Anionic rearrangement mechanisms

With appropriately substituted oxetanes, aluminum-based initiators (321) impose a degree of microstmctural control on the substituted polyoxetane stmcture that is not obtainable with a pure cationic system. A polymer having largely the stmcture of poly(3-hydroxyoxetane) has been obtained from an anionic rearrangement polymerisation of glycidol or its trimethylsilyl ether, both oxirane monomers (322). Polymerisation-induced epitaxy can produce ultrathin films of highly oriented POX molecules on, for instance, graphite (323). Theoretical studies on the cationic polymerisation mechanism of oxetanes have been made (324—326). 

No firm decision, between an anion diradical mechanism and a concerted S ->0 1,2-anionic shift, could be made from the available evidence106. Interestingly, the use of a stronger base such as ethylmagnesium bromide results in rearrangement to trans-1,2-diphenylcyclopropanesulfinic acid in highly stereoselective manner (equation 36)107. 

Haeffner, F., Houk, K. N., Schulze, S. M., Lee, J. K. Concerted Rearrangement versus Heterolytic Cleavage in Anionic [2,3]- and [3,3]-Sigmatroplc Shifts. A DFT Study of Relationships among Anion Stabilities, Mechanisms, and Rates. Journal of Organic Chemistry 200Z, 68, 2310-2316. 

One can therefore conclude that the base-catalyzed cyclopropane - propene rearrangement 93a(b) - 96a(b) is induced by an acid-base and not by an electron transfer reaction. The mechanism of the facile cyclopropyl - allyl anion rearrangement 94a(b) - 95a(b), however, is not clear74). 

The anion of 3-alkoxycarbonyl-3 f-azepines exhibits a reaction that is an allyl anion to cyclopropyl anion rearrangement (see Section 2.3.1.2.) and is also formally an azacyclohep-tatriene to azanorcaradiene rearrangement with an additional alkoxycarbonyl shift. Treatment of 13 (X-ray analysis) with lithium 2,2,6,6-tetramethylpiperidide (LTMP), followed by iod-omethane, provides 14 (X-ray analysis of picrate) in 50-60% yield. The mechanism as suggested by the authors is shown on the following page. 

Insertion into Carbon-Carbon o-Bonds. First examples of this concept were reported with anionic nucleophiles in the context of the addition of a-lithioalkyl and a-lithioarylacetonitriles to arynes. In 1984, Meyers and Pansegrau proposed a cyclization rearrangement mechanism to account for the formation of products 77 in the reaction of a-lithioacetonitriles to 3-oxazolylbenzyne. Initial attack of the nucleophile takes place at C-2 probably due to chelation of the lithium atom from the lithiated nitrile to the oxazoline moiety (Scheme 12.40) [67]. 

A probable mechanism for this rearrangement postulates the intermediate formation of a hydroxide-ion addition complex, followed by the migration of a phenyl group as an anion ... 

Interest in this reaction was revived when the relevance of a carbene mechanism was realized, particularly following the demonstration (cf. SectionI,B) of a similar ring expansion of indene to 2-chloro-naphthalene by dichlorocarbene via the cyclopropane adduct. Indeed, at this time Nakazaki suggested that these reactions occurred by the addition of dichlorocarbene to the indolyl anion and subsequent rearrangement to the indolenine and, with loss of chloride ion, to the quinoline [Eq. (12)]. The preference of dichlorocarbene for... 

As for compounds 37, the rearrangements of 39 are considered to occur by a mechanism involving heterolytic N-C bond cleavage followed by in-termolecular recombination of the carbenium cation and benzotriazolyl anion so formed. 

Evidence in support of a carbocation mechanism for electrophilic additions comes from the observation that structural rearrangements often take place during reaction. Rearrangements occur by shift of either a hydride ion, H (a hydride shift), or an alkyl anion, R-, from a carbon atom to the adjacent positively charged carbon. The result is isomerization of a less stable carbocation to a more stable one. 

Fig. 8.9 Possible mechanisms of the bioluminescence reaction of dinoflagellate luciferin, based on the results of the model study (Stojanovic and Kishi, 1994b Stojanovic, 1995). The luciferin might react with molecular oxygen to form the luciferin radical cation and superoxide radical anion (A), and the latter deproto-nates the radical cation at C.132 to form (B). The collapse of the radical pair might yield the excited state of the peroxide (C). Alternatively, luciferin might be directly oxygenated to give C, and C rearranges to give the excited state of the hydrate (D) by the CIEEL mechanism. Both C and D can be the light emitter.

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