Cyclopentadiene 1,5-shifts

The first step is a simple displacement of two molecules of carbon monoxide. In the second step the acidic hydrogen of 7r-bonded cyclopentadiene shifts to the iron atom to give w-cyclopentadienyliron dicarbonyl hydride, a known compound. Two molecules of this iron hydride then add hydrogen to cyclopentadiene to form cyclopentene and the dimer of w-cyclopentadienyl-iron dicarbonyl. This reaction proceeds at temperatures between 150° and 220° C. Above 220° C, the remaining carbon monoxide molecules are displaced by a second molecule of cyclopentadiene forming ferrocene [42). 

On the basis of the studies described in the preceding chapters, we anticipated that chelation is a requirement for efficient Lewis-acid catalysis. This notion was confirmed by an investigation of the coordination behaviour of dienophiles 4.11 and 4.12 (Scheme 4.4). In contrast to 4.10, these compounds failed to reveal a significant shift in the UV absorption band maxima in the presence of concentrations up to one molar of copper(ir)nitrate in water. Also the rate of the reaction of these dienophiles with cyclopentadiene was not significantly increased upon addition of copper(II)nitrate or y tterbium(III)triflate. 

The preparation of cyclopentadienes with up to four trimethylsilyl groups can be performed easily on a large scale starting with monomeric cyclopentadiene by repeated metalation with n-butyllithium and treating the resulting anion with chlorotrimethylsilane [84], Any complication caused by formation of regioisomers does not occur, since all trimethylsilyl-substituted cyclopentadienes are fluxional by virtue of proto- and silatropic shifts [85], Upon deprotonation with n-butyllithium the thermodynamically most favorable anion is formed selectively (Eqs. 20, 21). Thus, metalation of bis(trimethylsilyl)cyclopentadiene 74, which exists preferentially as the 5,5-isomer, selectively affords the 1,3-substituted anion 75. Similarly, tris(trimethylsilyl)cyclopentadiene 76, which is found to be mainly as the 2,5,5-isomer, affords the 1,2,4-substituted anion 77. 

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." ... 

In both cyclic and acyclic dienes which can achieve the necessary geometry the [1, 5] shift is commonly observed because the activation energy is lowest for the transition states involving minimal distortion. This is particularly so in cyclopentadienes and indenes. The preference for [1, 5] over [1, 3] shifts is demonstrated by thermal rearrangement of 1-duterioindene. At 200°C, deuterium became scrambled over all three non-benzenoid carbons. 

In the case of 1,3-butadiene, the chemical shifts of inner (H2, H3) protons and outer (HI, H4) is large, while in the case of cycloalkadienes (e.g. 1,3-cyclopentadiene and 1,3-cyclohexadiene), the difference is very small. It is interesting to note that in 1,3,5-cycloheptatriene, the chemical shifts of three kinds of olefinic protons are very diverse. The effect of the ring size and in the chemical shifts of radialenes was also included. 

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. 

Using quantum-chemical calculations (MINDO/3, MNDO) the migrations of SH and OH groups in the cyclopentadiene system were discussed138. The calculations have confirmed a preference for 1,2-shift with j2-structure of the transition state. 

The other necessary reaction for a BN to VN isomerization is a well precedented 1,5 H shift to convert the linearly conjugated substituted cyclopentadiene (LCC) into the cross conjugated cyclopenta-diene (CCC). The relative lability of BN relative to VN is thus a reflection of the stabilizing conjugation of the substituent in the vinyl isomers and the fact that the formation of LCC from BN is more favorable than the formation of CCC from the retro Diels Alder of VN. The relative energetics for all of these processes is represented in a combined reaction profile diagram shown in Figure 1. 

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-... 

According to new data for the vinylcyclopropane-cyclopentadiene rearrangement,372 particularly concerning the stereochemistry of the process, the [1,3] sig-matropic carbon shift proceeds through all four stereochemical reaction paths,... 

Reactions of the HNiL3CN complex with 1,3-cyclopentadiene, 1,3-cyclo-hexadiene, and 1,3-cyclooctadiene gave intermediates with decreasing stabilities in that order the 1,3-cyclooctadiene intermediate was not spectroscopically observable. The cyclohexadiene adduct was shown to be the cyclohexadienyl complex 12 by its proton spectra, with resonances of H , Hb, and —(CH2)3— at 14.53, 6.06, and 8.47, respectively these values are close to the chemical shifts found earlier (51) for 13 14.52,5.86, and 8.48. The reaction of DNi[P(OMe)3]X with cyclopentadiene gives 13-d, with addition of D and Ni to the same side of the ring (52). Backvall and Andell (55) have shown, using Ni[P(OPh)3]4 and deuterium cyanide (DCN), that addition of D and CN to cyclohexadiene is stereospecifically cis, as expected for jt-allyl intermediate 12. 

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... 

SCHEME 12. Silatropic shifts and deprotonation/silylation reactions in silylated cyclopentadienes... 

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