"Conducted Tour" Migration of Li+ during the cis/trans Stereoinversion of α-Arylvinyllithiums.

A "conducted tour" migration keeps a mobile client on a profitable route even though an occasional side-step may seem attractive. A stereochemical manifestation of such a migration had been suggested by Donald J. Cram (1964), and we present now a different example that concerns the cis/trans stereoinversion of monomeric H2 C=C(Li)-aryl compounds: Upon tetrahydrofuran (THF)-assisted heterolysis of the Li-C bond with formation of a solvent-separated ion pair (SSIP), the unchained "mobile client" Li+ (THF)4 is proposed to surmount the rim of the electronically fixed aryl group and to disdain the less encumbered pathways across the H2 C=C region. This interpretation is based on knowledge from a previously published series of monomeric α-arylalkenyllithiums in combination with two new members: 4-(α-lithiovinyl)-2,2-dimethylbenz[f]indane (1) revealed both a barrier against α-aryl rotation and a route-distinguishing retardation as compared with the corresponding migration-dependent cis/trans stereoinversion rate constant of 1-(α-lithiovinyl)naphthalene (2). Monomeric and dimeric ground states of 1 and 2 and their microsolvation numbers were determined by using the recently developed primary and secondary NMR criteria.

Kinetics of α-(2,6-Dimethylphenl)vinyllithium: How To Control Errors Caused by Inefficient Mixing with Pairs of Rapidly Competing Ketones.

Kinetic studies are a suitable tool to disclose the role of tiny reagent fractions. The title compound 2 reacted in a kinetic reaction order of 0.5 (square root of its concentration) with an excess of the electrophiles ClSiMe3, 1-bromobutane (n-BuBr), or 1-iodobutane (n-BuI) at 32 °C in Et2O or in hydrocarbon solvents. This revealed that the tiny (NMR-invisible) amount of a deaggregated equilibrium component (presumably monomeric 2) was the reactive species, whereas the disolvated dimer 2 was only indirectly involved as a supply depot. Selectivity data (relative rate constants κobs) were collected from competition experiments with the faster reactions of 2 in THF and the addition reactions of 2 to carbonyl compounds. This provided the rate sequences of Et2C═O > dicyclopropyl ketone > t-Bu-C(═O)-Ph > diisopropyl ketone ≫ t-Bu2C═O > ClSiMe3 > n-BuI > n-BuBr ≈ (bromomethyl)cyclopropane (but t-Bu2C═O < ClSiMe3 in THF only) and also of cyclopropanecarbaldehyde > acetone ≥ t-Bu-CH═O. It is suggested that a deceivingly depressed selectivity (1 < κobs < kA/kB), caused by inefficient microscopic mixing of a reagent X with two competing substrates A and B, may become evident toward zero deviation from the correlation line of the usual inverse (1/T) linear temperature dependence of ln κobs.

Unusual traits of cis and trans-2,3-dibromo-1,1-dimethylindane on the way from 1,1-dimethylindene to 2-bromo-, 3-bromo-, and 2,3-dibromo-1,1-dimethylindene.

Do not rely on the widely accepted rule that vicinal, sp(3)-positioned protons in cyclopentene moieties should always have more positive (3) J NMR coupling constants for the cis than for the trans arrangement: Unrecognized exceptions might misguide one to wrong stereochemical assignments and thence to erroneous mechanistic conclusions. We show here that two structurally innocent-looking 2,3-dibromo-1,1-dimethylindanes violate the rule by means of their values of (3) J(cis) = 6.1 Hz and (3) J(trans) = 8.4 Hz. The stereoselective formation of the trans diastereomer from 1,1-dimethylindene was improved with the tribromide anion (Br3 (-)) as the brominating agent in place of elemental bromine; the ensuing, regiospecific HBr elimination afforded 3-bromo-1,1-dimethylindene. The addition of elemental bromine to the latter compound, followed by thermal HBr elimination, furnished 2,3-dibromo-1,1-dimethylindene, whose Br/Li interchange reaction, precipitation, and subsequent protolysis yielded only 2-bromo-1,1-dimethylindene.

Unpaired spin densities from NMR shifts and magnetic anisotropies of pseudotetrahedral cobalt(II) and nickel(II) vinamidine bis(chelates).

The distribution of unpaired electron spin over all regions of the organic ligands was extracted from the large positive and negative 1H and 13C NMR paramagnetic shifts of the title complexes. Owing to benevolent line broadening and to very high sensitivities of approximately 254,000 and approximately 201,000 ppm/(unpaired electron spin) for Co(II) and Ni(II), respectively, at 298 K in these pseudotetrahedral bis(N,N'-chelates), spin transmission through the sigma- (and orthogonal pi)-bonding system of the ligands could be traced from the chelate ring over five to nine sigma bonds. Most of those "experimental" spin densities DeltarhoN (situated at the observed nuclei) agree reasonably well with quantum chemical DeltarhoDFT (DFT = density functional theory) values and provide an unsurpassed number of benchmark values for the quality of certain types of modern density functionals. The extraction of DeltarhoN became possible through the unequivocal separation of the nuclear Fermi contact shift components from the metal-centered pseudocontact shifts, which are proportional to the anisotropy Deltachi of the magnetic susceptibility: Experimental Deltachi values were obtained in solution from measured deuterium quadrupole splittings in the 2H NMR spectra of two deuterated model complexes and were found to be nonlinear functions of the reciprocal temperature. This provided the reliable basis for predicting metal-centered pseudocontact shifts for any position of a topologically well-defined ligand at varying temperatures. The related ligand-centered pseudocontact shifts were sought by using the criterion of their expected nonlinear dependence on the reciprocal temperature. However, their contributions could not be differentiated from other small effects close to the metal center; otherwise, they appeared to be smaller than the experimental uncertainties. The free activation energy of N-aryl rotation past a vicinal tert-butyl substituent in the Ni(II) vinamidine bis(N,N'-chelates) is DeltaG++(+74 degrees C) approximately 17.0 kcal/mol and past a vicinal methyl group DeltaG++(-6 degrees C) approximately 13.1 kcal/mol.

Total synthesis of flustramine C via dimethylallyl rearrangement.

The marine natural product flustramine C from the bryozoan Flustra foliacea was synthesized in five steps and 38% yield starting from Nb-methyltryptamine. The key step is the biomimetic oxidation of the natural product deformylflustrabromine causing selective 1,2-rearrangement of the inverse prenyl group. By 1H,15N HMBC experiments, it is unambiguously shown that the reaction with t-BuOCl commences with chlorination of the side chain nitrogen. Deformylflustrabromine itself was synthesized via Danishefsky inverse prenylation. [reaction: see text].

Carbenoid chain reactions: substitutions by organolithium compounds at unactivated 1-chloro-1-alkenes.

The deceptively simple "cross-coupling" reactions Alk(2)C=CA-Cl + RLi --> Alk(2)C=CA-R + LiCl (A = H, D, or Cl) occur via an alkylidenecarbenoid chain mechanism in three steps without a transition metal catalyst. In the initiating step 1, the sterically shielded 2-(chloromethylidene)-1,1,3,3-tetramethylindans 2a-c (Alk(2)C=CA-Cl) generate a Cl,Li-alkylidenecarbenoid (Alk(2)C=CLi-Cl, 6) through the transfer of atom A to RLi (methyllithium, n-butyllithium, or aryllithium). The chain cycle consists of the following two steps: (i) A fast vinylic substitution reaction of these RLi at carbenoid 6 (step 2) with formation of the chain carrier Alk(2)C=CLi-R (8), and (ii) a rate-limiting transfer of atom A (step 3) from reagent 2 to the chain carrier 8 with formation of the product Alk(2)C=CA-R (4) and with regeneration of carbenoid 6. This chain propagation step 3 was sufficiently slow to allow steady-state concentrations of Alk(2)C=CLi-Aryl to be observed (by NMR) with RLi = C6H5Li (in Et2O) and with 4-(Me3Si)C6H4Li (in t-BuOMe), whereas these chain processes were much faster in THF solution. PhC[triple bond]CLi cannot perform step 1, but its carbenoid chain processes with reagents 2a and 2c may be started with MeLi, whereafter LiC[triple bond]CPh reacts faster than MeLi in the product-determining step 2 to generate the chain carrier Alk(2)C=CLi-C[triple bond]CPh (8g), which completes its chain cycle through the slower step 3. The sterically congested products were formed with surprising ease even with RLi as bulky as 2,6-dimethylphenyllithium and 2,4,6-tri-tert-butylphenyllithium.