Synthesis and structural characterisation of alkali metal complexes of heteroatom-stabilised 1,4- and 1,6-dicarbanions

Abstract

A straightforward Peterson olefination reaction between either [{(Me₂PhSi)₃C}Li(THF)] or in situ-generated [(Me₃Si)₂{Ph₂P(BH₃)}CLi(THF)_n] and paraformaldehyde gives the alkenes (Me₂PhSi)₂CCH₂ (1) and (Me₃Si){Ph₂P(BH₃)}CCH₂ (2), respectively, in good yield. Ultrasonic treatment of 1 with lithium in THF yields the lithium complex [{(Me₂PhSi)₂C(CH₂)}Li(THF)_n]₂ (3), which reacts in situ with one equivalent of KOBu^t in diethyl ether to give the potassium salt [{(Me₂PhSi)₂C(CH₂)}K(THF)]₂ (4). Similarly, ultrasonic treatment of 2 with lithium in THF yields the lithium complex [[{Ph₂P(BH₃)}(Me₃Si)C(CH₂)]Li(THF)₃]₂.2THF (5). The bis(phosphine-borane) [(Me₃Si){Me₂(H₃B)P}CH(Me₂Si)(CH₂)]₂ (6) may be prepared by the reaction of [Me₂P(BH₃)CH(SiMe₃)]Li with half an equivalent of ClSiMe₂CH₂CH₂SiMe₂Cl in refluxing THF. Metalation of 6 with two equivalents of MeLi in refluxing THF yields the lithium complex [[{Me₂P(BH₃)}(Me₃Si)C{(SiMe₂)(CH₂)}]Li(THF)₃]₂ (9), whereas metalation with two equivalents of MeK in cold diethyl ether yields the potassium complex [[{Me₂P(BH₃)}(Me₃Si)C{(SiMe₂)(CH₂)}]₂K₂(THF)₄]_∞ (10) after recrystallisation. X-Ray crystallography shows that, whereas the lithium complex 5 crystallises as a discrete molecular species, the potassium complexes 4 and 10 crystallise as sheet and chain polymers, respectively.