(PPh3)]X, has
been postulated.8 Despite our research, we could not
observe any evidence for the formation of the intermedi-
ate [{η5:η1-(Ind-P)n}Rh(CO)R′]X. In contrast, when an
iridium-carbonyl complex was employed, the corre-
sponding cationic complex could be isolated.8b Reaction
of {η5:η1-(Ind-P)n)4}IrCO (8a ) with MeI in CH2Cl2 gave
the cationic iridium(III) complex [{η5:η1-(Ind-P)n)4}Ir-
(CO)(Me)]I (9a ) as a pale yellow powder in 94% yield
OM9800997
(12) The present stereoselective reaction may involve a mechanism
similar to that proposed for the oxidative addition of (η5-C5H5)RhCO-
(PPh3) with methyl iodide:8b (1) nucleophilic attack by the metal on
the alkyl halide opposite the indenyl group, (2) methyl migration to
the CO under the benzene ring, and (3) coordination of the halide to
the vacant site on the metal, which could explain the stereochemical
results. However, because of lack of an X-ray structure for the starting
carbonyl complexes as well as of a kinetic study, the exact mechanism
is not clear at present. Moreover, from the preliminary studies on the
stereochemistry for the reaction of [{η5: η1-(Ind-P)n}Rh(CO)R′]+ with
a halide X, giving the complex {η5: η1-(Ind-P)n}RhX(COR′), we suspect
that the reaction mechanism may not be explained simply.