Cis stereospecificity

The flipped orientation of the heme in HPII and PVC results in the oxidized ring being sufficiently well removed (7 A) from the essential histidine (His 128 in HPII) and the presumed peroxide binding site to complicate an explanation of the reaction mechanism. The explanation is further complicated by the cis-stereospecificity of the reaction that results in both oxygens being situated on the proximal side of the heme away from what is considered to be the normal reaction center on the distal side. This stereochemistry dictates that the hydroxyl group on the heme d have originated on the proximal side of the heme, and a mechanism has been proposed to explain the reaction in both PVC and HPII 93). The mechanism assumes that compound I is formed as a first step... 

Scott, Frost, Belt and O Reilly (118) reported that the strongly cationic polymerization of butadiene with aluminum chloride can be easily intercepted by the addition of trace amounts of cobalt chloride and made to produce cis stereospecific polymers. The cobalt chloride... 

Enophile activation by an alkoxy substituent was also observed with the particularly smooth and cis stereospecific additions of dialkylallylboranes (43) to ethoxyacetylene (44).The resulting dienylbo-ranes (45) are interesting polyfunctional building blocks and provide 2-ethoxy-1,4-dienes (46) or 1,4-enynes (48) (75-81%) by protonolysis or alumination/elimination (Scheme 10 Table 6). 

Allylnickel species undergo analogous additions to strained alkenes, as exemplified the cis stereospecific metallo-allylationAnethoxycarbonylation of norbomene, (60) -> (61) (62), employing a stoi-... 

Thus, cis-stereospecificity results from Jt-allylic stabilisation of terminal unit of the growing polymer chain, when the lifetime of the terminal unit in the o-state is not sufficient for the elementary event of insertion to take place. Otherwise, there is no need for o-stabilisation of the centre by the formation of complex shown in Figure 3.1c, and the dienes directly attack the Nd-C bond with one of its double bonds. Naturally, the dienes take the conformation that is thermodynamically more advantageous in solution, that is, the transoid conformation. 

Monoalkylcopper reagents add to a, -acetylenic sulfoxides with high cis stereospecificity to form -alkylated a, -ethylenic sulfoxides (Truce and Lusch, 1974 Vermeer et al., 1974b). 

Catalytie systems based on lanthanide compounds are effective catalysts of polymerization of dienes (mainly of isopiene and butadiene). A distinctive feature of these systems is high cis stereospecificity. In particular, the S5mthesized polybutadiene and polyisoprene contain approximately 98% 1,4-cis units [1]. 

Z)-Alk-2-enenitriles are also available from the hydrocyanation of acetylenes, which occurs with cis stereospecificity when catalysed by a nickel(O) complex, and from the stereoselective cyanation of vinyl halides, catalysed by tetracyano-cobaltate(i). The latter procedure is equally applicable to the stereoselective synthesis of the corresponding ( )-isomers. Also, the ( )- and (Z)-isomers (17) and (18) react with piperidine with retention of configuration to provide (19) and (20) respectively (Scheme 30). In contrast, the corresponding reaction with sodium methoxide gives rise to the (Z)-isomer only. 

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