Contents
Organometallic Alkali Metal Bases
Lochmann-Schlosser superbases with their metalating power of unreactive C-H bonds could prove their usefulness in synthetic chemistry over the last five decades as an alternative to Lithium compounds. These reagents are formed by mixing Alkyllithium compounds with Potassium tert-Butoxides, a reaction which can also be used to form the corresponding Alkylpotassium:
Despite many efforts, very little is known about the structure and properties of the potential Alkyl/Alkoxy Aggregates. By using Neopentyllithium in this reaction we managed to isolate two mixed aggregates, containing Lithium and Potassium, and as well Alkyl and Alkoxy groups side by side:
The structural similarity of Neopentyl and tButoxy groups leads to substitutional disorder, some of the Neopentyl groups are partially replaced by tButoxy groups, leading to a deviation from the ideal stoichiometry.
The very good Alkane-solubility and the stability at ambient temperature, which can have important implications towards further developing the applications of these polar reagents in synthesis.
Large Organometallic Structures
The combination of smaller organometallic units can lead to large, more complex arrangements. Each building block can introduce structural motifs (linear arrangements, angled arrangements,...) which can be found in the resulting complex.
In this example we used tetrameric Lithium amid (LiTMP, Lithium 2,2,6,6-tetramethylpiperidide), polymeric Lithium cyclopentadienide (LiCp) and hexameric nButyllithium (LinBu).
Mixing LiTMP and LiCp in Alkanes produced Solutions, from which a tetrameric complex containing 8 Lithium atoms and four Cp and TMP units each (see figure, left). Adding an excess of nButyllithium leads to a second metalltion of the Cp unit and the incorporation of a Li6nBu2 unit into the square arrangement (figure right).
Andrew A. Fyfe, Alan R. Kennedy, Jan Klett,* Robert E. Mulvey, Angew. Chem. Int. Ed. 2011, 50, 7776.
Deprotonative Ether Cleavage
A constant problem in organometallic chemistry is the reaction of alkali metal compounds with organic solvents, which leads to thermal instability or prevents their characterisation. Especially ethers, which are very useful because of their coordinating abilities, quickly undergo deprotonation and immediate decomposition. In case of Tetrahydrofuran (THF) the deprotonation occurs at the α-Position. The formed α-lithio-tetrahydrofuran usually decomposes undergoing a [3+2] cycloreversion (a reversed [3+2] cycloaddition), forming ethene and Lithium enolate:
By using a Sodium Zincate we were able to stabilise the α-deprotonated THF. This compound (see figure below) is stable at ambient temperature and was fully characterised and was used in a reaction with an electrophile (PhCOCl) to produce an α-substituted THF.
Alan. R. Kennedy, Jan Klett, Dominic S. Wright, Robert E. Mulvey, Science 2009, 326, 706.
When a Sodium Magnesiate is used, a "catastrophic" ether cleavage takes place: both C-O bonds and four C-H bonds are broken, leaving back an O(-II)-oxide Anion and a dimetalated butadiene dianion. The former fragment was isolated as a so called "inverse crown ether", the latter fragment was isolated coordinated by two bimetallic base-units (see figure below).
"Inverse Crown" trapped oxide Dimetalated butadiene with two bimetallic fragments
Alkyl Chemistry of Transition Metals