4- Methyl-l,3-pentadienes

Another type of steric effect results from interactions between diene substituents. Adoption of the s-cis conformation of the diene in the TS brings the d.v-oricnlcd 1- and 4-substituents on a diene close together. /(-1,3-Pcnladicnc is 103 times more reactive than 4-methyl-l,3-pentadiene toward the very reactive dienophile tetracyanoethylene. This is because the unfavorable interaction between the additional methyl substituent and the C(l) hydrogen in the s-cis conformation raises the energy of the TS.20... 

The 2,5-dihydro-l,2-oxaphosphole-2-oxide derivatives 196 were also obtained from the reaction of esters of 3-chloro-4-methyl-l,3-pentadiene-2-phosphonic acid with halogens (Scheme 78) [155, 156],... 

For the synthesis of permethric acid esters 16 from l,l-dichloro-4-methyl-l,3-pentadiene and of chrysanthemic acid esters from 2,5-dimethyl-2,4-hexadienes, it seems that the yields are less sensitive to the choice of the catalyst 72 77). It is evident, however, that Rh2(OOCCF3)4 is again less efficient than other rhodium acetates. The influence of the alkyl group of the diazoacetate on the yields is only marginal for the chrysanthemic acid esters, but the yield of permethric acid esters 16 varies in a catalyst-dependent non-predictable way when methyl, ethyl, n-butyl or f-butyl diazoacetate are used77). 

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). 

The change in selectivity is not credited to the catalyst alone In general, the bulkier the alkyl residue of the diazoacetate is, the more of the m-permethric acid ester results 77). Alternatively, cyclopropanation of 2,5-dimethyl-2,4-hexadiene instead of l,l-dichloro-4-methyl-l,3-pentadiene leads to a preference for the thermodynamically favored trans-chrysanthemic add ester for most eatalyst/alkyl diazoacetate combinations77 . The reasons for these discrepandes are not yet clear, the interplay between steric, electronic and lipophilic factors is considered to determine the stereochemical outcome of an individual reaction77 . This seems to be true also for the cyclopropanation of isoprene with different combinations of alkyl diazoacetates and rhodium catalysts77 . 

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-... 

An analogous reaction applied to l,l-dichloro-4-methyl-l,3-pentadiene would lead to a selective synthesis of permethric acid, of which the cis isomer was more desirable. It is interesting that this seemingly simple structural change provided... 

Figure 8 l3C NMR spectrum of l-trimethylsilyl-4-methyl-l, 3-pentadien-2-yl cation 31 (R = H) in S02CIF/S02F2 at -137 °C (37). 

Another protecting group of amines is 1-isopropylallyloxycarbonyl. which can be deprotected by decarboxylation and a /3-elimination reaction of the (w-l-isopropylallyl)palladium intermediate under neutral conditions, generating CO2 and 4-methyl-l,3-pentadiene. The method can be applied to the amino acid 674 and peptides without racemization[437]. 

So Cataldo et al. [101] have focused their studies on polymers like 1-2 polybutadiene, 3-4 polyisoprene and poly(4-methyl-l,3 pentadiene). These polymers are very sensitive to ozone, and their own tacticity seems to have a very low influence on their reactivity. In a usual way, substituents make... 

Cyclobuten-Bildung findet auch bei zahlreichen substituierten 1,3-Buta-dienen statt (z.B. bei Isopren (92,277), 2,3-Dimethylbutadien-(l,3) (92, 277), trans-l,3-Pentadien (277), l-Cyclohexylbutadien-(l,3) (92), Sorbin-alkohol (92) und 2,3-Diphenyl-butadien (322)). Besonders erwahnt sei das 4-Methyl-l,3-pentadien (Formel 60), das bei der UV-Bestrahlung nicht 3,3-Dimethylcyclobuten-(l) (Formel 59), sondem 1,3-Dimethyl-cyclobuten-(l) (Formel 62) (92,126) liefert. 

The polymers obtained from unsymmetrically terminally disubstituted butadiene such as 4-methyl-1,3-pentadiene are made up of 1,2 monomeric units only, irrespective of the catalyst used this is due to the presence of two methyl substituents at the C4 atom in the monomer. Two stereoregular polymers have so far been obtained from 4-methyl-l,3-pentadiene, one with a 1,2-isotactic structure and one with a 1,2-syndiotactic structure. The isotactic polymer has been yielded by heterogeneous Ziegler-Natta catalysts, e.g. TiCU—AlEt3 and a — TiCl3— [AlEt3 [182]. The factors that determine the orientation of the coordinating monomer in this case are not, however, completely clear [41]. 

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],... 

Scheme 3.15 Products in partial hydrogenation of 4-methyl-l,3-pentadiene.

The system Cp2TiCl2/MAO is suggested to be less active than CpTiCl3/MAO and Cp2TiCl/MAO in the polymerization of 1,3-butadiene, 4-methyl-l,3-pentadiene, and styrene to give predominantly cis-, 4-polybutadiene, 1,2-syndiotactic poly(4-methyl-l,3-pentadiene), and syndiotactic polystyrene.1214... 

The catalytic activity of the titanocene derivatives increases in the order Cp2TiC2dienes also polymerize in the presence of CpTiCl3/MAO catalyst with the selectivity depending on the monomer structure. The polymerization of (Z)-l,3-pentadiene forms cis-... 

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