Sommelet-Hauser rearrangement, Stevens

Several limitations of this base-promoted rearrangement are worthy of mentioning (a) when structurally feasible, both Stevens and Sommelet-Hauser reactions will compete (b) certain substituents on the aromatic ring, e.g. Cl, CN, NO2, prevent the formation of the ylide and no rearrangement takes place (c) if a p-hydrogen is present on the ammonium salt, elimination becomes yet another undesirable side reaction, In an attempt to overcome these limitations, Sato has utilized the fluoride-induced desilylation of... 

Giumanini et al.7B have studied rearrangements of N,/V-dimethyl-/V-bis(2-thenyl)ammonium salts (39, X = Cl, I). The two products, 40 and 41 (Eq. 16), arise by the Stevens and Sommelet-Hauser rearrangements, respectively it was found that the product ratio depended on the nature of the base, the solvent, and the temperature. The highest proportion of the dithienylmethane 41 (75%) in the product was obtained with sodamide in liquid ammonia at —45°C. 

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. 

For a method that uses nonbasic conditions, and gives high yields of the Sommelet-Hauser product, with little or no Stevens rearrangement, see Nakano, M. Sato, Y. J. Org. Chem., 1987, 52, 1844 Shirai, N. Sato, Y. J. Org. Chem., 1988, 53, 194. 

Ammonium ylides can isomerize to (1,2) rearrangement products (Stevens rearrangement) or to (2,3) shift products (Sommelet-Hauser sigmatropic rearrangement) when allyl or benzyl are located on the nitrogen atom. A strong microwave effect is noticed (Eq. 66) [116]. 

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

Reaction of benzyne with tertiary aliphatic amines to give anilines via Sommelet-Hauser and/or Stevens-type rearrangements of the intermediate ylid has been documented.362 363... 

Fluoride ion-assisted desilylation has been extensively used to create an ylid from a /V-silyl methyl-quaternary ammonium salt. Its evolution to final produces) is variable and Sommelet-Hauser and Stevens rearrangement products were obtained (often as major products) in a ratio that can be shifted from one structure to another very close one, as in examples 1 and 2 dealing with //-benzyl salts.246,366 Differences in the solvents used are not significant because in the first example, HMPA does not reverse the ratio, yields and selectivity being just a bit lower, /so-toluene was proposed as an intermediate in example 1 it might also be the intermediate in example 2. Thus product partition reflects the relative ability of the C-H or the C-C bonds to be cleaved to produce aromatization with proton or a-amine carbocation migration. 

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 variants with allylic ammonium and sulfonium ylidesl are well known and correspond to the Sommelet-Hauser and the Stevens rearrangement. Two representative examples are shown in Schemes 50 and 51. 

The base-induced 2,3-shift in a benzyltrialkylammonium halide (Sommelet-Hauser synthesis) efrects an orr/io-selective aminomethylation of aromatics. A variation (orr/to-selective formylation) is shown in equation (37). Unfortunately, the 1,2-Stevens rearrangement (— 10% 143) of the intervening ylide competes with the 2,3-pathway (— 142). A reliable tool to circumvent this very general difficulty is still unknown. Nonetheless, 2,3-rearrangement pathways can be very efficient as demonstrated by the synthesis of the cyclic amine (144 equation 38). ... 

In 1930, Ingold and Jessop reported the rearrangement of the fluorenyl ylide (412).Compound (414) was obtained in what can be considered as the first example of a Sommelet-Hauser reaction (Scheme 106). In 1937, Sommelet described the base-catalyzed transformation of the benzhydryl ammonium bromide (415) into the substituted benzylic amine (416 Scheme 107, equation a)." Wittig found later that reacting the salt (417) with phenyllithium produced two amines, (418) and (419). The former is derived from a Sommelet-Hauser 3,2-rearrangement, whereas the latter comes from a Stevens... 

In some cases, the Stevens rearrangement or an elimination (when a (3-hydrogen atom is present) can compete with the Sommelet-Hauser rearrangement (ref. 104) as shown in Table I. 

R Elimination % Sommelet-Hauser rearrangement % Stevens rearrangement %... 

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