A-ketocarbene intermediate

The Irradiation of -quinone dlazldes gives rise to the loss of a nitrogen molecule and to the formation of a ketocarbene intermediate which undergoes a Wolff rearrangement to a ketene derivative. This ketene can further react with water to form In-dene carboxylic acid. This reaction pathway Is normally used to Increase the solubility of specific coatings in alkaline developers. This typical positive-working mechanism Is of great... 

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 ... 

Photochemical or thermal extrusion of molecular nitrogen from ot-diazocarbonyl compounds generates a-carbonylcarbenes. These transient species possess a resonance contribution from a 1,3-dipolar (303, Scheme 8.74) or 1,3-diradical form, depending on their spin state. The three-atom moiety has been trapped in a [3 + 2] cycloaddition fashion, but this reaction is rare because of the predominance of a fast rearrangement of the ketocarbene into a ketene intermediate. There are a steadily increasing number of transition metal catalyzed reactions of diazocarbonyl compounds with carbon-carbon and carbon-heteroatom double bonds, that, instead of affording three-membered rings, furnish hve-membered heterocycles which... 

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... 

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). 

Prevention of the Wolff rearrangement is much more likely to occur through alternative reactions of the ketocarbene intermediate (11 Scheme S). Lack of rearrangement to the ketene (14) may be due to a structural feature present in the ketocarbene (e.g. functional groups a to the C=0 interfere with the Wolff rearrangement, or lead to secondary reactions) " but, not infrequently, it can be the result of an... 

The electron deficiency in the ketocarbene intermediate can be satisfied through various types of addition reactions. Cyclopropane derivatives are the result of inter- or intra-molecular cycloaddition to al-kenic double bonds. In the case of Z-group-activated C=C, the initial reaction of the diazo ketone is usually a 1,3-dipolar cycloaddition to give a pyrazoline derivative (see equation 9) on heating these decompose with nitrogen elimination, resulting in cyclopropane formation (e.g. 29 30 31). The... 

There have been relatively few applications to the contraction of larger rings. Irradiation of solutions of 4-diazo-/ra j-bicyclo[6.1.0]nonan-5-one afforded the strained carboxylate derivatives (107 XR = OH, OMe) in ca. 22-45% yield.Small to medium ring diazo ketones afford the ring-contracted carboxylic acids (108 n = 4-10) in 25-95% yield on photolysis in dioxane-H20 or THF-H20. Thermolysis of the diazo ketones in aniline at 150-160 C gave the anilides of (108) in 78-91% yield. Transannular reactions of the ketocarbene intermediates can be a complication under some conditions. 4-Carboxy[8]paracyclophane (109) has been prepared (25%) by the photochemical method. ... 

In aqueous solutions, 2-halophenols, such as 2-bromophenol, are known to undergo photo-Wolff rearrangement (see also Scheme 6.171). In this reaction, the cyclopentadie-necarboxylic acid derivatives 487 are formed via the singlet a-ketocarbene (488) and ketene (489) intermediates, followed by nucleophilic addition of water (Scheme 6.238).1344-1346... 

Cyclic a-diketones (acenaphthenequinone, aceanthrenequinone, and tV-alkyUsatins) were deoxygenated by tris(diethylamino)phosphine in the presence of fiillerene CgQ. a-Ketocarbenes were probable intermediates in the formation of polycyclic methanofullerene derivatives, which were investigated for their electrochemical behaviour and structures. 

The Buchner reaction describes cyclopropanation of an aromatic double bond with the a-ketocarbene derived from an a-diazocarbonyl compound 1 to produce an unstable norcaradiene intermediate 2, which is in thermal equilibrium with the more stable cycloheptatriene tautomer 3. This tautomer undergoes thermally or photochemically induced electrocyclic ring opening to give other cycloheptatriene isomers A-6)... 

In dimethylacetylene, the triplet ground state intermediate biradical has a ketocarbene structure with a C=0 double bond, a central C-C single bond, and a carbene C atom ... 

Like other diazo compounds, diazo ketones are sensitive to both heat and light. Upon exposure to either, nitrogen is lost and a carhene intermediate (p. 431), in this case, a ketocarbene, is formed (Rg. 18.65). 

Tricyclanes and Quadricyclanes.—Oxidation of camphor hydrazone with silver carbonate on celite gave the tricyclane (565) and a trace of bomylene, whereas other hydrazones are found to rearrange to azines. Diazocamphor (566) in THF is decomposed by catalytic silver perfluorobutyrate and affords the tricyclanone (567) in nearly quantitative yield the a-ketocarbene is the presumed intermediate. The carbomethoxylation of a-bonded organomercurials in the presence of stoicheio-metric amounts of lithium tetrachloropalladate and CO occurs with predominant retention of configuration at carbon. Thus (568) gave (569 38 %) and various minor components. 

The reaction of a-diazocarbonyl compounds with nitriles produces 1,3-oxazoles under thermal (362,363) and photochemical (363) conditions. Catalysis by Lewis acids (364,365), or copper salts (366), and rhodium complexes (367) is usually much more effective. This latter transformation can be regarded as a formal [3 + 2] cycloaddition of the ketocarbene dipole across the C=N bond. More than likely, the reaction occurs in a stepwise manner. A nitrilium ylide (319) (Scheme 8.79) that undergoes 1,5-cyclization to form the 1,3-oxazole ring has been proposed as the key intermediate. 

When the Wolff rearrangement is carried out photochemically, the mechanism is basically the same,162 but another pathway can intervene. Some of the ketocarbene originally formed can undergo a carbene-carbene rearrangement, through an oxirene intermediate.168 This was shown by l4C labeling experiments, where diazo ketones labeled in the carbonyl group... 

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