Oxidation Oxyallyl cations

Tricarbonyl[t/M-(ethoxycarbonyl)-l//-azepine]iron(0) (30) with the 2-oxyallyl cation derived from 2,4-dibromo-2,4-dimethylpentan-3-one and nonacarbonyldiiron(O) yields a mixture of adducts which, after oxidative decomplexation with tetrachloro-l,2-benzoquinone (o-chloranil), affords the tetrahydrofuro[2,3-A)azcpine derivative 33 and the 3-substituted 1H-azepine-l-carboxylate 34.227... 

If a suitable (1,3-di-f-butyl) allene is epoxidized with m-CPBA the unstable allene oxide can actually be isolated. On heating, this epoxide gives a stable fra s-di-f-butylcycl6propanone. It is very difficult to see how this reaction could happen except via the oxyallyl cation intermediate. 

The same cyclopropanone gives a cycloadduct with furans—this must surely be a reaction of the oxyallyl cation and we can conclude that the three isomeric reactive intermediates (allene oxide, cyclopropanone, and oxyallyl cation) are all in equilibrium and give whichever product is appropriate for the conditions. 

Allenamides have been used as precursors for oxidation reactions <2002JOC1339>. The allene oxide products that are produced are useful oxyallyl cation equivalents for cycloaddition chemistry (Scheme 30) <2001JA7174, 2003JA12694>. 

Hoffmann s cycloaddition (67) of the oxyallyl cation 272, generated from 2,4-dibromopentan-2-one with LeGoff s zinc-copper couple (68). Hydrogenation followed by Baeyer-Villiger oxidation gave the lactone 274, with C-2, C-3 and C-6 correctly set up. Methanolysis gave a single hydroxyester,... 

Phosphole oxides also participate in a cycloaddition reaction with oxyallyl cations <75T53>, a process that gives rise to novel phosphatropolone structures. Preformed 1,2,5-triphenylphosphole oxide gave a derivative of the 8-phosphabicyclo(3.2.1]octan-6-ene-3-one ring system ((157), 91%). The oxyallyl cation (156) was prepared from a,a -dibromodibenzylketone and Nal in acetonitrile (Scheme 31). 

The sensitivity of S to electrophilic reagents proved to be a complication when an attempt was made to perform cycloaddition with the oxyallyl cation, which had worked well with phosphole oxides. The reagent attacked on S, presumably to give a spirophosphorane, such as (177), which rearranged to leave oxygen on phosphorus as in (178) (Scheme 38) (31% <75T53 . Other products... 

Solvolysis of the bicyclic mesylate (69) gave a mixture of rearranged olefins which were hydrogenated to give the hydroazulene (70). Oxyallyl cations react with phosphole sulphides and phosphole oxides to give the expected 8-phosphabicyclo [3,2,l]octanoic systems together with other products. ... 

Dimethylcyclopropanone remains in equilibrium with zwitterionic oxyallyl cation 92 in situ and undergoes D-A reaction with furan. Similarly, allene oxide 92a generated from silyl epoxide gives cyclopropanone and oxyallyl cation, which is trapped as furan adduct [79]. 

This looks as though each of the C—C bonds is independently the result of both HOMO/LUMO interactions, with an endo selectivity as well. In the presence of dienes, these species behave as allyl cations (see p. 259) and undergo clean [4 + 2] cycloadditions, as in the reaction of the oxyallyl 6.372 giving the tricyclic ketone 6.373, which is similar to the diene 6.369. Normally, oxyallyls are in equilibrium by disrotatory electrocyclic ring closures with cyclopropanones and with allene oxides, but the presence of the five-membered ring in these particular examples makes these pathways counter-thermodynamic. 

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