Pseudomonomolecular, Ionic sp2-Stereoinversion Mechanism of 1-Aryl-1-alkenyllithiums

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• 作者：Rudolf Knorr ; Thomas Menke ; Claudia Behringer ; Kathrin Ferchland ; Johann Mehlst盲ubl ; Ernst Lattke
• 刊名：Organometallics
• 出版年：2013
• 出版时间：August 12, 2013
• 年：2013
• 卷：32
• 期：15
• 页码：4070-4081
• 全文大小：539K
• 年卷期：v.32,no.15(August 12, 2013)
• ISSN：1520-6041

文摘

The trans/cis stereoinversion of the trigonal carbanion centers C-伪 in a series of monomeric 2-(伪-aryl-伪-lithiomethylidene)-1,1,3,3-tetramethylindanes (known to be trisolvated at Li) is rapid on the NMR time scales (400 and 100.6 MHz) in THF solution. The far-reaching redistribution of electric charge in the ground-state molecules caused by lithiation (formal replacement of 伪-H by 伪-Li) is illustrated through NMR shifts, 螖未. The transition states for stereoinversion are significantly more polar and charge-delocalized than the ground states (Hammett 蟻 = +5.2), pointing to a mechanism that involves heterolysis of the C鈥揕i bond via a solvent-separated ion pair (SSIP). This requires immobilization of only one additional (the fourth) THF molecule at Li⁺, which accounts for part of the apparent activation entropies of ca. 鈭?3 cal mol^鈥? K^鈥? and constitutes a kinetic privilege of THF depending on microsolvation at Li. Thus, the sp²-stereoinversion process is 鈥渃atalyzed鈥?by the solvent THF; its mechanism is monomolecular with respect to the ground-state species because the pseudo-first-order rate constants, measured through NMR line shape analyses, are independent of the concentrations (inclusive of decomposition) of the dissolved species (hence no associations and no dissociation to give free carbanion intermediates). In the deduced pseudomonomolecular mechanism (bimolecular through solvent participation), the angular C-伪 of the SSIP undergoes rehybridization (approximately in-plane inversion) through a close-to-linear transition state; this motion occurs with a concomitant 鈥渃onducted tour鈥?migration of Li⁺(THF)₄ and is unimpaired by additional ortho-methylations at 伪-aryl. The synthetic route started with preparations of three 伪-chloro congeners through the carbenoid chain reaction, followed by vinylic substitution of 伪-Cl by 伪-SnMe₃ (most efficient in THF despite steric congestion). The final Sn/Li interchange reaction afforded the new 1-aryl-1-alkenyllithium samples, initially uncontaminated by free Li⁺.