Cyclo- and cyclized diene polymers. XVI. Nature of the active species and a proposed mechanism of cyclopolymerization of isoprene with catalysts containing ethylaluminum halides

The polymerization of isoprene with C₂H₅AlCl₂ to yield solid cyclopolyisoprene is markedly accelerated by the addition of TiCl₄. The polymer yield passes through a maximum on increasing the catalyst reaction time with or without monomer present. The active species are probably cations formed by dissociation of the reaction product of C₂H₅AlCl₂ and TiCl₄. The polymerization of isoprene with (C₂H₅)₂AlX–TiCl₄ (X = F, Br, Cl) has maximum activity at an Al/Ti mole ratio of 0.75 corresponding to conversion of R₂AlX to RAIX₂ which then reacts with remaining TiCl₄. A proposed mechanism of cyclopolymerization of conjugated dienes involves monomer activation, i.e., conversion to cation radical by one-electron transfer to catalyst cation which is itself neutralized, addition of cation end of monomer cation radical to terminal or internal unsaturation of fused cyclohexane polymer chain, one-electron transfer from “neutral” catalyst to cation on polymer chain which is then transformed to a diradical which undergoes coupling to form a cyclohexene ring. The mechanism of the “living” polymerization involves addition of catalyst-activated monomer to a “dead” polymer with a terminal cyclohexene ring and regeneration of the active catalyst.     

Orthoesters versus 2-O-acyl glycosides as glycosyl donors: theorectical and experimental studies

n-Pentenyl orthoesters (NPOEs) undergo routine acid catalyzed rearrangement into 2-O-acyl n-pentenyl glycosides (NPGs). The reactant and product can both function as glycosyl donors affording 1,2-trans linked glycosides predominantly. However, both donors differ in their rates of reactions, the yields they produce, and the nature of their byproducts, indicating that the NPOE/NPG pair may not be reacting through the same intermediates. We have therefore applied quantum chemical calculations using DFT methods and MP second order perturbation theory to learn more about orthoesters and their 2-O-acyl glycosidic counterparts. The calculations show that in the case of a manno NPG and NPOE pair, each donor goes initially to a different cationic intermediate. Thus, the former goes to a high-energy oxocarbenium ion before descending to a trioxolenium ion in which the charge is distributed over the pyrano ring oxygen, as well as the carbonyl and ether oxygen atoms of the putative C2 ester. On the other hand, ionization of the NPOE produces a dioxolenium ion lying slightly above the more stable trioxolenium counterpart. For the gluco pair, the NPG also goes to a very high-energy oxocarbenium ion, which also descends to a trioxolenium ion. However, unlike the manno analogue, the gluco NPOE does not give a dioxolenium ion; indeed, the dioxolenium is not energetically distinguishable from the trioxolenium counterpart. The theoretical observations have been tested experimentally. Thus, it was found that with manno derivatives, the orthoester is a more reactive donor than the corresponding NPG donor, whereas, for gluco derivatives, there is no advantage to using one over the other, unless one resorts to carefully selected promoters.     

Plasma-chemical methane conversion under nonthermal and thermal conditions: An attempt toward uniform kinetic modeling

It is shown that it is basically possible to model plasma-chemical methane conversion using a kinetic concept regardless of the kind of plasma, i.e., the kind of activation. While the temporal plasma-chemical decomposition of methane is controlled by a first-order rate equation, the temporal product formation can be described by a set of first-order consecutive reactions. Prolonged portions of constant product concentrations in the temporal product formation curves were explained by the assumption of an equilibrium between forward and reverse reactions. The modeling revealed the special role of the re-formation of dissociated molecules.     

Preparation of halogen-modified titanium(II) arene complexes and their electronic spectra

Summary Complexes of overall formula TiAl₂X₈ · Ar (X = Cl, Br or I, Ar = benzene or hexamethylbenzene) and the complexes containing various amounts of Br or I in addition to Cl were prepared both by a direct synthesis — the reduction of titanium tetrahalide by aluminium in the aromatic solvent and in excess of the corresponding aluminium halide — and by halogen exchange between the complexes and aluminium halides. The interpretation of the electronic spectra of the complexes is given on the assumption of pseudooctahedral symmetry of the ligand field. The values of 10 Dq and B obtained are compatible with the assignment.     

Higher Sucrose Analogs: Homologation of a Glucose Unit of Sucrose by Two Carbon Atoms

Abstract

The primary hydroxyl groups (at C-6 and C-6′) in 2,3,4,3′4′-penta-O-benzyl-l′-O-methoxymethyl sucrose (2) can be reactively differentiated with tert-butyldiphenylsilyl chloride. Reaction of 2 with TBDPSCl afforded only one monosilylated product protected at C-6′ (6). The regioisomeric monoprotected sucrose 8 was prepared by selective deprotection of the double silylated derivative 7. Compound 6 was converted into 2,3,4,3′,4′-penta-O-benzyl-6-carbomethoxymethylidene-1′-O-methoxymethylsucrose 10 in three steps. Osmylation of the double bond in 10 afforded stereoisomeric homologated sucroses: 11a [6(S),7(R)] and 11b [6(R),7(S)] in the ratio 3:2. A large downfield shift of the H-1 (up to 0.5 ppm) was observed for 6′-silylated derivatives.