Cyclopentadiene 1,5 -sigmatropic shifts

It is a [1,5]-sigmatropic rearrangement. The figure T in the square brackets shows that the same atom is at one end of the new o bond as was at one end of the old c bond. One atom has moved in a 1,5 manner and these are often called [1,5]-sigmatropic shifts. This is often abbreviated to [1,5]H shift to show which atom is moving. This particular example is important because sadly it prohibits a most attractive idea. The cyclopentadiene anion is.very stable (Chapter 8) and can easily be alkylated. The sequence of alkylation and Diels-Alder reaction looks very good. 

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... 

However, the equivalent of 49 is known from the reaction of diphenyl ketene with cyclopentadiene. The adduct 50, formed at low temperature, rearranges through [3,3] sigmatropic shift to form what appears to be the [2 + 2] adduct 51. The reaction of 1-methoxybutadiene with diphenyl ketene to form 52 at low temperature is akin to [2 + 2] reaction involving nc=c of the ketene. However, this species also rearranges further by [3,3] sigmatropic shift to form 53, which is akin to [4 + 2] cycloaddition of the butadiene with 7rc=o °f the ketene, just as in 49 [ 18]. 

These [l,5]-sigmatropic shifts are not restricted to cyclopentadienes. In Chapter 35 we bemoaned the lack of Diels-Alder reactions using ,Z-dienes. 

Hoffmann and co-workers have made extensive kinetic studies of the [l,5]sigmatropic shift in pentakis(methoxycarbonyl)cyclopentadiene (88). First-order rate constants for the process (88) (89) were determined by n.m.r. at 373 K... 

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. ... 

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. 

---