Sterically Congested Molecules, 16 A Convenient Synthesis of Di-tert-butyl Ketone via its Imine, 2,2,4,4-tetramethyl-3-pentanimine

Doubly Diastereoconvergent Preparation and Microsolvation-Controlled Properties of (Z)- and (E)-1'-Lithio-1'-(2,6-dimethylphenyl)propenes

A doubly diastereoconvergent reaction can ad libitum generate either one or the other of two diastereomeric products with complete consumption of the diastereomeric precorsors or their mixtures. Thus, the preparation of configurationally pure (Z)-1'-lithio-1'-(2,6-dimethylphenyl)propene [(Z)-1] from any Z,E mixture of the corresponding bromoalkenes with n-butyllithium succeeded by means of a user-friendly (E)-1 → (Z)-1 configurational interconversion. The subsequent treatment of (Z)-1 with a minimum amount of THF afforded exclusively (E)-1 as the other diastereomeric product and was mediated by a beneficial (Z)-1 → (E)-1 interconversion. This behavior provided microsolvation-controlled choices of highly diastereoselective derivatizations of 1. Low-temperature ¹³C NMR spectra established that (Z)-1 was dissolved as a trisolvated monomer in THF but as a disolvated dimer in monodentate, ethereal, non-THF solvents, whereas (E)-1 was always monomeric. Backed by such knowledge, kinetic experiments indicated that the electrophiles 1-bromobutane or ClSiMe₃ in Et₂O reacted at 32 °C with the tiny (NMR-invisible) population of monomeric (Z)-1 that was formed in a mobile equilibrium from the inactive, predominantly dimeric (Z)-1. The equilibration of monomeric (Z)-1 and (E)-1 in THF as the solvent was fast (seconds on the ¹H NMR time scale), whereas the corresponding stereoinversion of both solvated and unsolvated (E)-1 → (Z)-1 in non-THF solvents occurred on the laboratory time scale (minutes at ambient temperatures). Dicyclopropyl ketone added rapidly to the monomers (Z)-1&3THF and (E)-1&3THF with a rate ratio of at least 14:1 in THF at -78 °C. Di-tert-butyl ketone added less rapidly to the less shielded (Z)-1 [but never to (E)-1]; this singly diastereoconvergent process was much more slowly reversible in THF.

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 κ_obs) 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 < κ_obs < k_A/k_B), 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 κ_obs.