Angle strain cyclopropene

The C—C=C angle in alkenes normally is about 122°, which is 10° larger than the normal C—C—C angle in cycloalkanes. This means that we would expect about 20° more angle strain in small-ring cycloalkenes than in the cycloalkanes with the same numbers of carbons in the ring. Comparison of the data for cycloalkenes in Table 12-5 and for cycloalkanes in Table 12-3 reveals that this expectation is realized for cyclopropene, but is less conspicuous for cyclobutene and cyclopentene. The reason for this is not clear, but may be connected in part with the C-H bond strengths (see Section 12-4B). 

There is no doubt that the driving force for cyclopropene as the dienophile for a Diels-Alder reaction is the release of angle strain energy in the course of the reaction. This is demonstrated by its relatively low activation barrier. For example, cyclopropene reacts with cyclopentadiene and butadiene at 0°C or at room temperature, producing almost exclusively the endo cycloadduct [53]. This addition can be explored by computing activation barriers for two isomeric transition state structures. In this way, nonbonding interactions between diene and dienophile in two isomeric transition state structures can be closely evaluated. The reactivity and selectivity for two concurrent reaction pathways can also be computed. 

There is interest in cycloaddition of diazoalkanes to cycloalkenes for various reasons. Small rings (cyclopropene and cyclobutene) react easily and often in good yield as a result of their angle-strained double bond. Their products with diazomethane, 2,3-diazabicyclo[3.1.0]hex-2-ene (6.56) and 2,3-diazabicyclo[3.2.0]hept-2-ene (6.57) and their substituted derivatives are interesting for synthetic purposes, e. g., by azo-extrusion leading to ring contraction. 

Among the cycloalkenes, cyclobutene, cyclopropene and cylcohexene are most common. Cyclobutene is about 4 kcal/mol more strained than cyclopentene. The smaller bond angles mean more deviation from 120°, and this makes cyclobutene more reactive than cyclopentene. 

Cyclopropene Cyclopropene has bond angles of about 60°, compressing the bond angles of the carbon-carbon double bond to half their usual value of 120°. The double bond in cyclopropene is highly strained. 

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