Wolff thermal

E. G. Wolff, Thermal Expansion in Metal/Lithia-Alumina-Silica (LAS) Composites, Int. Journal of Thermophysics 9, 221-232 (1988). 

Beckmann rearrangement, 4, 292 phototransposition, 4, 204 synthesis, 4, 223 Wittig reaetion, 4, 294 Wolff-Kishner reduetion, 4, 291 Pyrrole, 1-aeyl-barrier to rotation, 4, 193 IR speetra, 4, 21, 181 rearrangement, 4, 41 synthesis, 4, 82 thermal rearrangement, 4, 202 Pyrrole, 2-aeyl-aeidity, 4, 297 cleavage, 4, 289 conformation, 4, 33... 

The intermediacy of a ketocarbene species 4 is generally accepted for the thermal or photochemical Wolff rearrangement oxirenes 8 that are in equilibrium with ketocarbenes, have been identified as intermediates ... 

There are several reactions that are conceptually related to carbene reactions but do not involve carbene, or even carbenoid, intermediates. Usually, these are reactions in which the generation of a carbene is circumvented by a concerted rearrangement process. Important examples of this type are the thermal and photochemical reactions of a-diazo ketones. When a-diazo ketones are decomposed thermally or photochemically, they usually rearrange to ketenes, in a reaction known as the Wolff rearrangement.232... 

Finally, a nice combination of a light-induced Wolff reaction of a diazoketone (e. g., 5-108) with a thermal Cope rearrangement, a light-induced Norrish type I cleavage and a recombination was developed by Stoltz and coworkers (Scheme 5.22) [38]. Here, irradiation of 5-108 at 254 nm in a photoreactor afforded the bicy-... 

The Wolff rearrangement is well known as a reaction of diazo ketones, i.e. of diazoalkanes with a carbonyl group in a-position. Reaction 34 demonstrates that diazotized aminonaphthols are mesomeric with naphthoquinone diazides (48b) and that they have therefore also the character of quinonoid diazo ketones (see also Section II.C of this chapter). Wolff rearrangements take place also thermally and catalyzed by silver ions. 

Most reactions with hydrazine are carried out with aldehydes and ketones in the presence of alkali. The reduction proper is preceded by formation of hydrazones that decompose in alkaline medium at elevated temperatures to nitrogen and compounds in which the carbonyl oxygen has been replaced by two hydrogens. The same results are obtained by alkaline-thermal decomposition of ready-made hydrazones of the carbonyl compounds. Both reactions are referred to as Wolff-Kizhner reduction [280]. 

Intramolecular insertion and addition reactions are almost unknown for alkyl nitrenes. In fact, it is not clear that the nitrenes are formed as discrete species. The migration may be concerted with elimination, as in the case of the thermal Wolff rearrangement.164... 

A similar synthesis of mechanistic interest rather than preparative value involves the thermal reaction of dimethyl 2,5-bisdiazo-3,4-diketoadipate (89, Scheme 23) with benzofuran (91)." The presumed intermediate is the pyrone cation 90 produced from the adipate 89 by the Wolff rearrangement, cyclization, and loss of nitrogen. Electrophilic substitution then affords the benzofuran 92, which can be isolated. Ring opening and cyclization of the resultant ketene 93 then affords the dibenzofuran 94 in poor (0.4%) yield. 

Direct sulfination of azibenzils in benzene yields cycloadducts 86 and 87. The diazoketones thermally decompose and easily undergo partial Wolff... 

The cycloaddition reactions of the unsymmetrical a-diazo-/3-diketone, 2-diazo-l-phenyl-l,3-butanedione 330, with diaryl imines 331 took place with high regioselectivity, affording exclusively the 6-methyl-5-phenyl-substituted 477-l,3-oxazin-4-ones 332 via the acetylphenylketene, generated by the thermal Wolff rearrangement of 330 (Equation 32) <2002HAC165>. 

Also, the Wolff rearrangement of diazo Meldmm s acid 33, studied by DFT at the B3PW91/6-311+G(3df,2p) level of theory, proved to be a concerted process because the product of the photochemical or thermal decomposition in methanolic solution was the ketoester 34 (Scheme 3) while the expected products of the singlet carbene 35, for example 36, were not detected <2003JA14153, 2005CJC1382>. 

Four major methods have been used to separate the organic elemental carbon thermal methods, digestion, extraction, and optical techniques. These are discussed in detail in the volume on particulate carbon edited by Wolff and Klimisch (1982) and in the article by Cadle et al. (1983). 

Huntzicker, J. J., R. L. Johnson, J. J. Shah, and R. A. Cary, Analysis of Organic and Elemental Carbon in Ambient Aerosols by a Thermal-Optical Method, in Particulate Carbon Atmospheric Life Cycles (G. T. Wolff and R. L. Klimisch, Eds.), pp. 79-88, Plenum, New York, 1982. 

In a more general approach, eight examples of the Wolff—Kishner reduction of aromatic aldehydes and ketones are described using 80% hydrazine hydrate in toluene64 (Scheme 4.37). The reaction times are longer than described in the previous paper because less reactive substrates were used. Still, both the formation of the hydrazone and the reduction step are considerably faster than under thermal conditions the reduction proceeds at ambient pressure and in the absence of a solvent. The microwave reduction is compatible with other reducible functional groups such as aromatic OMe, Me, Cl or COOMe, which can otherwise cause problems under conventional reaction conditions64. 

Ketocarbenes (1) are usually generated from the corresponding diazo compounds (3).s Other sources which are occasionally used are a,a-dibromo compounds (4),9 sulfur ylides (5)10 and iodonium ylides (6 Scheme 2).11 The thermal or photochemical decomposition of diazo compounds in the presence of ir-systems is often complicated by indiscriminate side reactions, such as Wolff rearrangements,12 C—H insertions and hydride migrations. To avoid such problems, the use of metal-catalyzed decomposition of diazo compounds is generally preferred.1 2... 

The key step of the Amdt-Eistert Homologation is the Wolff-Rearrangement of the diazoketones to ketenes, which can be accomplished thermally (over the range between r.t. and 750°C, photochemically or by silver(I) catalysis. The reaction is conducted in the presence of nucleophiles such as water (to yield carboxylic acids), alcohols (to give alcohols) or amines (to give amides), to capture the ketene intermediate and avoid the competing formation of diketenes. 

A thermal Wolff rearrangement of diazophosphonates 143 forms the intermediate ketenes 144, which undergo [1,5]-hydrogen shift and ring closure to afford l-amino-4-phosphonate-l/f-isochromenes (Scheme 46) <1998T6457,... 

Similarly, the acetal 146 can undergo a thermal Wolff rearrangement to afford the intermediate ketene. Intramolecular nucleophilic attack of the acetal oxygen onto the ketene forms the zwitterion 147. Subsequent C-0+ bond cleavage and cyclization then furnishes dimethyl l,3-dimethoxy-l//-isochromen-4-yl phosphonate in excellent yield (Scheme 47) <1998T6457>. 

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