Dimethylketene dimerization reactions

The preparation of ketenes has been discussed by Hanford and Sauer in Organic Reactions Dimethylketene has been prepared by the treatment of a-bromoisobutyryl bromide with zinc,3 and by the pyrolysis of isobutyrylphthalimide,4 dimethylmalonic anhydride,6 or a-carbomethoxy-a,j3-dimethyl- -butyrolactone. Dimethylketene dimer has been prepared by heating isobutyryl chloride with a tertiary amine. Pyrolysis of the dimer yields dimethylketene.7... 

Although these reactions can be duplicated in most cases with the normal dimer of dimethylketene,11 the more reactive lactone dimer is the preferred reagent. The liquid form of this dimer is convenient to handle. A distinct difference in behavior of the dimethylketene dimers is noted when they are pyrolyzed. The normal dimer is dissociated at 600° to dimethylketene,12 but the lactone dimer is decarboxylated almost quantitatively at 450° to tetramethylallene.13... 

Phosphines Phosphines have also been utilized as effective asymmetric catalysts for ketene dimerization reactions. Initial reports by Elam [60] and Bentrude and Johnson [61] demonstrated that trialkylphosphites were capable of acting as effective nucleophilic catalysts in the dimerization of dimethylketene, while Kerrigan and coworkers also demonstrated that disubstituted ketenes could be dimerized with catalytic quantities of trialkylphosphines (Scheme 3.27) [62]. Kerrigan and coworkers extended this... 

In the presence of catalytic amounts of sodium methoxide, dimethylketene /3-lactone dimer is polymerized at moderate temperature to a polyester.3 At higher temperatures (above 100°), disproportionation to the cyclic trimer, hexamethyl-1.3,5-cyclohexanetrione, takes place.9 Addition of a stoichiometric amount of sodium methoxide to the lactone dimer generates the sodium enolate of methyl 2,2,4-trimethyl-3-oxovalerate. This reaction provides a convenient entry into certain ester anion chemistry that formerly required the use of a strong base like tritylsodium.10... 

The reaction mixture is then cooled to 35-40° and poured into a stirred solution of 230 g. of sodium chloride and 6.0 g. of sodium acetate in 600 ml. of water at 40°. The mixture is stirred for 15 minutes and then is transferred to a separatory funnel the layers are separated, and the lower, aqueous layer is discarded. The crude product is distilled at reduced pressure through a stainless steel spinning-band column (Note 5). The yield of the /3-lactone dimer of dimethylketene is 122-132 g. (61-67%) b.p. 69-71.5° (14 mm.) (Note 6). The product may be redistilled and the fraction boiling at 119.5-120° (150 mm.), 20d 1.4380, collected. 

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... 

The third type of cycloaddition results from the reaction of cyclopropanones with activated olefins. For example, dimethylketene adds to methyl substituted cyclopropanones affording the spiro lactones 153 a—c. 96,n8,i22b) Similarly the ortho ester 154 is formed from 1,1-dimethoxy-ethylene and 2,2-dimethylcyclopropanone 154 dimerizes to 155 upon standing. ng>... 

Oxaphospholans.—Full details have appeared of the reactions of the lactone and dione dimers of dimethylketen with a series of tervalent phosphorus esters and amides, and the postulated quinquecovalent intermediate (49) from the lactone dimer has been isolated in one case. Of potential mechanistic significance is the preferred migration of exocyclic substituents in the steps corresponding to (49) (50). 

Dimethylketene.u A modification of the ordinary ketene lamp is necessary to permit the pyrolysis of the comparatively high-melting di-methylketene dimer (m.p. 115°), which tends to sublime out of the reaction zone. The apparatus illustrated in Fig. 2 has a triple filament made... 

Mixed ketene dimers are also obtained by generating haloketenes in the presence of dimethylketene, by mixing of solutions of two different ketenes, and by cogeneration of two different ketenes from the carboxylic acid precursors Bis(trifluoromethyl)ketene does not dimerize thermally, but it reacts with Me2C=C=0 to form cyclobutanone and /3-propiolactone-type dimers ". The cycloaddition always proceeds across the C=C bond in the dimethylketene. In the reaction with ketene and methylketene, only the jS-propiolactone-type mixed dimers are formed. 

Disubstituted ketenes usually form the symmetric dimers. However, Staudinger and Klever also obtained a small amount of the j8-lactone dimers. This dimer is the only reaction product if the dimerization of dimethylketene is conducted in the presence of aluminum chloride as the catalyst Diphenylketene produces the -lactone dimer on addition of a catalytic amount of sodium methoxide . 

The [2+2] cycloaddition reaction of ketenes with olefins was already observed by Staudinger and his coworkers, who postulated the four-membered ring structure for the cycloadducts. The rate of reaction of ketenes with olefins is as follows diphenylketene > dimethylketene > butylethylketene > ketene. Because of its slow rate of dimerization butylethylketene is an especially useful reagent and its cycloaddition to slow reacting olefins can be forced by using elevated temperatures. 

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