Vacant Jt—orbital

Aromatic nucleophihc substitution proceeds by an addition-elimination mechanism, where one of the vacant Jt orbitals of aromatic ring is used for bonding interaction with the nucleophile [162]. In this way, the addition of nucleophile to the aromatic ring occurs with the formation of -complexes - Meisenheimer adducts - without displacement of any of the existing substituents (Scheme 100). The leaving group is then expelled in the second step [162]. 

Transition from the tropylium ion 3 to its neutral heteroaromatic counterparts is possible by replacement of a CH+ group by a heteroatom with a vacant p orbital. The latter effectively accepts Jt-electrons, thus providing ring-electron delocalization. A typical example is the boron atom in l//-borepine 11 <1992AGE1255>. Correspondingly, this type of heteroatom can be referred to as borepine-like. Other little-known representatives of this family are alumopine 12 and gallepine 13. 

Replacement of the methylene group in cyclopropyne by oxygen leads to changes in electronic structure similar to those already noted for aziridinediylidene (72). The lowest energy electronic configuration has only two 7t-electrons and corresponds to the dicarbene oxiranediylidene (16) rather than the triply bonded species (which would have four sr-electrons). There is then transfer of t-electrons from oxygen to the formally vacant 2pn orbitals on the carbene centers. In the STO-3G structure, C2 symmetry was assumed and this jt-electron donation (and associated partial double bond character) is reflected in short C—O bonds (1.361 A). Oxiranediylidene is isomeric with ketenylidene ( C=C=0) and is found to be 51.4 kcal/mol less stable than the latter (4-31G). >... 

The tropylium cation is a planar carbocation with three double bonds and a positive charge contained in a seven-membered ring. This carbocation is completely conjugated, because the positively charged carbon is sp hybridized and has a vacant p orbital that overlaps with the six p orbitals from the carbons of the three double bonds. Because the tropylium cation has three Jt bonds and no other nonbonded electron pairs, it contains six it electrons, thereby satisfying HuckeTs rule. 

It is known that group 14 metals facilitate the electron transfer from the neighboring jt-system by virtue of so called interaction [196]. The LUMO level is decreased by such interaction, which in turn favors the reduction of the system. The silyl group also stabilizes the anion radical intermediate formed by a one-electron reduction [197,198]. These effects can also be explained in terms of the orbital interaction with a low-lying a orbital of the sily group instead of the vacant d orbital. 

Molecular orbitals demonstrate the smooth transition from the allyl silane, which has a jt bond and a C-Si a bond, to the allylic product with a new Jt bond and a new a bond to the electrophile. The intermediate cation is mainly stabilized by o donation from the C-Si bond into the vacant p orbital but it has other o-donating groups (C-H, C-C, and C-E) that also help. The overall process is electrophilic substitution with allylic rearrangement. Both the site of attachment of the electrophile and the position of the new double bond are dictated by the silicon. 

In general, the n,Jt excited ketones undergo much faster cleavage than those with a lowest 7t,7t excited state, because the O-orbital of a bond being cleaved overlaps with the half-vacant n-orbital on the oxygen atom.905,911,919 The cleavage rate constant of the n,jt triplet excited benzyl phenyl ketone (PhCH2COPh) is, for example, more than three orders... 

In many of their complexes PF3 and PPh3 (for example) resemble CO (p. 926) and this at one time encouraged the belief that dieir bonding capabilities were influenced not only by the factors (p. 198) which affect the stability of the a P-> M interaction which uses the lone-pair of elecu-ons on P " and a vacant orbital on M, but also by the possibility of synergic it back-donation from a nonbonding d , pair of electrons on the metal into a vacant 3d r orbital on P. It is, however, not clear to what extent, if any, the bonds reinforce each other, and more recent descriptions are based on an MO approach which uses all (cr and jt) orbitals of appropriate symmeu-y on both the phosphine and the metal-containing moiety. To the extent that cr and n bonding effects on the stability of metal-phosphoms bonds can be isolated from each odier and from steric factors (see below) the accepted sequence of effects is as follows ... 

Step 2 Ethylene reacts with the active form of the catalyst. The Jt orbital of ethylene with its two electrons overlaps with the vacant titanium orbital to bind ethylene as a ligand to titanium. 

The addition of a singlet carbene to an alkene involves simultaneous interactions of the vacant carbenic p orbital (LUMO) with the filled alkene k orbital (HOMO), and of the filled carbenic a orbital (HOMO) with the vacant alkene Jt orbital (LUMO). [70] These interactions are illustrated in Figure 5, where the orbital symmetries are also shown. 

Tanida showed that the efficiency of the n-participation of the benzene ring in the stabilization of the developing cation centre depends largely on the position of the vacant p-orbital relative to this ring. Thus, sulphonates 279, 247 and 280 yield on acetolysis only acetates with the same skeleton and with configuration retention indicating a ring Jt-participation in the transition state... 

Similarly, the existence of FgP BHg as a stable compound may be due to additional yr-bonding involving overlap of a vacant 3d-orbital of phosphorus with the pseudo Jt-orbital provided by the BH3 group. 

Figure lA shows a simphfied picture of the arrangement of MOs in 6C MNO hnear complexes. The d Jt— Jt No back-bonding model describes the hnearity in the MNO units and the multiple character of the Fe—NO and N—O bonds in terms of the orbital mixing of two fiaUy occupied (hc-bonding (dxz.dy ) and vacant antibonding orbitals, with a minor... 

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