Long-range allylic coupling

The magnitude of the long-range allylic coupling, (V ) is controlled by the dihedral angle between the C-H bond and the plane of the double bond in a relationship reminiscent of the Karplus relation. 

The exocyclic olefinic methylene protons show obvious COSY correlations to one another. In addition, we note weak cross peaks between the olefinic protons at 4.86 and 4.97 ppm and an apparent diastereotopic methylene group (2.11 and 2.37 ppm) and a quartet at 2.60 ppm, respectively. These interactions are reminiscent of the long-range allylic coupling that we saw in ipsenol we could assign these correlations to the diastereotopic methylene C-7 and the methine at C-9. For now, we will be cautious and conservative, and return to this point later in the chapter. 

The structure resembles the corresponding anthracene complex [203]. The ligand is bonded in an allylic fashion with Sm-N bonds in the range of Ln-N (r-bonds and Ln-C bonds at the long range end of allylic interactions (2.866(3), 2.877(2) A). In contrast the reaction of SmCpf with acridine again yielded a coupled product, which contains a similar allylic bonding pattern (Sm-C, 2.944(5), 2.970(5) A) [203]. 

M. Barfield, DFT/FPT studies of the structural dependencies of long-range H/H coupling over four bonds 4/(H,H ) in propanic and allylic systems, Magn. Reson. Chem., 41 (2003) 344-358. 

Delayed-COSY Enhances detection of small and long-range couplings (<2 Hz) such as between protons in allylic systems or those in w-relationships. Requires magnitude-mode presentation. Crosspeaks due to larger couplings can be significantly attenuated. 

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