Group transfer pericyclic reaction

The transfer of hydrogen from donors to the coal substrate during liquefaction occurs by concerted pericyclic reactions of the type termed group transfers by Woodward and Hoffman (1 ). ... 

The actual rates of thermally-allowed pericyclic reactions vary vastly, and frontier-orbital theory (14, 15, 16) has proven to be the primary basis for quantitative understanding and correlation of the factors responsible. It is therefore of interest to find the dominant frontier orbital interactions for the group transfer reactions hypothesized to occur. 

The preceding experiments offer preliminary support to our notion that pericyclic pathways might be intimately involved in the mechanism of coal liquefaction. More specifically, the results indicate that pericyclic group transfer reactions constitute a plausible pathway for the transfer of hydrogen from donor solvents to coal during liquefaction. 

Paquette has reported the most detailed investigation of this sort, of the relationship between (crystal) structure and reactivity for a series of compounds [78] (Paquette et al., 1990). These undergo a thermal pericyclic reaction in which two hydrogen atoms are transferred between alkene and alkane C-C bonds across a gap whose dimensions can be accurately measured, and can be varied by varying the group X bridging the central... 

In this primer, Ian Fleming leads you in a more or less continuous narrative from the simple characteristics of pericyclic reactions to a reasonably full appreciation of their stereochemical idiosyncrasies. He introduces pericyclic reactions and divides them into their four classes in Chapter 1. In Chapter 2 he covers the main features of the most important class, cycloadditions—their scope, reactivity, and stereochemistry. In the heart of the book, in Chapter 3, he explains these features, using molecular orbital theory, but without the mathematics. He also introduces there the two Woodward-Hoffmann rules that will enable you to predict the stereochemical outcome for any pericyclic reaction, one rule for thermal reactions and its opposite for photochemical reactions. The remaining chapters use this theoretical framework to show how the rules work with the other three classes—electrocyclic reactions, sigmatropic rearrangements and group transfer reactions. By the end of the book, you will be able to recognize any pericyclic reaction, and predict with confidence whether it is allowed and with what stereochemistry. 

Alternatively, covalent propagation may proceed by a pericyclic reaction involving a multicenter rearrangement such as a group transfer polymerization no examples of these type of reactions have been reported so far. 

The first step 202 is now a pericyclic reaction. It looks like a cycloaddition, though it involves a hydrogen transfer as well. It is in fact a carbonyl (or oxo- ) ene reaction. It is like a Diels-Alder cycloaddition in which a C-H bond has replaced one of the double bonds in the diene and a C=0 group is the dienophile. Many Prins reactions are probably carbonyl ene reactions. In his excellent review30 in Comprehensive Organic Synthesis, B. J. Snider says The (carbonyl) ene reaction and the Prins reaction are not mechanistically distinct . Though this step is pericyclic, it is very polar and the transition state 203 no doubt contains partial charges. It is therefore stabilised and the reaction accelerated by protic acids 205 and Lewis acids 207. 

The Alder ene reaction is like a Diels Alder reaction in which one Jt-bond in the diene has been replaced by a C-H bond 121. It does not therefore form a ring and does not fit easily into any of the three classes of pericyclic reaction (cycloaddition, electrocyclic, and sigmatropic). Since a hydrogen atom is transferred from one component to the other it is best described as a group transfer reaction.21 The regioselectivity is determined by the interaction 123 with the Jt-bond of the ene (the HOMO) with the LUMO of the enophile. ... 

In addition to these three basic types of pericyclic reactions there are also some other, more special processes as, e.g., the cheletropic reactions (6), valence isomerizations (7) or the reactions of the transfer of the group (8), the typical examples of which are shown bellow. 

Pericyclic reactions are classified into four classes. These are electrocyclic reactions, cycloadditions, sigmatropic rearrangements, and group transfer reactions. 

An aryne-ene reaction has been reported and evidence supports it as a pericyclic reaction [59], Write a mechanism for the reaction below. When the allylic methyl groups are substituted with deuterium, greater than 95% deuterium transfer is observed. How does this support the conclusion that the reaction is conceited ... 

Pericyclic reactions were defined in 1969 by R. B. Woodward and R. Hoffmann as reactions in which all first-order change.s in bonding relationships take place in concert on a closed curve that is, as concerted reactions in which all bonds are formed or brokeasimultaneously around a circle. This definition arose from the systematic study of the conservation of orbital symmetry in a series of reactions electrocyclic reactions, cycloadditions, sigmatropic shifts, cheletropic reactions, and group transfer and elimination reactions. Excellent reviews on the historical evolution of pericyclic reactions have been published. ... 

Because of condition (iii) all pericyclic reactions may formally be regarded as cyclo-addition processes or their retrogressions, but it is generally more useful to divide pericyclic reactions into a number of more distinct reaction series. These are electrocyclic reactions (e.g. Equations 3.3 and 3.4), cycloaddition reactions (e,g. Equations 3.5 and 3,6), sigmatropic reactions (e.g. Equations 3.7 and 3.8), cheletropic reactions (e.g. Equations 3.9 and 3.10), group transfers (e.g. Equation 3.11), and eliminations (e.g. Equations 3.12 and 3,13). Examples in other categories are less numerous, and will not be considered in this volume. 

Volume 1 includes nine chapters, the first six of these deal with the main established families of organic photochromes which have a few real and many potential applications. Their photochemical processes are based on pericyclic electrocyclic reactions. The three other chapters concern hydrogen or group rearrangement, and electron transfer. Seven out of the nine main authors, selected from all over the world, have not written chapters for previous books and importantly, three are from companies. Four chapters cover families not reviewed... 

If both positions 3 and 4 of the substrate are stereogenic centers, 1,3-chirality transfer along both sides of the pericyclic system allows a diastereocontrolled Cope rearrangement to be performed. In anionic oxy-Cope rearrangements with a substituent adjacent to the hydroxy group, e.g., 5, the equatorial/axial orientation of the alkoxide group is most important for the stereochemical outcome of the reaction. 

If a carbonyl group is attached a to position 1 of the pericyclic system, the oxy-Cope rearrangement of divinylcyclohexanes results in the formation of medium ring enolates, which after proton transfer undergo a transannular vinylogeous aldol-type reaction (probably a transannu-lar, intramolecular Michael-type reaction). For example, tricyclic alcohol 4 is obtained via such a reaction sequence from 1 in 45% yield1129. 

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