Generation of Hafnium Hydride and its Application to Chemo- and Diastereoselective Reactions

Abstract

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.

Key words

hafnium - hydrides - transmetalation - reduction - selectivity

References and Notes

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• 2a Ishihara K. Ohara S. Yamamoto H. Science  2000,  290:  1140 
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• 3 Baba A. Shibata I. Yasuda M. In Comprehensive Organometallic Chemistry III   Vol. 9:  Crabtree RH. Michael D. Mingos P. Elsevier; Oxford: 2006.  Chap. 8. p.341-380  
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6

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

Typical Procedure
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).

12

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.