Ketene dimers mixed

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... 

If ketene dimer is added to an inert solvent oontaining a little p> ridine, .47 .w, W.4B1 trimethylainine or alkaH pheno late, or if it is treated with an add, - dehydroaoetic add is formed. IIub ako occurs during various reactions of ketene dimer, with mixed or symmetrical oiganomagnCBnim compounds, and even during distillation under insufficiently low pressuro ... 

Stereoselective formation of unsymmetrical ketene dimers is conveniently achieved by a process where a more reactive ketene is generated in the presence of a less reactive ketene in the presence of a chiral catalyst. The catalyst adds to the more reactive ketene and the resulting reactive eno-late reacts with the less reactive ketene to form the unsymmetrical dimer. By this process methylketene formed by dehydrochlorination in methylene chloride in the presence of the chiral catalyst TMS-quinine reacts with diphenylketene forming the mixed dimer 44 (Eqn (4.24)). 

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. 

The most general method for preparing tlic dimers of the aJkyl-ketenes, and possibly the mixed dimers, - consists of treating an acid chloride with A carefully selected tertiary It ie often chumed that the amino hydrochloride foimed catsjyxes the dimerization of the elkylkeitene, but [Pg.341]

KETEN or KETENE (463-51-4) Slowly hydrolyzes in water, releasing ammonia and forming acetate salts. This process is accelerated with heat in an acid or caustic environment. Reacts vigorously with water and a wide variety of organic compounds. Forms explosive compound when mixed with hydrogen peroxide. Can dimerize to diketene even at low temperatures diketene forms an explosive mixture with air (flash point 90°F/32°C). Reacts with ammonia, forming acetamide. Reacts violently with oxidizers, forming unstable and explosive diacetyl peroxides violent polymerization is possible. 

Mixed dimers are obtained in good yield by the generation of two different ketens in situ by dehydrohalogenation of acid chlorides. These cross-additions lead regio-... 

Mixed aldoketene dimers have been prepared by treating mixtures of two acid chlorides with tertiary aliphatic amines. Presumably the two moncxneiic ketenes are formed and combine to give the simple and mixed dimers. Three of the four possible products have been isolated from the treatment.of a mixture of acetyl and lauroyl chlorides with tiiethylamine. 

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