Two very crowded, twisted acenes with dibenzosuberene "wings", 19,20,21,22-tetraphenyl-5H,14H-bisdibenzo[3,4:6,7]cyclohepta[1,2-a:1 ', 2 '-c]naphthalene (7) and 19,28-Diphenyl-5H,14H-bisdibenzo[3,4:6,7]cyclohepta[1,2-a:1 ', 2 '-c]dibenzo[h,j]anthracene (8), were prepared by addition of crowded arynes to the known bis(dibenzosubereno)cyclopentadienone 6. Their X-ray structures showed the naphthalene and anthracene cores to have end-to-end twists of 32 degrees and 67 degrees, respectively. Compound 8 crystallized as a chiral conglomerate; thus, individual crystals contained only one enantiomer. The rate of racemization of 8 in solutions made from individual crystals was monitored by circular dichroism spectroscopy. Thermal racemization in the dark has a high barrier (t 1/2 = 6.5 h at 450 K; Delta G double dagger rac = 36 kcal/mol), but exposure to even ambient light yields rapid racemization at room temperature. Computational studies of possible mechanisms of racemization of 7 and 8 are also reported.

The 1H NMR spectrum of (R*)-4-[(S*)-1-methanesulfonyloxypropyl]-3-methoxy-1,3-oxazinane (3) is incomprehensible at room temperature due to line broadening from the dynamic exchange of many conformations. VT-NMR studies show two equal populations at low temperatures. As the temperature increases, they coalesce into a single population with a free energy of activation for this process of 14.0-14.6 kcal/mol. DFT studies indicate that each of the low-temperature populations is a collection of rapidly interconverting species, and the critical barrier to interconversion is an inversion at nitrogen in compound 3 with calculated barriers of 14.3-16.0 kcal/mol, depending on the computational method employed.

When 8-bromo-1,2,3,4,5,6,7-heptaphenyl-1,4-dihydro-1,4-epoxynaphthalene (5) is heated with copper powder and no solvent at 340 degrees C under argon, the chief product is 1,8-dibenzoyl-2,3,4,5,6,7,9,10-octaphenylphenanthrene (6) in yields as high as 68 %. The reaction appears to involve Ullmann coupling of 5 and expulsion of two molecules of diphenylacetylene to give a decaphenyl bis(isobenzofuran) which then undergoes intramolecular cyclization and fragmentation to yield phenanthrene 6.

The highly substituted naphthalenes 1,2,3,4,5,6,7-heptaphenylnaphthalene (13), 2,3,4,5,6,7,8-heptaphenyl-1-naphthol (12), 1-bromo-2,3,4,5,6,7,8-heptaphenylnaphthalene (4), and 1-(phenylethynyl)-2,3,4,5,6,7,8-heptaphenylnaphthalene (5) were prepared by a variety of methods, and all but 5 were crystallographically characterized. The attempted Ullmann coupling of 4 to give tetradecaphenyl-1,1'-binaphthyl (3), at both 270 °C and 350 °C, yielded instead 1,2,3,4,5,6-hexaphenylfluoranthene (17) via an intramolecular cyclization reaction. When the alkyne 5 was heated with tetracyclone (6) at 350 °C, 1-(pentaphenylphenyl)-2,3,4,5,6,7,8-heptaphenylnaphthalene (7) was formed in 3 % yield. However, greater amounts of 5,6,7,8,9,14-hexaphenyldibenzo[a,e]pyrene (20, 11 %) and 1,2,3,4,5,6,7-heptaphenylfluoranthene (21, 11 %) were produced, the former by intramolecular cyclization and dehydrogenation of 5 and the latter by an intramolecular Diels-Alder reaction of 5 followed by extrusion of acetylene. The X-ray structure of 7 shows it to be an exceptionally crowded biaryl, and the X-ray structure of 20 shows it to be a saddle shaped polycyclic aromatic hydrocarbon.

Decaphenylbiphenyl (1) and 2,2',4,4',6,6'-hexaphenylbiphenyl (2) are bulky molecules expected to be greatly destabilized by steric crowding. Herein, through a combined experimental and computational approach, we evaluate the molecular energetics of crowded biphenyls. This is complemented by the study of phase equilibria for 1 and 2. Compound 1 shows a rich phase behavior, displaying an unusual interconversion between two polymorphs. Surprisingly, the polymorph with distorted molecules of C1 symmetry is found to have the highest melting point and to be the one that is preferentially formed. The thermodynamic results also indicate that the polymorph displaying the more regular D2 molecular geometry has larger heat capacity and is probably the more stable at lower temperatures. The melting and sublimation data clearly reveal the weakening of cohesive forces in crowded biphenyls due to the lower molecular surface area. The experimental quantification of the intramolecular interactions in 1 and 2 indicated, using homodesmotic reactions, a molecular stabilization of about 30 kJ mol-1. We attribute the origin of this stabilization in both compounds to the existence of two parallel-displaced π⋯π interactions between the ortho-phenyl substituents on each side of the central biphenyl. Computational calculations with dispersion-corrected DFT methods underestimate the stabilization in 1, unless the steric crowding is well balanced in a homodesmotic scheme. This work demonstrates that London dispersion forces are important in crowded aromatic systems, making these molecules considerably more stable than previously thought.

In attempts to make octaphenyldibenzofuran ( 7 ) and octaphenyldibenzothiophene ( 8 ), 2,5‐dibromofuran ( 4 ) and 2,5‐dibromothiophene ( 5 ), respectively, were heated with tetracyclone ( 2 ) under forcing conditions, but only single addition products, such as 2‐bromo‐4,5,6,7‐tetraphenylbenzofuran ( 10 ) and 2‐bromo‐4,5,6,7‐tetraphenylbenzothiophene ( 12 ) were observed. However, when 2,5‐dibromothiophene‐1,1‐dioxide ( 6 ) was heated with tetracyclone, the chief product was 1,2,3,4,6,7,8‐heptaphenyltriphenylene ( 14 ). Similarly, when compound 6 was heated with acecyclone ( 15 ), the product was 11,18,20‐triphenyldiacenaphtho[ a,h ]triphenylene ( 16 ). Both 14 and 16 have been characterized by X‐ray crystallography. They are proposed to form from double Diels‐Alder addition products of the cyclopentadienones by extrusion of sulfur dioxide and rearrangement of the resulting radicals.

Tetradecaphenyl-p-terphenyl (2) was synthesized from 2,3,5,6-tetraphenyl-1,4-diiodobenzene (11) by two methods. Ullmann coupling of 11 with pentaphenyliodobenzene (9) gave compound 2 in 1.7 % yield, and Sonogashira coupling of 11 with phenylacetylene, followed by a double Diels-Alder reaction of the product diyne 12 with tetracyclone (6), gave 2 in 1.5 % overall yield. The latter reaction also gave the monoaddition product 4-(phenylethynyl)-2,2',3,3',4',5,5',6,6'-nonaphenylbiphenyl (13) in 4 % overall yield. The X-ray structures of compounds 2 and 13 show them to possess core aromatic rings distorted into shallow boat conformations. Density functional calculations indicate that these unusual structures are not the lowest energy conformations in the gas phase and may be the result of packing forces in the crystal. In addition, while uncorrected DFT calculations indicate that the strain energy in compound 2 is approximately 50 kcal/mol, dispersion-corrected DFT calculations suggest that it is essentially unstrained, due to compensating, favorable, intramolecular interactions of its many phenyl rings. An attempted synthesis of tetradecaphenyl-o-terphenyl (4) by reaction of diphenylhexatriyne (14) with three equivalents of tetracyclone at 350 °C gave only the diadduct 2-(phenylethynyl)-2',3,3',4,4',5,5',6,6'-nonaphenylbiphenyl (15) in 17 % yield. Even higher temperatures failed to produce 4 and lowered the yield of 15, perhaps due to rapid decomposition of the starting materials. Ullmann coupling of 3,4,5,6-tetraphenyl-1,2-diiodobenzene (16) and compound 9 also failed to give compound 4.

Metallodithiolene complexes of the type [(R2C2S2)M(η2-tpbz)] [R = CN, Ph, or p-anisyl; M = Ni2+, Pd2+, or Pt2+; tpbz = 1,2,4,5-tetrakis(diphenylphosphino)benzene] chelate transition metals ions to form trimetallic arrays [[(R2C2S2)M(tpbz)]2M']n+, where M' is square planar Pt2+, tetrahedral Cu+, Ag+, or Au+, or octahedral {ReBr(CO)}/{Re(CO)2}+. Forcing conditions (190 °C reflux in decalin, 72 h) are demanded for the Re+ compounds. With third-row metals at the nexus, the compounds are stable to air. Twelve members of the set have been characterized by X-ray diffraction and reveal dithiolene centroid-centroid distances ranging from 22.4 to 24.0 Å. Folding around each tpbz intrachelate P···P axis such that the MP2/M'P2 planes meet the tpbz P2C6P2 mean plane at non-zero values gives rise to core topologies that appear "S-like" or herringbone-like for M' = Pt2+ or {ReBr(CO)}/{Re(CO)2}+. Calculations reveal that departure from idealized D2h/D2d/C2v symmetries is induced by steric crowding between Ph groups and that dynamic, fluxional behavior is pertinent to the solution phase because multiple, lower-symmetry minima of comparable energy exist. Spectroscopically, the formation of the trimetallic arrays is marked by a shift of the open end 31P nuclear magnetic resonance signal from approximately -14.5 ppm to approximately +41, approximately +20.5, and approximately +28.5 ppm for M' = Pt2+, Au+, and {ReBr(CO)}/{Re(CO)2}+, respectively. Electrochemically, dithiolene-based oxidations are observed for the R = Ph and M' = Pt2+ or Au+ compounds but at potentials that are anodically shifted relative to charge-neutral [[(R2C2S2)M]2(μ-tpbz)]. The compounds reported clarify the possibilities for the synthesis of assemblies in which weakly coupled spins may be created in their modular (R2C2S2)M and M' parts.

Highly congested derivatives of biphenyl were prepared by double Diels-Alder reactions of cyclopentadienones with substituted butadiynes. The reaction of 2,3,5-tri(tert-butyl)cyclopentadienone (5) and diphenylbutadiyne (3) gave only the single adduct, 1-(phenylethynyl)-2-phenyl-3,5,6-tri-tert-butylbenzene (6), and even extreme conditions gave no second addition. When tetracyclone (4) was added to bis(trimethylsilyl)butadiyne (8), two additions were achieved, but one silyl group was lost either during, or immediately following, the second addition to give 2-(trimethylsilyl)-2′,3,3′,4,4′,5,5′,6-octaphenylbiphenyl (11). However, when 3,4-diphenyl-2,5-dimethylcyclopentadienone (12) was added to 8, the fully substituted 2,2′-bis(trimethylsilyl)-4,4′,5,5′-tetraphenyl-3,3′,6,6′-tetramethylbiphenyl (14) was formed. The X-ray structures of compounds 11 and 14 show them to be quite crowded, but the central biphenyl rings do not exhibit the distortions previously observed in decaphenylbiphenyl. In an alternative approach, arynes were added to 5,5′-bis(4-chlorophenyl)-3,3′,4,4′-tetraphenyl-2,2′-bis(cyclopentadienone) (18). Simple benzyne added twice to give 4,4′-bis(4-chlorophenyl)-2,2′,3,3′-tetraphenyl-1,1′-binaphthyl (19) in low yield, but tetraphenylbenzyne, generated from tetraphenylanthranilic acid, added only once.

We discovered a way to funnel high-frequency vibrational quanta rapidly and unidirectionally over large distances using oligo(p-phenylene) chains. After mid-IR photon photoexcitation of a —COOH end group, the excess energy is injected efficiently into the chain, forming vibrational wavepackets that propagate freely along the chain. The transport delivers high-energy vibrational quanta with a range of transport speeds reaching 8.6 km/s, which exceeds the speed of sound in common metals (∼5 km/s) and polymers (∼2 km/s). Efficiencies of energy injection into the chain and transport along the chain are found to be very high and dependent on the extent of conjugation across the structure. By tuning the degree of conjugation via electronic doping of the chain, the transport speed and efficiency can be controlled. The study opens avenues for developing materials with controllable energy transport properties for heat management, schemes with efficient energy delivery to hard-to-reach regions, including transport against thermal gradients, and ways for initiating chemical reactions remotely.

1,2,3,4,5,6,7,8-Octaphenylphenanthrene (4) and decaphenylphenanthrene (5) were prepared by very short syntheses (two or three steps) from tetraphenylfuran and polybrominated benzene derivatives. The X-ray structures of compounds 4 and 5 show them to be quite crowded, with the phenanthrene cores twisted by about 40° due to the clash of the C4 and C5 phenyl groups. Compound 4 was resolved by chromatography on a chiral support, and its free energy of activation for racemization was determined to be 24.6 kcal/mol at 40 °C. Computational studies indicate that compound 5 has a racemization barrier approximately 6 kcal/mol lower than 4, and thus 5 would not be configurationally stable at room temperature.

Dodecaphenyltetracene (4), the largest perphenylacene yet prepared, was synthesized from known tetraphenylfuran, hexaphenylisobenzofuran, and 1,2,4,5-tetrabromo-3,6-diphenylbenzene in three steps. The X-ray structure of the deep red, highly luminescent 4 shows it to be a D2 -symmetric molecule with an end-to-end twist of 97°. The central acene is encapsulated by the peripheral phenyl substituents, and as a result, the molecule is relatively unreactive and even displays reversible electrochemical oxidation and reduction.

1,2-Bis(pentaphenylphenyl)benzene (2) was synthesized by the cycloaddition of 1,2-bis(phenylethynyl) benzene and tetracyclone. Its X-ray structure was determined, and the molecule adopts a C-2-symmetric conformation in the crystal. Monomethoxy and dimethoxy derivatives of compound 2 were also prepared, and dynamic NMR studies of these compounds yielded a free energy of activation for racemization (Delta G(rac)(double dagger)) of 20.3 kcal/mol at 423 K. The results are compared with estimates of Delta G(rac)(double dagger) for 2 by various DFT methods. (C) 2019 Elsevier Ltd. All rights reserved.

Oxidation of benzo[1,2‐b:3,4‐b′:5,6‐b′′]trithiophene (1) with MCPBA at room temperature gives the corresponding monosulfone 3. This material readily undergoes a Diels–Alder dimerization with extrusion of SO2 to form the dihydroheterohelicene 5. This, in turn, is easily converted into the heterohelicene 6 in a one‐pot NBS bromination and elimination. In a similar manner, oxidation of phenanthro[9,10‐b]thiophene (12) gives the dihydroheterohelicene 13, and bromination/elimination forms the corresponding heterohelicene 8. The X‐ray structures of compounds 5, 6, and 13 are reported, as well as computational studies that illuminate the unusual regiochemical outcome of the dimerization reactions.

Two in,in-cyclophanes that contain methyl groups in their central cavities have been synthesized, and their X-ray structures have been determined. One of these molecules contains a very short nonbonded contact between a hydrogen atom and a methyl group, and the other is the first example of a macrobicyclic compound that contains two inwardly directed methyl groups.

Three cyclophanes with the formula (C28H31PS3)n, where n = 1 or 2, were isolated from the base‐promoted macrocyclization of tris(2‐mercaptophenyl)phosphine (4) and tris(3‐bromopropyl)methane (5). Spectroscopic and X‐ray analysis showed them to be the in,in‐isomer 1, the in,out‐isomer 2, and a dimeric in,out,in,out‐isomer 3. Compounds 1 and 3 display approximately C3‐symmetric and Ci‐symmetric structures, respectively, in the crystal, but compound 2 adopts two distinct low‐symmetry conformations in the solid state. In each case, the observed structures correspond to the lowest energy conformations of their respective isomers as calculated at the B3PW91/6‐31G(d) level of theory. The in,in‐isomer 1 displays a close contact between the phosphine in‐lone pair and the in‐methine group, with through‐space spin–spin coupling constants of JPH = 9 Hz and JPC = 24 Hz, the latter of which appears to be the largest reported phosphorus‐carbon through‐space coupling constant.

AbstractCondensation of 1,8,13‐tris(mercaptomethyl)triptycene and tris(bromomethyl)methane yields an in,in‐cyclophane with two inwardly directed methine groups. Based on X‐ray analysis and DFT and MP2 calculations, the hydrogen–hydrogen non‐bonded contact distance is estimated to be 1.50–1.53 Å. Furthermore, the two in‐hydrogen atoms show obvious spin–spin coupling with J=2.0 Hz.