7r-Bonding effects

Theoretical considerations of the enthalpy of the reactions represented by the intersystem constants based on the bond-energy approach show that the values of AH assuming only cx-bond contributions differ considerably from the AH values that were calculated from the intersystem constant according to Eq. (13). These differences are attributable to 7r-bonding effects which, therefore, appear to be the major driving force for the nonrandom equilibrations described by the data in Table VIII. 

Resonance effect (Section 16.4) The donation or withdrawal of electrons through orbital overlap with neighboring 7r bonds. For example, an oxygen or nitrogen substituent donates electrons to an aromatic ring by overlap of the O or N orbital with the aromatic ringp orbitals. 

Theoretical explanation of the trans-effect (and /rans-influence) has centred on two theories, one based on cr-bonding the other on 7r-bonding. The tr-bonding argument considers two frans-ligands sharing a metal p orbital (Figure 3.83). 

Figure 3.84 Effect of a 7r-bonding ligand in acting as a 7r-acceptor. (Reproduced with permission from S.A. Cotton and F.A. Hart, The Heavy Transition Elements, published by Macmillan Press...

A 7r-bonding explanation notes that several ligands high in the trans-effect series are good -acceptors and thus siphon off 7r-density, making the region trans to it electron deficient and thus attractive to ligands that are electron rich. 

As already mentioned, a purely 7r-bonding mechanism cannot account for the position of hydride in trans-effect and trans-influence series. Overall, therefore, a major role (though not necessarily the only one) for <7-bonding is implied. 

This Pd(0)/formic acid system was effective for the cyclization of substituted 5-allene-l-ynes to give diene 140 (Eq. 25) through initial insertion into the internal 7r-bond of the allene followed by insertion into the alkyne [79]. All of the examples provided were geminally substituted within the backbone to facilitate cyclization. Intramolecular allene-alkyne reductive couplings to generate six-membered rings were not achieved. 

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