Exclusive Formation of Bridge-Substituted [2.2]Paracyclophane by Topochemical Photocycloaddition Reaction of Unsymmetrical Substituted p-Quinodimethane

Abstract

Unsymmetrical 7-(2′-bromoethoxycarbonyl)-7,8,8-tris­(methoxycarbonyl)-p-quinodimethane (2) underwent a quantitative intermolecular [6 + 6] photocycloaddition reaction through a single crystal-to-single crystal transformation to afford a bridge-substituted [2.2]­paracyclophane (3). The crystal structure of 2 indicates that the bromoethoxy groups conveniently form σ-halogen bonds with the carboxyl groups to yield 2-fold helical assemblies of an isolated pair of 2. 3 has a relatively long distance between bridged carbon–carbon bonds in comparison with the known ones. Such bonds caused an one-side insertion reaction of molecular oxygen in solution to afford the peroxide bridge-substituted [2.2]­paracyclophane in a quantitative yield.

Computational and Experimental Confirmation of the Diradical Character of para-Quinonedimethide

“…The contrasting cyclization behavior of p -QDM versus TCNQ and TEQ reflects the additional steric strain in the cyclodimers of these substituted compounds, 20 and 21 , respectively. Figure shows the optimized geometries of the respective cyclodimers where, for example, the (sp 3 )­C–C­(sp 3 ) bond is considerably longer (1.7 Å versus 1.6 Å) in the substituted derivatives. , While compounds with even longer C–C bonds are known, compounds 20 and 21 are likely to be less kinetically stable since they carry not one but two such bonds, and they also lack the structural features to hold these long bonds together . Noncovalent interaction (NCI) analyses of the cyclodimers (Figure ) reveal additional repulsive interactions involving the cyano (TCNQ) and ethynyl (TEQ) substituents.…”

“…23,24 While compounds with even longer C−C bonds are known, 25 compounds 20 and 21 are likely to be less kinetically stable since they carry not one but two such bonds, and they also lack the structural features to hold these long bonds together. 26 Noncovalent interaction (NCI) analyses of the cyclodimers (Figure 5) reveal additional repulsive interactions involving the cyano (TCNQ) and ethynyl (TEQ) substituents. This steric strain also contributes to the significantly reduced thermodynamic favorability of their dimerization reactions and their reactions with TEMPO.…”

A highly substituted pyrazinophane generated from a quinoidal system via a cascade reaction

“…Cyclophanes are a class of compounds containing at least one aromatic ring incorporated into a larger macrocycle. [1][2][3][4][5][6][7][8][9] Perhaps the most well-known and industrially-useful members of this class of compounds are the [2.2]paracyclophanes. [2.2]Paracyclophanes and their derivatives frequently exhibit unique reactivity and electronic properties (Scheme 1a).…”

“…[2.2]Paracyclophanes and their derivatives frequently exhibit unique reactivity and electronic properties (Scheme 1a). [4][5][6][7][8][9][10] The unique properties of [2.2]paracyclophanes can mostly be attributed to the instability of the strained transannular C-C bonds holding their two rings together, their bent aryl rings, and the intraannular π-electron density exhibited in this family of molecules. [6][7][8][9][10][11] Many [2.2]paracyclophane derivatives can be thermolyzed in the vapor-phase, yielding substituted paraquinodimethane, a compound that spontaneously polymerizes upon condensation to give industrially useful polymers called parylenes (Scheme 1a).…”

“…[6][7][8][9][10][11] Many [2.2]paracyclophane derivatives can be thermolyzed in the vapor-phase, yielding substituted paraquinodimethane, a compound that spontaneously polymerizes upon condensation to give industrially useful polymers called parylenes (Scheme 1a). [1][2][3][4][5][6][7][8][13][14][15][16] Two isomeric forms of [2.2](2,5)pyrazinophane-a nitrogenous paracyclophane-have also been prepared and polymerized by the above method to yield nitrogen-substituted parylenes (Scheme 1a,b). 2,[7][8][9]17 Additionally, [2.2]paracyclophanes have been of recent interest to the organic electronics research community as models of intramolecular charge transfer.…”