Ketene cycloaddition reactivity

Why do ketenes participate in [2+2] cycloadditions with their $\ce{C=C}$ bond and not the $\ce{C=O}$ bond? Can the reactivity be reversed somehow?




Ketenes can participate in [2+2] cycloadditions with their $\ce{C=O}$ double bond. Ketene dimerization is the best known example of such a cycloaddition.


![enter image description here](https://i.stack.imgur.com/OcWnZ.png)


([image source](http://en.wikipedia.org/wiki/Ketene_cycloaddition))


As you add substituents to the ketene $\ce{C=C}$ double bond, you begin to favor cycloadditon across the carbon-carbon double bond at the expense of cycloaddition across the carbonyl double bond.


Some Diels-Alder cycloadditions also involve adding across the ketene carbonyl.


![enter image description here](https://i.stack.imgur.com/Tj0SC.gif)


([image source](http://pubs.rsc.org/En/content/articlelanding/2006/ob/b611311g#!divAbstract))


With both the [2+2] and [4+2] cycloadditions often the kinetic product involves cycloaddition across the carbonyl, but on mild heating this cycloadduct can rearrange to the product from cycloaddition across the carbon-carbon double bond, the thermodynamic product.


A very nice review of the synthetic and mechanistic aspects of ketene cycloadditions can be found [here](http://evans.rc.fas.harvard.edu/pdf/smnr_1999-2000_Janey_Jake.pdf).


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