Thermal concerted reactions

We summarize here a procedure to predict the feasibility and the stereochemistry of thermally concerted reactions involving cyclic transition states. The 1,2 rearrangement of carbocations will be used to illustrate the approach. This is a very important reaction of carbocations which we have discussed in other chapters. We use it here as an example to illustrate how qualitative MO theory can give insight into how and why reactions occur ... 

Show the expected stereochemistry of the product of each of the following thermally concerted reactions ... 

The preferred sp3 hybridization for saturated fluorocarbons is a major driving force for the easy polymerization of TFE to PTEF with more exothermic enthalpy change (Scheme 1.34) [2]. The it-bond energy of tetrafluoroethylene is 52 kcal/mol, which is smaller than that of ethylene, 64-65 kcal/mol [3].The easy [2 + 2] cycloadditions of gem-difluoroalkenes (thermal concerted reactions of which are forbidden) proceed stepwise via biradical intermediates. The intermediacy of a biradical is supported by the fact that a 1 1 mixture of the cis- and trans-dideuteriocyclobutanes (7 and 8) was obtained in the [2 + 2] cycloaddition of TFE with cis- (4) and frans-dideuterioethylene (5) [4] (Scheme 1.34). 

Sulfolenes are masked 1,3-dienes [33]. At about 150°C they undergo [4+1] cycloreversion (cycloelimination) into 1,3-dienes and sulfur dioxide. As this is a thermal concerted reaction, it proceeds in a... 

Arranging these orbitals in relative order of energy and connecting orbitals of like symmetry leads to the diagram shown in Fig. 10.10 and to the conclusion that the thermal concerted reaction between butadiene and ethylene is allowed, since bonding levels of starting materials and products correlate. 

Sulfolenes are masked 1,3-dienes [96]. At about 150 °C they undergo (4+ 1)-cycloreversion into 1,3-dienes and sulfur dioxide. As this is a thermal concerted reaction, this cycloreversion proceeds in a disrotatory manner according to the Woodward-Hoffmann rules. Thus, a ds-2,5-disubstituted 3-sulfolene stereospecifically yields an (F,E)-l,3-diene, while the trans-diastereomer gives an (F,Z)-1,3-diene ... 

When the orbitals have been classified with respect to symmetry, they can be arranged according to energy and the correlation lines can be drawn as in Fig. 11.10. From the orbital correlation diagram, it can be concluded that the thermal concerted cycloadditon reaction between butadiene and ethylene is allowed. All bonding levels of the reactants correlate with product ground-state orbitals. Extension of orbital correlation analysis to cycloaddition reactions involving other numbers of n electrons leads to the conclusion that the suprafacial-suprafacial addition is allowed for systems with 4n + 2 n electrons but forbidden for systems with 4n 7t electrons. 

The prediction of the Woodward-Hofrmann rules that thermal concerted cycloadditions are allowed for combinations in which 4 -1- 2 7c electrons are involved has stimulated the search for combinations with 10 and larger numbers of participating electrons. An example of a [6 -1- 4] cycloaddition is the reaction of tropone with 2,5-dimethyl-3,4-diphenylcyclopentadienone ... 

Is the reaction concerted As was emphasized in Chapter 11, orbital symmetry considerations apply only to concerted reactions. The possible involvement of triplet excited states and, as a result, a nonconcerted process is much more common in photochemical reactions than in the thermal processes. A concerted mechanism must be established before the orbital symmetry rules can be applied. 

Acyl azides can undergo photolytic cleavage and rearrangement upon irradiation at room temperature or below. In that case acyl nitrenes 8 have been identified by trapping reactions and might be reactive intermediates in the photo Curtius rearrangement. However there is also evidence that the formation of isocyanates upon irradiation proceeds by a concerted reaction as in the case of the thermal procedure, and that the acyl nitrenes are formed by an alternative and competing pathway " ... 

This section describes reactions in which elimination to form a double bond or a new ring occurs as a result of thermal activation. There are several such thermal elimination reactions that are used syntheses, some of which are concerted processes. The... 

Chapter 6 looks at concerted pericyclic reactions, including the Diels-Alder reaction, 1,3-dipolar cycloaddition, [3,3]- and [2,3]-sigmatropic rearrangements, and thermal elimination reactions. The carbon-carbon bond-forming reactions are emphasized and the stereoselectivity of the reactions is discussed in detail. 

This is a general situation for thermal, concerted additions those involving Ane + 2ne systems proceed readily, e.g. the Diels-Alder reaction, whereas those involving 2ne + 2ne systems, e.g. the cyclo-dimerisation of alkenes, do not. We might, however, expect that photochemical cyclodimerisation of alkenes would be symmetry... 

Within the isolobal formalism, the conversion of 47 to 48 is a symmetry-allowed process, if it were to proceed as a concerted reaction (50). Structure 47 represents a transoid-2-meta.Wa-1,3-butadiene. In the bonding description, complex 48 represents formally a 1-metalla-bicyclo[1.1.0]butane. Therefore, the conversion of 47 to 48 represents a thermally allowed, concerted [ 2a + 2S] ring closure, in analogy to the pericyclic ring opening of bicyclo[1.1.0]butanes to give trans,trans-, 3-butadienes. 

Cycloadditions give rise to four-membered rings. Thermal concerted [2+2] cycloadditions have to be antarafacial on one component and the geometrical and orbital constraints thus imposed ensure that this process is encountered only in special circumstances. Most thermal [2+2] cycloadditions of alkenes take place by a stepwise pathway involving diradical or zwitterionic intermediates [la]. Considerably fewer studies have been performed regarding the application of microwave irradiation in [2+2] cydoadditions than for other kinds of cydoaddition (vide supra). Such reactions have been commonly used to obtain /1-lactam derivatives by cycloaddition of ketenes with imines [18-20,117,118],... 

Photoinduced versus thermal processes 165 Quantum yield for concerted reactions 166 Quantum yield for stepwise reactions 170 Examples 176... 

It is a pleasure to acknowledge the essential contribution of Dr. C. P. Andrieux to most of the work reported above as well as that of Dr. D. Lexa in the field of porphyrins, Professor Moiroux and Dr. A. Anne to cation radical reactivity, Dr. M. Robert to photoinduced dissociative electron transfer and to the stepwise/concerted competition and Drs. P. Hapiot and Medebielle to recent work on thermal SRn1 reactions. Many students from our group have also contributed effectively to the work, namely, C. Costentin, G. Delgado, V. Grass, A. Le Gorande, C. Tardy and D. L. Wang. Fruitful and pleasant... 

Eq. 17 is meant to represent the possibility for a concerted formation of oxetane product. A problem that always exist in cycloadditions is the question of whether the reaction takes place by a two-step biradical reaction pathway or through a concerted mechanism. Such questions have not even been resolved for purely thermal reactions. 4> A recent speculation on this point proposes almost universal concertedness for all cycloaddition reactions. 79> In that work, mixed stereochemistry in the products of [2+2] cycloaddition reactions is generally attributed to a mixture of two concerted reactions, suprafacial-suprafacial, and supra-facial-antarafacial. It will be seen later that the PMO calculations generally do not support this idea. A mixture of biradical and concerted reactions is in better agreement with experimental facts. 

Scheme 3.22), thus diverting the reaction to the radical pathway. The differences in the products observed in Scheme 3.22 as compared to Scheme 3.23 require that the two mechanisms be different and are consistent with a concerted reaction for the thermal deazetization of unhindered nitrosimines. 

Thermal concerted 2 + 2 reactions are predicted to occur between an alkene and a ketene. According to Woodward-Hoffmann rules, addition must be suprafacial to one component and antarafacial to the other if the process is to be concerted. 

In the transition state a boat like structure appears and there will be a suprafacial cis addition to the termini of the n bond. The ene reaction does not have a symmetrical transition state and it is a thermally allowed concerted reaction. Its transition state involves a suprafacial interaction of six electrons (4 from the k bonds and two from the o bond) So it is a Huckel system and transition state is aromatic. In the terminatlogy of Woodward and Hoffmann it can be regarded as o2s + n2s + 7t2s reaction. 

The RDA reaction is often observed from steroid molecular ions, and it can be very indicative of steroidal stmcture. [107,110,113,114] The extent of the RDA reaction depends on whether the central ring junction is cis or trans. The mass spectra of A -steroidal olefins, for example, showed a marked dependence upon the stereochemistry of the A/B ring juncture, in accordance with orbital symmetry rules for a thermal concerted process. In the trans isomer the RDA is much reduced as compared to the cis isomer. The effect was shown to increase at 12 eV, and as typical for a rearrangement, the RDA reaction became more pronounced, whereas simple cleavages almost vanished. This represented the first example of such apparent symmetry control in olefinic hydrocarbons. [114]. 

This system covers concerted reactions of the n electron systems on two reactants to form new a bonds yielding carbocyclic rings with a single unsaturation. If the reaction follows the rule of maximum orbital overlap, then it is a suprafacial, suprafacial process and is termed a [,r4 + r t] reaction. By the Woodward-Hoffmann rules this is a symmetry-allowed thermal reaction [13]. 

An ab initio RHF/3-21 G study has shown that the decomposition of 3-hydroxy-3-methylbutan-2-one is a concerted process with hydrogen transfer and bond breaking via a five-membered cyclic transition state.AMI and PM3 methods using UHF calculations were applied to study the thermolysis of 2-cyanofuroxan. The reaction proceeds via a two-step pathway in which the second step is rate determining. The effect of solvent in the thermal decomposition reaction of fran -3,3-dimethyl-5,6-tetramethylene-l,2,4-trioxacyclohexane was studied. ... 

The interpretation of chemical reactivity in terms of molecular orbital symmetry. The central principle is that orbital symmetry is conserved in concerted reactions. An orbital must retain a certain symmetry element (for example, a reflection plane) during the course of a molecular reorganization in concerted reactions. It should be emphasized that orbital-symmetry rules (also referred to as Woodward-Hoffmann rules) apply only to concerted reactions. The rules are very useful in characterizing which types of reactions are likely to occur under thermal or photochemical conditions. Examples of reactions governed by orbital symmetry restrictions include cycloaddition reactions and pericyclic reactions. 

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