Trienes Octatrienes

Electrocyclic reactions of 1,3,5-trienes lead to 1,3-cyclohexadienes. These ring closures also exhibit a high degree of stereospecificity. The ring closure is normally the favored reaction in this case, because the cyclic compound, which has six a bonds and two IT bonds, is thermodynamically more stable than the triene, which has five a and three ir bonds. The stereospecificity is illustrated with octatrienes 3 and 4. ,Z, -2,4,6-Octatriene (3) cyclizes only to cw-5,6-dimethyl-l,3-cyclohexadiene, whereas the , Z,Z-2,4,6-octa-triene (4) leads exclusively to the trans cyclohexadiene isomer. A point of particular importance regarding the stereochemistry of this reaction is that the groups at the termini of the triene system rotate in the opposite sense during the cyclization process. This mode... 

The molecular orbitals derived from overlap of six p atomic orbitals as found in trienes are shown in Figure 8.7. Since there are six electrons to accommodate, as two paired electrons per orbital, the HOMO is n3. Thus, for 2,4,6-octatriene the disrotatory mode of reaction gives the trans isomer (Scheme 8.5). 

Dimerization of conjugated dienes and trienes is generally accomplished at elevated temperatures or in the presence of metal catalysts. Linear dimerization of butadiene occurs readily at room temperature on nickel catalysts bearing aminophosphinite (AMP) ligands, and the reaction rate is reportedly twice that observed in other nickel systems employing either morpholine, ethanol or P-methyloxaphospholidines as modifiers62. 1,3-Pentadiene dimerizes in the presence of 1 mol% nickel catalyst to give a diastereomeric mixture of 4,5-dimethyl-l,3,6-octatriene as shown in equation 42. 

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 ). 

The highly substituted 3,4-diazabicyclo[4.2.0]octatrienes (177) have been obtained by cyclization of cis- 3,4-dibenzoylcyclobutenes with hydrazine (69CB1928). The triene (R = H) undergoes thermal ring opening to give a vinylpyridazine, but with bridgehead substituents (e.g. R = Me) conversion to the substituted benzene (181) occurs in pyridine at 70-80 °C... 

The ergosterol system mentioned earlier falls in this category. Before the system is discussed, it is worthwhile to point out that, in general, substitution of even a simple group such as an alkyl seems to change the details of the photochemical primary processes in a startling manner among the dienes and trienes. Thus, in solution, 1,3,5-hexatriene cannot be converted to 1,3-cyclohexadiene photochemically, but 2,4,6-octatriene does cyclize photochemically to (rans-5,6-dimethyl-l,3 Cyclohexadiene under the same conditions. ... 

Systematic studies were carried out on copolymers of isobutene with 1,3,5-hexatriene (HTI) and 2,4,6-octatriene (OTI). The structure of the triene units present in the copolymer chains was determined by riKans of C-NMR by making reference to the cationic triene homopolymers and to the same copolymers previously hydrogenated. In the case of HTI there are only traces of repeat units descending from the 1,2- and 1,4-opening of the trienic system, the 1,6-enchainment being prevalent some cyclic structures are also present The cationic homopolymer of 2,4,6-octatriene results from the preval t (ca. S)%) 2,7-opening of the monomer, while the remainder unit results from 2,5-addition. A similar situation also seems to be present in OTI ... 

The block distribution of 2,4,6-octatriene existing in OTI (Sect. III.A.2.) can be another reason accounting for the lower grafting efficiency of OTI with respect to HTI, since allylic positions are more sterically hindered in the triene blocks. 

By contrast, OTl reacts incompletely, even at 150 °C. The blodc distribution of 2,4,6-octatriene in OTI seems the main reason for the incomplete reaction of the adjacent diene groups, because Bell observed largely incomplete reactions of poly-13,5 Jiexatriene with sewral dienophiles in particular, with bulky ones, more than 50% conversion was never achieved. In fact, severe steric constraints arise for a diene group belonging to a 1,6-triene homosequence wlwn the two adjacent Diels-Alder adducts are formed. 

The interconversions of the corresponding dimethylcyclohexadienes and the 2,4,6-octatrienes are also stereospecific (Fig. 29.13). Here, too, thermal and photochemical reactions differ in stereochemistry. If we examine the structures closely, we see something else the stereochemistry of the triene-cyclohexadiene interconversions is opposite to that of the diene-cyclobutene inlerconversions. For the thermal reactions, for example, cis methyl groups in the cyclobutene become cis and trans in the diene cis methyl groups in the cyclohexadiene are trans and trans in the related triene. 

Skeletal rearrangements are rare, but are found for strained bi- and polycyclic alkenes, e.g., bullvalene , bicyclo[4.2.2]deca-2,4,7,9-tetraene , 4,5-exo-trimethylene-2-norbornene , 6-methylenebicyclo[3.1.1]heptane ", 5-methylbicyclo[2.1.1] hexene N-benzyl-9-aza-bicyclo[4.2.2]deca-2,4,7-triene , dibenzobicyclo[2.2.2]-octatriene derivatives and bornylene ... 

The first report of a topochemical 1,6-polymerization of a conjugated triene was recently described by Lauher and co-workers [39]. In particular, the double ester of l,8-dihydroxy-2,4,6-octatriene and nicotinic acid (Scheme 2.3.5) was found to adopt a solid-state packing arrangement conducive for the 1,6-polymerization. Specifically, the molecules were oriented at a repeat distance of 7.2 A (Fig. 2.3.3(a)). Although the separation between terminal carbon atoms of adjacent triene moieties was somewhat large (4.09 A), the solid underwent a SCSC topochemical... 

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]. 

AI3-00737 Alloocimene 2,6-Dimethyl-2,4,6-octatriene 2,6-Dimethylocta-2,4,6-triene EINECS 211-614-5 NSC 406263 Ocimene, alio 2,4,6-Octatriene, 2,6-dimethyl-. 

Figure 10.27 summarizes the energy relationships for the four-, six-, and eight-electron systems relative to the polyenes. We see that for cyclobutene-1,3-butadiene, ring opening is favored and the is 32 kcal/mol. The is similar for the 1,3,5-triene cyclization (30 kcal/mol), but ring closure is favored. The E drops to 17.0 kcal/mol for the Z,Z-l,3,5,7-octatriene to 1,3,5-cyclooctatriene cyclization, whereas the E for the reverse reaction is 28.2 kcal/mol. 

The more activated TpRe(CO)(MeIm)(q -anisole) complex has demonstrated a slightly broader range of cycloaddition reactivity. The anisole complex 110 cychzes with DM AD to give the diastereomeric bound bicyclo [2.2.2] octatriene 111 isolated as a 1 1 ratio of coordination diastereomers (Fig. 23). Oxidation of these complexes liberates the corresponding triene 112 as well as the disubstitut-ed anisole 113, which presumably is generated with acetylene from the cycloreversion of 112. 

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