Labelling experiments rearrangement

A 1,2-shift has been observed in radicals bearing an OCOR group at the p-carbon where the oxygen group migrates as shown in the interconversion of 36 and 37. This has been proven by isotopic labeling experiments and other mechanistic explorations. A similar rearrangement was observed with phosphatoxy alkyl radicals such as 38. ... 

The enzymes in the so-called "isomerase reactions contain 5 -deoxy-adenosylcorrinoids. Labeling experiments have been used to identify the C, H, and O atoms vv hich have moved in the course of the rearrangement and to show that during the reaction the hydrogen atoms of the substrate exchange with the hydrogen atoms of the C-5 atom coordinated to the cobalt ion, but not with the solvent. There is also some spectroscopic evidence that the Co—C bond is broken during the reaction. 

The isomerization mechanism is clearly established by labeling experiments. The rearrangement of a to c via a 7i-allyl hydride complex b in the coordination sphere of the metal is a key step in this cayalytic cycle (Scheme 54) [174, 175]. hi case of polyunsaturated derivatives, formation of a stable q" complexes (Scheme 55) is preferred over the rearrangement (a c). 

As a mechanistic hypothesis, the authors assumed a reduction of the Fe(+2) by magnesium and subsequent coordination of the substrates, followed by oxidative coupling to form alkyl allyl complex 112a. A ti—c rearrangement, followed by a syn p-hydride elimination and reductive elimination, yields the linear product 114 with the 1,2-disubstituted ( )-double bond (Scheme 29). This hypothesis has been supported by deuterium labeling experiments, whereas the influence of the ligand on the regioselectivity still remains unclear. 

The rearrangements of b-(acyloxy), (i-(phosphatoxy)alkyl, and related systems have been reviewed [51,52] and representative kinetic data are given in Table 2 above. As revealed by isotopic labeling experiments, the acyloxy... 

Labelling experiments using both 15N and 2H indicate that the rearrangement is intramolecular. Reactions are also acid-catalysed and are believed to occur via the Wheland intermediates 74 and 75. The most likely interpretation is that the rearrangement occurs within the Wheland intermediate by a direct 1,3-shift rather than by consecutive 1,2-shifts, and that the process can be regarded as a typical [l,5]-sigmatropic rearrangement. 

Aromatization of dihalocarbene adducts to 1,4-cyclohexadiene or synthetic equivalents is the method of choice for the synthesis of the parent benzocyclo-propene (1). ° The mechanism of the aromatization step of the intermediate 7,7-dihalogenobicyclo[4.1.0]hept-2-ene (51) has been shown by labeling experiments with 51 depleted of C at Cl, to proceed via a series of elimination and double bond migration steps via cyclopropene- and alkylidenecyclopropane intermediates 52 to 54 with preservation of the original carbon skeleton. The synthesis of the benzannelated homologue, l//-cyclopropa[b]naphthalene (42), by the same route confirms these findings. Some skeletal rearrangement has, however, been observed in an isolated case. ... 

Based on the labeling experiments, a plausible mechanism involving mthenium vinylidene intermediates SS is proposed in Scheme 6.21. Cydization of this vinylidene intermediate leads to the formation of the epoxy carbenium 56, which then undergoes an epoxide opening to form l,4-dien-3-ol 57. A subsequent pinacol rearrangement of this alcohol furnishes ketone 58, providing the required skeleton for the observed phenol product 54. 

Rearrangement of a silene to a silylene (40) via migration of a Me3Si group has been suggested as a step in the gas-phase silylene to silylene rearrangement. Labeling experiments, however, have indicated an alternative mechanism (Scheme 14.23). ... 

An unusual sulfur-nitrogen donor, benzothiazole-2-thiolate (58), has been reacted with Vaska s complex to produce 59 in high yield no bidentate adducts of 58 are produced even in refluxing solvent (156). Upon reaction with dioxygen, the extremely sensitive and reactive complex 60 is produced. Addition of water to 60 caused rearrangement to the carboxylate complex 61, while the addition of sulfur dioxide to 60 produces 62 (see Scheme 11). A proposed mechanism for the reaction of water with 60, based on labeling experiments, was outlined and can be found in Scheme 12. 

When the Wolff rearrangement is carried out photochemically, the mechanism is basically the same,162 but another pathway can intervene. Some of the ketocarbene originally formed can undergo a carbene-carbene rearrangement, through an oxirene intermediate.168 This was shown by l4C labeling experiments, where diazo ketones labeled in the carbonyl group... 

Basic information concerning the mechanism of skeletal rearrangement was provided by labeling experiments and kinetic studies. The use of specifically prepared catalysts, such as metal films and alloys, and structure sensitivity studies supplied additional data. The information resulted in establishing two basic processes the bond shift and the cyclic mechanisms.151-154... 

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