Iridium-catalyzed crossed aldol coupling and a new concept to form an Ir-Si species

Article abstract of DOI:10.1016/S0040-4039(99)02303-5

[Ir(COD)(PPh₃)₂]X (X=PF₆, ClO₄, and OTf) activated by H₂ molecule catalyzes Mukaiyama-type aldol coupling between enoxysilanes and acetals or aldehydes. An Ir-Si species formed in the first stage of t

Full text of DOI:10.1016/S0040-4039(99)02303-5

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 41 (2000) 1405–1408
Iridium-catalyzed crossed aldol coupling and a new concept to
form an Ir–Si species
Isamu Matsuda,∗ Yuki Hasegawa, Tatsuya Makino and Kenji Itoh
Department of Molecular Design and Engineering, Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa,
Nagoya 464-8603, Japan
Received 20 October 1999; revised 22 November 1999; accepted 3 December 1999
Abstract
[Ir(COD)(PPh3)2]X (X=PF6, ClO4, and OTf) activated by H2 molecule catalyzes Mukaiyama-type aldol co-
upling between enoxysilanes and acetals or aldehydes. An Ir–Si species formed in the first stage of the catalytic
cycle plays an important role in these reactions. © 2000 Elsevier Science Ltd. All rights reserved.
Keywords: iridium catalyst; aldol coupling; enoxysilanes.
It is well known that iridium complexes possess efficient potential as catalysts for hydrogenation and
isomerization of olefinic double bonds.1 In particular, Ir(I) cationic complexes are utilized as an active
catalyst for the synthesis of unsymmetrical allylsilanes2 and enoxysilanes,3 whereas examples reporting
carbon–carbon bond formations catalyzed by Ir complexes are quite limited.4 Thus, we focused our study
to explore the characteristic aspects of Ir complexes. At the initial stage of our projects, we selected the
Mukaiyama-type aldol coupling to evaluate the catalytic efficiency of [Ir(COD)(PPh3)2]X (COD=1,5-
cyclooctadiene) for the carbon–carbon bond formation. We report here that Ir(I) cationic complexes are
active for the coupling reactions of enoxysilanes with aldehydes or acetals and that the formation of
an Ir–Si species plays an important role in initiating the catalytic cycle for the carbon–carbon bond
formation.
An aldol-type product 4aa (13%) was isolated when a CH2Cl2 solution of benzaldehyde dimethylace-
tal (2a) with two equivalent moles of trimethylsilyloxypropene (3a) was heated for 1 h at 80°C in a sealed
tube containing a catalyst solution prepared from [Ir(COD)(PPh3)2]PF6 (1a) and H2 molecule (Scheme
1). This catalyst system also showed moderate efficiency in the coupling of benzaldehyde (5a) with 3a
under similar conditions followed by protodesilylation to give 6aa (41%). When similar types of the
complex, [Ir(COD)(PPh3)2]ClO4 (1b) and [Ir(COD)(PPh3)2]OTf (1c, OTf=trifluoromethanesulfonyl),
were used as a catalyst precursor for aldol couplings, the reaction rate and yield of the product were
remarkably improved as shown in Table 1. In particular, the triflate complex 1c worked quite efficiently
∗
Corresponding author. Fax: +81-52-789-5116; e-mail: matsudai@apchem.nagoya-u.ac.jp (I. Matsuda)
0040-4039/00/$ - see front matter © 2000 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(99)02303-5
tetl 16231

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even at 25°C in the corresponding reaction within 3 h to give 4aa and 6aa in 93% and 86% yield,
respectively (entries 4 and 8 in Table 1). Activation of the catalyst precursor with H2 molecule is crucial
for a sufficient yield of products. The yield of 4aa lowered to 48% without this preliminary operation
(entry 5 in Table 1).
Scheme 1.
Table 1
Ir-Catalyzed aldol-type coupling of 3a with acetal 2a or aldehyde 5aa
Since 1c was highly effective as a catalyst precursor for the aldol coupling of 3a with 2a or 5a, some
other types of starting materials were used to reveal the scope and limitations of 1c in aldol couplings. The
results are summarized in Table 2. Acetal 2a reacted with enoxysilanes, 3c, 3d, and 3e which need slightly
severer conditions for sufficient yield of products than the reaction with 3a because of the steric hindrance
of the substituents on the nucleophilic site. Diastereoselectivity to form 4ac seems to depend on the
stereochemisitry of enoxysilane 3c, though the extent of the selectivity is fair at present (entries 1 and 2
in Table 2). When cyclic enoxysilanes (3d and 3e) were used as a nucleophile, appreciable syn-selectivity
was observed (entries 3 and 4 in Table 2). Aliphatic acetals (2b and 2c) and α,β-unsaturated acetal (2d)
reacted readily with enoxysilanes, the substituents of which seem to retard the rate of coupling. This
defect is surmounted by raising reaction temperature to 70°C. It should be noted that dimethoxymethane
(2e) is also available as an electrophile in the present catalytic system (entry 12 in Table 2). In contrast to
5a and 5d, aliphatic aldehydes gave unacceptable results (entries 16 and 17 in Table 2).
Cationic complexes 1b and 1c activated by H2 molecule possess much higher catalytic ability for
Mukaiyama-type aldol coupling than the Rh analogues,5 though they are far less reactive than TiCl4,6
trityl perchlorate7 and Me3SiOTf.8 Moreover, the anion part of 1 exerts remarkable catalytic efficiency,

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Table 2
Aldol-type coupling catalyzed by [Ir(COD)(PPh3)2]OTf (1c)a
for example, PF6<ClO4<<OTf. It is intuitively considered that 1 interacts with 3a to form Me3SiX which
is a genuine catalyst for aldol coupling. This idea, however, can be ruled out by the fact that the signal
ascribed to Me3SiX is not detected at all in the 1H NMR spectrum of a mixture containing an equivalent
mole of 1 and 3a, either in the presence or in the absence of H2 molecule. A new species showing a
Me3Si signal (δ 0.16 ppm) and acetone appeared with the signal intensity corresponding to the quantity
1
of the consumed 3a in the H NMR spectrum of a CDCl3 solution of 1, H2 molecule, and 3a at 25°C,
regardless of the anionic part X. The identical species increases proportionally with 3a added until the
quantity reaches two equivalent moles. Although this new silyl species has not been isolated in pure form
at present, the signal is elucidated by assuming the presence of an Ir–Si species 8 of which formation is
described as shown in eq.1. Since it has been demonstrated that H2 molecule adds oxidatively to 1a to
form 7 at −78°C,9 the activation of 1 with H2 molecule in our catalyst system would correspond to this
step. Therefore the reaction of 7 with 3a is regarded as a new method to form an Ir–Si bond. The formed
8 reacted rapidly with 2a to form Me3SiOMe. Thus, we can propose Scheme 2 as a plausible rationale for

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the present Ir-catalyzed aldol-type coupling. Further topics on the application of an Ir(I) complex catalyst
are actively underway.
(1)
Scheme 2.
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