Fragmentation reactions orbital interaction

M(tj-CsH5)2 fragment with 7r-acceptor and -donor ligands and have emphasized the orbital interactions with conformational consequences, e.g., for carbene and olefin ligands. They have also discussed insertion reactions of olefins and carbon monoxide coordinated to this metal fragment 134). 

Consider the electrocyclic ring-opening reaction of cyclobutene. The molecule is formally divided into two fragments the double bond and the single 0 bond which is cleaved.9 The frontier orbital interactions (0,7t ) and (0, it) relevant to the conrota-tory and disrotatory reactions are given in diagrams 4, 5, 6 and 7, respectively. The net overlap is positive for 4 and 5, but zero for 6 and 7. The conrotatory process is therefore allowed, and the disrotatory process forbidden. 

Lastly, in the reaction of LiH with 2-chloropropanal, five TSs were located, 30a-e (Figure 6.16). 30a and 30b lead to the major product, while the other three lead to the minor product. The two lowest energy structures correspond with the Felkin-Anh major (30a) and minor (30c) TSs. Their energy difference corresponds with the energy difference for their aldehyde fragments when LiH is removed. 30b and 30d display the chelation effect, but unlike with 29, chelation is not enough to make up for the favorable orbital interactions in the Felkin-Anh approach. 

The following very useful approach for the analysis of photochemical reactions is due to Dauben, Salem, and Turro [13]. A bond, a—b, made from fragment orbitals and of molecular fragments, a and b, is broken in the process under consideration. The relationship to the orbital interaction diagram is transparent (see Figure 15.3). The orbitals... 

The longest standing model to rationalize this effect emphasizes so-called secondary orbital interactions. These are molecular orbital interactions other than those primarily used to define whether the reaction is allowed or forbidden, and that are frequently invoked to describe additional features of pericyclic reactions. Consider the LUMO of the CH2=CHA fragment in Figure 15.12 C (orbital n). The p orbital associated with A is in-phase with the adjacent carbon. Now consider the HOMO of a diene. C2 and C3 are similarly in-phase with Cl and C4, respectively. Thus, if we tuck the substituent A under the diene in the transition state, an additional interaction between the HOMO and LUMO can occur, and this must be stabilizing (see margin). 

The reaction path for addition of the simplest nucleophilic reactant, the hydride ion, calculated by an ab initio and the MINDO/3 method is depicted in Fig. 5.7. At large distances between the hydride ion and the electrophilic center, predominant are the electrostatic forces of interaction between the reactants, and the nucleophile occupies a position collinear with the C=0 bond, as in XXXIa. At the distances less than 3 A, the role of the orbital interaction, i.e., the exchange repulsion and charge transfer effects, becomes decisive. The attacking nucleophile leaves the plane of the carbonyl group and approaches the carbonyl fragment, in the zone of formation of the new bond, at an angle of 109.5°. 

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