Anti-electron entries

The fourth factor becomes an issue when anti betaine formation is reversible or partially reversible. This can occur with more hindered or more stable ylides. In these cases the enantiodifferentiating step becomes either the bond rotation or the ring-closure step (Scheme 1.12), and as a result the observed enantioselectivities are generally lower (Entry 5, Table 1.5 the electron-deficient aromatic ylide gives lower enantioselectivity). However the use of protic solvents (Entry 6, Table 1.5) or lithium salts has been shown to reduce reversibility in betaine formation and can result in increased enantioselectivities in these cases [13]. Although protic solvents give low yields and so are not practically useful, lithium salts do not suffer this drawback. [18]... 

Ando and coworkers conducted isotope effect studies (entry 11) on the direct displacement reaction of benzyl arenesulfonates with dimethylaniline (DMA)38. They found that an electron-withdrawing substituent in the substrate (Y = 3-Br) caused the TS to shift to a later position along the reaction coordinate, which is consistent with that predicted by the Thornton rule (or anti-Hammond effect). The anilinolysis of phenylethyl arenesulfonates (entries 12 and 13) proceeds also by an SN2 mechanism. The reaction was found to proceed by a dissociative SN2 mechanism with a relatively small degree of aryl participation. The fraction of the phenonium ion intermediate captured by the aniline nucleophile in the aryl-assisted pathway has been shown to increase with a stronger nucleophile, and a four-center TS in an intermolecular SNi mechanism is suggested for the aryl-assisted pathway39,40. Under the same reaction conditions, benzylamine nucleophiles react at a rate ca two times faster than that of anilines. 

The data of Table 8 reveal that the three activation modes may lead to similar (Table 8 e.g. entries la, c, e) or to completely different (Table 8 e.g. entries 13a,b) product distributions. Even within a series of PET experiments dramatic differences in the product distribution were observed by variation of the electron acceptors (Table 8 entry 2f). Furthermore, stereochemical factors are of considerable importance. Thus, syn-5-methylbicyclo [2.1.0]pentane easily rearranges to 1-methylcyclopentene on treatment with [Rh(CO)2Cl]2 (Table 8 entry 2a), whereas the anti isomer does not react even at elevated temperature (Table 8 entry 2b). Unfortunately, the activation modes for the individual substrates have not been completely examined in all cases, which limits comprehensive understanding of the reaction mechanisms. Nevertheless, several examples will be discussed below, in order to obtain a concrete description of the reaction mechanisms that are involved. 

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