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According to the above generalization, 3a may behave as a catalyst capable of Michael-type additions
of enoxysilanes toward α,β-enones. In fact, this type of coupling is realized by using a catalyst system
prepared from 1a. We describe here an Ir(I)-catalyzed Michael-type coupling between α,β-enones and
enoxysilanes, and the application to a consecutive modification of the β- and α-positions of α,β-enones.
Michael-type coupling product 7aa was isolated after protodesilylation in 33% yield when a CH2Cl2
solution of benzalacetone (4a) and two equivalent moles of trimethylsilyloxypropene (5a) was heated for
13 h at 70°C in a sealed tube containing 1 mol% of 1a which was preliminarily activated by H2 molecule
at −78°C. The 1H NMR spectrum of the crude mixture obtained before protodesilylation suggests that the
initial product of this reaction is the enoxysilane 6aa which is difficult to isolate from a multi-component
mixture. Though increase of the catalyst precursor 1a (3 mol%) did not improve the isolated yield of 7aa,
the use of four equivalent moles of 5a resulted a good yield of 7aa (67%).
(2)
Despite the moderate yield of 7 at present, it is an important finding that the Michael-type coupling
is also catalyzed by an identical system for aldol couplings. Thus, several combinations of α,β-enones
and enoxysilanes were used to reveal the scope and limitations of the Michael coupling based on this
concept. The results are summarized in Table 1.
Linear α,β-enones, 4a, 4b, 4c, and 4d reacted with 5 to give 7 in a yield of almost equal level, except
a combination of 4c and 5a (entry 7). The substituent on the nucleophilic carbon of enoxysilanes may
cause lowered reactivity for Michael-type coupling. For example, 5b needed a higher concentration of
the catalyst and prolonged reaction time to obtain 7ab in an acceptable yield (entry 4). On the other hand,
ketene acetal 5e was more effective than 5a. Two equivalent moles of 5e were sufficient to obtain 68%
of 7ae (entry 5). α,β-Enones bearing two substituents on the β-carbon hindered severely the coupling
(entry 11). Cyclic α,β-enones (4f and 4g) gave the corresponding 7 in good to excellent yields under
similar conditions with the reaction of 5. In particular, 7ga and 7gb were isolated in 97% and 93%
yield, respectively, in the reaction of 4g with four equivalent moles of 5a or 5b (entries 14 and 17).
The corresponding precursors, 6ga and 6gb were isolated by simple distillation. When ketene acetal
5e was used as a nucleophile, a moderate yield of 7ge was obtained in the reaction of 4g with an
equivalent mole of 5e (entry 18). Although (S)-(+)-carvone (4h) resulted in poor conversion under similar
conditions because of the two substituents on the cyclohexenone ring, the yield was improved by a slight
modification of conditions, namely, high concentration of the catalyst and extended reaction time (entries
20 and 21). The substituent on the β-carbon of cyclic enones prevented severely the coupling (entry 22).
Choice of the counter anion is also crucial for the success of the Michael couplings. Complexes, 1a
(X=OTf) and 1b (X=ClO4) showed almost similar efficiency as a catalyst, whereas 1c (X=PF6) resulted
in poor yields of 7 (entries 1, 2, 3, 14, 15, and 16).
Mukaiyama aldol couplings and Michael-type couplings using enoxysilanes are successfully achieved
by an identical Ir(I) cation complex. The structure of the resultant enoxysilane is retained in the step just
before protodesilylation in the Ir-catalyzed Michael coupling. Thus, a one-pot modification toward α,β-
enones is designed by a combination of Michael and aldol couplings. Into a CH2Cl2 solution containing
1a (3 mol%) activated by H2 molecule, were added 4g and 5e (1.3 equiv. moles) successively. After
the completion of Michael coupling (50°C, 12 h), benzaldehyde dimethylacetal (1.3 mol equiv.) was
added into the same reaction vessel. The resulting mixture was stirred for a further 24 h at 25°C to
give 81% of 8a after chromatographic purification. When other acetals such as dimethoxymethane and
Matsuda, Isamu
