Pericyclic reactions transformations

Apart from the thoroughly studied aqueous Diels-Alder reaction, a limited number of other transformations have been reported to benefit considerably from the use of water. These include the aldol condensation , the benzoin condensation , the Baylis-Hillman reaction (tertiary-amine catalysed coupling of aldehydes with acrylic acid derivatives) and pericyclic reactions like the 1,3-dipolar cycloaddition and the Qaisen rearrangement (see below). These reactions have one thing in common a negative volume of activation. This observation has tempted many authors to propose hydrophobic effects as primary cause of ftie observed rate enhancements. 

Two sequential pericyclic reactions are involved in the following furan synthesis. Identify them, and propose a mechanism for the transformation. 

By retrosynthetic analysis, identify a precursor that could provide the desired product by a single pericyclic reaction. Indicate appropriate reaction conditions for the transformation you identify. 

The following transformations involve two or more pericyclic reactions occurring in tandem during the process. Suggest a plausible sequence of reactions that can lead to the observed product. 

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. 

In this section are described those domino reactions which start with a retro-pericy-clic reaction. This may be a retro-Diels-Alder reaction, a retro-l,3-dipolar cycloaddition, or a retro-ene reaction, which is then usually followed by a pericyclic reaction as the second step. However, a combination is also possible with another type of transformation as, for example, an aldol reaction. 

The main reason for the rapid development of metathesis reactions on a laboratory scale (the reaction itself had been known for quite a long time) has been the development of active and robust second-generation ruthenium catalysts (6/3-14 to 6/3-16), which usually provide better yields than the first-generation Grubbs catalysts (6/3-9 or 6/3-13) (Scheme 6/3.2). This also reflects the huge number of domino processes based on ruthenium-catalyzed metathesis, which is usually followed by a second or even a third metathesis reaction. However, examples also exist where, after a metathesis, a second transition metal-catalyzed transformation or a pericyclic reaction takes place. 

A pericyclic reaction is forbidden if its orbital symmetry is not conserved. In such reactions the occupied orbitals of the reactant do not transform into the occupied orbitals of the product with the same symmetry. 

Cope himself formulated this transformation as what would now be called a synchronous pericyclic reaction . This interpretation was supported by Woodward-Hoffmann s analysis of pericyclic processes. The Cope rearrangement of 1,5-hexadiene derivatives was regarded therefore for a long time as a classical example of an allowed pericyclic reaction... 

The proposed mechanism involves either path a in which initially formed ruthenium vinylidene undergoes nonpolar pericyclic reaction or path b in which a polar transition state was formed (Scheme 6.9). According to Merlic s mechanism, the cyclization is followed by aromatization of the ruthenium cyclohexadienylidene intermediate, and reductive elimination of phenylruthenium hydride to form the arene derivatives (path c). A direct transformation of ruthenium cyclohexadienylidene into benzene product (path d) is more likely to occnir through a 1,2-hydride shift of a ruthenium alkylidene intermediate. A similar catalytic transformation was later reported by Iwasawa using W(CO)5THF catalyst [14]. 

Pericyclic reactions are unimolecular, concerted, uncatalyzed transformations. They take place in a highly stereoselective manner governed by symmetry proper-ties of interacting orbitals. - Characteristic of all these rearrangements is that they are reversible and may be effected thermally or photochemically. The compounds in equilibrium are usually interconverted through a cyclic transition state,224 although biradical mechanisms may also be operative. A few characteristic examples of pericyclic rearrangements relevant to hydrocarbon isomerizations are presented here. 

Each of the following transformations is the result of two successive pericyclic reactions. Draw the structures of the intermediates A-E, and identify the class of pericyclic reaction to which each step belongs ... 

Identify the two and the three pericyclic steps involved in the formation of the major and minor products, respectively, in this transformation (hint all four classes of pericyclic reaction are represented) ... 

Two extreme pathways may be envisaged for this transformation. One involves a pericyclic reaction followed by desilylation and elimination of methoxide from the product silyl enol ether. The alternative proceeds via a Lewis acid promoted aldol reaction to give an intermediate which cyclizes to the product dihydropyrone (Figure Si3.6). The actual pathway in an individual case is affected to a considerable degree by the nature of the Lewis acid used and lies somewhere between these two extremes. 

Frequently, the intramolecular [2 + 2] photocydoaddition of an alkene to a benzene ring is followed by further pericyclic reactions. Such transformations yield... 

We now turn to a different type of photochemistry and different mechanistic questions, namely photochemical pericyclic reactions and the utility of the Mobius-Hiickel treatment of these transformations. 

The symmetry properties of orbitals, and the pattern of electron occupation of the orbitals, is important in concerted reactions, that is, those that transform substrates directly into products without going through intermediates. A large group of concerted reactions called pericyclic reactions have been widely studied for alkenes and... 

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