Electrophilic reaction pathway

The electrophilic reaction pathway leads, with an olefin, to the epoxide and a free site as symbolized in Figure 1.6. A purely nucleophilic site would catalyze oxidative dehydrogenation as discussed above. The frequent case of co-existence of nucleophilicity and electrophilicity leads to selective or total oxidation as seen in... 

Ab initio molecular orbital theory has been applied by Olah and coworkers to investigate the reactions of NO and the protonitrosonium ion HNO with methane. The reaction path was found to involve attack of NO on carbon instead of C-H bond insertion in accord with the studies of Schreiner et al. It was, however, pointed out that this is the consequence of the ambident electrophilic nature of NO and does not represent a general electrophilic reaction pathway for the reactions of methane. In fact, Schreiner and coworkers suggested that the electrophilic substitution of methane occurs by substitution of the nonbonded electron pair of methane instead of insertion of the electrophile into a C-H bond via 3c-2e bonding. Nonbonded electron pair formation in methane, however, can be considered only when methane would tend to flatten out (58) from its tetrahedral form, but this would be prohibitively energetic (>100 kcal mol ) and thus unlikely. 

This genera] scheme could be used to explain hydrogen exchange in the 5-position, providing a new alternative for the reaction (466). This leads us also to ask whether some reactions described as typically electrophilic cannot also be rationalized by a preliminary hydration of the C2=N bond. The nitration reaction of 2-dialkylaminothiazoles could occur, for example, on the enamine-like intermediate (229) (Scheme 141). This scheme would explain why alkyl groups on the exocyclic nitrogen may drastically change the reaction pathway (see Section rV.l.A). Kinetic studies and careful analysis of by-products would enable a check of this hypothesis. 

The soft-nucleophile-soft-electrophile combination is also associated with a late transition state, in which the strength of the newly forming bond contributes significantly to the stability of the transition state. The hard-nucleophile-hffld-elechophile combination inqilies an early transition state with electrostatic attraction being more important than bond formation. The reaction pathway is chosen early on the reaction coordinate and primarily on the basis of charge distributiotL... 

In other work Rozen added molecular fluorine to a steroidal ene-one dissolved in ethanol at low temperatures to produce a vicinal difluonde in a cleaner, better yield reaction than previously obtainable [55] Although the reaction was not general, the stereoselectivity was very high, and contrary to addition of other halogens, addition was r>ii, characteristic of an electrophilic addition pathway... 

For toluene fluorination, the impact of micro-reactor processing on the ratio of ortho-, meta- and para-isomers for monofluorinated toluene could be deduced and explained by a change in the type of reaction mechanism. The ortho-, meta- and para-isomer ratio was 5 1 3 for fluorination in a falling film micro reactor and a micro bubble column at a temperature of-16 °C [164,167]. This ratio is in accordance with an electrophilic substitution pathway. In contrast, radical mechanisms are strongly favored for conventional laboratory-scale processing, resulting in much more meta-substitution accompanied by imcontroUed multi-fluorination, addition and polymerization reactions. 

Because of the role of precursor structure on film processing behavior (consolidation, densification, crystallization behavior), the reaction pathways are typically biased through the use of the catalyst, which is simply an acid or a base. This steers the reaction toward an electrophilic or nucleophilic attack of the M—OR bond.1,63 Hydrolysis sensitivity of singly or multiply hydrolyzed silicon alkoxides is also influenced by the catalyst, which contributes to the observed variations in oligomer length and structure. Figure 2.3b illustrates... 

When a carbonyl group and an amino group are present within the same molecule, reaction with dichlorocarbene favours, somewhat unexpectedly, electrophilic attack on the carbonyl group [ 14, 15]. Although no confirmatory evidence is available, such a reaction pathway (Scheme 7.17) would explain the formation of the ring... 

A similar reaction pathway was found for the Sn2 substitution of an epoxide with a lithium cuprate cluster [124]. In contrast to that in the MeBr reaction, the stereochemistry of the electrophilic carbon center is already inverted in the transition state, providing the reason for the preferred trans-diaxial epoxide-opening widely observed in synthetic studies. The TS for the Sn2 reaction of cyclohexene oxide is shown in Eq. 10.12. 

An alternate pathway is possible for systems containing silylamino substituents at phosphorus. This most likely involves attack of the CCl3 anion at the electrophilic silicon resulting in elimination of Me3SiCCl3 as shown in pathway B. In the systems investigated thus far, the reaction pathway preference appears to be influenced by (1) solvent polarity, and (2) steric and electronic effects of the substituents at phosphorus ( ). 

While electrophilic reactions of alkylidenecycloproparenes are generally rapid, the nucleophilic pathways require more vigorous conditions. The only nucleophilic addition of cycloproparenes known is that of /-butoxide across the bridge bond of 237,240 and 241 which leads to the corresponding heptafulvenes 407. ... 

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