Ene reaction, of arynes

Despite the long history of arynes and their potential as a powerful enophile in the ene reaction (because of their low-lying LUMOs), reports on the ene reaction of arynes remain rare, especially with alkynes. Recently, an ene reaction of arynes with alkynes has been reported by Cheng and coworkers, which readily affords arylallenes (Equation 12.49) [106]. 

Efficient Alder-ene reactions of arynes generated from bis-l,3-diynes have been reported and the formation of aryne intermediates and their ene reactions revealed to be less sensitive to the catalyst than to the structural features of the substrates (Scheme 58). 

It should be noted that because of the extreme reactivity of arynes, other reactions such as 2+2 cycloaddition or the ene reaction may complete with the Diels-Alder mode of addition more frequently than is the case with less reactive dienophiles. This is especially true if the diene component is not conformationally predisposed to the 4+2 cycloaddition mode. Also, as with other dienophiles, Diels-Alder reactions of arynes are stereoselective. 

In 1953, Robert s experiments on the conversion of C-labeled chlorobenzene with KNH2 into aniline gave strong support to the intermediacy of ortho-benzyne in this and related reactions. Additional direct evidence for the existence of ortho-benzyne was provided by the observation of its IR spectrum, sohd-state dipolar NMR spectrum, and NMR in a molecular container, and by UV photoelectron spectroscopy. Even at low temperatures, arynes are extraordinary reactive. The reactions of arynes can be divided into three groups (i) pericyclic reactions, (ii) nucleophilic additions, and (iii) transition-metal catalyzed reactions. The pericyclic reactions can be divided into several categories such as Diels-Alder reactions, [2-f2] cycloadditions, 1,3- and l,4-dipolar cycloadditions, and the ene reactions. Arynes react with practically aU kinds of nucleophiles. More recently, the transition-metal catalyzed reactions of arynes have been studied, in particular those involving palladium. 

Arynes, generated in situ from o-silylaryl triflates (229), undergo ene reaction with alkynes (230) possessing propargylic hydrogen in the presence of KF-18-crown-6 in... 

One of the two reaction types which dominate the synthetic utility of arynes as reactive intermediates is cycloaddition. Cycloadditions can be subdivided into several categories. For example, benzynes undergo [4+2] and [2+2] cycloadditions as well as 1,3-dipolar cycloadditions, the ene reaction, and miscellaneous others. These reactions may occur in an inter- or intramolecular mode. Further, multiple and tandem aryne reactions can be used for multiple-bond construction in a single step. Each of these reaction types, as well as a few miscellaneous reactions, is discussed in the following sections. 

Candito DA, Dobrovolsky D, Lautens M. Development of an intramolecular aryne ene reaction and application to the foemal synthesis of (+/-)-Crinine. J Am Chem Soc 2012 134(37) 15572-80. 

Scheme 6.11 A consecutive ene reaction as one of many trapping modes for the HDDA-generated aryne.

A three component coupling reaction involving carbopallada-tion of the aryne followed by a Fleck coupling with tert-butyl acrylate affords oFtAo-substituted cinnamic acids in good yields (eq 11). An ene reaction between the aryne, generated in THF at room temperature, and an alkyne creates allenylbenzenes in moderate yields. The reaction of jr-allylpalladium species with the benzyne created from 2-(trimethylsilyl)phenyl triflate provides access to several types of products in multicomponent... 

The arynes generated by various methods can act as enophiles and undergo intramolecular reactions with various types of ene donors as shown in Scheme 6.15. 

The common reaction mode for arynes and olefins is the ene process that results in a phenylation accompanied by a double bond shift. Such transformations can also be achieved with aza analogs (Schiff bases) of terminal or internal olefms. ... 

---