Coupling constants in cyclopropanes

W. Makulski and M. Wilczek, Nuclear magnetic shielding and indirect spin-spin coupling constants in cyclopropane. Gaseous and liquid NMR measurements. Polish J. Chem., 2006, 80(7), 1055-1060. 

One of the early determinations of vicinal coupling constants in cyclopropanes was that of the chrysanthemumic acids, trans acid (75) shown. For the ethyl esters in benzene,... 

Geminal proton-proton coupling constants in cyclopropanes are usually found to be negative The values reported by Dauben and Wipke had an average of... 

The one-bond CH coupling ranges from about 100 to 320 Hz, and much of the variation may be interpreted in terms of the 7-s relationship. The coupling constant in cyclopropane (162 Hz) demonstrates that the carbon orbital to hydrogen is approximately sp hybridized. Intermediate values in hydrocarbons may be interpreted in terms of fractional hybridization. The J = 144 Hz for the indicated CH bond in tricyclopentane (4-15) corresponds to 29% s character (sp " ), 160 Hz in cubane (4-16) to 32% s character (sp ), and 179 Hz in quadricyclane (4-17)... 

Watts and Goldstein have provided, by analysis of the satellite spectra, accurate values of proton—proton coupling constants in cyclopropane. The directly bonded coupling constant has... 

TABLE 4. Proton-proton coupling constants in monosubstituted cyclopropanes... 

The results of the carbon-proton coupling constants of some of these monosubstituted cyclopropanes are given in Table 7. Long-range carbon-proton coupling constants in monosubstituted cyclopropanes are such that they may be construed either as two bond or three bond coupling in some cases the latter have been found to be larger. Cyclopropane itself shows Jaim — 2 55 Hz data for a number of monosubstituted derivatives are shown in Table 8. 

TABLE 7. Carbon-proton coupling constants in monosubsti tuted cyclopropanes (1) (in Hz)... 

The H-NMR spectrum of fluorocyclopropane (199) with that for cyclopropane (1) for comparison is given in Table 16 The vicinal coupling constants in Table 16 are... 

Table 3. Coupling constants (in Hertz) and geometry of cyclopropane. The structure parameters have been obtained by fitting Djj and D, (values taken from Meiboom and Snyder [50]).

In the C NMR spectrum two signals with unusually small shift values [(07/2)2 5c = 7.7 CH 5c = 10.6] and remarkably large CH coupling constants Jch= 161.9 and 160.1 Hz) indicate a mono-substituted cyclopropane ring A. The protons which belong to this structural unit at = 0.41 (AA ), 0.82 BB ) and 1.60 (M) with typical values for cis couplings 8.1 Hz) and trans couplings 4.9 Hz) of the cyclopropane protons can be identified from the CH COSY plot. 

The formation of alkyl shifted products H and 14 can be explained in terms of the formation of endo-intermediate 21 formed by endo attack of bromine to 2 (Scheme 4). The determined endo-configuration of the bromine atom at the bridge carbon is also in agreement with endo-attack. Endo-Intermediate 21 is probably also responsible for the formation of cyclopropane products 12 and 15. The existence of cyclopropane ring in 12 and 15 has been determined by and 13c NMR chemical shifts and especially by analysis of cyclopropane J cH coupling constants (168 and 181 Hz). On the basis of the symmetry in the molecule 12 we have distinguished easily between isomers 12 and 15. Aryl and alkyl shift products IQ, H, and 14 contain benzylic and allylic bromine atoms which can be hydrolized easily on column material. 

Both the 13C chemical shifts and the F—C coupling constants for CF2 carbons are quite characteristic in value, as can be seen from the examples in Scheme 4.9. A review article on 13C NMR spectra of fluorinated cyclopropanes has recently appeared.3... 

In addition to H and 13C chemical shifts, vicinal 1H,1H coupling constants provide another tool for the stereochemical assignment of disubstituted cyclopropanes, as the cis or trans relationship of the substituents can be determined from the absolute values, often without the need to compare diastereomers. The relationship is ... 

Phenylcyclopropane radical cation (9 ) has divergent hyperfine coupling constants for the secondary cyclopropane protons (aptrans = 0.78 mT upcis = 0.07 mT CIDNP, B3LYP/6-31G calculations), apparently because the cis protons are located in a nodal plane. " Similarly, vinylcyclopropane radical cation is... 

For radical cations of norcaradiene and derivatives, the interaction of the cyclopropane in-plane e orbitals with the butadiene frontier MO favors the type B structure. The assignments are based on ab-initio calculations, CIDNP results, and the ET photochemistry. The norcaradiene radical cation (lla ) has a electronic ground state (Cj symmetry). The Cl—C6 bond is shortened on ionization (—3.4 pm) while the lateral bonds are lengthened (+2.8 pm). The delocalization of spin density to C7 (py = 0.246 py 5 = 0.359) and the hyperfine coupling constants of the cyclopropane moiety a e = 1.36 mT oysyn = —0.057 mT flvanti = —0.063 mT) support a type B structure. 

---