Of pericyclic

The special case of pericyclic reactions is an appropriate means of introducing the subject These reactions are very common, and were extensively studied experimentally and theoretically. They also provide a direct and straightforward connection with aromaticity and antiaromaticity, concepts that mm out to be quite useful in analyzing phase changes in chemical reactions. 

Adopting the view that any theory of aromaticity is also a theory of pericyclic reactions [19], we are now in a position to discuss pericyclic reactions in terms of phase change. Two reaction types are distinguished those that preserve the phase of the total electi onic wave-function - these are phase preserving reactions (p-type), and those in which the phase is inverted - these are phase inverting reactions (i-type). The fomier have an aromatic transition state, and the latter an antiaromatic one. The results of [28] may be applied to these systems. In distinction with the cyclic polyenes, the two basis wave functions need not be equivalent. The wave function of the reactants R) and the products P), respectively, can be used. The electronic wave function of the transition state may be represented by a linear combination of the electronic wave functions of the reactant and the product. Of the two possible combinations, the in-phase one [Eq. (11)] is phase preserving (p-type), while the out-of-phase one [Eq. (12)], is i-type (phase inverting), compare Eqs. (6) and (7). Normalization constants are assumed in both equations ... 

There are several general classes of pericyclic reactions for which orbital symmetry factors determine both the stereochemistry and relative reactivity. The first class that we will consider are electrocyclic reactions. An electrocyclic reaction is defined as the formation of a single bond between the ends of a linear conjugated system of n electrons and the reverse process. An example is the thermal ring opening of cyclobutenes to butadienes ... 

For reviews of several concerted reactions within the general theory of pericyclic reactions, see A. R Marchand and R. E. Lehr, eds., Pericyclic Reactions, Vols. I and II, Academic Press, New York, 1977. 

The participation of the lone-pair orbital in the cyclization process allows its classification as a so-called pseudopericyclic reaction (76JA4325 97JA4509), which is a subset of a general type of pericyclic reactions... 

Biological examples of pericyclic reactions are relatively rare, although one much-studied example occurs during biosynthesis in bacteria of the essential amino acid phenylalanine. Phenylalanine arises from the precursor chorismate,... 

These selection rules are summarized in Table 30.4, thereby giving you the ability to predict the stereochemistry of literally thousands of pericyclic reactions. 

The following synthesis of dienones occurs readily. Propose a mechanism to account for the results, and identify the kind of pericyclic reaction involved. 

Karahanaenone, a terpenoid isolated from oil of hops, has been synthesized by the thermal reaction shown. Identify the kind of pericyclic reaction, and explain how karahanaenone is formed. 

LUMO (Sections 14.4, 30.2) An acronym for lowest unoccupied molecular orbital. The symmetries of the LUMO and the HOMO are important in determining the stereochemistry of pericyclic reactions. 

Suprafacial (Section 30.6) A word used to describe the geometry of pericyclic reactions. Suprafacial reactions take place on the same side of the two ends of a -n electron system. 

In a more general view, the most promising choices in this selection of pericyclic carbonyl additions are allylboronates [M = B(OR)2, Section D.1.3.3.3.3.], allyltitanates [M = Ti(OR)3,... 

A complex sequence of pericyclic reactions, intramolecular and intermolecular cycloadditions and cycloreversions, was studied in an attempt to readily achieve bicyclic five-membered heterocycles, the methyl 4,6-dihydrothieno- and methyl-... 

For a classification of pericyclic reactions, see Hendrickson, J.B. Angew. Chem. Int. Ed. Engl., 1974, 13, 47. Also see Fleming, I. Pericyclic Reactions-, Oxford University Press Oxford, 1999. 

The first pair of examples we would like to discuss occurs in a field which lends itself naturally to be conquered by theory. Indeed, the past three decades have seen the exploration of mechanistic details of pericyclic reactions as one of the major success stories of computational chemistry. Rooted in qualitative molecular orbital theory, the key concept of... 

Wiest, O., Houk, K. N., 1996, Density Functional Theory Calculations of Pericyclic Reaction Transition Structures Top. Curr. Chem., 182, 1. 

An analogy can be drawn between pericyclic reactions in water and under high pressure. Water s internal pressure on hydrophobic substrates acts on the volume of activation of a reaction in the same way as an externally applied pressure does. Thus, the internal pressure of water influences the rates of pericyclic reactions in water in the same direction as external pressures. The use of salting-out salts will further increase the rate of pericyclic reactions. Recently, Kumar quantified the relationship between internal pressure and the rate of the aqueous Diels-Alder reaction. A linear relationship between the two was observed.5... 

Exactly such steric repulsion seems to be responsible for the further rearrangements observed with heptaphenylborepin (109 or 114 with R = Ph). The end result of a reasonable, but amazing sequence of pericyclic reactions is 1,2,3,3a,4,5-hexaphenyl-5-bora-3a,4-dihydro-5//-benz[>]in-dene (119), whose generation is initiated with 109 and continues through 116-118 (Scheme 11). The last step, 118-119, is an impressive intramolecu-... 

As pericyclic reactions are largely unaffected by polar reagents, solvent changes, radical initiators, etc., the only means of influencing them is thermally or photochemically. It is a significant feature of pericyclic reactions that these two influences often effect markedly different results, either in terms of whether a reaction can be induced to proceed readily (or at all), or in terms of the stereochemical course that it then follows. Thus the Diels-Alder reaction (cf. above), an example of a cycloaddition process, can normally be induced thermally but not photochemically, while the cycloaddition of two molecules of alkene, e.g. (4) to form a cyclobutane (5),... 

The third major category of pericyclic reactions can be looked upon as involving the migration of a a bond—hence the name—within a 7t-electron framework. The simplest examples involve the migration of a a bond that carries a hydrogen atom. 

By analogy with the categories of pericyclic reactions we have already considered, the feasibility of the migration will then be decided by the relative phase of the terminal lobes, i.e. the symmetry, of the HOMO of the pentadienyl radical (38). As this is a 5ne system, its... 

The combination of pericyclic transformations as cycloadditions, sigmatropic rearrangements, electrocydic reactions and ene reactions with each other, and also with non-pericyclic transformations, allows a very rapid increase in the complexity of products. As most of the pericyclic reactions run quite well under neutral or mild Lewis acid acidic conditions, many different set-ups are possible. The majority of the published pericyclic domino reactions deals with two successive cycloadditions, mostly as [4+2]/[4+2] combinations, but there are also [2+2], [2+5], [4+3] (Nazarov), [5+2], and [6+2] cycloadditions. Although there are many examples of the combination of hetero-Diels-Alder reactions with 1,3-dipolar cycloadditions (see Section 4.1), no examples could be found of a domino all-carbon-[4+2]/[3+2] cycloaddition. Co-catalyzed [2+2+2] cycloadditions will be discussed in Chapter 6. 

The second largest group of pericyclic domino reactions starts with a sigmatropic rearrangement, which is most often a Claisen or an oxa- and aza-Cope rearrangement however, some processes also exist with a 2,3-sigmatropic rearrangement as the second step. 

Frontier molecular orbital (FMO) theory 62) has provided new insights into chemical reactivity. This, and the simplicity of its application, has led to its widespread use, particularly in the treatment of pericyclic reactions 63). An FMO treatment depends on the energy of the highest occupied (HOMO) and lowest unoccupied molecular... 

The dienes and polyenes are compounds which intervene in a large number of organic reactions, as will be seen in different chapters of this book. Several excellent reviews have been devoted to theoretical studies about their reactivity, with special emphasis on the mechanism of pericyclic reactions3-5. As was mentioned in the introduction, this section will only treat, as an example, the Diels-Alder reaction, since it has been the most studied one by theoreticians. Our goal is not to cover all aspects, but instead to show the high potential and usefulness of theoretical methods in order to interpret and rationalize the experimental results. In the rest of the chapter we will concentrate on the last ab initio calculations. 

C8—Ge—C18 137.7° (3)] (Figure 24). The distortion of the internal O—Ge—C angles leads to an enhanced Lewis acidity, which was illustrated by the promotion of pericyclic reactions involving activation of aldehyde carbonyl groups. 

Perhaps the most successful application of Fukui function and local softness is in the elucidation of the region-selective behavior of different types of pericyclic reactions including the 1,3-dipolar cycloadditions (13DC), Diels-Alder reactions, etc. These reactions can be represented as shown in Scheme 12.4. Considering the concerted approach of the two reactants A and B, there are two possible modes of addition as shown in Pathway-I and Pathway-II. 

---