Mode selectivity. 1,3-dienes

Edman and Simmons [146] synthesized bicyclo[2.2.1]hepta-2,5-diene-2,3-dicar-boxylic anhydride 80 as a facially perturbed dienophile on the basis of the norbornadiene motif, and its top selectivity in Diels-Alder reactions with cyclopentadiene (top-exo top-endo = 60 70 1) was observed by Bartlett (Fig. 14) [147], The most preferred addition was top-exo addition, along with the minor addition modes, top-endo bottom-enrfo addition (Fig. 14). The addition of butadiene to this anhydride preferentially afforded the top-adduct (top bottom = 6 1). In the addition of anthracene, a top-adduct was formed exclusively. 

The syn addition mode was also confirmed by ab intio calculation of the reaction between thiophene 1-oxide 99 and ethylene. They stated that the selectivity can be explained by the orbital mixing rule (Scheme 51). The ir-HOMO of the diene part of 99 is modified by an out-of-phase combination with the low lying n-orbital of... 

Also the a-n interaction in Diels-Alder additions, which occurs with sy -fashion with regard to both diene and dienophile, is explained (Fig. 7.38). For the first place, the p-a type interaction is allowed, by the selection rule already mentioned, between the jr-part of butadiene and the ji-part of ethylene. Once this weak p-a type interaction starts, the p AO part forms a six-electron system. The HO of this -part will come from HO of butadiene jr-part interacting with LU of ethylene jr-part will interact with er-LU s of both butadiene and ethylene. The mode of interaction is as indicated in Fig. 7.38. 

The cycloadditions between tetrahalogenocyclopropenes and butadienes are usually carried out at about 100 °C in an inert solvent, in the presence of a small amount of K2CO3 and hydroquinone in order to prevent polymerization of the diene. Originally endo selectivity was assumed for additions of substituted open-chain dienes to 1,2-dihalogeno- and tetrahalogenocyclopropenes, but more recent X-ray and NMR investigations showed that the preferred mode of addition is exo. The cycloadducts are conveniently aromatized to cydoproparenes by reaction with r-BuOK in THF at low temperature. Since 1,1-dihalogenocyclo-proparenes solvolyze veiy readily, an anhydrous work-up procedure is usually indicated. 

The fact that the reactions take place in the direction of ring-opening is determined by thermodynamics, but the stereochemistry is most certainly not, for the cyclobutene 4.36 gives the thermodynamically more strained product 4.37 with one of the double bonds cis. Thermodynamics affects the stereochemistry only with the opening of the cyclobutene 4.38, which shows a preference for one of the conrotatory modes, that giving the trans,trans diene 4.39, where the rules could have led to the cis,cis diene equally well. This type of selectivity is called torqueoselectivity. 

Shibasaki and co-workers have described a regioselective Heck cyclization of aryl triflate 12.1, which ultimately provides tricyclic enone 12.4, a key intermediate in a number of diterpene syntheses (Scheme 8G.12) [25], Treatment of 12.1 under typical cationic conditions resulted in preferential 6-exo closure to give 12.2 and 12.3 as a 3 1 mixture in 62% overall yield and with 95% ee for both products. The complete selectivity for 6-exo cyclization is noteworthy because 6-endo, 5-exo, and 7-endo cyclization modes were also possible. An analysis of the steric interactions involved in the various cyclization modes was presented and was used to rationalize the observed selectivity. Non-conjugated diene 12.2 could be isomerized to the fully conjugated diene 12.3 in quantitative yield by using catalytic naphthalene Cr(CO)3. Both Heck products could be converted to the enone 12.4,... 

In view of the important role played by PMR spectroscopy in determining the structure and the mode of rearrangement of the i 3-allyl metal complexes, it is surprising that the 13C-NMR spectra of relatively few examples have been reported. Complexes of this type are of considerable interest as intermediates in the metal-catalyzed transformations of 1,3-dienes, and we have recently systematically investigated a number of systems. Selected examples will be discussed. 

As antiscuff, anti-wear properties of additives depend on the covalent-bonded sulfur contents, the selection of optimum conditions to effect sulfuring processes has been set with introducing the greatest amount possible of covalent sulfur. For this purpose functionalizing of diene and olefin hydrocarbons was conducted on a wide time-temperature mode. Considering that the temperature of boiling pipeiylene fraction low (42-44°C), sulfuring was carried out in a constantly temperature-controlled autoclave in the presence of the catalyst cobalt phthalocyanine [3], in the medium of non-polar solvents (for example, heptane). Under... 

Cycloheptatriene and related heterocyclic analogs have been examined to some extent with regard to their reactivity with various conjugated diene partners. Characteristically, higher-order cycloaddition pathways often suffer relative to alternative modes of reaction in these systems and usually a select type of highly reactive diene partner is a prerequisite for viable cycloaddition to occur. Selectivity in these species can also be compromised by die intervention of the corresponding norcaradiene tautomer in certain circumstances. Moctest periselectivity coupled with low chemical yields limit the potential synthetic utility of these cycloadditions. 

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