Alkenes Polyalkenes

Contemporary advances in computer hardware and software have brought about a immense increase in our ability to calculate (as distinct from measuring) Ahyd Him. Modem computational thermochemistry includes various empirical, semiempirical, and quantum mechanical methods for calculating AhydH29g of linear and cyclic alkenes, polyalkenes, alkynes and polyalkynes. There is at present no critical survey of computational methods for determining Ahyd- 298 which Chapter 3 supplies. 

A interesting and useful reaetion is the intramolecular polycyclization reaction of polyalkenes by tandem or domino insertions of alkenes to give polycyclic compounds[l 38]. In the tandem cyclization. an intermediate in many cases is a neopentylpalladium formed by the insertion of 1,1-disubstituted alkenes, which has no possibility of /3-elimination. The key step in the total synthesis of scopadulcic acid is the Pd-catalyzed construction of the tricyclic system 202 containing the bicyclo[3.2. Ijoctane substructure. The single tricyclic product 202 was obtained in 82% yield from 201 [20,164). The benzyl chloride 203 undergoes oxidative addition and alkene insertion. Formation of the spiro compound 204 by the intramolecular double insertion of alkenes is an exam-ple[165]. 

The chain-carrying catalytic species of alkene-polymerization reactions is commonly a tri-coordinate group 4 transition-metal cation of the general form L2M+P , where P is the polyalkene chain. A family of commercially important examples is based on the complex titanium ion57... 

In principle, carbometallation of an alkene (RCH=CH2) with a coordinatively unsaturated organotransition metal compound (R1 M I. ) can produce a monomeric carbometallation product 1 (Scheme 6). This reaction may not, however, stop at this stage. It can be accompanied by other processes of which (i) hydrogen-transfer hydrometallation to produce a potentially thermodynamically more favorable mixture of a 1,1-disubstituted alkene and a hydrometallation product 2 and (ii) polymerization to produce polyalkenes 3 are representative. The extents to which these side-reactions occur are functions of relative rates of various competing processes. For example, accumulation of the monomeric carbometallation product 1 can be favored in cases where the starting R1 MTL is more reactive toward alkenes than 1. The organometal/alkene ratio is also an important parameter, since neither of the two side-reactions can proceed after all of the starting alkene has reacted. 

For cyclic alkenes which involve a degree of ring strain, this may be eliminated by metathesizing the halves of the strained alkene, resulting in linear polyalkenes (polyenamers) (Figure 5.25). This is illustrated by the ROMP of norbornene to provide poly(norbornene). The Diels-Alder... 

Complexes with alkenes and arenes are formed when the hydrocarbons are shaken with aqueous solutions of silver(I) salts. Di- or polyalkenes often give crystalline compounds with Ag+ bound to one to three double bonds. The formation of alkene complexes of varying stability may be used for the purification of alkenes, or for the separation of isomeric mixtures (e.g., 1,3-, 1,4-, and 1,5-cyclooctadienes), or of the optical isomers of a- and /3-pinene. There is very little back-bonding contribution in the formation of Ag1 rr-complexes. For example, the planar complex (hfa)Ag(Ph-C= C-Ph) contains an almost linear acetylene ligand with a C=C... 

There have been far fewer studies on the reactions between tetranuclear clusters and alkynes or alkenes than have been reported for trinuclear systems. The reactions that have been investigated have largely been with alkenes. Both R H CO) 2 and Os4H4(CO)12 react with mono- and polyalkenes, and substitution of hydrides and carbon monoxide ligands is observed in most cases (180-187), as shown by Eq. (10). There are a few examples, however, where metal-metal bond cleavage does occur and trinuclear clusters are isolated (181-183,188). In contrast, when Rh4(CO)12 and Ir4(CO)12 are treated with dienes, cluster growth occurs (189-193). 

The organometalhc chemistry of Ceo is dictated by its spherical geometry and localized polyalkene tt-electronic structure. All the derivatives reported to date are rf- complexes see Hapticity) in which the metal coordinates at a six-six ring fusion (formal double bond) no analogous jj" -diene or jj -friene complexes have been prepared to date see Alkene Complexes). The instability of these polyene complexes is postulated to be due to the curvature of the Ceo molecule and the corresponding divergence of the surface p-orbitals, which results in poor overlap with the metal orbitals. ... 

Polyalkenes are invariably made by polymerization of alkenes. Why is an analogous route, the polymerization of disilenes, R2Si=SiR2, not used to make polysilanes The reason is that the barrier to polymerization of disilenes is simply too low, so that in most cases they polymerize or oligomerize as soon as they are generated. Stable disilenes can be made and isolated, but only with very bulky substituents at the silicon which make the polymer less stable than the disilene. An example is tetramesityldisilene (7), a highly reactive compound that undergoes many novel chemical reactions but does not polymerize. 

Figure 11.5 shows that the functional group compositional analysis of the pyrolysis oil/waxes derived from the fixed-bed pyrolysis of PVC, PS and PET is very different from the polyalkene plastic pyrolysis oil/waxes. The spectra of the PVC pyrolysis oil/wax shows that the characteristic peaks of alkanes and alkenes are present as described for the polyalkene plastics. Since the PVC plastic polymer is based on a similar backbone structure to the polyalkene plastics, a similar degradation product oil/wax composition may be expected. However, the spectra for PVC in Figure 11.5 show that there are additional peaks in the region of 675-900 cm and 1575-1625 cm The presence of these peaks indicates the presence of mono-aromatic, polycyclic aromatic and substituted aromatic groups. Benzene has been identified as a major constituent in oils derived from the pyrolysis of PVC whilst other aromatic compounds identified included alkylated benzenes and naphthalene and other polycyclic aromatic hydrocarbons [19, 32, 39]... 

The involvement of SBM in alkene polymerization is another area of research interest. Although chain growth in Z-N polymerization occurs by 1,2-insertion into an M-C bond, chain termination may involve SBM. For example, H2 is sometimes added to curtail the length of polymer chains. Watson113 observed that H2 reacts with Lu-alkyls according to equation 11.38. The reaction is undoubtedly a SBM and, as such, may provide a good model for what actually occurs when H2 is used to limit polyalkene chain length. 

In commercial polyalkene production, Ziegler or Phillips catalysts based on early transition metals (Ti, Zr, Cr, V) are employed. However, due to their high oxophi-licity, such catalysts require strictly anhydrous conditions. In recent years, discovery of several new types of catalysts for ethylene and 1-alkene polymerization based on late transition metals have inspired a strong interest in this area. Due to their lower oxophilicity, such catalysts can be much more stable towards polar media. 

The bonding description for a coordinated alkene can be extended to other unsaturated organic ligands. Polyalkenes may be non-conjugated or conjugated. In complexes of non-conjugated systems (e.g. cycloocta-1,5-diene (cod), 23.20, or... 

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