Cyclopropane, coupling constants

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. 

There is much evidence, chiefly derived from nmr coupling constants, that the bonding in cyclopropanes is not the same as in compounds that lack small-angle strain.280 For a... 

The large CH coupling constants Jcll of cyclopropane and - to a smaller extent of cyclobutane are attributed to a higher s character of the carbon bond orbitals. 

The allyl radical (Figure 18.6) has been the subject of very many studies. It has been prepared in the liquid phase by electron irradiation of cyclopropane. The sign of the CH proton coupling constant was deduced by comparison with a single-crystal study of the radical CH(COOH)2 formed by y-irradiation of a single crystal of malonic acid. A straightforward UHF calculation gives a wavefunction... 

Table 3. H Hyperfine coupling constants [G] for the radical cations of selected 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. 

---