The absorption and circular dichroism spectra of chiral triquinacenes

Abstract

The three deuterated, optically active 2,3-dihydrotriquinacenes lla-c of known absolute configuration have been prepared from (+)-( 15)-2,3-dihydrotriquinacen-2-one (4). The optically active monodeuterated triquinacene 12 was obtained from the same precursor and (-)-(lS)-2-methyltriquinacene ( 15) from (-)-triquinacene-2-carboxylic acid. The chirality in 11 and 12 is due solely to isotopic substitution. Although the three dihydrotriquinacenes have CD effects smaller than could be measured, presumably because of the distance between the deuterium atoms and the chromophore, the absorption and circular dichroism spectra of 12 and 15 proved very revealing. The contributions of the composite double bonds and peripheral substituents to the observed Cotton effects are analyzed in a manner which is consistent with the observed transitions.The effect of deuterium as an isolated perturber on the electric dipole forbidden n -* * transitions of chiral deuterio ketones has recently been examined intensively and with considerable success.* 1 Simultaneously, there have appeared a smaller number of reports dealing with experimental assessment of the extent to which chiral isotopic perturbation is seen in the weaker electric dipole allowed -* ir* transitions of conformationally rigid olefins.2-4 The placement of a C-D bond at various positions in relatively inflexible ketones generates chiroptical contributions which are dissignate (antioctant). The same effect has been observed in (1S)-(1 -2 3H)apobornene (1) but not in 2 where the octant con-2 D tribution is consígnate.4 Such net contributions of deuterium substituents to the rotary strength have been attributed to shorter C-D bond lengths, effectively smaller size, and reduced polarizability relative to their C-H counterparts.4 Thus, a change in steric environment alone is presently considered capable of reversing the observed isotope-induced chiral perturbation.Because new and different chiral molecular systems can be expected to constitute interesting test cases for advanced models of chirality function, we were led to examine the absorption and CD spectra of optically active {\R,4SJR, 10S)-(2-2H)triquinacene,5 the related methyl derivative, and several 2,3-di-

An Experimental Thermochemical and Theoretical Study of Triquinacene: Definitive Disproof of Its Neutral Homoaromaticity

“…While homoaromaticity clearly is present in ionic systems, such stabilization, if at all detectable, is often considered to be very small in neutral hydrocarbons. , Spectroscopic (IR, UV, CD, PES) and structural data, as well as the calculated overlap integral of nonbonded interaction, offer no evidence of homoaromatic character in 1 . We determined for triquinacene ( 1 ) for the first time by measuring its energy of combustion.…”

Expansion of Thiele’s Acid Chemistry in Pursuit of a Suite of Conformationally Constrained Scaffolds

“…The second obstacle to the use of Thiele’s acid or esters in the applications described above is that a dearth of general reaction chemistry has been reported for this system, although the photochemically promoted intramolecular [2 + 2] cycloaddition of 4a was shown by Dunn and Donohue to afford the corresponding cyclobutane, which was subsequently utilized by Marchand in the synthesis of a larger cage structure . In addition, Deslongchamps and co-workers converted the diacid 3 to the corresponding ketone, en route to a synthesis of triquinacene; the route was later repeated by the Paquette group to prepare a series of triquinacene derivatives for absorption and circular dichroism studies . Notwithstanding these seminal contributions, however, very little (nonphotochemical) reactivity has been described for the two alkenes 3 or 4 , aside from Thiele’s initial description of perbromination of 3 and Peters’ report of hydrogenations and epoxidations occurring at both alkenes of 4 .…”

“…28 In addition, Deslongchamps and co-workers converted the diacid 3 to the corresponding ketone, en route to a synthesis of triquinacene; 29 the route was later repeated by the Paquette group to prepare a series of triquinacene derivatives for absorption and circular dichroism studies. 30 Notwithstanding these seminal contributions, however, very little (nonphotochemical) reactivity has been described for the two alkenes 3 or 4, aside from Thiele's initial description of perbromination of 3 1 and Peters' report of hydrogenations and epoxidations occurring at both alkenes of 4. 31 However, in neither of these cases were the structures of the products established.…”

A theoretical study of the chiroptical properties of molecules with isotopically engendered chirality

“…10,11,[21][22][23][24][25][26][27][28][29] Furthermore, the experimental observations have been rationalized with a different polarizability/refractivity of the isotope 9,21,23,24 or a different electronegativity. 25 So far, quantum-mechanical calculations on semiempirical 21,25,26,28 and ab initio 27 levels of theory and also model calculations 9,22 have been reported in literature which at least qualitatively support the different explanations mentioned above. However, up to now no conclusive answer has been given about the dominating effect responsible for the experimental observations.…”