Pentadienes formation, methyl

Methyl Pentadienes Formation. Rate data for methyl pentadienes formation in the presence of H2S are presented in Figure 8. At both temperatures, methyl pentadienes formation is found to be/v/ 0.3 order in H2S. The observed E for the formation of methyl pentadienes based on these data is 26.5 kcal/ mole. When one considers the possible free radical homogeneous mechanism for methyl pentadienes formation, the following mechanism may be proposed ... 

A striking example for the preferred formation of the thermodynamically less stable cyclopropane is furnished by the homoallylie halides 37, which are cyclopro-panated with high c/s-selectivity in the presence of copper chelate 3891 The cyclopropane can easily be converted into cw-permethric acid. In contrast, the direct synthesis of permethric esters by cyclopropanation of l,l-dichloro-4-methyl-l,3-pentadiene using the same catalyst produces the frans-permethric ester (trans-39) preferentially in a similar fashion, mainly trans-chrysanthemic ester (trans-40) was obtained when starting with 2,5-dimethyl-2,4-hexadiene 92). 

Although the reactions in the above ligand effect studies are designed to minimize the formation of isomers of 1,4-hexadiene, approximately 10% isomers are still formed. These are 3-methyl-1,4-pentadiene and 2,4-hexadiene and a very small amount of higher oligomers. Whatever effect the donor might have on the rate of formation of these isomers (i.e., with respect to the 1,4-hexadiene formation) or on their distributions appeared to be insignificant (24). 

The isomer distribution of the nickel catalyst system in general is similar qualitatively to that of the Rh catalyst system described earlier. However, quantitatively it is quite different. In the Rh system the 1,2-adduct, i.e., 3-methyl-1,4-hexadiene is about 1-3% of the total C6 products formed, while in the Ni system it varies from 6 to 17% depending on the phosphine used. There is a distinct trend that the amount of this isomer increases with increasing donor property of the phosphine ligands (see Table X). The quantity of 3-methyl-1,4-pentadiene produced is not affected by butadiene conversion. On the other hand the formation of 2,4-hexadienes which consists of three geometric isomers—trans-trans, trans-cis, and cis-cis—is controlled by butadiene conversion. However, the double-bond isomerization reaction of 1,4-hexadiene to 2,4-hexadiene by the nickel catalyst is significantly slower than that by the Rh catalyst. Thus at the same level of butadiene conversion, the nickel catalyst produces significantly less 2,4-hexadiene (see Fig. 2). 

The formation of the same cyclopropylamine from 2-methyl-l,3-pentadiene as from 4-methyl-l,3-pentadiene (entries 2 and 3 in Table 11.11) can most probably be attributed to initial isomerization of the former to the latter under the conditions employed. The fact that the conjugated 6-methyl-l,3,5-heptatriene yields only the 2,3-dialkenylcyclopro-... 

Exercise 15-22 Methylmagnesium iodide with 2-butenal gives an addition product that, when hydrolyzed with dilute sulfuric acid and extracted with ether, yields an ether solution of impure 3-penten-2-ol. Attempted purification by distillation of the ether extract gives only 1,3-pentadiene and di(1-methyl-2-butenyl) ether. Write equations for each of the reactions involved. How could you avoid ether and diene formation in the preparation of 3-penten-2-ol by this method ... 

As with the Aratani catalysts, enantioselectivities for cyclopropane formation with 4 and 5 are responsive to the steric bulk of the diazo ester, are higher for the trans isomer than for the cis form, and are influenced by the absolute configuration of a chiral diazo ester (d- and 1-menthyl diazoacetate), although not to the same degree as reported for 2 in Tables 5.1 and 5.2. 1,3-Butadiene and 4-methyl- 1,3-pentadiene, whose higher reactivities for metal carbene addition result in higher product yields than do terminal alkenes, form cyclopropane products with 97% ee in reactions with d-men thy 1 diazoacetate (Eq. 5.8). Regiocontrol is complete, but diastereocontrol (trans cis selectivity) is only moderate. 

The formulas of 37-39 show the substitution of three carbonyls by a conjugated diene. Due to the fact that dienes are four-electron donors, the complexes are electron-deficient species. The electron deficiency is reduced by formation of C—H—Cr three-center, two-electron bonds, previously found for a number of other complexes (43). When 2-methyl-5-isopropyl-l,3-cyclohexadiene is used as a potential ligand, the expected complexes 37r and 39r are not obtained, but instead complexes with the isomeric l-methyl-4-isopropyl-l,3-cyclohexadiene group (37r and 39r ) are formed [Eq. (19)]. Similarly, lk and 11 form not only the corresponding complexes 39k and 391, but also the isomeric complexes with (Z)-l,3-hexadiene (39k ) and with (Z)-2-methyl-1,3-pentadiene (391 ). 

Syndiotactic l,2-poly(4-methyl-l,3-pentadiene) has been formed by polymerisation with homogeneous catalysts, e.g. TiBz4—[Al(Me)0]x and CpTiCl3—[Al (Me)0]x [41,43]. The coordination of the monomer as an s-trans-t/2 ligand rather than an s-cis-r A ligand at the Ti atom has been postulated to be involved in the polymerisation. The s-cis-r A monomer coordination is less favoured for steric reasons in the case of 4-methyl-1,3-pentadiene. A possible scheme for the formation of the 1,2-syndiotactic polymer from this monomer is presented in Figure 5.7 [41,43],... 

Figure 5.7 Schematic presentation of the formation of 1,2-syndiotactic poly(4-methyl-1,3-pentadiene)...

Formations of 6-dimethylamino-2,4-hexadiene (163) from 1,6-dimethyl-3-piperideine,87 5-dimethylamino-4-methyl-1,3-pen tadiene (164) from l,3-dimethyl-3-piperideine,82 5-dimethylamino-3-methyl-1,3-pentadiene (165) from l,4-dimethyl-3-piperideine,83 and 6-di-methylamino-2,4-heptadiene (166) from l,2,6-trimethyl-3-piperi-deine94 may serve as further examples of the Hofmann exhaustive methylation in the 3-piperideine series. 

According to Blake and Hole , methyl ketene decomposes into CO2 and pentadiene-2,3 as well as into CO and butene-2 subsequent polymerization and decomposition processes produce other products, especially at higher temperatures. The results indicate that the overall orders of both CO2 and CO formation are 1.5 and each reaction path is inhibited by isobutene. Blake and Hole suggested tentative chain mechanisms to account for the observed product formation and for the kinetics of the decomposition. Initiation and termination was assumed to occur at the surface of the vessel. 

In many synthetically useful radical chain reactions, hydrogen donors are used to trap adduct radicals. Absolute rate constants for the reaction of the resulting hydrogen donor radicals with alkenes have been measured by laser flash photolysis techniques and time-resolved optical absorption spectroscopy for detection of reactant and adduct radicals Addition rates to acrylonitrile and 1,3-pentadienes differ by no more than one order of magnitude, the difference being most sizable for the most nucleophilic radical (Table 8). The reaction is much slower, however, if substituents are present at the terminal diene carbon atoms. This is a general phenomenon known from addition reactions to alkenes, with rate reductions of ca lOO observed at ambient temperature for the introduction of methyl groups at the attacked alkene carbon atom . This steric retardation of the addition process either completely inhibits the chain reaction or leads to the formation of rmwanted products. 

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