Homoallylic coupling

Homoallylic coupling (5,/nn) is closely related to allylic coupling in its stereochemical dependence. In cyclic compounds the existence of homoallylic couplings can be used for configurational assignments. For example, tru s-2,5-disubstituted bislactim ethers show a 4 Hz difference in the couplings between H-2 and H-5200. 

The relative configuration of the stereogenic units in bis- and tris-TBs has been unequivocally determined on the basis of homoallylic couplings and NOE experiments (Figure 23). For instance, homoallylic couplings between H-6n and H-7x,... 

Hz and H absorbs in the range d 1.2-1.75. A rather smaller coupling Vh h = 0.8 Hz was determined for the enol acetate 142 (X = OAc). The homoallylic coupling constants, h = 0.7 Hz for 142 (X = I) and 1.2 Hz for 141 are in accord with earlier cited work on related compounds. The effect on the chemical shift of a vinyl proton brought about by the introduction of a cis or trans cyclopropyl ring has been found to be rather smalF" ... 

The magnitude and signs of 4- and 5-bond couplings in formate esters have been determined (31).The spectrum of methyl a-apopo-, dophyllate (32) shows several examples of homoallylic coupling. 

In general. Protocol III in Scheme 1 is far superior to Protocol II, as exemplified by comparative results shown in Scheme 4. As mentioned earlier, the main difficulties associated with Protocol II are /3-elimination and the inability of Pd complexes to undergo facile oxidative addition with y,5-unsaturated aUcyl electrophiles. In some special cases where /3-dehydropalladation is not possible, some Pd-catalyzed aryl-homoallyl coupling reactions have been observed (Scheme 5). 

Interaction between even more remote spins is possible in conjugated systems such as alkynes and allenes. is also commonly observed in homoallylic fragments (H-C-C=C-C-H). Large couphngs of this type are observed in in l,4-cyclohexadienes. Small homoallylic coupling is also observed across amides. Its magnitude is a function of conformations across peptide bonds (Figure 12.81)." ... 

Scheme 3.58 Cyanthiwigins B, F, and G synthesized by Pd-catalyzed alkenyl-homoallyl coupling [228].

Allylic and homoallylic couplings with methyl groups are often comparable Jh-C=C-CH3 " JCH3-C=C-CH3... 

In acyclic systems, Jlcisoid l itransoid usually holds. Large homoallylic coupling constants are generally observed in cyclic systems ... 

If there is a double bond between two coupling nuclei, coupling across four (allylic coupling) or five (homoallylic coupling) bonds may be observed. 

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