New Applications of Monoorganocopper/Iodotrimethylsilane Reagents in Conjugate Additions
The thesis reports new applications of monoorganocopper compounds, RCu(LiI), and iodotrimethylsilane in conjugate additions. The present investigation demonstrates conjugate addition of alkylcopper compounds and iodotrimethylsilane to reactive .alpha.,.beta.-unsaturated ketones and esters. The reactions are fast and in many cases proceed at -78 °C. Conjugate addition of alkylcopper compounds and iodotrimethylsilane at -78 °C to allylic .alpha.,.beta.-unsaturated esters gives allylic silyl ketene acetals, which are essentially allyl vinyl ethers. These undergo Claisen rearrangement at room temperature to afford diastereomeric mixtures of .gamma.,.delta.-unsaturated carboxylic acids after hydrolytic work-up. Monoorganocopper compounds, RCu(LiI), and iodotrimethylsilane give mainly addition via the s-trans conformation of an allylic acrylate. Lithium diorganocuprates, alone or in the presence of chlorotrimethylsilane, give mainly reaction via the s-cis conformation. In strong contrast, lithium diorganocuprates in the presence of bromo- or iodotrimethylsilane give mainly reaction via the s-trans conformation. The ratio of diastereomers in the organocuprate reactions is strongly influenced by the type of halotrimethylsilane used as additive.
Copper acetylides, normally unreactive in conjugate additions, readily add to reactive .alpha.,.beta.-unsaturated ketones and aldehydes in the presence of iodotrimethylsilane. The preferred conformation of the substrate is important for the reactivity. Enones with fixed or preferred s-trans geometries give high yields of product. The conjugate addition of copper acetylides requires the presence of LiI in the reaction mixture. Tetrahydrofuran as solvent gives faster reactions than diethyl ether or dichloromethane. Several different additives were investigated and iodotrimethylsilane was superior to other additives such as trimethylsilyl triflate or bromotrimethylsilane. The choice of copper(I) salt used for the preparation of the acetylides is important and copper iodide gives far better results than copper bromide or copper cyanide.
The present results further support coordination of iodide to copper as mechanistically important in iodotrimethylsilane-promoted conjugate additions of organocopper compounds.