Methylketene, dimerization

Which of the two plausible structures given for methylketene dimer is more consistent with its observed pK of 2.8 Why ... 

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... 

The Calter group next sought to demonstrate the utility of this asymmetric dimerization process by applying their methodology in the preparation of high-value synthetic intermediates. Taking the established methylketene dimerization procedure, in situ treatment of the -lactone 100 with a secondary amine followed by further in situ selective reduction provided access to aU four stereoisomers of the target dipropionate synthon 102 or 103 (Scheme 3.21) [56]. 

Further work from Calter and Bi extended this successfid methylketene dimerization to the synthesis of an antibiotic subunit from the Pamamycin family at the time an unsynthesized class of natural products [57]. Extensions of the methodology demonstrated the stereocontrolled synthesis of polypropionates [58] via an in situ aldol reaction. Both anhydrides 106 (Scheme 3.23) and acid chlorides 1 (Scheme 3.24) were exploited as suitable ketene precursors. 

Dimerization of methylketene is catalyzed by an amine, trimethylsilylquinine, to give the P-lactone enantioselectively (Scheme 27) [129]. The catalyst amine attacks the ketene to form an ammonium enolate, an electron donating alkene. The donor is strong enough to react with a ketene across the C=0 bond. That is why the P-lactone is obtained instead of the 1,3-cyclobutandione, the uncatalyzed dimerization product of the monosubstituted ketene. 

Nach Reid und Groszos (165) liegt das dimere Methylketen (III) im festen Zustand nicht als Enol, sondern als Diketon vor, da in seinem Ultrarotspektrum die Banden der charakteristischen OH-Frequenz niclit nachgewiesen werden konnen. 

Ketene dimerization is the principal synthetic route to 4-methylene-2-oxetanones. This reaction proceeds very satisfactorily for ketene and methylketene, but disubstituted ketenes dimerize only to cyclobutane-1,3-diones. The cycloaddition reaction of r-butylcyanoketene to ketene and to methylketene gives a-cyanoalkylidene-/3-lactones in about 40% yield in addition to the cyclobutane-1,3-dione dimer of f-butylcyanoketene. A mechanism has been proposed for the formation of both types of dimers from a common zwitterionic intermediate (equation 111), with the relative amount of each product determined by the configurational equilibrium of the intermediate (80JOC4483, 75JOC3417). 

Exercise 17-41 1-Propen-1-one (methylketene) forms a dimer, C6H802, by [2 + 2] cycloaddition with itself. The nmr spectrum of the dimer is shown in Figure 17-5. Assign a structure to the dimer consistent with the spectrum. 

Which of the two possible structures for the dimer of methylketene is in best accord with the observed pA of 2.8 Explain. 

Scheme 117 Cinchona alkaloid-catalyzed dimerization of methylketene (549)...

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)). 

Relatively few such preparations have been reported. -The dimers of ketene, methylketene, and dimethylketene have been converted to the monomers in yields of 86-100% by decomposition over hot filaments or in hot (550-600°) tubes. Ethylcarbethoxyketene has been prepared in 80-90% yield by heating the dimer under a pressure of 15 mm. in an oil... 

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. 

A trimer of methylketene with the cyclobutenone structure 85 is obtained in high yield when methylketene is generated from the corresponding carboxylic acid bromide with diisopropylethylamine on attempted [2-1-2] cycloaddition reaction with aldehydes This reaction presumably proceeds via the symmetrical dimer as the intermediate. 

Tertiary Amines It is significant to note that in both Pracejus asymmetric ketene alcoholysis and Wynberg s ketene-chloral cycloaddition, the catalysts of choice were both members of the cinchona alkaloid family that promoted the desired asymmetric process in remarkably high levels of stereocontrol. In 1996, Calter reported a catalytic, asymmetric dimerization of methylketene 89 using cinchona alkaloid catalysts to afford enantiomerically enriched (3-lactones 90 that were reduced in situ using lithium aluminum hydride (LiAlH4) to afford l-hydroxy-3-ketones 91 (Scheme 3.20) [54]. 

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