2-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... 

An endo-Selective Sequential Epoxidation-Oxyallyl Cycloaddition and the First Nitrogen-Stabilized Oxyallyl Cations. 

Epoxidations of chiral allenamides lead to chiral nitrogen-stabilized oxyallyl catioins that undergo highly stereoselective (4 + 3) cycloaddition reactions with electron-rich dienes.6 These are the first examples of epoxidations of allenes, and the first examples of chiral nitrogen-stabilized oxyallyl cations. Further elaboration of the cycloadducts leads to interesting chiral amino alcohols that can be useful as ligands in asymmetric catalysis (Scheme 2). 

A rare example of a higher-order [5 + 3]-cycloaddition was reported by Liesbeskind and Arrayas. Homochiral 7]3-pyranyl- and // -pyridinyl-molybdcnum 7r-complexes react with an oxyallyl cation that is generated in situ from the precursor 88. After decomplexation of the molybdenum, oxa- and azabicyclo[3.3.1]nonenes are obtained in moderate yields and in excellent ee s (Scheme 27).89 Related work using 3-pyranyl- and 3-pyridinyl-molybdenum 7r-complexes as five-carbon components in [5 + 2]-cycloadditions is discussed in Section 10.13.2.1.2. 

Ring opening reaction of alkylidenecyclopropanone acetals readily proceeds in the presence of Lewis or Bransted acids to produce l-alkylidene-2-oxyallyl cation, which is provided for the reaction with nucleophiles such as chloride, alcohols, siloxyalkenes, and furans. The reaction of this cation with the carbon nucleophiles gives products of [4 + 3] and [3 + 2] cycloaddition as well as those of nucleophilic addition. The modes of addition reactions are controlled by the oxy group of the cation and by the reaction conditions including solvent. 

The ring-opened intermediates, which are known as oxyallyl cations, can also be generated by a number of other reaction processes.97 98 99 100... 

The azepine-carbonyliron complex is also attacked at C-3 by the 2-oxyallyl cation (98) to give initially the charged intermediate (99), and finally either the furanoazepine (100) or the ketone (97 Me2CHCOCH2 in place of COR), as their tricarbonyliron complexes... 

The intramolecular 4 + 3-, 3 + 3-, 4 + 2-, and 3 + 2-cycloaddition reactions of cyclic and acyclic allylic cations have been reviewed, together with methods for their generation by thermal and photochemical routes.109 The synthetic uses of cycloaddition reactions of oxyallyl cations, generated from polybromo and some other substrates, have also been summarized seven-membered rings result from 4 + 3-cycloadditions of these with dienes.110 The use of heteroatom-stabilized allylic cations in 4 + 3-cycloaddition reactions is also the subject of a new experimental study.111 The one-bond nucleophilicities (N values) of some monomethyl- and dimethyl-substituted buta-1,3-dienes have been estimated from the kinetics of their reactions with benzhydryl cations to form allylic species.112 Calculations on allyl cations have been used in a comparison of empirical force field and ab initio calculational methods.113... 

Debromination. C24Fe effects stereospecific anri-debromination of wc-dibrom-ides(THF, 70°).1 It is an effective substitute for Fc2(CO)<,2 for generation of 2-oxyallyl cations from a,a -dibromo ketones (5, 222-223 6, 195-196 9,477-478). 

Cycloadditions. Silver perchlorate converts the l,l-dimcthyl-2-(tri-methylsiloxy)allyl chloride (1) and related allylic chlorides into an oxyallyl cation (a), which reacts with 1,3-dienes to form seven-membered ketones, often in high yield, particularly in reactions conducted in nitromethane. 

The competing pathways in the reactions of intermediate l-alkylidene-2-oxyallyl cations (47) with furan have been investigated.54 Cycloaddition pathways compete with the electrophilic substitution pathway which initially forms the cation (48) from which a number of products may form. 

In an effort to provide experimental evidence for the formation of a second oxyallyl cationic intermediate, a gem-dichlorocyclopropane substrate was envisaged with two internal nucleophiles one to participate in the initial interrupted Nazarov reaction, and the other to capture the second cationic species. Surprisingly, when the carefully designed substrate 80 was subjected to the optimized reaction conditions, an alternate mode of trapping occurred to generate the intriguing bridged bicyclic product 81 (Scheme 4.25). 

The formation of cyclic oxyallyl cations from dehydrochlorination of chlorocyclopenta-nones is precedented in the literature see Harmata, M. Elomari, S. Barnes, C. F., J. Am. Chem. Soc. 2004, 82, 375-385. 

This chapter does not cover oxyallyl cations which are usually formed from halides, since these have been reviewed recently2, as were halo-containing heterocumulenes3. 

The scope of this chapter is based on the analogous one by Naso and Marchese published in 19831. Since the time of their review, the focus of work in this area has shifted somewhat, and sections have been added or omitted to reflect that focus. New sections covering chromium(II)-mediated reactions of halides and covering cobalt-mediated radical reactions of halides have been added. In view of the relatively mature nature of the areas, sections dealing with 7c-allylnickel complexes, iron oxyallyl cations and cyanation reactions have been omitted. 

Wright and co-workers developed a novel entry into spirocyclic, six-membered O-containing heterocycles as they combined oxyallyl cation chemistry profitably with a ROM-RCM process <02AG(E)4560> (Scheme 41). An analogous process, involving the Diels-Alder reaction of 57 with iV-phenylmaleimide followed by ROM-RCM, gave 58 in excellent yield. 

Allylsilanes act as good acceptors of nitrones and oxyallyl cations. The 1,3-dipole species arising from electronically activated cyclopropanes can be trapped by allylsilanes.203 204 2043 Epoxides as well as aziridines act as 1,3-dipole precursors for inter- and intramolecular [3 + 2]-cycloadditions with allylsilanes.205 2053 206 2063... 

Other chemists prefer a pericyclic description of the ring-closure step. The same enolate simply loses chloride to give an oxyallyl cation —a dipolar species with an oxyanion and a delocalized allylic cation. This species can cyclize in a two-electron disrotatory electrocyclic reaction (Chapter 36) to give the same cyclopropanone. We shall return to this discussion in the next chapter but, whatever the mechanism, there is no doubt that a cyclopropan one is an intermediate. 

Now comes the most interesting step in the whole process—a step that unites a cycloaddition and a rearrangement and sets the scene for a fragmentation. The idea was to treat the bromoketone with base to make an oxyallyl cation as an unstable intermediate. 

The oxyallyl cation with its two electrons delocalized over the allylic system would add to furan in a [2 + 4] cycloaddition to give a new cation stabilized by the oxyanion or, in more familiar guise, a ketone. The reaction was supposed to go like this. 

What is happening here is that the oxyallyl cation is in equilibrium with the cyclopropanone by an electrocyclic reaction (Chapter 36) and the alcohol is capturing this unstable ketone by nucleophilic li addition. Hemiacetals of cyclopropanones form spontaneously in alcoholic solution (Chapter 6)... 

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