Cyclophanes, 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). 

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. 

Cyclizations by formation of carbon—selenium bonds represent a modern method with a high synthetic potential in the chemistry of cyclophanes. Selenocyanates such as 16 are accessible usually in excellent yields through the reaction of bromides with KSeCN [27], The reaction with benzylic bromides under reductive conditions using the dilution principle results in good to excellent yields of [3.3]di-selenacyclophanes which can be deselenized photochemically, pyrolytically (without previous oxidation), or by reaction with arynes, Stevens rearrangement and subsequent reaction with Raney nickel. [2.2]Metacyclophane (18), for example, is accessible in 47% total yield by using this sequence of reactions starting with... 

The dibenzodihydrothiepin 231 reacts in the same way as 229 with benzyne to give phenanthrene in high yield.115 The same pattern of reaction has been used to extrude sulfur atoms, usually two at a time, from larger rings (e.g., 232) for the synthesis of cyclophanes.116 However, in such cases elimination of thiophenol does not follow spontaneously after the Stevens rearrangement, and indirect methods for subsequent removal of the SPh groups (e.g., from 233) are necessary. 

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. 

A rhodium(II)-catalyzed double Stevens rearrangement of cyclophanes 92 led to ring expansion affording benzimidazolidinone cyclophanes 94 via the sulfonium intermediates 93.Irradiation of 93 in triethyl phosphite resulted in extrusion of the bridging sulfur atoms to give ring-contracted [3,3]heterophanes. 

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