Ketene dimers tables

Allene ketene cycloadditions are of greater synthetic utility than cither mixed allene dimerization or mixed ketene dimerization. In this class of reaction the ketene is the more reactive species and homodimerization of ketene can be minimized by use of excess allene. Such cycloadditions always result in 2-alkylidenecyclobutanones with the sp carbons of both moieties forming the initial bond. In substituted allenes and ketenes, mixtures of stereoisomers of 2-alkylidenecyclobutanones are obtained with very little stereoselectivity, the stereoisomers arise from cisUrcins isomerism in the cyclobutane ring and EjZ isomerism of the exocyclic double bond. In unsymmetrically substituted allenes some regiochemical preference for ketene cycloaddition is observed. Examples of dimethylketene allene cycloadditions are summarized in Table 1,2... 

Schollkopf and co-workers have synthesized a number of cyclopropanone acetals by the addition of various sulfur- and oxygen-containing carbenes to ketene diethylacetals (Table 3).26>27> Similarly, cyclopropanone dithioacetals may be prepared by the addition of the Ws-thiomethyl and Ws-thiobenzylcarbenes 12a, b to olefins.29) However, cyclopropanone acetal formation by this method requires double bonds with considerable electron enrichment and the yields are generally low. With unsubstituted olefins such as cyclohexene, the carbenes 12 a, b tend to form dimeric and trimeric products such as 13 and 14, instead of the double bond addition products. 

The history of the establiehmeat of thin structure U Mimmoiized in Table 10 which inoludee only a part of the work of C. D, Hwd and his school. We have also not mentioned some of the work of O. Sohroeter, of H. Staudingcr and of othera who have dlroumed the structure of the ketene dimer incidentally without furnishing any new evidence. 

TABLE 5. Ph7 ioia Propariin of the Ketene Dimer (Btilid State)... 

Specific examples for ketene dimers are summarized in Table 5.9. Ketene dimers are commercially available from Hercules Inc. Wilmington, Delaware under trade names such as Aquapel and Hereon (29), or NovaSize AKD 3016, made by Georgia-Pacific Corp. (30). 

Table IV. Preparation of Ketenes by Pyrolysis of Ketene Dimers. . . 120...

The various ketene dimers which have been reported are listed in Table VII. No attempt has been made to give all the references to the preparation of a particular dimer only those references which, in the authors opinion, give the best preparative methods are listed. Experimental procedures for preparing ketene dimers are described on pages f 137 and 140. 

TABLE VII—Contimiid Ketene Dimers B. Ketokelene Dimer —Continued... 

Physical Properties. Dimeric ketenes are colorless to dark brown Hquids or crystalline soHds with a broad range of melting and boiling points. Table 2 Hsts examples of dimeric ketenes and thioketenes. 

Simple a,3-unsaturated aldehydes, ketones, and esters (R = C02Me H > alkyl, aryl OR equation l)i, 60 preferentially participate in LUMOdiene-controlled Diels-Alder reactions with electron-rich, strained, and selected simple alkene and alkyne dienophiles, - although the thermal reaction conditions required are relatively harsh (150-250 C) and the reactions are characterized by the competitive dimerization and polymerization of the 1-oxa-1,3-butadiene. Typical dienophiles have included enol ethers, thioenol ethers, alkynyl ethers, ketene acetals, enamines, ynamines, ketene aminals, and selected simple alkenes representative examples are detailed in Table 2. - The most extensively studied reaction in the series is the [4 + 2] cycloaddition reaction of a,3-unsaturated ketones with enol ethers and E)esimoni,... 

Dimer of ketene. The mase spectrum of the dimer of ketene has been studied by the ssme authois (Table 16). These cleavages... 

Much wphysical properties of the dimer of ketene which has the stnioture of the /3-lactone of 3-hydroxy-S-butenoio acid these values an given in Tables Tables 8 and 9 are concerned with the dimers of the substituted hetenes fcv whioh ft /8-lactone structure has been demonstrated. 

In general, the higher the pK of an initiator, the more readily a monomer can be induced to polymerize anionically (Table 18-1). In alkali-metal ketene, for example, dimers—the alcoholate anions similar to the triphenylcarbinol anion—are the real initiators (see below). According to Table 18-1, pK values of 19-20 are to be expected for these anions (cf. the values for the alkali compounds of acetophenone and triphenylcarbinol). Accordingly, these initiators should be able to polymerize acrylonitrile but not styrene, as is indeed the case. 

The sixth item in Table 9.6 illustrates the reaction between ketene (CH2=C=0) and ethanoic acid (acetic acid,CH3C02H). As shown in Equation 9.99,ketene (CH2=C=0) can be prepared by the dehydration of ethanoic acid (acetic acid, CH3CO2H), and the isolation of ketene is complicated because of its ready dimerization to diketene (4-methylideneoxacyclobutane-2-one). 

The known tetrakis(trifluoromethyl)allene has been observed as a byproduct in the preparation of bis(trifluoromethyl)thioketen via pyrolysis of its dimer" at atmospheric pressure and l,l-dimethyI-3,3-bis(trifluoromethyl)-allene, apparently a new compound, has been synthesised from bis(tri-fluoromethyl)keten (see Scheme 15)." The new allenes CFa CFa-CClrCiCMea and CF2CbCF2-CCl C CMe2 have been obtained by dehydrochlorination of adducts prepared from isobutene (see Table 3, p. 12). 

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