Cyclopentadiene, -sigmatropic hydrogen shifts

The formation of Thiele s ester 6.217 is a remarkable example of several of the kinds of selectivity that we have been seeing in the last few sections, all of which can be explained by frontier orbital theory. The particular pair of cyclopenta-dienes which do actually react together 6.215 and 6.216 are not the only ones present. As a result of the rapid 1,5-sigmatropic hydrogen shifts [see (Section 6.3.1.3) page 197], all three isomeric cyclopentadiene carboxylic esters are present, and any combination of these is in principle possible. As each pair can combine in several different ways there are, in fact, 72 possible Diels-Alder adducts. 

As a result of the easy reversibility of these reactions there appears to be no report on a directed synthesis of a stereogenic carbon center by a [1,5] sigmatropic hydrogen shift in simple cyclopentadienes. 

The thermal [1,5] sigmatropic hydrogen shift is an allowed suprafacial process, which has been investigated at the SCF level in 1,3-pentadiene, in -hydroxyacrolein and in cyclopentadiene. ... 

Liu and coworkers reported the cycloisomerization of c -3-en-l-ynes to cyclopentadiene derivatives through 1,5-sigmatropic hydrogen shifts of catalytic ruthenium vinylidene intermediates (Scheme 44) [158]. 

In contrast to the [l,5]-sigmatropic rearrangement of pentadienes, that of cy-clopentadienes proceeds at an appreciable rate at room temperature along a suprafacial pathway [36, 37]. For example, the enthalpy of activation for the hydrogen shift in 5-methylcyclopentadiene, is 20. kcal/mol [38] and that in cyclopentadiene itself, which has been proven isotopically to be intramolecular, is 24. kcal/mol [39]. The latter reaction is illustrated in Fig. 8.8, in which the circled hydrogen atom is depicted as migrating from Ci to C2. 

The sigmatropic shift of hydrogen in, for example, cyclopentadiene (equation 3-23), has its equivalent in the much faster shift of the stannyl substitutent in a stannylcyclopenta-diene (equation 3-24).8... 

Because a [1,5] sigmatropic rearrangement involves three electron pairs (two IT bonds and one orbital-symmetry mles in Table 30.3 predict a suprafacial reaction. In fact, the [1,5] suprafacial shift of a hydrogen atom across two double bonds of a -ar system is one of the most conrmonly observed of all sigmatropic rearrangements. For example, 5-methyl-l,3-cyclopentadiene rapidly rearranges at room temperature to yield a mixmre of 1-methyl-, 2-methyl-, and 5-methyl-substituted products. 

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