Carbon nucleophiles, reactions with aryne

Arynes react readily with simple alkenes to give either benzocyclobutenes or substituted benzenes (Scheme 7.31). The formation of benzocyclobutenes by [2+2] cycloaddition reaction of the aryne to the alkene proceeds best for strained and electron-rich carbon-carbon (C=C) double bonds. For example, dicyclopentadiene reacts to give the ex o-isomer of the corresponding four-membered ring in good yield. The addition to cyanoethene (acrylonitrile) and the reaction with the electron-rich ethoxyethene (ethyl vinyl ether) gives the cyano- and ethoxy-benzocyclobutenes in 20% and 40% yields, respectively. The latter reaction almost certainly involves nucleophilic addition of the enol ether to the electrophilic aryne followed by coUapse... 

The reactive zwitterions arising from the nucleophilic attack of imines 479 on the benzyne generated in situ from 2-(trimethylsilyl)phenyl triflate 478 proved to be an appropriate molecular scaffold for the capture of CO2 with sufficient electrophilicity to yield 2-aryl-3,l-benzoxazin+-ones 480 (Equation 53). Both substituents of the C=N bond affected the course of the reaction considerably the best yields were achieved by using imines with electron-rich or neutral aryl groups on the carbon, and benzyl or nonbranched chain alkyl substituents on the nitrogen atom. With substituted derivatives of 478, the unsymmetrically substituted arynes led to regioisomeric products <2006JA9308>. 

The aryne intermediate is usually written with a triple bond and a delocalized aromatic system, as shown in 3-44. The anion in the side chain reacts as a nucleophile with the electrophilic aryne. The resulting anion, 3-45, can remove a proton from ammonia to give 3-46. Because the product has been reached, we usually stop writing the reaction mechanism at this point. However, in the reaction mixture, amide will remove a proton from the carbon a to the cyano group of 3-46. Only during workup will the anion be protonated to give back 3-46. 

Insertion into Carbon-Carbon o-Bonds. First examples of this concept were reported with anionic nucleophiles in the context of the addition of a-lithioalkyl and a-lithioarylacetonitriles to arynes. In 1984, Meyers and Pansegrau proposed a cyclization rearrangement mechanism to account for the formation of products 77 in the reaction of a-lithioacetonitriles to 3-oxazolylbenzyne. Initial attack of the nucleophile takes place at C-2 probably due to chelation of the lithium atom from the lithiated nitrile to the oxazoline moiety (Scheme 12.40) [67]. 

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