Ketenes dimerisation

Silane thiones have been shown to be formed as intermediates in a range of reactions, and either oligomerize or react with other suitable acceptors. Thus, pyrolysing the propar-gylthiosilane 9 at 600 °C gives the silanethione which dimerises, while in the presence of ketene at 800 °C the 4-membered ring 10 results. The latter decomposes at 900 °C to give silanone and thioketene (equation 26)51. 

Ketenes are rarely isolated as they dimerise easily. Ketene itself 2 gives the lactone 5 but dimethylketene 6 gives the diketone 7. Other ketenes may give either type of dimer. Only a few ketenes, such as diphenyl ketene, are normally isolated. 

Unlike ordinary alkenes, ketenes do 2 + 2 cycloadditions with themselves—the dimerisation above—and with other alkenes.1 Reaction of dichloroketene with cyclobutadiene 11 to give the... 

The cycloaddition of ketenes to carbonyl compounds also shows the expected regioselectivity. Both HOMO,keI() c/LlJMO(kctcnc) and LUMO(ketone)/ HOMO(ketcnc) interactions may be important, but they lead to the same conclusions about regioselectivity. Lewis acid catalysis is commonly employed in this reaction presumably the Lewis acid lowers the energy of the LUMO of the ketene (or that of the ketone) in the same way that it does with dienophiles. Ketenes also dimerise with ease, since they are carbonyl compounds. The regiochemistry, whether it is forming a /3-lactone 6.256, 6.257 or a 1,3-cyclobutanedione 6.258, is that expected from the frontier orbitals of Fig. 6.39. 

Ketenes also dimerise with ease, since they are carbonyl compounds, and the regiochemistry, whether it is forming a /3-lactone 6.386 or a 1,3-cyclobutanedione 6.387, is that expected from the frontier orbitals of Fig. 6.51.886... 

We have met ketenes (1) already as intermediates in the Arndt-Eistert procedure (Chapter 31) for chain elongation. They are highly electrophilic at the curious sp carbon atom ( in 1). Nucleophiles give acyl derivatives (3) via enolate (2). Ketenes are unstable and eannot be stored. In the absence of nucleophiles they dimerise ketene itself gives ester (4) (which is available commercially) and disubstituted ketenes give cyclobutadiones (5). Mono-substituted ketenes may give either type of product. 

Diazocyclopentadienes lose nitrogen by the action of light or heat, generating carbene species, whose reactions with alkenes and with other caibene acceptors have been extensively studied. Cyclo-pentadienylidene has been directly observed in a photolysis of diazo-cyclopentadiene at very low temperature in a matrix [83,84]. IVhen warmed it dimerised or reacted with carbon monoxide present in the matrix to give a ketene ... 

Moving to a related type of [2 + 2] cycloaddition, not involving aldehydes or imines as partners, Calter and coworkers reported Cinchona alkaloid (TMSO-QN) catalysed asymmetric dimerisation of ketenes, generated in situ from the corresponding acid chlorides, yielding (3-lactones via a formal Claisen condensation (Scheme 14.29). The unstable ketene dimers were trapped with an alko>yamine to afford p-keto amides i.e. Weinreb amides) with variable yields and excellent enantioselectivities. 

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