Trienes cyclic

I Benzene is unusually stable, having a heat of hydrogenation 150 kj/mol less negative than we might expect for a conjugated cyclic triene. 

This class of compounds is defined to have some of the three conjugated double bonds found in the ring and others not. This class includes the isomeric 3,3 -bis(cyclohexenylidenes), 100 and 101. Roth shows us that the two isomers have the same enthalpy of formation within ca 1 kJmol-1, a difference somewhat smaller than the 4 kJmol-1 found for the totally acyclic 1,3,5-hexatrienes, 79 and 80 respectively. Naively these two sets of trienes should have the same (E)/(Z) enthalpy difference. Given experimental uncertainties, we will not attempt to explain the difference69. We may compare 100 and 101 with phenylcyclohexane, 102, an isomeric species which also has the same carbon skeleton. There is nearly a 110 kJ mol-1 enthalpy of formation difference between the semicyclic and cyclic trienes. We are not surprised, for the word cyclic is customarily replaced by aromatic when in the context of the previous sentence. 

The metal-mediated and metal-catalyzed [6 + 2]- and [6 + 4]-cycloaddition reactions, pioneered by Pettit and co-workers105 106 and Kreiter and co-workers,107 respectively, involve the cycloaddition of metal-complexed cyclic trienes with 7r-systems such as alkenes, alkynes, and dienes. The [6 + 2]-reactions produce bicyclo[4.2.1]nonadiene derivatives and the [6 + 4]-reactions produce bicyclo[4.4.1]undecatrienes (Scheme 32). Trienes complexed to chromium, which can be prepared on large scale (40 g) as reported by Rigby and co-workers,108 react with 7r-systems upon thermolysis or irradiation.109-111 Chromium and iron-catalyzed [6 + 2]-reactions of cycloheptatrienes and disubstituted alkynes... 

A similar result obtained with nopadiene is presented above (Scheme 13.9). Linear and cyclic trienes furnish alkenyl-r 3-allyltitanium complexes, which can react with aldehydes to produce dienyl alcohols (Scheme 13.13) [24],... 

Dauben and coworkers produced a number of lovely examples of the reaction in the course of their studies of the photochemistry of large-ring (Cs-Cn) cyclic trienes, many of which were produced by photochemical electrocycloreversion of the isomeric annulated cyclohexadiene derivatives (cf Reference 172 and references cited therein). Two examples... 

Several examples of the photochemistry of larger-ring (C9-C12) cyclic trienes have already been cited (vide supra), and a great many more have been studied, particularly by Dauben and coworkers. These have been reviewed thoroughly elsewhere in general,... 

Cycloaddition reactions using tropone or another cyclic triene as the 6ji partner have been abundantly described in the literature. It has been found that virtually all metal-free [6 + 4] cycloadditions of cyclic trienes afford predominantly exo adducts. This has been rationalized by consideration of the HOMO-LUMO interactions between the diene and triene partners. An unfavorable repulsive secondary orbital interaction between the remaining lobes of the diene HOMO and those of the triene LUMO develops during an endo approach. The exo transition state is devoid of this interaction (Figure 9). 

Diynes are treated with Ic in the presence of an allylsilane to give cyclic trienes (Equation (12)). ... 

Trans-cis trans-ocXzXr eae (47) cyclizes at 130°C in the predicted disrotatory sense to m-5,6-dimethylcyclohexa-l,3-diene.106 Closure in cyclic trienes also occurs readily according to the generalized structures in Equation 12.60, where... 

The carbolithiation of six- to nine-membered 3-methylenecycloalka-l,4-dienes has been investigated and shown to be an exceptionally facile and general process.69 Primary, secondary, and tertiary organolithium reagents may be employed for carbolithiation of cyclic trienes with uniform efficiency, generating cyclic pentadienyl carbanions. The six-electron pentadienyl systems display unique reactivity as a function of ring size. 

As regards the metathesis polymerisation of cyclic trienes, it has been carried out in an attempt to find alternative routes for preparing soluble and meltable precursors of polyacetylene [149, 150], Hence, several substituted or unsubstituted tricyclic or other polycyclic trienes were subjected to polymerisation in the presence of metathesis catalysts such as WCl6-SnMe4 [151-154] and the tungsten neopentylidene complex [Me(F3C)2CO]2W(=NAr)(=CHCMe3) [155]. A successful solution of the problem is outlined below [125,150] ... 

An interesting instance of the ring-opening metathesis polymerisation of cyclic trienes is the polymerisation of the novel conjugated diene 3,4-diisopropylidene-cyclobutane in the presence of a bis(cyclopentadienyl)titanacyclobutane derivative as a catalyst, which affords a linear cross-conjugated polymer [156] ... 

The reverse reaction is a linear cheletropic extrusion of SO2 with conrotatory twisting of the terminal methylenes. With cyclic triene SO2 forms only the 1,4 adduct. In this case the antarafacial addition to the triene is geometrically impossible and the alternative non-linear n s + (O s process does not compete with the concerted 71 5 + (O s linear cheletropic addition to a diene component. So a nonlinear cheletropic addition containing eight electrons becomes highly unfavorable. This is illustrated by the fact that SO2 is eliminated 60,000 times more slowly (even at 180°) from (c) than from (b). In (b) + (ip s, retro process is allowed. 

Cyclic triene (nonaromatic) and tetraene reactions form a series of new complexes. While some cyclotrienes yield mostly unstable species, heating or hgand trapping can generally lead to stable, isolable compounds. Hydrogen transfer also plays an important role in the synthesis ofrf-rf sandwich complexes. With cot and Cr atoms a Ci Cr bonded Cr2(CgH8)3 complex is formed, while in the case of Ti-cot, a novel Triple Decker Sandwich is obtained. ... 

Trienes and isonitriles, cyclic trienes (nonaromatic) codeposited with metal atoms yield new complexes. Often hydrogen transfers from one ring to another are observed, e.g., with cycloheptatriene for the metals Ti, Hf Zr Cr Mo, W, Co, Co and Fe... 

In dramatic contrast to the normal course of higher-order cycloadditions with most cyclic triene substrates, the photochemically induced [6 h- 4] cycloaddition reactions exhibited by tricarbonyl(T -l,3,5-cy-cloheptatriene)chromium(O) complexes appear to be quite suitable for preparative applications. Yields of the resultant diene cycloadducts are usu ly good and the metal can be removed in high yield on treatment of the cycloadduct complex with triethylphosphine. Scheme 14 shows the results of several cycloadditions between the complex and substituted diene addends. While not specifically delineated in the original work, these reactions provide the corresponding endo adducts exclusively. ... 

Two isomeric 1,5,9-cyclododecatrienes, namely, trans,trans,cis-CijH 18 (XLVI) and trans,trans,trans-CuHis (XLVII), are formed in good yield by the cyclic trimerization of butadiene using certain Ziegler-type catalysts 247, 250, 251, 252). The formation of these 12-membered ring hydrocarbons probably proceeds via metal 7r-complexed intermediates. When the cyclic triene (XLVII) is treated with nickel acetylacetonate and... 

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