Improved preparations of 2,6-dimethylstyrene (5) and its abromo derivative (10) are described. The Br/Li exchange reaction of 10 provides single crystals of the title compounds 11 or 12, which were characterized as disolvated dimers by X-ray analyses. A similar dimer persists in diethyl ether, tertbutyl methyl ether, and toluene at all accessible temperatu-res, with significant lithiation NMR shifts (relative to 5) partially due to charge delocalization from the sp2-carbanionic center. Some NMR coupling constants are typical of the dimeric aggregate. The configurational (E,Z) lability is quantified in toluene solution.

The trans/cis stereoinversion of the trigonal carbanion centers C-α in a series of monomeric 2-(α-aryl-αlithiomethylidene)-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 α-H by α-Li) is illustrated through NMR shifts, Δδ. The transition states for stereoinversion are significantly more polar and charge-delocalized than the ground states (Hammett ρ = +5.2), pointing to a mechanism that involves heterolysis of the C−Li bond via a solventseparated 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. −23 cal mol −1 K −1 and constitutes a kinetic privilege of THF depending on microsolvation at Li. Thus, the sp 2 -stereoinversion process is "catalyzed" 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-α of the SSIP undergoes rehybridization (approximately in-plane inversion) through a closeto-linear transition state; this motion occurs with a concomitant "conducted tour" migration of Li + (THF) 4 and is unimpaired by additional ortho-methylations at α-aryl. The synthetic route started with preparations of three α-chloro congeners through the carbenoid chain reaction, followed by vinylic substitution of α-Cl by α-SnMe 3 (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 + .

The title compound 4 is a trisolvated monomer 4&3THF in THF solution and dimerizes endothermically to form (4&THF)2 with a strongly positive (!) dimerization entropy in toluene as the solvent. In the absence of electron-pair donor ligands, 4 aggregates (>dimer) in hydrocarbon solutions. These results followed from the (13)C-α splitting patterns and the magnitudes of the one-bond (13)C,(6)Li NMR coupling constants in combination with lithiation NMR shifts as secondary NMR criteria. The rate constants of cis/trans sp(2)-stereoinversion could be measured on the (1)H NMR time scale in THF, in which solvent the preinversion lifetime is 0.24 s at 25 °C. This inversion proceeds according to the pseudomonomolecular, ionic mechanism with the typical, strongly negative pseudoactivation entropy. In a different mechanism, the lifetimes are much longer at 25 °C for the dimer (4&t-BuOMe)2 in toluene (ca. 2.5 min) and for donor-free, aggregated 4 in hexane solution (roughly 1 min). The olefinic interproton two-bond coupling constants (2)JH,H of the H2C═CLi part are proposed as an indicator of microsolvation at Li, because they were found to increase linearly with the "explicit" solvation of α-arylvinyllithiums by 0, 1, 2, and 3 electron-pair donor ligands.

No abstract

Die rasche und vollstnndige Umwandlung der Vinyllithiurn-Verbindung 1 in die Allyllithium-Verbindung 7 wird 'H-NMR-spektroskopisch verfolgt und durch chemische Umsetzungen belegt. Diese scheinbare Umlagerungsreaktion erfolgt unter katalytischer Mitwirkung des 2-Methyl-lphenyl-1-propens (2). Aus den Reaktionsordnungen, Aktivierungsparametern und der Solvensabhangigkeit wird auf einen intermolekularen Ummetallierungsmechanismus mit deaggregierten und ionisierten Zwischenstufen geschlossen. >Methyl-1-phenyl-1-propenyllithiurn. A Vinyllithium Derivative Showing Catalyzed TransmetalationFull preparative and kinetic details are given for the apparent vinyl-to-ally1 anion rearrangement 1) of the title compound 1. The rapid and quantitative formation of the allyllithium derivative 7 is shown to be catalyzed by 2-methyl-1-phenyl-1-propene (2). An intermolecular transmetalation mechanism with de-aggregation and ionization steps is suggested to explain the orders of reaction, parameters of activation, and solvent dependency.Die im Protonenresonanzspektrum der Titelverbindung 1 getrennt sichtbaren Methylsignale sollten zu einer einzigen Resonanzabsorption verschmelzen, wenn die E/Z-Diastereotopomerisierung I + l a geniigend rasch (auf der NMR-Zeitskala) erfolgt. Bei der Fahndung nach derartigen Koaleszenzerscheinungen beobachteten wir statt dessen eine scheinbare Umlagerungsreaktion') zum Allyl-Anion 7. Wir berichten hier iiber die strukturelle Sicherung und die kinetischen Details dieser unerwarteten Urnmetallierung. A. Synthesen und StrukturbeweiseDie reinsten Praparate 2-Methyl-1-phenyl-1-propen (2) erhielten wir aus 2-Methyl-1-phenyl-I-propanol, wenn dieses aus Isobutyraldehyd hergestellt 2*3) und mit Kaliumhydrogensulfat katalytisch dehydratisiert wurde. Hierbei bildete sich auch das isomere Olefin 3, das sich aber durch erneute Behandlung mit dem wasserfreien Katalysator vollstandig in 2 umlagern lien. Das bekannte Bromolefin 4 gewannen wir aus 2 analytisch rein nach Perkin ').