A-Ketocarbenes

Decomposition of the diazoimide (551) by heating in the presence of copper acetylaceton-ate also generated a ketocarbene (552). This undergoes an intramolecular condensation to give the anhydro-4-hydroxy-3-methyl-4-p-nitrophenyl-2-phenyloxazolium hydroxide (553), which cannot be prepared by more classical means (75CL499). 

In this review an attempt is made to discuss all the important interactions of highly reactive divalent carbon derivatives (carbenes, methylenes) and heterocyclic compounds and the accompanying molecular rearrangements. The most widely studied reactions have been those of dihalocarbenes, particularly dichlorocarbene, and the a-ketocarbenes obtained by photolytic or copper-catalyzed decomposition of diazo compounds such as diazoacetic ester or diazoacetone. The reactions of diazomethane with heterocyclic compounds have already been reviewed in this series. ... 

With a few recent exceptions the reactions in this group have been with the a-ketocarbenes, CH COOMe, CH COOEt, and CH-COCHs, derived from the corresponding diazo compounds. Sorm and coworkers have reported the ring-opening of furan and methylfurans upon reaction with diazoacetone decomposed by copper, via attack at... 

An a-diazo ketone 1 can decompose to give a ketocarbene, which further reacts by migration of a group R to yield a ketene 2. Reaction of ketene 2 with water results in formation of a carboxylic acid 3. The Woljf re arrangement is one step of the Arndt-Eistert reaction. Decomposition of diazo ketone 1 can be accomplished thermally, photochemically or catalytically as catalyst amorphous silver oxide is commonly used ... 

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 ... 

The photolysis of o-quinone diazides was carefully investigated by Stis in 1944, many years before the development of photoresists. Scheme 10-102 shows the photolysis sequence for the diazoquinone 10.75 formed in the diazotization of 2-amino-1-naphthol. The product of the photolytic step is a ketocarbene (10.76), which undergoes a Wolff rearrangement to a ketene (10.77). In the presence of water in-dene-3-carboxylic acid (10.78) is formed this compound is highly soluble in water and can be removed in the development step. The mechanism given in Scheme 10-102 was not postulated as such by Stis, because in 1944 ketocarbenes were unknown (for a mechanistic discussion of such Wolff rearrangements see review by Zollinger, 1995, Sec. 8.6, and Andraos et al., 1994). 

The reaction, formally speaking a [3 + 2] cycloaddition between the aldehyde and a ketocarbene, resembles the dihydrofuran formation from 57 a or similar a-diazoketones and alkenes (see Sect. 2.3.1). For that reaction type, 2-diazo-l,3-dicarbonyl compounds and ethyl diazopyruvate 56 were found to be suited equally well. This similarity pertains also to the reactivity towards carbonyl functions 1,3-dioxole-4-carboxylates are also obtained by copper chelate catalyzed decomposition of 56 in the presence of aliphatic and aromatic aldehydes as well as enolizable ketones 276). No such products were reported for the catalyzed decomposition of ethyl diazoacetate in the presence of the same ketones 271,272). The reasons for the different reactivity of ethoxycarbonylcarbene and a-ketocarbenes (or the respective metal carbenes) have only been speculated upon so far 276). 

Insertion of a ketocarbene moiety into a C—O bond of orthoesters is normally performed with catalysis by BF3 Et20. Copper(II) trifiouromethanesulfonate was found to be a similarly efficient catalyst also, at least in some cases, whereas Rh2(OAc)4 was much less suited to promote this transformation l60). Besides the C/O insertion product 343, the alcohol insertion product 344 and, in reactions with ethyl diazoacetate, the formal carbene dimers were obtained. In agreement with BF3 EtzO, Cu(OTf)2 did not bring about insertion into a C—O bond of trimethyl... 

In the list of diazoketones studied by us95 mostly derivatives were included which have in solution no or only a small tendency for a Wolff rearrangement. Nevertheless we found not a single diazoketone 71 which enabled us to identify a ketocarbene 72, only the corresponding ketenes 73 could be detected. The same observation was made when we studied in collaboration with Yannoni et al." the photochemically induced deazotation of l-diazo-2-propanone in an organic matrix at 77 K, using 13C CPMAS NMR spectroscopy as the analytical tool. 

Unsaturated ketones, ring-contracted ketones, and bicyclic ketones resulting from transannular insertion are the main products isolated in the oxidation of cycloalkynes.738 Product distribution was interpreted in terms of an oxirene-a-ketocarbene equilibrium. 

Photolysis of the diazocompound 171 caused ring contraction to the pyrazolo[3,2-c]-s-triazole (172) via rearrangement of the first-formed a-ketocarbene and hydrolysis of an intermediate ketene.161... 

Silver clusters 2.5 nm in diameter displayed unusual electrocatalytic properties in Wolff rearrangements of diazoketones.67 The reaction proceeds with electron transfer to and from the silver cluster. The presence of an a-ketocarbene/ketene was confirmed using pyridine as a nucleophilic probe and by UV-visible spectroscopy. Electrochemistry was used to support the role of the silver particles in the rearrangement. 

Fig. 9.1 Generating an oxo carbene (a ketocarbene ) from a labelled diazo ketone sometimes leads to a ketene in which the label is scrambled. This indicates that a species with the symmetry of oxirene is formed...

Pyridinium (trifluoroacetyl)methylide forms [3-1-2] cycloadducts with a wide variety of perfluorinated and partially fluorinated olefins, alkynes, and nitriles [86JFC(34)275]. Photolysis of a mixture of hexafluoro-3-diazobutan-2-one and perfluoro-2-butyne in the gas phase results in the formation of tetrakis(trifluoromethyl)furan a ketocarbene is the key intermediate of this reaction sequence (87JOC2680) (Scheme 79). 

Diketones. Coppcr(l) aoetylauetonate is a very effective copper catalyst for the insertion of an a-ketocarbene into C-C double bonds. Thus (3) can be obtained in 55% yield from the reaction of l-dia7 o-2-butanone (I) with isopropenyl acetate (2). When (3) is refluxed with 4% methanolic sodium hydroxide, 2,3-dimethyleydo-pentene-2-one (5) is obtained in 85% yield. 

The iodonium salt 193 derived from methyl acetoacetate behaves as an a-ketocarbene equivalent and adds to methyl ketones to give the dioxoles 194. Similar addition to C=S occurs for thioketones to give 195 and with CS2 to give the oxathiole-2-thione 196 (Scheme 18) <2002TL5997>. The corresponding diazo compound 197 reacts with succin-imide and a rhodium catalyst to give the spiro dioxole 198 <2006TL2643>. 

Bachmann, C., N Guessan, T. Y. Photoactivated a-ketocarbenes formation and isomerization reactions. RRKM calculations with semiempirical parameters (AMI, MNDOC). Int. J. Chem. Kinet. 1994, 26, 643-664. 

Calvo-Losada, S., Suarez, D., Sordo, T. L., Quirante, J. J. Competition between Wolff Rearrangement and 1,2-Hydrogen Shift in P-oxy-a-ketocarbenes Electrostatic and Specific Solvent Effects. J. Phys. Chem. 61999,103, 7145-7150. 

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