Organic reaction mechanisms Diels-Alder reactions

Organic Reaction Mechanism DIELS-ALDER REACTION... 

The domino cycloaddition-iV-acyliminium ion cyclization cascade has been extensively reviewed. Tandem reactions combining Diels-Alder reactions and sigma-tropic rearrangement reactions in organic synthesis have been extensively reviewed. The tandem Diels-Alder reaction between acetylenedicarboxaldehyde and N,N -dipyrrolylmethane has been extensively studied at the RHT/3-21G and RHF/6-31G levels.The molecular mechanism of the domino Diels-Alder reaction between hexafluorobut-2-yne and A,A -dipyrrolylmethane has been studied using density functional theory. 

Maleimide. Maleimide and maleic anhydride are simple olefins and very useful molecules in organic syntheses for introducing various substituents. Addition reactions, cyclodimerization, Diels-Alder reactions and polymerization reactions are as well-known as their photochemical reactions. They are also known as electron acceptors in photoinduced electron-transfer reactions. The photosensitized reaction of maleimide with xanthone in 2-propanol has been investigated by TR EPR. The emissive CIDEP spectrum observed in 2-propanol is predominantly assigned to two kinds of maleimide alkyl-type radicals. On the other hand, the absorptive spectrum of the maleimide radical-anion was observed in 2-propanol in the presence of hydrochloric acid. The hydrochloric acid addition effect on the CIDEP patterns indicates the existence of two mechanisms for the photosensitization of maleimide by xanthone. One is a T-T energy transfer to maleimide followed by hydrogen abstraction by maleimide... 

The concepts we have been discussing come into focus in an important reaction in synthetic organic chemistry, the Diels-Alder reaction. In this reaction, a diene with its 4 7t electrons reacts with an alkene with its two 7t electrons to give a cyclohexene ring. Since the product is a ring, it is a cycloaddition reaction. Since the 4 n electrons of the diene and the two n electrons of the alkene participate in the reaction. We will discuss the mechanism of this reaction in Chapter 25. In this chapter, we will focus upon its synthetic versatility. The simplest example of a Diels-Alder reaction is the cycloaddition reaction of 1,4-butadiene and ethene to give cyclohexene. In every case, a cyclohexene ring is a product of a Diels-Alder reaction. 

This thesis contributes to the knowledge of catalysis in water, us it describes an explorative journey in the, at the start of the research, unh odded field of catalysis of Diels-Alder reactions in aqueous media. The discussion will touch on organic chemistry, coordination chemistry and colloid chemistry, largely depending upon the physical-organic approach of structural variation for the elucidation of the underlying mechanisms and principles of the observed phenomena. 

The Diels-Alder reaction, probably the most widely used methodology in organic synthesis today, has contributed greatly to the development of mechanistic and theoretical chemistry. The recent discovery of a Diels-Alderase enzyme has provided insights into the mechanism of biosynthetic cycloaddition. 

The Diels-Alder reaction is among the most useful tools in organic chemistry. It has been the object of a great number of theoretical studies95-131 dealing with almost every one of the experimental aspects reactivity, mechanism, selectivity, solvent effects, catalysis and so on. 

The article is organized as follows. In the next Section we present a brief outline of the theoretical background for the present work. Section 3 contains summaries of the SC models for the electronic mechanisms of the gas-phase Diels-Alder reaction between butadiene andethene [11] and the 1,3-dipolar cycloaddition of fulminic acid to ethyne [12]. In Section 4 we provide, for the first time, a description of the SC model for the electronic mechanism of the gas-phase disrotatory electrocyclic ring-opening of cyclohexadiene. Conclusions and final comments are presented in Section 5. 

Perez-Prieto, J., Stiriba, S.E., Gonzalez-Bejar, M., Domingo, L.R. and Miranda, M.A. (2004) Mechanism of triplet photosensitized Diels-Alder reaction between indoles and cyclohexadienes theoretical support for an adiabatic pathway. Organic Letters, 6, 3905-3908. 

Key Mechanism 15-3 The Diels-Alder Reaction 684 15-12 The Diels-Alder as an Example of a Pericyclic Reaction 692 15-13 Ultraviolet Absorption Spectroscopy 696 15-14 Colored Organic Compounds 701... 

Catalysis of the [2+2+2] cycloaddition of alkynes by transition metal complexes has been extensively exploited for the synthesis of complex organic molecules [30-34]. The accepted mechanism for this transformation, shown in Scheme 10, involves coordination of two alkyne molecules to the metal centre followed by oxidative coupling to form the coordinatively unsaturated metallocyclo-pentadiene 49, which can coordinate a third molecule of alkyne to afford 50. Insertion of the alkyne in a metal-carbon bond of this complex leads to met-allocycloheptadiene 51, and reductive elimination then affords cyclotrimer 52 and regenerates the catalytic species. Alternatively, the transformation of 49 into 52 might involve a Diels-Alder reaction giving intermediate 53, followed by reductive elimination [35]. 

The kinetics of the Diels-Alder reactions of di(2-pyridyl)-l,2,4,5-tetrazine with substituted styrenes have been investigated in aqueous media and in organic solvents. The second-order rate constants of this reaction increase dramatically in water-rich media. A decrease in pH accelerates the aqueous Diels-Alder reaction even more. The Hammer p-values and also the electronic demand 77-values of the reaction are solvent sensitive. In protic solvents, the dipolar character of the activated complex is increased but, simultaneously, hydrogen-bond interactions stabilize the activated complex. These effects are most pronounced in 2,2,2-trifluoroethanol, which shows that the aqueous accelerations cannot be attributed solely to solvent-induced changes of the reaction mechanism <1996JOC2001>. 

The introduction of an atom other than carbon to either dienes or dienophiles will result in the hetero-Diels-Alder reaction. Owing to the nature (steric effect, electronic effect, etc.) of substituents on dienes and dienophiles, the Diels-Alder reaction might occur through a synchronous concerted, an asynchronous concerted, or a stepwise reaction mechanism. The stepwise and asynchronous concerted Diels-Alder reactions proceed via diradical intermediates, whereas the synchronous concerted mechanisms does not. It should be pointed out that most of the Diels-Alder reactions are concerted as a result, both the rate constants and the stereoselectivities of Diels-Alder reaction are only moderately sensitive to the changes in the nature of organic sol vents. However, it has been clearly shown that the applications of water to the reaction system can greatly accelerate such reactions. Other modifications on this reactions include the application of high pressure, Lewis acid, and ultrasound radiation.More information about this reaction can be easily attained from reviews and relevant books. 

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