Methylcyclopentadiene: hydrogen shift

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. 

Fig. 4 Stress tensor analysis of the reactant in the 1,5-sigmatropic hydrogen shift reaction from 5-methylcyclopentadiene to 1-methylcyclopentadiene. The eigenvectors of the stress tensor at the heps of the two active C-H bonds are shown. The stress tensor was computed at the B3LYP/ 6-31 lG(d,p) level. Further details can be found in the paper of Guevara-Gareia et al. [32]...

Since a [1,5] Slgmatropic rearrangement involves three electron pairs (two 7T bonds and one cr bond), the orbital-symmetry rules in Table 30.3 predict a suprafacial reaction. In fact, the fl,51 suprafacial shift of a hydrogen atom across two double bonds of a tt system is one of the most commonly observed of all slgmatropic rearrangements. For example, 5-methylcyclopentadiene rapidly rearranges at room temperature to 3rield a mixture of 1-methyl-, 2-methyl-, and 5-methyI-substituted products. 

It is interesting to note that this correlation allows one to estimate at least roughly the rates of 1,2-shifts of hydrogens in neutral compounds — methylcyclopentadienes (Sib) and (52i>) cf. Ref. — calculated values of AGh amount to 24.2 and 24.4 kcal/mole, experimental ones — to 22.5 and 23.7 kcal/mole, respectively 279,282)... 

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