Ethylene additions

The addition of maleic anhydride can occur by excitation of either dienone or the anhydride. It is tempting to ascribe the 4,5-adduct (264) to a reaction between the excited dienone (260) and unexcited maleic anhydride by analogy with the observed major products of ethylene addition [cf. (261), (262)]. The 6,7-adducts (265) and (266) would then imply that these cycloadditions proceed by way of excited maleic anhydride which adds preferentially to the more electron-rich y,5-double bond of the groundstate dienone. 

Ethyl 6,7-dimethoxy-3-methylin-dene-2-carboxylate, 40, 43 Ethylene, addition to piperidine, 43, 45 5,5 -Ethylene-l,2-bis-(2-furyl)-ethane,... 

The ethylene addition product is saturated, that of benzene, on the other hand, is more unsaturated than is benzene itself, since the symmetrical cancelling of residual valency (Thiele) is upset and the aromatic character is destroyed. In order to re-establish this character it is only necessary for hydrogen bromide to be eliminated, which takes place with liberation of energy. The elimination occurs with extraordinary speed, even before the other double bonds, which have become reactive, have time to take up bromine. 

In discussing the mechanism, there has been a tendency to take as evidence the results obtained on alumina with a single reactant, mostly ethanol. Almost all of the deductions have hinged on the relationship between the formation of ether and of ethylene. Additionally, the various investigators failed to realize that the structure and the mode of preparation of the catalyst were important. 

Back in Chapter 3, Murphys Law stated that anything that can happen, will happen. The control of the rate of ethylene addition is not all that good. So, the rates vary from molecule to molecule and out comes a distribution of alpha olefins. 

Initiation of the ethylene addition should precede the introduction of the first portion of bromine by 2-4 min. Ethylene is then added continuously at a slow rate until all the bromine is consumed (see Note 4). 

The first 2-3-mL portion of bromine is added until the reaction mixture is intensely violet. Subsequent portions of bromine are added when the reaction mixture fades to an amber color. After all the bromine has been added, ethylene addition is continued until the color fades to amber. 

Although the actual oxidation state of chromium in the active catalyst is unclear, the reaction has again been interpreted as an ethylene dimerization leading to a metallacycle, in this case followed by ethylene addition and p-hydrogen elimination (Figure 16). 

In the case of ethylene, additional insertions occur to give mixtures of long-chain trialkylaluminum compounds via the Aufbau, or growth, reaction (equation 2). The resulting... 

The interaction of ethylene with triethylaluminium has been studied in the gas phase under conditions under which the formation of 1 -butene is predominant The resulting kinetic parameters of this reaction are given in Table 8. It should be taken into account that in this case for the calculation of the rate constants of the addition reaction several additional assumptions are introduced concerning the concentrations of various OAC forms and the ratio of the rate constants of various reactions. Apparently not all the assumptions are fulfilled and the activation energies of the ethylene addition listed in Table 8 are overestimated. Ziegler et al. have found the following order of reactivity of olefin addition to organoaluminium compounds ... 

Termination is usually thought to occur through an agostic fi-hydride coordination as shown in Scheme 13. Two reactions are shown, to explain the dependence of chain transfer on ethylene concentration, which is similar for many industrial catalysts. H transfer can occur either (a) to the chromium (Cr-H then begins a new chain in a separate step with ethylene addition) or (b) directly to the incoming monomer [32,40-42,47,52,307,351,364—367]. In the first case, H elimination to chromium is not dependent on the ethylene concentration but chain growth is. Therefore, the polymer MW (more accurately, MN, the number average MW) should be proportional to the monomer concentration. In the latter case, H elimination to monomer is dependent on monomer concentration and... 

This striking effect of a strong donor base on the catalytic activity of an organolithium compound contrasts with the reverse effect of donor bases on the growth reaction of ethylene with trialkylaluminum compounds. The catalytic activity of the latter is connected with the electron deficient nature of the uncoordinated, monomeric, trialkylaluminum species (10). These facts point to a difference in the mechanism of ethylene addition between the amine-coordinated organolithium catalyst and the trialkylaluminum compounds. 

The ethylene addition step can be interpreted as an insertion of the ethylene molecule between an amine-coordinated lithium cation and a carbanionic residue. This insertion reaction necessarily leads to the formation of a linear paraffinic carbon chain which agrees with experimental findings. 

These products are consumed consecutively, probably to form benzene and polycyclic compounds. Toluene may also react consecutively to benzene. The ratio of toluene or xylenes to benzene was about twice that obtained in the thermal reaction of ethylene, respectively, at temperatures from 703° to 854°C and at conversions up to 40 mole %. The ratio of styrene to benzene was about one-third as large as that obtained in the thermal reaction of ethylene. Addition of butadiene in the thermal reaction of propylene increased the selectivity of cyclic compound formation, although the increase was smaller than in the case of ethylene. These facts support the mechanism for the formation of monocyclic aromatic compounds proposed by Wheeler and Wood (24) this is discussed in detail later. 

Whether a simple carbonyl reaction or an ethylenic addition oc-... 

Truffer and Renken [27] found a positive effect in the reaction of ethylene addition to acetic acid that involves substantial reaction inhibition by an adsorbed product. The effect is achieved at high frequency oscillation of acetic acid concentration because the cycle average concentration of inhibiting intermediate product is lower than in steady-state. This facilitates ethylene adsorption and a favorable distribution between the two adsorbed species. 

Table 44. Activation barriers (kcal/mol) for ethylene addition calculate by using the 6-3l4-G(d) basis set...

To confirm this assumption, we have computed activation barriers for acetylene, ethylene and cyclopropene addition to 4,4-dimethyl-[4H]-l,2-diazole. To our delight, the B3LYP/6-31G(d)/AMl computed activation barrier for ethylene addition to 4,4-dimethyl-[4//]-1,2-diazole is almost identical (Table 47) to the value obtained with full B3LYP/6-31+G(d) calculation on [4H]-1,2-diazole as dienophile (Table 44). The activation barrier for the acetylene addition is 22.3 kcal/mol indicating that this reaction should be also experimentally feasible. As indicated... 

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