P/ -acceptor

Accentuation of the p-acceptor of an enone by substitution with an iminium moiety greatly contributes to the facile intramolecular Michael addition involving an enolate. The reactive components are generated from an enollactone and an ynamine [133],... 

This interaction raises the energy of the metal t2g orbitals, and decreases D0. On the other hand, p-acceptor ligands such as CO have empty antibonding p orbitals. Overlap with the metal in this case causes the t2g orbitals to be lowered in energy so that D0 is increased. 

This ground-state phenomenon is known as the trans influence, and depends mostly on the a bonding capability and the polarizability of the ligand. Some ligands such as CN- do not show much trans influence but nevertheless have a large kinetic trans effect, because their p-acceptor properties help in the stabilisation of the transition state. 

Evidence for the p-acceptor interaction comes from different sources. 

Many compounds are known, containing CO in conjunction with other ligands, which may include p acceptors such as phosphines, and / or s bonding ligands. For example, there is a series of compounds Mn(CO)5X, where X = H, halogen or an... 

The 18-electron rule can be a useful guide to stable organometallic compounds, especially when p-acceptor ligands are present, although it has the limitations referred to in Topic H9. Compounds 3, 5 and 6 obey this rule, but 1 without p bonding ligands has an electron count of only 12. Metallocenes [M(h5 C5H5)2 ] are known for the 3d series elements V-Ni, with 15-20 valence... 

Soft acceptors and donor atoms are generally large and van der Waal s forces may contribute to bonding, p-acceptor behaviour is shown by some soft bases (e.g. CO). 

Mechanistically, this new insertion-CI-Diels-Alder hetero domino sequence can be rationalized as follows (Scheme 64) After the oxidative addition of the aryl halide 115 or 118 to the in situ generated Pd(0) species the arylpalladium halide 120 intramolecularly coordinates and inserts into the tethered triple bond via a syn-carbopaUadation to furnish cyclized vinylpalladium species 121 with a p-acceptor substitution in a stereospecific fashion. Transmetallation of the in situ generated copper acetylide 122 gives rise to the diorganylpalladium complex 123 that readily undergoes a reductive elimination and liberates the electron poor vinylpropargylallyl ether 124. The triethylamine catalyzed propargyl-allene isomerization furnishes the... 

RC cofactors. This comes about because the lower exciton state of P, P, carries 88% of the dipole strength and is therefore strongly coupled to B according to the dipole approximation, but has only a small overlap with the B emission, as shown in Fig. 16. The net effect is that P is not predicted to be an effective acceptor for B. On the other hand, the upper exciton state P+ overlaps significantly with B emission (see Fig. 16), but since it carries only 12% of the total dipole strength, this state is predicted to be too weakly coupled to B to be an effective acceptor. The answer to the conundrum lies in the idea that the absorption spectra of the P acceptor states do not contain the... 

Here, reactions are classified by the types of component that are combined. Hence, addition of a N donor to a P acceptor is denoted [N, P], Similarly, reactions of P donors and N acceptors are labeled [P, N]. 

The absorption of the G and D lines of the Znin acceptor in InP have been reported by Causley and Lewis [30], and this appears to be the only known acceptor absorption result for this compound. Their positions are 241.5 cm-1 (29.94 meV) for the G line (2P3/2 (Ts-) and 286.0 cm-1 (35.46 meV) for the D line (2P5/2 (Ts-). The scarcity of absorption results is due to the fact that such spectra would be close to the one-phonon absorption of InP (37.7 and 42.8meV for the TO and LO phonons, respectively). However, shallow acceptor transitions have been identified in InP by selective pair luminescence (SPL) and excitation spectroscopy (Dean et al. [48]). The separation from the IS3/2 ground state of some S and P acceptor states of Znin and Cdin have also been measured by Raman scattering [188]. 

In general, electrophilicity is favored by a low-lying HOMO or a orbital, which makes electron donation by the carbene unfavorable, and by a low-lying, accessible LUMO or p acceptor orbital. Conversely, carbenic nucleophilicity is favored by a high-lying HOMO, for efficient electron donation, and a high-lying inaccessible LUMO. [66]... 

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