Stevens-type rearrangement

The reaction of 1,3-dibromopropane with methyl methylsulfanylmethyl sulfoxide in the presence of base gave cyclobutanone via the intermediate formation of ftve-membered cyclic sulfonium salt 6, which was finally converted to 1-methylsulfinyl-l-methylsulfanylcyclobutane (7 a) by a Stevens-type rearrangement in 70% yield which on acid hydrolysis gave cyclo-butanone.142... 

The synthesis of 3-benzylcyclobutanone (3) is an illustration of an overall intramolecular alkylation of an acyl anion equivalent (Section 5.9). The a,a>-dihalide is 2-benzyl-l,3-dibromopropane, and the acyl anion equivalent is methyl methylthiomethyl sulphoxide2 the product is 1-methylsulphinyl-l-methylthio-3-benzylcyclobutane which is obtained as a mixture of cis/trans isomers [(9) and (10)] (Expt 7.3). Aqueous acid hydrolysis in ethereal solution unmasks the carbonyl group. The possible mechanism of the reaction is via a Stevens-type rearrangement of the intermediate sulphur ylide, which may proceed in a pericylic, radical or ion pair fashion. 

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... 

The azathiaphenanthrene 1045 undergoes a thermal rearrangement in refluxing xylene involving ring expansion to yield the dibenzothiazepine 1047 via a Stevens-type rearrangement of the methylide intermediate 1046. 

Less common as a synthetic method is the rearrangement under mild conditions of some hydrazinium salts. In contrast to the severe conditions necessary (200 C, solid KOH) to rearrange 1,1,1-trialkylhydrazinium salts [60], 1,1-dimethyl-l-phenacylhydrazinium bromide is transformed merely by refluxing in pentanol or pyridine into 2-benzoyl-4-phenylimidazole (65-73%). This is a Stevens-type rearrangement [61, 62]. 

The most popular mechanisms at present invoke oxonium ylides as intermediates. van den Berg et al. [20] proposed that DME is protonated by a Bronsted site, and the resultant ion suffers nucleophilic attack by a second molecule of DME to form TMO with release of MeOH. The TMO ion is then deprotonated by a basic site to form the dimethyloxonium methylide, which undergoes a Stevens-type rearrangement to give methylethyl-oxonium ion. MeOEt is subsequently formed upon p— elimination. No experimental evidence was offered in support of the scheme. 

At this point, three questions must be addressed. Do oxonium ylides indeed exist If so, will they undergo a Stevens-type rearrangement Finally, will a zeolite conjugate base be sufficiently basic for proton abstraction from an oxonium ion to generate the ylide ... 

The observed deuterium distribution in MeUEt appeared to rule out the intermolecular Stevens-type rearrangement. 

According to Huisgen [26] the reaction proceeds via ylide formation followed by a Stevens-type rearrangement. This was disputed by Franzen and Fikentscher [27] who found, using CH2, that EtOPr from EtOEt was labelled in the 7-position rather than the a-position expected from an ylide rearrangement. 

More modem technology has also product some interesting results. Quatemization of -NfJ-di-methylaminopyrrole and treatment of the product with sodium methoxide in DMSO leads to Stevens-type rearrangement products. No CIDNP was observed in an NMR study of the rearrangement and it was concluded that the ylide fragments to an ion pair, which recombines as shown in Scheme 4. 

The intermolecular nature of the C1-C2 transformation step was shown by experiments using mono- C labelled dimethyl ether and analyzing the isotopic composition of the product ethylene. The intramolecular Stevens-type rearrangement under the reaction conditions was clearly ruled out. 

Without additional reagents Semicarbazides from carbamylaminimides Thermal Steven-type rearrangement... 

A variety of differently fused indoles 155 can be efficiently assembled via the W(0)-catalyzed cycloisomerization of o-alkynylaniline derivatives 154 under photoirradiation conditions. The proposed mechanistic rationale featured a 1,2 Stevens-type rearrangement in the metal-containing ammonium ylide 157, followed by a 1,2-alkyl... 

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