Large-ring acetylenes

The reaction of 1,2-dibromocycloalkenes (13) with magnesium in the presence of l,3-diphenylbenzo[c]furan (16) affords a cycloalkyne adduct (17). Also, the mercuric oxide oxidation of bishydrazone, which has been used extensively in the preparation of open-chain and medium or large cyclic acetylenes " , is adapted for small ring bishydrazones (18) . Treatment of 18 with mercuric oxide in the presence of 16 or of phenyl azide results in the adducts, 17 and 19, respectively. It seems... 

Intramolecular Friedel-Crafts-like acylation of a trimethylsilyl acetylene had previously been used to prepare unstrained large ring alkynes K. Ultimoto, M. Tanaka, M. Kitai, H. Nozaki, Titrahedron Lett. 1978, 2301-2304. 

In general, Rh catalysts are not very effective in stereoseleetively polymerizing sterically crowded monosubstituted acetylenes such as t rt-butylacetylene and ortho-substituted phenylacetylenes. Rh catalysts are also not capable of polymerizing disubstituted acetylenes. One exception is cyclooctyne, whose very large ring strain ( 38 kJ/mol) enables fast polymerization with [(nbd)RhCl]2, giving an insoluble polymer in good yield. ... 

Meier has provided an excellent review of medium- and large-ring cyclic acetylenes. Cycloheptyne and cyclo-octyne may be obtained by pyrolysis of (144 n = 7 or 8), which can be readily obtained by selenium dioxide oxidation of the corresponding semicarbazones the photochemistry of cyclo-octyne in the presence of molybdenum hexacarbonyl is discussed. Compound (145) reacts with sodium methoxide to give (146) the acetylene (147) is proposed as an intermediate. 

Hoger S (2005) Shape-persistent phenylene-acetylene macrocycles large rings-low yield Angew Chem Int Ed 44(25) 3806-3808... 

This stereoelectronic requirement would lead to a large distortion of the normal geometry of a five-membered ring and introduce strain. It is this distortion and strain that disfavor the 5-endo-trig cyclization. In contrast, 5-endo-dig cychzation is feasible because the acetylenic system provides an orbital that is available for a nearly planar mode of approach. 

Arynes present structural features of some interest. They clearly cannot be acetylenic in the usual sense as this would require enormous deformation of the benzene ring in order to accommodate the 180° bond angle required by the sp1 hybridised carbons in an alkyne (p. 9). It seems more likely that the delocalised 7i orbitals of the aromatic system are left largely untouched (aromatic stability thereby being conserved), and that the two available electrons are accommodated in the original sp2 hybrid orbitals (101) ... 

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