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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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9 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.
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10 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%).
11 The use of other hydrosilanes [Et3SiH, PhMe2SiH, poly(methylhydrosiloxane)] instead of PhSiH3 resulted in no reduction under the same conditions.
12 The use of a half amount of PhSiH3 lowered the yield to 62%.
13
Abernethy CD.
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14
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.
15a
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16 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.
17 The In(OAc)3-catalyzed reduction of 6a with PhSiH3 (r.t., 1.5 h) gave 1-naphthylmethanol in 82% yield.
18 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.
19
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).
20 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:
21a
Montgomery J.
Savchenko AV.
Zhao Y.
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21b See also the following paper for the preparation of 12a and 12b: Huddleston RR.
Cauble DF.
Krische MJ.
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2003,
68:
11
22 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]
.
23 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]