Alkylative Reduction of Titanium(IV) Isopropoxide with EtMgBr: Convenient Method for the Generation of Subvalent Titanium Alkoxide Reagents and their Reactivity in Pinacol Coupling Reactions

Abstract

The composition of the gaseous products from the reaction of Ti(IV) isopropoxide with ethylmagnesium bromide in diethyl ether evidences the formation of subvalent titanium isopropoxide species in various oxidation states depending on relative amounts of the reactants. Reaction of titanium(IV) isopropoxide with one equivalent of the Grignard reagent gives presumably titanium(III) isopropoxide. The latter is generated as a result of disproportionation of starting Ti(IV) species and titanium(II) isopropoxide-ethene complex which is formed as a result of ethane extrusion from diethyltitanium(IV) isopropoxide. Titanium(III) isopropoxide ­prepared by this way transforms the aldehydes and the aromatic ­ketones into the corresponding pinacols in good yields.

titanium compounds - reduction - Grignard reagents - carbonyl compounds - pinacol coupling

References

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9

Procedure for Pinacol Coupling: To a solution of Ti(i-PrO)₄ (2.05 g, 7.3 mmol) in THF (6 mL) under argon atmosphere, a solution of EtMgBr prepared from Mg (0.19 g, 7.9 mmol) and EtBr (0.9 g, 8.2 mmol) in THF (8 mL) was added for a 2 min. The initially colorless solution turned blue, green-brown and black-brown over the course of EtMgBr addition. The reaction mixture was stirred and refluxed for 30 min under argon, then a solution of carbonyl compound in THF (2 mL) was added in one portion at r.t. or at reflux. The reaction mixture was stirred for a denoted time (Table [2] ) and then diluted with 20 mL of Et₂O and quenched with 20 mL of 10% H₂SO₄ (in the case of the example in entry 5 the reaction mixture was quenched with aq NH₄CI and filtered). Organic layer was separated, washed with H₂O, aq NaHCO₃ and dried over MgSO₄. After removing of the solvent in vacuo the resulting reaction mixtures were analyzed by ¹H NMR and then crystallized from CCI₄ or EtOH.

10

The treatment of benzophenone with 1 equiv of Ti(II) isopropoxide, prepared by alkylative reduction of Ti(IV) isopropoxide with EtMgBr in THF at r.t., led to 39% of pinacol 8, 3% of benzhydrol 9, and 47% of the starting compound was recovered.

11

The treatment of benzophenone with 1 equiv of Ti(II) isopropoxide, prepared by alkylative reduction of Ti(IV) isopropoxide with EtMgBr in THF at -78 °C, led to 58% of pinacol 8, 1% of benzhydrol 9, and 21% of the starting compound was recovered.

12

Selected NMR data for pinacol coupling products:
dl - and meso -1,2-bis(4′-Chlorophenyl)ethane-1,2-diol. ¹H NMR (lit. [13] ) (400 MHz, CDCI₃), dl: δ = 2.93 (s, 2 H, OH), 4.61 (s, 2 H, CH), 7.00 (d, 4 H, J = 8.6 Hz), 7.20 (d, 4 H, J = 8.6 Hz); meso: δ = 2.36 (s, 2 H, OH), 4.83 (s, 2 H, CH), 7.09 (d, 4 H, J = 8.6 Hz), 7.25 (d, 4 H, J = 8.6 Hz).
dl - and meso -2,3-Diphenylbutane-2,3-diol. ¹H NMR (lit. [14] ) (400 MHz, CDCI₃), dl: δ = 1.50 (s, 6 H, CH₃), 2.59 (s, 2 H, OH), 7.17-7.26 (m, 10 H, Ph-H); meso: δ = 1.58 (s, 6 H, CH₃), 2.29 (s, 2 H, OH), 7.17-7.26 (m, 10 H, Ph-H).
dl - and meso -Dodecane-6,7-diol. [15] ¹H NMR (400 MHz, CDCI₃), dl: δ = 0.85-0.93 (m, 6 H, CH₃), 1.21-1.54 (m, 16 H, CH₂), 1.92-2.05 (s, 2 H, OH), 3.37-3.41 (m, 2 H, CH); meso: δ = 0.85-0.93 (m, 6 H, CH₃), 1.21-1.54 (m, 16 H, CH₂), 1.92-2.05 (s, 2 H, OH), 3.57-3.63 (m, 2 H, CH).

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