Benzyne-Stevens rearrangement

Ramberg-Backlund reaction (equation 29), ° can be prepared in higher yield using the benzyne-Stevens rearrangement, but, when this approach is used, it is always contaminated with an equal amount of the inseparable regioisomeric [3,3]paracyclophanediene. 

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. 

Another example is the reaction of benzyne with 1-methyl- (and benzyl)-3, 3-dimethylbenzo[d]-l,3-azasiloline (see their preparation in Section IV.A.2.p). Stevens rearrangement product is obtained (48%) as a single compound. If a Me on nitrogen is replaced by an Et, then Hoffmann elimination occurs.251 It should be noted that an Y-alkyl MSMA is converted into an Y-aryl RSMA in the former case and into a Y-aryl MSMA in the latter case. 

N,AT-Dimethylpiperazine reacts similarly with benzyne (from bromoben-zene and sodium amide) to give N-methylaniline (214 R = Me), 217 (corresponding to 213 R = Me), and a third product 216 which is formed via Stevens rearrangement of the ylid 215.108... 

Reaction of the naphthothiin 229 with benzyne produces acenaphthylene in high yield via formation and Stevens rearrangement of the sulfur ylid 230 (Scheme 22).115... 

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. 

In an nmr study of the reaction of N,N-dimethylbenzylamine with benzyne, Lepley 98> observed a free radical intermediate (path 8) which is formed from the ylid and which rearranges to N-methyl-N-(a-phen-ethyl)aniline via a Stevens rearrangement. Prior to this report 8 ") radical intermediates had not been observed in the Stevens rearrangement but their presence had been suggested m>. This radical path must be considered in all reactions of nitrogen ylids for it may be the route by which many rearrangement reactions occur. 

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 ylides 31 and 39, the key intermediates of both the Stevens and Sommelet rearrangement discussed above, can be generated also by addition of benzyne 38 to N-alkylamines. Thus, generation of benzyne 38 in the presence of A /V-dimethylbenzylamine... 

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