Strained triple bonds

Its Strained triple bond makes benzyne a relatively good dienophile and when benzyne IS generated in the presence of a conjugated diene Diels-Alder cycloaddition occurs... 

When benzyne is generated in the presence of potential dienes, additions at the highly strained triple bond occur. Among the types of compounds that give Diels-Alder addition products are furans, cyclopentadienones, and anthracene. 

The carbanion can expel bromide ion to become a neutral species. As bromide leaves with its bonding electrons, an empty sp2 orbital remains. This orbital overlaps with the filled orbital adjacent to it, giving additional bonding between these two carbon atoms. The two sp2 orbitals are directed 60° away from each other, so their overlap is not very effective. This reactive intermediate is called a benzyne because it can be symbolized by drawing a triple bond between these two carbon atoms. Triple bonds are usually linear, however, so this is a very reactive, highly strained triple bond. 

A reactive intermediate in some nucleophilic aromatic substitutions, benzyne is benzene with two hydrogen atoms removed. It can be drawn with a highly strained triple bond in the six-membered ring. (p. 789)... 

The generally accepted mechanism for this reaction is a two-step elimination-addition mechanism. In the / elimination step, NIk promotes the / elimination of the elements of HI to give benzyne. In the addition step, Nik adds to the very strained triple bond to give an aryl anion, which undergoes proton transfer. Workup gives the neutral product. 

I he strained triple bond of (1) to form the mono adduct (2, red). Hydrolysis of the... 

Arynes are novel reaction intermediates that react with dienes or 1,3-dipoles to give the corresponding cycloadducts. Recently, many researchers have reported the reaction of benzyne prepared from 2-(trimethylsilyl)phenyl triflate or benzenedia-zonium carboxylate with imines, aminobenzoate, 2-aminobenzophenones, azides, and diazo compounds, which provides various N-containing cycloadducts, such as acridines, acridones, triazoles, and indazoles. Formally, the reaction proceeds in a [2-1-2], [3-1-2], or [4-1-2] manner. Benzyne is an extremely reactive species because of the presence of a strained triple bond and undergoes polar and pericycHc reactions. The lifetime of benzyne in the gas phase has been estimated to around 20 ns (2 X 10" s) by mass spectroscopic techniques. Some spectroscopic properties of benzyne have been determined by Orville Chapman using matrix isolation techniques (Scheme 7.20). 

Organometals add onto strained triple bonds with particular ease. -Butyllithium reacts with cyclooctyne at -75 °C slowly and at 0 °C instantaneously. The important... 

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