Intermediate 1,2 -rearrangement

Enamino ketones and esters also react with dimethyl acetylenedicarboxylate (67). Again cycloaddition appears to occur and the unstable cyclobutene intermediates rearrange to give insertion of two carbon atoms. 

Allyl sulfones formed from allyl sulfinates (cf. equation 1) can easily tautomerize to give a, /J-unsaturated sulfones26 in cases for which R1, R2 are part of an (hetero) aromatic system, this tautomerization occurs spontaneously. Similarly, sulfinic acid esters from jV-phenylhydroxamic acids as reactive intermediates rearrange to give o-(major part) and p-sulfonylanilines (minor part)27 ... 

Moreover, because of the involvement of cationic intermediates, rearrangements can occur in systems in which a more stable cation can result by aryl, alkyl, or hydrogen migration. Oxymercuration-reduction, a much milder and more general procedure for alkene hydration, is discussed in the next section. 

The role of oxazirdines as intermediates in the rearrangement of heteroaromatic N-oxides has not been fully established although in most cases the formation of photoproducts can best be rationalized in terms of such intermediates. Rearrangement appears to be singlet-derived and competes with triplet-derived deoxygenation. 

Several compounds containing Tt-bonds show reactions with 1 which most likely proceed via [2+1] or [4+1] cycloaddition processes, but no detailed mechanistic studies have been performed so far. Not unexpectedly, the electron-rich species 1 preferentially reacts with electron-poor substrates, and ring-strained or dipolar intermediates rearrange or react further to more stable products in a sometimes rather complicated and surprising fashion. In a few cases even the pentamethylcyclopentadienyl substituents at silicon are involved in the reaction pathways. 

The experimental observations were interpreted by assuming that the redox cycle starts with the formation of a complex between the catalyst and the substrate. This species undergoes intramolecular two-electron transfer and produces vanadium(II) and the quinone form of adrenaline. The organic intermediate rearranges into leucoadrenochrome which is oxidized to the final product also in a two-electron redox step. The +2 oxidation state of vanadium is stabilized by complex formation with the substrate. Subsequent reactions include the autoxidation of the V(II) complex to the product as well as the formation of aVOV4+ intermediate which is reoxidized to V02+ by dioxygen. These reactions also produce H2O2. The model also takes into account the rapidly established equilibria between different vanadium-substrate complexes which react with 02 at different rates. The concentration and pH dependencies of the reaction rate provided evidence for the formation of a V(C-RH)3 complex in which the formal oxidation state of vanadium is +4. 

In the first step 61 is dehydrocyclized to 67. This intermediate rearranges to 68, that yields 66 by nitrogen elimination. Further oxidation dehydrogenates 67 to 69, which decomposes to 63 and 64. Benzil (65) is produced by hydrolysis, 59 by oxidative cleavage of 61. 

A similar pathway seems operative in the 2-substituted benzo[6]thiophene case also <79JCS(Pi 13207). Here, however, the spiro intermediate rearranges to both the benzothiazine (258) and benzothiazole (259), the two products being isolated in almost equal amounts (Scheme 70). The dihydrobenzo[6]thiophene unit present in (259) does not seem to be dehydrogenated as the dihydrothiophene unit does (Scheme 69). 

The iron-catalyzed [3 + 2]-cycloaddition (Huisgen reaction) of nitriles and carbonyl compounds as reported by Itoh et al. is one of the rare examples reported where an iron reagent can be utilized for the synthesis of 1,2,4-oxadiazoles (Scheme 9.35) [93]. In this reaction, methyl ketones are nitrated at the a-position by Fe(N03)3 to generate an a-nitro ketone. This intermediate rearranges to an acyl cyanate, which reacts further with the nitrile to give the heterocyclic product 48 in good to excellent yields (R1 = Ph, R2 = CH3 95% yield). 

In low thiol environments, the intermediate rearranges through several steps to the sulfenic acid. 

Pyrolysis of dibenzothiophene 5,5-dioxide at 690° gave a 95% yield of two products, dibenzofuran and dibenzothiophene in the ratio 6 1 (Fields and Meyerson, 1966a). The predominant reaction, therefore, involved overall loss of SO, presumably through an intermediate rearrangement... 

The reaction mechanism of photocyclization of aryl vinyl ethers was derived from results obtained by means of flash photolysis. The ground state intermediate rearranges by mono-or bi-molecular 1,4-hydrogen shifts to yield the products (Scheme 62) (81JOC978). The photocyclization of 2-aryloxyenones was used in the total synthesis of ( )-lycoramine (77JA8065). The formation of dihydrobenzo[6 ]furans by radical cyclization from o-allenyl-oxyarenediazonium salts with tri-n-butyltin(IV) hydride was successful (81CC136). 

The pathway which gives rise to the unrearranged 5-sulfinates [see Eq. (20)] is not obvious. A reasonable mechanism comprises dissociation of the olefin intermediate, rearrangement of the allylsulfinate anion, and then recombination of the ions [Eq. (23)]. Such a scheme accords with... 

Figure 3.15 Mustard oil formation. After hydrolysis of glucosinolates, the unstable intermediates rearrange. In general, the main reaction products are isothiocyanates, but nitriles and thiocyanates are also produced.

In the case of acylating or sulfonylating electrophiles, however, the acyl (sulfonyl) diaziridinium intermediate rearranges to give acyl (sulfonyl) hydrazones (Scheme 15) <2003HCA2490>. 

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