Siloxanes

Siloxane is compound with a Si-O-Si bond functional group in silicone chemistry. The parent siloxanes include oligomeric and polymeric hydrides with the formulae H(OSiH₂)_nOH and (OSiH₂)_n. Siloxanes also include branched compounds, in which each pair of silicon centers is separated by one oxygen atom. The siloxane functional groups form the backbone of the siloxane, a prime example of which is polydimethylsiloxane. And the functional group (RO)₃Si is called siloxy.

Applications

• Industry use: Cyclomethicone is a class of liquid siloxane with low viscosity and high volatility. It is a skin emollient and a useful cleaning solvent in some cases. Cyclomethicone has short backbones that form closed or nearly closed cycles with methyl groups, giving them many of the same properties as dimethicones, but making them become more volatile. So it is used in many cosmetics where the ultimate complete evaporation of the siloxane carrier fluid is desired. In this way, it is useful for products such as deodorants and antiperspirants that need to be applied to the skin but will not be sticky afterwards.

• Organic chemistry: Molecular capsules provide micro spaces within the molecule, where unstable species are protected and can be stabilized. Cyclic oligomers of trisilanol are considered as transient intermediates in the polycondensation of trialkoxysilanes, resulting in the formation of siloxane networks or ladder polymers. The preparation and separation of these stable forms of intermediates are not only particularly important for understanding the condensation process, but also important for the modeling of the silica surface in homogeneous systems and further conversion to silicon-based functional materials. If the condensation reaction of the trialkoxysilane is performed in the nanopore cavity of the self-assembled coordination cage, which shows significant binding ability to the neutral molecule, a cyclic trimer can be prepared as a stable form.

  Figure 1. Cyclic oligomers of trisilanols.

  In the selective chemodivergent conversion of aldehydes to esters and secondary alcohols, siloxanes and POPd can be used as reaction switches. The reaction applies to a series synthesis of aldehydes. Compared to other methods, this method promotes arylation of aldehydes with aryl siloxanes to provide corresponding secondary alcohols while avoiding the use of transition metal catalysts and aryl fluorosilanes.

Figure 2. Chemoselective nucleophilic arylation and single-step oxidative esterification.

References

1. Yoshizawa M, Kusukawa T, Fujita M, et al. Ship-in-a-bottle synthesis of otherwise labile cyclic trimers of siloxanes in a self-assembled coordination cage[J]. Journal of the American Chemical Society, 2000, 122(26): 6311-6312.
2. Lerebours R, Wolf C. Chemoselective nucleophilic arylation and single-step oxidative esterification of aldehydes using siloxanes and a palladium− phosphinous acid as a reaction switch[J]. Journal of the American Chemical Society, 2006, 128(40): 13052-13053.