Unsaturated carbon chains

A similar pre-orientation involving unsaturated carbon chains was operative on generating twelve-membered enediyne 23 and arenediyne lactams 24 [7]. The seco methylesters 21 and 22 were cleaved with LiOH, the corresponding carboxylic acids underwent cyclizations after activation with 2-fluoro-pyridinium tosylate 25 [8]. Dimerization products were found as by-products (<10%). It should be pointed out, that the lactamization succeeded in a single step in about 75% yield by treating the seco-methylesters 21 and 22 with Me3Al in refluxing methylene chloride. Obviously, the latter route was more convenient (Scheme 5). 

OH,46-49-51,57 cclj44 C10,58 CNj47 NHj47 CHg52 Nh2,56 small Si- and Ge-containing species,54 and unsaturated carbon chain species.55... 

In contrast, the chemistry of higher metaUacumulenylidene complexes containing longer odd carbon chains has been comparatively less studied due to the synthetic difficulties in preparing such species. This fact relies on the increased reactivity of the unsaturated carbon chains towards electrophilic and nucleophilic attacks. In fact, only a limited number of stable pentatetraenylidene complexes have been described, while others have been proposed as highly reactive transient intermediates. 

For a review that summarize the chemistry of complexes containing unsaturated carbon chains end-capped by various metal-ligand combinations, see Bruce MI, Low PJ (2004) Adv Organomet Chem 50 179 44 Jiao H, Gladysz JA (2001) New J Chem 25 551-562 Marrone A, Colletti C, Re N (2004) Organometallics 23 4952-4963 Rigaut S, Maury O, Touchard D, Dixneuf PH (2001) Chem Commun 373-374... 

Main routes to 71-donor-substituted allenylidene complexes include (1) the nucleophilic addition of secondary amines to Fischer-type carbenes [M =C(OR ) C=CR (C0)5] (M = Cr, W) [9], (2) the Lewis-acid induced abstraction of NR2 groups from anionic complexes [M C=CC(NMe2)3](CO)5] (M = Cr, W) [9], and (3) the regioselective addition of protic nucleophiles to metallacumulenylidenes with more extended unsaturated carbon chains, such as butatrienylidenes or penta-tetraenylidenes [10]. In the following sections updated syntheses are presented by Periodic Group. 

In an aromatic cycle or in an unsaturated carbon chain, the doublets 11 of the double bonds may move and thus modify the reactivity of the molecule. For instance, this modification may specifically happen with acrolein - a corrosive (and toxic) molecule (Fig. 3.25) ... 

Reactions between 3(2//)-isoquinolinones and substituted maleic anhydrides, after hydrolysis of the adducts and oxidative decarboxylation, result in derivatives bridged with an unsaturated carbon chain (34)... 

The reactions often proceed in high yield and predominantly with retention of configuration at the vinyl—aluminum bond. Many experimental variations of this substitution reaction remain to be studied systematically and the method appears to have great promise for the synthesis of unsaturated carbon chains. Because only one C—Al bond in R2AIR responds to cleavage the method has little appeal for application to the R2AIH adducts of alkenes (Scheme 3). Little preference for cleavage of the three C—Al bonds would be expected, so conversion to the R E would be low. 

Polymers with unsaturated carbon chain backbone form another important class of macromolecules, many of the compounds from this class having properties of elastomers. The most common polymers from this class are obtained from 1,3-butadiene, 2-methyl-1,3-butadiene (isoprene) and their derivatives. Natural rubber, which is poly(c/s-isoprene), as well as the natural polymers gutta-percha and balata also have an unsaturated carbon chain backbone. For many practical applications, the polymers from this class are subject to a process known as vulcanization, which consists of a reaction with sulfur or S2CI2, and leads to the formation of bridges between the molecular chains of the polymer. This process significantly improves certain physical properties of practical interest. A separate subclass of polymers with unsaturated carbon chain backbone is formed by polyacetylene. 

If the sulfur atoms are not part of the polymeric backbone, the polymers are not included in this class. For example, in the vulcanization process of the polymers with unsaturated carbon chain backbone, -S-S- bonds are introduced in the polymer, but the resulting product is not classified as polymer with C-S bonds in the backbone, although in hard rubber (ebonite), for example, the content of sulfur can be as high as 32%. 

A rather different type of macromolecule is generated from the polymerization of certain terpenes such as limonene and a-pinene. The polymers generated from these compounds retain a double bond in the structure of the molecule, and it can be argued that they have a mixed saturated and unsaturated carbon chain backbone as shown below ... 

Polymers with Unsaturated Carbon Chain Backbone... 

Polymers with unsaturated carbon chain backbone... 

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