Allyl cation, trapping

Further variations of the general scenario described in Scheme 4 consist in trapping adduct radical 48 before oxidation occurs7. This can be achieved if intramolecular radical additions are possible, as is the case in radical 62. Oxidation of 62 to the corresponding allyl cation is slower than 6-ew-cyclization to the chlorobenzene ring to form radical 63, which subsequently is oxidized to yield tetrahydronaphthalene 64 as the main product (equation 27). This sequence does not work well for other dienes such as 2,3-dimethyl-1,3-butadiene, for which oxidation of the intermediate allyl radical is too rapid to allow radical cyclization onto the aromatic TT-system. 

Iron(II) salts, usually in conjunction with catalytic amounts of copper(II) compounds, have also been used to mediate radical additions to dienes91,92. Radicals are initially generated in these cases by reductive cleavage of peroxyesters of hydroperoxides to yield, after rearrangement, alkyl radicals. Addition to dienes is then followed by oxidation of the allyl radical and trapping by solvent. Hydroperoxide 67, for example, is reduced by ferrous sulfate to acyclic radical 68, which adds to butadiene to form adduct radical 69. Oxidation of 69 by copper(H) and reaction of the resulting allyl cation 70 with methanol yield product 71 in 61% yield (equation 29). 

This scheme can be extended by using mixtures of dienes with electron-deficient alkenes such as acrylonitrile. Due to its nucleophilic nature, addition of radical 68 to acrylonitrile is faster than addition to butadiene. The resulting ambiphilic adduct radical then adds to butadiene to form a relatively unreactive allyl radical. Oxidation and trapping of the allyl cation by methanol lead, as before, to products such as 72 and 73, which are composed of four components the radical precursor 67, acrylonitrile, butadiene and methanol (equation 30)17,94. 

The allylic cation (40), formed in a specific acid-catalysed process, is relatively stable thermodynamically, stable enough towards trapping by nucleophiles that the reaction product obtained is almost invariably the naphthalene elimination product. di-Enediynes (42) are formed regiospecifically when the allylic cation (41) is trapped as shown. The walking of methanol around optically active l-methyl-3-ethylallyl... 

The cyclopropyl-allyl rearrangement has been shown to proceed with nucleophilic assistance,87i232<233 and the intermediate allyl cation can be trapped by nucleophiles leading to synthetically useful derivatives. An example is the formation of an unsaturated acetal and the propiolic acid ortho ester (equations 92 and 93).232... 

The allyl cation generated by the electrocyclic cleavage of dibromocyclopropanes (cf. Section 4.7.3.7.1) has been trapped by a carboxyl group in a highly efficient synthesis of furanones (equation 111) and pyranones.236 Acid-catalyzed openings of dihalocyclopropanes also give similar results.237... 

An elegant synthesis of the neurotoxic alkaloid anatoxin has exploited the electrocyclic opening of the dibromobicyclo[5.1.0]octane followed by transannular cyclization (Scheme 13).238 Similarly, the thermal electrocyclic opening of the dichlorocyclopropane followed by intramolecular trapping of the developing allylic cation by a suitably positioned amine has been used in a homoaporphine synthesis.238... 

Sometimes, probably due to the acidity of the DDQ hydroquinone generated during DDQ oxidations, unsaturated alcohols can generate allylic cations that can be trapped with nucleophiles.133... 

The silver(I)-mediated ring opening of halocyclopropanes has been used to construct complex frameworks through the inter- and intramolecular trapping of cationic intermediates with heteronucleophiles. An obvious extension of this work is the involvement of carbon-based nucleophiles to form new carbon-carbon bonds. In 1996, Kostikov and coworkers reported the participation of aromatic solvents in the capture of halocyclopropane-derived allyl cations even in the absence of silver(I).30 However, this early example of intermolecular attack by a carbon nucleophile is one of very few such reports. In the same year, Gassman et al. reported cationic cyclizations of gem-dibromocyclopropanes tethered to remote diene moieties (Scheme 4.16).31... 

Scheme 4.19. Intermolecular trapping of the allyl cation by indole nucleophiles.

The addition of an electrophile to the cyclopropene double bond formally leads to a cyclopropyl cation this may be expected to undergo ring opening to an allyl ion unless it is rapidly trapped by a nucleophile. In some cases, however, electrophilic attack may occur at one of the a-bonds, leading directly to an allylic cation. 

In the second example, analysis of the neutral obtained via halide abstraction by the ionic intermediate allowed one to establish the structure (2-propenyl vs allyl) of the gaseous c3h5+ ionic species formed in the protonation of allene and propyne in radiolytic experiments7. Using 1,4-dibromobutane as the trapping agent it was demonstrated that 2-propenyl ions are formed almost exclusively and trapped before they can rearrange to the more stable allyl cation (Scheme 2)7 (See also Section II.B.l). 

Contrary to what is observed during tandem addition reactions to [Os]-toluene (vide supra), electrophilic additions to [Os]-bound ortho- and meta-xylenes result in regioselective attack at C6 (Table 3). A coordination isomer having the metal across C4-C5 (19) is the only isomer observed for both ortho- and meta-xylene. Electrophilic addition of HOTf (entry 1) or dime-thoxymethane (entries 2 and 3) at C6 generates the complexed allyl cation 20, which can be trapped with MMTP to form the complexed diene 21. Demetalation using AgOTf releases the free diene 22, which potentially possesses two adjacent quaternary centers (entry 3) [15]. 

Another intermediate for which Die Is-Alder trapping provided convincing evidence is the oxy-allyl cation. This compound can be made from a,oc -dibromoketones on treatment with zinc metal. The first step is the formation of a zinc enolate (compare the Reformatsky reaction), which can be drawn in terms of the attack of zinc on oxygen or bromine. Now the other bromine can leave as an anion. It could not do so before because it was next to an electron-withdrawing carbonyl group. Now it is next to an electron-rich enolate so the cation is stabilized by conjugation. 

In 1988, Gassman et al. described ionic Diels-Alder addition of vinyl ortho ethyl ester 25 to a series of 1,3-dienes in the presence of la (Sch. 20) [51]. This concept came from their previous report that allylic cations are powerful Diels-Alder dieno-philes toward 1,3-dienes. When an attempt was made to clarify the nature of the intermediate 27 by trapping with trimethylsilyl cyanide, 24 % 28 was accompanied by 25 % adduct 26. 

This preliminary rationale is accommodated in a mechanistic model that begins with the departure of alcohol as water prompted by the strongly acidic medium, and the trapping of the resulting allyl cation IV with the acetylene function. Although vinyl cation V would have been a proposition open to public ridicule years ago, today it is a well established reactive intermediate. Its actual participation in this reaction was seen by the isolation of ketone VIII from the reaction, which may have been derived from the attack of water on this cation (Va) (see Scheme 44.1). Also, ample precedent exists for the attack of alkynes on cationic centers. ... 

Brewster et al. have investigated reductions of a,3-unsaturated carbonyls with a 1 3 mixture of LiAlH4/AlCb. The reduction is closely similar to the results mentioned above, with an allyl cation intermediate trapped by hydride attack to afford an alkenic product. As seen in equations (17) and (18),... 

The preparative electrochemical oxidation of allylsilanes proceeds smoothly and the C-Si bond is cleaved selectively without affecting other allylic C-H bonds [110-113]. This selectivity is ascribed to the selective cleavage of the C-Si bond in the cation radical intermediate. The resulting allyl radical intermediate is further oxidized to give the allyl cation intermediate, which is trapped by nucleophiles such as alcohols, water, carbamates, and tosylamides to give the corresponding allylic substitution products as shown in Eq. (25). Usually, the nucleophiles are introduced to both ends of the allyl cation, and therefore a mixture of two regioisomeric products is formed. 

A transformation will be regarded as a thermolysis if the halocyclopropane is heated either neat, in the gas phase or in an inert solvent. This solvent should neither serve as a nucleophile HY, nor have a high dielectric constant (to favor cation formation). If the solvent does have one or both of the aforementioned properties, the transformation will be regarded as a solvolysis. Although all these methods in principle obey the same general rules vide supra), their synthetic results are clearly distinct. Conditions in thermolysis are harsh. Since there is no other nucleophile present, the allylic cation can only be trapped by the halide ion that has left the cyclopropane. At the elevated temperature of thermolysis, the resulting allylic halides... 

Depending on the reaction conditions, the allylic cation can be trapped by the halide ion expelled during cyclopropane ring opening or by an external nucleophile. For example, 3,3-dibromotricyclo[4.1.1.0 ]octane (14) rearranged to give 15. ... 

Dibromobicyclo[3.1.0]hexane (16) underwent ring opening to give an allyl cation that was trapped by morpholine to give 17. ... 

C-H acidic compounds can be used as nucleophiles to trap the allylic cation, e.g. formation of 24. 3... 

These reactions formally involve addition of HX to the n-bond and ring opening of an intermediate cyclopropyl cation to an allyl cation, although the details of the mechanism are unclear and may, at least in some cases, involve direct attack at the cr-bond. In some cases, the intermediate allylic cation may be trapped intramolecularly to produce methylenefurans or dihy-dropyrans [see Sections 3.2.3.4., 3.3.2.5. and l.B.2.4.1.3. (E17c, p2313fi)]. In other cases, dienes are produced, e.g. formation of 6. ... 

---