Addition-rearrangement mechanism

Methoxy[(trimethylsilyl)ethoxy]carbene - obtained by thermolysis of the corresponding oxadiazoline 337 - reacts with Cgo in an unprecedented reaction pathway [383,384]. The expected methanofullerene-[2-i-l]-cycloadduct could not be observed. Instead the two dihydrofullerene adducts 338 and 339 were isolated (Scheme 4.69). They are formed by an unusual addition-rearrangement mechanism that includes the migration of the trimethylsilyl group. 

Fig. 31.15 Addition-rearrangement mechanism for arene oxide formation and proton-catalysed rearrangement of an arena oxide to a phenol (After Silverman ).

The free energies of reaction and activation have been calculated by the semiempirical molecular orbital method for the oxidation of ammonia and mono-, di- and trimethylamine by singlet and triplet oxygen atoms as models for oxidation by cytochrome P-450 [56]. For the non-radical oxidation (closed-shell path), the results indicate a two-step addition-rearrangement mechanism leading to both N-hydroxy and N-methoxy products via N-oxide intermediates. In the triplet path both a-C- and N-oxidation are competitive. N-oxidation via an addition mechanism seems to be favored over the H-abstraction mechanism, but no stable N-oxide radical intermediate is found on the triplet surface. 

Double-bond isomerization can also take place in other ways. Nucleophilic allylic rearrangements were discussed in Chapter 10 (p. 421). Electrocyclic and sigmatropic rearrangements are treated at 18-27-18-35. Double-bond migrations have also been accomplished photochemically, and by means of metallic ion (most often complex ions containing Pt, Rh, or Ru) or metal carbonyl catalysts. In the latter case there are at least two possible mechanisms. One of these, which requires external hydrogen, is called the nwtal hydride addition-elimination mechanism ... 

The evidence is in accord with an addition-elimination mechanism (addition of ArPdX followed by elimination of HPdX) in most cases. The reactions are stereospecific, yielding products expected from syn addition followed by syn elimination. Because the product is formed by an elimination step, with suitable substrates the double bond can go the other way, resulting in allylic rearrangement, for example, ... 

A new domino lithium acetylide addition/rearrangement procedure on trans-1,2-dibenzoyl-3,5-cyclohexadiene furnished 3-alkylidene-2,3-dihydrofurans via an intriguing mechanism involving three bond formations and two bond cleavages in one single operation <06SL1230>. The reaction of dimedone with meso-diacetoxycyclohexene in the presence of a palladium catalyst led to the formation of the tricyclic product as depicted below <06S865>. 

Mechanistic studies of the rearrangement activity of the ring-opening metathesis polymerization catalyst [Ru(H20)6]2+ were reported for unfunctionalized alkenes (112). The mechanism was found to be intermolecular, the alkene isomerization proceeding through an addition-elimination mechanism with a metal hydride catalytic species. This interpretation was... 

Methoxy(methyl)(trifluoromethyl)phosphane (1) undergoes an Arbuzov rearrangement with iodomethane to give dimelhyl(trifluoromethyl)phosphane oxide (2). The reaction does not follow an intramolecular 1,2-shift mechanism, but rather occurs by the normal Arbuzov addition-substitution mechanism, as demonstrated by the reaction of 1 with iodomethane-phosphane oxide 2 containing a deuterated methyl group. 

Adenine also may undergo an additional reaction at its C-6 amine group using a Fischer-Dimroth rearrangement mechanism. Alkylation at N-l can result in a rearrangement to give the C-6 alkylated product. The reaction at N-l usually requires... 

Although the two competing intermediates, the hypothetical ketyl-aryl radical pair (4) and the oxaspirooctadienyllithium (3), are not the rate-determining transition states, they should lie at almost the same energetic level. The rearrangement is in accord with the intramolecular nucleophilic addition/elimination mechanism rather than with homolytic cleavage/recombination. 

Reactions of alkynyliodonium salts 119 with nucleophiles proceed via an addition-elimination mechanism involving alkylidenecarbenes 120 as key intermediates. Depending on the structure of the alkynyliodonium salt, specific reaction conditions, and the nucleophile employed, this process can lead to a substituted alkyne 121 due to the carbene rearrangement, or to a cyclic product 122 via intramolecular 1,5-carbene insertion (Scheme 50). Both of these reaction pathways have been widely utilized as a synthetic tool for the formation of new C-C bonds. In addition, the transition metal mediated cross-coupling reactions of alkynyliodonium salts are increasingly used in organic synthesis. 

Radical cations derived from a variety of hexadiene systems constitute an interesting family of intermediates, since they are related to the potential mechanistic extremes of the Cope rearrangement. The electrocyclic reaction of a hexadiene radical cation has three mechanistic extremes a) addition precedes cleavage (associative mechanism) b) cleavage preceeds addition (dissociative mechanism) c) addition and cleavage occur in coordinated fashion (concerted mechanism). To date, radical cations corresponding to all three mechanistic extremes have been characterized. This illustrates remarkable differences between... 

The reaction of phenyl vinyl sulfoxide 234 with isobutene, in the presence of trifluoroacetic anhydride, yielded the to-alkylated product 238 (Scheme 59).128 It was suggested that this reaction proceeded by a different mechanism than the usual additive Pummerer mechanism. The alkene reacts with the electrophilic sulfur atom of intermediate 235, giving, after loss of a trifluoroacetate ion and a proton, the sulfonium ion 236. Thio-Claisen rearrangement of the ion then gives the thonium ion 237 which reacts with a further molecule of isobutene to give the product 238. 

Scheme 21 Mechanism of the Lewis acid catalyzed reactions of benzhydryl chloride with (Ey and (z)-2-butene. Ref. 89 describes the formation of some additional rearranged products in low yields, which do not affect the general conclusions, drawn in this discussion.

A number of reactions which at first sight appear to be substitution reactions, particularly at sp centres, in fact proceed via an addition-elimination mechanism (e.g. Scheme 1.2). Oxidation and reduction reactions may often be regarded as subsets of elimination and addition reactions, respectively. Other oxidation reactions may involve the substitution of a hydrogen atom by an oxygen atom, while some reductions involve the displacement of a substituent by hydrogen (hydrogenol-ysis). Rearrangement reactions (Scheme 1.3) may be considered as internal substitution reactions. 

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