1.3.5- Octatriene intermediate

If a stepwise cyclization mechanism is assumed—for example, for /j-Cg—two octatriene intermediates may be formed, viz. 1,3,5-octatriene would lead to ethylbenzene, and 2,4,6-octatriene to o-xylene (Scheme II). The dehydrogenation of the latter would give octatetraene, which, in turn, gives styrene via vinylcyclohexadiene. Dehydrogenation and cyclization of octatriene were reported to compete over chromia and molybdena catalysts (67) with less hydrogen present (e.g., in a pulse system with in helium carrier gas), styrene formation is enhanced. 

Pd-cataly2ed reactions of butadiene are different from those catalyzed by other transition metal complexes. Unlike Ni(0) catalysts, neither the well known cyclodimerization nor cyclotrimerization to form COD or CDT[1,2] takes place with Pd(0) catalysts. Pd(0) complexes catalyze two important reactions of conjugated dienes[3,4]. The first type is linear dimerization. The most characteristic and useful reaction of butadiene catalyzed by Pd(0) is dimerization with incorporation of nucleophiles. The bis-rr-allylpalladium complex 3 is believed to be an intermediate of 1,3,7-octatriene (7j and telomers 5 and 6[5,6]. The complex 3 is the resonance form of 2,5-divinylpalladacyclopentane (1) and pallada-3,7-cyclononadiene (2) formed by the oxidative cyclization of butadiene. The second reaction characteristic of Pd is the co-cyclization of butadiene with C = 0 bonds of aldehydes[7-9] and CO jlO] and C = N bonds of Schiff bases[ll] and isocyanate[12] to form the six-membered heterocyclic compounds 9 with two vinyl groups. The cyclization is explained by the insertion of these unsaturated bonds into the complex 1 to generate 8 and its reductive elimination to give 9. 

Azabicyclo[4.2.0]octatriene systems (e.g. 8), formed as nonisolable intermediates during the photochemical addition of benzonitrile or 1-naphthonitriles to phenols,16 -19 isomerize to 2-hydroxyazocines which exist predominantly as the lactam tautomers. 

Dupont and co-workers studied the Pd-catalyzed dimerization [108] and cyclodimerization [109] of butadiene in non-chloroaluminate ionic liquids. The biphasic dimerization of butadiene is an attractive research goal since the products formed, 1,3,5-octatriene and 1,3,6-octatriene, are sensitive towards undesired polymerization, so that separation by distillation is usually not possible. These octa-trienes are of some commercial relevance as intermediates for the synthesis of fragrances, plasticizers, and adhesives. Through the use of PdCl2 with two equivalents of the ligand PPhj dissolved in [BMIM][Pp6], [BMIM][Bp4], or [BMIM][CF3S03], it was possible to obtain the octatrienes with 100 % selectivity (after 13 % conversion) (Scheme 5.2-23) [108]. The turnover frequency (TOP) was in the range of 50 mol butadiene converted per mol catalyst per hour, which represents a substantial increase in catalyst activity in comparison to the same reaction under otherwise identical conditions (70 °C, 3 h, butadiene/Pd = 1250) in THF (TOP = 6 h ). 

Homogeneous nickel complexes proved to be versatile catalysts in dimerization and trimerization of dienes to yield different oligomeric products.46-55 Depending on the actual catalyst structure, nickel catalyzes the dimerization of 1,3-butadiene to yield isomeric octatrienes, and the cyclodimerization and cyclotrimerization to give 1,5-cyclooctadiene and all-trans-l,5,9-cyclododecatriene, respectively46 56 [Eq. (13.13)]. Ziegler-type complexes may be used to form cis,trans,trans-1,5,9-cyclododecatriene37,57 58 [Eq. (13.14)], which is an industrial intermediate ... 

Unique linear dimerization of butadiene is catalysed by a Pd—PI13P complex [50]. This Pd-catalysed linear dimerization affords 1,3,7-octatriene (89) by //-elimination of 128 and reductive elimination of 129. Transfer of H from C(5) to C(3) in the intermediate bis-7r-allylpalladium 68 occurs to give 1,3,7-octatriene (89). 

Soft nucleophiles or halide ions may also coordinate to C to give [Pd(l, 2,3-r)3-octa-2,7-dien-l-yl)(X)(L)] (F) (X = Nu, Cl ), which is known to be less active [47] and may even lead to catalyst deactivation. Finally, the by-product 1,3,7-octatriene can also be formed from intermediate C, by proton abstraction at the 4-position of the r 3,r l-octadienyl ligand. Note that the proton that is lost upon 1,3,7-octatriene formation is different from the proton added by the nucleophile (4 position vs. 6 position, respectively), which points at the general irreversibility of the step going from B to C. 

The intermediate bicyclo[5.1.0]octatriene (225) may be trapped as a Diels-Alder adduct, while at elevated reaction temperature, the heptafulvene (224) may likewise be trapped. The chloride (223, X = Cl, Y = H) may similarly be converted to the parent heptafulvene by reaction with the same base in tetraglyme at 90 °C and low pressure 152). 

Alkynes undergo cycloaddition on irradiation with benzene or naphthalene derivatives or with other aromatic compounds. With a benzene derivative the product is usually a cyclo-octatetraene which results from thermal electrocyclic ring-opening of the bicyclo-octatriene formed initially by 1,2-addition of the alkyne to the benzene ring (equation 61) . The intermediate can be trapped using a dienophile such as tetracyanoethylene (equation 62) ". The first step of the photoaddition process involves excitation of the alkyne , and orbital symmetry considerations suggest that concerted 1,2-addition is allowed if the alkyne is excited but not if the benzene is excited ... 

A metal-complexed carbene (75) was postulated as an intermediate in order to account for the unexpected formation of octatriene derivatives (76) in the copper(l)-catalysed dimerization of tri- and tetramethylcyclopropene (74) (equation 48) ". ... 

The preparation of substituted diphosphabicyclo-octatrienes or diphosphabarre-lenes (lO) was achieved by Krespan, McKusick and Cairns by direct reaction of red phosphorus with fluorinated acetylenes in the presence of a catalytic amount of iodine at 200 for 8 h under pressure. The reaction might possibly go through a diphosphorin intermediate (9), since phosphorin readily... 

The quantum chain process has also been observed in benzophenone-sensitized isomerization of 2,6-dimethyl-2,4,6-octatriene (25) [120]. The effect of azulene on the photostationary state isomer composition suggests that both tt and tc are the stable conformers in the triplet state and are equilibrated. However, equilibration of all excited intermediates is not complete within the lifetime of the excited triplet state ( 50 ns). The triplet lifetime of 1,3,5-hexa-triene (26) is reported as 100 ns (Table 5) [121]. 

When the starting material 71 was unsubstituted, this reaction route is unavailable. Reaction of 75 proceeds via 76, a similar intermediate to 73, but then adds another molecule of PhC=CCH3 to give the bicyclo[4.2.0]octatriene 77. 

Fig. 15 [4+2] Cycloaddition reactions of r -anisole complexes via a 4H-anisolium intermediate and the formation of various bicyclo[2.2.2]octadiene and bicyclo[2.2.2.]octatriene complexes...

Beyond the few simple substitution reactions summarized below, the dominant chemistry reported for the fully-unsaturated diazocines (15) and (16) involves their thermal and photochemical ring contraction. Both the thermal and photochemical reactions appear to proceed via intermediate formation of bicyclo[4.2.0]octatriene valence tautomers. However, the product distribution differs considerably between the thermal and photochemical reactions. Thus, thermolysis of (15) in dode-cane solution at 140°C affords benzene (43.2%) and pyridine (56.8%) at 175°C, benzene becomes the major product (55.7% vs. 44.3% pyridine) (Scheme 3). Pyridazine, however, is not formed. In contrast, photolysis in tetrahydrofuran at >300 nm affords only benzene neither pyridine nor pyridazine are detected <79JOCl264>. 

Beller and co-workers find that the ratio of 47 to 48 is influenced by the reaction temperature, the ligand to metal ratio, and the ratio of methanol to butadiene. A high 47/48 ratio can be obtained at relatively low temperature with one phosphine per palladium and a 1 2 methanol/butadiene ratio. For example, at 30 °C (2.5 h, 36% conversion, TON = 1376, turnover frequency (TOF) = 556 h ), the 49-catalyzed reaction affords 34% combined yield of 47 and 48 in a 36 1 ratio. Addition of a second equivalent of phosphine per palladium decreases the regioselectivity (12 1 47/48). The authors suggest that disruption of the chelated 7r-allylpaUadium intermediate 50 by displacement of the complexed alkene by the second phosphine to 51 accounts for the lower regioselectivity. At 90 °C, impressive TON of 10,500 and TOF of 21,000 h are observed, although at higher temperature formation of 1,3,7-octatriene competes more effectively and comprises 23% of the product mixture. 

MINDO/3 calculations suggest that the thermal isomerizations of syn- and anti-tricyclo-octadienes (125) and (126) to cyclo-octatetraene proceed via triplet intermediates, and thermal isomerization of syn-benzotricyclo-octene to benzo[c]-1,3,5-cyclo-octatriene in the presence of 9,10-dibromoanthracene was accompanied by the generation of visible light corresponding to fluorescence from the 9,10-dibromoanthracene. ... 

In the presence of added ligand (Pd L= 1 1), 1,3, 7-octatriene isomers are obtained (Fig. 12.8). The conformation of the C chain in the intermediate (C) has been determined in solution by n.m.r. spectroscopy and also in the solid state (L = Me3 ) by X-ray diffraction. While the analogous nickel intermediate rearranges at room temperature, leading to cyclic products, the palladium compound does not. If the dimerization is carried out in the presence of active hydrogen... 

A number of CSl additions have been shown to produce much less stable j5-lactams. The azetidinone (51) was isolated from the reaction of cyclo-hexadiene with CSI for five minutes at room temperature, while after thirty hours (52) was formed, and the reaction in refluxing chloroform gave (53). Analogous reactions with bicyclo-octadiene did not involve the tautomeric cyclo-octatriene. Initial formation of a 1,4-dipolar intermediate was... 

Another fairly general class of photochemical isomerizations are the di-Ti-methane photoisomerizations. These reactions involve the photochemical conversion of a divinyl methane unit to a corresponding vinyl cyclopropane through intermediate biradical states. The example shown below is interesting in that the naphthalene chromophore in (66) absorbs the light, undergoes intersystem crossing, and sensitizes the benzo-bicyclo-[2.22]-octatriene for reaction. 

Bergamini et al. reported an unusual variation on the linear dimerization, wherein Pd-catalyzed dimerization in acetonitrile/water and in the presence of CO2 affords 37 selectively (52% yield, turnover number (TON) = 420). Under these conditions, only small amounts of the water-trapped product and 1,3,7-octatriene are observed as side products (Scheme 11). The formal intramolecular [3 + 2] cycloaddition product 37, particularly the 1,4-diene subunit imbedded in it, is reminiscent of the products obtained from cyclization of Tp-allylpalla-dium intermediates onto pendant alkenes, a process termed a palladium-ene reaction by Oppolzer and Gavdin, which has also been investigated by Negishi et al. and Trost and Luengo. One can account for the formation of 37 from the chelated Tr-allylpalladium intermediate 39 by ligand insertion to 40 followed by /3-hydride elimination. 

Isopropylidenedioxy-substituted BCOD-fused pyrrole 52d was deprotected to afford the dihydroxy derivative, which was transformed via cyclic thiocarbonate by treatment with thiocarbonyldiimidazole to bicyclo[2.2.2]octatriene-fused (BCOT-ftised) pyrrole 54 by l,3-dimethyl-2-phenyl-1,3,2-diazaphospholidine [57]. Quadmply BCOT-fused porphyrin 55 was obtained in 66% yield by the usual cyclic tetramerization method [58]. In this reaction, an interesting by-product, triply BCOT-fused porphyrin, was obtained in a very small amount. This by-product was thought to be formed by the rDA reaction of intermediates during the oxidation of porphyrinogen to porphyrin [59]. 

Butadiene allowed to react at 0° with phenol in the presence of i r-allylpalladium chloride and Na-phenoxide, then triphenijlphosphine added to the mixture containing l-phenoxyocta-2,7-diene, and distilled under reduced pressure 1,3,7-octatriene. Y 85% purity 98%. - This is a convenient prepn. of a hitherto inaccessible butadiene dimer. Also isolation of the intermediate, use of other Pd-catalysts such as Pd-chloride, and f. anti-Markownikoff additions of nucleophiles to the linear butadiene dimer s. E. J. Smutny, Am. Soc. 89, 6793 (1967) cf. S. Takahashi et al.. Bull. Ghem. Soc. Japan Al, 254, 454 (1968). 

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