Cyclopentane, chemical shifts

The use of 13C-NMR spectroscopy in stereochemical assignment of disubstituted cyclopentanes has been investigated using dimethylcyclopentanes and mcthylcyclopentanols495. In 1,2-disubstituted derivatives the chemical shifts of the substituted and the methyl carbons are significantly smaller (2 to 5 ppm) in the cis than in the trans series. In 1,3-disubstituted cyclopentanes the sequence is the same, but the differences are often so small that a reliable assignment is possible only if both isomers are available. 

Although pyrrolidines are important structural fragments in alkaloid chemistry, little has been documented on comparisons of diastereomeric species. It was found that the 13C characteristics of methylated pyrrolidines are similar to those of cyclopentanes (see Section 4.1.4.3., Cyclopentanes), i.e., the chemical shifts of the substituted ring carbons in cis-2,3-dimethylpyrrolidine are clearly smaller than in the rrani-diastereomer503. Corresponding effects in 2,4-dimethylpyrro-lidines are much less pronounced503. 

Table 4.6. Methyl Substituent Effects on 13C Chemical Shifts of Cyclopentane and Methylcyclopentane Resonances [210]...

Fig. 2. The chemical shift (in cps, 60 MHz) upheld from cyclopentane of the methyl protons vs. the mole ratio (CH3)/[(CH3) + (C2H3)]. The lower curve was obtained at + 40°, while the upper one was obtained at — 50°.

The observation that the chemical shift of added methyl chloride in Al2Me6-cyclopentane solutions is very close to the chemical shift of the same concentration of methyl chloride in cyclopentane solutions (Figure 3, Tables I and II) indicates that there is little if any Al2Me6 MeCl complex formed in solutions in cyclopentane. Therefore, the chemical shift of MeCl in Al2Me6-cyclopentane solutions is a convenient measure of MeCl concentration, and the plot of the W /2 of AlMe3 vs. the chemical shift of MeCl in cyclopentane solutions should be linear. This is observed and is illustrated in Figure 4. 

Figure 3. Concentration dependence of the chemical shift of methyl chloride in cyclopentane solutions...

Table II. Chemical Shifts of Cyclopentane Solutions of Methyl Chloride...

CHEMICAL SHIFT OF METHYL CHLORIDE, HZ FROM CYCLOPENTANE AT 100 MHZ... 

Figure 5 shows the chemical shift of the methylene protons of Al-i-Bu3 as a function of the ratios of concentrations of [Al-i-Bu3]/[MeCl] in cyclopentane solution at ambient temperature. The sharp break in the plot at 1.7 [Al-i-Bu3]/[MeCl] suggests complex formation between these compounds. Similarly, the plot of the corrected chemical shift of methyl chloride—i.e., the chemical shift of MeCl in Al-i-Bu3-cyclopen-tane solutions minus the chemical shift of MeCl in the same concentration in cyclopentane solutions vs. [Al-i-Bu3]/[MeCl] (Figure 6), indicate complex formation. Obtaining more chemical shift measurements on... 

Figure 5. Chemical shift of the methylene protons of triisobutylalu-minum as a function of the ratios of concentrations of AliBus and MeCl-cyclopentane solutions...

Solution Using as a base value the chemical shift of cyclopentane (8 25.6, Table 7.1) and the internal substituent parameters for the -OH group, we calculate ... 

To utilize the substituent parameters given in Table 3-5, one adds the appropriate values to the chemical shift of the carbon in the analogous hydrocarbon, rounding off to the nearest ppm. As seen in Figure 3-15, the chemical shift of the 1 carbon of 1,3-dichloropropane may be calculated from the value (16) for the methyl carbon of propane and from the figures in Table 3-5. The chemical shift of the 3 carbon of cyclopentanol similarly may be calculated from the value (27) for cyclopentane. 

Applying the additivity of chemical shift analysis to the 2-norbornyl cation also supports the nonclassical bridged nature of the ion. The chemical shift difference of 168 ppm between 2-norbomyl cation (C7H11+) and its parent hydrocarbon norbornane 129 is characteristic of the <200 ppm difference observed between a nonclassical ion and its parent hydrocarbon. In contrast, an ordinary classical trivalent carbocation such as the cyclopentyl cation (75) reveals a chemical shift difference of >360 ppm (between the ion and the parent hydrocarbon, cyclopentane). This is consistent with the 350ppm difference characteristic of classical carbocations and their precursor hydrocarbons. 

AH of the protons found in chemically identical environments within a molecule are chemically equivalent, and they often exhibit the same chemical shift. Thus, all the protons in tetramethylsilane (TMS), or aU the protons in benzene, cyclopentane, or acetone—which are molecules that have protons whieh are equivalent by symmetry considerations— have resonance at a single value of 8 (but a different value from that of each of the other molecules in the same group). Each such compound gives rise to a single absorption peak in its NMR spectrum. The protons are said to be chemically equivalent. On the other hand, a moleeule that has sets of protons that are chemically distinct from one another may give rise to a different absorption peak from each set, in which case the sets of protons are chemically nonequivalent. The following examples should help to clarify these relationships. 

The model compounds used to estimate the averaged chemical shift were isopropyl cation and cyclopentane ... 

Solid state static C-NMR was applied to alkanes in order to address the fundamental issue of the anisotropy and alignment of the chemical-shift tensor of a CH2 group. The spectra of cyclopropane, spiropentane, cyclooctane, cyclopentane and various other compounds were obtained. It was shown that the <7cc component lying perpendicular with respect to the CCC plane of the molecules is related to the CCC bond angle. The... 

FIGURE 4. a) Influence of DIPIP/Li ratio on chemical shifts of the y carbon in isoprenyllithium in cyclopentane at 20°. Major peak-cis isomer, minor peak-trans isomer. 

The chemical shift of the parent compound (e.g., 22.9 for cyclobutane, 25.6 for cyclopentane, and 27.1 ppm for cyclohexane) and the same increments as for alkanes (see Chapter 4.1) can be used to estimate the chemical shifts of sp -hybridized carbon atoms of alicyclic compounds. Appropriate use of the conformational correction terms, K (page 79), is especially important with axial and equatorial substituents in cyclohexanes. The additivity rule is, however, not suitable for estimating chemical shifts of substituted cyclopropanes. 

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