Cyclopropane molecular structure

The molecular structure of the parent compound was investigated in the vapor and in the solid phase using X-ray, XN and GED methods. The reported data are shown in Table 16. In both phases a clear bond length separation could be detected with a localized three-membered ring and its three adjacent double bonds. The symmetry-equivalent cyclopropane bonds are rather long in C3v-symmetric BUL (1.533-1.542 A), which can be explained by the common electron-withdrawing effect of the 7r-systems in a. svM-ciinal conformation. For comparison, the unaffected bonds in unsubstituted cyclopropane are 1.499 A in the crystal and 1.510 A in the gas phase. Therefore, the bond lengths in BUL... 

The two most commonly used derivatization methods for exohedral functionalization are the nucleophilic cyclopropanation with malonates (Bingel, 1993) and the formation of fulleropyrrolidines (Maggini et al., 1993). Both of these protocols have been used extensively to produce water-soluble fullerenes for biomedical applications. Other stable water-soluble fullerene adducts have also been reported (Hirsch and Brettreich, 2005). Sections 3.2.2-3.2.5 will give a short overview on the state-of-the-art of water-soluble fullerene derivatives and outline some general trends for designing such molecular structures. 

In an approach to the stereocontrolled creation of the acyclic side-chain of tetracyclic triterpenoids and other natural products, Trost and his colleagues have converted the acyclic starting compound (29) into the cyclopropanoid intermediate (31) via the diazo-ketone (30). The key step in the scheme is the cleavage of the cyclopropane with lithium dimethylcuprate to give (32). The stereochemistry at C-7 is determined by the configuration of the double bond in (29). The c.d. and u.v. spectra of a series of triterpenoid olefins have been measured. The Scott-Wrixon rules can be used to correlate the sign of the c.d. curves with molecular structure. A... 

Cyclopropane (D3 , symmetry) has a degenerate pair of in-plane e orbitals (S, A). Vertical ionization leads to a doubly degenerate E state, and JT distortion results in two nondegenerate electronic states, A and 82 (Cav symmetry), corresponding to two different molecular structures. The Ai state (orbital S singly occupied) corresponds to a structure with one lengthened C—C bond it is lowest in energy for many cyclopropane radical cations (Fig. 6.10). 

The results for cyclopropane are given as an example for the poor correlation in these highly strained molecules. These extreme molecular structures have an electron distribution that cannot be adequately described using the approximations of the bond polarization model and must be subjected to ab initio calculations. 

L. C. Snyder and S. Meiboom, Molecular structure of cyclopropane from its proton NMR in a nematic solvent.. Chem. Phys., 1967, 47(4), 1480-1487. 

Figure 1-2. Molecular structures of cyclopropane- and aziridine-based analogs of epothilones. Numbers in parentheses are ICso-values for growth inhibition of the human colon carcinoma cell line HCT-116. Data are from Johnson et al. (12 and 13) and Regueiro-Ren et al. " (14 and 15).

Cremer, D. Gauss, J. Theoretical determination of molecular structure and conformation. 20. Reevaluation of the strain energies of cyclopropane and cyclobutane - CC and CH bond energies, 1,3 interactions, and o-aromaticity, J. Am. Chem. Soc. 1986, 108, 7467-7477. 

The molecular structure of methylenecyclopropane has been determined by micro-wave spectroscopy 129) and may be compared with that of l-(diphenylmethylene)-cyclopropane, which has been established by a three-dimensional X-ray analysis 130> (Fig. 5). 

Cyclopropane is somewhat less stable than cyclohexane, CgHi, which has a molecular structure with a zigzag ring of six carbon atoms with no bending of bonds and with the stable (staggered) orientation for each of the six carbon-carbon bonds, as you will see when you turn the page. 

This chemical lability may be understood better if the molecular structure of ptaquiloside, rather peculiar in various respects, is examined closely. Firstly, by looking at a 3D representation of Fig. (4) one can discern that the cis-AB ring fusion forces out the spyro-cyclopropane in such a way that, while it contributes to block the approach of possible nucleophiles on the carbonyl carbon from the [1-side, at the same time it exposes like a tail the cyclopropane methylenes to possible reactants in the medium. The approach to the carbonyl carbon is further impeded from... 

Another aspect of conformational flexibility in general though is the effect this may have on the properties which are computed in order to describe molecular structures. As an example of this, consider the pyrethroid structure shown in Figure 8.3. There are five flexible torsion angles in the two side chains of the cyclopropane ring, although torsion 3 is effectively fixed as it has some double bond character in resonance with the carbonyl oxygen. 

Bastiansen O, Fritsch FN, Hedberg K (1964) Least-squares refinement of molecular structures from gaseous electron-diffraction sector-microphotometer data. III. Refinement of cyclopropane. Acta Crystallogr 17 538... 

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