Hexa-l,5-dienes

Examine the geometry of methylbenzyne. Measure carbon-earbon distances. Which 7C bonds are deloealized and whieh are localized Is there really a triple bond (Compare bond distance to triple bond in hexa-l,5-dien-3-yne and to partial double bonds in benzene). Are you able to draw a single Lewis structure whieh adequately represents the geometry of the molecule ... 

The Cope rearrangement of hexa-l,5-diene does not allow for differentiation of starting material and product this is called a degenerate Cope rearrangement. Another example is the automerization of bicyclo[5,l,0]octa-2,5-diene 7 ... 

Aryl group substituted butatrienes and hexapentaenes can be selectively reduced with Zn-ZnCl2-H20 to result in aryl-substituted 1,3-butadienes and hexa-l,5-dien-3-ynes, respectively (equations 191 and 192)315. 

Simple alkenes such as 1-octene are completely resistant to this cation-radical hydrogenation. This makes it possible to reduce a more ionizable double bond selectively in the presence of a simple alkene moiety as illustrated for l,l-bis(anisyl)hexa-l,5-diene in Scheme hexa-1,5,7.5 (Mirafzal et al. 1993). 

The kinetically controlled Cope rearrangement of 2,5-bis(4-methoxyphen-yl)hexa-l,5-dienes induced by photosensitized electron transfer to DCA was examined by Miyashi and co-workers [101-103]. Remarkable in this context was the temperature-dependent change of the photostationary ratio of this rearrangement, yielding the thermodynamically less stable compound at — 80°C in 96%. A radical cation-cyclization diradical cleavage mechanism (RCCY-DRCL) is... 

Another metathesis polymerization procedure uses terminal dienes such as hexa-l,5-diene (16) acyclic diene metathesis (ADMET)). Here again, the escape of the gaseous reaction product, i.e. ethylene, ensures the irreversible progress of the reaction ... 

Although it is generally agreed that the thermal isomerization of bicyclo[2.2.0]hexanes to hexa-l,5-dienes takes place via diradical intermediates,113 118 121,123 125 experimental evidence has been obtained which implies otherwise.115,116 While a radical stabilization energy of approximately 4 kcal mol"1 was obtained for the pyrolysis of methyl 4-chlorobicy-clo[2.2.0]hexane-l-carboxylate (28 b) to methyl 5-chlorohexa-l,5-diene-2-carboxylate (29b),115116 as related to the parent 2-chlorohexa-1,5-diene (29a),115-l16-118 kinetic studies have indicated that there is a small but significant increase in activation energy of about 1 kcal mol-1 for the gas-phase and solution pyrolysis of l-chloro-4-methylbicy-clo[2.2.0]hexane (28c), as compared to l-chlorobicyclo[2.2.0]hexane (28a).115-116 In the light of this result, the commonly accepted diradical mechanism must be questioned and it is likely that the isomerization of these compounds occurs via a concerted process. 

Calculations of the interactions between matured catalytic antibody AZ28 and the different conformers and enantiomers of 2,5-diaryl-hexa-l,5-dien-3-ol, allowing for the flexibility of the antibody, have reproduced, qualitatively only, the observed selectivity of the antibody for the S,equatorial substrate.7 Doubly a,/9-unsaturated A-arylimines (3) have been shown to react with a,j3-unsaturated ketenes to give azocinones (4),... 

Tandem RCM and cross-metathesis reactions of allyl hexa-l,5-dien-3-yl ether with alkenes leads to 2-alkylidene 3,6-dihydro-27f-pyrans (Scheme 6) <07TL1417>. 

Initial concentration of isoprene On the basis of H NMR integration Determined by SEC Determined by DSC Not given Hexa-l,5-diene Hex-l-ene... 

Diblock C2 [W]=CHCMe2Ph Hexa-l,5-diene-l-oligomers of Mi -Ppolystyrene Wagener 1991b... 

High optical yields were obtained using [Rh(hexa-l,5-diene)L] complexes, where L =. BPPM(Scheme 7.19), or 1 and 2 in Scheme 7.25. 

SCHEME 3.18 Anionic Oxy-Cope rearrangement of hexa-l,5-dien-3-ol. 

The metathesis reaction of cycloalkenes yields linear unsaturated polymers, so-called polyalkenamers. This ROMP is driven by the release of ring strain in the starting material. Several interesting polymers are commercially produced via the ROMP of different types of unsaturated cyclic monomers such as cyclooctene, norbornene, and dicyclopentadiene, using homogeneous catalyst systems [6]. As an alternative process, the cross-metathesis between a cyclic and an acyclic olefin allows to synthesize certain poly-unsaturated compounds for the special chemical market. Shell [7] developed the FEAST process for the manufacture of hexa-l,5-diene via cross-metathesis of cycloocta-1,5-diene with ethene. 

Some heterocyclizations have been used for the preparation of 3-perflu-oroalkylthiophenes. As an illustration, treatment of l,l,6,6-telrakis(ethylsulfanyl)-2,5-bis(trifluoromethyl)-hexa-l,5-dien-3-yne 135 with a mixture of trifluoroacetic acid and water for 2 h at 75 °C led to the thiophene derivative 136 in high yield. The starting compound 135 was obtained by reaction of perfluoroketene dithioacetal 134 with bis(trimethylsilyl)acetylene [76]. 

The oxy-Cope rearrangement of hexa-l,5-dien-3-ols to give Se-unsaturated carbonyl compounds often gives low yields because of the high temperatures that are needed to elfect the reaction. However, in the presence of mercury tri-fluoroacetate, the reaction proceeds at room temperature and the required Se-unsaturated carbonyls (93) are obtained bydemercuration of the intermediates (92) with sodium borohydride in good yield ( 60%). Nakai and his co-workers... 

Mercury(n) triiluoroacetate has been shown to induce the oxy-Cope rearrangement of tertiary hexa-l,5-dien-3-ols at room temperature. Further details of the stereochemical consequences of sequential tandem Cope-Claisen rearrangements have been disclosed, and the use of the tandem [2,3]-Wittig-oxy-Cope rearrangement has been demonstrated in the synthesis of (E S-octenal, a precursor of exo-brevicomin [equation (36)]. Investigation of the trans-... 

Oxy-Cope rearrangements (95, 98, 99) have been extensively used in natural product synthesis to generate l-oxo-hexa-l,5-diene systems which are then elaborated to introduce the functionality presented in the selected targets. Wender and his coworkers have utilized this methodology in a key step in the synthesis of reserpine (36, 37). They envisioned that the DE-ring precursor 128 could arise from the hydroisoquinoline 129 which could be generated by oxy-Cope rearrangement of isoquinuclidene 130 (Scheme 3.20). This key intermediate, in turn, could be prepared by Diels-Alder reaction of dihydropyridine 131 with an appropriately substituted dienophile 132. 

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