Norbornene, decarbonylative

On the other hand, in bi- or polycyclic molecules a decarbonylation reaction such as a [5 -> 4 + 1] fragmentation of a five-membered ring is quite common. However, in these cases the reaction is not accompanied by ring contraction. As an example, norbornen-7-one (16) affords cyclohexadiene (86CP307 90JA5089). 

Mitsudo et al. [34] then examined the reaction of cyclobutenedione with CO and norbornene (Eq. 19). The reaction requires a low pressure of CO. Regio-selective C-C bond cleavage followed by decarbonylation takes place to give the four-membered metallacyle 17, which reacts with norbornene to give the final product. 

In a similar conversion optically active methyl (5i ,6/ )-6-en(/o-formylbicyclo[2.2.1]hept-2-ene-5-exo-carboxylate (7) upon decarbonylation with [l,3-bis(diphenylphosphano)propane]rho-dium(I) chloride gives methyl ( + )-(/ )-tricyclo[2.2.1.0 ]heptane-3-carboxylate (8) while the analogous exo-aldehyde gives the decarbonylated norbornene derivative. ... 

Not all decarbonylations give the expected product, and the relative yields are influenced by the rhodium catalyst employed. Chlorotris(triphenylphosphine)rhodium(I) rapidly and selectively decarbonylated the e/ c/o-norbornene isomer shown in equation (4) to form a tricycline product, but [Rh(dppp)2]Cl additionally forms an isomeric alkene. In the case of the corresponding exo-isomer (equations), [RhCl(PPh3)3] is the less selective catalyst. It was also found that [Rh(dppp)2]Cl was a superior catalyst... 

The use of both LIU and HIU has been shown to increase the efficiency of the P-K reaction, which involves the formation of cyclopentenone from the annulation of a cobalt alkynyl carbonyl complex and an alkene. The use of low-power ultrasound, as for example, from a cleaning bath, although capable of producing intramolecular P-K reactions, generated relatively low cyclization yields. The motivation for the use of high intensity came from its ability, as previously described, to effectively decarbonylate metal carbonyl and substituted metal carbonyl complexes. Indeed, HIU produced by a classic horn-type sonicator has been shown to be capable of facile annulation of norbornene and norbornadiene in under 10 min in the presence of a trimethylamine or trimethylamine N-oxidc dihydrate (TMANO) promoter, with the latter promoter producing cleaner product mixtures. This methodology also proved effective in the enhancement of the P-K reaction with less strained alkenes such as 2,5-dihydrofuran and cyclopentene, as well as the less reactive alkenes -fluorostyrene and cycloheptene. The mechanism has been postulated to involve decarbo-nylation of the cobalt carbonyl alkyne, followed by coordination by the amine to the vacant coordination sites on the cobalt. 

The application of a rhodium catalyst to decarbonylative and direct coupling reactions of cyclobutenones with 2-norbornene provides stereoselective access to fused cyclopentenes (Scheme 6.23). Argon atmosphere is crucial for this reaction to give fused cyclopentenes, while in the presence of carbon monoxide the reaction affords cyclohexenones by direct coupling [27]. 

Decarbonylative annulation of cyclobutenone 89 with norbornene proceeded in the presence of [RhCl(CO)2]2 under argon to afford cyclopentenone 91, while the reaction under 30 atm CO yielded the [4-1-2] annulation product 92 (Scheme 3.52) [61]. 

Hoveyda et al. reported the enantioselective synthesis of (+)-africanol through asymmetric olefin metathesis followed by one-carbon contraction via decarbonylation (Scheme 8.16) [65]. When symmetrical norbornene 84 was treated with a chiral molybdenum catalyst, desymmetrization took place to afford the chiral bicycle 85 in an enantioselective fashion. The resulting vinyl terminus was subsequently transformed into a methyl group via hydroboration, oxidation to aldehydes, and decarbonylation. Further manipulations of the functional groups gave (+)-africanol. 

Similarly, photolysis of 3,3-dimethoxy-5-norbornen-2-one 109 in various solvents (dioxane, acetonitrile, and acetone) gave the 1,3-acyl shift product 110 in minor amounts and the decarbonylated product 111 as the major product (Scheme 25). Irradiation of 109 in methanol gave a complex mixture of products arising through an oxa-carbene formed from the biradical generated after initial a-cleavage. It was also observed that the decarbonylated product 111 was obtained at the expense of the initially formed 1,3-acyl shift product 110. 

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