Steven M. Bachrach

Following the recent preparation of infinitene (J. Am. Chem. Soc. 2022, 144, 862-871), a computational (ωB97XD/6-311G(d)) exploration of 42 isomeric compounds with 12 fused phenyl rings identified structures with linking number of zero (ring, saddle, and ribbon shapes), two (infinitene-like shape), and one (Möbius infinitene shape) is reported. An infinitene isomer composed of two [5]helicene fragments connected to two stacked phenyl rings and a Möbius infinitene isomer are identified that are more stable than the known infinitene. The energies of the structures are examined by assessing their macrocyclization (strain) energies, π-stacking, and possible aromaticity. Examples of fused phenyl molecules with linking numbers of 3, 4, 5, and 6 are shown, indicating the potential topological range that these molecules can possess.

Following the recent preparation of infinitene (J. Am. Chem. Soc. 2022, 144, 862-871), a computational (ωB97XD/6-311G(d)) exploration of 42 isomeric compounds with 12 fused phenyl rings identified structures with linking number of zero (ring, saddle, and ribbon shapes), two (infinitene-like shape), and one (Möbius infinitene shape) is reported. An infinitene isomer composed of two [5]helicene fragments connected to two stacked phenyl rings and a Möbius infinitene isomer are identified that are more stable than the known infinitene. The energies of the structures are examined by assessing their strain energies, π-stacking, and possible aromaticity. Examples of fused phenyl molecules with linking numbers of 3, 4, 5, and 6 are shown, indicating the potential topological range that these molecules can possess.

DFT computations (ωB97X-D/6-31G(d)) of eight different cycloparaphenylenes that have three internal linking groups are shown to have two different conformations that express interesting Möbius topologies. The D₃ conformers have linking numbers Lk of 3 (three half-twists as a trefoil), and the much more stable C₂ conformers have a single half-twist, Lk = 1.

A number of variants of the recently synthesized cycloparaphenylene lemniscates G have been explored at ωB97X-D/6-311G(d). Lemniscates with from four to eight phenyl rings in each loop having the N-N, C=C or B-N bond as the linkage all exhibit the figure-8 shape. Their ring strain energies (RSE) are relatively independent of the nature of the linkage bond, but the RSE does systematically decrease with increasing size of the loop. Selective nitrogen substitution into the ortho positions of the aromatic rings allows for the rings to become coplanar; this manifests in a more ribbon-like structure of the lemniscates. It also reduces the RSE of the lemniscates. Similarly, nitrogen substitution into the recently synthesized cycloparaphenylene trefoil E creates a more ribbon-like structure with reduced RSE.

Density functional theory and extrapolated CCSD(T) computations of several “anti‐Bredt” alkenes were carried to explore possible 1,2‐diyl “alkene” candidates with a triplet ground state. Ten candidates containing twisted double bonds at the bridgehead positions of bicyclic structures (1‐6) or adamantene (7‐10) derivatives were studied. Based on a combination of ring strain, rigid scaffolding, and steric crowding, four species were identified to have surprisingly low singlet‐triplet energy gaps (lower than 4 kcal/mol). A tert‐butyl substituted bicyclic structure (4) was identified to have a near‐zero singlet‐triplet energy gap, but no triplet ground‐state alkene was found. Ring strain energy (RSE) calculations, π‐orbital axis vector (POAV) analyses, and multiple linear regression models were performed to elucidate the geometric and energetic effects of double bond twisting in 1‐10. Based on our computational exploration, it appears unlikely that there is a ground‐state triplet olefin.

Nano-Saturn complexes with a planar horizontal ring can be constructed by selective nitrogen substitution into the ortho-positions in polyaryl macrocycles. A number of intriguing supermolecular complexes can be created with these planar or ribbon macrocycles, including nano-Saturns with a flat ring, nano-Saturns with an encircling belt, and nanotubes encircled by belts or rings. These systems are investigated using DFT (B3LYP-D3/6-31G(d)), which predicts their structures and finds them to have significant binding enthalpies.

Extended hydrogen bond networks can substantially increase basicity through stabilization of the resultant conjugate acid. In this study, the chiral superbases (9-11) having anilinyl substituents attached to the prototype superbase 1,8-bis(dimethylamino)naphthalene 1 are examined using DFT methods (PBE1PBE/6-311+G(d,p). While in the gas phase, these are more basic than 1, in THF solution they are slightly weaker bases than 1. While 9-11 are chiral, their rotational barrier about the Caryl-Caryl bond is about 20 kcal mol-1. Aminonaphthyl substituents (18-20) increase the rotational barrier by about 10 kcal mol-1. As a proof-of-concept, deprotonation of propanal by the chiral bases 9-11 is the predicted to have ee’s ranging from 58% (with 9) to 95% (with 11). The deprotonation of 4-t-butylcyclohexanone and cyclohexanoxide by 9 are predicted to have modest (23%) and excellent (97%) ee’s, respectively.

A number of 1,3,1′,3′‐tetrasubstituted bisadamantylidenes are examined with 4 different density functional methods (BP86‐D3, ωB97X‐D, B3LYP, and B3LYP‐D3) and the 6‐311G(d) basis set. With increasing steric bulk, these substituted bisadamantylidenes become ever more twisted about the central carbon‐carbon double bond. This manifests in significantly reduced singlet‐triplet gaps from that of typical alkenes, with the tetra‐t‐butylbisadamantylidene 17, twisted to almost 90°, having a gap of only 2.2 kcal mol⁻¹. While its large strain energy may preclude the synthesis of 17, other less‐strained tetrasubstituted analogues are more feasible synthetic targets and still possess small singlet‐triplet gaps, which should be testable by variable temperature NMR and EPR spectroscopy. The twisted alkenes also result in a low rotational energy barrier, and 4 examples with a low barrier to cis‐trans isomerization are presented. These too should be testable by experiment.

Cycloparaphenylenes (CPP) can serve as both guest and host in a complex. Geometric analysis indicates that optimal binding occurs when the CPP nanohoop differ by five phenyl rings. Employing C-PCM(THF)/omegaB97X-D/6-31G(d) computations, we find that the strongest binding, with binding free energy as high as 40 kcal mol-1, does occur when the host and guest differ by five phenyl rings. For CPP pairs that are not optimal in size difference, strong binding is often still found. The guest CPP is modestly inclined relative to the plane of the host CPP except when the host and guest differ by 4 phenyl rings, when the inclination angle is over 40deg. The distortion/interaction model shows that interaction dominates, and is best when the host and guest differ by five phenyl rings. The computed 1H NMR shifts of the guest CPP are shifted by about 1 ppm upfield relative to their position when unbound. This distinct chemical shift should aid in experimental detection of these CPP planetary orbit complexes.

Analogues of ExBox⁴⁺ 1 are proposed that possess triaryl fragments that are nearly flat. These two new hosts are predicted by density functional theory (ωB97X-D/6-311G(d,p)) to bind five small linear acenes more tightly than does 1. The “flatter” triaryl fragments provide a less congested interior along with improved π–π-stacking between these hosts and guests.