Fragmentation reactions stereoelectronic effects

These regioselectivities can be rationalized on the basis of stereoelectronic effects since it is assumed that colinearity between the scissible bond and the donating orbital (the ring it-orbital in the case of benzyl cation radicals, see Scheme 10) is required during fragmentation the reaction is faster for bonds... 

Although this is the only chapter in which stereoelectronics appears in the title, you will soon recognize the similarity between the ideas we cover here and concepts like the stereospecificity of E2 elimination reactions (Chapter 17) and the effect of orbital overlap on NMR coupling constants (Chapter 18). We will also use orbital alignment to explain the Karplus relationship (Chapter 32), the Felkin-Anh transition state (Chapter 33), and the conformational requirements for rearrangement and fragmentation reactions (Chapter 36). 

Several classes of such fragmentation reactions have been studied in detail in order to understand the effect of substituents, the stereoelectronic requirements, and the effects of the medium and of nucleophiles. The mesolytic fragmentation of bibenzyls with the reduction of the radical and nucleophilic trapping of the cation is the main process for stabilized fragments, typical examples being cumyl or benzhydryl (see Equation 4.12). The relative stability of radical and cation makes the cleavage selective for Ar Ar. " ... 

The stereoelectronically controlled reaction of hydroxide ion with an 0-labeled tertiary amide(J8 ) (Fig. 3) should give the intermediate 19 which can fragment in only two ways, yielding the starting labeled amide J8 or the hydrolysis products direct cleavage of W to give unlabeled amide 18 cannot take place with the help of the primary electronic effect. In order to form the unlabeled amide J8 with stereoelectronic control, intermediate Jj) must first be converted into another conformer such as 20. Oxygen... 

This so-called stereoelectronic factor operates to maximize or minimize orbital overlap, as the case requires, to obtain the most favorable energy. This was evident from the three- and four-center systems we have discussed by the VB and HMO methods. It was also implicit in favored anti-1,2-additions, 1,3-cyclizations (Fig. 23), fragmentations (e.g. (174)), etc. Here we have selected several reaction types to illustrate the principle. In this and other sections, we show that the tendency for reaction centers to be collinear or coplanar stems largely from orbital symmetry (bonding), but may also derive from steric and electrostatic effects, as well as PLM. 

These effects are minimized for n-o fragmentations, especially those where the intramolecular electron transfer takes place between spatially adjacent orbitals. Somewhat related stereoelectronic considerations apply to nucleophile or base assisted fragmentations where certain three-dimensional dispositions of existing bonds may favor the assistance, in a way related to, for example, the anti stereochemistry of elimination reactions (Sect. 5.2). 

In 1977, Trost published the first example of an asymmetric variant of the Tsuji-Trost reaction, termed the asymmetric allylic alkylation reaction (AAA). Much of the subsequent development of the AAA reaction can be attributed to the dedicated work of Trost and co-workers.There was a substantial time lag however, in the development of processes where high enantioselectivities were realized in a predictable fashion. This was due, in part, to the fact that chiral, asymmetrically pure ligands must create a chiral environment on the opposite face of the allyl fragment to the metal centre (a stereoelectronic requirement, vide infra)P This obviously represents a significant design challenge in the production of effective ligand systems. 

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