Coupling constants allylic

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. 

Some typical proton and carbon chemical shift and coupling constant data for allylic and benzylic systems are given in Scheme 3.54. An alkenyl substituent or a phenyl substituent on either a CH2F or a —CHF- group has virtually no effect upon that carbon s chemical shift, and they also only affect the proton chemical shift by about 0.5 ppm. 

In the above spectrum, the vinylic hydrogen appears as a pentet at 8 6.14 with a three-bond F—H coupling constant of 9 Hz the allylic... 

The allyl alkali-metal compounds give 2 line C-13 nmr spectra. The terminal positions are equivalent either because of rapid equilibrium between two covalent structures, or because the structure is a delocalized symmetrical ion. The chemical shifts and C-C coupling constants are recorded in table I. Reasonable agreement with literature 6 values are found (8, 2). 

Allylation of 3-alkoxy-3,6-dihydro-l,2-dioxin derivatives in the presence of TiCb or SnCl4 produced allylated dioxins 75 in moderate yields (Scheme 16) <2000JOC8407, 1993JA6458> the products were isolated as 3 2 cisttrans-mixtures, regardless of the stereochemistry of the starting material and identified by the Vh-3,h-4 coupling constant. 

For stereochemical dependencies of 1H,13C coupling constants through one bond and of allylic and homoallylic H. H couplings, see Section 4.1.1.2.. 

XLIV). The chemical shift and spin-coupling data for these two isomers is given in Table II. Of special interest is the almost zero value of the coupling constant for the gem-protons, viz., Jhc. This phenomenon is general for Tr-allylic complexes, and has usually been explained in terms of the... 

Many compounds are now known which contain simple allyl residues bonded sideways to a metal. Table IX is a summary of the proton resonances for the scries. The coupling constants in general are more characteristic than the chemical shifts of allyl groups bonded in this way (139), but the scattered data are not complete enough to permit a summary of this aspect, so far as the nickel family is concerned. 

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... 

We would expect that the spectrum of the latter compound would consist of two signals a two-proton triplet in the vinyl region and a four-proton doublet in the allylic region. This is because the coupling constant, / 3 is zero. It if were not zero, then a more complicated spectrum would result. Thus magnetic nonequivalence can lead to much more complicated spectra. 

The 1II NMR corroborates diis conclusion since two vinyl protons are observed both in the reactant and product however, a new two-proton doublet appears at 4.155 for die newly produced allylic methylene group. The acid O-H proton is moved far upheld as well. The coupling constants of die vinyl protons (./ = 16 Hz) show the starting compound to be dans, and die large splitting for the downheld vinyl doublet of die product (J = 16 Hz) shows die trails stereochemistry to be maintained in die unsaturated alcohol product. Moreover die splitting between the methylene group and the upheld vinyl proton clearly supports its allylic position. 

However, when a mixture of both crotyl regioisomers 60 and 61 were obtained and separated, unambiguous assignment of C-9 to an a orientation could be ascertained from NMR coupling constants (Eq. 17). After this comparison, it was clear C-9a allyl analogue 59 had been produced earlier. However, 59 underwent kinetic enolization with LDA at low temperature and subsequent acid quench afforded desired C-9P allyl analogue 57. 

The electronic spectrum of 1,5-diphenylpentadienyllithium in thf shows the presence of contact (tight) and.solvent-separated (loose) ion pairs. The smaller the cation and the more delocalized the anion, the greater is the tendency to form loose ion pairs. They are also favored as the temperature is lowered (64,65). 13C-NMR studies (66,67) have been interpreted in terms of appreciable covalency, but the present view is that organolithiums are predominantly ionic (68). Schlosser and Stahle have analyzed coupling constants 2J(C—H) and 3J(H—H) in the 13C- and H-NMR spectra of allyl derivatives of Mg, Li, Na, and K and of pentadienyls of Li and K (69). They conclude that considerable pleating of the allyl and pentadienylmetal structures occurs. The ligand is by no means planar, and the metal binds to the electron-rich odd-numbered sites. The lithium is considered to be q3 whereas the potassium is able to reach t]5 coordination with a U-shaped ligand. 

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