Dicarbon Cumulenes

Ketenes 2 are generally synthesized by dehydrochlorination of carboxylic acid chlorides 1, which is best conducted using ahydrogen chloride scavenger, such as triethylamine. Mono substituted ketenes (RCH=C=0) are less stable than disubstituted ketenes (R2C=0=0). 

Cumulenes in Click Reactions Henri Ulrich 2009 John Wiley Sons, Ltd 

Interestingly, trapping of the unstable o -oxoketenes with suitable dienophiles affords the [4-1-2] cycloadducts, often in very high yields (see Section 4.1.4.4). 

Thioacylketenes are only stable at very low temperatures and even at -196 °C only thietons, their rearrangement products, can be detected by infrared spectroscopy (band at 1790 cm ). a-Imidoylketenes are obtained by flash vacuum pyrolysis of Meldrum s acid derivatives, but only an oxoketenimine (band at 2076 cm ) rearrangement product is detected at -77 K . Also, the unstable methyleneketenes (R2C=C=C=0) are obtained from suitable precursors. In the absence of trapping agents their cyclodimers are isolated (see Section 4.1.2). 

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Dicarbon Cumulenes 301 give the [2+2] cycloadduct, indicating that the [4+2] cycloadduct is a kinetic product . 

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