And the Sommelet Hauser rearrangement

Although nitrogen ylides have been extensively used in organic transformations, especially those concerning the formation of nitrogen heterocycles,not many references are available for the use aza-sulfonium ylides and the Sommelet-Hauser rearrangement in the synthesis of indole derivatives. 

The Sommelet-Hauser rearrangement competes when Z is an aryl group (see 3-26). Hofmann elimination competes when one of the R groups contains a 0 hydrogen atom (7-6 and 7-7). 

The chlorosulfonium compounds can also be used for the synthesis of oxindoles, the stable tautomeric form of 2-hydroxyindoles. The a -methylthio ketone is replaced by ethyl methylthioacetate and the products of the Sommelet-Hauser rearrangement are cyclized to 3-methylthiooxindoles. The methylthiooxindoles are reductively desulfurized with Raney nickel (equation 102) (74JA5508). 

The presence of an alkyl group or not on the benzylic carbon atom has little influence on the result (example 2). The Sommelet-Hauser rearrangement product is not obtained as its exo-1 -alkylidene-cyclohexadi-2,4-ene precursor is stable (allowed typical cyclohexadiene chemistry) and isolable as a 1 5 mixture (35% yield) of the two stereoisomers. Styrene is also obtained as the major product (50%) whereas the Stevens product is formed in trace amounts. Triene A is converted quantitatively into 2-ethylbenzylamine under treatment with diluted KOH at 60 °C for 0.5 h in ethanol.368... 

Modification of the stereochemistry of the reaction has been demonstrated by studying desilylation reaction of cis- and mmy-2-methyl-1 -(phenyl)isoindolinium 2-methylides. It has been established that cw-isomer leads preferentially to the Sommelet-Hauser rearrangement product (eight-membered ring formed) whereas the trans-isomer leads predominantly to the Stevens rearrangement product.374... 

The Sommelet-Hauser rearrangement has rarely been used as a tool in organic synthesis and then only for very special systems [54], The reason is undoubtedly that the Hofmann elimination and the dealkylation reaction of quaternary nitrogen atoms are in direct competition with the ring expansion reactions of these substrates. 

Benzylic quaternary ammonium salts, when treated with aUcali-metal amides, undergo a rearrangement called the Sommelet-Hauser rearrangement Since the product is a benzylic tertiary amine, it can be further alkylated and the product again subjected to the rearrangement. This process can be continued aroimd the ring until an ortho position is blocked. ... 

Another type of ylidic rearrangement, the Sommelet-Hauser rearrangement, was discovered later, in 1937, by Sommelet and studied extensively by Sommelet and by Hauser. This transposition involves the base-promoted rearrangement of non stabilized ammonium and sulfonium ylides. Thus, the ammonium salt (11), when heated with alkali, gave cleanly the substituted diphenylmethane derivative (12 Scheme 5). Wittig found that the dibenzyl ammonium salt (13) reacted with phenyllithium giving two... 

Initial work on the Sommelet-Hauser rearrangement was mechanistically inspired. Hauser and Kantor provided a major contribution to the understanding of this transformation. They showed that two ylides were involved in the rearrangement. The more stable ylide (421) is initially generated but equilibrates with the less stabilized one (422). The latter ylide then undergoes a concerted, symmetry-allowed 3,2-sigmatropic shift, giving the triene (423). Aromatization finally affords the orrAo-substituted benzylic amine (424 Scheme 108). 

Finally, attempts have been made to induce asymmetry in the Sommelet-Hauser rearrangement, but with limited success. Campbell rearranged the optically active sulfonium salt (449) and obtained a mixture of the two Sommelet-Hauser products (450) and (451). The chiral product (451) displayed only low optical activity (Scheme 116). 

In the presence of NaNH2 benzylic quaternary ammonium salts generally lead to the Sommelet-Hauser rearrangement (refs. 90, 92-104). An ortho alkylation takes place via an exomethylene intermediate. If the two ortho and ortho positions are methylated, the methylene compounds can be isolated (refs. 92, 93). The first anion formed in this reaction can be trapped at a very low temperature (-80 °C) in an aldol reaction for example (ref. 103). At -30°C the isomerization and the rearrangement occur (Fig. 16). 

In some cases ring expansion has been obtained during the Sommelet-Hauser rearrangement (refs. 94, 100). This rearrangement also occurs with naphthalenic derivatives (Fig. 18) and with many other heterocyclic compounds (refs. 105-107). 

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