endolexo 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 methylene-cyclopropane/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 =CH2 < NH < =PH < =O for the considered substituents. The higher the ring-strain, the higher the propensity for exocyclic unsaturation. The low-strain 2H-phosphirenes favor endocyclic unsaturation, except for the 3-OH-substituted species where the strong C=O double bond is the determining factor. ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003).