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The Aromatic Transition State

For suprafacial pericyclic reactions, the cyclic transition state is stabilized if a Huckel number of electrons, 4n + 2, is involved. This will give faster reaction rates. [Pg.159]

Although Mobius molecules are rare. Mobius transition states, with one antarafacia component, are relatively common. For these systems, 4n electrons is the favourable number, giving a stabilized transition state and a taster reaction rate [Pg.161]

Uniquely unfavourable orhilal Ini even number of atoms in ring [Pg.162]

3 The Link between the Frontier Orbital and the Aromatic Transition State Approaches [Pg.162]


In the aromatic transition state approach, the basic criterion was that a reaction is allowed in the ground state if and only if there occurs in the transition state aromatic stabilization. This criterion led to the Dewar-Zimmerman selection rule (Equation 11.36), where p. i. = 0 signifies an even number of phase inversions, p. i. = 1 signifies an odd number of phase inversions, and N is the total number of electrons. [Pg.611]

A problem with this explanation is that it is a bit more difficult to explain those pericyclic reactions that we shall come to in Chapter 4, which smoothly take place in spite of their having a total of 4n electrons. We shall find that these all show stereochemistry involving an antarafacial component. It is possible to include this feature in the aromatic transition state model—if the... [Pg.32]

The biradicaloid transition state provided better agreement with the experimental data. It is unlikely that this model is the correct transition state for the process, however, since it has a much higher energy than the aromatic transition state model, according to the reported AMPAC calculations. Several factors can account for these results. First the AMPAC calculated frequencies of both transition state... [Pg.142]

Figure 6. Logarithmic plot values of LUm/fc versus pressure for the Cope rearrangement of bullvalene (torr at the experimental temperature of 356 K). Experimental values are signified by solid circles. Pressures are the total sample pressure at 356 K. Errors in frUni/fc are reported to 2o. The solid (upper) line represents the values calculated from RRKM theory using the biradicaloid transition state model. The lower line represents calculated rate constants using the aromatic transition-state model. The collision diameter was 3.6 A in both cases. Figure 6. Logarithmic plot values of LUm/fc versus pressure for the Cope rearrangement of bullvalene (torr at the experimental temperature of 356 K). Experimental values are signified by solid circles. Pressures are the total sample pressure at 356 K. Errors in frUni/fc are reported to 2o. The solid (upper) line represents the values calculated from RRKM theory using the biradicaloid transition state model. The lower line represents calculated rate constants using the aromatic transition-state model. The collision diameter was 3.6 A in both cases.
One may therefore tentatively conclude from this, in the case of a [2 + 2] cycloaddition or reversion, that a diradical transition state is most likely, if it is stabilized by substituents in the 2,5-position by means of a mesomeric interaction. Otherwise the equilibrium is shifted to the tetraphosphahexadiene. This explains the observations with respect to the reaction proceeding and hence the influence of the substituents at the two carbon atoms. If, in the case of a hindered orbital overlap between the substituents and the PC double bonds, a quasiaromatic interaction is blocked, the equilibrium is shifted to the tetraphosphahexadiene. On the other hand, if substituents in the 2,5-position are able to interact with the PC double bond, a mesomeric charge transfer into the side chain takes place, obstructing the aromatic transition state and so favoring the 1,4-cyclohexadiyl radical, which recombines to the bicyclic compound (Fig. 8). The carbon atom and its substituents... [Pg.297]

Chapter 8 covers extensively pericyclic reactions and also includes the aromatic transition state theory. Most of the examples are taken from latest literature and are useful for postgraduate and research students. [Pg.386]

We have established earlier in the chapter that there will be favourable Frontier Orbital HOMO-LUMO interactions when two molecules approach for a cycloaddition reaction if there are 4n + 2 electrons involved in a fully suprafacial reaction, or 4n electrons if there is an antarafacial component. For delocalization of electrons in the transition state, the fully suprafacial cycloaddition reaction will result in a continuous cyclic overlap of atomic orbitals in the transition state without a phase change, for which 4n + 2 electrons will give aromatic stabilization. For a cycloaddition with one antarafacial component, the cyclic overlap of orbitals will give a Mobius system for which 4n electrons will provide stabilization. Thus the two approaches, Frontier Orbitals and the Aromatic Transition State will always be in agreement favourable... [Pg.162]

Application of the Idea of the Aromatic Transition State to Pericyclic Reactions... [Pg.163]

In the remainder of this chapter we will consider further cycloaddition reactions and other examples of pericyclic reactions. We will use the aromatic transition state approach for simplicity, although in all cases an approach based on HOMO-LUMO interactions would give the same result. [Pg.163]

Using the concept of the aromatic transition state, show that, for pericyclic thermal reactions, 4/i + 2 electrons need to be involved if the reaction is entirely suprafacial, whereas 4n electrons are needed for reactions with one antarafacial component. [Pg.175]

Chapter 2 covers kinetics, which provides useful information about reaction mechanisms, and allows us to distinguish between possible mechanisms in many cases. Elementary reactions do not involve intermediates, but go through a transition state. Although this transition state cannot be isolated, it can be studied in various ways which provide insights into the reaction mechanism, and this forms the subject matter of Chapter 3. This is followed by three chapters on the most important intermediates in organic chemistry anions, radicals and cations. A final chapter on molecular reactions concerns thermal and photochemical processes. The concepts of frontier orbitals and the aromatic transition state allow us to predict which reactions are allowed and which are forbidden , and provide insights into why most reactions of practical interest involve multi-step processes. [Pg.206]

You can see that this mnemonic works if you look at the two products above the first has the two substituents X and Z on neighbouring carbon atoms, just like ortho substituents on a benzene ring, while the second has 1,4-related X and Z just like para substituents. The connection with aromaticity (the aromatic transition state ) simply means that the transition state is cyclic and has six electrons. We have not yet explored the consequences of this, but we will do shortly. [Pg.891]

This latter expression illuminates the connection between the avoided crossing diagrams and the orbital symmetry and related MO rules [1, 37]. It is apparent thus from Eq. (18) that B will get smaller the smaller becomes the HOMO-LUMO gap of the transition state in the general case). Since antiaromatic" transition states of forbidden reactions [37] possess small or vanishing HOMO-LUMO gaps, then according to Eq. (18) these transition states will possess much smaller B values than the aromatic" transition states of allowed reactions [7, 8]. [Pg.285]

The FMO analysis is as shown in Figure 15.10 C. The HOMO-LUMO interaction is now favorable and leads naturally to the formation of the two new bonds. Figure 15.10 D shows the aromatic transition state analysis. Using the looped lines, we have designated the full cyclic array of interactions. As shown, there is one node in the system, so this is a Mobius system. Since there are four electrons in the cyclic array, the reaction is allowed. By the generalized orbital symmetry rule, this approach trajectory ([ 2s + is thermally allowed [only the component fits the 4q + 2)s and (4r)a formulas]. In summary, it is incorrect to say that... [Pg.895]

Figure 15.17 B shows the aromatic transition state analysis of these reactions. We draw a picture of an opening pathway with the minimum number of phase changes and examine the number of nodes. The four-electron butadiene-cyclobutene system should follow the Mobius/conrotatory path, and the six-electron hexatriene-cyclohexadiene system should follow the Hiickel/disrotatory path. As such, aromatic transition state theory provides a simple analysis of electrocyclic reactions. The disrotatory motion is always of Hiickel topology, and the conrotatory motion is always of Mobius topology. Figure 15.17 B shows the aromatic transition state analysis of these reactions. We draw a picture of an opening pathway with the minimum number of phase changes and examine the number of nodes. The four-electron butadiene-cyclobutene system should follow the Mobius/conrotatory path, and the six-electron hexatriene-cyclohexadiene system should follow the Hiickel/disrotatory path. As such, aromatic transition state theory provides a simple analysis of electrocyclic reactions. The disrotatory motion is always of Hiickel topology, and the conrotatory motion is always of Mobius topology.
We also show the aromatic transition state analysis of a [3,3] sigmatropic shift (Figure 15.21 C). There are no nodes. This is a six-electron, Hiickel system, and so is allowed. A more realistic representation of the [3,3] sigmatropic shift is given in Eq 15.25, showing a chair-like transition state with all orbitals in-phase. [Pg.912]

Use the aromatic transition state theory method to determine whether the reactions given in Exercise 23 are allowed or forbidden as written. [Pg.931]

Figure 13. jr-MOs of the aromatic transition state struc ture 32 for the inversion of pyramidal configuration at the phosphorus atom of 2//-l,3,2-dioxaphosphole calculated with the 6-31G basis set. ... [Pg.17]


See other pages where The Aromatic Transition State is mentioned: [Pg.32]    [Pg.114]    [Pg.142]    [Pg.143]    [Pg.144]    [Pg.197]    [Pg.222]    [Pg.223]    [Pg.856]    [Pg.856]    [Pg.159]    [Pg.99]    [Pg.17]    [Pg.18]   


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