Generation of Ketenes

Ketenes are most often prepared from carboxylic acid derivatives, and a novel reductive pathway has been described for the conversion of biomass derived fatty acid esters in microalgae oils into hydrocarbons through intermediate ketenes. The conversion into alkanes using Zr02-promoted Ni catalysis is explained as involving conversion of the esters into acids and dehydration to ketenes (observed by strong 

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Another important issue of this reaction is the variable stereochemical outcome obtained depending upon the solvent used, the temperature, and the sequence of addition of the reactants. These aspects have been carefully studied experimentally by Xu et al. [71-73] in a brilliant series of papers. In particular, since one method for the generation of ketenes consists of the reaction between acyl chlorides (12) and bases (usually tertiary bases such as triethylamine), the direct reaction between compounds (12) with imines (66) was investigated at the B3LYP/6-31G level of theory [52], Under these conditions, it was calculated that formation trans-fi-lactams (70) from enolates (69) is preferred, in agreement with the experimental results [74—76] and with the careful analysis of Xu et al. [72] (Scheme 16). 

The Wolff Rearrangement allows the generation of ketenes from a-diazoketones. Normally, these ketenes are not isolated, due to their high reactivity to form diketenes. 

The generation of ketenes was confirmed, as illustrated in Scheme 29, by trapping experiments with methanol or benzylamine, producing the methyl ester or amide 196. Upon photolysis of precursor 194 (R = Ph R = H), the formation of phenyl ketene, which was monitored by IR spectroscopy, was indicated by the appearance of the C=C=0 band at 2120 cm. The even more efficient generation of ketenes was also achieved by photolysis of the selenium analogues of compounds 194. 

Taggi, A.E., Hafez, A.M., Wack, H. et al. (2000) Catalytic, asymmetric synthesis of P-lactams. Journalof the American Chemical Society, 122,7831-7832 Taggi, A.E., Wack, H., Hafez, A.M. et al. (2002) Generation of ketenes from acid chlorides using NaH/crown ether shuttle-deprotonation for use in asymmetric catalysis. Organic Letters, 4,627-629 France, S.,... 

While the acetylene based route to acetic anhydride would remain in practice in some locations until the 1940 s, a competitive technology was developed within the same decade that would dominate production even today. In this period, chemists discovered that the highly endothermic generation of ketene could be accomplished by heating either acetone (equation [10]) or acetic acid in the presence of a phosphate catalyst (equation [11]) at very high temperature (>700°C). Subsequent reaction of ketene with glacial acetic acid gave acetic anhydride (equation [12]). 

Some methods for the generation of ketenes are given below in Scheme 4.4. 

On the other hand, Marples [24] reported a new base mediated generation of ketene acetal obtained by hydrobromic acid elimination in bromoacetals 96. These compounds are prepared by condensation between allyUc alcohols 95 and 1,2-dibromo-l-ethoxyethane 94. However, the elimination of hydrobromic acid required rather harsh conditions (HMPA, EtjN, 150-160 °C, overnight) (Scheme 6.13). 

A second generation of ketene—azadiene cycloadditions involves [4 + 2] cycloaddition with chiral 2-aIkenyloxazoHnes and 2-aIkenylthiazohnes 250 (Scheme 79), which react with ketene, with excellent stereoselectivity to afford enantio-pure thiazolopyridones 251 (1999SL1379, 1979H(12)231, 2001JOC6756). 

The generation of ketene intermediates in reactions of activated carboxylic acid derivatives is never quite certain unless they are either... 

Treatment of acyl chlorides with pyridine and trifluoroacetic anhydride (TFAA) was assumed to involve generation of ketenes which can be subjected to electrophiUc attack by TFAA to afford jd-trifluoroacetylacetic acid derivatives 484. Quenching of the reactions with dienophiles resulted in construction of heterocycles 485 that clearly result from cycloaddition reactions of intermediate acylketenes generated by ehmination from 484 (Scheme 155) (1992TL1285, 1995T2573, 1995T2585). 

Synthesis of Ketene. Photolysis (270 nm) of 2-thiono-l,3-dioxol-4-ene results in the quantitative formation of ketene via Wolff rearrangement of the corresponding a-ketocarbenes (eq 2). Substituted dioxolenes can also be used in this fashion to generate substituted ketenes (eq 3). This serves as a convenient alternative to the use of a-diazo ketones in the generation of ketenes. 

This section deals with a juvenile but interesting field of acylpalladium, generation of ketene, to which little attention has been paid. 

Thermal generation of ketenes in the presence of alcohols sensitive to acid or base permits the formation of esters imder neutral conditions and is a valuable technique for use with such substrates. For example esterification of an acylketene generated by dioxinone thermolysis gives an intermediate designed for a synthesis of tragoponal (Eqn (4.106)). ° ... 

Taggi AE, Wack H, Hafez AM, France S, Lectka T. Generation of ketenes from acid chlorides using NaH/crown ether shuttle-deprotonation for use in asymmetric catalysis. Org. Lett. 2002 4(4) 627 29. 

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