Influence of Hydroxyl Substitution on Benzyne Properties. Quantum Chemical Characterization of the Didehydrophenols

Abstract

Geometries and singlet-triplet splittings for the 10 geometrical isomers of didehydrophenol are characterized at a variety of levels of electronic structure theory. The influence of the hydroxyl group is primarily to increase/decrease the weight of zwitterionic singlet mesomers that place positive/negative charge adjacent to oxygen in valence bond descriptions of the arynes. For some of the meta isomers, this interaction stabilizes distortion in the direction of a bicyclic geometry. The net effect, relative to the unsubstituted benzynes, is to increase the singlet-triplet splittings in 2,3-, 2,6-, and 3,5-didehydrophenol and to decrease that splitting in 2,4- and 2,5-didehydrophenol (3,4-didehydrophenol is essentially unaffected). As shown for other arynes, the singlet-triplet splittings can also be accurately estimated by correlation with proton hyperfine coupling constants in antecedent monoradicals, these values being accessible from very economical calculations.

The Cyclization of Parent and Cyclic Hexa-1,3-dien-5-ynes—A Combined Theoretical and Experimental Study

“…Hence, this makes the use of DFT for biradicals a case-to-case decision. [20,47,51,52] Based on our and others recent experience, the choice of functional/basis set combination used in the present work seems, therefore, well justified. We would like to point out that feasible multireference treatments (CASSCF(8,8)/6 ± 31G* and CASMP2(8,8)/6 ± 31G*) are not significantly better in describing these types of reactions.…”

Properties and Reactivity of Gaseous Distonic Radical Ions with Aryl Radical Sites

“…153,244 Substitution of 33 with a hydroxyl group results in a slight decrease in the calculated dehydrocarbon atom distance ( 53 and 55 ), while no change was observed for 54 (Figure 17). The C-O bond in 53 and 55 was calculated to be shorter than that of phenol (1.381 Å), while no real effect was found on the C-O bond length in 54 .…”

“…The C-O bond in 53 and 55 was calculated to be shorter than that of phenol (1.381 Å), while no real effect was found on the C-O bond length in 54 . 244 The shortening of the C-O bond in 53 was proposed to arise from stabilizing π-interactions between the oxygen lone pair and the cyclopropenium cation moiety in the zwitter ionic resonance structure of the bicyclic structure ( 33c , Chart 5; Scheme 13), while the C-O bond shortening in 55 arises from stabilizing inductive interactions between the electron-withdrawing oxygen atom and the allyl anion moiety. Incorporation of a neutral or protonated nitrogen atom into the aromatic ring affects all of the bond lengths and bond angles of 33 .…”

Does the 2,6‐didehydropyridinium cation exist?

“…The discovery that the DNA-cleaving ability of the enediyne family of naturally occurring compounds is associated with in situ formation of aromatic biradicals (often para-benzyne derivatives) has led to the publication of a large number of theoretical and experimental studies on organic biradicals since the 1990s. [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18] Some of the studies, aimed at facilitating the design of novel DNA-cleaving anticancer drugs based on formation of biradicals, were devoted to better understanding of the factors controlling the reactivity of aromatic biradicals. [3][4][5] For example, several experimental and theoretical studies have focused on metabenzynes and the possibility of tuning their properties for specific applications.…”