2.5- dimethyl-2,4-hexadiene, effect

Particularly the diazoacetic method has a certain appeal for this problem. The decomposition of the sterically hindered diazoacetic ester 74 is effective in the presence of 2,4-dimethyl-hexadien-2,5 75 involving transient unsymmetrical carbon-complexes with chiral copper complexes 76 of highly substituted schiff bases of saUcyhc aldehyde [116] affording the 1-R-trans ester 77 in high optical and chemical yield (Reaction scheme 45) [114]. 

A strong acceptor TCNE undergoes [2+2] rather than [4+2] cycloaddition reactions even with dienes. 1,1-Diphenylbutadiene [20] and 2,5-dimethyl-2,4-hexadiene (Scheme 5) [21] afford mainly and exclusively vinyl cyclobutane derivatives, respectively. In the reactions of 2,5-dimethyl-2,4-hexadiene (1) the observed rate constant, is greater for chloroform solvent than for a more polar solvent, acetonitrile (2) the trapping of a zwitterion intermediate by either methanol or p-toluenethiol was unsuccessful (3) radical initiators such as benzyl peroxide, or radical inhibitors like hydroquinone, have no effect on the rate (4) the entropies of activation are of... 

Decomposition of diazoacetate in 2,5-dimethyl-2,4-hexadiene (3 equiv relative to diazoacetate) leads to the pyrethroid chrysanthemic acid esters with moderate enantioselectivity (68% ee trans, 62% ee cis) but poor diastereoselectivity (58 42 trans to cis), Eq. 5. Both the dimer and the pyridyl monomer were found to be equally effective in this reaction. 

Diets-Alder catalysis.2 This cation radical enhances the reactivity of a neutral or electron-rich eis-1,3-diene in Diels-Alder reactions. Thus 1,3-cyclohexadiene undergoes Diels-Alder dimerization only at temperatures around 200°. The presence of 5-10 mole % of this salt effects dimerization even at —78°, with the usual endo/ exo selectivity (5 1). It also permits facile condensation of 1,3-cyclohexadiene with a hindered dienophile such as 2,5-dimethyl-2,4-hexadiene (equation 1) the dimer of the former diene is a minor product (20% yield). 

Wiberg and coworkers published relative rate constants and the products of reaction of silene 6 with a number of alkenes and dienes in ether solution at 100 °C6 106-108. These data are listed in Table 2 along with an indication of the type of product formed in each case. As is the norm in Diels-Alder additions by more conventional dienophiles, the rate of [2 + 4]-cycloaddition of 6 to dienes increases with sequential methyl substitution in the 2- and 3-positions of the diene, as is illustrated by the data for 2,3-dimethyl-1,3-butadiene (DMB), isoprene and 1,3-butadiene. The well-known effects of methyl substitution at the 1- and 4-positions of the diene in conventional Diels-Alder chemistry are also reflected with 6 as the dienophile. For example, lruns-1,3-pen tadiene reacts significantly faster than the f/.v-isorrier, an effect that has been attributed to steric destabilization of the transition state for [2 + 4]-cycloaddition. In fact, the reaction of c/s-l,3-pentadiene with 6 yields silacyclobutane adducts, while the trans-diene reacts by [2 + 4]-cycloaddition108. No detectable reaction occurs with 2,5-dimethyl-2,4-hexadiene. The reaction of 6 with isoprene occurs regioselectively to yield adducts 65a and 65b in the ratio 65a 65b = 8.5 (equation 50)106,107. 

The reaction thus always leads to a six-membered ring, by 1,4-addition. Of the three double bonds concerned in the reaction, only one is retained in the product, and that at a different position. Rearrangements rarely occur in these reactions. Moreover, they are stereospecific on addition of maleic acid to a diene the adduct produced is a cw-dicarboxylic acid, and on addition of fumaric acid it is a frms-dicarboxylic acid. Substituents on the diene or philodiene are not wholly without effect on the course of the addition for example, 1,2,3,4-tetramethylbutadiene (3,4-dimethyl-2,4-hexadiene) reacts smoothly,33 but the tetraphenyl analog shows no tendency to add a philodiene. Bulky substituents at the 2,3-positions can completely suppress the addition, presumably because they prevent free rotation and thus disfavor formation of the con-... 

Further, it is shown in Figure 6 that this methyl steric effect can be observed even when the methyls are placed alpha to the metathesizing olefin itself. Thus, 3,4-dimethyl-1,5-hexadiene does not polymerize, whereas 3-methyl-1,5-hexadiene dimerizes in the 5 position. These steric interactions are quite subtle, and by examining the ADMET polymerization cycle shown earlier, it becomes evident that the formation of a metallacyclobutane ring and its stability is influenced by simple steric interaction. 

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