Novel Diastereoselective Synthesis of (E)-gem-Dimetalloalkenes Containing Boron and Germanium: An Easy Access to Alkyl Trimethylgermyl Ketones and Carboxylic Acids

Abstract

A convenient, novel synthesis of (E)-gem-dimetalloalkenes containing boron and germanium from the (Z)-1-bromo-1-alkenylboronate esters has been developed. α-Bromo-(Z)-1-alkenylboronate esters readily prepared from the literature procedures smoothly underwent a reaction with freshly generated tri­methylgermyllithium in hexamethylphosphoramide (HMPA) from the hexamethyldigermanium at -78 °C to provide the corresponding ‘ate’ complexes. These ‘ate’ complexes underwent intramolecular nucleophilic substitution reaction to provide the corresponding (E)-1-alkenylboronate esters containing trimethylgermyl moiety. These intermediates were isolated in good yields (72-81%) and were characterized by the spectral data (¹H NMR and ¹³C NMR). Upon oxidation of these intermediates with trimethylamine-N-­oxide, the corresponding alkyl trimethylgermyl ketones were prepared in good yields (78-86%). Oxidation with alkaline hydrogen peroxide followed by acidification afforded the corresponding ­carboxylic acids in good yields (82-88%).

Key words

gem-dimetalloalkanes - trimethylgermyllithium - hexa­methylphosphoramide

References and Notes

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14

The preparation of n-hexanoic acid from the (E)-2-(1-tri-methylgermyl-1-hexenyl)-1,3,2-dioxaborinane is represen-tative: To a solution of (E)-2-(1-trimethylgermyl-1-hexenyl)-1,3,2-dioxaborinane (10 mmol, 2.85 g) in THF (10 mL) was added MeOH (5 mL). It was then cooled to 0 °C and NaOH (3 M, 5 mL) was added followed by the slow addition of 30% H₂O₂ (25 mmol, 2.5 mL). The reaction mixture was allowed to come to r.t. and the stirring was continued for 4 h at r.t. Also, 3 M NaOH (5 mL) was added. The reaction mixture was washed with Et₂O (2 × 30 mL). Acidification of the aqueous phase was performed with concd HCl acid. It was then extracted with Et₂O (2 × 30 mL), which was then removed via evaporation to provide n-hexanoic acid in 82% (0.95 g) isolated yield. IR, ¹H NMR, and ¹³C NMR spectral data were used characterize the compound. IR (neat): <2926, 1718 cm^-1. ¹H NMR (CDCl₃/without TMS): δ = 0.8 (m, 3 H), 1.28-1.57 (m, 6 H), 2.30 (m, 2 H), 11.75 (s, 1 H). CMR (CDCl₃/without TMS): δ = 13.79, 22.31, 24.37, 31.24, 34.11, 180.74.