Facile precursor

A fair amount of research has been conducted on so called facile precursor-mediated chemisorption systems. The section that follows is a review of selected existing studies and will serve as a useful introduction to these gas-surface dynamics and will provide a foundation for later discussions of activated chemisorption systems. 

More one example is connected with generation of unsubstituted ylides [35], As facile precursors for non stabilized pyridinium methylides N-(trimethylsilylmethyl) pyridinium triflates were synthesized. Cesium fluoride induced desilylation of the precursors liberated the nonstabilized pyridinium methylides which were trapped as the cycloadducts to dimethyl acetylenedicarboxylate. Trapping of 3-CF3-pyridinium ylide gave the mixture of isomers (50a 50b = 1 5) in 53 % overall yield. 

Tsuge O, Kanemasa S, Kuraoka S, Takenaka Shigeori (1984) N-(Trimethylsilylmethyl)-pyridinium trifluoromethanesulfonates as facile precursors for nonstabihzed pyridinium methyhdes. Chem Lett 13 279-280... 

In most of these examples, the chiral auxiliary is introduced to the allylic reagent at a very late stage in the synthesis of the precursor, thus providing a facile access. It is obvious that in most examples, the central metal atom is kept from becoming stereogenic, and in addition, a C2-symmet-ric cation is desirable, in order to minimize the possible number of competing transition states. 

The symmetric series provides functional cyclohexadienes, whereas the non-symmetric one serves to build deuterated and/or functional arenes and tentacled compounds. In both series, several oxidation states can be used as precursors and provide different types of activation. The complexes bearing a number of valence, electrons over 18 react primarily by electron-transfer (ET). The ability of the sandwich structure to stabilize several oxidation states [21] also allows us to use them as ET reagents in stoichiometric and catalytic ET processes [18, 21, 22]. The last well-developed type of reactions is the nucleophilic substitution of one or two chlorine atoms in the FeCp+ complexes of mono- and o-dichlorobenzene. This chemistry is at least as rich as with the Cr(CO)3 activating group and more facile since FeCp+ activator is stronger than Cr(CO) 3. 

An example of a surprisingly facile and stereoselective formation of an eight-membered lactone from an acyclic precursor diene ester was observed during the total synthesis of the antitumor agent octalactin A (148) (Scheme 27) [81]. The dense substitution pattern in cyclization substrate 146 presumably imposes... 

Recent progress of basic and application studies in chitin chemistry was reviewed by Kurita (2001) with emphasis on the controlled modification reactions for the preparation of chitin derivatives. The reactions discussed include hydrolysis of main chain, deacetylation, acylation, M-phthaloylation, tosylation, alkylation, Schiff base formation, reductive alkylation, 0-carboxymethylation, N-carboxyalkylation, silylation, and graft copolymerization. For conducting modification reactions in a facile and controlled manner, some soluble chitin derivatives are convenient. Among soluble precursors, N-phthaloyl chitosan is particularly useful and made possible a series of regioselective and quantitative substitutions that was otherwise difficult. One of the important achievements based on this organosoluble precursor is the synthesis of nonnatural branched polysaccharides that have sugar branches at a specific site of the linear chitin or chitosan backbone [89]. 

A number of proteins containing [3Fe-4S] centers form cubane-like clustes of the type [M,3Fe-4S] (115-120), which were prepared using this facile [3Fe-4S]/[4Fe-4S] conversion pathway. The [3Fe-4S] core present in D. gigas Fdll was the first precursor used for the synthesis of heterometal cores inside a protein matrix, and the first derivative synthesized was the [Co,3Fe-4S] core (121). Similar synthetic products have also been derived in D. africanus Fdlll (118, 119). 

In the early work on the thermolysis of metal complexes for the synthesis of metal nanoparticles, the precursor carbonyl complex of transition metals, e.g., Co2(CO)8, in organic solvent functions as a metal source of nanoparticles and thermally decomposes in the presence of various polymers to afford polymer-protected metal nanoparticles under relatively mild conditions [1-3]. Particle sizes depend on the kind of polymers, ranging from 5 to >100 nm. The particle size distribution sometimes became wide. Other cobalt, iron [4], nickel [5], rhodium, iridium, rutheniuim, osmium, palladium, and platinum nanoparticles stabilized by polymers have been prepared by similar thermolysis procedures. Besides carbonyl complexes, palladium acetate, palladium acetylacetonate, and platinum acetylac-etonate were also used as a precursor complex in organic solvents like methyl-wo-butylketone [6-9]. These results proposed facile preparative method of metal nanoparticles. However, it may be considered that the size-regulated preparation of metal nanoparticles by thermolysis procedure should be conducted under the limited condition. 

Since alkoxy radicals are known precursors to chain scission in autoxidation (24), the "hot" alkoxy radicals formed as shown should undergo facile chain scission or fragmentation. The chain scission is illustrated for the alkoxy radical derived from either the ethylene or propylene monomer unit in EPM ... 

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