Exocyclic 1,3-dienes, formation

A new entry to exocyclic dienes was reported by Sha who uncovered that a radical cyclization of the vinyl iodide 100 can lead to the formation of an exocyclic dienes fused with a tetrahydrofuran ring. The cyclization is proposed to proceed in a 5-(n-exo)-exo-dig fashion <00OL2011>. 3,4-Disubstituted tetrahydrofurans can also be constructed via the cyclization of O-stannyl ketyls and allylic 0-stannyI ketyls onto electron-rich or electron-poor alkenes <00TL8941>. 

For the first time, application of sequential Diels-Alder reactions to an in situ-generated 2,3-dimethylenepyrrole was shown with various dienophiles 548 to afford 2,3,6,7-tetrasubstituted carbazoles (549). This novel tandem Diels-Alder reaction leads to carbazole derivatives in two steps, starting from pyrrole 547 and 2 equivalents of a dienophile, and is followed by 2,3-dichloro-5,6-dicyano-l,4-benzoquinone (DDQ) oxidation of the intermediate octahydrocarbazole. Mechanistically, the formation of the intermediate octahydrocarbazole appears to involve two sequential [4+2] cycloadditions between the exocyclic diene generated by the thermal elimination of acetic acid and a dienophile (529) (Scheme 5.17). 

The mechanism of the formation of compound 1137 appears to be two sequential [4+2] cycloadditions between the exocyclic diene of compounds 1139 and 1141 and a dienophile (Scheme 223). The 2,3 d ethylenepyrrole required for the Diels-Alder reaction can be generated by the thermal elimination of acetic acid to form compound 1139, which is observed by mass spectroscopy. There are two possible pathway by which diene 1139 can proceed to tricycle 1137. The first is the elimination of a second molecule of acetic acid from diene 1139 to form 5-benzyl-aza[5]radialene 1140, which is also observed by mass spectroscopy. Attempts to improve the yield of compound 1137 by accelerating the elimination of acetic acid by acid or base catalysis failed, resulting in the decomposition of compound 1136 < 20000L73>. 

When these ketones are the product of a 1,3-acyl shift, a photoequilibrium is not established because 121 is drained off by the ketene fragmentation. A limited amount of reversible 1,3-shift stiU occurs in a few cases. 87) In compounds like 06 and 07, where the 2-vinyl cyclobutanone group on ketene formation would give rise to an exocyclic diene, decarbonylation occurs instead of ketene fragmentation. 53,74) Although the detailed mechanism of the elimination process is not known, from the examples at hand it appears to be efficient only when extensive overlap with the developing orbitals occurs, t.e., 121 is perfectly set up for overlap throughout the reaction, 06 and 97 are certainly not. Ketene formation is also prominent in the special ketones 122 87) and 123. 57,88) The... 

Grigg, R., Stevenson, R and Worakun, T. (1984) Rhodium- and palladium-catalysed formation of conjugated mono- and bis-exocyclic dienes. 5-exo-trig versus 6-emfo-trig cycUsation. J. Chem. 

Exocyclic Diene Complexation. Formation of iron carbonyl complexes of (6) illustrates the mild nature of the complexation conditions (eq 7). ... 

Benzo[Z)]furans and indoles do not take part in Diels-Alder reactions but 2-vinyl-benzo[Z)]furan and 2- and 3-vinylindoles give adducts involving the exocyclic double bond. In contrast, the benzo[c]-fused heterocycles function as highly reactive dienes in [4 + 2] cycloaddition reactions. Thus benzo[c]furan, isoindole (benzo[c]pyrrole) and benzo[c]thiophene all yield Diels-Alder adducts (137) with maleic anhydride. Adducts of this type are used to characterize these unstable molecules and in a similar way benzo[c]selenophene, which polymerizes on attempted isolation, was characterized by formation of an adduct with tetracyanoethylene (76JA867). 

Niggli and Neuenschwander294 studied the reaction of fulvene (461) with cyclopen-tadiene. The main product fraction consisted of three 1 1 adducts, as illustrated in equation 138. Diels-Alder Adducts 462 and 463 resulted from attack of cyclopentadiene at the endocyclic and exocyclic double bonds of fulvene, respectively. The formation of 464 was rationalized by a [6 + 4] cycloaddition reaction followed by two [1,5] hydrogen shifts. It was stated that due to the absence of electron-donating and electron-withdrawing groups on both triene and diene, fulvene may have reacted via its HOMO as well as its LUMO. 

A predominant isomerization of metalloles to transoidal dienes occurs when t-BuLi is used34. This isomerization involves the formation of an allylic carbanion. which is protonated by water at the position a to Si (or Ge) and is silylated at the exocyclic carbon... 

One of the most reported pathways for C=C and C=C bonds coupling involves the oxidative coupling and the ruthenacyde intermediate formation. The first ruthenium-catalyzed Unear codimerization of disubstituted alkynes and alkenes involved acrylates or acrylamides and selectively produced 1,3-dienes [33] (Eq. 23). The proposed mechanism involves a ruthenacyclopentene via oxidative coupUng on the Ru(0) catalyst Ru(COD)(COT). The formation of 1,3-di-ene results from intracyclic /1-hydride eUmination, this process taking place only when a favored exocyclic /1-elimination is not possible. 

The presence of two different isomers can be viewed through the competitive ruthenacycle formation, depending on the orientation of the alkyne via oxidative coupling. A /3-hydride elimination, which is favored with H exocyclic with respect to intracyclic /3-hydride, produces the 1,4-dienes (Scheme 2). 

There is a Fine balance between the acidity of the alcohol and the basicity of the trans.olefin. For example, 1-methylcyclohexenes undergo photoprotonation in methanol whereas cyclo-hexenes require the addition of small amounts of acid. In the present example, the addition of a small quantity of acid reduces the competing formation of the exocyclic isomer, p-mentha-lt7),H-diene. 

As 6-electron donors they are able to control the formation and the breaking of metal-metal bonds by just varying the steric bulk of their exocyclic P-substituent. As phospholium salts they stabilize unusual types of conjugated diene r-complexes, probably through steric protection of the metal by the phosphonium units. 

Dienyl acetate (256) (readily prepared l-hepten-6-one and lacking the disulfone moiety) was also easily transformed to ( )-1,4-diene (259 87%) (Scheme 53). This regiochemistry parallels that of stoichiometric additions of allylpalladum complexes to notbomadiene, but was subject to uncertainty in view of the relatively fast 1,3-Pd migration within the allyl component. Therefore the conversion (256) -> (259) demonstrates a new stereocontrolled access to exocyclic trisubstituted alkenes which implies C—C bond formation at the less-substituted allylpalladium terminal and thus shows a regio- and stereo-chemistry both opposite to the type II magnesium-ene process (c/. Section 1.2.3.2.2 81 -> 83). 

Doyle has put forward arguments against the intermediacy of such complexes in catalytic cyclopropanation . Firstly, metal coordination activates the alkene to nucleophilic attack. Hence, an electrophilic metal carbene would add only reluctantly or not at all. Secondly, the stable PdCl2 complexes of dienes 8 and 428 do not react with ethyl diazoacetate, even if Rh fOAc) or PdCljfPhCbOj is added. The diazoester is decomposed only when it is added to a mixture of the Pd complex and excess diene. These results exclude the metal-carbene-olefin intermediate, but they leave open the possibility of metal carbene interaction with an uncomplexed olefin molecule. The preferred formation of exo-cyclopropanes in the PdCyPhCN) -catalyzed reactions between 8 and N2CHCOOEt or N2CPh2, with exo. endo ratios virtually identical to those observed upon cyclopropanation of monoolefin 429, also rule out coordination of a palladium carbene to the exocyclic double bond of 8 prior to cyclopropanation of the endocyclic double bond. 

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