Cyclization-induced rearrangement

The mechanism of the rearrangement catalyzed by Pd(fl), typically by PdCl2(RCN)2, is explained by the oxypalladation of an alkene to form 810 as an intermediate, or cyclization-induced rearrangement. As a limitation, no rearrangement takes place when the allylie ester 812 is substituted at the C-2 position of the allyl group, while a smooth rearrangement of 811 takes place[500]. 

Pd(II)-catalyzed Cope rearrangement [225] occurs at room temperature, via chairlike transition states. A plausible mechanism is cyclization-induced rearrangement. Both the addition and fragmentation steps are assisted by the introduction and removal... 

Electrophilic transition metals, particularly palladium(II) salts, catalyze a number of heteroatom allylic transposition processes38 by a mechanism which almost certainly involves nucleophilic attack of a heteroatom on a metal-bound alkene (Scheme 10), often termed cyclization-induced rearrangement . 

The reaction of crotonaldehyde and methyl vinyl ketone with thiophenol in the presence of anhydrous hydrogen chloride effects conjugate addition of thiophenol as well as acetal formation. The resulting j3-phenylthio thioacetals are converted to 1-phenylthio-and 2-phenylthio-1,3-butadiene, respectively, upon reaction with 2 equivalents of copper(I) trifluoromethanesulfonate (Table I). The copper(I)-induced heterolysis of carbon-sulfur bonds has also been used to effect pinacol-type rearrangements of bis(phenyl-thio)methyl carbinols. Thus the addition of bis(phenyl-thio)methyllithium to ketones and aldehydes followed by copper(I)-induced rearrangement results in a one-carbon ring expansion or chain-insertion transformation which gives a-phenylthio ketones. Monothioketals of 1,4-diketones are cyclized to 2,5-disubstituted furans by the action of copper(I) trifluoromethanesulfonate. ... 

These authors also showed that the indolizidine skeleton can be prepared from cyclopropyl dipolarophiles (Scheme 1.16). The cycloaddition of alkyhdenecyclo-propanes 67 with various nitrones (e.g., 68) afforded the expected isoxazolidine adducts 69 and 70, commonly forming the C(5) substituted adducts 70 (97,105-108) predominantly but not exclusively (109-111). Thermally induced rearrangement of the spirocyclopropyl isoxazolidine adduct 70 afforded the piperidinones 71 (107,108). These authors propose reaction via initial N—O bond homolysis of 70 to diradical 72 followed by ring expansion through relief of the cyclopropyl ring strain forming the carbonyl of a second diradical intermediate 73, which cyclizes to afford the isolated piperidinone 71. 

Despite many attempts it has not been possible to oxidize 2-substituted 1,2,3-triazoles 382 to the corresponding 1-oxides 326. Peracetic acid, 3-chloroperbenzoic acid, dichloropermaleic acid, trifluoroperacetic acid, peroxydisulfuric acid, and f-pentyl hydrogen peroxide in the presence of molybdenum pentachloride all failed to oxidize 382 (1981JCS(P1)503). Alkylation of 1-hydroxytriazoles 443 invariantly produced the isomeric 3-substituted 1,2,3-triazole 1-oxides 448 (see Scheme 132). However, the 2-substituted 1,2,3-triazole 1-oxides 326 can be prepared by oxidative cyclization of 2-hydroxyiminohydrazones (1,2-hydrazonooximes, a-hydrazonooximes) 345 or by cyclization of azoxyoximes 169. Additional methods of more limited scope are reaction of nitroisoxazoles 353 with aryl-diazonium ion and base, and reaction of nitroimidazoles 355 with hydroxy-amine- or amine-induced rearrangement of nitro-substituted furoxanes 357. 

A mechanism involving Norrish type I cleavage followed by radical cyclization has been proposed to explain this photolytically induced rearrangement.An alternative possibility might involve bonding from the carbonyl carbon to the 6-carbon followed by alkoxy radical expulsion and ring closure. 

In the elaboration of cyclopentenones the Nazarov cyclization (not a cycloaddition reaction) also offers some advantages. Metal complexes including a V(IV) chelate of salen 60 and the Cu complex of ent-96B have been employed as catalysts. By structural demand of the substrate to induce rearrangement following the cyclization a synthesis of spirocycles is realized. 

The ene reaction (of, for example, the pinenes) with PhS02NS0 has been described, and the cyclization-induced [3,3] sigmatropic rearrangement of allylic carbamates e.g. linalyl A,A-dimethylcarbamate) catalysed by mercuric... 

The products from the thermolysis of anthranils bearing a heteroaryl residue at position 3 are less predictable. 3-(2-Pyridyl)anthranils in boiling trichlorobenzene yield pyrido[l,2-h]cinnolin-6-iums (e.g., 139) (cyclization of the initially formed nitrene taking place only at the electron-rich nitrogen center) and no rearranged products.191 In contrast, the 3-(2-imidazolyl)-anthranil 108 (X = N, Y = CH) yields a mixture of the nitrene-induced rearrangement product (157) (63%) and the nitrene insertion product (158) (3%).131 The 3-(5-pyrazolyl)anthranil 108 (X = CH, Y = N) behaves similarly. 

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