Ketenes from diradicals

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... 

A new photoreaction is described for the tricyclic triazoline shown in Scheme 44, which upon irradiation in methanol gives a pyrrole methyl ester205 by retro Diels-Alder reaction of an intramolecular pyrrole-ketene Diels-Alder adduct (88) formed from the diradical (Scheme 154).205 In benzene, ketene dimerization occurs. 

Olefin 6, aldehyde or ketone 8, acrylic acid 17 and ketene 18 can be found among the possible products generated unfinished states 15 and 19 did not seem to be promising. Complex 14 and 1,4-metalla diradical 4 formed from it by decarboxylation is an important intermediate. The ways terminating at olefin contain path 4-5-6 starting with decarboxylation and path 2-12-13-6, where the decarboxylation follows after the homolysis of the C —O bond of the original hydroxyl. Complex 14 can homolyse into 4 or 16, in both cases diradicals produce... 

The ratio of spirohexan-4-one/spirohexan-5-one was close to 1 1 which can be rationalized by assuming a diradical mechanism as proposed for the cycloaddition of dimethylketene to methylenecyclopropane. Dichloroketene also underwent nonregioselective addition to methylenecyclopropane (49% yield) however, 4,4-dichlorospiro[2.3]hexan-5-one was formed as the main product. When methylenecyclopropane was reacted with chloro(2,2,2-tri-chloroethyl)ketene, generated by elimination of chlorine from 2,2,4,4,4-pentachlorobutanoyl chloride with zinc and phosphoryl chloride, 4-chloro-4-(2,2,2-trichloroethyl)spiro[2.3]hexan-5-one (10) was isolated in 31% yield. The isolation of a single isomer instead of mixtures as described above is possibly due to the different reaction conditions which do not guarantee an uncatalyzed cycloaddition. 

In another example from Moore, cyclobutenone 148, rearranges to give enyne-ketene 149. The resulting diradical was intercepted by a tethered alkyne, which in turn led to spirocycle 153 (Scheme 27) [49],... 

Solution It is thought that 1,4-cycloaddition reactions proceed through a mechanism involving a diradical intermediate. If we assume that the mechanism of 1,2-cycloaddition is similar to 1,4-cycloaddition, there are four possible products from the reaction of cyclopentadiene and ketene. The reactions and their mechanisms are presented below (arrows show movement of one electron) ... 

In 1985 Harold W. Moore and co-workers (University of California, Irvine), described the generation and chemistry of 2-alkenylethynyl ketenes, which were accessible from the corresponding alkynylcyclobutenones. For example, 4-alkynyl-4-trimethylsililoxycyclobutenone 3 forms an enyne-ketene 4. The conjugated ketene 4 undergoes ring closure to produce the diradical 5 (or related zwitterion) which, in turn, proceeds to the substituted... 

Organometallic reagents were used for the synthesis of bicyclic aromatic compounds via Moore-type cyclization. Rahm and Wulff described the new synthesis of 5-hydroxyindolines with the use of a chromium carbene complex bearing alkynyl substituent 22." The amino-tethered bis-alkynyl carbene complex 22 was transformed into indoline 23 by thermolysis in the presence of a hydrogen source. The low yield of product 23 was improved when the reaction was carried out in the presence of the electrophile, added to protect the phenol function. This process involves the insertion of one carbon monoxide group from the chromium complex into the skeleton of an eneyne compound 24. The resulting enyne-ketene 25 undergoes a cycloaromatization reaction to afford the 1,4-diradical intermediate 26. Subsequent demetalation yields product 23."... 

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