Butadienes unsymmetrically terminally

Stereoisomerism of polymers derived from 1,3-pentadiene and other unsymmetric terminally monosubstituted butadienes (CH2 = CH CH = CHR) is more complex. Stereoregular polymers can be formed from these monomers via their... 

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]. 

The asymmetric dihydroxylation of dienes has been examined, originally with the use of NMO as the cooxidant for osmium [56a] and, more recently, with potassium ferricyanide as the cooxidant [56b], Tetraols are the main product of the reaction when NMO is used, but with K3Fe(CN)6, ene-diols are produced with excellent regioselectivity. The example of dihydroxylation of trans.trans-1,4-diphenyl-1,3-butadiene is included in Table 6D.3 (entry 21). One double bond of this diene is hydroxylated in 84% yield with 99% ee when the amounts of K3Fe(CN)6 and K2C03 are limited to 1.5 equiv. each. Unsymmetrical dienes are also dihydroxy-lated with excellent regioselectivity. In these dienes, preference is shown for (a) a bans over a cis olefin, (b) the terminal olefin in a,p,y,8-unsaturated esters, and (c) the more highly substituted olefin [56b],... 

A similar complex isomerism is also displayed by unsymmetric internally and terminally disubstituted butadienes (of the CH2 CH CR CHR and CH2=CR-CH=CHR type). 

The fact that the unsymmetrical product dominates in the cross-metathesis of a cyclic olefin and a terminal olefin is an advantage for the efficient synthesis of the brown algae pheromones multifidene (yield 31%) and viridiene (yield 30%) via the cross-metathesis between bicyclo[3.2.0]hepta-2,6-diene and but-l-ene or butadiene, respectively (molar ratio 1/2), in the presence of lmol% of Ru(=CHCH=CPh2)(Cl)2(PCy3)2 Scheme 15.1 (Randall 1995). 

---