Sigmatropic rearrangements mechanisms

M06/6-31-l-l-G calculations in the gas phase and using the CPCM (conductor-like polarizable continuum model) in solution have shown that the formal reaction of imidozirconocene with allylic ethers and an imidotitanium complex with allylic alcohols occurs by a [2 -I- 2] cycloaddition/retro- [2 -I- 2] pathway via central transition states rather than by the proposed [3,3] sigmatropic rearrangement mechanism. ... 

The 2-oxonia [3,3]sigmatropic rearrangement was also used by Ramachandran ti al. to explain their observation on the synthesis of 4-substituted homoallylic alcohols from crotylboronates [292]. The products from the ln(OTf)3 catalyzed reaction of crotylboronates and aldehydes were observed to be highly dependent on temperature, with the kinetic 2-substituted homoallylic alcohol exclusively observed at low temperature and the thermodynamic 4-substituted product observed at room temperature. This is provided that there is an excess of aldehydes, with less than stoichiometric amounts giving product mixtures (Figure 8.134). These results are consistent with the oxonia [3,3]sigmatropic rearrangement mechanism. The procedure was found to be useful for a variety of alkyl and aryl aldehydes. 

Apparatus 500-ml round-bottomed, three-necked flask, provided with a gas inlet tube, a mechanical stirrer and a thermometer-vent combination for the preparation of the a-chloroether 500-inl round-bottomed flask, provided with a gas inlet, a reflux condenser and a thermometer, dipping in the liquid for the elimination of HCT and the 3,3-sigmatropic rearrangement. 

A number of reaction pathways have been proposed for the Fischer indolization reaction. The mechanism proposed by Robinson and Robinson in 1918, which was extended by Allen and Wilson in 1943 and interpreted in light of modem electronic theory by Carlin and Fischer in 1948 is now generally accepted. The mechanism consists of three stages (I) hydrazone-ene-hydrazine equilibrium (II) formation of the new C-C bond via a [3,3]-sigmatropic rearrangement (III) generation of the indole nucleus by loss of... 

In accord with the experimental findings a mechanism via a [5,5]-sigmatropic rearrangement has been formulated. In a first step the hydrazobenzene is proto-nated to the dicationic species 3, in which the phenyl groups can arrange in such a way to allow for rearrangement ... 

In certain cases the reaction may proceed by a concerted mechanism. With allyl ethers a concerted [2,3]-sigmatropic rearrangement via a five-membered six-electron transition state is possible " ... 

An intramolecular rearrangement of the conjugate acid of the triazene compound to form the oc-complex without an additional molecule of amine would correspond to a thermal [l,3]-sigmatropic rearrangement. However, such a mechanism can be ruled out on the grounds of the antarafacial pathway required from orbital symmetry considerations (Woodward-Hoffmann rules). 

Since the ratio of the two sulfones 2 and 3 increases with the polarity of the solvent (from 1 4 in benzene to 16 1 in formamide) a possible concerted [2,3]sigmatropic rearrangement for the formation of sulfone 3 was first considered. However, other evidence such as the effects of solvent and added salts seem to support an ionization mechanism, with the formation of the two sulfones by recombination from two different ion-pair species40. 

In conclusion, it may be noted that the pioneering investigations by both the Mislow and Braverman groups have contributed to the elucidation of the mechanism of this concerted [2,3]-sigmatropic rearrangement, and have thus laid the ground for the following stereochemical and synthetic studies. 

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 mechanism is a concerted pericyclic [3,3] sigmatropic rearrangement and accounts for all these facts. For the ortho rearrangement ... 

At first glance the reaction does not seem to be a rearrangement. However, the key step of the mechanism is a [3,3] sigmatropic rearrangement... 

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