Ketene, derivative

Ketenes. Derivatives of the compound ketene, CH2=C=0, are named by substitutive nomenclature. For example, C4Hc,CH=C=0 is butyl ketene. An acyl derivative, such as CH3CH2—CO—CH2CH=C=0, may be named as a polyketone, l-hexene-l,4-dione. Bisketene is used for two to avoid ambiguity with diketene (dimeric ketene). 

Thermal Stability. The saturated C —C 2 ketones are thermally stable up to pyrolysis temperatures (500—700°C). At these high temperatures, decomposition can be controlled to produce useful ketene derivatives. Ketene itself is produced commercially by pyrolysis of acetone at temperatures just below 550°C (see Ketenes, ketene dil rs, and related substances). 

More definitive evidence for the formation of an oxirene intermediate or transition state was presented recently by Cormier 80TL2021), in an extension of his earlier work on diazo ketones 77TL2231). This approach was based on the realization that, in principle, the oxirene (87) could be generated from the diazo ketones (88) or (89) via the oxocarbenes 90 or 91) or from the alkyne (92 Scheme 91). If the carbenes (90) (from 88) and (91) (from 89) equilibrate through the oxirene (87), and if (87) is also the initial product of epoxidation of (92), then essentially the same mixture of products (hexenones and ketene-derived products) should be formed on decomposition of the diazo ketones and on oxidation of the alkyne this was the case. 

The photochemistry of cyclobutanones differs from that of less strained larger cycloalkanones. Fragmentation to ethylene and ketene (derivatives), decarbonylation and rearrangement to oxacarbenes predominate here. The oxacarbene formation, which occurs with retention of the configuration of the... 

Four-membered heterocycles are easily formed via [2-I-2] cycloaddition reac tions [65] These cycloaddmon reactions normally represent multistep processes with dipolar or biradical intermediates The fact that heterocumulenes, like isocyanates, react with electron-deficient C=X systems is well-known [116] Via this route, (1 lactones are formed on addition of ketene derivatives to hexafluoroacetone [117, 118] The presence of a trifluoromethyl group adjacent to the C=N bond in quinoxalines, 1,4-benzoxazin-2-ones, l,2,4-triazm-5-ones, and l,2,4-tnazin-3,5-diones accelerates [2-I-2] photocycloaddition processes with ketenes and allenes [106] to yield the corresponding azetidine derivatives Starting from olefins, fluonnaied oxetanes are formed thermally and photochemically [119, 120] The reaction of 5//-l,2-azaphospholes with fluonnated ketones leads to [2-i-2j cycloadducts [121] (equation 27)... 

Ketene derivatives are observed as by-products of elimination reactions involving hindered earboxylie acid derivatives, and it is important to understand their ehemistry. 

Pyran-4-ones are formed when acyl ketenes derived from dioxofurans and dioxinones react with vinyl ethers. Intermediate products are 1,3,5-triketones and reduced pyranones <96CPB956,96H(43)2457,96TL6499>. 

Additional evidence for a photochemically produced noncarbene precursor to ketene 29 is provided by product analysis. Photolysis of 25 in neat methanol leads to both carbene-derived (i.e., 30 in 75% absolute yield) and ketene-derived adducts (i.e., 31 in 18% absolute yield), but thermolysis of 25 in neat methanol (sealed tube at 170°C) provides only carbene-derived adduct 30 (91% absolute yield) The ratio... 

The last [2 + 2] cycloaddition performed onto methylenecyclopropane itself consists of the use of ketene derivatives, particularly of dimethylketene 508 (Table 40, entry 1) [133]. The result is an almost equimolar mixture of the two possible regioisomers 511, albeit no yield has been reported. Anyway, the particular reactivity of methylenecyclopropane was confirmed, since it was found to be around 15 times more reactive than isobutene [133]. The scarce regioselectivity of this cycloaddition was confirmed by the reactions of the same ketene 508 with 2,2-disubstituted methylenecyclopropanes (entries 2 and 3) [134], BCP has also been shown to be reactive towards chloro-substituted ketenes 509 and 510, affording the expected cycloadducts 514 and 515 in mild conditions (entries 4 and 5) [13b],... 

Cycloadditions involving ketene derivatives as one or both reaction partners are assumed to be rare examples of concerted [7r2 + jt2] cycloadditions146. The activation volumes determined for the [2 + 2] cyclodimerization and the [2 + 2] cycloadditions... 

As an alternative to hydrozirconation of acetylenic tellurides or selenides, Dabdoub and co-workers have more recently described the first additions of the Schwartz reagent (one equivalent) to acetylenic selenide salts 51 (Scheme 4.30) [52]. Subsequent alkylation at selenium produces 1,1-dimetallo intermediates 52, which are cleanly converted in a one-pot process to stereodefined products 53. It is noteworthy that ketene derivatives 52 are of ( )-geometry, the opposite regiochemistry to that which results from hydrozirconation of acetylenic tellurides (vide supra). This new route also avoids the mixtures of regio-isomers observed when seleno ethers are used as educts. The explanation for the stoichiometric use of Cp2Zr(H)Cl in these reactions, in contrast to the requirement for two equivalents with seleno ethers, may be based on cyclic intermediates 54, in which Li—Cl coordination provides an additional driving force. Curiously, attempted hydrozirconation of the corresponding telluride salt 55 under similar conditions was unsuccessful (Scheme 4.31) (Procedure 12, p. 143). 

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. 

From Ketene or Ketene Derivatives with Another C-C Double-Bond... 

Thomas Lectka of Johns Hopkins University has reported (J. Org. Chem. 68 5819,2003) that benzoylquinine (BQ) catalyzes the two-carbon homologation of a ketene, derived from the acid chloride, with chloroamide such 7, to give the (3-amino acid derivative 8 with control of both relative and absolute configuration. The authors suggest that the BQ is involved five times in the course of the transformation of 6 into 8. The two esters of the product are differentiated, so one can imagine, inter alia, reduction of... 

Enantioselective halogenation is a powerful transformation, directly installing an efficient leaving group. Thomas Leckta of Johns Hopkins University has shown (7. Am. Chem. Soc. 2004,126, 4245) that benzoylquinine 11 catalyzes the a-chlorination of ketenes derived from acid chlorides such as 10, to give 12 in high . 

The reaction of 1,2,4-triazines with ketene derivatives such as the 0,0-acetals (42la), 0,iV-acetals (421b), Af,Af-aminals (421c) and (V.S-aminals (421d) has been studied extensively. In all cases pyridines (422) were isolated (Scheme 14). These reactions follow the usual cycloaddition route. Again it is found that the orientation is dominated by secondary... 

Lactams.1 The ketene derived from (S)-phenyloxazolidylacetyl chloride (1), prepared from (S)-phenylglycine, undergoes cycloaddition with N-benzyl al-dimines to give two as-azetidinones with high stereochemical control (equation I). The chiral auxiliary and the benzyl group are cleaved by Birch reduction to provide enantiomerically pure azetidinones (3). 

Ketene derivatives S,S-Acetals and ketals Diphenyl-2-(l,3-dithianyl)phosphine oxide, 126... 

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