Towards the Next Generation of Lochmann–Schlosser Superbases: A Potassium Neopentyl/Alkoxy Aggregate used in the Tetra‐Functionalization of Ferrocene

Abstract

Lochmann-Schlosser superbases are formed by mixing alkyllithium with potassium alkoxides. These reagents could prove their synthetic usefulness and reliability in many reactions over five decades. However, despite many efforts, the real source of the exceptional reactivity remained a secret. The seemingly manageable system of four components (lithium, potassium atoms, alkyl groups, and alkoxy groups) and their interaction is obscured by poor solubility and fierce reactivity. Recent progress was achieved by using neopentyllithium, leading to alkane-soluble aggregates with varying lithium/potassium content and a flexible alkyl/alkoxy ratio. Herein, we isolated two new alkane-soluble alkyl/alkoxy mixed aggregates, [Li KNp (OtBu) ] and [K Np(OtAm) ]. The latter compound is a thermally stable three-component potassium alkyl/alkoxy base with well-defined stoichiometry, in contrast to lithium-containing Lochmann-Schlosser bases with variable metal and alkyl/alkoxy content. In a simple protocol, this potassium-base gave tetrametalated ferrocene, which was converted into 1,1',3,3'-ferrocenetetracarboxylic acid by reaction with CO . A subsequent conversion into the methyl ester allowed its separation from accompanying di- and tri-substituted ferrocenes.

Structural Motifs of Alkali Metal Superbases in Non‐coordinating Solvents

“…The simplification of a fractional Li/K ratio (a) and a fractional R/OR’ ratio (b) allows plotting all possible variations in a two dimensional coordination system referring to a and b (Figure 2). [54] …”

“…With the exception of ill‐defined methyl sodium with lithium atoms statistically replacing sodium atoms, [13] there is no information available about mixed alkali metal alkyl compounds. While there are numerous examples of combinations of alkali metals with less electropositive metals such as magnesium or zinc, [59] the structural motif of alkyl groups coordinated by a mixed alkali metal environment is so far restricted to compounds 1 (motif III ), [52] 2 (motifs III + IV ), [52] and 3 (motif II ) [54] with their mixed alkyl/alkoxy arrangement. Motif IV with a K 3 environment is also present in pure alkyl potassium compounds such as polymeric methyl potassium [63] or tetrameric trimethylsilylmethyl potassium coordinated by TMEDA [51] …”

“…Compound 4 was successfully used in a tetra‐metalation of ferrocene [54] . Ferrocene is a suitable test‐substrate for metalation for several reasons.…”

The Preparation of Tetramethyl 1,1′,3,3′-Ruthenocenetetra­carboxylate and Tetramethyl 1,1′,3,3′-Osmocenetetracarboxylate, and a Simplified Synthesis for Tetramethyl 1,1′,3,3′-Ferrocene­tetracarboxylate

“…The formation of the methyl esters allowed the isolation of 2 4 in a half gram scale and moderate yield (58%). 18 However, the limited access to neopentyllithium and the necessary amounts of potassium tert-amyloxide prevented this reaction in larger scale. Neopentyllithium is produced by the reaction of neopentyl chloride with lithi-…”

“…15 The use of readily soluble potassium tert-amyloxide 16,17 (or tertpentoxide) [KOCMe 2 Et] in a four-fold excess leads to the formation of a potassium neopentyl/tert-amyloxy mixed aggregate in n-hexane [K 4 Np(Ot-Am) 3 ]. 18 In fact, more than four equivalents (>4.5 equiv) of tert-amyloxide are needed: one equivalent forms neopentylpotassium, which is then co-aggregated by three more equivalents; further excess alkoxide is needed to bind lithium tert-amyloxide as mixed Li/K alkoxides. A five-fold excess of this base prepared in situ is able to poly-metalate ferrocene in a smooth reaction at 60 °C.…”