A Quantitative NMR Method for Silyllithium Analysis

Published as part of the Cluster Silicon in Synthesis and Catalysis

Abstract

A rapid and extremely simple method for silyl anion analysis is presented. The progress of silyllithium reagent preparation can be determined by quenching an aliquot with neat chloro(trimethyl)silane, evaporation, dilution with CDCl₃, and direct proton NMR analysis. This procedure is fast, simple, and allows for identification and relative quantification of the starting reagent, intermediates, and the silyllithium product.

Key words

silicon - lithium - lithiation - organometallic reagents - halogens - hydrides

• References and Notes

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• 43 1-[3-(Methoxymethoxy)propyl]-2,2,2-trimethyl-1,1-diphenyldisilane (11) Granular lithium shot [87 mg, 12.5 mmol, 14 equiv, in mineral oil (Aldrich)] was washed three times with hexane then transferred to a 25 mL round-bottomed flask fitted with a septum. The flask was evacuated and then filled with argon three times. THF (6 mL) was added, and the mixture cooled to 0 °C. TMSCl (0.3 mL) was then added, the mixture was stirred for 30 min, and the solution was removed by syringe. The cleaned lithium shot was washed with THF (3 × 3 mL). A solution of [3-(methoxymethoxy)propyl](diphenyl)silane (10; 259 mg, 0.90 mmol) in THF (3.0 mL) was added, and the mixture was stirred for 9 h. The solution was decanted and washed with H₂O. The aqueous phase was extracted with Et₂O (3 × 10 mL), and the combined organics were dried (Na₂SO₄, filtered, and concentrated. Purification by flash chromatography (gradient 100:1 to 10:1 hexanes–EtOAc) gave a clear, colorless oil; yield: 214 mg (60%); R_f = 0.53 (4:1 hexanes–EtOAc). IR (neat): 3067, 2947, 2086, 1954, 1428 cm^–1. ¹H NMR (400 MHz, CDCl₃): δ = 7.45–7.50 (m, 4 H), 7.32–7.37 (m, 6 H), 4.60 (s, 2 H), 3.50 (t, J = 6.6 Hz, 2 H), 3.34 (s, 3 H), 1.64–1.73 (m, 2 H), 1.17–1.24 (m, 2 H), 0.17 (s, 9 H). ¹³C NMR (125 MHz, CDCl₃): δ = 136.3, 135.4, 128.9, 128.0, 96.5, 70.7, 55.3, 25.1, 9.1, –1.1. HRMS: m/z [M + NH₄]⁺ calcd for C₂₀H₃₄NO₂Si₂: 376.2123; found: 376.2123.
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