Intramolecular reactions formate anion trapping

Another common trapping method is an intramolecular aldol reaction of the initially formed anion, as shown in equation (91) and Schemes 53 and 54.% In the first case, an aldol-like trapping of the iminium salt produced (411 equation 91 ).96b The initial heteronucleophile in the other two cases is ultimately lost from the product by oxidation and elimination, so that the overall process is C—C bond formation at the a-center of an enone. Thus, treatment of the formyl enone (412 Scheme 53) with an aluminum thiolate afforded in 60% yield the trapped product (413) which could be oxidized and eliminated to give (414).96c Addition of the corresponding aluminate species to the ketoacrylate (415 Scheme 54) produced only one diastereomer of the aldol product (416) which was converted into the alkene (417) in excellent yield.96 1... 

The formation of compound 175 could be rationalized in terms of an unprecedented domino allene amidation/intramolecular Heck-type reaction. Compound 176 must be the nonisolable intermediate. A likely mechanism for 176 should involve a (ji-allyl)palladium intermediate. The allene-palladium complex 177 is formed initially and suffers a nucleophilic attack by the bromide to produce a cr-allylpalladium intermediate, which rapidly equilibrates to the corresponding (ji-allyl)palladium intermediate 178. Then, an intramolecular amidation reaction on the (ji-allyl)palladium complex must account for intermediate 176 formation. Compound 176 evolves to tricycle 175 via a Heck-type-coupling reaction. The alkenylpalladium intermediate 179, generated in the 7-exo-dig cyclization of bro-moenyne 176, was trapped by the bromide anion to yield the fused tricycle 175 (Scheme 62). Thus, the same catalytic system is able to promote two different, but sequential catalytic cycles. 

The alkyne insertion reaction is terminated by anion capture. As examples of the termination by the anion capture, the alkenylpalladium intermediate 189, formed by the intramolecular insertion of 188, is terminated by hydrogenolysis with formic acid to give the terminal alkene 192. Palladium formate 190 is formed, and decarboxylated to give the hydridopalladium 191, reductive elimination of which gives the alkene 192 [81]. Similarly the intramolecular insertion of 193 is terminated by transmetallation of 194 with the tin acetylide 195 (or alkynyl anion capture) to give the dienyne 196 [82], Various heterocyclic compounds are prepared by heteroannulation using aryl iodides 68 and 69, and internal alkynes. Although the mechanism is not clear, alkenylpalladiums, formed by insertion of alkynes, are trapped by nucleophiles... 

Intramolecular trapping studies have verified the intermediacy of acyl radicals in the conversion of carboxylic acid chlorides to samarium acyl anions by Smli. Treatment of 2-allyloxybenzoyl chlorides with Sml2 resulted in a very rapid reaction, frtrni which cyclopropanol products could be isolated in yields of up to 60% (equation 8iy Apparently, initial formation of the acyl radical was followed by rapid radical cyclization. The 3-keto radical generated by this process undergoes cyclization by a radical or anionic process, affording the observed cyclopropanols (Section 1.9.2.3.1). 

There are many examples of such reactivity and some of these have been reviewed by Roth and coworkers, a research group that is extremely active in this area. An example that is typical of the processes encountered involves the cyclization of the diene geraniol (1). In this case the sensitizer is 9,10-dicyanoanthracene (DCA) and the reactions are carried out in methylene chloride. The authors state that a contact radical-ion parr is involved, i.e. the radical cation of the diene is in close proximity to the radical anion of the DCA. Reaction within this yields the cyclopentane derivatives 2 and 3 in the yields shown. The ring formation is the result of a five centre CC cyclization within the radical cation of 1. When a more powerful oxidant such as p-dicyanobenzene is used as the sensitizer in acetonitrile as solvent, separated radical-ion pairs are involved. This leads to intramolecular trapping and the formation of the bicyclic ethers 4 and 5 . The bicyclic ether incorporates an aryl group by reaction of the radical cation of the diene with the radical anion of the sensitizer (DCB). This type of reactivity is referred to later. Other naturally occurring compounds such as (/fj-f-bj-a-terpineol (6) and (R)-(- -)-limonene (7)... 

Whatever the exact mechanism of the conjugate-addition reaction, it seems clear that enolate anions are formed as intermediates and they can be trapped as the silyl enol ether or alkylated with various electrophiles. For example, addition of lithium methylvinyl cuprate (a mixed-cuprate reagent) to cyclopentenone generates the intermediate enolate 166, that can be alkylated with allyl bromide to give the product 167 (1.161). The trans product often predominates, although the transxis ratio depends on the nature of the substrate, the alkyl groups and the conditions and it is possible to obtain the cis isomer as the major product. Examples of intramolecular trapping of the enolate are known, as illustrated in the formation of the ds-decalone 168, an intermediate in the synthesis of the sesquiterpene valerane (1.162). 

It was shown that a variety of readily available (Z) and ( )-enol phosphates are good stereoselective synthons. Thus as5unmetric epoxidation of these phosphates using Jacobsen s (Salen)Mn(ii) complex afforded a-hydroxy ketones in enantio-selectivity up to 96%. The reaction of enantiomerically enriched 2-methyl-2-nitro-3-(diphenylphosphatoxy)alkyl radicals (80) with tributyltin hydride and AIBN in benzene results in the formation of alkene radical/anion pairs (81) which are trapped intramolecularly, leading to pyrolidine and piperidine systems (82) with memory of stereochemistry (Scheme 17). ... 

Photohomolysis reactions of cobalt(III) pseudohalide complexes can be used to effect photoreduction to cobalt(II) complexes. Thus, intramolecular photoelectron transfer in the complexes Co(CN)5N3 and Co(NH3)5N3 leads to oxidation of the azide ion to the azide radical, and reduction of the cobalt(III) center to cobalt(II). Evidence for the initial formation of the azide radical comes from the photolysis of solutions containing Co(CN)5N3 and iodide ion, when the iodine anion radical I2 is observed in the solution. This formation of I2 results from the photochemical generation of the azide radical, which then oxidizes the iodide ion to an iodine atom (Scheme 2.1). Subsequently, spin trapping experiments with phenyl-N-t rf-butyl nitrone has been used to verify the formation of azide radicals from the photolysis of Co(CN)5N and Co(NH3)sN3 / ... 

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