Anionic rearrangement Subject

As can be seen from the energy level structure diagram, the relative position of the HOMO and LUMO levels are not less important than the energy gap between them, since they control the possibility of charge injection. At this point, however, note, that a MO scheme is often used for illustration, but more properly the total energy states of the molecules and their radical cations and anions that may be subjected to electronic rearrangement have to be considered. Bearing this in mind, the measured values of redox potentials can be translated into the molecular orbital picture. 

Recently, analogues of nucleosides [60], natural products Huperzine-A [61] and Hydroartemisinin [62], and inhibitors of metallo-/ -lactamases have been synthesised [63]. With acylsilane electrophiles, the initial adducts undergo Brook rearrangement which is interrupted by -Si bond fission with loss of fluoride anion (Eq. 16), leading to the formation of extremely useful difluoro-enol silanes [64]. Of the various fluoride sources employed, the tetrabutylam-monium triphenyldifluorostannate described by Gingras appears to be particularly effective. The numerous other methods for trifluoromethylation formed the subject of an exhaustive review [65]. More recently, the Olah group described a chlorodifluoromethyl trimethylsilane which is expected to have a rich chemistry [66]. 

This preliminary result has been the subject of an extensive study that confirms the catalytic nature of this rearrangement. It was observed that aniline derivatives do not undergo the rearrangement, probably because the aza-anion is more stabilized than... 

In an extension of this methodology, it was found that the anion formed could also lead to a ring rearrangement depending on the conditions it was subjected to. The mechanism for this transformation is shown in Scheme 2. 

An experimental aspect of the Cope rearrangement particularly important in synthesis is the feature that it is frequently subject to catalysis. We have already seen that in the anionic oxy-Cope rearrangement enormous rate enhancements are realized by the incorporation of an alkoxide ion in the substrate. In many Cope rearrangements similar rate enhancements are achieved by the addition of catalysts, particularly acids and metals. ... 

Although the anion is used as a means of derivatizing the sulfone the key step to this method is the generation of a homoenolate anion and its subsequent rearrangement. The subject of homoenolate anions and their synthetic applications has been extensively reviewed elsewhere. ... 

The reduction of organic halides in the presence of aromatic hydrocarbons has been the subject of detailed kinetic studies, which provide rate constants for the homogeneous ET [130] and the follow-up reaction [131]. The theoretical basis for this kind of experiment ( homogeneous redox catalysis ) was laid by Saveant s group in a series of papers in 1978-1980 [132-134]. Homogeneous ET also plays an important role in the protonation of anion radicals [135], and it is also an essential step in the sigmatropic rearrangement in 1-arylindenes [136]. Both reactions are discussed here. 

In order to study this unusual dimerization in detail 49 [40] was synthesized and subjected to the same reaction conditions. The cumyl group inhibits the Grovenstein-Zimmerman rearrangement as in this case the formation of an anion at a tertiary center would be encountered, so only cleavage to the vinyllithium compound 18 is possible. Indeed, after reacting 49 with lithium metal for 4 h at room temperature and subsequent work-up with dimethyl sulfate the dimer 51 is isolated in 82 % yield, with t-butylbenzene (53) as the other product in 84 % yield (Scheme 11). 

In analogy to the preparation of bicyclic triene 37 from bisacetylene J449, the diethynyloxiranes 35 and thiiranes 36 (both cis and trans isomers can be employed tram- 36 leads to desulfurized product only) were subjected to gas phase flow-pyrolyses to effect clean, though low yielding, rearrangement to furo- and thienocyclobutadiene J S50 and 39sl respectively, systems that are isoelectronic with anion 33. Both 38 and 39 are planar Sir-systems and as such exhibit typical... 

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