Ethylene, concerted dimerization

The orbital correlation diagram for the concerted dimerization of ethylene to form cyclobutane or for the reverse reaction, the fragmentation of cyclobutane into two ethylenes, may be obtained most easily by applying the principle of conservation of orbital symmetry. A mirror plane perpendicular to the molec-... 

Consider now the concerted dimerization of ethylene in 11.28. The frontier orbital interactions of this reaction are the (tt c tiqq), (7T(J(j - ttcc)- 2nd (7r c -itqc) interactions shown in 11.29. The one destabilizing interaction (ttqc cc) is nonzero as can be seen from 11.30a, wliile the stabilizing interactions (ttqc cc) and (ttcc cc) vanish as shown in 11.30b and 11.30c, respectively. As two ground... 

FIGURE 11.1. An orbital correlation diagram lor the concerted dimerization reaction of ethylene. 

Consider now the concerted dimerization of ethylene. The frontier orbital interactions of this reaction are the (ttcc — cc)y i cc ind (7T cc cc)... 

A new process developed by Institut Francais du Petrole produces butene-1 (1-butene) by dimerizing ethylene.A homogeneous catalyst system based on a titanium complex is used. The reaction is a concerted coupling of two molecules on a titanium atom, affording a titanium (IV) cyclic compound, which then decomposes to butene-1 by an intramolecular (3-hydrogen transfer reaction. ... 

The frontier orbital interaction is forbidden by the symmetry for the dimerization of ethylenes throngh the rectangular transition state. The HOMO is symmetric and the LUMO is antisymmetric (Scheme 25a). The overlap integrals have the opposite signs at the reaction sites. The overlap between the frontier orbitals is zero even if each overlap between the atomic p-orbitals increases. It follows that the dimerization cannot occur throngh the fonr-membered ring transition states in a concerted and stereospecfic manner. 

The essential point that distinguishes between allowed and forbidden reactions is the role of the D+A configuration. If the D+A configuration is allowed by symmetry to mix into the transition state wave-function then the transition state will be stabilized and will take on character associated with that configuration. For the ethylene dimerization, D+A is precluded from mixing with DA due to their opposite symmetries. As was discussed in detail in Section 2 (p. 130), DA cannot mix with D+A" since and n orbitals are orthogonal (106). Thus for ethylene dimerization the concerted process... 

The concept of the conservation of orbital symmetry can be extended to intermolecular cycloaddition reactions which occur in a concerted manner. The simplest case is the dimerization of ethylene molecules to give cyclobutane, the 2n + 2je cycloaddition. The proper geometry for the concerted action would be for the two ethylene molecules to orient one over the other. Two planes of symmetry are thereby set up -perpendicular to the molecular plane bisecting the bond axes oy-parallel to the molecular plane lying in between the two molecules (Figure 8.10). 

A soluble titanium-based modified Ziegler-Natta catalyst [Ti(OR)4-Et3Al, R = n-Bu, isoPr] is employed in the reaction.42 Since similar catalysts may be used for the oligomerization and polymerization of ethylene, the nature and oxidation state of the metal and reaction conditions determine selectivity. Ti4+ was found to be responsible for high dimerization selectivity, whereas polymerization was shown to be catalyzed by Ti3+. According to a proposed mechanism,42,43 this catalyst effects the concerted coupling of two molecules of ethylene to form a metal-lacyclopentane intermediate that decomposes via an intramolecular p-hydrogen transfer ... 

Variations of this mechanism included the suggestion of a partially bonded alkene molecule,299 the participation of a titanium-aluminum ion pair,300 and a concerted alkene insertion.301 The development of the activator-alkyl mechanism was probably strongly influenced by the Aufbau reaction, studied originally by Ziegle.102 He observed that Group I—III alkyl compounds such as Et3Al catalyzed the oligomerization of ethylene to terminal alkenes. Additional evidence of such mechanism comes from the fact that alkylaluminum compounds exist in dimeric... 

In fact, the cycloaddition of butadiene to ethylene, as well as cycloadditions of similar non-polar dienes to non-polar alkenes seem experimentally to be cases where concerted and stepwise (biradical or biradicaloid) mechanisms compete. We have recently discussed a number of cases, such as the dimerization of butadiene, piperylene, and chloroprene, the cycloadditions of butadiene or methylated dienes to halogenated alkenes, and others, where non-stereospecificity and competitive formation of [2 + 2] adducts indicate that mechanisms involving diradical intermediates compete with concerted mechanisms10). Alternatively, one could claim, with Firestone, that these reactions, both [4 + 2] and [2 + 2], involve diradical intermediates1 In our opinion, it is possible to believe that a concerted component can coexist with the diradical one , and that both mechanisms can occur in the very same vessel 1 ). Bartlett s experiments on diene-haloalkene cycloadditions have also been interpreted in this way12). 

Li, Y Houk, K. N. Diels-Alder dimerization of 1,3-butadiene an ab initio CASSCF study of the concerted and stepwise mechanisms and butadiene-ethylene revisited, J. Am. Chem. Soc. 1993,115, 7478-7485. 

Now, consider a thermal [2 4 2] cyclization, dimerization of ethylene. This would involve overlap of the HOMO, tt, of one molecule with the LUMO, tt, of the other. But w and n are of opposite symmetry, and, as Fig. 29.21 shows, lobes of opposite phase would approach each other. Interaction is antibonding and repulsive, and concerted reaction does not occur. 

A group of papers dealing with the intramolecular photodimerization reaction of a,a>-bis(9-anthryl)alkanes (185 n = 2—10) fails to produce agreement about the detailed mechanism of the reaction. Measurements of quantum yields for fluorescence, photoreaction, and intramolecular deactivation as a function of temperature are said to provide no support for a biradical intermediate, but rather to support a concerted mechanism. In reply, the proposers of the biradical mechanism reinterpret these data and find them consistent with their mechanism. A third research group reports results in fluid solution at room tempera-ture " their concern is more with the question of excimer involvement in the mechanism, and they report that in many of the systems unidentified photoproducts are formed via excimers that do not lead to the normal 9,9 -linked photodimer. The internal photodimer from (185 n = 3) has been studied in a matrix at 10 and photodissociation is shown to lead to two different modifications of (185 n = 3) with different reactivities. A geometrical constraint on the intramolecular photoreactions of 9,9 -linked bisanthracenes is demonstrated by the failure of the di-substituted ethylenes (186) to give internal dimers. 

The essence of relay-race (or concerted) copolymerization thus lies in the coexistence of two different bound active sites in the same polymer matrix, the first being responsible for dimerization and the second for copolymerization of butene with ethylene [116], The general scheme of the process can be represented as in Eq. (12-16). 

MOs, while tlie two 7t c orbitals lead to the tt and tt MOs. In the initial stage of (he dimerization, the interaction between two ethylencs is weak so that 7t+ and tt. lie far below the n+ and tt levels, so that only 7t+ and rr are occupied. Of the a orbitals of cyclobutane described earlier, only those related to the tt., 7t1 and nl levels by symmetry are shown in Figure 11.1. Not all the occupied MOs of the reactant lead to occupied orbitals in the product. In particular, tt. correlates with one component of the empty set in cyclobutane. The tt+ combination ultimately becomes one component of the filled set in cyclobutane. So the reaction is symmetry forbidden. The reader should carefully compare the correlation diagram for ethylene dimerization here with the Ho + O2 reaction in ITgure 5.8. flie two correlation diagrams are very similar, as they should be, since in this instance the spatial dfstributions of tt and n " are similar to those of and respectively, in H2. These two reactions are probably the premier examples of symmetry-forbidden reactions. A related symmetry-allowed example is the concerted cycloaddition of ethylene and butadiene, the Diels-Alder reaction. We shall not cover the orbital symmetry rules for organic, pericyclic reactions. There are several excellent reviews that the reader should consult.But it should be pointed out that the orbital symmetry rules have stereochemical implications in terms of the reaction path and products formed. The development of these rules by Woodward and Hoffmann... 

Computations [12] show that concerted [ 2 2a]-cycloaddition of ethylene is disfavored relative to biradical formation even when the recommended orientation is adopted. The qualitative argument has been made [13] that the HOMO-LUMO interactions along this pathway are less favorable than in the orientation leading to a transoid biradical. All in all, it is hardly surprising that [t 2s +7r2a] cycloaddition of alkenes has only been observed in exceptional circumstances - if at all. For example, Kraft and Koltzenburg [14] observed that bicyclo[4.2.2]-deca- raTis-3,czs-7,9-triene dimerizes to a product with one cis and one trans junction ... 

The two cycloaddition reactions we have discussed so far illustrate some of these points. The [2+2] cycloaddition is forbidden. However, olefins can dimerize to make cyclobutanes. It is just that the reaction is not concerted, but rather involves a biradical intermediate. The [4+2] cycloaddition is allowed, but in fact the concerted cycloaddition of ethylene and butadiene requires high temperature and pressure. It does occur by a concerted allowed path, but the activation barrier is high. 

Look again at Figure 20.24, from which we derived the information that the thermal 2 + 2 dimerization of a pair of ethylenes was forbidden. It s only forbidden if we insist on smushing the two ethylenes together head to head exactly as shown in the figure. If we are more flexible, and allow one ethylene to rotate about its carbon-carbon O bond as the reaction takes place, the offending antibonding overlap vanishes. What orbital symmetry really says is that for the concerted thermal dimerization of two alkenes to take place, there must be this rotation (Fig. 20.28). 

Evans pointed out that one would therefore expect the Diels-Alder reaction to be a concerted one-step process because the activation energy for this should be less than that involving an intermediate biradical. This prediction has proved to be correct. He also pointed out that a one-step Diels-Alder-like dimerization of ethylene to cyclobutane. 

FIGURE 5.36. Overlap of 2p AOs in (a) the cyclic transition state for concerted pericyclic dimerization of ethylene (b) cyclobutadiene. 

A number of cases are known where ethylene derivatives undergo n cycloaddition to form cyclobutanes. Tetracycanoethylene [TCNE (NC)2C=C(CN)2] and fluorinated ethylenes are particularly prone to behave in this way. These reactions were discovered at the laboratories of duPont de Nemours Co. and have been studied in great detail by Bartlett and his collaborators. As we have already seen, concerted cis dimerizations of this kind should be less favorable than an path via an intermediate 1,4-butadiyl biradical all these reactions in fact take place in this way. An interesting example is the reaction of TCNE with bismethylenecyclobutene (230). The normal Diels-Alder reaction to form (231) is inhibited because this would be an antiaromatic cyclobutadiene derivative and because the transition state leading to it would be isoconjugate with benzocyclobutadiene. The product is therefore the spiran (232), formed as indicated by an process. 

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