Single enantiomers of 1,8-di(1-adamantyl)naphthalenes were synthesized by the [4+2]cycloaddition reaction of 6-adamantylbenzyne and 2-adamantylfuran. The enantiomers were resolved by conversion into diastereomeric ketopinic acid esters. The absolute configuration was determined by X-ray analysis. Kinetic studies by CD revealed an enantiomerization barrier of 29 kcal mol−1 for 1,8-(1-adamantyl)naphthalenes.
1,4-Bis(trimethylsilyl)buta-1,3-diyne in the presence of GaCl3 reacts with aromatic hydrocarbons at −90 to −100 °C yielding 2-arylbut-1-en-3-ynes; the reactions exhibit an unusually high tendency to alkenylate the o-position of alkyl substituents; toluene, ethylbenzene and isopropylbenzene react predominantly to exclusively at the o-position while o-xylene and 1,2,3,4-tetrahydronaphthalene react at the 3 and 5-position, respectively.
Two compounds with two hexa(ethynylhelicene) parts connected by a flexible hexadecamethylene and a rigid butadiyne linker were synthesized. The 1H NMR spectroscopic and CD analyses and vapor-pressure osmometry (VPO) of these two compounds revealed intramolecular double-helix formation. Upon heating a 5-microM solution in toluene, the double-helix structure unfolded to form a random coil, and on cooling it folded again into a double helix. The thermodynamic stabilities of both structures were dependent on temperature, and the structural change in both compounds is due to the large enthalpies and entropies under equilibrium. The rate constants of their unfolding were obtained by assuming a pseudo-first-order reaction; the compound with a rigid linker unfolded slower than that with a flexible linker. The former has a larger activation energy, and its double-helix and random-coil conformers were separated by chromatography. The rate of folding was also faster for the flexible-linker compound with larger activation energy. The rate constants for the folding of both compounds slightly decreased with increasing temperature, which was ascribed to the presence of exothermic pre-equilibrium and rate-determining steps. The folding was markedly accelerated with increasing random-coil concentration, which suggests the involvement of self-catalysis. A mechanism of folding was proposed. The involvement of different mechanisms of folding and unfolding was suggested by the kinetic studies, and it was confirmed by the presence of hysteresis in the melting profiles. The difference in linker structure also affected the thermal-switching profiles of the double-helix-random-coil structural changes.
Abstract Unstable monosilylated 1,3,5,7-octatetrayne was synthesized and lithiated with butyllithium. The resulted lithium acetylide added to aldehydes and a ketone giving tetraynols in high yields.
Abstract Chiral silica nanoparticles (70 nm) grafted with ( P )‐helicene recognized the molecular shape of double helix and random coil ( P )‐ethynylhelicene oligomers in solution. A mixture of the ( P )‐nanoparticles and double helix precipitated much faster than a mixture of the ( P )‐nanoparticles and random coil, and the precipitate contained only the double helix. The mixture of the ( P )‐nanoparticles and ( P )‐ethynylhelicene pentamer reversibly dispersed in trifluoromethylbenzene upon heating at 70 °C and precipitated upon cooling at 25 °C. When a 10:90 equilibrium mixture of the double helix and random coil in solution was treated with the ( P )‐nanoparticles, the double helix was precipitated in 53 % yield and was accompanied by equilibrium shift.
Abstract ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.
A novel carbonylative decomplexation of alkyne-dicobalt hexacarbonyls, i.e. hydrocarbamoylation of alkynes, was carried out by reaction of the complexes with 10 equiv. of primary and secondary amines.
SnCl4-Bu3N reagent promotes α-alkylidenation of ketones with 1-alkynes giving (E)- α-enones. When TMSCl is added to this mixture, α-alkenylation takes place giving β-enones.
Hysteresis is ubiquitous in nature and biology. It appears in ferromagnetism, ferroelectrism, traffic congestion, river sedimentation, electronics, thermoresponses, cell division, differentiation, and apoptosis. Hysteresis phenomena are beyond equilibrium and involve nonlinear, bistable, time delay, and memory events, which are described in input/output profiles by different outputs during continuous decreases and increases in input intensity. Although hysteresis profiles in these phenomena appear similar, the mechanisms underlying them are complex, and their basic understanding is desired. In this Account, I describe thermal hysteresis caused by molecules dispersed in dilute solutions containing optically active helicene oligomers, which form homo- and heterodouble helices, the cooling and heating processes of which cause different structural changes with regard to their relative concentrations. Reversible self-catalytic reactions are involved in the formation of a double helix, which catalyzes its own formation. The reactions accelerate as they progress, in contrast to ordinary reactions, which exhibit monotonic retardation as they progress. Thermal hysteresis involving reversible self-catalytic reactions exhibits notable phenomena, when various cooling/heating inputs are applied during the reaction; these phenomena are shown herein with profiles of experimental results of Δε outputs obtained by circular dichroism (CD) plotted against temperature inputs. Thermal hysteresis is discussed in terms of (1) two states of the homodouble helix and a random coil involving one reversible self-catalytic reaction and (2) three states of enantiomeric heterodouble helices and a random coil involving two reversible self-catalytic reactions. Repeated cooling and heating processes provide the same stable thermal hysteresis loops, when the initial and final high-temperature states are under equilibrium, and nonloop and unstable thermal hysteresis appears when whole the systems are beyond equilibrium. Diverse thermal hysteresis loops are obtained under different temperature change conditions for different oligomers. The mechanism of thermal hysteresis involves different macroscopic mechanisms at a fixed temperature, when the relative concentrations of substrates/products and the reaction direction differ. Microscopic mechanisms, which are shown by energy diagrams, are fixed at a temperature irrespective of cooling or heating. A comparison of thermal hysteresis loops and equilibrium curves provides distances to the metastable states on the loops from equilibrium, and reactions occur from the metastable states toward equilibrium. Notable phenomena described herein include bistability, high sensitivity to small concentration changes, equilibrium crossing, three-state one-directional structural change caused by a single heating procedure, reaction shortcuts, the memory effect on thermal history, figure-eight thermal hysteresis, chemical oscillation, stable and unstable thermal hysteresis, double-helix formation only under heating, and chiral symmetry breaking.
