Sulfides Stevens rearrangement

Two efficient syntheses of strained cyclophanes indicate the synthetic potential of allyl or benzyl sulfide intermediates, in which the combined nucleophilicity and redox activity of the sulfur atom can be used. The dibenzylic sulfides from xylylene dihalides and -dithiols can be methylated with dimethoxycarbenium tetrafiuoroborate (H. Meerwein, 1960 R.F. Borch, 1968, 1969 from trimethyl orthoformate and BFj, 3 4). The sulfonium salts are deprotonated and rearrange to methyl sulfides (Stevens rearrangement). Repeated methylation and Hofmann elimination yields double bonds (R.H. Mitchell, 1974). 

Eq. (25) 5 . Various products have been reported in the photocatalyzed oxidation of dimethyl sulfide depending on the initial concentration of thioether. On TiO2, CdS, or ZnSe, a Stevens rearrangement occurs, Eq. (26) The key intermediate appears to be a dimethylsulfide dimer cation radical, and the reaction is only efficient in protic solvents. 

Boekelheide and his collaborators [407] have described a two-step sequence for transforming sulfide linkages to carbon-carbon double bonds — Stevens rearrangement of sulfur ylides and Hofmann elimination — which they found particularly useful for the synthesis of cyclophane derivatives, such as the [2.2]metaparacyclophane-l,9-diene shown. The Ramberg-Backlund rearrangement (see Section 4.3.2) was unsatisfactory for such highly strained molecules. 

STEVENS REARRANGEMENT. Migration of an alkyl group from a quaternary ammonium salt to an adjacent carbanionic center on treatment with strong base. The product is a rearranged tertiary amine, sulfonium, or sulfide. 

In 1928, T. S. Stevens reported the first example of a reaction which was later to be called the Stevens rearrangement. He found Aat, upon treatment with aqueous alkali, phenacylbenzyldimethylammonium bromide (1) was converted into l-benzoyl-2-benzyldimethylamine (2) in high yield (Scheme 1, equation a). Soon after, in 1932, Stevens reported the analogous rearrangement of the corresponding sulfbnium derivative. The sulfonium bromide (3) was smoothly transformed into a sulfide, which was initially formulated as structure (4), when (3) was treated with hot alkali (Scheme 1, equation b). Subsequent work, however, established that the correct structure of the rearranged material was (84) (Scheme 19). 

Although each of these ions reacts differently toward butyllithium, treatment with phenyllithium always gives predominantly the Stevens rearrangement, and treatment with amide ion in liquid ammonia gives predominantly the orfAo-rearrangement. Sulfonium and benzyl sulfides 59) on treatment with potassium amide undergo a similar c Ao-rearrange-ment which proceeds via the sulfur ylid. 

The benzyne-Stevens rearrangement continues to find utility as a carbon-carbon bond-forming reaction, especially in the synthesis of cyclophanes ". One example, the synthesis of strongly bent 617 from bis(sulfide) 616 via a double Stevens rearrangement, is shown. 

Dichlorocarbene inserts into allyl sulfides (67) via the Stevens rearrangement of the ylide intermediates. Internal competition for a carbene is always won by sulfur atom over olefins (68, 69), even though attack on sulfur would give rise to strained molecules. 

The transformation of quaternary ammonium salts 1 and sulfonium salts 3 to the corresponding amines 2 and sulfides 4 in the presence of a strong base is known as Stevens rearrangement. The salts are usually obtained by the alkylation of the corresponding amines and sulfides. The competing reaction is the Sommelet-Hauser rearrangement. 

The sulfide 23 was prepared from 21 via 22 in a two step process. Alkylation of 23 with methyl iodide afforded the sulfonium salt 24, which underwent Stevens rearrangement upon reaction with base to give the ferrocenophane 25. Oxidation of 25 followed by elimination gave the strained ethene bridged ferrocenophane 26. 

These rearrangements involve 1,2-shifts, and benzylsulfonium salts (24) behave like the corresponding benzyl sulfides in undergoing 1,2-shifts (Scheme 13) (Stevens (i) or Sommelet (ii) rearrangement). The direction of the rearrangement is sensitive to the experimental conditions, the Stevens pathway (i) being favoured at higher temperatures (Scheme 13). 

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