Hydrogen Shift 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. 

The orbitals on both sides of the correspondence diagram are [2 x a -h a ] and the rearrangement via the suprafacial transition state in clearly allowed in agreement with both the Woodward-Hoffmann Rules and experiment. It is 

The alternative choice would exchange the symmetry labels of the two anasymmetrized tt orbitals, but the electron configuration would remain the same. 

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The [ 1,5] -hydrogen shift in cyclopentadiene can be observed at room temperature. At 60°C, migration is so fast that only one peak for all hydrogens appears in H-NMR (Scheme 8.2). 

Thermal 1,5 hydrogen shift of cyclopentadiene and 1,3,5-cycloheptatriene, and methyl shifts in the corresponding methyl-substituted derivatives and in methyl-1,3-... 

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. 

From the kinetic parameters it is evident that the [1,5] hydrogen shift in 1.3-cyclohexadienes demands a substantially higher activation energy (by 12-15 keal/mol)34-36 than the analogous reaction in 1,3-cyclopentadiene (vide supra), but the amount required is similar to that in acyclic (Z)-l,3-pentadienes. 

Methylbicyclo[2,l,0]pent-2-ene (300) rearranges thermally to 1-methyl-cyclopentadiene (301), showing the process as one ascribable to a [ 2g + concerted cycloreaction involving C-1—C-2 and C-4—C-5 bonds and not a biradical two-step process. The cyclopentadiene (301) was trapped as it was formed (43 °C) as its iV-phenylmaleimide adduct (302), a process which completes favourably with the 1,5-hydrogen shift in the diene itself. It is pointed out that this result undermines the usual either/or concerted/non-concerted question of mechanism in that an unusual or suppressed concerted mechanism may be brought to the fore when the more obvious concerted mechanism is prohibited. ... 

Due to the two electron-donating groups in the bicyclic product 150 and the unhydrolyzed precursor of 148, they should be quite reactive dienes in Diels-Alder reactions. However, such [4+2] cycloadditions were observed only for the cyclohexane-annelated cyclopentadienes 151b, which equilibrate with the more reactive isomers 154 by 1,5-hydrogen shifts (Scheme 33). The [4+2] cycload-... 

Note that the first example bears out the stereochemical prediction made earlier. Only the two isomers shown were formed. In the second example, migration can > continue around the ring. Migrations of this kind are called circumambulatory rearrangements. Such migrations are known for cyclopentadiene, pyrrole, and phosphole derivatives.[1,5] Hydrogen shifts are also known with vinyl aziridines." ... 

Niggli and Neuenschwander294 studied the reaction of fulvene (461) with cyclopen-tadiene. The main product fraction consisted of three 1 1 adducts, as illustrated in equation 138. Diels-Alder Adducts 462 and 463 resulted from attack of cyclopentadiene at the endocyclic and exocyclic double bonds of fulvene, respectively. The formation of 464 was rationalized by a [6 + 4] cycloaddition reaction followed by two [1,5] hydrogen shifts. It was stated that due to the absence of electron-donating and electron-withdrawing groups on both triene and diene, fulvene may have reacted via its HOMO as well as its LUMO. 

One of the most common [1,5] hydrogen shifts takes place in cyclopentadienes, where it is constrained to be suprafacial. Because the atoms C-l and C-5 are held close together by a a-bond, the reactions 5.12 —> 5.13 — 5.14 take place even at room temperature. It is important to recognize... 

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. 

The ability of an oxyanion substituent to facilitate 1,5-hydrogen sigmatropy was demonstrated by Paquette et al. in 1980. For example, rearrangement of the potassium salt of 2,4-cycloheptadienol occurs more than 1(P times faster than the rearrangement of the corresponding alcohol (equation 12). Accelerated 1,5-shifts of alkyl, vinyl, aryl and cyclopropyl groups in cyclopentadiene derivatives e.g. equation 13) have also been reported. ... 

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