Other Mechanistic Possibilities

The fact that primary carbenium ions are unstable suggests that the exchange with solvent is an Sn2 process with primary alcohols. If true, then are primary carbenium ions ever intermediates in dehydration reactions Studies have shown that it depends upon the case. Neopentyl alcohol does form a primary carbenium ion, whereas 1-propanol does not. Acid-catalyzed elimination of 1-propanol to form propene occurs by a concerted E2 reaction (Eq. 10.90). Similarly, whether a secondary alcohol eliminates in acid via an El or E2 pathway depends on the case. 

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Scheme 51), so it has been suggested that the initial reaction involves the dinitrosobenzene. It does seem, however, that this may be an over-simplification, as there are documented cases where mono-iV-oxides rather than the di-iV-oxides are formed for instance, the reaction of benzofuran-3(2//)-ones with benzofuroxan yields 3-(o-hydroxyphenyI)quinoxa-line 1-oxide (Scheme 52). Other mechanistic possibilities may also be put forward but it seems probable that more than one pathway may be operating, particularly in view of the more recent findings on the reactions of benzofuroxans (81AHC(29)251). 

A number of other mechanistic possibilities were examined and rejected in the course of this work. (a) The original suggestion (12) of a rate-determining step involving formation of radical pairs... 

There are two other mechanistic possibilities, halogen atom abstraction (HAA) and halonium ion abstraction (EL), represented in Schemes 4.4 and 4.5, respectively, so as to display the stereochemistry of the reaction. Both reactions are expected to be faster than outer-sphere electron transfer, owing to stabilizing interactions in the transition state. They are also anticipated to both exhibit antiperiplanar preference, owing to partial delocalization over the C—C—Br framework of the unpaired electron in the HAA case or the electron pair in the EL case. Both mechanisms are compatible with the fact that the activation entropies are about the same as with outer-sphere electron donors (here, aromatic anion radicals). The bromine atom indeed bears three electron pairs located in two orthogonal 4p orbitals, perpendicular to the C—Br bond and in one s orbital. Bonded interactions in the transition... 

All these observations point to the occurrence of a 8 2 rather than an outer sphere, dissociative electron-transfer mechanism in cases where steric constraints at the carbon or metal reacting centres are not too severe. It is, however, worth examining two other mechanistic possibilities. One of these is an electrocatalytic process of the Sg -type that would involve the following reaction sequence. If, in the reaction of the electron donor (nucleophile), the bonded interactions in the transition state are vanishingly small, the alkyl radical is formed together with the oxidized form of the electron donor, D . Cage coupling (144) may then occur, if their mutual affinity is... 

Other mechanistic possibilities also exist for the [5-1-2] cycloaddition reaction and cannot be mled out at this point In addition to the stepwise pathways depicted in Scheme 13.8, it is also possible that coordination of the 27r-component to 42 could lead directly to 48 through simultaneous insertion and cleavage. Variations on these events, such as direct insertion into the cyclopropane to form a metaUacyclobutane, are also possible. 

The efficient formation of dihalocyclopropanes could be extended to allyl alcohol adducts 74, which are oxidized to 75. The resulting ketones are very useful synthetic building blocks. Reaction with alkoxide, for instance, affords dihydrofuran derivatives 76 via alkoxysubstituted cyclopropyl ketones 43). This ring enlargement might be a purely thermal process (1,3-sigmatropic shift), but other mechanistic possibilities could also be conceived (cf. Scheme 2). 

If the C - Nu bond is formed first and then the C- X bond is broken, the mechanism has two steps, but this mechanism has an inherent problem. The intermediate generated in the first step has 10 electrons around carbon, violating the octet rule. Because two other mechanistic possibilities do not violate a fundamental rule, this last possibility can be disregarded. 

A detailed discussion of other mechanistic possibilities is given in 128>. 

Data is accumulating for pepsin and HIV protease to suggest that they may catalyze their reactions through a mechanism in which release of products leaves the enzyme in a conformation which must undergo isomerization before another round of catalysis can commence [65]. More work is needed here to eliminate other mechanistic possibilities. 

It is now known that the pinacol rearrangement is characteristic of all types of 1,2-diol, and that the process is promoted by most electrophilic catalysts. Most of the available data point to the involvement of a carbocationic intermediate, even when the hydroxyl groups are not tertiary. Evidence for a concerted process, i. e. loss of water with the synchronous migration of the substituent, has also been obtained. Other mechanistic possibilities, e. g. transformation through an epoxide or an eno-lic intermediate, can either be ruled out or regarded as playing a role in limited cases only [6]. 

There are still other mechanistic possibilities. Consider, for example, the following mechanism, which starts off with a sulfur-on-sulfiir attack. 

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