Double Stevens rearrangements

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. 

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). 

Such reductive ring contractions of sulfones are formally similar to two other methods capable of supplanting a sulfur atom by a carbon-carbon double bond the Ramberg-Backlundand Stevens rearrangements. The distinguishing feature of this novel approach to cyclobutenes consists in the resultant higher level of alkyl substitution at the sp -hybridized centers. 

At low temperatures cyclopropenones and enamines or ketene acetals were shown to yield 2-azonia-bicyclo(3,l, 0)hex-3-enolates-3 (371, X=0), which can be isomerized thermally to penta-2,4-diene amides(372, X=0). At elevated temperatures the amides were found to be the principal products arising from C-N-insertion 237) (insertion of the cyclopropenone three-carbon unit into the C-N bond of the enamine). These were accompanied in some cases by 3-aminoenones 373 arising from C-C-insertion 237) (insertion of the cyclopropenone into the C-C double bond of the enamine) and a-amino cyclopentenones 375 formed by Stevens rearrangement of the ylide 369 and cyclopentenones 374 ( condensation 237)). 

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. 

The addition of thiols to a non-activated double bond is catalysed by In(OTf)3 and when applied to 6-methylhept-5-en-2-thiol results in cyclisation to the tetrahydrothiopyran (Scheme 44) <06CC332>. Tetrahydrothiopyran-2-nitrile has been obtained by a base or electrochemically induced Stevens rearrangement of the S-ylide, 1-cyanomethyl-tetrahydrothiophenonium bromide <06CL98>. 

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