Wolff rearrangement of a-diazoketones

Aryl nitrenes also generally rearrange rather than undergo addition or insertion reactions. A few intramolecular insertion reactions in aromatic systems go in good yield.  

The nitrenes which most consistently give addition and insertion reactions analogous to carbenes are the carboalkoxynitrenes generated from alkyl azidofor- 

This nitrene is somewhat more selective than simple carbenes, showing selectivities of roughly 1 10 40 for the primary, secondary, and tertiary positions in 2-methyl-butane in insertion reactions. The relationship between nitrene multiplicity and stereospecificity in addition to alkenes is analogous to that described for carbenes. The singlet gives stereospecific addition, while the triplet gives nonstereospecific addition products. 

Acyl azides are well-known compounds. Their role in the thermal Curtius rearrangement, a reaction that apparently does not involve a nitrene, will be discussed in Section 9.3. Photochemical decomposition of acyl azides elicits nitrene reactivity. In particular, intramolecular C-H insertion reactions have been observed, but not usually in high yield. 

Sulfonylnitrenes are formed by thermal decomposition of sulfonyl azides. Insertion reactions occur with saturated hydrocarbons. With aromatic rings, addition is believed to occur, but the main products are sulfonanilides, which result from ring opening of the addition intermediate. 

CHAPTER 8 REACTIONS INVOLVING ELECTRON-DEFICIENT INTERMEDIATES 

There have also been a number of investigations of the stereochemistry of the Favorskii rearrangement. Although several compounds show stereoselectivity, the general result is that these reactions give the stereoisomeric reaction products that 

The thiirane dioxides decompose with elimination of sulfur dioxide. The reaction is useful for the synthesis of certain types of olefins. 

The most important reactions in this group are the Beckmann rearrangement, which converts oximes to amides a family of reactions that convert carboxylic add derivatives to amines with loss of the carbonyl carbon and the reaction of ketones with hy dr azoic acid to give amides (the Schmidt reaction). 

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Wolff rearrangement of a-diazoketones to give ketenes or subsequent products is an often used synthetic procedure the scope and limitations of which are well established 13 390), so that only a few new features of this reaction need to be considered here. Concerning its catalytic version, one knows that copper, rhodium and palladium catalysts tend to suppress the rearrangement390). A recent case to the contrary is provided by the Rh2(OAc)4-catalyzed decomposition of ethyl -2-diazo-3-oxopent-4-enoates 404 from which the p,y-unsaturated esters 405 are ultimately obtained via a Wolff rearrangement 236). The Z-5-aryl-2-diazo-3-oxopent-4-enoates undergo intramolecular insertion into an aromatic C—H bond instead (see Sect. 4.1). 

With very few exceptions ketenes cannot be isolated pure at room temperature (cf. Section 15.4). Consequently, they are prepared in situ and in the presence of the heteroatom nucleophile. The Wolff rearrangement of a-diazoketones is often used for this purpose (Section 14.3.2). a-Diazoketones can be obtained, for example, by the reaction between a carboxylic acid chloride and diazomethane (Figure 8.13 see also Figure 14.27) or hy treating the sodium enolate of a-formylketones with tosyl azide (Figure 14.29). 

Wolff rearrangement of a-diazoketones to give ketenes or subsequent products is an often used synthetic procedure the scope and limitations of which are well established so that only a few new features of this reaction need to be consi-... 

Microwave irradiation has been used to modify selectivity in rearrangement and coupling reactions. Indeed, the photo-Fries rearrangement described above could also be classified in this section [56]. Sudrik described the Wolff rearrangement of a-diazoketones under conventional heating and microwave irradiation [76]. The... 

Wolff rearrangement of a-diazoketones to carboxylic acid derivatives proceeds via ketenes under photochemical, thermal or metal ion catalysis. " The extrusion of nitrogen and the 1,2-shift can occur either in a stepwise manner via a carbene intermediate 8 or through a concerted process. 

Sudrik SG, Chavan SP, Chandrakumar KRS, Pal S, Date SK, Chavan SP, Sonawane HR. Microwave specific Wolff rearrangement of a-diazoketones and its relevance to the... 

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 rearrangement of a-diazoketones to carboxylic acids or esters shown in Equation 9 is known as the Wolff rearrangement. 

In the Wolff rearrangement, an a-diazoketone is heated to give a ketene. The mechanism of the Wolff rearrangement consists of one step the carbonyl susstituent migrates to the diazo C and expels N2. When the reaction is executed in H20 or an alcohol, the ketene reacts with solvent to give a carboxylic acid or an ester as the ultimate product. (The mechanism of this reaction was discussed in Section 2.3.1.)... 

Photolysis of the o-diazoacetylbenzenesulfonamide 13 (R = Me) over seven days gives a reasonable yield of 47 (R = Me),10 presumably via the Wolff rearrangement of the diazoketone to a ketene intermediate (53). Heating 13 (R = Ph) in refluxing chlorobenzene gave only 30% of 47 (R = Ph).10... 

The Wolff rearrangement of A -protected a-amino diazoketone 91 in the presence of silica gel and a catalytic amount of silver trifluoroacetate led to the formation of the A -protected acid 92. ... 

Rh (CO)i6 is a highly active catalyst in the reaction of ethyl diazoacetate with alkenes to give cyclopropyl esters. Yields obtained are better than those obtained using palladium and copper catalysts. Methyl ester derivatives of tricyclo[4.2.0.0 ]octane have been prepared by a photochemical Wolff rearrangement of a related diazoketone in methanol. ... 

The photolytic decomposition of a-diazoketones, accompanied by rearrangement to ketene (photolytic Wolff rearrangement), has been used successfully in the preparation of A-nor- and C-norsteroids. The method is reviewed in chapter 15 by R. M. Scribner. ... 

The Arndt-Eistert reaction (Scheme 2.1) which involves the Wolff rearrangement of diazoketones 13 (prepared from the corresponding commercially available N-protected-a-amino acids 12 by reaction of their mixed anhydrides with diazomethane a cautionary note is warranted here the generation and handling of diazomethane require special precautions) has been used extensively by Seebach and coworkers for the preparation of N-protected /9 -amino acids 14 and /9 -amino acid esters 15 and 16. 

The diazoketone first formed is decomposed in a high-boiling solvent (benzyl alcohol or octanol 2) at 160°-180°. Although no catalyst is required, but the addition of a collidine base improves the yield. The second step is the rearrangement of the diazoketone which is also called Wolff rearrangement. 

A density functional study has been made of the competition between Wolff rearrangement and [1,2]-H shift in /S-oxy-a-diazocarbonyl compounds. Silver-catalysed decomposition of a-diazoketones (88 n = 0), derived from A-tosyl a-amino acids in methanol, gave rise to mixtures of products of Wolff rearrangement (89) and direct insertion of the carbene into the NH bond (90). The -amino acid derived species (88 n = 1) gave rise to products of Wolff rearrangement. 

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