1.1- dichloro-4-methyl-pentadiene

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 . 

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... 

Dichlorination of tetramethylallene afforded 3-chloro-2,4-dimethyl-l,3-pentadiene as the single product, whereas the same reaction of 1,1-dimethylallene yielded a mixture of 2-chloro-3-methyl-l,3-butadiene, 2,3-dichloro-3-methyl-l-butene and 1,2-dichloro-3-methyl-2-butene, indicating the intermediacy of the 2-chloroallylic cationic intermediate 11 [13]. 

The electroreduction of l,l,l-trichloro-2-hydroxy-4-methyl-4-pentene (123) at a lead cathode led to two products, l,l-dichloro-4-methyl-2,4-pentadiene (124) and 1,1-dichloro-2-hydroxy-4-methyl-4-pentene (125) (equation 64). Compound 124 was favored (yields > 75%) in acidic medium using methanol or 2-propanol as solvent97. 

FIGURE 17.12 Dichloro[l,4-dithiabutanediyl-2,2 -bis(4,6-di-ferf-butyl-phenoxy)]titanium (10). an active catalyst for the isospecific 1,2-polymerization of 4-methyl-1,3-pentadiene. (Reprinted with permission from Proto, A. Capacchione, C. Venditto, V. Okuda, J. Macmmolecules 2003, 36, 9249-9251. Copyright 2003 American Chemical Society.)... 

Poly( 1,3-pentadiene), 1,4-2-methyl-Poly(pentamethylene) 1,2-dichloro-irans-... 

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