DPE

Light and photosynthetic electron transport convert DPEs into free radicals of undetermined stmcture. The radicals produced in the presence of the bipyridinium and DPE herbicides decrease leaf chlorophyll and carotenoid content and initiate general destmction of chloroplasts with concomitant formation of short-chain hydrocarbons from polyunsaturated fatty acids (37,97). 

The properties of optimized helical structures, which were derived from torus C54D and Cs7a, >yps (A), (proposed by Dunlap) and torus C ,o> Dpe (B), (proposed by us) by molecular dynamics were compared. (see Figs. 9 (a) and 10). (Although the torus Cs7f, is thermodynamically stable, helix 57 was found to be thermodynamically unstable 14]. Hereafter, we use helix C to denote a helix consisting of one torus (C ) in one pitch. 

Abbreviations acac, acetylacetonate Aik, alkyl AN, acetonitrile bpy, 2,2 -bipyridine Bu, butyl cod, 1,5- or 1,4-cyclooctadiene coe, cyclooctene cot, cyclooctatetraene Cp, cyclopentadienyl Cp, pentamethylcyclopenladienyl Cy, cyclohexyl dme, 1,2-dimethoxyethane dpe, bis(diphenyl-phosphino)ethane dppen, cis-l,2-bis(di[Atenylphosphino)ethylene dppm, bis(diphenylphosphino) methane dppp, l,3-bis(diphenylphosphino)propane eda,ethylenediamine Et,ethyl Hal,halide Hpz, pyrazole HPz, variously substituted pyrazoles Hpz, 3,5-dimethylpyrazole Me, methyl Mes, mesityl nbd, notboma-2,5-diene OBor, (lS)-endo-(-)-bomoxy Ph, phenyl phen, LlO-phenanthroline Pr, f opyl py, pyridine pz, pyrazolate Pz, variously substituted pyrazolates pz, 3,5-dimethylpyrazolate solv, solvent tfb, tetrafluorobenzo(5,6]bicyclo(2.2.2]octa-2,5,7-triene (tetrafluorobenzobanelene) THE, tetrahydrofuran tht, tetrahydrothicphene Tol, tolyl. 

Here Ceq is the ethylene concentration equilibrium to the concentration in a gaseous phase, Kp the propagation rate constant, N the concentration of the propagation centers on the catalyst surface, Dpe the diffusion coefficient of ethylene through the polymer film, G the yield of polymer weight unit per unit of the catalyst and y0at, ype are the specific gravity of the catalyst and polyethylene. 

In 1979, it was stated that poiybrominated aromatic ethers have received little attention (ref. 1). That statement is still applicable. Analyses to characterize this class of commercial flame retardants have been performed using UV (refs. 1-2), GC (refs. 1-6), and GC-MS (refs. 1-4). The bromine content of observed peaks was measured by GC-MS, but no identification could be made. The composition of poiybrominated (PB) diphenyl ether (DPE) was predicted from the expected relationship with polyhalogenated biphenyl, a class which has received extensive attention. NMR (refs. 3-6) was successfully used to identify relatively pure material which had six, or fewer, bromine atoms per molecule. A high performance liquid chromatography (HPLC) method described (ref. 1) was not as successful as GC. A reversed phase (RP) HPLC method was mentioned, but no further work was published. 

The work presented here describes a RP-HPLC method for characterizing PB-DPE preparations. Components were purified by preparative RP-HPLC and identified by NMR. Components observed by GC under conditions similar to a published analysis (ref. 4), are identified by relation to the HPLC. A clear description of the bromination path can be made. 

Samples of PB-DPE were subjected to RP-HPLC producing the chromatographic patterns shown in Figure 1. The two major peaks of Penta (Fig. lA, peaks 1 and 2), were isolated by preparative HPLC and analyzed by NMR. The spectra (Table 1) were in excellent agreement with previously published data (refs. 3-6). The two HPLC peaks were identified as 2,2, 4,4 tetrabromo DPF and 2,2, 4,4, 5 pentabromo DPF, respectively. 

Figure 1. HPLC Chromatogram of PB-DPE Fire Retardants For conditions, see text.

Peak 5 revealed two types of protons in the NMR spectrum (Table 1). Based on comparison with previous NMR assignments, peak 5 is 2,2, 3,4,4, 5,5, 6-octabromo DPE, where the protons are on the same ring. Peak 6 also revealed two types of protons (Table 1). With similar reasoning the structure is 2,2, 3,3, 4,4, 5, 6 octabromo DPE, where the protons are on separate rings. 

Since peak 8 is the largest one in the HPLC tracing (Figure 1C), high-melting Octa may be aptly named "Nona". It was predicted that this isomer would be the predominant isomer of nonabromo DPE in Octa. That prediction is apparently incorrect. 

Consistent with the foregoing data, the bromination of DPE may be described as proceeding according to the scheme shown in Figure 3. 

The DPE reduction is used as a test reaction to characterize the materials and optimize the preparation conditions of the catalyst. Since hydroaluminations can also be used for the synthesis of carboxylic acids, deuterated products, or vinyl halides via quenching with CO2, D2O or Br2 [44], the method is also a valuable organic synthesis tool. However, as compared with molecular catalysts like Cp2TiCl2 that are known to catalyze hydroaluminations [44], the titanium nitride materials described here are solid catalysts and can be separated by centrifugation. Moreover, they can be reused several times, which is an advantage as compared to molecular catalysts. 

Fig. 19.9 DPE reduction using TiN-activated NaAlH4 (a) and pure NaAlH4 (b).

Re-use of the catalyst The high activity of the solid catalysts also allows one to re-use high surface area titanium nitride several times. By centrifugation, the solid can be separated and used in a new reaction. The catalyst was used up to four times. Figure 19.10 shows the conversion of DPE with time. 

Fig. 19.10 DPE-conversion with time forTiN nanoparticles in four susbsequent runs (after two hours, the catalyst was separated and re-used).

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