Bicyclo[3.2.1]octa‐2,6‐dienylkalium. Synthese aus dem Dien und 13 C‐NMR‐Spektrum

50 years of superbases made from organolithium compounds and heavier alkali metal alkoxides

A review of reactions of organolithium compounds (RLi) with alkali metal alkoxides is presented. On the one hand, simple lithium alkoxides form adducts with RLi the reactivity of which differs only slightly from that of RLi. On the other hand, after mixing heavier alkali metal alkoxides (R’OM, M = Na, K, Rb, Cs) with RLi, a new system is formed, which has reactivity that dramatically exceeds that of the parent RLi. A metal interchange, according to the equation RLi + R’OM = RM + R’OLi, occurs in this system, giving rise to a superbase. This reaction is frequently used for the preparation of heavier alkali metal organometallic compounds. Similar metal interchange takes place between R’OM and compounds such as lithium amides and lithium enolates of ketones or esters, thus demonstrating the general nature of this procedure. Superbases react easily with many types of organic compounds (substrates), resulting in the formation of a heavier alkali metal derivative of the substrate (metalation). The metalated substrate can react in situ with an electrophile to yield the substituted substrate, a procedure that is frequently used in synthetic and polymer chemistry. An improved mechanism of metal interchange and reaction of superbases with substrates is proposed.

An ab initio study on the magnetic susceptibility and magnetically induced current density distribution of the bicyclo[3.2.1]octa-3,6-dien-2-yl anion

The molecular susceptibility and the atomic contributions including the basin and surface components calculated at the RHF/6-31+G(d)//6-31+G(d) level of theory along with vector current densities obtained at the 6-31++G(d,p)//6-31+G(d) and 6-31+G(d)//6-31+G(d) levels are reported for benzene (1) and correlated - the agreement is excellent - with the results obtained by Keith and Bader with the 6-311++G(2d,2p) basis. This validates our similar studies on the bicyclo[3.2.1]octa-3,6-dien-2-yl anion (2) - initially considered to be a bishomoaromatic species - its parent hydrocarbon bicyclo[3.2.1]octa-2,5-diene (3), the bicyclo[3.2.1]oct-3-ene-2-yl anion (4), and 5, the lithium complex of 2. While a magnetic susceptibility exhalation is found in going from 4 to 2, the increase in atomic surface contribution to Δχ arises predominately (42.6%) from the ethenyl bridge carbons C6 and C7, with C2 + C4 and C3 exhibiting substantially smaller increases (27.4 and 8.3%, respectively). This result, in conjunction with the nature of the vector current density maps of 2 and 4, provides further evidence that 2 does not possess a ring current as depicted in 2-bh, the usual representation of "bishomoaromatic" 2.Key words: ab initio calculations, IGAIM, CSGT, magnetic susceptibility, induced current density distributions, bicyclo[3.2.1]octa-3,6-dien-2-yl anion.

Applications and evaluation of IGAIM 13C and 1H chemical shift calculations for unsaturated hydrocarbons and organolithium compounds

Wave functions obtained at the RHF/6-31+G(d) level of theory were used with the new method IGAIM (individual gauges for atoms in molecules) developed by Keith and Bader to calculate the isotropic 13C and 1H NMR chemical shifts of a group of neutral molecules (bicyclo[3.2.1]octa-2,6-diene (1), bicyclo[3.2.1]oct-6-ene (2), bicyclo[2.2.1]hepta-2,5-diene (3), benzene (4)), carbanions (prop-2-en-1-yl (allyl) (5), bicyclo[3.2.1]octa-3,6-dien-2-yl (8)), and lithium complexes (prop-2-en-1-yllithium (6) and its dimer 7, bicyclo[3.2.1]octa-3,6-dien-2-yllithium (9)). The theoretical isotropic 13C NMR chemical shifts of the neutral molecules, relative to the calculated value for TMS(tetramethylsilane), are in excellent agreement with the experimental values, with differences between the sets of data ranging from +4.9 to −7.1 ppm. For the same group of compounds the theoretical 1H shifts are lower than the experimental values by increments ranging between 0.4 and 1.29 ppm. For allyllithium, which exists as an unsymmetrical fluxional dimer, the theoretical averaged 13C shifts are larger, 2.6 ppm for the terminal carbons and 16.7 ppm for the central carbon, than the experimental values. In the case of 8, originally considered to be a bishomoaromatic species, the theoretical 13C chemical shifts of its Li+ complex 9 differ from the experimental ones for THF-solvated 9 by values that range from +6.2 to −15.0 ppm. Yet, the relative theoretical chemical shifts — of special importance is the fact that the carbons of the vinylene bridge of this compound are unusually shielded relative to the parent diene 1 — correlate with the experimental data. The 1H chemical shifts calculated for the hydrocarbons 1, 2, 3, 4 and the lithium complexes 7 and 9 range from 0.08 to 1.38 ppm less than the experimental values. To gain information on whether variations in charge density play a significant role in determining the magnitudes of the chemical shifts, we used AIMPAC calculations to obtain the atom electron populations of diene 1, 5, 6, dimer 7, 8, and 9. We find no obvious correlation between the charges on the carbon atoms and the 13shifts for this set of compounds. Key words: IGAIM, calculations, 13C and 1H chemical shifts, unsaturated hydrocarbons, organolithium compounds.