Ketenes halogenation

Ketenes are oxo compounds with cumulated carbonyl and carbon—carbon double bonds of the general stmcture R R2C—C—O, where and R2 may be any combination of hydrogen, alkyl, aryl, acyl, halogen, and many other functional groups. Ketenes with R = sometimes called aldoketenes,... 

Simple olefins do not usually add well to ketenes except to ketoketenes and halogenated ketenes. Mild Lewis acids as well as bases often increase the rate of the cyclo addition. The cycloaddition of ketenes to acetylenes yields cyclobutenones. The cycloaddition of ketenes to aldehydes and ketones yields oxetanones. The reaction can also be base-cataly2ed if the reactant contains electron-poor carbonyl bonds. Optically active bases lead to chiral lactones (41—43). The dimerization of the ketene itself is the main competing reaction. This process precludes the parent compound ketene from many [2 + 2] cyclo additions. Intramolecular cycloaddition reactions of ketenes are known and have been reviewed (7). 

Ketene Insertions. Ketenes insert into strongly polarized or polarizable single bonds, such as reactive carbon—halogen bonds, giving acid hahdes (7) and into active acid haUdes giving haUdes of p-ketoacids (8) (46). Phosgene [77-44-5] (47) and thiophosgene [463-71-8] (48) also react with ketenes. 

General Reaction Chemistry of Sulfonic Acids. Sulfonic acids may be used to produce sulfonic acid esters, which are derived from epoxides, olefins, alkynes, aHenes, and ketenes, as shown in Figure 1 (10). Sulfonic acids may be converted to sulfonamides via reaction with an amine in the presence of phosphoms oxychloride [10025-87-3] POCl (H)- Because sulfonic acids are generally not converted directiy to sulfonamides, the reaction most likely involves a sulfonyl chloride intermediate. Phosphoms pentachlotide [10026-13-8] and phosphoms pentabromide [7789-69-7] can be used to convert sulfonic acids to the corresponding sulfonyl haUdes (12,13). The conversion may also be accompHshed by continuous electrolysis of thiols or disulfides in the presence of aqueous HCl [7647-01-0] (14) or by direct sulfonation with chlorosulfuric acid. Sulfonyl fluorides are typically prepared by direct sulfonation with fluorosulfutic acid [7789-21-17, or by reaction of the sulfonic acid or sulfonate with fluorosulfutic acid. Halogenation of sulfonic acids, which avoids production of a sulfonyl haUde, can be achieved under oxidative halogenation conditions (15). 

Mercury can be used to dehalogenate acyl halides to ketenes when a halogen IS present next to the COCl group [67] (equation 35)... 

The 2+2 cycloadditions of benzyne to cis- and trani-propenyl ether gave cis- and fran -benzocyclobntanes as the main products, respectively [ 117,118], Stereospecific [2+2] cycloaddition reactions were observed between the benzyne species generated by the halogen-Uthium exchange reaction of ort/io-haloaryl triflates and the ketene silyl acetals (Scheme 23) [119],... 

The best yields are obtained when the ketene has an electronegative substituent, such as halogen. Simple ketenes are not very stable and must usually be generated in situ. The most common method for generating ketenes for synthesis is by dehydrohalo-genation of acyl chlorides. This is usually done with an amine such as triethylamine.167 Other activated carboxylic acid derivatives, such as acyloxypyridinium ions, have also been used as ketene precursors.168 Ketene itself and certain alkyl derivatives can be generated by pyrolysis of carboxylic anhydrides.169... 

Ketene acetal synthesis by /1-elimination of haloacids from halogenated acetals under well controlled conditions using thermal activation (A), ultrasound (US) or micro-wave irradiation [92] (MW) has been described. From a mechanistic point of view, as the TS is more charge delocalized than the GS and the polarity is enhanced during the course of the reaction, a favorable microwave effect can therefore be observed (Eqs. (37) and (38) and Scheme 3.13). 

This section deals with alkylidene complexes L M=CR2 and vinylidene complexes LnM=(C)n,=CR2 in which the metal-bound carbon atom bears only hydrogen, alkyl, or aryl groups, but neither heteroatoms (halogen, nitrogen, oxygen, or sulfur) nor electron-withdrawing groups. Dimetallacyclopropanes and ketene complexes will not be discussed. 

The reaction involves an attack by the tellurium anion at the halogen atom followed by a rapid elimination of ketene from the resulting enolate. 

Truce and Norrell noticed in 1963 that sulfenes readily undergo 2 - - 2 cycloaddition to ketene diethyl acetal with the formation of 3,3-diethoxythie-tane 1,1-dioxides. The same reaction was carried out by Paquette, using 1,1-dioxide ketals with cyclic a-halo sulfones in which the corresponding chloroketal derivatives 47 were formed. This approach may be illustrated by the addition of ketene diethyl acetal to halogenated sulfenes and to... 

Chlorinated Novolak Resins. Mixtures of a cresol formaldehyde Novolak resin and a photoactive compound cross-link at electron doses far smaller than the dose required for the Novolak resin alone (11). The reason for this accelerated cross-linking is the reactions between the ketene (an intermediate formed from the photoactive compound upon irradiation) and the Novolak resin. This reaction may be reduced by using a Novolak resin modified for this purpose, or by using certain additives. The rationale for developing a halogen-substituted Novolak resin is the control of the reaction between the intermediate ketene and the Novolak. 

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 . 

Addition of halogens to ketenes 5-27 Addition of HOBr or HOC1 to triple bonds addition of chlorine acetate or other reagents to olefins 5-34 Addition of acyl halides to olefins... 

While there are no examples of the formation of fluorinated unsaturated aldehydes by de-halogenation, dehalogenation is the preferred method for the synthesis of reactive fluorinated ketenes. Examples of dehalogenations giving unsaturated fluorinated ketones and fluorinated ketenes are listed in Table 9. 

Keywords a-halogen acetal, a-haogen thioacetal, KOtBu, microwave irradiation, /(-elimination, ketene O,O-acetal, ketene 5,5-acetal... 

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