On the synthesis and deprotonation of bipyrromethyl silanes

Abstract

An important field in catalysis is transition-metal (TM) based carbon-carbon cross coupling. As noble metals are scarce, efforts are made to use first-row transition metals for these catalytic conversions instead. However, due to their current design, some base metal TM complexes contain phosphine ligands. These ligands are prone to dissociation, making these complexes difficult to characterize. The use of strongly binding anionic silyl ligands, isoelectronic to phosphines, might aid in the stability of the complex and thus allow for more facile characterisation. This would lead to a better understanding of the mechanism behind carbon-carbon cross coupling, and will eventually allow for better catalyst design. In this work, 1,9-diphenyl-5,5-dimethylbipyrromethane (H2dpbpm) and 1,9-dimesityl-5,5-dimethylbipyrromethane (H2dmbpm) are synthesized in a three-step synthesis, largely according to literature procedures. These compounds are then silylated in a two-step synthesis via lithiation, of which 1,9-dimesityl-5,5-dimethylbipyrromethyl phenyl silane is isolated in good yields. The crystal structure of this compound is presented as well. Attempts to deprotonate the silane are made, but most led to degradation of the scaffold and desilylation of the silane, if the reagents reacted at all. The potassium salt of the siloxane was synthesized and crystallized serendipitously by reaction of PhSi(dmbpm)H in the presence of potassium hydride, 18-crown-6 and traces of moisture. Methods to circumvent the problem of deprotonation included oxidative addition to platinum(0) tetrakis(triphenylphosphine), as well as incorporating halogens into the designed silane for subsequent reduction. These methods proved to be unsuccessful as well, as oxidative addition did not occur, and silylation with trihalogenated silanes resulted in an incomprehensible mixture of compounds, often not representing the bipyrromethyl backbone, from which no single compound could be isolated.