Ketene, acylating reactions dimerization

Acyl sulfenes, like all sulfenes, prefer to participate as 2ir components of [2 -I- 2] or [4 + 2] cycloadditions (Chapter 5). Nonetheless, a range of [4 -I- 2] cycloaddition reactions of acyl sulfenes have been described - (Scheme 8-XII), including their 47t participation in dimerization reactions and reactions with imines, carbodiimides, ketenimines, 1-azirines, vinyl ethers,and ketenes. The reactions often provide mixtures of [4 + 2] and [2 + 2] cycloadducts, and the observed course of the reaction usually depends on the reaction conditions. Consequently, many of the observed [4 + 2] cycloadditions of acyl sulfenes proceed by a stepwise, polar addition-cyclization reaction. 

Ketenes can be prepared by treatment of acyl halides with tertiary amines. The scope is broad, and most acyl halides possessing an a hydrogen give the reaction, but if at least one R is hydrogen, only the ketene dimer, not the ketene, is isolated. However, if it is desired to use a reactive ketene in a reaction with a given compound, the ketene can be generated in situ in the presence of the given compound. ... 

Route (b), based on an aliphatic Friedel-Crafts reaction, gives a good yield of (31) which is duly acylated with ketene dimer. 

Commencing with an acylation step, Mukaiyama et al utilized keten dimer to effect an annulation reaction and form the spirofuran (25). 

Monoalkylketenes are also prone to dimerization, but dialkylketenes have longer lifetimes. The remarkably crowded and unreactive di-teri-butylketene 5 bears strong steric protection and was first prepared in 1960 from the acyl chloride using a strong base (Eqn (4.4)), and identified by the characteristic ketenyl IR absorption. The dehydrochlorination reaction has also been carried out with triethylamine as the base using ultrasound in 86% yield or by reaction with neat tri- -butylamine at 80 °C, also in 86% yield. The use of the aldehyde i-Bu2CClCH=0 as an alternative precursor to 5 by an elimination reaction has also recendy been reported. This ketene is stable indefinitely as a neat liquid and reacts slowly with and there is no... 

Acylketenes generated by Wolff rearrangement undergo uncatalyzed Friedel-Crafts type reactions with different nitrogen heterocycles, including N-methylpyrrole, as in the example shown (Eqn (4.126)). The acylation was also successful with dihydrofuran and dihydropyran. At higher temperatures ketene dimerization also occurred. 

Dimers of other ketenes also can be prepared from the monomers, but frequently it is more convenient to prepare the dimers directly by extending the reaction time or increasing the reaction temperature. The most convenient preparation of higher aldoketene dimers is that of dehydro-halogenation of acyl halides with tertiary aliphatic amines." ... 

Theoretical studies [59] indicate that the lowest unoccupied molecular orbital (LUMO) of such molecules is localized primarily on the acyl carbon atom, similarly to the situation in Fischer complexes. An example of such a compound is shown in Fig. 23.9, where the shortness of the Th-O distance (2.37(2) A) relative to Th-Cg (2.44(2) A) is unprecedented for a dihapto-acyl. A second example of an actinide dihapto-acyl is shown in Fig. 23.10. It should be noted that the orientation of the C-O vector is in the opposite direction from that in Fig. 23.9. The relative magnitudes of the Th-O and Th-C distances appear to reflect both the orientation of the C-O vector and conjugation with the arene n system. The intricate chemistry exhibited by actinide dihapto-acyls is summarized in Fig. 23.11. Important reactions include C-C coupling to form monomeric (10) or dimeric enediolates [57,58,60,61], isomerization to yield enolates (//) [60,62], catalytic hydrogenation to yield alkoxides (/2) [63], CO tetramerization to form dionediolates (13) [62, 64], coupling with ketenes 14), coupling with CO and phosphines 15) [62, 64], and addition to isocyanides to yield ketenimines [62, 64] 16). 

Oxidative cyclization of 1,3,4,6-tetraketones RCOCH2COCOCH2COR by lead tetra-acetate to a 4-hydroxy-a-pyrones (144) has long been thought to involve oxidative fission of the tetraketone to 2 moles of acyl-keten (RC0CH=C=0), which then dimerize. However, incontrovertible evidence has now been presented" for an intramolecular reaction (Scheme 33) involving a 1,2-acyl migration in the initially formed carbene (143). 

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