Synthesis and structure of protic amine tethered rhodium(III) peralkylcyclopentadienyl complexes

Article abstract of DOI:10.1021/om100151m

A series of mononuclear tethered complexes, Rh^III(η ⁵:η¹-Cp--NHR), in which the η⁵- peralkylcyclopentadienyl group and the ligated protic amine group are connected by suitable carbon chains, were newly prepared and structurally characterized. Their catalytic performance in the reductive amination of carbonyl compounds was significantly influenced by the chain length of the tether, which possibly plays a key role in determining the reaction course.

Full text of DOI:10.1021/om100151m

1
886 Organometallics 2010, 29, 1886–1889
DOI: 10.1021/om100151m
Synthesis and Structure of Protic Amine Tethered Rhodium(III)
Peralkylcyclopentadienyl Complexes
†
Masato Ito, Noriko Tejima, Masahiro Yamamura, Yoshinori Endo, and Takao Ikariya*
Department of Applied Chemistry, Graduate School of Science and Engineering, Tokyo Institute of
Technology, 2-12-1-E4-1 O-okayama, Meguro-ku, Tokyo 152-8552, Japan. Present address: Institute of
Materials Chemistry and Engineering (IMCE), Kyushu University, Kasuga, Fukuoka 816-8580, Japan
†
Received February 24, 2010
III
Summary: A series of mononuclear tethered complexes, Rh -
molecular catalysts. Accordingly, we then focused on the
5
1
0
5
6
6
(
η :η -Cp -NHR), in which the η -peralkylcyclopentadienyl
“tethered” (η -arene)Ru complexes, in which an η -arene
ligand and a protic amine ligand is connected with a suitable
carbon chain in order to generate new reactivity in the cata-
group and the ligated protic amine group are connected by
suitable carbon chains, were newly prepared and structurally
characterized. Their catalytic performance in the reductive
amination of carbonyl compounds was significantly influenced
by the chain length of the tether, which possibly plays a key role
in determining the reaction course.
5
lysis. Our previous efforts in this area resulted in the
development of a versatile method for the introduction of
4a
aminoalkylarenes as well as N-triflylated 1-(aminoalkyl)-
4b
,4-cyclohexadienes into the ligand sphere of Ru to
II
1
furnish 1a,b (Chart 1). One of the logical extensions was
We have developed a series of bifunctional half-sandwich
type Ru catalysts bearing a chelating protic amine ligand for
the introduction of a similar “tethered” motif into an iso-
5
electronic half-sandwich type η -cyclopentadienyl complex.
Recently, we have found a convenient method for the pre-
1
-4
a wide range of molecular transformations.
The catalytic
performance of the bifunctional catalysts is significantly
influenced by a judicious choice of the coordinating element
in the chelating amine ligand as well as by ingenious design of
paration of tetramethylcyclopentadiene compounds with a
0
variety of pendant protic aminoalkyl groups (Cp H-NHR),
which serve as excellent precursors for the preparation of a
n
6
5
multidentate η -C R ligands such as η -arene and η -cyclo-
n
n
III
5
1
0
variety of Rh (η :η -Cp -NHR) complexes. Furthermore,
their structural features, which depend on the carbon chain
length of the tethered unit, were clarified by X-ray crystal-
lographic analysis. Described herein are the preliminary
results of this study.
pentadienyl. These results strongly indicated that a synthe-
tically more flexible method that provides a convenient
access to tailor-made ligands as well as a more efficient
method for their complexation with a targeted central metal
should lead to the development of further sophisticated
Our initial experiments focused on the synthesis of iso-
meric mixtures of (CH ) C H(CH ) NHR (2a, R = H; 2b,
*
To whom correspondence should be addressed. E-mail: tikariya@
apc.titech.ac.jp.
1) (a) Ikariya, T.; Murata, K.; Noyori, R. Org. Biomol. Chem. 2006,
3
1
3 4
5
2 2
R = CH ) from 2,3,4,5-tetramethylcyclopent-2-enone (4),
3
6
which was developed by Hessen and co-workers (Chart 1).
(
93–406. (b) Ikariya, T.; Blacker, A. J. Acc. Chem. Res. 2007, 40, 1300–
308. (c) Ikariya, T.; Gridnev, I. D. Chem. Rec. 2009, 9, 106–123.
We were also successful in the preparation of the N,N-
dimethyl derivative (CH ) C H(CH ) N(CH ) (2c) from
(
2) (a) Ito, M.; Ikariya, T. Chem. Commun. 2007, 5134–5142. (b) Ito,
M.; Ikariya, T. J. Synth. Org. Chem. Jpn. 2008, 66, 1042–1048.
3) (a) Ito, M.; Hirakawa, M.; Murata, K.; Ikariya, T. Organome-
3 4
5
2 2
3 2
2b through a similar treatment for the conversion of 2b from
2
(
a using formylation followed by LiAlH reduction. Unfor-
4
tallics 2001, 20, 379–381. (b) Ito, M.; Hirakawa, M.; Osaku, A.; Ikariya, T.
Organometallics 2003, 22, 4190–4192. (c) Ito, M.; Osaku, A.; Kitahara, S.;
Hirakawa, M.; Ikariya, T. Tetrahedron Lett. 2003, 44, 7521–7523. (d) Ito,
M.; Kitahara, S.; Ikariya, T. J. Am. Chem. Soc. 2005, 127, 6172–6173.
e) Ito, M.; Sakaguchi, A.; Kobayashi, C.; Ikariya, T. J. Am. Chem. Soc.
007, 129, 290–291. (f) Ito, M.; Osaku, A.; Shiibashi, A.; Ikariya, T. Org.
0
tunately, however, all our synthetic efforts to prepare Cp H-
NHR with a longer alkyl chain or with a branched alkyl
group from 4 met with little success. Therefore, we turned our
attention to the dehydrative cyclization of di-2-butenylcarbi-
nol derivatives, leading to tetramethylcyclopentadienyl com-
(
2
Lett. 2007, 9, 1821–1824. (g) Ito, M.; Koo, L.-W.; Himizu, A.; Kobayashi,
C.; Sakaguchi, A.; Ikariya, T. Angew. Chem., Int. Ed. 2009, 48, 1324–1327.
7
pounds. As illustrated in Scheme 1, N-Boc-γ-lactam (5) was
(
h) Ito, M.; Osaku, A.; Kobayashi, C.; Shiibashi, A.; Ikariya, T. Organome-
tallics 2009, 28, 390–393.
4) (a) Ito, M.; Komatsu, H.; Endo, Y.; Ikariya, T. Chem. Lett. 2009,
subjected to a nucleophilic ring-opening reaction with 2 equiv
of 2-butenyllithium to yield the corresponding tertiary alco-
hol with an N-Boc propylamino group, which subsequently
underwent both cyclization and deprotection of the N-Boc
group upon treatment with HCl to furnish an isomeric
(
3
6
1
8, 98–99. (b) Ito, M.; Endo, Y.; Ikariya, T. Organometallics 2008, 27, 6053–
055. (c) Ito, M.; Shibata, Y.; Watanabe, A.; Ikariya, T. Synlett 2009, 1621–
626.
(5) Seminal works by Wills and co-workers indicated the introduc-
n
tion of a suitable tether between the η -C
n
R
n
ligand and the mono-
mixture of tetramethylcyclopentadienylpropylamine HCl
3
tosylated chiral 1,2-diamine ligand in the Noyori-type asymmetric
transfer hydrogenation catalysts causes significant improvement of
catalytic performance: (a) Hannedouche, J.; Clarkson, G. L.; Wills,
M. J. Am. Chem. Soc. 2004, 126, 986–987. (b) Morris, D. J.; Hayes, A. M.;
Wills, M. J. Org. Chem. 2006, 71, 7035–7044. (c) Cheung, F. K.; Lin, C.;
Minissi, F.; Crivill ꢀe , A. L.; Graham, M. A.; Fox, D. J.; Wills, M. Org. Lett.
007, 9, 4659–4662. (d) Matharu, D. S.; Morris, D. J.; Kawamoto, A. M.;
Clarkson, G. J.; Wills, M. Org. Lett. 2005, 7, 5489–5491. (e) Matharu, D. S.;
Morris, D. J.; Clarkson, G. J.; Wills, M. Chem. Commun. 2006, 3232–3234.
(6) van Leusen, D.; Beetstra, D. J.; Hessen, B.; Teuben, J. H.
Organometallics 2000, 19, 4084–4089.
(7) (a) Manriquez, J. M.; Fagan, P. J.; Schertz, L. D.; Marks, T. J.;
Bercaw, J.; Mcgrady, N. Inorg. Synth., doi: 10.1002/9780470132524.ch41.
(b) Wochner, F.; Zsolnai, L.; Huttner, G.; Brintzinger, H. H. J. Organomet.
Chem. 1985, 288, 69–77. (c) Tobita, H.; Habazaki, H.; Shimoi, M.; Ogino, H.
Chem. Lett. 1988, 1041–1044. (d) Jutzi, P.; Dahlhaus, J. Synthesis 1993, 683–
686. (e) Krut'ko, D. P.; Borzov, M. V.; Petrosyan, V. S.; Kuz'mina, L. G.;
Churakov, A. V. Russ. Chem. Bull. 1996, 45, 940–949.
2
(
1
f) Matharu, D. S.; Martins, J. E. D.; Wills, M. Chem. Asian J. 2008, 3, 1374–
383.
pubs.acs.org/Organometallics
Published on Web 03/29/2010
r 2010 American Chemical Society

Communication
Organometallics, Vol. 29, No. 8, 2010 1887
Chart 1
Scheme 1. Preparation of Ligand Precursors
We next explored an efficient method for introducing
0
Cp H-NHR derivatives into the ligand sphere of late transi-
6
tion metals having a d electronic configuration. The desired
5
1
complexes, RhCl [η :η -(CH ) C (CH ) NR ] (13a-c, n =
2
3 4
5
2 n
2
2
; 14, n = 3), were successfully obtained by the reaction of
RhCl 3H O with 2a-c HCl and 3 HCl in refluxing metha-
3
3
2
3
3
nol followed by the treatment of the resulting intermediary
9
complexes 11a-c and 12 with KO-t-Bu in CH Cl at room
2
2
temperature (Scheme 2).
The solid-state structures of 13c and 14 were unequivo-
cally determined by X-ray crystallographic analysis
(Figure 1). It should be noted that the direct reaction of
chiral carbinol 10 with RhCl H O in refluxing methanol
3
3
2
followed by treatment with Ag CO furnished the mono-
2
3
nuclear Rh complex 15, having a chiral tether. Its solid-state
structure was also determined by X-ray crystallographic
analysis (Figure 2).
5
1
0
The protic tether moieties in a series of RhCl (η :η -Cp -
2
(
3 HCl) in 60% yield based on the tertiary alcohol. It should
3
NHR) were remarkably robust against a variety of reaction
conditions. For example, the reaction of 13a or 14 with
be noted that the use of N-Boc-azetidinone (6) or N-Boc-β-
alanine methyl ester (7) in place of 5 resulted in the formation
of 2a HCl via a similar two-step sequence. These results
demonstrated that the latter method is more synthetically
flexible than the former starting from 4, at least in the
preparation of Cp H-NHR derivatives.
However, we encountered a notable drawback of the latter
method in the preparation of a chiral Cp H-NH species
2
triphenylphosphine in the presence of AgSbF gave [RhCl-
6
5
1
3
{
n = 3), respectively. On the other hand, the reaction of 13a,
η :η -(CH ) C (CH ) NH }(PPh )]SbF (16a, n = 2; 17,
3 4 5 2 n 2 3 6
0
1
3b, or 14 with N,N -(1,2-phenylene)bis(trifluoromethane-
sulfonamide) (1,2-(NHTf) C H ) in the presence of KO-t-Bu
0
2
6
4
yielded the corresponding diamido complexes (18a,b, 19),
whose tethered moieties remained intact (Chart 2).
0
starting from (R)-N-Boc-2-azabicyclo[2.2.1]hept-5-en-3-one
(
The solid-state structural analysis of these diamido com-
plexes by X-ray diffraction revealed that the chain length of
the tether significantly influences the conformational con-
straints of the cyclopentadienyl ligand. As shown in Figure 3,
the bond lengths of Rh-Cipso (the tether-attached ring
8). Whilethe 2-fold nucleophilic2-butenylationof 8 afforded
analogously the ring-opened product 9, which underwent
selective hydrogenation at its endocyclic double bond to
furnish the corresponding di-2-butenylcarbinol 10 in 56%
yield, all attempts to cyclize 10 with a variety of acid catalysts,
including HCl, p-TsOH, and Momtmorillonite K10 have
failed (vide infra). These results may indicate that the acid
treatment of di-2-butenylcarbinol, having a secondary car-
bon adjacent to the carbinol center, causes the formation of
intractable materials containing no cyclopentadiene com-
carbon) in the C -tethered complexes 18a,b (2.094(4) and
2
˚
.092(2) A, respectively), are considerably shorter than that
2
in the C -tethered complex 19 (2.170(7) A). Additionally, the
˚
3
maximum difference in the bond lengths of five Rh-C bonds
in 18a,b (0.091 A each) is greater than that in 19 (0.032 A).
˚
˚
The structural difference characterized by the C and C
2
8
3
pound, although the reason still remains unclear.
1
tethers can be also seen in the H NMR spectra of cationic
phosphine complexes 16a and 17 in CD Cl . Two of the four
2
2
nonequivalent methyl groups in 16a appear as doublets at
values of 5.4 and 7.6 Hz,
PH
(
8) We have examined the cyclization of cyclopentyl(di-2-bute-
4
0.97 and 2.13 ppm with J
nyl)carbinol using HCl in diethyl ether to observe the formation of
intractable materials, which may support the notion that not the NHBoc
group in 10 but the secondary carbons can be an obstacle in the
cyclization.
respectively. Similarly, those in 17 appear as doublets at
0
4
.97 and 1.83 ppm with J
respectively. The fact that the difference between two J
PH
values of 5.9 and 6.8 Hz,
PH
4
(
9) The spectroscopicand elementalanalysisof11a-c and 12indicated
that the cyclopentadiene moiety of 2a-c HCl and 3 HCl was changed to
3
3
values is significantly greater in 16a (2.2 Hz) than in 17 (0.9
Hz) should be attributable to the more restricted rotational
mobility of the cyclopentadienyl ring of 16a within the time
scale of NMR measurement, which in turn suggests the
5
a η coordination mode but their amino termini remained unchanged as
HCl salts, although it was difficult to determine whether they adopt a
5
0
chloro-bridged dinuclear structure, [Rh
zwitterionic mononuclear structure, RhCl
2
Cl
4
(η -Cp -NHR
2
)
2
](Cl)
0
2
(η -Cp -NHR ). Very recently,
2
, or a
5
3
Eppinger and co-workers independently reported the similar preparation
of 11a, whose side chain was further acylated to give another non-tethered
complex. Reiner, T.; Jantke, D.; Raba, A.; Marziale, A. N.; Eppinger, J.
J. Organomet. Chem. 2009, 694, 1934–1937.
relatively rigid structure imposed by the C tether.
2
With a variety of tethered Rh (η :η -Cp -NR ) com-
plexes in hand, we briefly explored their potential catalytic
III
5
1
0
2

1
888 Organometallics, Vol. 29, No. 8, 2010
Ito et al.
Scheme 2. Preparation of 13a-c and 14
Figure 3. Molecular structures of diamido complexes 18a,b and
9. Substituents on sulfur and hydrogen atoms, except those on
protic amine, are omitted for clarity.
1
5
1
0
Figure 1. Molecular structures of RhCl (η :η -Cp -NR ) com-
2
2
plexes 13c (left) and 14 (right). Hydrogen atoms, except those on
protic amine, are omitted for clarity.
Chart 2
Figure 2. Molecular structure of 15. Hydrogen atoms, except
those on protic amine, are omitted for clarity.
1
2
reductive amination of carbonyl compounds, in which the
length of the tether plays a key role in determining the
reaction course. For example, 13a,b promote the reaction
of p-methoxybenzaldehyde with 1 equiv of dibenzylami-
activity in the hope that protic congeners present their
1
0,11
characteristic Rh/NH bifunctionality.
1
As a result, 13a,
3b, and 14 have been found to serve as catalysts for the
ne HCO
H at 70 °C in methanol to give N,N-dibenzyl-p-
3
2
methoxybenzylamine predominantly and the selectivity of
the amine and the undesired benzylic alcohol reaches as high
as 96:4. On the other hand, the same reaction with the
non-tethered complex (Cp*RhCl ) or the dimethylamino-
(10) (a) Mashima, K.; Abe, T.; Tani, K. Chem. Lett. 1998, 27, 1199–
1
200. (b) Mashima, K.; Abe, T.; Tani, K. Chem. Lett. 1998, 27, 1201–1202.
(
(
c) Murata, K.; Ikariya, T.; Noyori, R. J. Org. Chem. 1999, 64, 2186–2187.
d) Mao, J.; Baker, D. C. Org. Lett. 1999, 1, 841–843. (e) Hamada, T.; Torii,
2 2
tethered complex 13c resulted in selectivities of 57:43 and
5:25, respectively, under identical conditions (Scheme 3).
These results may indicate that the Rh/NH bifunctionality is
effectively operative in the case of 13a,b, although further
studies are required to clarify the relationship between the
chain length and the catalytic performance.
T.; Izawa, K.; Noyori, R.; Ikariya, T. Org. Lett. 2002, 4, 4373–4376.
f) Hamada, T.; Torii, T.; Onishi, T.; Izawa, K.; Ikariya, T. J. Org. Chem.
004, 69, 7391–7394. (g) Hamada, T.; Torii, T.; Izawa, K.; Ikariya, T.
Tetrahedron 2004, 60, 7411–7417. (h) Li, C.; Xiao, J. J. Am. Chem. Soc.
008, 130, 13208–13209.
11) For related Ir/NH bifunctionality: (a) Shirai, S.; Nara, H.;
Ikariya, T. Abstr. Symp. Organomet. Chem. Jpn. 2005, 52, PB148.
b) Heiden, Z. M.; Rauchfuss, T. B. J. Am. Chem. Soc. 2006, 128, 13048–
3049. (c) Ohkuma, T.; Utsumi, N.; Watanabe, M.; Tsutsumi, K.; Arai, N.;
7
(
2
2
(
(
1
In summary, we have developed convenient methods for
III
5
1
0
the preparation of Rh (η :η -Cp -NHR) complexes having
a variety of protic amine tether moieties. Their synthetic
flexibility would be beneficial for the sophisticated design of
well-defined transition-metal-based molecular catalysts.
Further studies on the preparation of other half-sandwich
type complexes with a protic amine tether are actively under-
way in our laboratory.
Murata, K. Org. Lett. 2007, 9, 2565–2567. (d) Li., C.; Wang, C.; Villa-
Marcos, B.; Xiao, J. J. Am. Chem. Soc. 2008, 130, 14450–14451. (e) Li,
Z.-W.; Wang, T.-L.; He, Y.-M.; Wang, Z.-J.; Fan, Q.-H.; Pan, J.; Xu, L.-J.
Org. Lett. 2008, 10, 5265–5268. (f) Shirai, S.; Nara, H.; Kayaki, Y.; Ikariya,
T. Organometallics 2009, 28, 802–809. (g) Heiden, Z. M.; Rauchfuss, T. B.
J. Am. Chem. Soc. 2009, 131, 3593–3600.
(12) Kitamura, M.; Lee, D.; Hayashi, S.; Tanaka, S.; Yoshimura, M.
J. Org. Chem. 2002, 67, 8685–8687.

Communication
Organometallics, Vol. 29, No. 8, 2010 1889
Scheme 3. Catalytic Reductive Amination with Rh Complexes
Acknowledgment. This research was supported by the
MEXT (Nos. 16750073 and 18065017 “Chemistry of Concerto
Catalysis”) and by the Asahi Glass Foundation (M.I.). This
work has been partially supported by the GCOE Program.
Supporting Information Available: Text giving experimental
procedures and CIF files giving X-ray crystallographic data for
13c, 14, 15, 18a,b, and 19. This material is available free of
charge via the Internet at http://pubs.acs.org.