Ketene, formation

Ketene formation is a common side reaction- scambling of chiral centers... 

In a noteworthy series of studies, Herndon has shown that cyclopropylcarbenes can be used as four-carbon components in molybdenum- and tungsten-mediated [4 + 2 + l]-reactions with alkynes and carbon monoxide (CO). These reactions give cycloheptadienones in moderate yields and with moderate selectivity (Equations (26)—(28)). The mechanism of this reaction is proposed to proceed through a series of steps involving metathesis, GO insertion, ketene formation, cyclopropane cleavage, and finally reductive elimination (Scheme 43).133... 

Since CH2 reacts with CO to form ketene,14,71 the presence of CH2 may be tested for under some conditions by addition of CO and observing ketene formation.23,88-91... 

Solid Catalysts. Nafion-H is an active catalyst for acylation with aroyl halides and anhydrides.60,61 The reaction is carried out at the boiling point of the aromatic hydrocarbons. Yields with benzoyl chloride using 10-30% Nafion-H for benzene, toluene, and p-xylene are 14%, 85% and 82%, respectively. Attempted acylation with acetyl chloride, however, led to HC1 evolution and ketene formation. Nation resin-silica nanocomposite materials containing a dispersed form of the resin within silica exhibits significantly enhanced activity in Friedel-Crafts acylations.62,63... 

CH2CH2COCH3 and CH3CH2COCH2, in addition to CH3CHCOCH3. The first of these yields ethylene readily, but there was no evidence of ketene formation by pyrolysis of the second, which therefore probably disappears by another path. 

Two other possible reactions of acyl cations, which do not seem to be of general importance under the conditions of ester hydrolysis and formation, are ketene formation... 

Enolates prepared by deprotonation of carboxylic acid derivatives can also undergo elimination to yield ketenes. This is rarely seen with amides, but esters, thiolesters, imides, or N-acylsulfonamides can readily decompose to ketenes if left to warm to room temperature (Scheme 5.58). At -78 °C, however, even aryl esters can be converted into enolates stoichiometrically without ketene formation [462, 463],... 

Scheme5.58. Temperature-dependent alkylation of N-(/3-aminopropanoyl)sulfonamides and decomposition via ketene formation [457].

If the alcoholate or the alkylating reagent contains a carboxylic acid ester, acylation of the alcoholate can compete with alkylation. This potential side reaction does not cause trouble in the examples sketched in Scheme 6.14 (first and third reactions), because these esters are sterically hindered and devoid of a hydrogen (no ketene formation can occur) but, as illustrated in Scheme 6.15, less hindered esters can readily undergo transesterification with alcoholates. 

Scheme 4.81 in situ ketene formation and (5-lactam synthesis. 

Intramolecular disproportionation of [1] could conceivably result in the formation of unsaturated aldehyde or ester. Both paths are well known to occur with cyclic alkanones (la). Abstraction of H by the acyl radical portion of [1] would produce alkenal [3] while abstraction of by the alkyl radical portion of [1] would produce ketene [4] which may be efficiently trapped in nucleophilic alcoholic solvents to yield ester [5]. The intramolecular nature of alkenal formation is supported by deuterium labeling experiments (5), while the intramolecular nature of ketene formation is supported by (a) deuterium labeling experiments (6,7), and (b) the observed decrease in ketene formation with decreasing ring size (8). 

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