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Generation of Hafnium Hydride and its Application to Chemo- and Diastereoselective Reactions
04 May 2009 (online)
Hafnium hydride was generated by the transmetalation between Bu3SnH and HfCl4 using either THF or EtCN as the solvent. This process effectively reduced aldehydes, aldimines, ketones, and esters. In the hafnium hydride reduction of α-alkoxy-ketones, the diastereoselectivity was dependent on whether THF or EtCN was used as the solvent.
hafnium - hydrides - transmetalation - reduction - selectivity
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References and Notes
Hafnium(IV) chloride and Bu3SnH were mixed in NMR sample tube under the conditions of -20 ˚C, and the mixture was immediately introduced NMR instrument and measured. The obtained chart was the one after 5 minutes from mixing.7
In transmetalated mixture in EtCN over -20 ˚C, fast decomposition occurred with quantitative generation of H2, hence we could not determine exact hafnium species such as HHfCl3 or H2HfCl2.9
A 10 mL round-bottom flask charged with HfCl4 (1.0 mmol) was dried by heating to 110 ˚C under reduced pressure (1.33˙10-³ bar) for 1 h. After the nitrogen was filled, EtCN
(2 mL) was added to dissolve HfCl4, and the solution was cooled to -20 ˚C. To the mixture was added Bu3SnH (1.0 mmol) and after 5 minutes benzoin methyl ether (9a, 1.0 mmol). After stirring for 3 h, the resulting mixture was quenched by aq MeOH (5 mL) and extracted with Et2O (3 × 10 mL). The combined organic layer was treated with NH4F, and then the precipitate was filtered to remove the tin compound. The organic layer was dried over MgSO4, and then filtered and evaporated. Yield and ratio of 10a/11a were determined by NMR. Further purification was performed by SiO2 column chromatography eluting with hexane-EtOAc = 85:15 afforded 10a and 11a as a mixture. threo -2-Methoxy-l,2-phenylethanol (10a) and erythro -2-Methoxy-l,2-phenylethanol (11a) Compound 10a: Mp 84-87 ˚C. IR (KBr): 3400, 1030, 1045 cm-¹. MS: m/z = 228 [M+]. ¹H NMR (400 MHz, CDC13):
δ = 2.45 (br, 1 H, OH), 3.30 (s, 3 H, OCH3), 4.12 (d, 1 H, J = 8.3 Hz, CHOMe), 4.65 (d, 1 H, J = 8.3 Hz, CHOH), 7.11-7.28 (m, 10 H, Ph).
Compound 11a ¹H NMR (400 MHz, CDC13): δ = 2.45 (br, 1 H, OH), 3.22 (s, 3 H, OCH3), 4.34 (d, 1 H, J = 5.4 Hz, CHOMe), 4.88 (d, 1 H, J = 5.4 Hz, CHOH), 7.11-7.28 (m, 10 H, Ph).
In the reduction of 9a in MeCN, the addition of ligand such as Ph3P=O afforded 79% anti selectivity. Hence Ph3P=O coordinates to hafnium center to prevent the chelate formation.