Rearrangement stereoelectronics

Carbocations, as we learned in Chapter 4 of Part A, can readily rearrange to more stable isomers. To be useful in synthesis, such reactions must be controlled and predictable. This goal can be achieved on the basis of substituent effects and stereoelectronic factors. Among the most important rearrangements in synthesis are those directed by oxygen substituents, which can provide predictable outcomes on the basis of electronic and stereoelectronic factors. 

A new [2,3]-sigmatropic rearrangement of the lithium salt 192 of the Af-benzyl-0-allylhydroxylamines (191) affording Af-benzyl-A-allylhydroxylamines (193) in moderate yields was reported (equation 56). The absence of crossover products confirms the intramolecular character of the transformation and an envelope transition state is proposed. The rearrangement proceeds via a transition state where facial selectivity is determined by stereoelectronic effects. 

Heating (2.S )-3-mctiiyl-3-phenyl-2-butyl tosylate in edianol leads to skeletal rearrangement and die formation of (3. S ) - 2 - e tho xy - 2 - me thy 1 -3-phenylbutane. What does diis information tell you about the stereoelectronic course of die skeletal rearrangement ... 

Considering that the present paper does not intend to analyze the chemical reactions of oxaziridines, we refer only to some publications in this field published in recent years. Certain oxaziridines undergo stereoelectronically controlled photochemical rearrangement into lactams (equation 49)204-2i9... 

In these rearrangements, there are two consecutive internal SN2 type displacement processes a) an electron pair of the oxygen atom displaces the electron pair of a C —C bond and b) the electron pair of a C—C bond displaces the leaving group. It is therefore pertinent to find out if these processes follow the stereoelectronic principle of the SNj reaction. 

On the basis of these results, the benzyl-benzylic acid rearrangement 179 180 -> 181 should occur with stereoelectronic control. The stereochemistry of the transposition should be as depicted by 182 +183. 

Sometimes, several rearrangements occur consecutively. A spectacular case has been observed (76-79) in the acid-catalyzed transformation of 3-a-fried-elanol (221) into 13(18)-oleanene (222). In this case, 221 gave presumably the carbonium ion 223 which underwent six stereoelectronically controlled... 

Conclusively, rearrangements of the Uagner-Meerwein type appear to be controlled by powerful stereoelectronic effects. 

They have further pointed out that the lower regioselectivity observed in the thermal rearrangement of oxaziridines does not rule out the above stereoelectronic requirement. In this case, the energy required for the reaction in sufficient to induce both nitrogen inversion in the starting oxaziridine and migration of the C-substituents. 

The term stereoselective is often confused with the term stereospecific, and the literature abounds with views as to the most satisfactory definition. To offer some clarification, it is perhaps timely to recall a frequently used term, introduced a decade or so ago, namely the stereoelectronic requirements of a reaction. All concerted reactions (i.e. those taking place in a synchronised process of bond breaking and bond forming) are considered to have precise spatial requirements with regard to the orientation of the reactant and reagent. Common examples are SN2 displacement reactions (e.g. Section 5.10.4, p. 659), E2 anti) elimination reactions of alkyl halides (e.g. Section 5.2.1, p.488), syn (pyrolytic) elimination reactions (Section 5.2.1, p.489), trans and cis additions to alkenes (e.g. Section 5.4.5, p. 547), and many rearrangement reactions. In the case of chiral or geometric reactants, the stereoisomeric nature of the product is entirely dependent on the unique stereoelectronic requirement of the reaction such reactions are stereospecific. 

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