Cycloalkenes Cyclobutenes, Cyclohexenes

In the case of cycloalkenes, it is found that the size of the ring is an important factor in product distribution. Photo [2 -I- 2] cycloaddition of cyclohexenone derivatives (4) to carbomethoxy cyclobutene [47], cyclopentene [48], and cyclohexene [49] (see Scheme 5) demonstrates a gradual reversal of regioselectivity from head-to-head to head-to-tail adducts as indicated in Table 2 [50]. This result of head-to-tail products is not consistent with the dipole-dipole interaction theory. Stability of biradical intermediates is suggested to explain the reversed regioselectivity. 

The bond angle of cycloalkenes can be used to estimate the LUMO and predict reactivity. Smaller bond angles correlate with lower LUMOs and greater reactivity. Thus the bond angles and reactivities for cycloalkenes follow the same order cyclopropene > cyclobutene > cyclopentene = norbomene > cycloheptene = cyclooctene > cyclohexene. Cyclopropene is the most reactive and cyclohexene is the least reactive. 

A student wished to prepare of series of cyclobutanes that can be considered to be dimers of cycloalkenes. Sensitized photolysis of the cycloalkenes proceeded efficiently for cyclopropene, cyclobutene, cyclopentene, cyclohexene, and cyclohcptene. However, cyclooctene produced little or no dimer. Rationalize this result. 

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