Thermal or photolytic decomposition

The nitrene can be generated by a variety of methods, the most popular being the thermal or photolytic decomposition of azidoformates. Other methods, particularly the base-catalyzed a-elimination of arylsulfonate ion from 7V-[(arylsulfonyl)oxy]urethanes, are useful as they avoid the use of the potentially explosive azido esters. 

Metal carbonyl compounds are other suitable precursors for the synthesis of NPs by thermal decomposition. The main advantage is the formation of CO that is expelled from the IL phase due to its poor solubility. However, high temperatures are commonly used to decompose such precursors. Metal NPs of Cr(0), Mo(0), and W(0) were prepared by thermal or photolytic decomposition of their respective monometallic carbonyl compounds [M(CO)6] dispersed in ILs [52]. Similarly, the precursors [Fe2(CO)9], [Ru3(CO)i2], and [Os3(CO)12] were employed in order to obtain stable metal NPs (1.5-2.5 nm) in BMI.BF4 [53]. The same procedure was extended to the preparation of lr(0), Rh(0), and Co(0) NPs in ILs [54]. 

An important route to the carbazoles is the thermal or photolytic decomposition of orf/zo-azidobiphenyls. Of the two procedures, thermolysis proceeds in higher yields. One advantage of photolysis is that it proceeds at room temperature and would therefore be compatible with thermally labile functional groups. 

Thermal or photolytic decomposition of ylides generates carbenes. 

The synthesis of 1-aryl-1-chlorocyclopropanes via thermal or photolytic decomposition of 3-aryl-3-chloro-3H-diazirines in the presence of alkenes is described in Houben-Weyl, Vol. El 9b, pp994-996. The preparation of l-chloro-tra A-2,3-dimethyl-l-phenylcyclopropane is described in Houben-Weyl, Vol. El 9 b, p994. 

Because oxidative decarboxylation of carboxylic acids by lead tetraacetate depends on the reaction conditions, the co-reagents, and the structures of the acids, a variety of products such as acetate esters, alkanes, alkenes, and alkyl hahdes can be obtained. Mixed lead(IV) carboxylates are involved as intermediates as a result of their thermal or photolytic decomposition decarboxylation occurs and alkyl radicals are formed. Oxidation of alkyl radicals by lead(IV) species gives carbocations a variety of products is then obtained from the intermediate alkyl radicals and the carbocations. Decarboxylation of primary and secondary acids usually affords acetate esters as the main products (Scheme 13.41) [63]. 

Heptafulvalene (39) is also formed by thermal or photolytic decomposition of 40 in solution i5), but not from 33 in solution. The existence of 37 in solution is established by its nucleophilic addition to alkenes when generated by de-... 

The thermal or photolytic decomposition of carbonyl azides in the presence of dipole acceptors such as acetylenes provides a valuable method for the construction of oxazoles. Thus the reaction of ethyl azidoformate with either diphenyl- or diethylacetylene produces mainly the 2-ethoxy-oxazole (77 ).166 166 The reaction involves the 1,3-dipolar cycloaddition of carbethoxy nitrene (76b) to the alkyne to give the oxazole (77). On the... 

Thermal or photolytic decomposition of a diazo compound which has a carbonyl group a to the diazo group often yields a ketene, which can be trapped by conventional techniques, such as reaction with water to give a carboxylic acid . 

Carbenes produced by photolysis of diazoalkanes are highly energetic species and often react indiscriminately. Thermal decomposition of diazoalkanes can be catalysed by certain transition metal (particularly copper or rhodium) salts, to produce less energetic and more selective carbenes. The active species in such a reaction is thought to be the metallocarbene rather than the free carbene. Another convenient and widely used route to alkylcarbenes is the thermal or photolytic decomposition of the lithium or sodium salts of toluene-p-sulfonylhydrazones. The diazoalkane is first formed (by elimination of the toluenesulfinate anion) and decomposes imder the reaction conditions to give the carbene (4.76). This process is often referred to as the Bamford-Stevens reaction. 

The carbenes formed by thermal or photolytic decomposition of diazocyclopentadienes have also been trapped by derivatives of Group and VI elements to form cyclopentadienylides and this has proved to be a valuable method for making these compounds, e.g. 

Dipolar cycloadditions are important as a means of synthesis of heterocyclic molecules. The cycloaddition of diazo compounds has also been used in the synthesis of cyclopropanes and other strained molecules. The cyclic adducts (pyrazolines) contain the azo linkage, and molecular nitrogen is lost on thermal or photolytic decomposition. Usually, the two carbon centers resulting from elimination of... 

Homolytic cleavage of the C—M bond is the most common process for thermal or photolytic decomposition of alkyl and aryl derivatives of the transition metals. In terms of a molecular orbital scheme, the process involves transfer of an electron from a bonding orbital to a C—M antibonding orbital. Electronic factors which broaden the energy gap between these orbitals should stabilize the compound toward homolytic decomposition. 

Investigations have shown that 9-anthrylcarbene °, generated by thermal or photolytic decomposition of 9-anthryldiazomethane in a polymer solution, can be inserted into the C-H bonds of the maciomolecules l As a result, anthracene groups are covalently bonded to the macromolecules (Table 1 LMn). 

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