Sigmatropic shift reactions 1,3-pentadiene

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. 

How are sigmatropic shifts formally described The first step is to identify the bond that is broken in the reaction and the bond that is made. In other words, write an arrow formalism. The bond that breaks in 1,3-pentadiene could he designated the l-l bond. Remember that these numbers have nothing to do with the lUPAC name. Next, note where the new bond is formed. In Figure 20.32a, the new bond is [1,5], in (b) it is [2,3], and in (c) it is [3,3].The numbers denoting the shift are always enclosed in brackets and separated by a comma. 

Stanton and Merz examined more complex organic and organometallic reactions. Of particular interest is their work on the Diels-Alder reaction, the [1,5] sigmatropic shift of 1,3-pentadiene, and the reaction of ZnOH and CO2 to produce Zn + and HCOf. These studies used the gradient-corrected Becke exchange and Perdew correlation functionals. All calculations employed double- plus polarization level basis sets. 

For the D + H2 reaction, the VTST/ST calculations mentioned above [5] indicate that tunneling increases the rate constant by factors of 3, 7, and 60 at 400, 300, and 200 K, respectively. This confirms that tunneling is an important ingredient in accurate reaction rate calculations. Table 1 shows similar rate enhancements for several other gas-phase hydrogen-atom transfer reactions we have studied in recent years, namely, the H + H2 reaction [16], the 1,5 sigmatropic shift in cis-pentadiene [7b], the CF3 + CH4 reaction, [17] and the OH + CH4 reaction [18]. I anticipate similarly large below-threshold tunneling contributions for many H, H+, and H transfer reactions in both the gas phase and solution. 

The kinetics of [1,5] hydrogen shifts in some alkyl-substituted (Z)-l,3-pentadienes have been determined23 50-52. From kinetic analyses53,54 of substituent effects on the reaction, several trends are evident. In the temperature range studied (130-250 l C), the [1,5] sigmatropic hydrogen shift is usually reversible. Only if there is an appreciable thermodynamic stabilization of the product is the reaction found to be unidirectional (Table 2, entries 7, 13, 16). 

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