And Bredt’s rule

By now many sorts of reactive intermediates, such as free radicals, carbocations, carbenes, benzynes, and Bredt s rule violating olefins, have been generated and isolated under conditions permitting full structural characterizations. Theory-based structural parameters are generally in nearly perfect agreement with experimentally determined values. 

Important to know the reason than to know the name, and Bredt s rule Is simply a consequence of the strain Induced by a planar bridgehead carbon. 

The impossibility of planar bridgehead carbons means that double bonds can almost never be formed to bridgehead carbons in bicyclic systems. This principle is known as Bredt s rule, but, as with all rules it is much more important to know the reason than to know the name, and Bredt s rule is simply a consequence of the strain induced by a planar bridgehead carbon. 

Figure 24, There are two linking patterns for each TRIAD core (a), subject to constraints due to ring-fusion geometry (b) and Bredt s rule, etc. (c).

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