Transient ketenes

The binary composition of the ketene transients was proven by low-temperature H-NMR spectroscopy [62] [72b] [74a]. Formal analysis of the reaction kinetics ) indicates that the two ketene components were generated concomitantly and not successively [74b]. The racemization, easily followed by monitoring the CD spectra (Fig. 3), gives indirect information about the temporary ring opening taking place ). 

Of the alternative conjectures, either that no ketene is produced in TFE or that the ketene produced is not capable of addition because of rapid cychzation due to TFE, only the latter merits consideration. The existence of the ketene transients has been affirmed not only indirectly, by the observed photoepimerization, but also directly, by detection by flash spectroscopy [72d]. 

The chemistry of ketenes is dominated by their high reactivity most of them are not stable under normal conditions, many exist only as transient Species. Nucleophilic attack at the j -carbon, [2 + 2] cycloadditions, and ketene iasertion iato single bonds are the most important and widely used reactions of such compounds. 

Several stable Group 6 metal-ketene complexes are known [14], and photo-driven insertion of CO into a tungsten-carbyne-carbon triple bond has been demonstrated [15]. In addition, thermal decomposition of the nonheteroatom-stabilized carbene complexes (CO)5M=CPh2 (M=Cr, W) produces diphenylke-tene [16]. Thus, the intermediacy of transient metal-ketene complexes in the photodriven reactions of Group 6 Fischer carbenes seems at least possible. 

The fragmentation of 1,2-O-cyclohexylidene-43 (92) and 1,2-0-isopropylidene-5-0-(methylsulfonyl)-Q -D-glucofuranuronic hydra-zide110 (93) to yield the corresponding 5-deoxy-D-xyZo-hexofu-ranurono-6,3-lactones 95 and 96, with transient formation of ketenes (94), is a special case of elimination caused by bases. 

The lifetime of formylcarbene was determined by transient absorption and grating spectroscopies.39 Photolysis of formyldiazomethane produces formylcarbene which can isomerize, kr, to ketene or react with pyridine (Scheme 4). Using the pyridine ylide method, the lifetime of singlet formylcarbene was estimated to be 150-730 ps in CH2CI2. This is in reasonable agreement with the lifetime of 900 ps determined by transient grating spectroscopy. 

Disubstituted 2,4-cyclohexadienones (112) undergo photoinduced electrocyclic ring opening to the transient ketene derivatives 113, which can be trapped by nucleophiles to prepare the corresponding carboxylic acid derivatives (114 equation 44)196 197 j le reaction has been employed successfully for the synthesis of various carboxylic acids, esters and amides. 

Direct evidence has been reported for the formation of methoxyvinyl- and methylthiovinyl-(carboxy)ketenes (55c and 55d) upon flash vacuum thermolysis of Meldrum s acid derivatives (54c) and (54d), respectively " the intermediates decarboxylate readily to give (56c) and (56d), respectively, and are more transient than those obtained previously from (54a,b). 

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 convenient method for generating and trapping reactive ketenes is the flash-vacuum pyrolysis (FVP) of substituted Meldrum s acid derivatives. The dispirocyclobutane-l,3-dione 9 is obtained from the transient ketene 8 generated by FVP of the diester precursor 7.56... 

As shown in Scheme 78, the transient bicyclo[3.3.0]oxonium ylide 255 that was generated from a THF-substituted diazoketone was first protonated by acetic acid to the corresponding bicyclo[3.3.0]oxonium ion, which then provided the ring expansion product <1996CC1077>. In the presence of the weakly acidic MeOH, the ylide underwent a concerted [3+2] cycloreversion to a ketene intermediate to form the ring cleavage product <2004JOC1331>. 

Summary. Intra- and intermolecular carbene or carbenoid reactions resulting from the photochemical and Cu(I)-, Rh(II)-, or Ru(I)-catalyzed decomposition of a-diazo-a-silylacetic esters are described. Among the products reported are (alkoxysilyl)ketenes, silaheterocycles, 1-trialkylsilylcyclopropane-l-carboxylates, and products derived from transient carbonyl ylides. 

Despite the pioneering work of Ando and Doyle, few synthetic applications of oxonium ylidic rearrangements have been reported. However, three examples of synthetic relevance appeared recently (Schemes 60 to 62). When a-allyloxyacetic esters are reacted with trimethylsilyl triflate and a base, the transposed material (252), resulting from a 3,2-sigmatropic rearrangement of the transient ylide (251), could be isolated in good yields. ° The same product ratio was also obtained upon treatment of the ketene acetal (253) with trimethylsilyl triflate. This second variant involves the direct formation of ylide (251) followed by its transformation into (252 Scheme 60). 

Similarly, a qualitative relation between the chemical behavior and the distortion from ideal C2v symmetry was suggested for a series of lithium ester enolates (Scheme 6.13) [108]. Enolate 1, furthest along the reaction coordinate to ketene, had to be handled at temperatures below -50°C and decomposed rapidly at temperatures higher than -30°C. The two other enolates, 2 and 3, were found to survive in crystalline form at 0°C and at room temperature, respectively. The decomposition occurs most likely through a ketene-like intermediate, whose transient existence was demonstrated by cleaving the lithium enolate of 2,6-di-/ert-butyl-4-methylphenyl-2-methylpropanoate at room temperature in the presence of excess -BuLi. 

The transient decays at the same rate that cyclic keten-imine 51 is formed, suggesting that the newly detected transient is singlet phenylnitrene ( 49). This assignment was secured with the aid of computational chemistry i and by studying the temperature dependence of the kinetics. 

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