Alkali Silica Gels

ChemFiles Volume 5 Article 9

Alkali metals have long been used in synthetic chemistry as reducing agents, but their pyrophoric nature has often prevented their use in larger-scale reactions. Alkali metals react violently with water, produce copious amounts of hydrogen gas that can ignite, and result in strongly alkaline solutions. The neutral metals usually need to be stored under vacuum or in an inert liquid to protect them before use. Typically, synthetic chemists have used these metals in reactions as dispersions, as solutions in liquid ammonia, or on inert supports to avoid some of these concerns. Sodium has been commonly employed as sodium mercury amalgam. However, a need still exists to have alkali metals and alloys available in a form that may be easily handled without a significant loss in metal reactivity.

SiGNa Chemistry has recently developed and reported1 a series of alkali metals and alloys absorbed into silica gel to create stable, free-flowing powders with varying reactivities (based on the intended application). These powders are an attractive alternative to other reagents for desulfurizations, dehalogenations (as Wurtz coupling reactions), and Birch reductions. Many of these reactions can be performed at room temperature, eliminating high pressure and high temperature systems.

SiGNa’s alkali silica gels are classified into three categories. Stage 0 powders are air-sensitive, but can easily be used in continuous-flow applications. Stage I powders are moisture-sensitive, nonpyrophoric, and air-stable; they can be stored for months without any change in reducing capacity. Stage II powders are easily handled in an open ambient environment, but readily react with water to produce stoichiometric yields of pure hydrogen gas. Stage II powders function well as a hydrogen source or drying agent.

Anthracene undergoes Birch reduction to form 9,10-dihydro-anthracene using a 2-mL Pasteur pipette containing either Stage 0 or Stage I alkali silica gel mixed with regular silica gel (Scheme 1). The reaction was carried out in 5 minutes (elution time) and formed the product in >99% purity.

Scheme 1


The Wurtz reduction of benzyl chloride can be completed using Stage 0 or I materials by a chromatographic process or using Stage II material in a batch process. By either method, bibenzyl is formed as the only product (Scheme 2). This methodology can be applied to the dehalogenation of both aromatic and aliphatic substrates.

Scheme 2


Normally, alkali metal-mediated desulfurizations in hydrocarbon solvents require high temperatures. However, the desulfurization of dibenzothiophene in THF to form biphenyl using Stage 0 or I materials in continuous or batch processes can be accomplished at room temperature (Scheme 3). The methodology was also successfully applied to the more difficult desulfurizations of 4,6-dimethyldibenzothiophene and diphenyl sulfide.

Scheme 3


Sigma-Aldrich is pleased to announce an agreement with SiGNa Chemistry to distribute research quantities of these powerful alkali silica gels for research applications.

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Materials

     

References

  1. Dye, J. L. et al. J. Am. Chem. Soc. 2005, 127, 9339.

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