endo / exo Preferences for Double Bonds in Three‐Membered Rings Including Phosphorus Compounds

Depending on the substituent and the heteroelements of an unsaturated three‐membered ring system of the methylenecyclopropane/1‐methylcyclopropene type, the double bond may be preferentially exo‐ or endocyclic. This preference may be considered as a combination of substituent and ring‐strain effects. The substituent effect is a composite one, encompassing both exocyclic olefinic strain and tautomeric preference. The preference for exocyclic unsaturation increases in the order =CH 2 < =NH < =PH < =O for the considered substituents. The higher the ring‐strain, the higher the propensity for exocyclic unsaturation. The low‐strain 2 H ‐phosphirenes favor endocyclic unsaturation, except for the 3‐OH‐substituted species where the strong C=O double bond is the determining factor. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003)