Sommelet-Hauser rearrangement mechanism

The mechanism of the Gassman indole synthesis bears some resemblance to that of the Fischer indole reaction. An Sn2 displacement of the TV-chlorinated aniline with the a-keto sulfide gives rise to a sulfonium ion. After deprotonation, a [2,3]-sigmatropic rearrangement (Sommelet-Hauser rearrangement) takes place where the weak N-S bond is broken and a strong C-C bond is formed. Similar to the Fischer indole reaction, the resulting imine was transformed into indole after re-aromatization and cyclization. 

The mechanism of the [2,3]-migration in a quaternary ammonium salt when treated with strong bases in the classic Sommelet-Hauser rearrangement is well studied, usually in contrast with the related [1,2]-migration observed in the same systems as a consequence of the Stevens rearrangement. Its mechanism was easily clarified by intermediate isolation and labeling experiments. ... 

Ghigo, G., Cagnina, S., Maranzana, A., Tonachini, G. (2010) The mechanism of the Stevens and Sommelet-Hauser rearrangements a theoretical study. The Journal of Organic Chemistry, 75, 3608 3617. 

The mechanism of the indolization of aniline 5 with methylthio-2-propanone 6 is illustrated below. Aniline 5 reacts with f-BuOCl to provide A-chloroaniline 9. This chloroaniline 9 reacts with sulfide 6 to yield azasulfonium salt 10. Deprotonation of the carbon atom adjacent to the sulfur provides the ylide 11. Intramolecular attack of the nucleophilic portion of the ylide 11 in a Sommelet-Hauser type rearrangement produces 12. Proton transfer and re-aromatization leads to 13 after which intramolecular addition of the amine to the carbonyl function generates the carbinolamine 14. Dehydration of 14 by prototropic rearrangement eventually furnishes the indole 8. 

There can be competition between Sommelet-Hauser and Stevens Rearrangement mechanisms ... 

Otter et al. (89JHC1851) obtained the thienopyrimidine 253 in 50% yield by heating 5-(2-propynylthio)pyrimidine 248 in DMSO at 145°C. The reaction proceeds via a Claisen rearrangement of 248 in the allene 249, which then cyclizes to the product 253. An 18% yield of the thiomethyl compound 252 was also isolated. This was explained by a mechanism involving solvent incorporation into allene 249 to give intermediate 250 this intermediate then undergoes a Sommelet-Hauser type of rearrangement to afford 251, which cyclizes to the product 252. 

The Gassman indole synthesis involves a one-pot process in which a hypohalite, a P-carbonyl sulfide derivative, and a base are added sequentially to an anihne or a substituted aniline to provide 3-thioalkoxyindoles. The mechanism of the Gassman indole synthesis involves a [2,3]-sigmatropic rearrangement (Sommelet-Hauser). The sulfur can be easily removed by hydrogenolysis or Raney nickel. 

On treatment with sodium amide in liquid ammonia, d undergoes a Sommelet rearrangement 188> (or/Ao-rearrangement) 81> to give 7 in 80— 90% yield. Kantor and Hauser 81> initially postulated an ylid mechanism... 

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