Pericyclic reactions, alkynes

In general the aromatic systems are more stable and hence less reactive especially towards an addition reaction and antiaromatic systems are more reactive. Moreover aromatic systems prefer to undergo substitution reactions rather than addition reactions (as in the cases of alkenes and alkynes). Pericyclic reactions have been analyzed via (anti) aromaticity of the transition state . ... 

A closely related method is the thermolysis of l-allylsulfinyl-2-cyanoethane in alkynes, which leads to the formation of thiolane oxide derivatives via consecutive pericyclic reactions . The low yield and formation of mixtures are somewhat compensated for by the convenience, but its practicality is as yet rather limited (equation 115). 

There are numerous reactions in organic chemistry that proceed through cyclic transition states. They may be classified generally as pericyclic reactions. An important and familiar example is the Diels-Alder reaction, in which a conjugated diene cycloadds to an alkene or alkyne ... 

Theoretical and computational studies of the reactivities and molecular interactions of acetylene have exploded during recent years. Most studies are aimed at gaining theoretical understanding of the difference in the reactivities between the triple bond in alkynes and the double bond in alkenes. In the following section, we will summarize the reactivities of acetylene involved in pericyclic reactions, electrophilic reactions, and nucleophilic additions. Then, we will give a brief review of the studies probing molecular interactions of acetylene. 

Other pericyclic reactions of alkynes that have been studied computationally include the addition of singlet methylene to acetylene [109], the addition of carbon monosulfide to acetylene [110], the [2 + 2] dimerization [100, 111], and the dihydrogen transfer reaction between acetylene and ethylene [112, 113]. 

The preparation of the quinoline system by pericyclic reactions is less important than the previous syntheses. Some cyclization/elimination reactions of iV-phenylimines 95 and 96 with alkynes, enol ethers or enamines are of preparative interest. Although they correspond to [4+2] cycloadditions, they are likely to proceed by SeAt mechanisms and are usually catalysed by Lewis acids. In the enolether cycloaddition to imines 95 ytterbium(III)triflate proved to be particularly effective [104]. 

Mock et al. [47] used the complexation of alkylammonium ions by Cucurbituril (53) to catalyze 1,3-dipolar cycloadditions of ammonium substituted alkynes 54 and alkyl azides 55. This pericyclic reaction is accelerated by a factor of 5.5 x 10 under the catalytic influence of 53. 

Hydroboration attracted several theoretical studies in 1978. Publications which have appeared include an ab initio study of the reaction of BHs with ethylene, a CNDO/2 study of the nature of the ethylene-borane complex, the transition state in the hydroboration reaction, a MNDO study of hydroboration of alkenes and alkynes, and a MNDO study of hydroboration and borohydride reduction with implications concerning cyclic conjugation and pericyclic reactions. Finally, HaBCHO was one of the molecules which has been studied with regard to banana bonds of the carbonyl group. ... 

Jones and coworkers200 found that a variety of sulphenic acids may be generated by thermolysis of the readily available /J-cyanosulphoxides (equation 81) and observed their highly regiospecific addition also to non-conjugated alkynes (Table 12). As expected for a pericyclic mechanism, the reaction afforded the product of a stereospecific cis-addition. However, the regioselectivity of the addition suggests that the partial carbon-sulphur bond in the transition state 148 is polarized in such a way that the carbon atom has some cationic character (equation 82). 

Transition metal-catalyzed transformations are of major importance in synthetic organic chemistry [1], This reflects also the increasing number of domino processes starting with such a reaction. In particular, Pd-catalyzed domino transformations have seen an astounding development over the past years with the Heck reaction [2] - the Pd-catalyzed transformation of aryl halides or triflates as well as of alkenyl halides or triflates with alkenes or alkynes - being used most often. This has been combined with another Heck reaction or a cross-coupling reaction [3] such as Suzuki, Stille, and Sonogashira reactions. Moreover, several examples have been published with a Tsuji-Trost reaction [lb, 4], a carbonylation, a pericyclic or an aldol reaction as the second step. 

The Diels-Alder reaction is one of the most important carbon-carbon bond forming reactions,521 522 which is particularly useful in the synthesis of natural products. Examples of practical significance of the cycloaddition of hydrocarbons, however, are also known. Discovered in 1928 by Diels and Alder,523 it is a reaction between a conjugated diene and a dienophile (alkene, alkyne) to form a six-membered carbo-cyclic ring. The Diels-Alder reaction is a reversible, thermally allowed pericyclic transformation or, according to the Woodward-Hoffmann nomenclature,524 a [4 + 2]-cycloaddition. The prototype reaction is the transformation between 1,3-butadiene and ethylene to give cyclohexene ... 

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