Indium(III) Acetate-Catalyzed 1,4-Reduction and Reductive Aldol Reactions of α-Enones with Phenylsilane

Katsukiyo Miura; Yusuke Yamada; Mitsuru Tomita; Akira Hosomi

doi:10.1055/s-2004-830862

LETTER

Indium(III) Acetate-Catalyzed 1,4-Reduction and Reductive Aldol Reactions of α-Enones with Phenylsilane

Katsukiyo Miura*, Yusuke Yamada, Mitsuru Tomita, Akira Hosomi*
Department of Chemistry, Graduate School of Pure and Applied Sciences, University of Tsukuba, and CREST, Japan Science and Technology Corporation (JST), Tsukuba, Ibaraki 305-8571, Japan
Fax: +81(29)8536503; e-Mail: hosomi@chem.tsukuba.ac.jp;

Abstract

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Abstract

A catalytic amount of In(OAc)₃ smoothly promoted 1,4-reduction of certain α-enones with PhSiH₃ in ethanol at ambient temperature. The intermediary enolates could be used for inter- and intramolecular aldol reactions and intramolecular Michael addition.

Key words

indium - reductions - enones - silicon - aldol reactions

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References

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Ranu et al. have reported the InCl₃-catalyzed 1,4-reduction of electron-deficient alkenes with NaBH₄. See:

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<A NAME="RU13104ST-9">9</A> Quite recently, Baba et al. have reported a similar catalytic system for reductive aldol reaction, in which InBr₃ and Et₃SiH are used as catalyst and reducing agent, respectively. See: Shibata I. Kato H. Ishida T. Yasuda M. Baba A. Angew. Chem. Int. Ed. 2004, 43: 711

The use of In(acac)₃ (acac = acetylacetonate) instead of In(OAc)₃ gave 2a and 3a in 34% and 65% yields, respectively. InCl₃ also promoted the reaction of 1a with PhSiH₃ in Et₂O at r.t. (2a, 29%; 3a, 49%).

The use of other hydrosilanes [Et₃SiH, PhMe₂SiH, poly(methylhydrosiloxane)] instead of PhSiH₃ resulted in no reduction under the same conditions.

The use of a half amount of PhSiH₃ lowered the yield to 62%.

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General Procedure for the In(OAc) ₃ -Catalyzed 1,4-Reduction of α-Enones with PhSiH ₃ : Under N₂, α-enone 1 (0.50 mmol) and PhSiH₃ (54 mg, 0.50 mmol) were added to a suspension of In(OAc)₃ (15 mg, 0.05 mmol) in EtOH (1.0 mL). The mixture was stirred at r.t. for 1.5 h and quenched with sat. aq NaHCO₃. The extract with t-BuOMe was dried over Na₂SO₄ and evaporated. The residual oil was purified by silica gel column chromatography.

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As shown here, the reductive aldol reaction is much slower than the 1,4-reduction. This observation is attributable to slow regeneration of the indium hydride species from the indium aldolate intermediate.

The In(OAc)₃-catalyzed reduction of 6a with PhSiH₃ (r.t., 1.5 h) gave 1-naphthylmethanol in 82% yield.

As proposed by Baba et al. (ref. [9] ), the syn-selectivity can be attributed by the formation of Z-4b by a concerted hydroindation and the subsequent aldol addition via a cyclic transition state. However, we have no evidence of the selective formation of Z-4b.

Typical Procedure for the In(OAc) ₃ -Catalyzed Reductive Aldol Reaction of α-Enones with Aldehydes: Under N₂, α-enone 1b (73 mg, 0.50 mmol), 6a (102 mg, 0.65 mmol), and PhSiH₃ (54 mg, 0.50 mmol) were added to a suspension of In(OAc)₃ (15 mg, 0.05 mmol) in EtOH (0.25 mL). The mixture was stirred at 0 °C for 36 h. The work-up and purification were performed by the procedure described in ref. [14] . Compound 7ba (syn:anti = 92:8): IR (neat): 3540 (br s, OH), 1680 (C=O) cm^-1. ¹H NMR (270 MHz, CDCl₃): δ = 0.69 (t, J = 7.6 Hz, 2.76 H), 0.86 (t, J = 7.6 Hz, 0.24 H), 1.63-1.79 (m, 1 H), 1.89-2.08 (m, 1 H), 3.52 (d, J = 5.9 Hz, 0.08 H), 3.80 (d, J = 1.7 Hz, 0.92 H), 3.96 (ddd, J = 9.1, 3.8, 3.6 Hz, 0.92 H), 4.11 (ddd, J = 8.2, 6.2, 5.9 Hz, 0.08 H), 5.82 (dd, J = 6.2, 5.9 Hz, 0.08 H), 5.85 (br s, 0.92 H), 7.31-7.96 (m, 12 H). ¹³C NMR (68 MHz, CDCl₃) for the major isomer: δ = 12.25 (CH₃), 20.17 (CH₂), 51.62 (CH), 70.12 (CH), 122.49 (CH), 124.51 (CH), 125.28 (CH), 125.34 (CH), 126.04 (CH), 127.94 (CH), 128.38 (CH × 2), 128.75 (CH × 2), 129.14 (CH), 129.86 (C), 133.63 (CH), 133.73 (C), 136.69 (C), 137.22 (C), 206.37 (C). For the minor isomer (only well-resolved peaks): δ = 11.88 (CH₃), 24.18 (CH₂), 53.14 (CH), 72.85 (CH), 123.01 (CH), 124.37 (CH), 125.48 (CH), 126.17 (CH), 128.07 (CH), 128.43 (CH), 129.03 (CH), 130.53 (C), 133.16 (CH), 138.15 (C), 206.08 (C).

The reaction of 1b with octanal was carried out in THF containing an equimolar amount of EtOH at 70 °C. However, both the yield of 7 and the syn-selectivity dropped to 52% and 56% syn, respectively.

According to the method reported by Montgomery et al., 8 and 14a were prepared by ozonolysis of cyclopentene and the subsequent Wittig olefination with Ph₃PCHC(O)Ph. This method was used also for the preparation of 12a, 12b, and 14b from 1-methylcyclopentene, 1,5-dimethyl-1,5-cyclooctadiene, and 1,5-cyclooctadiene, respectively. See:

<A NAME="RU13104ST-21A">21a</A> Montgomery J. Savchenko AV. Zhao Y. J. Org. Chem. 1995, 60: 5699

<A NAME="RU13104ST-21B">21b</A> See also the following paper for the preparation of 12a and 12b: Huddleston RR. Cauble DF. Krische MJ. J. Org. Chem. 2003, 68: 11

For the stereochemical assignment of 9, 15a, and 15b, see ref. [7a] . The relative configurations of 13a and 13b were determined by their NMR data reported in ref. [21b] .

Krische et al. have reported cis-selective reductive aldol reactions of 8 and 12a, and trans-selective reductive Michael reaction of 14. See ref. [7] and ref. [21b]