Carbohelicenes and heterohelicenes

Laarhoven, W. H.; Prinsen, W. J. C.

doi:10.1007/3-540-13569-3_3

Cited by 265 publications

(81 citation statements)

References 165 publications

(90 reference statements)

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“…[16] Sterically induced deformations from planarity can be seen in many polycyclic aromatic hydrocarbons, [17,18] including peri-substituted acenes [19] and, most notably, the helicenes. [20] Also, there exists a plethora of stable nonplanar geometrically constrained aromatic molecules; this class is exemplified by the cyclophanes, [21] buckminsterfullerene, the semibuckminsterfullerenes, and corannulene. [22] In a recent review on the subject, Herndon examined 38 examples of crystal structures of unsubstituted acenes.…”

Section: Introductionmentioning

confidence: 99%

On the Nature of Nonplanarity in the [N]Phenylenes

et al. 1999

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The [N]phenylenes display marked deformation from planarity in the crystalline state. In order to probe the generality of this phenomenon, several derivatives were synthesized and their single-crystal X-ray structures were obtained. All new compounds displayed some degree of nonplanarity. Thus, for example, the parent triangular [4]phenylene (4 b) has a median bend angle at the ring junction of 1.58 and a range of 0.38 to 3.58, whereas hexakis[triisopropylsilyl(ethynyl)] triangular [4]phenylene (4 c) possesses the bulkiest appendages and the largest median bend angle and range (3.88 and 1.78 ± 5.68, respectively). A detailed analysis of the bending and twisting angles at the ring junctions, however, revealed that the magnitude of deformations were independent of topology, molecular size, and substituent type. In contrast to the phenylenes, a Cambridge Structural Database (CSD) search of unsubstituted and non-peri-substituted naphthalenes and anthracenes shows these molecules to be virtually planar in the solid state. A comparison of the single-point energies (HF/6-31G*) of the phenylenes with the acenes calculated for molecules possessing a fixed bend angle at the ring fusion of 38, 68, 98, and 128 reveals the former to be 26 % to 45 % easier to deform than the latter. Based on these results, the nonplanarity seen for the phenylenes is most likely a consequence of crystalpacking forces deforming particularly flexible molecules.

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Section: Introductionmentioning

confidence: 99%

On the Nature of Nonplanarity in the [N]Phenylenes

et al. 1999

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“…39,40 The multistep conversion comprises isomerization of E-stilbenes to Z-stilbenes, followed by cyclization of intermediate radicals to dihydrophenanthrenes that are irreversibly dehydrogenated to phenanthrenes in the presence of oxidants. 37 The stilbenes 84-87 required for our investigation were prepared by Wittig reaction using the procedure of The preferential formation of the less hindered regioisomer has also been observed in hydroxylated precursors.…”

Section: Photocyclizationmentioning

confidence: 99%

Synthesis and electrochemistry of annoquinone-A, cypripedin methyl ether, denbinobin and related 1,4-phenanthrenequinones

Krohn¹,

Loock²,

Paavilainen³

et al. 2001

Arkivoc

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The natural product annoquinone-A (1), the 1,4-phenanthrenequinones 41-75 and the 9,10-dihydro-1,4-phenanthrenequinones 76-81 were prepared by Diels-Alder reaction of the styrenes 23-37 with the benzoquinones 38-40. The sterically hindered 5-methoxy compounds, which adopt a twisted conformation as shown by X-ray analysis, are only formed in trace amounts or as the less hindered 9,10-dihydro-1,4-phenanthrenequinones 76-81. The twisted conformation also leads to characteristic changes in the NMR spectra and the redox potential. Reaction of methoxybenzoquinone with styrenes preferentially affords the 3-methoxy-1,4-phenanthrenequinones. Selective ether cleavage of the 5-methoxy group in 60 leads to the natural product denbinobine (3). Small amounts of C-9 hydroxylated compounds (57, 66, 68, 73, and 75) were also formed in the Diels-Alder reactions. In an alternative synthesis, using the photocylization of the stilbenes 84-87 followed by CAN oxidation, the 1,4-phenanthrenequinones 45, 51, 59 [methyl ether of cyprepidine (2)

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“…[2] The two situations are illustrated in Figure 1, [2] which was adapted from theoretical results in the literature. [19,20] According to this model, two different complexes with different bonding characteristics can have the same unusually high n Ä(CO) value.…”

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confidence: 99%

“…[1±5] Examples are (average values for n Ä(CO) in parentheses; for free CO: n Ä(CO) 2143 cm ), [6] [Ag(CO) 2 ), [8] [ ), [9] and ).…”

mentioning

confidence: 99%

“…The first metal carbonyl derivative [Pt(CO) 2 Cl 2 ] reported in 1868 with an average n Ä CO value of 2175 cm À1 [well above that (2143 cm À1 ) for free CO] would be assigned ºnonclassical,ª which is in our view absurd.º [10] It is historically ironic, but hardly absurd, that the very first carbonyl complex has an average n Ä(CO) value higher than that of free CO, while more than 95 % of the metal carbonyls which were subsequently studied have average n Ä(CO) values less than 2143 cm…”

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confidence: 99%

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