ACETYLENE 3 Theoretical considerations

The betainic imidazo[l,2- 7][l,2,4]triazinium-olate 107 was found to react as a 1,3-dipole in 1,3-dipolar cycloaddition with ynamines to yield a bridged skeletone 108 <1999T13703> as shown in Scheme 15. This cycloadduct 108 underwent subsequent rearrangement upon heating, and resulted in formation of a fused eight-membered heterocycle 109. With acetylenes other than ynamines, the transformation was found to proceed slowly and in bad yields. The fact that ynamines were used successfully, as well as theoretical considerations (cf. Section 11.17.2) in this chapter, indicated that these Diels-Alder reactions are of inverse electron demand. 

Although there are several examples of transition metal-catalyzed addition of C-H bonds to acetylenes, there is neither experimental evidence nor theoretical consideration in respect of a reaction mechanism for C-H/acetylene coupling. This coupling reaction is believed to proceed through a pathway similar to that proposed for C-H/olefin coupling. 

Since the appearance of the theoretical consideration that the formation of tetrahedrane from two acetylenes would be a photochemically allowed process, a number of papers dealing with attempted syntheses of tetrahedranes have been published. It was reported that 2,2 -bis(phenylethynyl)biphenyl (26) was transformed thermally and photochemically into an isomeric hydrocarbon, for which a tetrahedrane (27) or a cyclobutadiene (28) structure would be possible . Actually, the structure of the hydrocarbon was confirmed to be 9-phenyl-l,2 3,4-dibenzo-anthracene (29) derived from transannular bond formation . ... 

Therefore, comparison of interatomic distances leads to the conclusion that acetylenes are more strongly bonded to the metal than olefins. This is also confirmed by theoretical considerations. Acetylene complexes containing a small... 

The ability of olefins and acetylenes to function as ligands towards transition metals has aroused considerable interest in the last ten years from both the theoretical and practical viewpoints. Elucidation of the nature of the bonding between the unsaturated hydrocarbon and the metal atom has made an important contribution to the modern concept of valency, and has led to a better understanding of the role metallic catalysts play in chemical reactions. 

Theoretical studies of simple acetylenes and the reactions of acetylene have reached a stage of considerable accuracy, yet a vast world of acetylene chemistry remains to be studied by theoretical methods. 

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