trimethylenemethanes

https://doi.org/10.1351/goldbook.T06500
The @D01765@ CH2=C(C.H2)2, 2-methylenepropane-1,3-diyl, for which no Kekulé structure can be written, and its @H02891@ derivatives.
Source:
PAC, 1995, 67, 1307. (Glossary of class names of organic compounds and reactivity intermediates based on structure (IUPAC Recommendations 1995)) on page 1373 [Terms] [Paper]