W.H. Laarhoven

Photodehydrocyclizations in stilbene-like compounds—III : Effect of steric factors

Abstract Photodehydrocyclizations in stilbene-like compounds take place only if the sum of the free valence numbers of the atoms concerned in the cyclization step exceeds in the excited state, a value of 1. If more than one cyclization is possible the photoconversion follows the pathway for which ΣF* is maximal; if ΔΣF* for various cyclizations is smaller than 0·1 a second product is sometimes formed. These rules also bold if only non planar and no planar products can arise. If planar as well as non planar compounds can be formed the main product is as a rule a planar polycyclic aromatic, even if ΣF* for its formation is lower than for other cyclizations. Sometimes but not always, a non planar product (with higher ΣF*) is found as a side product. The applicability of these rules in the planning of synthetic procedures for polycyclic aromatics is illustrated in the preparation of several new compounds (e.g. the four isomeric benzo hexahelicenes).Abstract Photodehydrocyclizations in stilbene-like compounds take place only if the sum of the free valence numbers of the atoms concerned in the cyclization step exceeds in the excited state, a value of 1. If more than one cyclization is possible the photoconversion follows the pathway for which ΣF* is maximal; if ΔΣF* for various cyclizations is smaller than 0·1 a second product is sometimes formed. These rules also bold if only non planar and no planar products can arise. If planar as well as non planar compounds can be formed the main product is as a rule a planar polycyclic aromatic, even if ΣF* for its formation is lower than for other cyclizations. Sometimes but not always, a non planar product (with higher ΣF*) is found as a side product. The applicability of these rules in the planning of synthetic procedures for polycyclic aromatics is illustrated in the preparation of several new compounds (e.g. the four isomeric benzo hexahelicenes).

Photodehydrocyclizations in stilbene-like compounds—II : Photochemistry of distyrylbenzenes

Abstract The photochemistry of distyrylbenzenes has been reinvestigated. It appears that formation of 1,12-benzperylene from the para isomer and picene from the ortho compound occur via an indirect route; the relevant cyclizations take place only in dimers of the parent compounds. Other photoproducts from ortho distyrylbenzene are stilbene and phenanthrene, probably arising via 2,3-diphenyl-2,3-dihydronaphthalene. These results show that the photoreactivity of the three isomers is entirely in accordance with the general rules previously formulated for photodehydrocyclizations in stilbene-like compounds.

The thermal racemization of methyl-substituted hexahelicenes

Abstract Rates of racemization of several Me-substituted hexahelicenes have been measured. It appeared that Me groups at the positions 3, 4, 13 and 14 have no influence; at the positions 2 and 15 the effect is small; Me-substitution at C(1) and C(16) leads, however, to a large increase of the free energy of activation. From an analysis of the data the suggestion is made that the conformation of highest energy during racemisation of 1-methyl- and 1,16-dimethylhexahelicene (and possibly also of hepta and octahelicene) is that with the terminal rings in orthogonal position.

Conformational studies on helicenes—V : Alteration of the conformation of helicenes by annelation of benzo groups☆

Abstract Analysis of the NMR spectra of five heptahelicenes and some related compounds reveals that annelation of heptahelicene by one or more benzo groups results in a conformational change of the helix. The extent and direction of this change was deduced correctly from the NMR spectra by application only of the ring current theory and the influence of Van der Waals forces, as could be established from the X-ray analyses of heptahelicene and the tribenzo derivative 12.

Conformational studies on hexahelicenes—II: Synthesis and spectral properties of alkyl hexahelicenes

Abstract The synthesis of eight alkyl substituted hexahelicene derivatives by photodehydrocyclization is described. Analysis of their NMR and UV spectra reveals that the conformation of the helix in hexahelicene is not noticeably disturbed by the introduction of substituents as large as t-butyl or p-tolyl at the 2 (or 3) position. Substitution at C1, at least with larger substituents (t-Bu) causes bending of the alkyl residue introduced or torsion of the substituted ring. The change in conformation is apparently necessary to alleviate steric crowding. The results conform better with the hexahelicene model of Kitaigorodsky than with that of Herraez.

Chirality and conformational changes in 4-phenylphenanthrenes and 1-phenylbenzo[c]phenanthrene derivatives

Abstract NMR data of several 4-phenylphenanthrenes (15, 16) have revealed that the crowding in these compounds does not lead to chirality at temperatures as low as −90°. The easy rotation of the phenyl substituent observed by NMR implies that notwithstanding the phenanthrene moiety in average behaves as a planar part the phenyl group does not experience steric hindrance. The analysis of temperature-dependent NMR spectra of several derivatives of 1-phenylbenzo[c]phenanthrenes (17-20) indicated that in these compounds exchange processes do occur. By calculations of the free energies of activation from the NMR data two processes could be distinguished: rotation of the phenyl substituent at one side of the helical benzo[c]phenanthrene moiety, for which ΔGXXXrot , is ca. 13.0 kcal mol or slightly larger when bulky substituents are present at C2, and racemisation by a rotation of the phenyl group around the opposite end of the benzo[c]phenanthrene skeleton with simultaneous inversion of the helical conformation. For this process ΔGXXXrac is ca. 16 kcal mol . The results have been compared with comparable data of related compounds like 1.8-diphenylnaphthalene, hexahelicene, and 4-methylbenzo[c] phenanthrenes, and gave evidence for the remarkably small, space-demanding properties of the phenyl substituent in these compounds.

Photodehydrocyclizations of stilbene-like compounds—XI : Synthesis and racemization of the double helicene diphenanthro[4.3-a; 3′.4′-o]picene

Abstract The synthesis of a new double helicene, diphenanthro[4.3-a;3′.4′-o]picene, is described. Of the two possible diastereoisomers only one was obtained as a racemate. The compound was partly resolved with TAPA yielding fractions enriched with one or the other enantiomer. They could not be converted into the other diastereomer by heating, but racemized under these conditions at a relatively low temperature and with a rate comparable to that of the racemization of hexahelicene at the same temperature. The NMR spectrum is discussed.

Conformational studies on hexahelicenes—IV : The conformation of 1,16-dimethylhexahelicene

Abstract From the analysis of NMR data of several mono- and dimethylhexahelicenes it has been concluded that the conformation of hexahelicenes substituted at C(1) or C(3) does not alter when a second substituent is introduced in a non-hindering position (C14) of the other terminal ring. From the NMR spectrum of 1,16-dimethylhexahelicene and the known structure of hexahelicene it was possible to give a general description of the conformational changes in the helix due to methyl groups in the overcrowded region (C1 and C16). The proposed conformation has been fully substantiated by an X-ray analysis of 1,16-dimethylhexahelicene.

Conformational studies on hexahelicenes-III - Synthesis and spectroscopic data of meso- and rac.-2,2'-Bis hexahelicyl

Abstract 2,2′-Bis-hexahelicyl has been synthesized and separated into meso and racemic forms. Analysis of the NMR spectra of both forms in CS2 as well as AsCl3 revealed that the central biphenylic part of the meso compound is rather planar, whereas in the racemic compound both halves of the molecule have been twisted around the central bond.

Chirality and conformational changes in 4,5-diaryltriphenylenes

Abstract Line-shape analysis of temperature dependent NMR spectra of several substituted 4,5-diphenyl-triphenylenes has been performed to determine the free energy of activation for rotation (ΔGrot*) of the phenyl groups. The rotational barrier (ΔGrot*) depends on the presence and position of substituents on the phenyl groups; it is the largest in compounds with ortho-substituents. The independent determined free energy of activation of racemization (ΔGrac*) is about equal to ΔGrot* in 4-phenyl-5-(3,5-dimethylphenyl)triphenylene, but in 4,5-bis-(3,5- dimethylphenyl) triphenylene ΔGrac* is much larger than ΔGrot*. It is concluded that the racemization does not occur via a process in which the phenyl groups remain parallel but via a molecular movement in which the phenyl groups turn around each other like cog wheels.