Suitable Precursor

There remained a niche market for carbon fibers made from a cellulosic precursor and production was maintained by Polycarbon Inc. in the USA (acquired by SGL Technic in 1980) and RK Carbon Fibers (who also became part of SGL in 1997, when production was transferred to Polycarbon Inc. in the USA). Demand and availability of a suitable precursor has added to production problems. 

To make a continuous carbon fiber, a continuous viscose rayon fiber is required, which immediately imposes a limitation, since the majority of current viscose production is chopped to form a staple product. A further limitation is that specific end users superimpose their own purchasing specifications, like NASA and the atomic energy industry where, for example, the presence of certain trace elements would have to be rigidly controlled. Availability of suitable 

In subsequent pyrolysis, in order to enable the precursor to be uniformly heated and breakdown products to be efficiently removed, the precursor must be presented in the pyrolysis oven as a single tow or wound loosely into a hank form. 

Typical rayon precursors will have a textile finish applied by the manufacturer to act as a lubricant to aid spinning and subsequent handling such as weaving. This finish will, at some stage before pyrolysis, have to be removed to avoid the formation of a breakdown product, which would bond individual filaments together and catastrophieally lower the strength. 

Mineral oil is one such finish which is applied and can be selectively removed by hot solvent followed by drying, to remove all traces of residual solvent. If a water based finish has been applied, then hot water can be used for the extraction process. 

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The absorption of a light pulse instantaneously generates reactive species in high concentrations, either tlirough the fomiation of excited species or tlirough photodissociation of suitable precursors. The reaction can... 

Substructure searches provide another method of searching for available starting materials. They arc used primarily for planning the synthesis of combinatorial libraries. After the target compound has been dissected into a set of suitable precursors, substructure searches can provide for each of them a series of representatives of a certain class of compounds, Siibsti ucturc searches enable the user to specify attributes such as open sites or atom lists at certain positions of the structure. Figure 10.3-38 shows the possible specification elements for the query in a substructure search. 

A symbol used to indicate a retrosynthetic step is an open arrow written from prod uct to suitable precursors or fragments of those precursors... 

Almost any nitrogen containing organic compound can be reduced to an amine The syn thesis of amines then becomes a question of the availability of suitable precursors and the choice of an appropriate reducing agent... 

Unsaturated hydrazones, unsaturated diazonium salts or hydrazones of 2,3,5-triketones can be used as suitable precursors for the formation of pyridazines in this type of cyclization reaction. As shown in Scheme 61, pyridazines are obtainable in a single step by thermal cyclization of the tricyanohydrazone (139), prepared from cyanoacetone phenylhydrazone and tetracyanoethylene (76CB1787). Similarly, in an attempted Fischer indole synthesis the hydrazone of the cyano compound (140) was transformed into a pyridazine (Scheme 61)... 

Photochemical elimination of carbon dioxide from suitable precursors has given a variety of reactive intermediates at low temperatures where they are often stable and can be studied further. This approach has been utilized in attempts to generate new 1,3-dipolar species, and photolysis of (515) gave an azomethine nitrene intermediate (516) (see Section 4.03.6)... 

Some tricyclic systems have been prepared by intramolecular cyclization from A-aryl-pyrazoles carrying substituents both in the pyrazole ring at C-5 and in the phenyl ring at the o-position. Thus pyrazolo[l,5-n]quinazolines (563) (69JHC947) and pyrazolo[l,5-n]-[l,4]benzodiazepines (564) (77JHC1163, 77JHC1171) can be prepared from suitable precursors. 

Photolysis of dlazirines to nitrogen and carbenes is a general reaction and plays a greater role in carbene chemistry than photolysis of linear diazo compounds. Whereas the latter are often obtained only under the conditions of their thermal decomposition from suitable precursors, diazirines are obtainable in a pure state in most cases. Photolysis has the further advantage to permit nitrogen extrusion at atmospheric pressure, even with low-boiling materials. 

Isoindolines comprise a group of well-characterized and easily synthesized substances, and being at the next stable reduction state below that of isoindoles, they constitute suitable precursors for synthesis of the latter. In principle, either oxidation or elimination from isoindolines should lead to isoindoles however, in view of the susceptibility of isoindoles to further oxidation, elimination has been preferred, and in all cases reported the leaving group has been placed on nitrogen rather than carbon. 

Fipases and esterases are often used for Idnetic resolution of racemates, variously by hydrolysis, esterification, or transesterification of suitable precursors. Scheme 8.3-3 illustrates the principal for the resolution of a secondary alcohol by esterification with vinyl acetate. 

Van der Schaaf et al. described a synthesis of the 14-electron complex [RuHCl(PPr13)2] (32) from [RuCl2(COD)]A.,PPr31,isopropanol,and abase.Compound 32 is a suitable precursor for ruthenium carbene complex 33, as outlined in Scheme 10. Although 32 was isolated and structurally characterized, it may also be generated in situ for the preparation of the carbene complex 33 [18]. 

A number of metal complexes with disulfurmonoxide as a ligand have been prepared either by oxidation of precursor complexes containing the 2 hgand or by trapping S2O formally produced in situ from a suitable precursor by heating [52]. Such molecules are cyclic di-, tri-, or tetrasulfane monoxides or dioxides such as the one shown in Scheme 1. 

However, until recently no data were available on the direct generation of allenyl cations, 242, via solvolysis of suitable precursor allenyl substrates. 

The bands of the SiCls radical (Milligan and Jacox, 1968b), have not been observed at all. Due to the high selectivity of its thermal decomposition within a wide temperature interval, Si2Cl6 has become one of the most suitable precursors of dichlorosilylene in preparative chemistry (Chernyshev and Komalenkova, 1990). 

Studies performed on CdS [282, 283] have revealed the importance of the microstructure, i.e., crystal structure, crystallite size, and geometrical surface area, in both the control of band structure and the concentration and mobility of charges, in relation to the photocatalytic performance of the photocatalyst. It has been shown also that the solubility product of CdS colloids prepared from acetate buffer aqueous solutions of suitable precursors increases from 7.2x 10 for large particles to about 10 for small (< 2.5 nm) particle colloids, this increase invoking a positive shift on the cathodic corrosion potential [284]. 

As for the synthesis of 5-e/j/-KDG, compound 6 seemed to be a suitable precursor of the methyl ester of 5-deoxy-KDG 20 since only the C-5 hydroxyl was unprotected. In this case the key step was not the epimerization but the removal of that hydroxyl. Our attempts of radicalar deoxygenation of 6 were unsuccessful because the intermediate radical was intramolecularly trappy by the C-2.C-3 double bound. Therefore we first reduced the double bond and then converted the resulting diastereoisomeric alcohols 14 into the corresponding triflates 15 which were submitted to the action of sodium iodide. Finally the iodides 16 Aus obtained were hydrogenolyzed in the presence of diisopropylethylamin to give 17. 

These experiments have recently been extended to the case of iron. The synthesis of small Fe particles has long been difficult due to the lack of suitable precursor. However, using the complex Fe[N(SiMe3)2]2 as precursor, we have obtained nanoparticles of low size dispersity and displaying a magnetization 5 T higher than in bulk iron and comparable to that of gas phase aggregates [57]. 

Possible applications include optical coatings [98], catalysts [99-101], substrates for Surface Enhanced Raman spectroscopy [102] or biosensor electrodes [103], Mesoporous gold can be prepared by de-aHoying a suitable precursor such as a... 

Gold Colloidal Nanoparticles Sized to be Suitable Precursors for Heterogeneous Catalysts... 

Pyrolysis method involves thermal decomposition of suitable precursors to produce free radicals. Pyrolysis sources based on continuous molecular beam nozzles are well developed (for example, methyl6 8 and benzyl9). Recently, Chen and co-workers have pioneered a flash pyrolysis/supersonic jet technique to produce free radical beams (Fig. I).10 In this radical... 

The compound truw-perfluoro-l-methyl-1-propenylsilver has been shown to be tetrameric, [CF3CF=C(CF3)Ag]4, and it is a suitable precursor to prepare pure silver films by plasma-enhanced CVD, or to deposit silver films under MOCVD conditions. 

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