Trans effect rationale

The Yu laboratory described the first selective pyridine C3-olefination reaction (Scheme 10.25). Using a similar rationale to that used in their arylation method, it was hypothesized that the success of this method was due to unique ability of the bidentate phen ligand to weaken the coordination of the palladium catalyst with the pyridyl N atom through the trans effect. 

Reaction of butyllithium with VBE tetrahydroisoquinolyl formamidine proceeds as shown in Scheme 56. The lack of a kinetic isotope effect in deuteriated substrates revealed that the slow step in the sequence is the complexation of the BuLi with the formamidine, followed by selective removal of the a proton as shown. A rationale for this selectivity is that the butyl group is oriented trans to the isopropyl of the 5-membered... 

Electronic effects become apparent in the M—NCS/M—SCN linkage switches observed in a series of Pd11 (iso)thiocyanate complexes. Ligands positioned trans to the pseudohalide and that are suited to accept electron density from the metal into empty orbitals (backbonding) stabilize the Pd—NCS linkage isomer. However, this rationale is contradicted by the trend in Co—(NCS) bonding in a series of cobalt complexes (see ref. 204b for a review). 

Scheme 1 on p. 4) whenever the principal complete bond diagrams describe similar core-ligand binding (e.g., trans-bent CH2 CH2 versus CH -CH). It goes without saying that much will have to be done before we know whether the rationale presented in this chapter amounts to a general procedure for qualitatively predicting "correlation effects . 

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