Synthetic Application of Arynes

 

Introduction

Arynes are reactive intermediates derived from substituted arenes which participate in some aromatic nucleophilic substitutions and some cycloaddition reactions. It is non isolable very reactive species with the hexagonal planar ring structure with six pi delocalizing and 2 additional pi electrons. The additional 2 pi electrons do not affect the aromatic character of ring as it does not interfere with the Huckel number. Arynes are so highly reactive that they exist as stable species only at very low temperatures, e.g., 8 K for benzyne. Arynes may be derived from substituted benzenes, fused arenes and pyridines.

Structure of Benzyne

The sp² orbitals involved in the ‘triple bond’ are at 90^o to the p-orbitals of the aromatic pi-system Instead of an overlap between two 2p orbitals (as in an alkyne) the “triple bond” is formedthrough overlap of two adjacent sp2 orbitals in the plane of the ring (i.e., at right angles to, and completely independently of, the aromatic pi system poor overlap leads to a very weak triple bond, easily broken by a nucleophile.

Synthetic Application of Arynes

Synthesis of substitution Phenanthrene

Reactions of substituted phenyl acetylene with benzyne yields substituted phenanthrene.

Synthesis of Aporphine Alkaloid

3,4-dimethyoxy acetylene reacts with benzyne, followed by H-migration to give the dimethoxyphenanthere. Similarly, 3-methoxybenzyne undergoes D-A reaction with aryl alkene to give aporphine alkaloids.

 

Synthesis of Xanthene derivative

Aryl trifolone can be prepared by inserting aryne into R-SO₂CF₃

Synthesis of Taxodione

Taxodione was synthesized by [2+2] cycloaddition between an aryne and di-methoxyethylene 

Synthesis of Ellipticine

Antitumor alkaloid Ellipticine is synthesized by Diel’s alder addition to 3,4 pyridyne.

Synthesis of cyclobutene

Benzyne react with wide range of olefin to give [2+2] cycloaddition products.

Synthesis of aryl triflones

Aryl triflone can be prepared from arynes by inserting aryne into C-SO₂F₃, this reaction proceed via addition of C-SO₂ F₃ containing nucleophiles.

Synthesis of Lysergic acid N,N-diethylamide precursor

References                                                                                                                      .     Aten, C. F., and Greene, E. F. (1961). Combuetim & Flame 5,55.

      Berry, R. S., Spokes, G. N., and Stiles, M. (1962). J. Am. Chem. SOC. 84, 3570

      Bradley, J. N., and Kistiakowsky, G. B. (1961). J. Chem. Phye. 35,264.

      Fields, E. K., & Meyerson, S. (1968). Mechanisms of formation and reactions of arynes at high temperatures. Advances in Physical Organic Chemistry, 6, 1-61.