Bredt's rule

https://doi.org/10.1351/goldbook.B00732
A double bond cannot be placed with one terminus at the @B00736@ of a bridged ring system unless the rings are large enough to accommodate the double bond without excessive @S06037@. For example, while bicyclo[2.2.1]hept-1-ene is only capable of existence as a @T06451@, its higher homologues having a double bond at the @B00736@ position have been isolated: e.g.
B00732.png
Source:
PAC, 1994, 66, 1077. (Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994)) on page 1077 [Terms] [Paper]