Inexpensive, operationally simple flow reactor for rapid, chromatography free synthesis of siloxane based liquid crystals

John W Goodby,Richard J Mandle

doi:10.1039/c8re00027a

Abstract

We report on the hydrosilylation of terminal alkenes in flow using a “Karstedt-like” platinum-on-silica catalyst to afford liquid-crystalline materials.

Highlights

The addition of a silyl hydride across an olefin is exothermic;[16] with increased reaction temperature there is the possibility for increased formation of the Markovnikov addition product
We demonstrate an inexpensive and operationally simple flow reactor for the hydrosilylation of terminal alkenes, and apply this to the synthesis of a family of liquid-crystalline materials
The reaction affords only the desired beta-addition product (1H/13C{1H}/DEPT135/29Si{1H} NMR) and complete conversion of the alkene to the siloxane was achieved with a throughput of 6 mmol h−1; increased throughput may be possible with modifications, this quantity is sufficient for the synthesis and evaluation of LC materials

Summary

Introduction

The two most common ways of forming carbon–silicon bonds are the reaction of a nucleophile with a silyl halide, or the addition of a silyl hydride across an olefin using transition metal catalysis (typically platinum, e.g. Karstedt's catalyst).[1,2,3,4] The incorporation of siloxanes into liquid crystals is often performed as a means to attain lower melting points,[5] suppress nematic liquid crystal phase formation[6] as well as to control smectic mesophase layer spacing[7] and induce De Vries behaviour.[8,9,10,11,12] One of the difficulties in preparing liquid-crystalline siloxanes is the need for chromatographic separation of unreacted olefin and silyl hydride from the desired product;[13] the similar retention factors of the product and starting material make this time consuming. Given that the liquidcrystalline state is extremely sensitive to chemical purity,[14] careful chromatographic purification of these materials is often unavoidable. In addition to their cost and inherent lack of reusability, homogenous catalysts such as Karstedt's catalyst may contaminate the resulting material with platinum colloids which, in addition to having catalytic activity of their own,[15] can be difficult to remove and may affect properties of the resulting materials. The addition of a silyl hydride across an olefin is exothermic;[16] with increased reaction temperature there is the possibility for increased formation of the (usually undesired) Markovnikov addition product.

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Conclusion