Aromaticity and electron affinity of Carbo(k)-[3]radialenes, k=0, 1, 2

Aromaticity enhancement is a possible driving force for the low reduction potentials of buta-1,3-diynediyl-expanded [N]radialenes: this hypothesis is theoretically analyzed for the expanded [3]radialene prototype. This study is undertaken within a more general prospect, namely the evaluation of the variation of aromaticity with endocyclic and peripheral carbomeric expansion of [3]radialene and its mono- and dianions. The structures, denoted as [C-H](6) (h)[C-C](3) (k)carbo-[3]radialene(q) (h=0, 1; k=0, 1, 2; q=0, -1, -2), were optimized in relevant singlet, doublet, or triplet spin states at the B3PW91/6-31G** level. They were found to be all planar. The structural aromaticity was measured through the average bond length d(av) over the [C-C](3) (k)carbo-[3]radialene core, and by the corresponding bond-length equalization parameter sigma(d), related to Krygowski's GEO. The magnetic aromaticity was measured by Schleyer's NICS values at the center of the rings. Regarding the relative variation of NICS and sigma(d), two classes of species can be distinguished according to their endocyclic expansion level. The species with a nonexpanded (k=0) or doubly expanded (k=2) ring constitute the first class: they exhibit D(3h) symmetry and a strong correlation of NICS with sigma(d). The species with a singly expanded ring (k=1) fall far from the correlation line, and constitute the second class. This class distinction is related to the degeneracy scheme of the frontier orbitals of the neutral representative. A finer appraisal of the electron (de)localization is brought by the ELF (Electron Localization Function) analysis of the electron density. It allows for a weighting of relevant resonance forms. Unsubstituted species are well described by the superimposition of two or three resonance forms. For (doublet spin state) monoanionic species, their respective weights are validated by comparison with AIM spin density. The weighted mean, n, of the formal numbers of paired pi(z) electrons in the resonance forms was calculated and compared with the closest even integer of either forms 4m+2 or 4m. A density-based continuous generalization of the orbital-based discrete Hückel rule is then heuristically proposed through an analytical correlation of NICS versus sigma(d), n, and S, the spin of the species. The frontier-orbital-degeneracy pattern of neutral species is discussed with respect to structural and magnetic aromaticity criteria. A decreasing HOMO-LUMO gap versus endocyclic expansion is obtained, but [C-C](3) (1)carbo-[3]radialene possesses the highest HOMO and LUMO energies. Vertical and adiabatic electron affinities of neutral and monoanionic species were also computed and compared with related experimental data.     

Synthesis and Derivatization of Expanded [n]Radialenes (n=3, 4)

Versatile, iterative synthetic protocols to form expanded [n]radialenes have been developed (n=3 and 4), which allow for a variety of groups to be placed around the periphery of the macrocyclic framework. The successful use of the Sonogashira cross‐coupling reaction to complete the final ring closure demonstrates the ability of this reaction to tolerate significant ring strain while producing moderate to excellent product yields. The resulting radialenes show good stability under normal laboratory conditions in spite of their strained, cyclic structures. The physical and electronic characteristics of the macrocycles have been documented by UV‐visible spectroscopy, electrochemical methods, and X‐ray crystallography (four derivatives), and these studies provide insight into the properties of these compounds as a function of pendent substitution in terms of conjugation and donor/acceptor functionalization.     

On the lack of ring-current aromaticity of (heteroatom) [N]radialenes and their dianions

Current-density maps, calculated at the ab initio RHF//6-31G**/CTOCD-DZ level, show no significant pi ring current in planar equilateral geometries of neutral and dianionic [N]radialenes, oxocarbons and thiocarbons C(N)Y(N) (q-) (Y=CH(2), O, S; N=4, 5, 6; q=0 (1 a-12 a), 2 (1 b-12 b)). Only the N=3 deltate dianions C(3)Y(3) (2-) (Y=CH(2), O, S (1 b, 5 b and 9 b)) have discernible pi ring current, and then with at most 20-25 % of the strength of the standard benzene current. On the magnetic criterion, lack of current is definitive evidence against aromaticity. Pictorial molecular-orbital analysis within the ipsocentric approach shows this to be an inevitable consequence of the nodal structure of the pi and pi* orbitals of [N]radialene-like systems. On grounds of angular-momentum symmetry, spatial distribution, or both, the HOMO-LUMO excitation does not contribute a significant central diamagnetic ring current.     

Radialenes are minimally conjugated cyclic π-systems

Conjugation energy (CE) in benzene is larger than its aromatic stabilisation energy (ASE). A far-reaching conclusion offered by this work is that per π-electron, CE is energetically larger than aromaticity. If a diene has a doubly degenerate HOMO, then its Diels–Alder reaction will be kinetically faster than a similar diene with a nondegenerate HOMO. The topological conjugation energy (TCE) for the radialene, monocyclic, dendralene, and linear polyene series has quite different trends. Radialenes are minimally conjugated cyclic systems with the TCE/No. π-bond = 0.432 β; the members of the dendralene series approach this same value from smaller values with increasing size. With increasing size, the members of the monocyclic and linear polyene series have, respectively, decreasing and increasing TCE/No. π-bond values approaching 0.547 β. Topological resonance energy (TRE) for radialenes, dendralenes, and linear polyenes all have TRE = 0, and the TRE/π-electron for monocyclic polyenes has alternating declining values between antiaromatic (?0.3066 β, ?0.07435 β, ?0.03287 β,?…) and aromatic (0.04543 β, 0.01594 β, 0.00807 β,?…). For benzene, TRE/No. π-bond = 0.0909 β and TCE/No. π-bond = 0.576 β.     

Expanded radialenes with bicyclo[4.3.1]decatriene units: new precursors to cyclo[n]carbons

A new method for the formation of conjugated polyynes has been developed based on both the rearrangement of vinylidenes to alkynes and the [2+1] cheletropic fragmentation of dialkynylmethylenebicyclo[4.3.1]deca-1,3,5-triene derivatives. A model study of the photolysis of simple dialkynylmethylenebicyclo[4.3.1]deca-1,3,5-trienes resulted in cheletropic fragmentation followed by 1,2-migration to give the corresponding linear polyynes, although undesired isomerization to methylenebicyclo[5.3.0]triene derivatives took place concurrently. Expanded [3]-, [4]-, [5]-, and [6]radialene derivatives with exocyclic bicyclo[4.3.1]decatriene units were prepared by oxidative coupling of the monomeric units as precursors to the corresponding cyclo[n]carbons, monocyclic forms of carbon clusters. The spectroscopic properties of the expanded radialenes were investigated in connection with cross conjugation of the core pi system and with its perturbation by the extraannular bicyclic pi system. In negative-mode laser-desorption time-of-flight (LD-TOF) mass spectra, the expanded radialenes exhibited peaks due to the corresponding cyclo[n]carbon anions (n = 18, 24, 30, and 36) formed by the stepwise loss of the aromatic indane fragments.