Of cyclohexa-l,3-diene

For instance, the reaction of cyclohexa-l,3-diene and ethyl glyoxylate in the presence of (S)-31 proceeds smoothly to give the cycloadduct in 66% yield with 97% ee. The synthetic application of this process was demonstrated by the preparation of a highly interesting synthon for sesquiterpene lactones in high yield and diastereoselectivity, and with a very high ee as illustrated in Scheme 17 [35]. 

Direct electrophilic attack on the exo side of a coordinated diene is also possible, even to complexes of modestly electron-rich metal centers. For example, Friedel-Crafts acetylation of (cyclohexa-l,3-diene)Fe(CO)3 gives principally the exo isomer. However, such direct exo attack occurs more commoiJy on the uncoordinated portion of a partially coordinated polyene or polyenyl ligand (recall that uncoordinated olefins and dienes are generally more nucleophilic than coordinated ones). An example of such exo attack on the uncoordinated portion of a pentadienyl ligand is shown in Equation 12.74. In ttiis case, coordination of a CO ligand is thought to displace one or more carbons of the dienyl ir-system from the iron center to trigger exo acylation. Similar results have been reported for electrophilic attack on the uncoordinated portion of the polyene system in [(C )Fe(CO)3]". ... 

The palladium-catalysed 1,4-hydrosilylation of cyclohexa-l,3-diene with Cl3SiH in the presence of ligand (55) was discussed in the paragraph on silicon. ... 

The photolyses of cyclohexa-1,3-diene, cycloocta-l,3,5-triene, and bicyclo[4.2.0]octa-2,4-diene were investigated by matrix-isolation techniques at a relatively early date and certainly before reliable computational methods for IR transitions were available. As an example, both IR and UV-visible spectroscopy were employed to monitor the photoreaction of cyclohexa-l,3-diene at 240-270 nm in Ar matrices at 20 K. It was discovered that the diene photolyzed irreversibly to Z-hexa-l,3,5-triene, which photolyzed more slowly but still irreversibly to -hexa-l,3,5-triene and several other thermally unstable products. The latter could not be identified with certainty, but hexa-l,2,4-triene and exo-2-vinylbicy-clo[1.1.0]butane were proposed as possibilities. 

Cookson et al. first reported prior to 1965 that photochemical reaction of the Diels-Alder endo-adducts of cyclopentadiene or cyclohexa-1,3-diene with p-benzoquinone gave cage compounds by formation of a cyclobutane ring from the two double bonds. Irradiation of an ethyl acetate solution of the cyclopentadiene adduct 36 with a medium pressure Hg lamp for 6 h afforded cage 37 (90%). This photoreaction proceeded even in the soHd state, and the cage compound 37 was obtained in 80% yield after 80 h irradiation. Similarly, the 1 1 adduct 38 of cyclohexa-l,3-diene and p-benzoquinone was converted into cage 39 by irradiating the ethyl acetate solution for 10 h (80%) or the soHd state for 90 h (90%). This reaction was successfully appKed to the adduct 40 of cyclooctatetraene (COT) and p-benzoquinone (92%... 

The first examples of this reaction (which was reviewed several times85), i.e. the addition of nitrosoarenes to 2,3-dimethylbutadiene to give 2-aryl-3,6-dihydro-2//-l,2-oxazines (equation 94), were reported in 194786. In general, the addition of nitroso compounds to 1,3-dienes to form dihydro-1,2-oxazines is only observed if the nitroso compound is activated by an electron-withdrawing group87. Kinetic studies of the reaction of cyclohexa-1,3-diene with para-substituted nitrosobenzenes (equation 95) show the accelerating effect of such groups (Hammett constant p = +2.57)88. 

The complexation of 2b in the presence of 12.5 mol% of the azadiene catalyst 4 provides quantitatively tricarbonyl[r] -cyclohexa-l,3-diene]iron 5... 

An alternative method for the oxidative cyclization of the arylamine-substituted tricarbonyl(r -cyclohexa-l,3-diene)iron complex (725) is the iron-mediated arylamine cyclization. Using ferricenium hexafluorophosphate in the presence of sodium carbonate provided hyellazole (245) directly, along with the complex 727, which was also converted to the natural product (599,600) (Scheme 5.71). 

We reported the first total synthesis of l,T-bis(2-hydroxy-3-methylcarbazole) (213) using our molybdenum-mediated construction of the carbazole framework (560). The required monomer, 2-hydroxy-3-methylcarbazole (52), was obtained in three steps, and 22% overall yield, starting from dicarbonyl(ri -cyclohexa-l,3-diene)(ri -cyclopentadienyl)molybdenum hexafluorophosphate (663) and 3-meth-oxy-4-methylaniline (655) (560) (see Scheme 5.52). Finally, oxidative coupling of the monomer 52 using p-chloranil afforded l,T-bis(2-hydroxy-3-methylcarbazole) (213) in 38% yield (560) (Scheme 5.167). 

Whereas the hexa-1,3,5-triene/cyclohexa-l,3-diene rearrangement serves as a model reaction in the hydrocarbon series, the perfluoro analog behaves differently, as presented in Section 5.3.2.1. Photolysis of perfluorohexa-l,3,5-triene (8) in the gas phase yields per-fluorobicyclo[2.2.0]hex-2-ene (10) as the major product, presumably via perfluorocyclohexa-1,3-diene (9).4 Perfluoro(3-vinylcyclobutene) is formed as the minor product. Thermolysis of hexa-tricnc 8 gives cyclohexadiene 9.4 Photolysis of 9 provides bicyclohexene 10, which reverts to 9 thermally.5... 

Scheme 56 Cl-cycloaddition-cyclocondensation three-component synthesis of l-acetyl-2-amino-cyclohexa-l,3-dienes 104...

The efficient trapping of the cyclohexadienyl anionic intermediates with protons raises the possibility of qnenching with carbon electrophiles. The process is not as general as the proton quench. However, when the nucleophile adds essentially irreversibly, quenching with a limited set of carbon electrophiles is successful. For example, addition of 2-lithio-l,3-dithiane to benzene-Cr(CO)2T, followed by addition of ethyl iodide and then oxidation or addition of a donor ligand (CO, PhsP), produces a cyclohexa-l,3-diene substituted by both acetyl (Me + CO) and the nucleophile (Scheme 47).134,209 insertion of CO occurs, without... 

The earliest reduction reactions of arene oxides to be reported involved catalytic hydrogenation of 1 (H2-Pd) to yield oxepane and reaction with lithium aluminium hydride to give cyclohexa-l,3-dien-5-ol. An alternative type of reduction reaction... 

One example of such a reaction was reported in 1971 by Brown and Davidson [49], who studied oxidation reactions of cyclohexa-1,3-diene and cyclhexa-1,4-diene. They observed that reaction of cyclohexa-1,3-diene with p-benzoquinone in acetic acid in the presence of catalytic amounts of Pd(OAc)2 produced l,4-diacetoxycyclohex-2-ene of unknown stereochemistry. At the time, they were uncertain about the mechanism and suggested a possible involvement of radicals. A related palladium-catalyzed 1,4-diacetoxylation of... 

In the catalytic cycle of the palladium-benzoquinone-based 1,4-oxidation of 1,3-dienes, benzoquinone is reduced to hydroquinone. The diacetoxylation reaction is conveniently performed with p-benzoquinone in catalytic amounts, employing Mn02 as the stoichiometric oxidant. In this process, the hydroquinone formed in each cycle (cf. Scheme 8-6) is reoxidized to p-benzoquinone by MnO,. For example, the catalytic reaction of cyclohexa-1,3-diene using catalytic amounts of both Pd(OAc)2 and p-benzoquinone with stoichiometric amounts of Mn02 in acetic acid in the presence of lithium acetate afforded a 93% yield of rranj-l,4-diacetoxycyclohex-2-ene (>91% tram) [51b]. The corresponding reaction in the presence of lithium chloride gave ci5-l,4-diacetoxycyclohex-2-ene in 79% yield (>96% cis). 

The chloroacetoxylation proceeds via the same type of intermediate as that involved in the palladium-catalyzed 1,4-diacetoxylation, i.e. via a 4-acetoxy-l,2,3-(jr-allyl)palladium intermediate (cf. Scheme 8-7). The high selectivity for iinsymmetrical product formation (usually >98%) is remarkable. Since chloride is the stronger nucleophile of the two amions present (CI and AcO ), the 4-chloro-l,2,3-( r-allyl)palladium intermediate 60 is initially formed. However, the chloride in the 4-position is rapidly exchanged for acetate to give a more stable, jr-allyl intermediate 61, which leads to product. The presence of intermediate 60 was confiimed by its trapping by a faster oxidant (isoamyl nitrate) than benzoqiiinone (BQ), which furnished l,4-dichloroalk-2-ene [78,84], In the case of cyclohexa-1,3-diene, this product was c -1,4-dichlorocyclohex-2-ene [84]. 

The benzene cyclohexa-l,3-diene complexes (CgHg)(l,3-CgHg)M (M = Fe, Ru, Os) have been prepared by reacting the metal trichlorides with i-PrMgBr in the presence of 1,3-CgHa 143, 144). The iron and... 

The quenching kinetics of 02( Ag) by the colourless trap cyclohexa-l,3-dien-... 

Cyclohexa-l,3-dien-5-one—see o-Isophenol Cyclohexadienones 1035—see also 4-AIkoxycyclohexadienones, Halocyclohexadienones, o-Isophenol, Nitrocyclohexadienones heats of formation of 40 ionized 264, 265 synthesis of 1080, 1082... 

The structure of the complex formed from tricarbonyl(isoprene)iron and hexafluoropropene has been established crystallographically as [77], allowing the assignment of the parameters shown with the structure, and similar assignments for the products from the butadiene- and 2,3-dimethylbutadiene-tricarbonyliron complexes then follow. (49) With tricarbonyl(cyclohexa-l,3-diene)iron, hexafluoropropene gave [78], in... 

In some cases the method chosen for dehydration has a decisive effect on the outcome of the reaction. Thus, the adduct of 4,4-dimethylcyclohex-2-enone underwent dehydration with (methoxycarbonylsulfamoyl)triethylammonium hydroxide, inner salt (Burgess reagent) in refluxing benzene to give 5,5-dimethyl-2-[l-(phenylsulfanyl)cyclopropyl]cyclohexa-l,3-diene (5) in 93% yield. 

---