Forbidden reaction

Figure 10 12 shows the interaction between the HOMO of one ethylene molecule and the LUMO of another In particular notice that two of the carbons that are to become ct bonded to each other m the product experience an antibondmg interaction during the cycloaddition process This raises the activation energy for cycloaddition and leads the reaction to be classified as a symmetry forbidden reaction Reaction were it to occur would take place slowly and by a mechanism m which the two new ct bonds are formed m separate steps rather than by way of a concerted process involving a sm gle transition state... 

Symmetry forbidden reaction (Section 10 14) Concerted re action in which the orbitals involved do not overlap in phase at all stages of the process The disrotatory ring opening of cyclobutene to 1 3 butadiene is a symmetry forbidden reaction... 

It has been found that certain 2 + 2 cycloadditions that do not occur thermally can be made to take place without photochemical initiation by the use of certain catalysts, usually transition metal compounds. Among the catalysts used are Lewis acids and phosphine-nickel complexes.Certain of the reverse cyclobutane ring openings can also be catalytically induced (18-38). The role of the catalyst is not certain and may be different in each case. One possibility is that the presence of the catalyst causes a forbidden reaction to become allowed, through coordination of the catalyst to the n or s bonds of the substrate. In such a case, the... 

Although the orbital-symmetry rules predict the stereochemical results in almost all cases, it is necessary to recall (p. 1070) that they only say what is allowed and what is forbidden, but the fact that a reaction is allowed does not necessarily mean that the reaction takes place, and if an allowed reaction does take place, it does not necessarily follow that a concerted pathway is involved, since other pathways of lower energy may be available.Furthermore, a forbidden reaction might still be made to go, if a method of achieving its high activation energy can be found. This was, in fact, done for the cyclobutene butadiene interconversion (cis-3,4-dichloro-cyclobutene gave the forbidden cis.cis- and rran.y, ra i -l,4-dichloro-1,3-butadienes,... 

Aromatic transition state = allowed reaction —, antiaromatic transition state — forbidden reaction. 

III. Other Spin-Forbidden Reactions of Transition Metal Compounds 606... 

Rather than focusing on the short-time photochemical reactivity, our interest in the spin-forbidden reactions of iron carbonyl fragments has been mainly in the longer-time thermal chemistry of the fragments produced. This is summarized in Scheme 3. As already stated, iron tricarbonyl and tetracarbonyl are known to have triplet ground states, and for many ligands, it is assumed that Fe(CO)3L would also have a triplet ground state. Hence many of the indicated processes are spin-forbidden. 

In 2003, we presented two reviews concerning spin-forbidden reactions in transition metal chemistry (10,16). A number of other reviews in 2004 addressed closely related topics (90,91). However, since then, we are not aware of any review on this topic. We, therefore, present a short discussion of a number of recent studies of reactivity in transition metal chemistry for processes involving changes in spin-state. The aim here is not to be exhaustive, as there are now quite a lot of studies of transition metal chemistry that address the topic of spin-state changes. Instead, this is an attempt to give a flavor of the types of question that are being addressed now and the techniques used to investigate them. 

A final aspect of the review is an attempt to make some general statements and predictions concerning the rate of spin-forbidden reactions of transition metal compounds. Doing this for all such... 

Let us consider two ethylene molecules approaching each other in such a way that the top of one n system interacts with the bottom of the other. The HOMO of one n system must be matched with the LUMO of the other as shown. In both the cases the phase relationships at one end of the system are wrong for bond formation. So a concerted process in which both new G bonds are formed simultaneously is not possible and the reaction will be termed a symmetry-forbidden reaction. 

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