Cyclopropenes ring strain

On the other hand, the observation that hydrogenation of cyclopropene is much more exothermic than would be expected for a typical cycloalkene, reveals the effect of ring strain. 

This method constitutes a convenient synthesis of substituted tricyclo[2.2.0.02,6]hexane derivatives. It is surprising that the ring strain associated with these derivatives would permit their preparation at such high temperatures. A homologous reaction involves the intramolecular [2 + 2] cycloaddition of 2-vinylphenyl substituted cyclopropenes 5 which give benzotricy-clo[3.2,0.02,7]heptenes 6.74 This reaction also proceeds by sensitized photolysis but gives a more complex mixture. 

Step 3 Regioselective E2-elimination (no cyclopropene is formed due to ring strain). 

Many chemists once believed that a cyclopropene could never be made because it would snap open (or polymerize) immediately from the large ring strain. Cyclopropene was eventually synthesized, however, and it can be stored in the cold. Cyclopropenes were still considered to be strange, highly unusual compounds. Natural-product chemists were surprised when they found that the kernel oil of Sterculia foelida, a tropical tree, contains sterculic acid, a carboxylic acid with a cyclopropene ring. 

The cyclopropenyl cation is the simplest aromatic system, and thus of some theoretical interest. In addition, the chemistry of cyclopropane derivatives is full of Interesting rearrangements to other novel structures,9 reflecting the great strain energy Of the cyclopropene ring. 

In this 3-phcnylalkyne system the triple bond becomes incorporated into a cyclo-propene ring, whereas from the enynes 10 and 11 there is no sign of a vinylcyclo-propene product. The preference for reaction to take place at the alkene unit rather than at the alkyne unit in the photochemistry of enynes is seen again in the photochemistry of enepoly-yne chlorides, where only cyclopropyl chlorides are produced (equation 18). This preference is probably a reflection of the lower strain energy in a cyclopropane than in a cyclopropene ring. 

The effect of the cyclopropene double bond on acidity of the allylic (C3-H) protons is striking in comparison to the situation for the vinyl (Cj-H) protons. As the data in Table 1 reveal, cyclopropene is at least 10 pX units less acidic than cyclopropane. On classical grounds, resonance stabilization of the cyclopropenyl anion (D31, structure. Scheme 1) should provide an acid-strengthening effect however, increased ring strain associated with planarization of the final ring carbon could offset this stabilization. If 7c-conjugative effects are considered unimportant, then by analogy to the cyclopropyl anion the nonplanar Q... 

The rapid and quantitative isomerization of 1-methylcyclopropene , and related methyl substituted cyclopropenes to the corresponding methylenecyclopropanes is indicative of relief of ring strain in the transition state which is predominantly carbanionic... 

As has been discussed briefly in Section III above, the ring strain of the cyclopropenes is accommodated, in part, by somewhat unusual bonding to the substituents. Structural and spectroscopic studies suggest that the vinylic C-H bonds have ca. 44% s character and that this is approximately constant in bonding to other substituents. Consequently aspects of the chemistry of the cyclopropenes are expected to reflect this with enhanced acidity of the vinylic proton. Cleavage of an exocyclic bond at C(3) will result in the cyclopropenyl cation, radical or anion with consequent interaction with the n bond and perturbation of the energy of the system. By comparison addition to the n-system to yield a cyclopropane relieves ring strain by ca. llOkJmol" ... 

As noted earlier (Section I V.A.5), the addition of nucleophiles to cyclopropenes relieves ring strain. Electrocyclic opening of the anion generated in this manner competes with capture by an ambient electrophile (E" ) (equation 85). Cyclopropene 278 reacts with... 

Because of the reduction in double bond character, and hence ring strain, complexation of cyclopropenes to metal centres takes place readily. Although no simple relationship between strain and complexing ability has been developed, the argentation constant for cyclopropene > 10 ) as determined from the expression... 

It has already been mentioned that cycloaddition across the cyclopropene double bond reduces the ring strain by about 109 kJ/mol. [2+2] cyclo- and -codimerizations of cyclopropenes should therefore be thermodynamically favored processes (see also Sect. 3.2). Indeed, when catalyzed by Zeolites70), Lewis acids71) and transition metal complexes 30,72 74) anii-tricyclop.l.O.O hexane derivatives can be synthesized efficiently (Eq. 15). 

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