Dialkenes

Dialkenes having different structures than butadiene, isoprene, and 1,3-pentadiene can be described as substituted butadienes bearing alkyl groups at the Cl or C2 carbon atoms, as shown in Eigure 17.10. 

Under favorable conditions, low molecular weight organics may polymerize on surface of adsorbent (dialkenes, 1-alkenes, alkynes, conjugated double-bond systems, and epoxides are especially susceptible to this behavior). 

It is clear that a detailed mechanism for the metathesis reaction of alkenes cannot yet be given with certainty. In view of the fact that, for similar reactions which are formally cyclobutane-dialkene transformations, a nonconcerted reaction pathway has been demonstrated, a concerted fusion of two alkenes to form a cyclobutane complex and its decomposition in the same way with a change in the symmetry plane is less probable. On the basis of the information on the two other mechanisms to date, the mechanism involving a metallocyclic intermediate is more plausible than a mechanism involving carbene complexes. 

The preferred kinetic model for the metathesis of acyclic alkenes is a Langmuir type model, with a rate-determining reaction between two adsorbed (complexed) molecules. For the metathesis of cycloalkenes, the kinetic model of Calderon as depicted in Fig. 4 agrees well with the experimental results. A scheme involving carbene complexes (Fig. 5) is less likely, which is consistent with the conclusion drawn from mechanistic considerations (Section III). However, Calderon s model might also fit the experimental data in the case of acyclic alkenes. If, for instance, the concentration of the dialkene complex is independent of the concentration of free alkene, the reaction will be first order with respect to the alkene. This has in fact been observed (Section IV.C.2) but, within certain limits, a first-order relationship can also be obtained from many hyperbolic models. Moreover, it seems unreasonable to assume that one single kinetic model could represent the experimental results of all systems under consideration. Clearly, further experimental work is needed to arrive at more definite conclusions. Especially, it is necessary to investigate whether conclusions derived for a particular system are valid for all catalyst systems. 

Selective epoxidation of one of the double bonds in dialkenes is of practical interest (Table XVI). Although monoepoxides predominate at low H2O2 concentrations, the diepoxides are also formed at higher concentrations. The diallyl epoxides of bisphenol A are major intermediates in the adhesives industry, and their synthesis in solid-catalyzed reactions in an eco-friendly manner remains a challenge. 

With acids, the dialkene 63 is liberated from complex 61. 

Strained alkenes, including cyclobutenes 29 [120] and benzcyclobutadiene derivatives 30 [121], can be prepared by this route. Particularly advantageous is the wide range of reduction potentials for 1,2-dibromides, which allows some selective reactions. The dibromide 26 can be purified from any of the isomer 25 by reduction at -0.86 V vs. see when the diequatorial bromide remains unchanged and the diaxial compound is converted to the alkene [110], Bromination of dialkenes followed by selective debromination to recover one alkene leads to protection of the other alkene as the dibromide. Subsequently the second alkene can be recovered by reduction at more negative potentials [122],... 

Since then, a number of studies of model systems have confirmed that dialkenes, cyclic alkenes, and aromatics form substituted monocarboxylic acids, dicar-boxylic acids, and organic nitrates in the condensed phase (e.g., see O Brien et al., 1975a Grosjean and Friedlander, 1979 Dumdei and O Brien, 1984 Izumi and Fukuyama, 1990 and Forstner et al., 1997a, 1997b). For example, Table 9.21 shows the products identified in particles formed in the 1-octene- and 1-decene-NO,-ambient air systems. In both bases, only 40% of the total particle mass could be identified, and the yields shown in Table 9.21 are those relative to the total identified compounds. That is, the absolute product yields are about factor of 2.5 larger. As expected from the known oxidation mechanisms (see Chapter 6.E), heptanal and heptanoic acid are the major condensed-phase oxidation products of 1-octene and nonanal and nonanoic acid from 1-decene (see Problem 4). The mechanism of formation of the fura-nones, which are formed in relatively high yields, is not... 

From among the variety of non-carbohydrate precursors, acetylenes and alkenes have found wide application as substrates for the synthesis of monosaccharides. Although introduction of more than three chiral centers having the desired, relative stereochemistry into acyclic compounds containing multiple bonds is usually difficult, the availability of such compounds, as well as the choice of methods accessible for their functionalization, make them convenient starting-substances for the synthesis. In this Section is given an outline of all of the synthetic methods that have been utilized for the conversion of acetylenic and olefinic precursors into carbohydrates. Only reactions leading from dialkenes to hexitols are omitted, as they have already been described in this Series.7... 

In 1973, Bond et al. performed a groundbreaking experiment when they achieved efficient alkene hydrogenation with several gold supports and at only 100-217 °C. Reaction took place with high chemoselectivity to monohydrogenation for alkenes and dialkenes however, the process was only diastereoselective in the case of monoalkenes [9] (Scheme 8.31). 

It is quite likely that 2H- thiin is very similar in structure to the dioxide though perhaps with a widened C(6)—S(l)—C(2) angle and consequently less twisted dialkene this is supported by the Diels-Alder reactivity it displays (albeit with difficulty), while the sulfone (34) is inert to similar reactions. Structures (36) and (37) are tentatively suggested as possible pseudo chair and pseudo boat conformations for the AH-thiin system. 

Reactions of Pt(O,0-MeCOCHCOMe)(MeCOCH2COMe)X (X = Cl, Br) with a wide variety of donors, including phosphines, arsines, amines, alkenes, diamines, etc. have been examined. In all cases the coordinated neutral acetylacetone is displaced. When monodentate ligands, L, are employed, products are of the type Pt(0,0-MeCOCHCOMe)XL. Bidentate ligands, LL, yield Pt(C-MeCOCHCOMe)X(LL) and bridging alkenes form PtCl(0,0-MeCOCHCOMe) (dialkene).494... 

Non-oxidative hydrocarboxylation of alkenes to carboxylic acids with CO and H20 is catalyzed by palladium complexes such as PdCl2(PhCN)2 or PdCl2(PPh3)2, and a-methyl acids predominate in the presence of HC1.374,443 A recent improvement of this reaction consisted of the use of a PdCl2/CuCl2/HCl catalyst under oxidative conditions.377 Almost quantitative yields of a-methyl carboxylic acids and dicarboxylic acids were obtained from terminal alkenes and terminal dialkenes respectively, at room temperature and atmospheric pressure (equation 174).377... 

The second intermediate, a 6 -<9-acroyl 6-0-allyl-dialkene was subjected to RCM (with Grubbs II catalyst) and afforded small amounts of the C2-sym-metrical derivative, accompanied by E- and Z-cyclic monomers.316... 

The dialkene 1,3-butadiene is widely used in the manufacture of polymers, particularly synthetic rubber. The first synthetic rubber to be manufactured on a large scale and used as a substitute for unavailable natural rubber during World War II was a styrene-butadiene polymer ... 

The cyclic dialkene cyclopentadiene has the structural formula shown below ... 

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