The stepwise (one-electron) chemical oxidation of the tetraphenylethylene donor and its substituted analogues (D) can be carried out by electron exchange with aromatic cations or antimony(V) oxidants to selectively afford the cation radical (D +•) initially and then the dication (D 2+). The ready interchange of the latter establishes the facile disproportionation (i.e., 2D +• ⇌ D 2+ + D) that was originally examined by only transient electrochemical techniques. The successful isolations of the crystalline salts of the tetraanisylethylene cation radical (1 +•) as well as the tetraanisylethylene dication (1 2+) allow X-ray diffraction analysis (for the first time) to quantify the serial changes in the molecular structure upon successive oxidations. Five structural parameters (d, l, θ, φ, and q) are identified as quantitative measures of changes in bond (Cα Cβ, Cα anisyl) lengths, dihedral (Cα Cβ)/torsional (anisyl) angles, and quinoidal (anisyl) distortion attendant upon the removal of first one-electron and then another electron from the tetraanisylethylene framework. The linear variation of all five parameters in Chart 3 point to a strongly coupled relaxation of tetraanisylethylene (involving simultaneous changes of d, l, θ, φ, and q) to a severely twisted dication. Most noteworthy is the structure of the cation radical 1 +• with d, l, θ, φ, and q values that are exactly one-half those of the dication. The complex molecular changes accompanying the transformation: D → D +• → D 2+ bear directly on the donor properties and the disproportionation processes of various tetraarylethylenes.
Triethyloxonium hexachloroantimonate [Et(3)O(+)SbCl(6)(-)] is a selective oxidant of aromatic donors (ArH), and it allows the facile preparation and isolation of crystalline paramagnetic salts [ArH(+)(*), SbCl(6)(-)] for the X-ray structure determination of various aromatic cation radicals. The mechanistic relationship between the Meerwein salt [Et(3)O(+)SbCl(6)(-)] and the pure Lewis acid oxidant SbCl(5) is based on a prior ethyl transfer from oxygen to chlorine within the ion pair.
The thermal decomposition of 3,5-di hydroxybenzoyl azide, to generate in situ the corresponding AB2-type dihydroxy isocyanate monomer, was found to give hyperbranched polyurethanes, whose structures were established using IR and N M R spectroscopy.
Hyperbranched polyesters based on 3,5-dihydroxybenzoic acid and
its derivatives were
prepared by self-condensation of the corresponding ester under standard
trans-esterification conditions.
The spacer segment length that connects the branching points was
systematically varied by starting
from the appropriate ethyl
3,5-bis(ω-hydroxyoligo(ethyleneoxy))benzoate. The
thermal properties of the
hyperbranched polyesters were studied using DSC, and they have been
compared with those of the linear
analogues prepared from the corresponding p-hydroxybenzoic
acid derivatives and also with the
molecularly “kinked” analogues prepared from the meta isomers.
These hyperbranched polyesters were
also terminally functionalized by using a potentially mesogenic
4-butoxybiphenylcarboxylic acid derivative
in an attempt to prepare novel hyperbranched liquid crystalline
polyesters. This was achieved by
copolymerization of the AB2 monomer with the mesogenic
A-type capping unit. These polymers were
found to be amorphous and did not exhibit any liquid crystalline
phases, probably due to the random
distribution of the mesogenic segments on the polymer framework, making
it difficult to both crystallize
and form mesophases.
All reactions were carried out using distilled solvents. Reactions were monitored by using precoated silica TLC plates. Mass spectra were recorded on EI, and ESI (TOF) modes. NMR spectra were recorded in at 400 MHz spectrometers in CDCl 3 , DMSO-d 6 , tetramethylsilane (TMS; δ = 0.00 ppm) served as an internal standard for 1 H NMR. The corresponding residual non-deuterated solvent signal (CDCl 3 ; δ = 77.16 ppm and DMSO-d 6 ; δ = 39.52 ppm) was used as internal standard for 13 C NMR. Column chromatography was carried out on silica gel 230-400 mesh or 100-200 mesh (Merck) and thin-layer chromatography was carried out using SILICA GEL GF-254. Chemicals obtained from commercial suppliers were used without further purification. All sulfoxonium ylides derivatives 1 and 4-hydroxy alkynoate derivatives 2 were synthesized according to a reported literature procedure. Experimental Section (A) General Experimental Procedure In a 8-mL screw cap reaction vial, sulfoxonium ylide derivative (0.2 mmol), 4-hydroxyalkynoate derivative (0.3 mmol), chloroacetic acid (19 mg, 0.2 mmol), [Cp*RhCl 2 ] 2 catalyst (6.2 mg, 5 mol %), AgNTf 2 (16 mg, 20 mol %) were added followed by the addition of ethyl acetate (3 mL). This vial was sealed with a screw cap after flushing with argon and placed in a pre-heated metal block at 80 °C and the reaction mixture was stirred at the same temperature for 16 h. After completion of the reaction (monitored by TLC), the reaction mixture was cooled to room temperature filtered through a short silica gel bed (100-200 mesh) and concentrated under
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