Cyclopentadiene rings, rotation

The examples where jlSR provides Arrhenius parameters similar in magnitude to those measured by NMR and QENS are summarized in Table 7.3. The xSR measured dynamic process in all these cases is likely to be the same as that measured by the other two techniques, namely the metallocene (benzene or cyclopentadien) ring rotation. The detailed investigation of ferrocene illustrates how this can be rationalized on the basis of the occupancy of a nonbonding orbital, with respect to the dynamic process, by the unpaired electron of the radical species. These were named as the electron playing a passive role. 

For M = Rh, the H and H can be observed separately in the NMR spectrum at -20 C, but the peaks collapse to one at 57°C. The analogous C2F4 compound shows no collapse of the F spectrum up to 100°C. The rotational barrier also is increased by CH3 groups on the cyclopentadiene ring and by changing from Rh to Ir. These differences can be explained by steric and better tc back bonding factors, but they do not determine whether rotation is about the metal-olefin or C=C bond. 

The major resonance form leaves the electrons in the cyclopentadiene ring to make both rings aromatic. Therefore, the central bond has more single bond character and rotation around that bond is easily achieved. 

The relative reactivity of cyclopentadiene and ds-dichloroethylene toward triplet cyclopentadiene was found to be greater than 20 1 while that for cyclopentadiene and trans-dichloroethylene is less than 5 1. Thus the trans isomer is about four times more reactive toward the triplet cyclopentadiene than the cis isomer. An interesting temperature dependence of the product distribution of this reaction has been reported (Table 10.8). The data in Table 10.8 indicate that the relative amount of 1,4 addition [products (39) and (40)] is much more sensitive to temperature than 1,2 addition [products (35)—(38)], especially for the trans-olefin. The data also indicate that some rotation about the CHC1-CHC1 bond occurs in intermediate radicals derived from both cis- and trans-dichloroethylene. However, rotational equilibrium is not established at ring closure since the ratios of ds-dichlorocyclobutanes... 

Fig. 1.4. Rotational Zeeman spectra of the lio - 2xi rotational transition in propene, methyl-enecyclopropene, cyclopentadiene, and fluorobenzene. For better comparison, spectra calculated for the same magnetic field strength are shown. The calculation is based on the experimentally determined -values and susceptibility anisotropies. While the order of magnitude of the M 1 splitting (j -tensor contribution) remains essentially the same, the shifts of the M = 0 satellite and of the M = + 1 doublet due to the jj-tensor contribution increase almost by a factor of ten when going from the small open chain molecule propene to the aromatic ring fluorobenzene. These susceptibility shifts are indicated by the horizontal arrows to the right for M 1 shifts and to the left for M = 0 shifts.

Stereogenic plane A planar structural fragment that, because of restricted rotation or structural requirements, cannot lie in a symmetry plane. If the stereogenic plane is reflection variant, the element may be called a chirality plane. For example with a monosubstituted paracyclophane, the stereogenic plane includes the plane of the benzene ring. For a 1,2-disubstituted ferrocene, the disub-stituted cyclopentadiene lies in a chirality plane. The absolute configuration may be specified by either R, S or P, M. See also stereogenic element. 

If the reaction is concerted then there should be a high level of stereoselectivity, as is indeed observed. However, this does not rule out a two-step mechanism should rotation about the bonds in the intermediate be slow compared with the rate of ring-closure. In this connection, it is noteworthy that cycloaddition of trans-and c/5-l,2-dichloroethene to cyclopentadiene is completely stereospecific (3.5). A two-step mechanism via a biradical intermediate might have been expected to be sufficiently long-lived to allow some interconversion, resulting in a mixture of products. Addition of dichlorodifluoroethene to cis,cis- and / ran5,/ ra 5-2,4-hexadiene is, however, non-stereospecific and is thought to proceed by a two-step mechanism with a biradical intermediate. 

The p-acetonaphthone-sensitized reaction of cyclopentadiene with cis- and trans-but-2-enes has been investigated in some detail. It has been shown that both butene isomers give the same ratio of total ci> to total trans-adducts, and both also give the same ratio of [2 -I- 2]- to [2 + 4]-addition. The evidence is interpreted in terms of initial formation of two diastereoisomeric triplet biradicals in unequal amounts (and different unequal amounts from cis- and trans-but-2-enes). Both biradicals achieve rotational equilibrium before spin inversion and final ring-closure take place (see Scheme 18). Dialkylvinylene carbonates (146)-(148) have been prepared and, like the parent compound, undergo [2 + 2]cycloadditions with olefins in high yields. ... 

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